Note: Descriptions are shown in the official language in which they were submitted.
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FIBER REINFORCED COMPOSITE CORES AND PANELS
Field Of The Invention
This invention relates to sandwich panel composite structures comprising
fiber reinforced low density cellular material, resin, fibrous and non-fibrous
skin
reinforcements, and in particular to improved structural configurations,
improved
methods of resin infusion and methods of production.
Background of the Invention
00011 Structural sandwich panels having cores comprised of low
density closed cell material, such as closed cell plastics foam material, and
opposing skins comprised of fibrous reinforcing mats or fabrics in a matrix of
cured resin; have been used for many decades in the construction of a wide
variety of products, for example, boat hulls and refrigerated trailers. The
foam
core serves to separate and stabilize the structural skins, resist shear and
compressive loads, and provide thermal insulation.
[0002] The structural performance of sandwich panels having foam
cores may be markedly enhanced by providing a structure of fibrous reinforcing
members within the foam core to both strengthen the core and improve
attachment of the core to the panel skins, for example, as disclosed in
applicant's U.S. Patent No. 5,834,082, When porous and fibrous reinforcements
are introduced into the closed cell foam core and a porous and fibrous skin
=
reinforcing fabric or mat is applied to each face of the core, adhesive resin,
such
as polyester, vinyl ester or epoxy, may be flowed throughout all of the porous
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skin and core reinforcements by differential pressure, for example under a
vacuum bag. While impregnating the fibrous reinforcements, resin does not
saturate the plastic foam core because of its closed cell composition. The
resin
then co-cures throughout the reinforced structure to provide a strong
monolithic
panel.
[0003] It is desirable to produce sandwich panels of enhanced
structural performance by improving the structural connections and support
among reinforcing members within the foam core and between the core and the
panel skins. This is desirable in order to resist buckling loads in the
reinforcing
members, to prevent premature detachment of reinforcing members from one
another and from the skins under load, and to provide multiple load paths for
the
distribution of forces applied to the panel. Existing fiber reinforced core
products offer important improvements over unreinforced foam in this regard
but
fail to integrate fully the separate reinforcing elements of the core into a
unified
and internally supported structure. For example, in a grid-like configuration
of
fibrous reinforcing sheet-type webs in which a first set of continuous webs is
intersected by a second set of interrupted or discontinuous webs, the webs do
support each other against buckling. However, under severe loading conditions,
the discontinuous webs tend to fail at the adhesive resin bond to the
continuous
webs along their narrow line of intersection. This tendency may be
substantially
reduced by providing resin filled fillet grooves in the foam along the lines
of
intersection as disclosed in the above mentioned patent. Moreover, since the
reinforcing fibers of interrupted webs terminate at each intersection with a
continuous web, the structural contribution of those fibers is substantially
less
than that of the fibers of the continuous webs.
[0004] In the case of strut or rod type core reinforcements
comprising rovings of fiberglass or carbon fiber or other fibers which extend
between the faces of the core, individual struts within a given row of struts
may
intersect each other in a lattice configuration. This supplies buckling
support to
each strut, but only in the plane of the strut row. To achieve bidirectional
support, struts of a first row must extend through the filaments of struts of
an
intersecting row. This requires difficult and costly levels of accuracy and
control
in machine processing, since all struts must be precisely positioned in three
dimensions.
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Summary of the Invention
[0005] One embodiment of the present invention overcomes the
limitations of both web type and strut type reinforced foam cores by combining
these two types of reinforcing elements into hybrid reinforcement
configurations.
In hybrid architecture the foam core is provided with parallel spaced rows of
fibrous reinforcing webs or sheets which extend between the faces of the foam
board at an acute or right angle. A second set of parallel spaced rows of
reinforcing elements comprising rod-like fibrous rovings or struts also extend
between the faces of the. foam board at acute or right angles, and the rovings
or
struts intersect the webs and extend through them. Thus webs and struts
constitute an interlocking three dimensional support structure in which all
reinforcing fibers within the core are uninterrupted. The interconnected webs
and struts provide multiple load paths to distribute normal loads efficiently
among
the reinforcing elements of the core and between the core structure and the
panel skins. Impact damage tends to be limited to the immediate area of
impact,
since the complex reinforcement structure resists the development of shear
planes within the core.
[0006] In an alternate hybrid architecture, the webs comprise a
continuous sheet of fabric or mat which is formed into corrugations having
segments which extend between the faces of the core, and the voids between
the corrugations are filled with foam strips of matching cross-section. The
corrugations, together with the intersecting panel skins, may form, in cross-
section, rectangles, triangles, parallelograms or other geometric shapes which
are structurally efficient or which offer manufacturing advantages.
[0007] In a particularly cost efficient version of hybrid core,
the core
reinforcing webs are produced by winding relatively low cost fibrous rovings
in
a helical manner onto rectangular foam strips, rather than by adhering
substantially more expensive woven or stitched fabric to the surface of the
foam
strips. Additional rovings may be applied axially along the length of the
strips
during the winding operation to enhance structural properties of the strips or
to
serve as low cost components of the finished panel skins. The fiber-wound foam
strips may also be attached together to form a structural core without the
addition of rows of structural struts. In this configuration, the contiguous
or
adjacent sides of wound strips of rectangular cross section form web elements
having I-beam flanges for attachment to panel skins. In contrast to the
disclosure of U. S. Patent No. 4411939, the fibrous extensions of each core
web
are attached to panel skins on both sides of the web rather than only one,
greatly
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increasing the shear strength of the resulting panel. This permits the use of
lighter and less expensive webs for a given strength requirement. Similarly,
the
present invention provides markedly improved core-to-skin attachment and
shear strength when compared to the structure disclosed in Applicant's U.S.
Patents No. 5,462,623, No. 5,589,243 and No. 5,834,082. In tests, webs
comprised of circumferentially wound rovings exhibit 75% greater shear
strength
than those whose end portions terminate adjacent the panel skins. Each wound
strip may be provided with internal transverse reinforcing webs to provide bi-
directional strength and stiffness. Roving-wound cores may also be formed
using strips of triangular cross section.
[0008] The winding of rovings by machine and the consolidation
of
the fiber-wound strips into a single core have both economic and handling
advantages. It is common for a single composite bridge deck panel or yacht
hull
constructed in accordance with U. S. Patent Nos. 5,701,234, 5904972 or
5958325 to comprise a thousand or more individual core blocks. The labor
component of producing these individual cores is very high. Reinforcement
fabric is cut into sheets which are wrapped and glued around each separate
core, or smaller pieces of fabric are glued to the separate faces of each
core, or
tubular fabrics are first formed and the cores inserted into them. These
processes become increasingly difficult as the dimensions of the core
components decrease. Arrangement of these cores in a mold is also labor
intensive, expensive and time consuming, which restricts the number of panels
which may be produced from a mold in a given period of time. Positioning of
individual core blocks becomes increasingly awkward as the curvature of the
Mold increases or as the mold surface departs from horizontal. The cores which
are the subject of the present invention substantially eliminate these
deficiencies
by unitizing a large number of components into a single, easily handled core.
[0009] In addition to their superior structural performance,
hybrid
design allows economical production of extremely complex and structurally
efficient configurations through relatively simple processes at high machine
throughput and without requiring extreme levels of manufacturing precision. As
mentioned above, bidirectional strut type cores require the insertion of
roving
reinforcements into the foam board with a degree of accuracy which is
difficult
to achieve when it is desired that rovings of intersecting rows extend through
one
another. It is also necessary to make multiple passes through strut insertion
devices in order to place struts angled in two to four directions within the
board.
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[0010] In contrast, bidirectional hybrid cores may be produced
in
as little as a single pass through a strut insertion device. The reinforcement
webs cooperate with the intersecting struts to resist loads in the plane of
the
struts. The webs also provide strength in the direction transverse to the
struts,
since the webs extend transversely to the rows of struts. Further, a much more
limited degree of accuracy is required in production, since the struts have
only
to intersect the plane of the webs, rather than a narrow bundle of filaments.
[0011] Hybrid cores improve production of molded panels by
increasing the rate and reliability of resin impregnation or infusion of both
the
core reinforcing elements and the sandwich panel skins which overlie the core.
In vacuum assisted resin transfer molding (VARTM) processes, panels
comprising dry and porous skin reinforcements are placed in a closed mold or
a single sided mold in which the panel is covered by a sealed bag impermeable
to air. The panel is then evacuated, and resin under atmospheric pressure is
allowed to flow into and infuse the reinforcements. Because of the complex
interconnections between the webs and struts in the cores of the present
invention, both air and resin are able to flow rapidly and pervasively
throughout
the structure. The porous webs and struts form natural resin flow paths
between
the skins and carry resin rapidly from its source of introduction to a
multiplicity
of points at the porous skins. This minimizes the problem of race tracking, in
which areas of dry skin fabric become isolated from the vacuum source by an
unevenly advancing resin front, preventing the skins to wet out fully before
the
resin begins to thicken and cure.
[0012] In one embodiment of the present invention, no resin
distribution medium of any kind is required between the panel skins and the
mold
surface or vacuum bag membrane. This not only eliminates the cost of such
distribution medium but also allows the production of panels having smooth
faces on all sides. Also, in contrast with prior art such as disclosed in U.S.
Patent No. 5,958,325, the foam core need not be provided with micro grooves
located on the periphery of the core adjacent the panel skins, or with slots
or
holes in the foam which extend between the skins, as the means for
distributing
resin to the skins. In the present invention, all resin flows to the skins
through
the core reinforcing structure, whereas Patent No. 5,958,325 specifically
describes impregnation as resulting from resin infusion originating at the
core
surface. A disadvantage of peripheral micro grooves is that the size and
spacing
of the micro grooves must be selected to match the type and quantity of the
panel's fibrous fabrics in order to insure full impregnation of the skin and
core
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reinforcements before the resin cures. In the present invention, all of the
resin
which infuses the skins passes through the porous reinforcing structure of the
core to reach the skins, and since panel skins are typically intersected by
two or
more porous reinforcing elements per square inch of panel surface, resin tends
to spread both rapidly and evenly across the skin surface. Thorough
impregnation of the panel skins, which can be seen, is a reliable indicator
that
the core reinforcing structure does not have dry, and therefore weak areas.
This
is an important advantage over other infusion systems, in which resin is
introduced adjacent the skins.
[0013] In accordance with the present invention, resin is
supplied
to the core reinforcing structure through a network of grooves within the
interior
of the foam core and adjacent the core reinforcing webs and extending parallel
to the webs, and not adjacent the panel skins. The ends of these grooves
intersect feeder channels which usually have a larger cross-sectional area.
Resin
supplied to the feeder channels rapidly flows through the grooves adjacent the
webs and substantially all of the resin then flows through the fibrous core
reinforcing elements to reach and impregnate the panel skins. If the resin
grooves are located in a plane near the center of the panel thickness, resin
need
only flow through half the thickness of the panel, in each direction from the
center plane, before full resin saturation is achieved. This is markedly
faster
than common resin infusion techniques in which resin is introduced across a
single panel face and must flow through the entire panel thickness to reach
and
infuse the opposing face. Panels with thick cores or thick skins may be
provided
with one or more additional sets of resin grooves and feeder channels for
faster
infusion. The sets of grooves and feeder channels describe a plurality of
planes
parallel to the panel faces.
[0014] The infusion method of the present invention is
particularly
well suited for the production of molded panels in which both faces of the
panel
require a superior surface finish. Because resin is introduced into the
interior of
the core and flows rapidly under differential pressure throughout the core to
the
skin reinforcing structure, both faces of the panel may be adjacent rigid mold
surfaces of desired shape and finish, without seriously increasing the time
required for infusion compared to infusion conducted under a flexible surface,
such as a vacuum bag. In contrast, common differential pressure molding
processes such as VARTM, in which the skin reinforcements are consolidated
by pressure prior to the introduction of resin, require that one side of the
panel
be covered with a flexible membrane, such as a vacuum bag, enclosing a resin
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distribution medium if it is desired to both maintain substantial pressure and
introduce resin rapidly over the skin surface. If this arrangement is not
used, the
pressure of rigid mold surfaces against both panel faces necessitates a long
and
slow infusion path, in which the resin impregnates the skins by flowing along
their length and width, rather than through their thickness.
[0015] The inside-out core infusion method of the invention may
be
used to infuse into the fibrous core reinforcements and inner skin layers a
resin
which differs in properties from the resin which infuses the outer skin layers
of
the panel. It may be used, for example, to produce a sandwich panel having an
outer skin layer comprising fire resistant phenolic resin and an inner skin
layer
and core reinforcement structure comprising structural vinyl ester resin. This
is
achieved by providing an adhesive barrier, for example of epoxy resin in film
form, between inner and outer layers of porous, fibrous skin reinforcements. A
first resin is supplied by infusion from within the core as previously
described,
and a second resin is infused directly into the outer skin reinforcements,
with the
barrier film serving to keep the resins separate while creating a structural
adhesive bond between them.
[0016] In a useful variation of the hybrid core of the
invention, the
reinforcing webs do not extend between the faces of the panel. Instead, two or
more foam boards are interleaved with porous, fibrous web sheets and stacked
in a sandwich configuration. Porous roving struts or rods extend between the
faces of the core and through the intermediate web sheet or sheets. The web
or webs stabilize the struts against buckling under load and also serve to
distribute resin to the struts and skins. Resin may be introduced through
parallel
spaced grooves in the foam adjacent the web. Alternately, resin may be flowed
into the core through a feeder channel which is perpendicular to the panel
faces
and which terminates in radial grooves adjacent the webs. This arrangement is
useful in infusing circular panels, for example, manhole covers. In a third
variation, the web sheet may incorporate low density fibrous mat or
non-structural, porous infusion medium through which resin supplied through
feeder channels flows across the center plane of the panel to the struts and
through the struts to the panel skins.
[0017] An additional feature of the present invention is the
provision
of improved connections between strut or rod -type core reinforcing elements
and sandwich panel skins. This improvement is applicable to hybrid panels
having both web and strut-type core reinforcing members, as well as to panels
whose core reinforcing comprises only struts. The porous and fibrous struts
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which extend between the faces of the core may terminate between the core and
the skins, may extend through the skins and terminate at their exterior
surfaces,
or may overlie one or more layers of the panel skins. Under load, the struts
are
subject to substantial forces of tension or compression at the point of
intersection with the skins, and these forces may cause failure of the
adhesive
bond between reinforcing element and skins.
[0018] Prior art, for example, as disclosed in European Patent
No.
0 672,805 B1, discloses the provision of looped end portions of the
reinforcing
elements adjacent the skins. Under pressure during molding, the loops formed
in the end portions of the struts provide an expanded area of adhesive contact
with the skins. However, a serious disadvantage of this design is that the
loops,
which are doubled-back bundles of fibers, form lumps which cause the panel
skins to deform out of plane under molding pressure. This results in excess
resin accumulation in the skins, an increase in the tendency of the skin to
buckle
under in-plane compressive loads, and undesirable surface finishes.
[0019] In the present invention, terminating ends of strut type
reinforcing elements are cut to allow the filaments which comprise the struts
to
flare laterally under molding pressure, which both significantly flattens the
end
portions against the skins and provides an expanded area of adhesive bond
between each strut end portion and skin in the region immediately adjacent the
strut end portion. Skin surface flatness may be further improved by applying
sufficient pressure, sometimes simultaneous with heat, to the faces of the
panel
before molding to provide recesses for embedding any reinforcement lumps or
ridges into the foam core during the molding process. Alternately, grooves may
be formed in the faces of the foam along the lines of strut insertion, into
which
strut end portions or overlying stitch portions are pressed during molding.
