Note: Descriptions are shown in the official language in which they were submitted.
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The invention relates to a sandwich structure of the kind `-
having the form of a laminate wich a core of foamed plastic
material enclosed between surface-layers of a hard material
resistant to pressure, tension, and wear.
Sandwich structures are used in several constructional
fields, e.g. for the production of hulls of smaller and larger
boats, for bulkheads in boats, in the production of containers,
open as well as closed, and in the car industry for the pro-
duction of platforms and truck bodies, e.g. for refrigerated
vans, and generally for all purposes where the lightness,
strength, particularly the large bending strength, and insulat-
ing properties of the material is advantageous.
Thus, sandwich structures are known, in which the core ma- `~
terial consists of cross-cut balsa wood,`i.e. wood~which has
been cut so that the fibers are substantially perpendlcular to;~ ~
the surfaces. The wood has been cut to form small flat blocks `
which are placed edge against edge in the structure, where they
may be temporarily kept together by adhesively applied threads
or web. Making up the core from such small blocks makes it pos-
sible to produce more or less curved sandwich structures.
However, balsa wood is comparatively costlyj and the avail-
ability thereof is limited, and~there~is thus a need for~a
cheaper and more available core material.~As such, various
types of foamed plastics have been used where, after foaming~
and curing the plastic in large blocks, the blocks are sliced,
and the slices are possibly further cut into small blocks.
In several respects, however, this is inexpedient. Above~
all this relates to the compression strength, since this may
vary up to 50~ along the surface of a slice cut from such a
larger block. Moreover, the surface will have numerous pits
originating from cut cells, which partly causes a reduction
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of the compression strength, and partly results in an addition-
al consumption of adhesive in building-in of the material in
the sandwich structure, since all of the small pits have to
be filled with adhesive in order to get a proper bonding to
the surface layers.
The object of the invention is to redress the said disad-
vantages in the use of foamed plastic as core material, and
this is attained by a core material as described in claim 1.
It is very surprising that the use of such core material,
usually called integral foam, has not formerly been proposed,
since it is advantageous in many ways in preference to the
hitherto used blocks produced by cutting larger blocks.
Thus, the integral foam has a foamed interior surrounded
by a hard and tight surface layer, and because the small blocks
making up the core material are cast and foamed in a great
number at a time, but each in its own mould, you get blocks
with a hard and tight surface on all sides, which means that
each block represents a kind of box-structure with good cor-
ner and side rigidity. Besides contributing to a greater
strength of the sandwich structure as a whole, the kight and
hard surfaces result in a cushioning over the surface and in-
to the block of local compressive actions, so that the struc-
ture is not so easily damaged by such actions. Also the tight
surfaces involve a minimal consumption of adhesive in khe
building up of the sandwich structure. Finally, khe tight
surfaces counteract or prevent accumulation of moiskure in
the blocks, and even if khe surfaces of some few blocks should `
be damaged, so khat moisture can penetrate, such moisture can-
not spread to the undamaged adjacent blocks.
In order to simplify and facilikate the production, the
core material units in an appropriate embodiment of the sub-
ject of the invention are in the form of hexagonal parallele-
pipeds. They may, for example, be of khe dimensions 30 x 30 x
10 mm, the sides being ak righk angles to one another.
In anokher embodiment according to the inven-
tion, two opposite sides have the form of parallelograms, by
which is atkained that the individual block funckions as a
~kind of brace in khe sandwich skructure.
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According to the invention, it can further be expedient
to have the core material units reinforced. This increases
the strength and rigidity to a substantial degree, which is
of special importance in sandwich structures of a thickness
from 30 mm and upwards. The sturdiness and strength of a
sandwich structure is namely not only a q~estion of being
able to resist actions perpendicular to the surface thereof,
but also of being able to resist actions parallel to the
surface, that is to say that the core material should be re-
sistant to such actions.
By way of example, the reinforcement can consist of fi-
bres, particularly glass fibres, which are incorporated in
the plastic prior to the foaming thereof, but also other
kinds of reinforcements, such as paper,textiles,or plastic
films in sheets or strips, can be built-in by pIacing the
reinforcing material in the moulds, in which the foaming of ~ "
the plastic is to be carried out. The reinforcing material
can also be continuing over partitions between the moulds,
thus serving to lock together a number of units to make
larger sheets.