[0020] The present invention also provides an alternate method
of
anchoring strut ends and which is effective even when the strut end portions
do
not overlie panel skins. In this configuration, parallel grooves or slits are
so
located in the faces of the foam board that the end portions of strut-type
reinforcing members pass through the grooves. Porous reinforcing rovings
having sufficient depth to adhesively anchor the strut ends are inserted into
the
grooves prior to insertion of the strut members, and resin which flows into
the
structure during molding provides structural attachment of struts to the
rovings
within the grooves. The rovings, having a substantial area of contact with the
overlying panel skins complete the transfer of structural loads between skins
and
cores. An important additional benefit of this construction is that the groove
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rovings and strut members may be sized so as to constitute a unitized truss
structure, with the groove rovings serving as truss chords. Since rovings cost
substantially less than woven fabrics, this allows for economical panel
fabrication
in cases where relatively thin skins are adequate between the truss rows.
[0021] In the present invention, low cost rovings may also be
applied directly to the faces of the foam boards to form panel skins during
the
process of inserting reinforcing members into the foam and in lieu of applying
skins of more costly woven or knitted fabric reinforcements to the faces of
the
core. In this method, multiple rovings are supplied along parallel lines
transverse
to the core length and are drawn in a longitudinal direction continuously from
supply creels by the forward progress of the foam core through the strut
insertion
machine, in sufficient number to more or less cover the faces of the foam.
Prior
to strut insertion, groups of rovings are drawn transversely, at right or
acute
angles, across the faces of the core from creels and advance with the core
while
strut rovings are stitched through the core. Overlying portions of the
stitches
hold all surface rovings in position to form a structural panel skin once
resin has
been applied to the panel. If desired, a light veil of reinforcing material
may be
applied over the surface rovings before stitching to improve the handling
characteristics of the core prior to molding. In lieu of continuous rovings,
random
or oriented chopped rovings may be applied between the core faces and surface
veils to form a structural mat.
[0022] Sandwich panels comprising helically wound rovings which
overlie and restrain axial rovings which have been substituted for skin fabric
reinforcements are effective at resisting skin delamination, even if the skins
are
not stitched through the core. This is quite useful in areas of non-uniform
core
thickness, for example at panel edge step-downs and tapers, which are subject
to delamination due to buckling or tensile loads in the skins.
[0023] The present invention includes several useful variations
of
reinforced core panel having bi-directional core strength and in which all of
the
core reinforcing members are provided by means of a helical winding process.
In the most economical embodiment, a unidirectional core panel comprised of
parallel wound foam strips is cut in a direction perpendicular to the axis of
the
strips into uniform second strips, which are then rotated 90 degrees and
consolidated to form a second unitized core panel. The original helically
wound
rovings then extend between the faces of the core panel as separate strut-like
roving segments whose end portions terminate adjacent the faces of the core.
This core architecture provides bi-directional shear strength and high
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compressive strength, but reduced attachment strength of the core to panel
skins. Skin attachment may be enhanced by helically winding the second strips
prior to their consolidation, to provide layers of wound reinforcements which
extend continuously between the foam strips and across the faces of the core
panel adjacent the skins. Depending upon the structural properties desired,
the
wound second strips may be oriented, prior to consolidation, to provide
doubled
layers of rovings either between or adjacent the skins. Bi-directional core
panels
may also be provided with parallel rows of continuous rovings which are
inserted
into slits in the faces of the core panels to form support members between the
core reinforcing webs for thin panel skins. Skin support between wound
reinforcing webs may be provided in unidirectional cores by winding pairs of
foam strips which have been provided with reinforcing webs between the strips
prior to winding.
[0024] An important advantage of all of the bi-directional cores
described herein is that the intersecting reinforcing webs stabilize each
other
against buckling under load into the adjacent low density and low strength
foam
strip. Web buckling resistance in unidirectional cores may be improved by
increasing the effective width of the web by providing a spacer strip, for
example
high density foam plastic, between adjacent wound foam strips. In an
economical form of unidirectional core panel, roving-wound foam strips
alternate
with plain foam strips, thus permitting the doubling of panel output for a
given
amount of winding machine output. To stabilize the webs against buckling in
this
embodiment, the spacer strip is provided between the opposing wound layers
on opposing sides of each wound strip. Unidirectional strips may be modified
to
provide bi-directional strength, by providing strips of serpentine or other
configuration in which the edges of the strips are not parallel and thus
provide
structural properties in directions other than the general direction of the
strip.
Core panels comprising strips of all configurations and incorporating
thermoplastic resin may be economically produced by applying reinforcing
fibers
and low cost thermoplastic materials to the strips as separate components for
subsequent consolidation under heat and pressure.
[0025] The structural performance of helically wound strips may
be
improved by providing rovings which extend axially along the corners of the
strips
and beneath the wound rovings. This addition causes the reinforcing web on
each side of each foam strip to take the general form of a bar joist having
top
and bottom chords which are separated by rod-like shear members. This
structure is more resistant to impact, and the axial rovings may permit the
use
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of less reinforcing fiber in the panel skins. Individual strips so constructed
may
be used as discrete structural members, for example columns or box beams,
whose performance may be further enhanced by providing the strips with
transverse reinforcing members and by providing additional axial rovings
between the corners of the strips.
[0026] The structural efficiency of certain panels comprising
wound
strips may be enhanced by varying the feed rate of the strip through the
roving
winding apparatus, in order to vary the angle and density of the wound
reinforcements along the length of the foam strips. This may provide improved
compressive strength for the panel at load bearing points, or core shear
resistance which is tailored to match predicted shear loads along the length
of
the panel.
[0027] Shear loads in core panels comprising unidirectionally-
wound foam strips may be may be transferred to the ends of the strips and
thence to intersecting panel reinforcements by spacing continuously wound foam
strips during the winding process and folding the strips back and forth before
consolidating them to form a core panel. This positions the wound rovings of
the
spaced segments across opposing ends of the foam strips and provides a strong
structural connection to panel edge reinforcements or to adjacent core panels.
It may also be desirable to produce sandwich panels of generally cylindrical
or
other closed configuration and having continuous core panel reinforcements
which do not end in core joints and thereby avoid structural discontinuities.
This
embodiment may be used for example to form jet engine casings, which are
designed to resist very high energy impacts while maintaining the overall
integrity
of the casing. The core panel is produced by helically winding reinforcing
rovings around a continuous foam strip, then wrapping the strip helically
around
a cylindrical mandrel. Continuous axial rovings may be provided underneath the
wound rovings for additional hoop strength and resistance to impact.
[0028] In a useful embodiment of the present invention, thin-
walled
tubes are substituted for the foam strips onto which reinforcing rovings are
wound. The tubes may comprise material of low structural properties, for
example stiffened paper, or of high structural properties, for example roll
formed
or extruded aluminum, preferably treated for strong adhesion to the resins
used
as the matrix for the fibrous reinforcements. This embodiment is useful when
it
is desirable to provide a hollow structure, or to eliminate the weight of the
low
density solid core, or to incorporate the structural properties of the tubular
material into the panel.
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[0029]
Another means of enhancing the impact resistance of
sandwich panels comprising helically wound cores and thermoset resins is to
incorporate thermoplastic resins, which are generally substantially less
brittle
than thermoset resins, into the outer portions of the panel skins. This may be
accomplished by several means. A thermoplastic film may be heated to flow into
the outer portion of a fibrous reinforcing mat or fabric, leaving the inner
portion
porous, for subsequent impregnation with the thermoset resin used to
impregnate the core reinforcements. If desired, a layer of fabric comprised of
commingled fiberglass and thermoplastic fibers may be substituted for the
thermoplastic film. The commingled fabric is heated to form a reinforced
thermoplastic outer surface and to flow the thermoplastic resin partially
through
the thickness of the inner reinforcing mat. In
still another embodimentõ
commingled fabric skin may be placed adjacent the reinforced core and infused
without application of heat, so that both the fiberglass and the thermoplastic
fibers of the skin are impregnated by the thermoset resin used to infuse the
core.
Brief Description of the Drawings
[0030] FIG. 1
is a fragmentary perspective view of a reinforced
foam core composite panel constructed in accordance with the invention;
[0031] FIG. 2
is a fragmentary section of a reinforced foam core
composite panel constructed in accordance with another embodiment of the
invention;
[0032] FIG. 3
is a fragmentary section of another embodiment of
a reinforced foam core composite panel constructed in accordance with the
invention;
[0033] FIG. 4
is a fragmentary section of another embodiment of
a reinforced foam core composite panel constructed in accordance with the
invention;
[0034] FIG. 5
is a fragmentary section of another embodiment of
a reinforced foam core composite panel constructed in accordance with the
invention;
[0035] FIG. 6
is a fragmentary section of another embodiment of
a reinforced foam core composite panel constructed in accordance with the
invention, with a center portion broken away;
[0036] FIG. 7
is a fragmentary section taken generally on the line
7--7 of FIG. 6 and with a center portion broken away;.
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[0037] FIG. 8 is a fragmentary section of another embodiment of
a reinforced foam core composite panel constructed in accordance with the
invention;
[0038] FIG. 9 is a fragmentary perspective view of a reinforced
foam core composite panel constructed in accordance with another embodiment
of the invention;
[0039] FIG. 10 is a fragmentary perspective view of a reinforced
foam core composite panel constructed in accordance with another embodiment
of the invention;
[0040] FIG. 11 is a fragmentary perspective view of a reinforced
foam core composite panel constructed in accordance with a modification of the
invention;
[0041] FIG. 12 is a diagrammatic view of apparatus for producing
fiber-wound foam strips in accordance with the invention;
[0042] FIG. 13 is a fragmentary perspective view of a fiber-
wound
foam strip constructed in accordance with the invention;
[0043] FIG. 14 is a fragmentary perspective view of a reinforced
foam core composite panel constructed in accordance with the invention;
[0044] FIG. 15 is a diagrammatic view of apparatus for producing
fiber reinforced foam core panels in accordance with the invention.
[0045] FIG.16 is a fragmentary perspective view of a reinforced
foam component constructed in accordance with the invention;
[0046] FIG.17 is a fragmentary perspective view of a reinforced
foam component using the component of FIG. 16;
[0047] FIG.18 is a fragmentary perspective view of a reinforced
foam core constructed in accordance with the invention and using the
component of FIG. 17;
[0048] FIG.19 is a fragmentary perspective view of another
embodiment of a reinforced foam core constructed in accordance with the
invention;
[0049] FIG. 20 is a fragmentary perspective view of a core panel
constructed in accordance with a modification of the invention;
[0050] FIG. 21 is an enlarged fragmentary portion of FIG. 20;
[0051] FIG. 22 is a fragmentary perspective view of a section
cut
from the panel shown in FIG. 20;
[0052] FIG. 23 is a fragmentary perspective view of a core panel
formed with the strips shown in FIG. 22 and partially exploded;
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14
[0053] FIG. 24 is a perspective view of the strip shown in FIG.
22
with helically wound rovings;
[0054] FIG. 25 is an enlarged perspective view of a portion of
the
wound strip shown in FIG. 24;
[0055] FIG. 26 is a fragmentary perspective view of a core panel
constructed with strips as shown in FIG. 24;
[0056] FIG. 27 is a fragmentary perspective view of a core panel
constructed with strips shown in FIG. 24 in accordance with a modification of
the
invention;
[0057] FIG. 28 is a fragmentary perspective view of a core strip
formed in accordance with another modification of the invention;
[0058] FIG. 29 is an enlarged perspective view of a portion of
the
core strip shown in FIG. 28;
[0059] FIG. 30 is a fragmentary perspective view of a core panel
constructed using core strips as shown in FIG. 28;
[0060] FIG. 31 is a fragmentary perspective view of a core panel
formed in accordance with another modification of the invention;
[0061] FIG. 32 is a fragmentary perspective view of a core panel
constructed in accordance with another modification of the invention;
[0062] FIG. 33 is a fragmentary perspective view of a core strip
formed in accordance with a modification of the invention;
[0063] FIG. 34 is a fragmentary perspective view of another core
panel formed in accordance with a modification of the invention;
[0064] FIG. 35 is a fragmentary perspective view of an annular
core
assembly formed helically winding a core strip constructed in accordance with
the invention;
[0065] FIG. 36 is a fragmentary perspective view of a core panel
formed of tubular core strips each having helically wound rovings and formed
in
accordance with a modification of the invention;
[0066] FIG. 37 is fragmentary plan view of a core strip
constructed
in accordance with another further modification of the invention;
[0067] FIG. 38 is a fragmentary plan view of a core panel formed
with the core strip shown in FIG. 37 in accordance with the invention;
[0068] FIG. 39 is a fragmentary perspective view of a core panel
formed in accordance with another modification of the invention;
[0069] FIG. 40 is a fragmentary perspective view of a panel
formed
in accordance with another modification of the invention;
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[0070] FIG. 41 is a fragmentary perspective view of a composite
panel formed in accordance with another modification of the invention;
[0071] FIG. 42 is a fragmentary perspective view of a modified
core
panel formed in accordance with the invention;
[0072] FIG. 43 is a fragmentary perspective view of another
composite panel formed in accordance with the invention;
[0073] FIG. 44-47 are fragmentary perspective views of core
panels
formed in accordance with the invention;
[0074] FIG. 48 is a diagrammatic perspective view of apparatus
showing the method of making a composite panel in accordance with the
invention;
[0075] FIG. 49 is another diagrammatic perspective view of
another
apparatus showing another method of making a composite panel in accordance
with the invention;
[0076] FIG. 50 is a further diagrammatic perspective view of
apparatus for producing another form of composite panel in accordance with the
invention; and
[0077] FIG. 51 is a fragmentary exploded end view of a composite
panel formed in accordance with the invention.
Description of the Preferred Embodiments
[0078] FIG. 1 illustrates a structural composite sandwich panel
30
which may be used, for example, as the floor of a highway truck cab, the hull
or
transom of a boat, the roof of a factory building, or as a vehicular or
pedestrian
bridge deck. Panel 30 comprises a fiber reinforced closed cell plastic foam
core
31 and opposing fiber reinforced skins 32. Foam core 31 comprises a plurality
of foam strips 33, whose structural properties are insufficient to resist
loads in
the core which would correspond with loads for which skins 32 are designed.
[0079] The core reinforcing fibers, which are selected to impart
the
required structural properties to the core, are of fiberglass or carbon fiber
or
other reinforcing fibers. In one direction, the reinforcing fibers comprise a
plurality of parallel sheets or webs 34 of porous, fibrous fabric or mat which
extend between the faces of the core 31 and which have been adhesively
attached to one face of each foam strip 33 while maintaining substantial
porosity
in the web material. If desired, the webs 34 may incorporate reinforcements
comprising a plurality of individual rovings adhesively applied to foam boards
(not shown) from which strips 33 are cut. In a crossing direction, generally
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perpendicular to the webs 34, the core reinforcing fibers comprise a plurality
of
parallel rows of spaced rods or struts 35, which extend between the faces of
the
core and are made up of bundles or rovings of porous reinforcing filaments.
[0080] Each row of struts comprises a plurality of struts 35
inclined
at opposing acute angles, for example +58 degrees and -58 degrees or + 45
degrees and -45 degrees, to the panel skins. The two sets of opposing struts
in each row lie in the same plane and intersect each other to form a
triangulated
or lattice type structure. The diameter and spacing of struts 35 within a row
of
struts are determined by structural considerations, but are commonly in the
range of .01 inch to .12 inch diameter and .25 inch to 2.0 inch spacing. In
some
cases struts may exceed .50 inch diameter and 7.0 inch spacing. Rows of struts
35 are commonly spaced 0.5-in. to 1.0-in. apart. The closed cell foam strips
or
pieces 33 may be of polyurethane, polyvinylchloride, polystyrene, phenolic,
polyethylene, polymethacrylimide or other foam material having the desired
properties for a specific application. Typically, foam density is low, in the
range
of 2 to 5 pounds per cubic foot, but much higher densities may be used where
appropriate.
[0081] As shown in FIG. 1, the struts 35 intersect webs 34, and
the
fibers which comprise the struts extend through the fibers which comprise the
webs. Since the fibrous rovings which comprise the struts are inserted into
the
foam core and through the webs in a stitching operation, the filaments which
comprise the struts pass through the filaments of the webs without breaking
either set of filaments, so that the continuity of all elements of the core
reinforcing structure remains intact. In a preferred embodiment, panel skins
32
comprise inner skins 36 and outer skins 37. The end portions 38 of reinforcing
struts 35 also extend through the inner skins 36 and flare laterally to
overlie the
inner skins 36. The inner skins 36 are covered by outer skins 37 prior to
molding
panel 30 with resin. The struts are thus mechanically attached to the skins,
providing high resistance to delamination of skins 32 from core 31 under load.
If desired, the end portions of strut rovings may terminate adjacent the faces
of
the reinforced core 31.
[0082] The porous and fibrous reinforcements of both core and
skins are impregnated or infused with an adhesive resin which flows,
preferably
under differential pressure, throughout all of the reinforcing materials and
cures
to form a rigid, load bearing structure. Before panel 30 is molded and cured,
inner skins 36 and foam strips 33 with their attached webs 34, are held
together
as a unitized structure by friction caused by pressure of the plastic foam and
the
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skin fibers against the roving fibers which form the struts 35, as well as by
the
roving segments or end portions which overlie the panel skins. While the core
30 may vary widely in dimensions for specific applications, practical core
sizes
include, for example 0.25-in. to 5.0-in, thick and 2-ft. to 8-ft. wide x 2-ft.
to 40-ft.
long. Cores are commonly produced in continuous lengths and cut to the
desired length. To mold sandwich panels which are larger in area than a single
reinforced core constructed in accordance with the present invention, two or
more cores may be arranged adjacent each other in the mold prior to the
introduction of resin.
[0083] Shear loads in the core 31 are resisted in one direction
primarily by the struts 35 and in the transverse direction primarily by the
webs
34. In addition, a complex integration of webs and struts is achieved through
the
rigid resin bond at each point of intersection of strut and web and through
the
continuity of reinforcing fibers through all such intersection points. Webs
and
struts support each other against buckling loads, which permits the use of
lighter
weight reinforcing members in thick panels, where the slenderness of the core
reinforcing members makes them prone to buckling failure. The configuration
shown in FIG. 1 is able to resist large compressive loads perpendicular to the
skins, since the webs 34 are oriented at right angles to skins 32 and are
restrained from buckling by the struts 35. The structural integration of webs
and
struts also provides multiple load paths to increase the sharing of localized
compressive loads among the core reinforcing elements and provides
substantial resistance to the initiation and spread of planes of shear failure
separation within the core. Adhesive and mechanical attachment of core
reinforcing members to skins provides high resistance to pull-through of
fasteners in the panel skins.
[0084] The fiber reinforcements of the foam core and skins are
commonly impregnated or infused with resin by flowing the resin throughout the
porous reinforcing fibers under differential pressure in processes such as
vacuum bag molding, resin transfer molding or vacuum assisted resin transfer
molding (VARTM). In VARTM molding, the core and skins are sealed in an
airtight mold commonly having one flexible mold face, and air is evacuated
from
the mold, which applies atmospheric pressure through the flexible face to
conform panel 30 to the mold and compact the fibers of the skins 32. Catalyzed
resin is drawn by the vacuum into the mold, generally through a resin
distribution
medium or network of channels provided on the surface of the panel, and is
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allowed to cure. The present invention may, if desired, incorporate an
improved
method of VARTM infusion.
[0085] Reinforced core 31 may be provided with resin grooves 39
machined into foam strips 33 and located adjacent webs 34 within the interior
of
the foam core 31. The grooves 39 terminate at a resin feeder channel 40 (FIG.
1) which is usually larger in cross sectional area than individual grooves 39,
but
may be of the same size. Channel 40 serves to distribute the resin under
differential pressure to the grooves 39. Feeder channels 40 may be located
either along one or both of the edges of the reinforced core 31 at which
reinforcing webs 34 terminate. Alternately, channel 40 may be located entirely
within the interior of the core. For purposes of illustration, FIG. 1 shows
channel
40 at the core edge, and FIG. 7 shows the feeder channel in the core interior.
If channel 40 is provided on only one edge of core 31, grooves 39 may extend
to the opposing edge of core 31 or alternately may terminate within foam strip
33, depending upon the dynamics of resin flow within the reinforced foam core
and panel skin reinforcements.
[0086] Catalyzed resin flows to channel 40 through a tube (not
shown) connected to a resin source, commonly a drum of resin. The tube
opening may be located at any point along channel 40. In a preferred method
of infusing the reinforced cores of the present invention using a vacuum bag,
the
mold is sealed and evacuated prior to attaching any resin plumbing apparatus
to the mold. A rigid resin connection or insertion tube is provided with a
sharp,
pointed end and is then inserted through the vacuum bag membrane and panel
skins 36 and 37, or through the vacuum bag at the edges of panel 30, and into
reinforced core 31, intersecting feeder channel 40. The insertion tube has
been
provided with openings in its circumference which permit the flow of resin
into
channel 40. A tape sealant is applied at the point of insertion to prevent
loss of
vacuum, the insertion tube is connected to the resin supply, and resin is
drawn
by the vacuum through the insertion tube and into channel 40.
[0087] In addition to the speed, simplicity and low material
cost of
this method of introducing resin into the panel, additional resin connection
tubes
may be inserted into the panel at other locations, while the infusion is in
progress, to bring additional resin to specific areas of the panel. The tube
insertion method may also be used to infuse panels 30 which are enclosed
entirely within a rigid mold, by providing in a mold surface one or more holes
through which resin connection tubes may be inserted. As resin fills grooves
39,
it flows into and throughout the porous and fibrous webs 34, into and
throughout
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19
the intersecting porous and fibrous struts 35, and into and throughout
intersecting panel skins 32, after which the resin cures to form a rigid
reinforced
sandwich panel structure. Reinforced cores 31 which have been provided with
channels 40 may be placed in a mold with channels 40 adjacent each other and
forming a single, larger channel. Resin which flows into this larger channel
cures
to form a structural spline which is keyed into the edge portions of webs 34
and
resists shear forces between the adjacent cores 31.
[0088] The resin distribution system incorporated into the
reinforced core 31 has significant advantages over existing VARTM processes.
Resin fills grooves 39 rapidly and flows throughout the web and strut
reinforcing
structure to panel skins 32 through numerous, relatively evenly distributed
connections with the skins by the webs and struts, thereby minimizing the
likelihood of non-impregnated areas in the skins. No resin micro grooves or
distribution medium material are required on the periphery of the core 31.
Resin
is introduced into the plurality of grooves 39 located in the mid-plane of the
panel
and travels a relatively short distance to both skins 32. Vacuum may be
applied
at any desired location or locations on outer skins 37 or panel edge fabrics.
If
desired, multiple rows of perforated vacuum tubing, fibrous drain flow media
or
other means of introducing vacuum may be provided against the surface of outer
skins 37 to ensure that small areas of dry, porous skin reinforcements are not
isolated from vacuum by surrounding resin flow. Panels having unusually thick
cores or skins may be provided with additional sets of resin grooves 39 and
associated feeder channels 40 located in planes parallel to panel skins 32.
Resin introduced into the center of the panel travels a relatively short
distance
to both skins 32. The internal core infusion system just described is also
effective in cores comprising webs which extend between the skins without
intersecting fibrous struts. Closer web spacing may be required for uniform
resin
distribution.
[0089] The mold surfaces in contact with the reinforced core
panel
may be either rigid or flexible without impairing the rapid flow of resin
throughout
the core reinforcing structure or skins. For example, a reinforced core with
associated porous and fibrous skins may be placed between a rigid mold table
and a rigid caul plate, with the caul plate covered by a vacuum bag sealed to
the
mold table. Evacuating the bag from one edge of the panel applies atmospheric
pressure to the panel, and resin introduced at the opposing edge of the panel
flows rapidly throughout the core and skin reinforcing structure, without
having
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to flow longitudinally through the entire length or width of the panel skins
as in
conventional VARTM processes in which both mold faces are rigid.
[0090] Reinforced panel 30 may be constructed to permit
simultaneous infusion of the core with two resins of differing properties. For
example, the exterior skin of the panel may be impregnated with fire resistant
phenolic resin, and the interior skin and core reinforcing structure may by
impregnated with structurally superior but less fire resistant vinyl ester
resin. If
such a structure is desired, panel 30 is provided, prior to resin infusion,
with
adhesive barrier films 41 located between the inner skins 36 and outer skins
37.
The barrier film 41 is comprised of adhesive material, for example epoxy,
which
prevents the passage of liquid resin from one side of the film to the other
and
which, under application of heat and moderate pressure, cures to form a
structural bond between the inner skins 36 and outer skins 37.
[0091] To infuse the panel, the reinforced core 31, together
with the
attached inner skins 36, adhesive barrier films 41 and outer skins 37, are
placed
in a closed mold which is then evacuated by vacuum pump. A first resin is
introduced into the interior of the core 31 through channels 40 and 39 and
allowed to flow throughout the core reinforcing structure and inner skins, as
previously described. Simultaneously, a second resin, of differing
composition,
is introduced directly into the outer skin through the mold surface or the
outer
skin edge. The adhesive barrier film 41 serves to prevent the mingling of the
two
different resins, and heat generated by the curing of the two resins also
advances the cure of the adhesive film, thus providing a structural bond
between the inner and outer skins. If adhesive film is applied to both sides
of
panel 30, three individual resins may be infused into the panel. If adhesive
film
41 is applied to one side of panel 30 only, the resin which infuses core 31
will
also infuse both inner and outer skins on the opposite side of the panel.
[0092] The embodiments of the present invention illustrated in
FIGS. 1, 2, 6, 7, 13, 14 and 18 have been shown as provided with internal
resin
distribution grooves adjacent the core reinforcing webs and with an associated
resin feeder channel. It is understood that this feature may, if desired, be
omitted from the embodiments of FIGS. 1, 2, 6, 7, 13, 14 and 18 and that the
feature may be added in the embodiments shown in FIGS. 3, 4, 5, 9 and 19 or
in any other embodiment having porous and fibrous web sheets within the foam
core.
[0093] A sandwich panel 50 (FIG. 2) utilizes a reinforced foam
core
52 which can be produced at improved rates of output compared to the
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embodiment shown in FIG. 1, because reinforcing struts need only be inserted
into the foam core at a single angle, rather than at two opposing angles.
Parallel
fiber reinforced webs 51 extend between the faces of foam core 52 at an acute
angle, for example 58 degrees or 45 degrees, to the faces of the core. The
rows
of webs 51 are intersected, generally at right angles, by a set of parallel
rows of
fiber reinforced struts 53, whose fibers extend through webs 51 and skins 54
in
the manner described in connection with FIG. 1.
[0094] In the embodiment shown in FIG. 2, all struts are
inclined
at an angle with respect to the panel skins, and the angle matches the angle
of
the webs 51 but in the opposite direction. Webs 51 and struts 53 support each
other against buckling and cooperate to resist shear loads in one direction,
and
the webs also resist shear loads in the transverse direction. While any number
of web reinforcement fabrics or mats may be selected, the dual direction
structural function of the webs may be enhanced through the use of web
reinforcing fabric having a portion of its fibers oriented at an angle
opposing the
angle of struts 53. Transverse shear strength may be efficiently achieved by
orienting the remaining fibers of webs 51 at angles of +45 degrees and -45
degrees to the panel skins, since shear forces in the core resolve themselves
generally into these angles. The core reinforcing webs 34 of FIG. 1 and 51 of
FIG. 2 terminate adjacent panel skins 32 and 54 respectively. Thus, the direct
structural connection between webs and skins is provided by the adhesive bond
of the resin matrix Which surrounds all reinforcing fibers in the panel. The
strength of this web-to-skin connection may by improved by providing the webs
34 and 51 with protruding and flared fibers at their edge portions or with web
edge resin fillets formed by grooving foam strips 55 adjacent the edge
portions
of the webs, as described in U. S. Patent 5,834,082.
[0095] The webs 34 and 51 also have an indirect structural
connection with skins 32 and 54 through struts 35 and 53, respectively, which
are attached to both webs and skins and thus carry a portion of the loads
between webs and skins. Panel skins are also tied together by the
configuration
of the roving struts shown in FIG. 2, which comprise rows of continuous
inclined
separate staples each having flared strut end portions. The inclined staple
form
of strut construction may also be provided in panels having opposing struts
and
is more fully described in connection with FIG. 8.
[0096] If it is desired to increase further the strength and
stiffness
of composite panels having intersecting webs and struts, the core reinforcing
webs may comprise a single, continuous fiber reinforced mat or fabric, rather
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22
than a plurality of discrete web strips. This embodiment is illustrated in
FIGS. 3,
4 and 5. Referring to FIG. 3, composite sandwich panel 60 comprises fiber
reinforced skins 61 and fiber reinforced foam core 62. The foam core 62
comprises foam pieces or strips 63, spaced rows of spaced fibrous roving
struts
64 , and a fibrous web sheet 65 which has been formed into a plurality of
rectangular corrugations extending between the panel skins and transverse to
the rows of struts. As in FIG. 1, struts 64 are inclined at equal opposing
angles
to the skins and intersect and extend through opposing struts and skins 61.
The
struts also intersect and extend through corrugated web segments 66, which
extend between the skins and through web segments 67 which lie adjacent the
skins. The architecture shown in FIG. 3 offers several structural enhancements
to that shown in FIG. 1. Corrugated web segments 67 provide an expanded
area of adhesive attachment to skins 61, and struts 64 provide a stitched
mechanical attachment between web segments 67 and skins 61. Also, the
corrugations of the web structure provide substantial additional strength and
stiffness in the direction transverse to the rows of struts.
[0097] Reinforced sandwich panel 70, shown in FIG. 4, also
provides the advantages of web-to-skin attachment and corrugation strength and
stiffness described in connection with FIG. 3. In FIG. 4, foam strips 71 are
of
parallelogram cross section, and web segments 72 of a continuous corrugated
web sheet 73 extend between the faces of the core 76 at an acute angle to
skins
74. A plurality of parallel rows of spaced fibrous roving struts 75 also
extend
between the faces of the reinforced core 76, and the struts 75 are inclined at
an
angle equal to but opposing the angle of web segments 72. The struts intersect
and extend through corrugated web segments 72, through web sheet segments
76 adjacent skins 74, and preferably extend through one or more layers of the
skins. Fiber orientation in the webs may be optimized for overall core
structural
properties as more fully described in connection with FIG. 2. Also as in the
case
of FIG. 2, the orientation of the struts at a single angle permits rapid and
efficient
production of the reinforced core because only a single strut insertion step
is
required.
[0098] Another reinforced sandwich panel 80 shown in FIG. 5 and
also employs a continuous corrugated web sheet 81 as part of the reinforcement
of foam core 82. Foam pieces or strips 83 are triangular in cross section, and
web segments 84 and 85, which extend between skins 87 are inclined at
opposing angles to the skins. A plurality of rows of spaced fibrous roving
struts
86 are inclined at equal but opposing angles to each other and intersect and
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23
extend through web segments 84 and 85. The struts also intersect and
preferably extend through one or more layers of skins 87.
[0099] In contrast to the configurations shown in FIGS. 3 and 4,
the
triangulated web architecture of FIG. 5 provides substantial strength and
stiffness to panel 80 both longitudinally and transversely, even in the
absence
of reinforcing struts 86. The struts enhance these properties by stabilizing
web
segments 84 and 85 and by tying skins 87 together. The struts 86 also provide
additional strength and stiffness in the direction of the strut rows. The
angle of
the struts is selected on the basis of overall structural considerations and
need
not correspond to the angle of web segments 84 and 85. For example, the
struts 86 may, if desired, be perpendicular to the skins. This not only
provides
increased compressive strength to panel 80, but also requires only a single
angle
of strut insertion, thus simplifying panel production.
[0100] FIGS. 6 and 7 illustrate a sandwich panel 90 having in
the
reinforced foam core 91 a plurality of parallel rows of spaced reinforcing
roving
struts 92, a plurality of intersecting parallel rows of spaced reinforcing
roving
struts 93, and a single continuous reinforcing web sheet 94 which is parallel
to
skins 95. Foam core 91 comprises stacked foam boards 96 separated by web
94. If required by structural design, struts 92 may differ from struts 93 in
spacing, diameter, fiber composition and angle. Struts may be provided as a
single set of parallel rows of struts if structural requirements of the panel
are
primarily unidirectional. Compressive and shear properties of panel 90 are
provided primarily by struts 92 and 93. As the thickness of core 91 increases,
or the diameter of the struts decreases, the struts are increasingly
susceptible
to buckling failure under structural load conditions. The struts 92 or 93 in
each
row intersect each other in a lattice-like configuration, providing buckling
support
for each other in the plane of the strut rows. However, only weak and often
insufficient transverse buckling support is provided by the low density foam
96.
The continuous fiber reinforced web 94, through which all of the struts 92 and
93 extend, provides the required additional buckling support. If needed, one
or
more additional support webs 94 may be provided, all spaced from each other
and parallel to the panel skins 95.
[0101] FIG. 6 also shows strut end portions 97 and web edge
portions 98 protruding from foam boards 96 to provide means of securing
enhanced structural continuity between the reinforcing members of core 91 and
the reinforcing members of adjacent foam cores molded as components of a
single sandwich panel, or to other adjacent composite structures (not shown).
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If structural attachment of adjacent cores within a given sandwich panel is
desired, edge portions of foam boards 96 and of foam boards of adjacent
reinforced cores (not shown) are abraded or otherwise removed to expose
fibrous strut end portions 97 and web edge portions 98, before introducing
resin
into the core and skin reinforcements. The reinforced cores are then pressed
together, for example in a mold, and exposed end and edge portions from
adjacent cores become intermingled and subsequently embedded in resin which
is flowed into the panel reinforcements under differential pressure and cures
to
form a strong adhesive bond with strut end portions and web edge portions.
Preferably, a strip of fibrous reinforcing mat or fabric extending between
skins
95 is arranged in the mold between adjacent cores to enhance the load bearing
properties of the joint between cores.
[0102] A strong structural connection between adjacent
reinforced
cores 31, or between cores 31 and sandwich panel edge skins, may also be
achieved by providing cores 31 with fibrous webs 34 which extend beyond their
intersection with the edges of core 31. The extensions of webs 31 are folded
at
right angles against foam strips 33 in the form of a tab. These web-end tabs
provide an expanded area of contact for adhesively bonding the web reinforcing
members to adjacent reinforcements when panel 31 is impregnated with resin.
If it is desired to achieve a strong structural bond between a resin
impregnated
and cured panel 90 and an adjacent composite structure, foam boards 91 are
abraded to expose stiff, hardened strut end portions 97 and web edge portions
98, and the area adjacent the end and edge portions is filled with adhesive
resin,
mastic or potting compound and pressed against the panel to which panel 90 is
to be bonded while the resin cures.
[0103] The reinforced core 91 shown in FIGS. 6 and 7 has been
provided with an integral resin infusion system, as generally described above
in
connection with FIG. 1. Sandwich panel 90 comprises porous and fibrous skin
and core reinforcements and is placed in a closed mold from which air is
evacuated. Resin is then introduced into feeder channel 99 at the end of the
channel or through a hole drilled from the panel face (not shown). The resin
then fills resin feeder channel 99, located within the interior of reinforced
core 91,
and fills connecting spaced resin grooves 100 located within the interior or
core
91 and adjacent the porous and fibrous web 94. Resin then flows from grooves
100 throughout porous web 94, from the web 94 throughout porous struts 92 and
93, and from the struts throughout porous skins 95, after which the resin
cures
to form a structural panel. If the core 91 is to be used to produce a circular
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panel, resin grooves 100 may be arranged radially from the center of the panel
and with the resin supplied from the panel face to the center.
[0104] The core reinforcement strut architecture shown in FIGS.
1,
3, 5, 6 and 7 takes the form of planar rows of opposing struts which intersect
each other within the foam core. The number of such intersections and the
density of the resulting lattice-like structure is dependent upon core
thickness,
the spacing between struts, and the steepness of the strut angle with respect
to
the panel skins. An alternate strut architecture is shown in FIG. 8 and may be
substituted for that of FIGS. 1, 3, 5, 6 and 7, but is most appropriate in the
case
of relatively thin panels or relatively thick struts. The core reinforcing
architecture
of FIG. 8 comprises either unidirectional rows of struts, as shown, or sets of
intersecting rows of struts and may be used with or without core reinforcing
webs, depending upon structural requirements.
[0105] Referring to FIG. 8, a sandwich panel 110 comprises
opposing skins 111 and reinforced foam core 112 having a plurality of rows of
fibrous roving struts 113 which extend between panel skins 111 and which are
inclined at equal but opposing angles to the skins. Opposing struts 113
intersect
each other adjacent panel skins 111 in a simple triangulated configuration and
extend through the skins. In the production of the reinforced core 110,
continuous fibrous rovings 114 are stitched through skins 111 and foam core
112 from opposing faces of the foam core. If desired, both sets of roving
struts
may be stitched through the skins and foam core from the same face of the
core.
In the stitching process, continuous rovings 114 exit skins 111 and protrude
in
the form of loops 115 (shown in phantom). The rovings then double back along
the line of insertion to form struts 113 comprised of double roving segments.
[0106] As the panel 110 advances through the stitching
apparatus,
roving segments 116 overlie the skins 111. Protruding roving loops 115 formed
during the stitching process are severed at a desired distance, for example
0.2
inches, from the surface of the skins to form protruding strut end portions
117
(shown in phantom). When pressure is applied to the panel skins during the
resin molding process, the protruding strut end portions 117 flare out and
form
flattened end portions 118 against the skins 111, forming a strong adhesive
bond to the skins and a mechanical resistance to pulling flattened strut ends
118
through skins 111.
[0107] The mechanical attachment may be improved by the
addition of outer skins as shown in connection with FIG. 1. Cut and flared
strut
ends 118 also provide substantially improved skin characteristics, compared to
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that achieved with intact loops, which tend to form lumps adjacent the skins
or
which prevent the panel from fitting tightly against the mold surface,
allowing
excess resin to accumulate at the skin surface. Surface flatness may be
further
improved by applying sufficient pressure to panel 110 to conform the foam core
112 to any roving segments which protrude beyond the surface of skins 111 or
by providing the foam core with grooves or indentations into which protruding
roving segments may be pressed under moderate molding pressure.
[0108] The inclined staple configuration comprising struts 113,
cut
and flared strut end portions 118, and roving segments 116 which overlie
skins,
as shown in FIG. 8, provides an efficient and effective means of securing
structural attachment between core reinforcing struts and panel skins and a
preferred method of producing all of the reinforced cores which are the
subject
of the present invention. It is understood that other methods of stitching and
other treatments of roving segments which are exterior to the faces of the
foam
core may also be used, for example, conventional patterns of lock stitching or
chain stitching of continuous fibers.
[0109] The sandwich panels and cores illustrated in FIGS. 1-8
typically have a width greater than their depth. Core reinforcing members
comprising porous and fibrous webs and struts may also be incorporated into
sandwich panels having a depth greater than its width. FIG. 9 illustrates a
beam-type panel or beam 120 incorporating a strut-type core reinforcing
architecture and designed for use as a roof support in corrosion resistant
buildings. The beam 120 comprises opposing fiberglass or carbon fiber,
reinforced plastic skins 121, and a reinforced foam core 122 which comprises
foam boards or pieces 123 and opposing porous fiberglass or carbon fiber
reinforcing member struts 124 which extend through the foam core 122 at acute
angles to the skins 121 in the general form of a bar joist. If required by
structural
design, additional struts may be added to intersecting struts 124 to form a
lattice-like configuration, as illustrated in FIGS. 6 and 7, or one or more
additional parallel rows of reinforcing struts may be incorporated into the
panel
or beam 120. Skins 121 function as structural chord flanges, the fibers of
which
are primarily oriented longitudinally. Skins 121 comprise inner skins 125 and
outer skins 126 having fibrous reinforcements, with end portions 127 of the
reinforcing members 124 flared and sandwiched between the skin layers as
described in connection with FIG. 8. If desired, the skins 125 and 126 may be
more strongly attached to the flared end portions 127, by stitching the skins
to
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the end portions using flexible fibers or thin rigid rods which extend through
the
fibers of end portions 127 and adjacent skins 125 and 126.
[0110] One or more porous and fibrous support webs 128 may be
incorporated into the beam 120 if required to stabilize the struts 124 against
buckling under load. The faces of the foam boards 123 which extend between
opposing skins 121 are provided with a second set of skins 129 of porous,
fibrous reinforcing fabric, such as fiberglass, to stabilize beam 120 against
lateral
deflection under load. As previously described, a curable resin introduced
under'
differential pressure impregnates all of the porous and fibrous reinforcing
materials which form the beam 120 and cures to form a rigid, load-bearing
beam. If required by structural considerations, the beam may be of non-uniform
cross section, that is, varying in depth from beam ends to beam center, and
may
also be in curved or arch form. If desired, skins 120 may be substantially
reduced in thickness, and the truss chord structural function may be provided
byl
roving bundles inset in grooves in the foam boards adjacent the skins, as more
fully described below in connection with FIG. 10.
[0111] The core reinforcing structure of sandwich panels in
which
panel width is greater than depth may take the form of a plurality of parallel
true
truss-type structures, in which rod- or strut-type reinforcing members extend
at
opposing angles in a triangulated configuration between top and bottom chord
members, into which the end portions of the struts are anchored. This
arrangement provides superior attachment of strut end portions. It also
utilizes,
as truss chord members, fibrous reinforcing materials, for example carbon
fiber
or fiberglass, in their relatively low cost roving form to replace a
substantial
portion of the more expensive fabric skin reinforcements. As shown in FIG. 10,
a sandwich panel 140 comprises a reinforced closed-cell foam core 141 and
opposing fibrous reinforcing skins 142. The reinforced core 141 is provided
with
a plurality of parallel rows of trusses 143 which extend between skins 142.
Each
truss 143 comprises parallel bundles of fibrous reinforcing rovings 144, such
as,
fiberglass or carbon fiber, which are located in grooves formed in the foam
core
141 and which serve as top and bottom chord members for each truss 143.
Fibrous reinforcing rods or struts 145 penetrate the chord members and are
anchored in chord members 143, and extend between panel skins 142 at
opposing acute angles, preferably penetrating and overlying one or more layers
of skins 142. A cured resin impregnates all of the reinforcing materials, as
previously described. The truss structure, comprising struts 145 and chord
members 143, may also be incorporated into cores having reinforcing webs
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which extend between or parallel to panel skins, as shown for example, in
FIGS.
1 and 7.
[0112] Referring to FIG. 11, the use of relatively economical
fibrous
rovings in place of woven or knitted fibrous reinforcing fabrics may be
extended
to form the entire panel skin structure. A sandwich panel 150 comprises a
reinforced closed cell foam core 151 and opposing fibrous skins 152. The core
151 comprises a foam board 153 and fibrous reinforcing members or struts 154
which extend between the skins. Each of the skins 152 comprises a first layer
of parallel reinforcing rovings 155 adjacent the foam core 153 and
substantially
covering the faces of the foam. A second layer of parallel reinforcing rovings
156 overlie and cross first roving layer 155 and substantially covering the
surface of first layer 155. If desired, a layer of fibrous mat or veil 157 may
overlie
second roving layer 156.
[0113] In the production of panel 150, the ends of the rovings
which
comprise first skin layer 155 are secured in a line across the leading edge of
foam board 153. The board advances through stitching apparatus such as that
shown in FIG. 15, and the forward motion of the board pulls the rovings to
form
the skin layer 155 from supply creels to cover the opposite faces of the
board.
Prior to the insertion of struts 154 by the stitching apparatus, a plurality
of
parallel skin rovings 156 are applied across first roving layer 155 by a
reciprocating mechanism having guides which maintain the desired spacing and
tension of the rovings 156. The second skin layer 156 is then covered by a
fibrous veil 157 drawn from a supply roll. Core reinforcing struts 154 are
stitched
through the veil 157, the layers of skin rovings 156 and 155, and the foam
board
153 to produce sandwich panel 150.
[0114] If required by structural considerations, additional
layers of
skin rovings may be applied to the panel faces at various angles before
stitching.
Alternately, oriented or non-oriented roving fibers may be chopped to desired
lengths and applied to the core faces in lieu of continuous rovings. Overlying
segments 158 of the stitched strut rovings 154 hold all of the skin rovings
155
and 156 in position until the panel 150 is placed in a mold where a curable or
hardenable resin is flowed throughout all of the fibrous reinforcements to
produce the structural panel. This method of forming panel skins directly from
rovings may be incorporated into any of the embodiments shown in FIGS. 1-10.
[0115] In a preferred embodiment of the invention, substantial
cost
savings are achieved by producing the web-type core reinforcing members
directly from fibrous rovings, rather than by using as the webs woven or
stitched
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fabrics, which are significantly more expensive than rovings. In this method,
rovings are wound circumferentially around a continuous foam strip to create a
structural tube reinforcement structure around the strip. A particularly cost-
effective means of forming the wound structure is by spiral or helical
winding.
The wound strip is cut to desired length and fed into a roving stitching
machine
in the manner described in connection with FIG. 15.
[0116] Referring to FIG. 12, plastic foam strips 170 of
convenient
length are fed end-to-end through a helical winding apparatus 171, illustrated
diagrammatically. Helical winding of core reinforcements offers major economic
advantages compared to existing processes. Fibers in roving form cost
approximately 50- to 60-percent of those incorporated into double-bias 45-
degree fabrics, and winding machine production rates are five to ten times
those
of braiding machines. If desired, the foam strip may be provided with one or
more grooves 39 as described in connection with FIG. 1 to facilitate the flow
of
resin in a subsequent molding operation. The foam strip 170 has a thickness
equal to the thickness of the sandwich panel core to be produced from the
strip
and a width equal to the desired spacing of reinforcing webs within the core.
[0117] As the strip 170 advances through the winding apparatus
171, it passes through the axes of a rotating bobbin wheel 172 rotating in one
direction and a bobbin wheel 173 rotating in the opposite direction. Each
wheel
is loaded with a number of bobbins 174 wound with fibrous reinforcing rovings
175. Rotating bobbin wheel 172 winds a layer 176 of rovings onto the foam
strip at a single angle which is determined by the rate of advance of strip
170
through the apparatus 171 and the rate of rotation of the bobbin wheel 172.
The
single-wound strip then advances through the counter-rotating bobbin wheel 173
which winds a second layer 177 of rovings over wound roving layer 176.
[0118] Winding apparatus 171 may be scaled to efficiently
process
a wide range of foam strip sizes, for example, from one-quarter inch to one
foot
or more in thickness. The rovings may be of different thicknesses and may be
closely spaced, so as to cover the surface of the foam strip or more widely
spaced, depending upon structural requirements of the finished wound strip and
the composite panel into which it will be incorporated. Rovings applied to the
surfaces of the foam strip may have a weight totaling as little as 0.1 ounces
or
less per square foot and as much as 5.0 ounces or more per square foot. The
rovings shown in FIGS. 12-14 are thicker than normal, so that details of
construction may be understood. The rovings may be wound at angles of +45
degrees and -45 degrees for maximum resistance to shear stresses in
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applications in which the strip is subjected to bending loads, or the rovings
may
be applied at other angles dictated by structural requirements of specific end
products into which they will be incorporated.
[0119] The continuous foam strip 170 with overlying wound layers
176 and 177, is cut to length by a traveling cutting apparatus, such as a
circular
saw (not shown) to form finished wound strips 178. Since the wound foam strips
178 are used as the foam and web elements of a hybrid sandwich panel such
as the one shown in FIG. 14, their length is equal to the desired width of the
sandwich core panel. Prior to being cut, the wound rovings 174 are secured
against unraveling, for example, by being wrapped on either side of the cut
with
yarn 179 impregnated with hot melt adhesive, or by applying adhesive tape
around the cut location, or by applying adhesive to the rovings. If desired,
foam
strips 170 may be wound with a barrier film applied before the roving layers
to
protect the foam from moisture, resin attack or the like.
[0120] Finished strips 178 are advanced to the infeed end of
core
forming apparatus 200 illustrated in FIG. 15 and are inserted into the
apparatus
as described in connection with FIG. 15, or are advanced into an apparatus
(not
shown) for attaching strips together with an adhesive veil 241, as shown in
FIG.
18. Labor cost per square foot of core produced is very low. In a variation of
the
winding process described in connection with FIG. 12, a layer 180 of
longitudinal
fibrous rovings is applied to the surface of the foam strip 170, in a
direction
parallel to the longitudinal axis of the strip and prior to rovings 174 being
wound
around the strip so that the layer 180 is held in place by the wound rovings
174.
The rovings of longitudinal layer 180 are supplied from stationary roving
packages 181 and are pulled through winding apparatus 171 by the forward
motion of the advancing foam strip 170. The longitudinal rovings may be
applied
to two opposing faces of the strip, as shown in FIG. 12, to serve as sandwich
panel skin elements as will be described in connection with FIG. 14.
Alternately,
the longitudinal rovings may be applied to all faces of the foam strip in
order to
provide compressive and buckling properties required for structural columns.
[0121] FIG. 13 provides a detailed view of a wound foam strip
178,
showing the layering and orientation of the four sets of porous and fibrous
rovings applied during the winding process illustrated in FIG. 12. In FIG. 13,
all
rovings are shown as having flat cross section and are closely spaced to cover
the surface of closed cell plastic foam strip 170. The longitudinal roving
layers
180 cover the top and bottom faces of foam strip 170. The first layer 176 of
wound roving, shown at an angle of +45 degrees, covers longitudinal roving
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layers 180 and the side faces of the foam strip 170. The second layer 177 of
wound rovings, at an angle of -45 degrees, covers the first wound layer 176.
When subsequently impregnated with a curable thermosetting resin or
hardenable thermoplastic resin, all of the fibrous rovings, along with the
cured
or hardened resin, produce a structural element having the general properties
of a beam of rectangular tubular cross section.
[0122] FIG. 14 illustrates a reinforced foam core sandwich panel
of the intersecting web and strut hybrid construction described above in
connection with FIG. 1, but in which the roving-wound strips 178 shown in FIG.
13, are substituted for the foam strips 33 with the attached web sheets 34
shown
in FIG. 1. Additionally, FIG. 14 incorporates rovings in place of woven or
knitted
fabrics to form the sandwich panel skins, in the production method shown in
FIG.
15. This combination of roving-wound foam core strips and roving-applied panel
skins provides important structural and cost advantages.
Referring again to FIG. 14, a structural composite panel 190 comprises a fiber
reinforced closed cell plastic foam core 191 and opposing fiber reinforced
skins
192. The reinforced foam core 191 comprises a plurality of parallel strips 178
shown in FIG. 13. If desired, foam strips 178 may be provided with diagonally
wound rovings in only one direction by alternating right hand and left hand
wound strips while forming the sandwich panel core, so that adjacent wound
edges are at plus and minus angular orientation, rather than both with the
same
orientation and therefore structurally unbalanced.
[0123] The wound foam strips 178 are intersected at right angles
by a plurality of parallel rows of spaced rods or struts 193 which extend
between
the faces of the core, and are made up of porous and fibrous reinforcing
rovings.
The struts 193 within each row are inclined at opposing acute angles to each
other, to the panel skins 192, and to the plane surfaces of the wound strips
178.
Overlying the wound strips 178 is a layer of parallel porous and fibrous skin
rovings 194 which extend in a direction parallel to the plane of the rows of
struts
193 and perpendicular to the wrapped strips 178 and their longitudinal rovings
layer 180. A light weight fibrous veil, mat or scrim 195 overlies the skin
roving
layer 194 which may be applied to the panel 190 in the form of either a
plurality
of discrete rovings or as a unidirectional fabric having rovings adhered in
advance to a light weight veil. The end portions of the struts 193 penetrate
all
layers of longitudinal rovings 180, wound rovings 176 and 177, skin rovings
194
and veil 195, and these end portions overlie veil 195.
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[0124] The panel illustrated in FIG. 14 has been inverted from
the
position in which it is produced in the apparatus of FIG. 15 in order to show
the
continuous rovings which comprise the struts 193. As shown in FIG. 14, a
plurality of continuous rovings have been stitched through sandwich panel 190
at opposing angles and from the same side of the panel, with each continuous
roving segment 196 interlocked with itself in a chain stitch configuration. It
is
understood that alternate stitching methods may be used, for example lock
stitching or cut loops as shown in FIG. 1.
[0125] An important feature of the fibrous reinforcing structure
shown in FIG. 14 is that the longitudinal roving layer 180 on the wound strips
178
comprises the transverse reinforcements of the sandwich panel skins 192, and
the +45 degrees and -45 degrees roving layers 176 and 177 which overlie
longitudinal layer 180 also constitute elements of the sandwich panel skins.
That
is, the web elements of the core reinforcements are comprised of the same
continuous wound rovings as the +45 degrees and -45 degrees skin elements.
This results in greater resistance to delamination between core and skin
structure, since the web-type core reinforcing webs do not terminate adjacent
the
panel skins as in FIG; 1. The roving layers 180, 176 and 177, which cover foam
strips 178, also anchor the end portions of struts 193.
[0126] Reinforced core 190 shown in FIG. 14 may also be
produced omitting the roving layers 180 and 194 and veil 195, which comprise
skin elements continuous across the length and/or width of the panel. This may
be desirable when the reinforced cores are used to produce large sandwich
panels, for example boat hulls, which generally consist of a plurality of
cores
adjacent one another and between the skins of the panel. In such panels, it is
generally preferred to use skins of sufficient length and width to provide
structural continuity across a number of cores, rather than to use cores
having
pre-attached skins, whether such pre-attached skins comprise reinforcing
fabrics
or of rovings integrated into the core as described in connection with FIG.
14.
When continuous skin elements 180, 194 and 195 are omitted, the wound strips
178 remain tightly held together as a unitized core by the friction of strut
rovings
193 which intersect adjacent cores and by the continuous strut roving segments
which are stitched along the top and bottom faces of strips 178. In this
configuration, the end portions 196 of struts 193 do not extend through the
skins
of the sandwich panel, but rather are trapped between the wound outer roving
layer 177 and the panel skins applied to the surface of the core.
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[0127] The roving-wound foam strips 178 of FIGS. 12-14 are shown
as rectangular in cross section. If desired, these strips may be of other
cross
sections, for example, parallelogram or triangular, as shown in FIGS. 4, 5 and
19.
[0128] U.S. Patent No. 5,904,972 discloses sandwich panel core
elements comprised of discrete plastic foam blocks or strips wrapped with
reinforcing fabrics. A plurality of the wrapped blocks are stacked between
sandwich panel skins in a mold in honeycomb configuration, with the end
portions of the foam blocks and edge portions of the wrapped fabric adjacent
the
panel skins. The helically wound foam strips 178 shown in FIG. 13 of the
present application may be substituted for these wrapped blocks to provide
comparable structural properties at substantial savings over the cost of
fabrics
and the labor of fabrication.
[0129] As described in Patent No. 5,904,972, it may be desirable
to extend the edge portions of the reinforcing fabric beyond the ends of the
foam
blocks, so that they may be folded over to form a flange for improved
structural
attachment to the sandwich panel skins. A similar extension of the wrapped and
longitudinal roving layers 180, 176 and 177 of FIG. 13 may be achieved by
alternating sacrificial foam blocks (not shown) end-to-end with core foam
strips
170, winding the foam as described above, cutting the wrapped strips through
the middle of the sacrificial foam blocks, and removing the sacrificial
blocks.
Foam strips 170 may also be provided with surface microgrooves prior to
insertion into winding apparatus 171. Other suitable core materials may be
substituted for the plastic foam used for the wound strips or blocks, for
example
balsa wood or hollow, sealed plastic bottles of similar geometric shape.
[0130] Since the structural properties of the sandwich panel
cores
shown in FIGS. 1-19 are usually provided primarily by the fibrous core
reinforcing
structure, the closed-cell plastic foam which comprises the cores may be
selected on the basis of other desired panel properties, such as water or fire
resistance, thermal insulation or light transmission. For example, translucent
polyethylene foam and fiberglass reinforcing materials may be impregnated with
translucent resin to produce a light-transmitting and load bearing panel for
use
as the roof of highway trailers or building roofs. It is also within the scope
of the
invention to substitute for the plastic foam other cellular materials, such as
carbon foam or balsa wood.
[0131] FIGS. 1-8, 10,11 and 14 illustrate fiber reinforced cores
and
sandwich panels which are produced in part by inserting, or stitching, porous
and
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fibrous reinforcing elements such as fiberglass rovings through the thickness
of
foam plastic core materials. This may be accomplished by the apparatus 200
illustrated in FIG. 15. A plurality of foam strips 201 are inserted adjacent
one
another into stitching apparatus 200. Strips 201 may be of rectangular or
other
cross section and may be provided with attached porous and fibrous webs of
reinforcing fabric or with wound porous and fibrous reinforcing rovings, as
previously described. It is understood that, if desired, foam boards having a
length substantially greater then the width of strips 201 may comprise the
foam
plastic material.
p1321 The strips 201 are advanced in generally equal steps by,
for
example, a reciprocating pressure bar (not shown) or movable endless belts
202, to stitching heads 203 and 204, to which are rigidly attached a plurality
of
tubular needles 205, cannulae or compound hooks, adapted for piercing and for
inserting fibrous rovings. Stitching heads 203 and 204 are inclined at
opposing
acute angles to the surface of strips 201. When the strips 201 stop advancing
at the end of each forward step, the reciprocating stitching heads 203 and 204
insert the needles 205 into and through the strips 201. The needles are
accurately positioned at their points of entry into strips 201 by needle
guides
207. The porous and fibrous rovings 208, which have been supplied from wound
roving packages (not shown), are inserted by the needles 205 through the
strips
201 and emerge on the surface opposite their points of entry in the general
form
of the loops 115 as shown in FIG. 8.
[0133] Referring again to FIG. 15, the loops 115 are gripped by
apparatus (not shown) which retains the loops formed beyond the surface of the
strips from which they have emerged and, if desired, engages them with other
loops to form a chain stitch as shown in FIG. 14 or with separately supplied
rovings to form a lock stitch. The stitching heads 203 and 204 then retract,
which advances into the needles 205 a predetermined length of rovings 208
sufficient to form the next stitch. After retraction, the row of strips 201
advances
a predetermined step or distance and stops, and stitching heads 203 and 204
reciprocate to insert the next pair of opposing struts. The unitized assembly
of
strips 201 held together by stitched rovings 208 which intersect the strips,
is cut
by a saw or other suitable means into cores 209 of desired length.
p134] The stitching apparatus 200 may be used to produce panels
209 having pre-attached porous and fibrous skins as shown in FIG. 1. Referring
again to FIG. 15, reinforcing skin fabric 210 is supplied from rolls and
advances
adjacent the opposing faces of the panel 206 to stitching heads 203 and 204.
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As rovings are stitched through the strips 201 which form the panel 206, the
rovings overlie the skin fabric 210 and mechanically attach the fabric 210 to
panel 206.
[0135] The apparatus 200 shown in FIG. 15 may also be used to
produce sandwich panels in which all structural reinforcing components of both
core and skins comprise low cost fibrous rovings, as shown in FIG. 14. A layer
of longitudinal skin rovings 194 (FIG. 14) is applied as the surface of panel
206
during its production in the stitching apparatus 200 shown in FIG. 15. A
plurality
of porous and fibrous rovings 211 sufficient to cover the faces of the panel
are
pulled by the advancing panel 206 from roving supply packages (not shown) and
advance adjacent the exposed faces of strips 201 to the stitch heads. A thin,
porous veil, mat or scrim 210 is pulled from rolls by the advancing panel 206
to
overlie skin rovings 211 and hold them in place after the rovings 208 have
been
stitched through panel 206. The strips 201 have been provided with a
longitudinal roving layer 180, as shown in FIG. 14, so that layers 180 and 194
of FIG. 14 comprise the transverse and longitudinal skin reinforcements of
panel
206 produced in FIG. 15. It is also within the scope of the invention to
provide
panel producing apparatus 200 with a reciprocating mechanism (not shown)
which applies transverse and double-bias angle rovings to the faces of panel
206. This permits the production of the panels 150 shown in FIG. 11, in which
the foam core does not comprise wound strips 178 containing roving layer 180.
[0136] In another preferred embodiment of the present invention,
bi-directional panel strength is achieved by providing wound foam strips 177
with
internal transverse reinforcing members, rather than by inserting structural
rovings 193 through the strips 177. Referring to FIG. 16, reinforced foam
strip
220 comprises a plurality of blocks or pieces 221 of foam plastic separated by
sheets 222 of web-like fibrous reinforcing material, such as fiberglass or
carbon
fiber fabric or mat. Foam pieces 221 and reinforcing webs 222 are adhesively
connected to each other for ease of processing and handling, while maintaining
substantial porosity of the web material, as described in U. S. Patent
5834082.
Reinforced strip 220 may be provided with a groove 223 for the flow of resin.
.
It is understood that other materials may be substituted for foam pieces 221,
for
example balsa wood or plastic blow-molded cubes, without compromising the
form or structural integrity of the core.
[0137] Referring to FIG. 17, reinforced strip 230 is provided
with
layers 176 and 177 of fibrous rovings, as shown in FIGS. 12 and 13, to form
wound reinforced strip 233. If needed for increased bending or axial strength,
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roving layer 180 shown in FIG. 13 may also be provided. Referring to FIG. 18,
reinforced core 240 is comprised of a plurality of wound reinforced strips 233
held together as a unitized structure by veils 241 adhered with heat activated
binder to opposite faces of core 240. If desired for greater bending
flexibility, veil
241 may be applied to only one surface of the core. Other means of unitizing
the core structure include adhering parallel bands of hot melt yarn or scrim
across the wound strips or applying pressure sensitive adhesive to the faces
of
the strips which are in contact with each other. In lieu of veils 241,
structural skin
fabric or mat may be adhered to the core surface to form a sandwich panel
preform ready for impregnation. When one or more cores 240 is placed in a
mold between fabric skin reinforcements and resin is flowed throughout the
core
and skin structure and cured to form a structural composite panel, fabric webs
222 and roving webs 242 comprised of four wound roving layers 176 and 177
form a grid-like reinforcing structure, and the portions of wound layers 176
and
177 adjacent the panel skins provide exceptional adhesive attachment for
resistance of shear forces. The articulated construction of core 240 also
permits
a high degree of conformability to curved mold surfaces.
[0138] FIG. 19 illustrates an embodiment of a fiber-wound core
250
in which bi-directional strength and stiffness are achieved without the
addition
of either internal webs or roving struts. Fiber reinforced core 250 comprises
a
plurality of triangular foam strips 251 which have been provided with layers
252
and 253 of helically fibrous rovings to form wound strips 254. The wound
triangular strips 254 are held together as a unitized core structure by veils
255
adhered with a heat activated binder to outer wound roving layer 253 of wound
strips 254. The angles to which the triangular strips 251 are cut may be
selected
for the desired balance of shear and compressive strength.
[0139] It is within the scope of the present invention to use
either
of two general types of hardenable resin to infuse or impregnate the porous
and
fibrous reinforcements of the cores and skins. Thermoset resins, such as
polyester, vinyl ester, epoxy and phenolic, are liquid resins which harden by
a
process of chemical curing, or cross-linking, which takes place during the
molding process. Thermoplastic resins, such as polyethylene, polypropylene,
PET and PEEK, which have been previously cross-linked, are liquefied by the
application of heat prior to infusing the reinforcements and re-harden as they
cool within the panel.
[0140] As an alternate to infusion of the porous reinforcement
materials of the assembled panel structure with liquid resin, the reinforcing
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37
materials may comprise fabrics and rovings which have been pre-impregnated
with partially cured thermoset resins which are subsequently cured by the
application of heat. Similarly, reinforcing roving and fabric materials may be
pre-impregnated with thermoplastic resins or intermingled with thermoplastic
fibers which are subsequently fused together through the application of heat
and
pressure.
[0141] It is further within the scope of the invention to bond
to the
faces of the reinforced foam cores rigid skin sheet materials such as steel,
aluminum, plywood or fiberglass reinforced plastic. This may be achieved by
impregnating the core reinforcements with a curable or hardenable resin and
applying pressure to the rigid skins while the resin cures, or by impregnating
and
curing the core reinforcement structure prior to bonding rigid skins to the
core
with adhesives.
[0142] FIGS. 20-23 show the steps in the construction of a fiber
reinforced foam core panel comprising helically wound strips and having
improved bi-directional strength and useful manufacturing advantages. In FIG.
20, helically wound foam strips 178 are connected together to form
unidirectionally reinforced core panel 260. If desired, strips 178 comprising
wound layers of rovings 176 and 177 (FIG. 2) may incorporate web sheets 94
generally parallel to the faces of core panel 260, as shown in FIGS. 6 and 7,
to
stabilize the rovings 176 and 177 against buckling under load. A preferred
method of connecting together a plurality of strips comprising low density
foam
and helically wound reinforcing rovings is shown in FIG. 23, in which
fiberglass
scrim 271, which has been coated with hot melt adhesive, is attached to
opposing faces of the core panel by application of heat and pressure. Scrim
271
or rows of adhesive coated individual fibers may be used to connect adjacent
strips in all of the core panel embodiments shown herein and comprising a
plurality of strips or blocks.
[0143] Layers of rovings 176 and 177 may comprise materials
resistant to adhesive bonding, for example, partially cured prepreg resin or
thermoplastic fibers. When such materials are used, rovings 176 and 177 may
be provided with additional spaced rovings comprising bondable fibers such as
non-impregnated fiberglass or carbon fiber. Referring to FIG. 21, the layer of
rovings 177 crosses and overlies the layer of rovings 176. If desired, the
rovings
may be wound onto the foam strip in a braiding process in which rovings 176
and 177 alternately overlie each other. This braiding option applies to all of
the
embodiments of the present invention which comprise two or more layers of
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reinforcing fibers wound onto a single strip of foam plastic or other low
density
cellular material. Strips 170 comprise closed cell foam if the core panel is
intended for infusion with a liquid thermoplastic resin in a pressure
differential
process. Both closed and open cell foams may be suitable for core panels
comprising prepreg rovings 176 and 177, or comprising hardened thermoplastic
resin components. After molding with skins and hardenable resin, foam may be
removed from reinforced strips 178 by grit blasting, solvent or otherwise to
produce hollow composite panels.
[0144] Referring to FIGS. 20 and 22, core panel 260 is cut in a
direction C perpendicular to the length of strips 178, by gang saw or other
means, into a plurality of first narrow fiber reinforced core panels 261 of
desired
thickness. During the cutting process, the severed end portions 262 of rovings
176 and 177 are frayed and are caused to protrude from the surface of foam
strips 170 due to removal of a layer of foam by the cutting process. Referring
to FIG. 23, a plurality of first narrow core panels 261 are connected
together,
using adhesive scrim 271, to form a bi-directional core panel 270 having
reinforcing webs extending both longitudinally and transversely. The
protruding
end portions 262 of reinforcing rovings 176 and 177 aid in making adhesive
connection to opposing panel skins (not shown) when the panel is infused with
a hardenable resin. If desired each strip 170 may be helically wound with a
single layer of rovings 176 and adjacent layers of rovings 176 will still
comprise
crossing layers having balanced structural properties. Similarly, all core
panels
described herein and comprising adjacent strips may be wound with a single
layer of helically extending rovings.
[0145] Cores of higher compressive strength may be produced by
providing wound strips 178 with axial rovings 180 on one or more sides of foam
strips 170 prior to winding, as shown in FIG. 13. In a finished core panel
270,
these axial rovings, which may be Similarly applied to core panels 290 and
300,
extend perpendicularly between the faces of the panel. An important advantage
of bi-directionally reinforced core panel 270 is that it can be quickly
produced in
any desired thickness from a pre-existing inventory of unidirectional core
panels
260, by simply slicing panel 260 into first narrow core panels 261 whose width
corresponds to the desired panel thickness and connecting the strips together
as previously described.
[0146] Core panel 270 may be provided with substantially
enhanced structural connection to panel skins as shown in FIGS. 24-26. That
is a narrow core panel 261 (FIG. 24), comprising foam strips 170 and wound
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layers of rovings 176 and 177, is provided with additional helically wound
roving
layers 281 and 282, which overlie layers 176 and 177, to form second narrow
core panel 280. A plurality of panels 280 are connected together, using
adhesive scrim 271 or other means, to form reinforced core panel 290, shown
in FIG. 26. Layers of wound rovings 281 and 282 form continuous webs
extending between the faces of core panel 290, while layers of rovings 176 and
177 form discontinuous webs intersecting the continuous webs. All four layers
of rovings are connected to sandwich panel skins 291 when hardenable resin is
introduced into the sandwich panel. FIG. 25 shows in detail the greatly
increased area of attachment of fibrous core reinforcing rovings to the panel
skins. Referring again to FIG. 24, if the layer of rovings 282 is omitted,
layers of
rovings 281 on adjacent wound strips 280 will form reinforcing webs in which
the
rovings 281 cross at opposing angles.
[0147] FIG. 27 shows a variation of bi-directionally reinforced
core
panel 290, in which second narrow core panels 280 are rotated 90 degrees from
the orientation shown in FIG. 26 before being connected together. In the FIG.
27 configuration, the densest layers of rovings on each wound core panel 280
are positioned within the core rather than adjacent the skins. The orientation
of
wound panel 280 is selected to produce either core panel 290 or core panel
300,
as determined by the desired balance of strength and stiffness between the
reinforcing webs and the panel skins.
[0148] Bi-directional core panels produced by helically winding
reinforcing members, such as those illustrated in FIGS. 23 and 26, are
comprised of a plurality of foam blocks which are attached together. This
articulated configuration allows the panel to conform to curved surfaces,
provided that the convex face of the panel is unitized by scrim fibers of
relatively
low tensile strength, or the curvature is achieved by applying heat to soften
the
adhesive which connects the scrim to the panel face. Referring to FIG. 23,
adhesive scrim 271 of high tensile strength, such as fiberglass, may be
applied
to opposing faces of core panel 270 after the panel is formed to simple or
compound curvature against a forming tool. After the scrim adhesive has set,
the pressure may be released and core panel 270 retains its curvature. This
method is useful for the production of preforms which may be efficiently
loaded
into curved molds. Adhesive scrim may also be used in this manner to produce
curved preforms comprising non-reinforced foam plastic.
[0149] Core panels which are used with thin skins, for example
roofs for trailers, may provide adequate shear strength and stiffness in the
core
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but insufficient support for the skins under conditions of impact or
compressive
loads. The poor skin support may be due to the absence of core reinforcements
which overlie the core panel faces, as in FIG: 23, or to the use of relatively
wide
strips of the helically wound foam comprising the core panel, which results in
widely spaced webs supporting the skins. A means of providing additional skin
support is shown in FIG. 27, in which bi-directional core panel 300, which
comprises a plurality of narrow core panels 280, has been provided with rigid
skin support members 301. In a preferred embodiment, support members 301
comprise fibrous rovings, for example fiberglass, which are inserted into
slits
formed in narrow core panels 261, shown in FIG. 22, prior to panels 261 being
helically wound with reinforcing rovings 281 and 282 to form narrow core panel
280, shown in FIG. 24. Support members 301, described a generally beam-like
rectangular cross section and are in turn supported at each point at which
they
intersect core reinforcing webs 302, which comprise wound layers of rovings
176
and 177, shown in FIG. 22. Referring again to FIG. 27, compression or impact
loads applied to panel skins 291 are transferred by skin support members 301
to reinforcing webs 302, thus preventing damage to skins 291.
st)] FIGS. 28-30 illustrate another embodiment of the present
invention, in which fiber reinforced strips 310 are provided with reinforcing
rovings 311 which extend axially along one or both sides of the corners of
foam
strips 170 and beneath one or more helically wound layers of rovings 176 and
177. This construction is shown enlarged in FIG. 29. When a plurality of
reinforced strips 310 are connected together as previously described to form
reinforced core panel 320 as shown in FIG. 30, adjacent pairs of reinforcing
webs comprised of crossing helically wound rovings cooperate with corner axial
rovings 311 to form, in effect, a plurality of structural bar joists having
top and
bottom chords which are separated by rod-like shear members. This structure
provides superior impact strength and enhanced attachment strength between
web reinforcements and panel skins, and permits the use of reduced skin
reinforcements. If desired, axial corner rovings 311 may also be added in the
construction of bi-directional core panels such as shown in FIGS. 24-26.
(0151] Additional axial rovings may be provided beneath wound
rovings to cover any or all of the surfaces of foam strips 170 in any of the
forms
of the present invention having helically wound reinforcing members. Single
reinforced strips 310 (FIG. 28), after molding with hardenable resin, may be
used
as discrete structural members, such as columns or box beams. Performance
of such structural members may be further enhanced by providing transverse
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reinforcing members as shown in FIGS. 17 and 24 and by providing additional
axial rovings to cover all exposed foam surfaces. Columns may be further
reinforced by helically wrapping layers of reinforcing material, for example,
fiberglass or carbon fiber fabric, around foam strips 170 at the end portions
of
the strips, or in other desired areas of the strips, prior to winding roving
layers
onto the strips, for purposes of providing enhanced strength in areas of
structural
attachment.
[0152] Molded column-like structural members may be
economically produced by a continuous process in which the fiber reinforced
foam output of a helical winding apparatus feeds directly and continuously
into
a molding apparatus, for example a resin injection pultrusion apparatus (not
shown) for the application and cure of thermoset resins. Similarly, helically
wound fiberglass rovings commingled with thermoplastic filaments, such as
"Twintex" rovings manufactured by Saint-Gobain Vetrotex , may be commingled
and hardened by being continuously advanced through an apparatus (not
shown) which successively applies heat and cooling to the fiber reinforced
foam
structure. It is also within the scope of the invention to provide a
continuous
process in which the fiber reinforced product of a helical winding apparatus
is cut
to form components of predetermined length and said components are delivered
into a mold for subsequent application and hardening of resin.
[0153] FIG. 31 illustrates a unidirectional fiber reinforced
core panel
330 comprising a unitized plurality of helically wound strips 331 in which
support
for panel skins is provided between helically wound core reinforcing webs. At
least two foam strips 170 are provided on one or both sides with facings 332
which may comprise rigid strip material or may comprise porous and fibrous
material, for example fiberglass mat, into which resin flows and hardens
during
molding of the core panel. In a particularly economical embodiment, foam
strips
170 are cut from low cost plastics foam insulation boards produced in a
continuous process in which the foam is introduced between continuous sheets
of fiberglass mat 332. Pairs of adjacent mats 332 provide substantial support
to panel skins between the core reinforcing webs comprising helically wound
rovings. Those segments of fiberglass mat which are adjacent the wound
rovings cooperate to form structurally enhanced reinforcing webs 333, which
are
comprised of two layers of fiberglass mat 332 and four layers of wound rovings
176 and 177. This structure provides both an increased amount of reinforcing
fibers, compared to webs which are helically wound only, and improved
resistance to web buckling under load, due to the greater overall thickness of
the
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webs. In lieu of fiberglass mat, strips 332 may comprise a variety of other
materials, including, for example, aluminum foil, which may be used to protect
foam strip 170 during the application of radiant heat applied to strip 331 in
order
to melt thermoplastic components of rovings 176 and 177.
[0154] FIG.
32 illustrates a form of reinforced core panel which can
be produced in greatly increased quantity from a given roving winding
apparatus.
Reinforced core panel 340 comprises alternating strips of roving wound
plastics
foam 178 and plain plastics foam strips 170. By increasing the weight of
reinforcing rovings wound on strips 178, structural properties roughly
equivalent
to those of uniform strip core panel 260 shown in FIG. 20 may be achieved in
the
alternating strip core panel shown in FIG. 32.
[0155] The
method of helically winding foam strips permits the
production of sandwich panels having cores whose structural properties vary
along the length of the core. This configuration is achieved by varying in a
controlled manner the spacing and angle of the rovings as they are wound onto
the foam strips which will be subsequently unitized to become core panels.
FIG.
33 shows wound strip 350 comprising foam strip 170 and spaced helically wound
rovings 176 and 177. Referring to FIG. 12, the angle and spacing of the
rovings
on foam strips 170 are controlled by varying the speed at which the strips are
advanced through winding heads 172 and 173 at a given rate of rotation of the
heads. This
relationship may be closely controlled through the use of
programmed strip conveyer drive motors. For example, as strip feed speed is
decreased the spacing of the wound rovings decreases and the angle at which
the rovings cross the axis of the strip decreases. The spacing of winding
heads
172 and 173 from each other is preferably adjustable to correspond to the
desired length of strip 350. Wound strip 350 shown in FIG. 33 illustrates a
foam
strip in which the density and angular steepness of the rovings with respect
to
the faces of strip 350 are highest at the ends of the strip, for the purpose
of
providing enhanced compressive strength to resist concentrated loads over
panel supports. For improved bi-directional strength, reinforced strip 261
shown
in FIG. 22, or reinforced strip 310 shown in FIG. 28 may be substituted for
non-
reinforced foam strip 170 shown in FIG. 33.
[0156] FIG.
33 also illustrates a means of providing improved skin
strength in composite panels of non-uniform core thickness. It is common in
structural sandwich panels for edge closeout portions of the panel to taper or
step down to lesser thickness, and thickness variations are sometimes required
=
within the interior of the panel. When the fibers comprising panel skins
deviate
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from a plane surface, tensile or compressive stresses in the skins may lead to
failure of the skin reinforcements and delamination of the skins from the
panel
core. The helically wound strip 350 shown in FIG. 33 has been provided with
layers of axial rovings 180, as described in connection with FIGS. 12 and 13,
on
the opposing faces of strip 350 which will comprise the faces of a reinforced
core
panel. As described in connection with FIG. 14, the axial layer of rovings 180
serves the function of skin fibers extending in the direction of the strip,
and the
axial rovings are helically overwound by layers of rovings 176 and 177. Under
conditions of bending stress, the tendency of axial rovings 180 to fail at or
near
core thickness transition area 351 is reduced because the helically wound
roving
layers constrain the axial rovings from moving outward. Stability of the axial
rovings may be further enhanced by providing strip 350 with transverse
reinforcements, as previously described, to prevent roving layer 180 from
buckling inward.
[0157] In helically wound unidirectional core panels comprising
low
density foam, the resistance of relatively thin reinforcing webs in relatively
thick
panels to buckling under compressive or shear loads may be substantially
improved by decreasing the slenderness of the webs. FIG. 34 shows core panel
360 comprising fiber reinforced foam strips 178 and web spacer strips 361,
whose function is to cooperate with layers of rovings 176 and 177 to form
compound reinforcing webs 362. Spacer strips 361 may comprise foam plastic
of greater compressive strength than that of foam strips 170, porous matting,
or
other material of sufficient strength to cause compound reinforcing web 362 to
function as a structural web of increased thickness. The spacer and roving
components of compound web 362 are structurally bonded together by the resin
used to infuse the sandwich panel. Spacer strips 361 serve to divide the mass
of resin present between foam strips 170 and thereby to reduce the shrinkage
normally induced in a local mass of resin during the curing process. This
reduced shrinkage along the reinforcing webs increases the flatness of molded
panel skins which improves appearance and may permit the use of lighter skin
reinforcements.
[0158] Sandwich panels comprising helically wound strips have
proven effective in retaining substantial structural integrity after high
energy
ballistic impact, for use in applications such as casings for jet engines or
structural backup for armor designed to prevent penetration by projectiles.
FIG.
35 illustrates a cylindrical or annular embodiment of the present invention
useful
as a jet engine casing, in which structural continuity of core properties is
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44
optimized by eliminating joints between the ends of helically wound foam
strips,
so that every helically wound roving within the entire panel is unbroken.
Cylindrical or annular core panel 370 is produced from a single helically
wound
foam strip 371, by wrapping strip 371 continuously around a cylindrical or non-
cylindrical mandrel in a helical pattern.
[0159] Wound strip 371, which comprise plastics foam strips 170
and layers of helically wound rovings 176 and 177, may be of cross sectional
shapes other than rectangular, for example, triangular, as shown in FIG. 19,
or
trapezoidal and in which the reinforcing webs within the core are oriented at
opposing angles to provide transverse shear strength to the core. Transverse
shear strength may also be provided by providing wound strip 371 with internal
transverse reinforcements, for example as shown in FIG. 24. If desired, a
second continuous strip 371 may be helically wound over core panel 370, =
preferably at a crossing angle, for greater strength. Hoop strength and impact
resistance of core panel 370 may also be enhanced by providing axial rovings
180 beneath wound rovings 176 and 177, as shown in FIG. 13. Ballistic impact
resistance of sandwich panels having helically wound core reinforcements and
structural skin reinforcements may be increased by stitching fibrous
reinforcements through the panel skins and core, at crossing angles or
perpendicular to the panel skins, as previously described in connection with
FIGS. 14 and 15. Continuous reinforced strips 371, in one or more layers, may
also be used to form enclosed containers of cylindrical or box-like
configuration
and intended to resist explosion, by forming strip 371 around all faces of the
container and providing skins applied by a filament winding process.
[0160] Continuous strip 371 may be wound using a relatively low
weight or relatively brittle reinforcing fibers, for example carbon tow, in
order to
allow a ballistic object such as a jet engine fan blade, to penetrate the
cylindrical
casing without seriously compromising the shape or structural integrity of the
panel, and the penetrating object is arrested outside the casing, for example
by
a surrounding wrap of non-resin-impregnated aramid fabric, such as Kevlar.
Alternately, the panel may be designed to contain the impacting object while
still
maintaining the integrity of the panel. In this configuration, it may be
desirable
to employ, as a core, skin and through-panel stitched reinforcements, fibers
such
as aramid or steel which will elongate under impact and resist penetration. By
employing resin film barriers 41 described in connection with FIG. 1, specific
layers of these impact resistant reinforcements may be kept generally free of
resin during molding, to optimize ballistic impact performance.
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[0161] FIG. 36 shows an embodiment of the present invention in
which hollow tubes are substituted for foam strips to produced a non-insulated
structural sandwich panel which may be used for the distribution of air or
water
or as an efficient heat exchanger, especially when provided with reinforcing
fibers of high thermal conductivity, such as carbon. Reinforced core panel 380
comprises a plurality of thin-walled tubes 381, which may be of rectangular,
triangular or other cross sectional shape, and which are helically wound with
layers of reinforcing rovings 176 and 177. Tubes 381 may serve primarily as
mandrels on which the structural rovings are wound and may therefore comprise
structurally weak material such as stiffened paper. Alternately, tubes 381 may
comprise material having significant structural properties, such as roll
formed or
extruded plastic or aluminum, preferably surface treated for structural
bonding
to the wound reinforcing layers and to subsequently applied panel skins.
[0162] The walls of tubes 381 comprising thin flexible material
may
be provided with convex curvature to resist pressure during the molding
process.
Molding pressure may also be resisted by sealing the ends of tubes 381 during
the process of producing core panel 380 or during the molding process. Sealed
helically wound flexible tubes of circular cross section containing air or
other gas
and comprising film plastic or other material impervious to resin, may be
unitized
to form core panel 380 and may be made to conform to generally rectangular
cross section during the molding process by applying pressure to the core
panel
faces using rigid platens. Core panels 380 which are sealed to prevent the
intrusion of resin may be combined with skin reinforcements and molded using
liquid resins. When rovings 176 and 177 comprise partially cured pre-preg
thermoset resins or heat-softened thermoplastic resins, core panel 380 may be
molded by the application of heat without sealing the ends of tubes 381.
[0163] FIGS. 37 and 38 show an embodiment of reinforced core
panel in which the helically wound core reinforcements which extend between
and over the faces of the core panel also extend over the edges of the core
panel. This construction provides superior transfer of structural loads in the
core
panel to adjacent core panels and to the edges of the sandwich panel and is
illustrated in FIG. 37. Spaced foam strips 170, preferably provided with axial
corner rovings 311 as described in connection with FIGS. 28-30, are passed
through a helical winding apparatus as previously described, to form
continuous
reinforced strip 390. Strip 390 comprises a plurality of axially spaced,
helically
wound foam strips 178, which may be provided with spaced transverse
reinforcing members as described previously, and which are connected to each
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other by layers of rovings 176 and 177, and the roving layers are supported
between strips 178 by axially extending rovings 311, to form hollow wound
segments 391. The wound roving layers are maintained intact across the
spaces between the foam strips.
[0164] In a second step, shown in FIG. 38, the wound strips 178
are folded back-and-forth, so that successive strips are adjacent one another
to
form reinforced core panel 400. The reinforcing rovings comprising hollow
wound segments 391 are folded and collapse across the ends of strips 178, to
provide superior adhesive attachment of the strip ends to adjacent panel
components in order to transfer structural loads between interior core panel
reinforcements and exterior core panel edges. Reinforced core panel 400 may
be produced in continuous lengths by applying continuous adhesive scrim to
connected strip segments 178 after they are moved or folded into contact with
adjacent strips. In its continuous form, core panel 400 is well adapted for
continuous molding processes, such as pultrusion, linked to the roving
helically
winding apparatus.
[0165] In another embodiment of the invention, fiber reinforced
foam core panels may be provided with bi-directional strength by helically
winding reinforcing rovings onto foam strips of serpentine shape. FIG. 39
illustrates reinforced core panel 410 comprising helically wound foam strips
411,
each having a serpentine configuration and shown with sandwich panel skin
reinforcements 291. The serpentine webs 412, which comprise crossing layers
of helically wound reinforcing rovings 176 and 177, provide core panel 410
with
shear strength in both longitudinal and transverse directions, and the ratio
of
strength in each direction is determined by the angular deviation of webs 412
from a straight line. Foam strips 170 may have parallel edges of serpentine
configuration in lieu of the symmetrical non-parallel edges shown in FIG. 39
and
may be cut from foam boards, using multiple gang saw water jets, or hot or
abrasive wires or may be formed by applying heat to thermoformable linear foam
strips. The winding angle of the wound rovings on strips having non-parallel
edges, may be controlled by varying strip feed through the winding apparatus,
as described previously.
[0166] The impact resistance of sandwich panels comprising fiber
reinforced cores impregnated with thermoset resins may be substantially
increased by incorporating thermoplastic resins of superior impact properties
into
the outer portions of the sandwich panel skins, instead of allowing the more
brittle thermoset resins to extend to the outer surfaces of the panel. FIG. 40
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illustrates a greatly enlarged section of composite sandwich panel 420
comprising helically wound fiber reinforced core 260 and panel skins 421 and
422. Foam strip 170 has been provided with resin distribution grooves 223,
previously described as grooves 39 in connection with FIGS. 13 and 14. Panel
skin 421 comprises fibrous reinforcing mat or fabric whose outer portions 423
are impregnated with thermoplastic resin, for example polypropylene, which
extends from the outer surface of skin 421 and partially through the thickness
of
the skin.
[0167] This layer of thermoplastic resin may be provided by
applying thermoplastic film to one side of fibrous skin 421 under heat and
pressure prior to infusing panel 420 with thermoset resin. If desired, a layer
of
fabric comprised of commingled fiberglass and thermoplastic fibers, for
example
"Twintex" fabric from Saint-Gobain Vetrotex, may be substituted for the
thermoplastic film. The commingled fabric is heated to form a reinforced
thermoplastic outer surface and to flow the thermoplastic resin partially
through
the thickness of the underlying reinforcing fabric. Enhanced impact resistance
may also be achieved by applying "Twintex" skin fabric 422, which has not been
consolidated by application of heat, to reinforced core panel 260, and
infusing
all core and skin reinforcements with thermoset resin. The thermoplastic
filaments which comprise skin 422 impart enhanced impact resistance to the
infused skin, and the skin may be heated after infusion to melt the
thermoplastic
fibers.
[0168] In a preferred method of producing helically wound fiber
reinforced composite panels having low density cellular cores such as foam
plastic, core panels are provided with separately applied fibrous
reinforcements
and hardened thermoplastic material, rather than with commingled-filament
roving such as "Twintex" fabric. Referring to FIG. 20, foam strips 170 may be
provided with a surrounding layer of thermoplastic resin, for example
polypropylene, by applying heated and liquefied resin to the strips in a
continuous extrusion process, after which the resin is cooled and solidified
prior
to helically wrapping reinforcing rovings 176 and 177 over the strips. Wrapped
strips 178 may be connected together, and the thermoplastic resin impregnates
the reinforcing fibers by application of heat and pressure, and skins
comprising
fibrous reinforcements and thermoplastic resin may be similarly attached to
the
core panel. In lieu of extrusion, strips of thermoplastic material may be
provided
adjacent the layers of rovings 176 and 177 and between foam strips 170.
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[0169] In still another method, foam strips 170 are helically
wound
with layers of rovings 176 and 177, each of which is comprised of a plurality
of
reinforcing rovings, such as fiberglass, and thermoplastic rovings. In all of
these
methods of separately applying fibrous reinforcing and thermoplastic
components to the foam strips, subsequent impregnation of the reinforcing
fibers
by application of heat and pressure is generally less complete than that
achieved
by using commingled-filament rovings. The advantage of the present methods
is that very low cost materials, including recycled thermoplastics, may be
used
in the production process. It is understood that monofilament fibers of
various
flexible materials, including metals and high tensile strength plastics, may
be
used as reinforcements in all of the fiber reinforced panels described in the
present invention, in lieu of fibrous rovings comprising a plurality of
filaments.
[0170] As previously described, embodiments of the present
invention are adapted for use with liquid thermoset molding resin in processes
in which the resin flows throughout and impregnates the internal core
reinforcing
elements under differential pressure. These embodiments are illustrated in
FIGS. 1-40 and comprise porous reinforcing elements within the core panel.
Major portions of the sandwich panel industry employ processes in which
differential pressure is not utilized or is insufficient to cause the resin to
wet out
the core reinforcements. As the thickness of the sandwich panel core
increases,
the absence of differential pressure severely limits the extent to which
molding
resin can penetrate and flow throughout the core reinforcing members, for
example fiberglass rovings, within the core. Penetration and hardening of the
resin is essential to achieving the structural properties of the fiber
reinforced core
and sandwich panel.
[0171] Several embodiments described herein adapt the present
invention for use in sandwich panel manufacturing processes which do not
employ differential pressure. Such processes include, for example, open
molding with liquid resins, open-bath pultrusion, and adhesive lamination of
rigid
skins to panel cores. In embodiments adapted for these processes, those
portions of the reinforcing members situated within the sandwich panel core
are
impregnated and hardened during the production of the core panel, and those
portions of the reinforcing members adjacent the faces of the core panel
remain
porous. The hardening of the internal reinforcing members secures the desired
core structural properties, and the porosity of those portions of the
reinforcing
members adjacent the core panel faces adapts the core for especially strong
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structural attachment to sandwich panel skins which are subsequently attached
to the core using adhesive resins.
[0172] Hardened web core panels may also be used
advantageously in molding processes employing differential pressure, such as
resin infusion, injection pultrusion and resin transfer molding. In these
exothermic resin curing processes, resin temperatures within the core are
significantly reduced by decreasing or eliminating the amount of uncured resin
in the webs, thus reducing the likelihood of foam damage or generation of
volatile gasses. It may be useful to perforate the hardened web core panel to
allow flow of skin molding resin from one face of the core panel to the other.
Alternately, the webs of the core panel may be only partially impregnated and
hardened, so that some residual porosity remains in the web reinforcements to
permit flow of resin during the molding process.
[0173] FIG. 41 illustrates structural composite sandwich panel
430,
useful as the wall of a refrigerated trailer or recreational vehicle,
comprising
reinforced core panel 431 and panel skins 432. Core panel 431 comprises a
plurality of helically wound strips 178 of plastics foam or other low density
cellular
material constructed generally as described in connection with FIGS. 12-14.
Axial roving layers 180 are not shown in FIG. 41 but may be provided if
desired.
If desired, wound foam strips 178 may omit second roving layer 177 and, if
desired, may also be provided with pre-attached reinforcing mats 332 as shown
in FIG. 31 or with transverse reinforcing members 222 as described in
connection with FIG. 16.
[0174] Referring again to FIG. 41, prior to consolidation of a
plurality of strips 178 to form core panel 431, a hardening adhesive resin
433,
for example polyester or polyurethane, is applied to those portions of porous
wound roving layers 176 and 177 which comprise the reinforcing webs of core
panel 431. Resin 433 may be applied to both opposing web faces of each foam
strip, or it may be applied to only one face, in sufficient quantity to wet
out the
porous fibers of the adjacent web face when strips 178 are connected together.
If desired, some porosity may be retained by limiting the amount of resin
applied.
Heat may be applied to the roving layers prior to application of resin to
facilitate
wet-out of the reinforcing fibers by reducing the viscosity of the resin when
it
contacts the heated reinforcements. The increased temperature also
accelerates the rate of resin cure subsequent to application of the resin. Web
strips 178 are connected together by pressing adjacent strips against each
other
in a stack while resin 433 hardens to form composite reinforced webs 434.
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Alternately, the web portions of individual strips 178 may be hardened, and
adhesive scrim or other connection means may be used as previously described
to consolidate a stack of strips 178 to form core panel 431.
[0175] In the embodiment shown in FIG. 41, web hardening resin
433 is withheld from those portions of the core panel webs immediately
adjacent
the faces or opposite side surfaces of the core panel, for example for a
distance
of one-eighth inch from the faces of the core panel, in order to permit
wicking or
flow of skin attachment resin into the outer portions of the web
reinforcements
for improved structural attachment of webs 434 to skins 432. It is understood
that, if desired, hardening resin 433 may extend fully to the opposite side
surfaces or faces of the core panel, or the resin may further extend partially
or
entirely across the faces of the core panel.
[0176] FIG. 51 illustrates core panel 500, in which web
hardening
resin 433 extends laterally across a portion of the exposed surfaces or faces
of
adjacent fiber wound strips 178 to form a series of structural I-beams 501.
This
embodiment is useful for increasing the strength and stiffness of sandwich
panels in which an adhesive of relatively low structural properties is used to
attach skins to the core panel. Resin 433 impregnates the wound fibers between
adjacent strips 178 and also impregnates a portion of the wound fibers 502
extending across the faces of core panel 500, and resin 433 hardens to form
structural I-beams 501. Skins 432 are attached to core panel 500 using
adhesive 435 which penetrates porous wound fiber portions 502 to form a strong
skin-to-core bond, while hardened I-beams 501 provide enhanced panel strength
and stiffness.
[0177] If the opposite side surfaces or faces of core panel 431
are
entirely impregnated with resin 433 and hardened, core panel 431 becomes a
rigid sandwich panel. Structural properties of this resulting sandwich panel
and
of the I-beams 501 shown in FIG. 51 may be enhanced by providing the wound
strips 178 with longitudinal fibrous rovings 180 as described in connection
with
FIGS. 13 and 14. Web hardening resin 433 may be applied by roll coater,
extrusion, spray or flow apparatus, resin film or otherwise. The resin may be
thermoset, for example polyester, epoxy or urethane or it may be
thermoplastic,
for example polypropylene, PET or nylon. The rate of hardening of thermoset
resins may be accelerated by the application of high catalyst levels, heat,
ultraviolet radiation or otherwise, in order to increase the rate of
attachment of
wound strips 178 to each other to form core panel 431.
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[0178] Thermoplastic resin may be incorporated into roving
layers
176 and 177 during the winding process by providing rovings comprising
commingled structural and thermoplastic filaments, for example "Twintex"
manufactured by Saint-Gobain Vetrotex, or structural rovings surface coated
with
thermoplastic resin as manufactured by Hexcel Corporation. Strips 178
comprising thermoplastic resins are connected to each other by pressing the
strips together after applying sufficient heat to the web portions of the
strips to
melt the thermoplastic matrix. Alternately, electrically conductive fibers,
for
example, carbon fiber may be provided adjacent wound layer 176 and 177, and
electrical current may be passed through the conductive fibers to melt the
thermoplastic matrix. Layers 176 and 177 may, if desired, comprise hardened
fiber reinforced thermoplastic tapes, such as "Zen icon" manufactured by Crane
Composites, in lieu of Twintex rovings. The thermoplastic tapes may be wound
onto foam strips 170 by providing sufficient heat to soften the tapes prior to
contact with strips 170. Tape-wound strips are connected together as described
for Twintex. It is also within the scope of the invention for layers 176 and
177 to
comprise high tensile strength polymer fibers, for example MFT by Milliken and
Curv by Propex.
[0179] Finished core panel 431 (FIG. 41) is moved to a molding
or
lamination process in which sandwich panel skins 432 are attached to the core
panel as previously described, using adhesive resin 435. Resin 435 used to
attach the skins may be, but need not be, of the same type as resin 433 used
to
harden webs 434. Resin 433 may, for example, comprise catalyzed polyester
resin, and resin 435 may comprise moisture curing polyurethane resin, or one
resin may be thermoplastic and the other thermoset. Skin attachment resin 435
wets out the porous portions of wound roving layers 176 and 177 which
comprise the opposite side surfaces or faces of core panel 431 and may
comprise the edge portions of the webs adjacent the core panel faces,
providing
a strong structural attachment of skins to core.
[0180] Adhesive resin for bonding skins, is similarly applied if
all
portions of roving layers 176 and 177 have been impregnated and hardened as
previously described. Sandwich panel skins 432 may be porous and fibrous
prior to attachment of resin 435, for example fiberglass fabric, or they may
be
rigid, for example aluminum or fiberglass reinforced plastic sheet. The skin
attachment resin may be applied by any convenient application process and
does not require differential pressure for flowing into webs 434, since these
have
already been hardened, as previously described. When core panel 431
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comprises roving layers which incorporate a thermoplastic matrix, skins may be
attached by heating the core faces to liquefy the thermoplastic matrix of the
exposed roving layers.
[0181] Sandwich panel 430 may be used as a construction panel
or building wall by incorporating skins 432 comprising sheet materials common
in the construction industry, for example decorative plywood or thin painted
metal. Adhesive resin 435 may also be used to adhere a plurality of pieces of
individual cladding materials, for example glazed tiles brick or stone. In a
useful
variation of the panel shown in FIG. 41, resin layer 435 may comprise a mastic-
like material such as fiber reinforced polymer stucco, or other hardening wall
surfacing material. In this embodiment, the material comprising layer 435
penetrates fibrous roving layers 176 and 177 prior to hardening to form a
permanent structural bond to the faces of core panel 431 and cooperates with
hardened webs 434 to resist structural loads applied to the building panel. If
desired, hollow tubes may be substituted for foam strips 170 as previously
described in connection with FIG. 36, and the tubes may be filled with a dense
material, for example sand or concrete, to render sandwich panel 430 shown in
FIG. 41 useful as a soil retaining wall or highway noise barrier.
[0182] Hardened webs having porous portions adjacent the panel
skins may also be provided in core panels in which the core reinforcing
members
comprise planar web sheets of fibrous reinforcing material, for example
fiberglass cloth or mat. FIG 42 shows reinforced core panel 440 comprising a
plurality of foam strips 33 having attached porous fibrous web sheets 34 as
previously described in connection with FIG. 1. The steps of providing fibrous
struts 35 shown in FIG. 1 are omitted. Referring again to FIG. 42, hardening
resin 433 is applied to porous web sheets 34, and a plurality of foam strips
33,
with attached web sheets 34, are connected together as described in connection
with FIG. 41. Web hardening resin 433, shown in FIG. 42, may be withheld from
the edge portions of webs 34 adjacent the opposite side surfaces or faces of
core panel 440 so that adhesive resin used to attach skins to the core panel,
as
described in connection with FIG. 41, will penetrate into the webs to provide
an
improved structural bond. Webs 434 may, if desired, comprise Twintex
commingled fiberglass and thermoplastic fabric, and the webs may be hardened
by application of heat and pressure, retaining porosity in the web edge
portions
for attachment to skins using liquid resins.
[0183] The embodiment shown in FIG. 43 illustrates sandwich
panel 450 having spaced reinforced core strips. A plurality of roving-wound
foam
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strips 178 having hardened web portions 451 and porous face portions 452 are
assembled in a spaced-apart array or relation and are attached, using a
lamination process, to opposing rigid panel skins 453, using adhesive resin
435.
This embodiment substantially reduces the volume of plastic foam required and
is useful in structural sandwich panels which do not require thermal
insulation.
If thermal insulation or continuous support for panel skins is required,
alternating
strips of plain foam and wound foam having hardened webs 451 may be
connected together as generally described in connection with FIG. 32. The
embodiments of the present invention shown in FIGS. 41 and 43 may, if desired,
incorporate hollow tubes 381 as shown in FIG. 36, in lieu of foam strips 33.
In
an alternate embodiment, higher density materials, for example dimensional
lumber, may be substituted for foam strips 170 to achieve improved structural
properties.
[0184] FIGS. 44-47 show the steps in the construction of a
reinforced core panel comprising helically wound strips and hardened
structural
webs and having improved bi-directional strength. Core panel 431 shown in FIG.
44 and having hardened webs 434, as described in connection with FIG. 41, is
cut in a direction perpendicular to the length of strips 178 into a plurality
of first
narrow fiber reinforced core panels 462 of desired thickness. Referring to
FIG.
46, first core panels 462 are helically wound with crossing roving layers 281
and
282 to form second reinforced strip 464. Referring to FIG. 47, hardening resin
433 is applied to adjacent faces of a plurality of second reinforced strips
464.
Resin 433 wets out roving layers 177, 178, 281 and 282, shown in detail in
FIG.
46, to form hardened webs 465 shown in FIG. 47, and strips 464 are pressed
together and connected as resin 433 hardens, to form reinforced core panel 460
having hardened webs 465 extending longitudinally and hardened webs 434,
shown in phantom, extending transversely. Sandwich panel skins may be
applied to core panel 460 as described in connection with FIG. 41. Referring
to
FIG. 45, bi-directional core panels may also be produced by applying hardening
resin 433 to the wound rovings of narrow core panel 462 and pressing the
wound edges together as the resin cures, to form a core panel similar in
architecture to that shown in FIG. 23.
[0185] FIGS. 48-50 illustrate schematically advantageous means
of producing continuous sandwich panels comprising foam strips having layers
of helically wound reinforcements. In panel molding apparatus 470 shown in
FIG. 48, a plurality of continuous lengths of foam strips 471 having layers of
porous reinforcing rovings are pulled from reel 472 into pultrusion apparatus
473
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comprising resin bath or resin injection module 474 and heated die 475, by a
pulling apparatus (not shown) commonly used in the pultrusion art. Continuous
wound strips 471 are taken up onto reel 472 during the process of winding
strips
177 as shown in FIG. 12, omitting the step of cutting strips 177 into lengths.
If
desired, a single continuous strip 471 may be provided in lieu of the
plurality of
strips shown in FIG. 48, and if desired, strips 471 may be pulled into
pultrusion
apparatus 473 simultaneously from a plurality of reels 472.
[0186] Strips 471 may be provided with transverse reinforcing
members, axial reinforcements or other improvements previously described
herein. As strips 471 progress through apparatus 470, skin materials 476, for
example fiberglass cloth, are applied to the surfaces of strips 471, the skin
and
core reinforcements are wet out in resin module 474, the resin is hardened in
heated die 475 to form reinforced sandwich panel 477 having reinforced core
478, and the sandwich panel is cut to desired length (not shown). Continuous
strips 471 provide unbroken reinforcing layers 176, 177 and 180 within
sandwich
panel core 478, regardless of where sandwich panel 477 is cut, thus producing
a panel of uniform strength throughout its length.
[0187] As previously described herein, the helically wound forms
of the present invention are well adapted for continuous process production of
molded composite panels. FIG. 49 illustrates an economical method of
producing a continuous sandwich panel, useful as a trailer wall or building
wall
and having a core comprising fiber reinforced foam strips 178 transverse to
the
length of the panel. The efficient incorporation of transverse reinforcing
members is especially difficult in traditional methods of continuous panel
production such as pultrusion. Panel production apparatus 480 comprises
winding apparatus 171 (FIG. 12), wound strip advance device 482, and molding
module 483. Winding apparatus 171, described in connection FIG. 12, produces
fiber wound foam strips 178 which, as shown in FIG. 49, are advanced
successively by advance device 482 into and through resin module 483. The
strips 178 may be advanced perpendicular to the length of the strips (FIG. 49)
or at an acute angle relative to the direction of advance of the strips.
[0188] The wound strips may incorporate features previously
described herein, for example, transverse reinforcing members within the
strip.
If desired, wound foam strips 178 may be fed from reels 472, as described in
connection with FIG. 48, and cut to the desired length before being advanced
into resin module 483. Prior to entering the molding module, the stack of
strips
178 is provided with porous skin materials 484. Resin wets out porous skins
484
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and the porous rovings in foam strips 178, and cures in molding module 483 to
form continuous sandwich panel 485. In a particularly economical embodiment
of the present invention, wound strips 178 are provided with axial roving
layers
180, and a plurality of reinforcing rovings supplied from reels are
substituted for
skins 484, eliminating the cost of weaving reinforcing fabric.
[0189] Molding module 483 may be a pultrusion apparatus, as
described in connection with FIG. 48, an extrusion apparatus as to be
described
in connection with FIG. 50, or other molding device known in the industry. An
important advantage of this method is that panels of any desired width may be
produced either directly from the output of a winding machine or alternately
from
a single reel of continuous fiber reinforced foam strip. Roving wound foam
strip
178 may, if desired, comprise a pre-stiffened web 332 adjacent one or opposing
faces of the foam strip, as shown in FIG. 31. In this configuration, webs 332
provide substantial compressive and shear strength to the core, and
penetration
of roving layers 176 and 177 by the molding resin used to attach skins 484
may,
if desired, be omitted.
[0190] FIG. 50 illustrates an economical method of producing a
continuous sandwich panel, useful as a construction plank, board or post of
high
strength, low material usage and low weight, and incorporating a plastics
resin
extrusion process. Panel production apparatus 490 comprises winding
apparatus 171' and 173', and extrusion module 491. Winding apparatus 171'
and 173', described in connection FIG. 12, produces continuous fiber wound
foam strip 178 which, as shown in FIG. 50, is advanced through extrusion
module 491. In the module 491, heated liquid thermoplastic resin, for example
PVC or polyethylene, is applied to wet out fibrous reinforcing layers 180, 176
and
177, and the resin is cooled and hardens to form continuous sandwich panel
plank 492. Strip 178 comprises a plastics foam composition, for example,
polyisocyanurate or phenolic, able to withstand the temperature of the heated
extrusion resin.
[0191] If desired, fiber wound foam strips 178 may comprise
fibrous
mat reinforcements 332 to supply enhanced compressive strength to sandwich
panel 492, as described in connection with FIG. 31, and additional skin
materials
may be provided to sandwich panel 492 as described in connection with FIGS.
48 and 49. The reinforced foam core may, if desired, be supplied from reels as
described in connection with FIG. 48. Also, if desired, the extrusion resin
may
comprise a filler material, for example cellulose wood flour, to produce
surface
properties useful in, for example, deck boards, in which case the extrusion
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process may include a first non-filled resin stage to ensure full wet-out of
the
fibrous reinforcements of sandwich panel 492. Also, panel board 492 may be
provided with surface embossing or with additional surface layers of extruded
resin to resist ultraviolet radiation, as commonly practiced in the extrusion
art.
Pultrusion module 473, described in connection with FIG. 48, may be
substituted
for extrusion module 491 shown in FIG. 50, depending upon the specific
materials and properties required in sandwich panel 492. Fiber wound hollow
tubes as described in connection with FIG.. 36 may be substituted for wound
foam strips 178, provided that the hollow tubes are sufficiently strong to
resist
the pressure of the extrusion process.
[0192] Any of the fiber reinforced core panels disclosed herein
may
be used to produce structural molded composite panels of a thickness
=
exceeding that of the individual core panels. Two or more core panels may be
stacked in a mold, with the fiber reinforcements of adjacent core panel faces
in
contact with each other or with a layer of reinforcing material, for example
fiberglass fabric, separating the core panels. If desired,
the fibrous
reinforcements of adjacent core panels may be placed in crossing orientation
to
achieve specific structural properties, for example by stacking in crossing
orientation two layers of the core panel shown in FIG. 18. Wound strips 178,
shown in FIG. 32, may be provided with transverse reinforcing members as
previously described, and two or more core panels 340 having said transverse
reinforcing members may be stacked with strips 178 in crossing arrangement to
form a second core panel having enhanced bi-directional strength. If desired,
stacked core panels 340 may be separated by a reinforcing mat or fabric.
[0193] For purposes of clarity and comparison, core panels herein
have been shown as rectangular in shape and as having sets of fibrous
reinforcements generally parallel to the edges of the core panels. If required
by
structural considerations, the sets of reinforcements may be oriented to any
desired angle to the direction or edge of the core panel. For example,
referring
to FIG. 18, transversely reinforced foam strips 233 may intersect the edges of
rectangular core panel 240 at an angle of 45 degrees.
[0194] The forms of the reinforced foam cores and core
panels herein described and their method, steps of construction constitute
preferred embodiments of the invention.