Particularly in the production of larger sandwich struc-
tures, it will often be expedient to have the core material
units thus locked together or connected to a sheetlike pro-
duct in other manner, e.g. by being attached to a plastic
film, a supporting web, or supporting threads. Such attach-
ment is easy to carry out in connection with the production
of the units, and the resulting product is easily handled
and can be rolled up and transported to the field of appli-
cation, where it can also be built in as so large plates or
sAeets as is practical at a time. The sandwich structures are
namely mostly built up in situ, whereby first one surface
layer, reinforced with fibres or a web, is built up in a -
mould, then the core material is adhesively joined to said
first surface layer in suitably great pieces at a time, and
finally the other surface layer is applied.
It is of the greatest importance to the strength of the
sandwich structure that no air is captured between the core
material units and the surface layer during the glueing of
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the former to the latter, whether the gluing is performed with
a separate adhesive, or is performed simply by pressing the
units into a surface layer, before the latter has been cured.
When using balsa blocks as core material this is no great
problem, because of the balsa wood being porous to some ex-
tent, which together with capillary action upon the adhesive
causes the air to disappear and the adhesive to penetrate
with the result that such blocks almost fasten themselves.
In the present core material units, the same possibili-
ties do not exist, and when obtainment of the greatest pos-
sible strength is aimed at, special precautions have to be
taken to avoid the said inclusion of air. In an embodiment
of the subject of the present invention this is attained by
providing ventilating openings across the layer of core ma-
terial units.
Thus, according to the invention, the units can have a
central opening crossing the largest surfaces of the unit.
In the production, such openings can be brought about in
known manner, using retractable core pins, but this makes the
moulding apparatus fairly complicated. This can be avoided by
placing the openings at the corners of the core material ~;
units when, according to the invention, the corners of a unit
are bevelled. Thus, a throughgoing aperture will be formed
where the corners of four units placed in a quadrangle meet
one another, whether only one corner or all four corners are
bevelled. Whethe~ the aperture has a cylindrical or an angu-
lar surface is of no importance for the ~untion as an escape
passage for air trapped behind the units.
Providing such apertures ma~es it also possible to fasten
the units by injection moulding, in that adhesive is simply
injected under pressure through some of the apertures. The in-
jected adhesive will then chase possibly trapped air before
it to other apertures or fissures between the units, where
the air can escape. Also the adhesive can emerge at the same
places to provide a control of the penetration of the adhe-
sive which can be used to determine the number and distribu-
tion of the injection sites.
In order to facilitate the laterial distribution of the
adhesive, the units can also, according to the invention, have
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gully-shaped recesses in either one or both of their largest
surfaces. Further, by placing one such unit on top of another,
a thicker unit with apertures lengthwise and crosswise in the
core can be formed, whereby all interspaces are more easily
filled for attaining great strenght of the structure. Possib-
ly, filling of the interspaces may be supported by creating
a vacuum therein.
If the sandwich structure is to be curved, as for example
in parts of a boat's hull, the narrow sides of the individual
core material units according to the invention can be formed
as respectively convex and concave cylinder surfaces with the
same radius of curvature. This results in that the units can
be laid down along a curved surface without wedge-shaped fis-
sures being formed between neighbouring units, because a con-
vex side fits into and turns in a concave side. Thus, the
core structure will become more compact, and the consumption
of adhesive will be less, since the fissures do not expand in
the curving.
Each unit can have two convex and two concave sides, or
every second unit can have only convex sides, and the others
only concave sides.
An embodiment of a sandwich structure according to the
invention is illustrated by the accompanying drawings, in which
Fig. 1 is a cross-sectional view of ths structure,
Fig. 2 shows some of the core material units of the struc-
ture, affixed to a wide-meshed web,
Fig. 3 shows a core material unit with two inclined sides,
Fig. 4 shows a core material unit with two convex sides `
and two concave sides,
Fig. S shows a special embodiment of a core material
unit, and
Fig. 6 shows an arcuate sandwich structure.
As it will appear from the drawings, the sandwiah struc-
ture consists of a core material composed of small parallele- -~
pipedic units 5 and 6 of a foamed plastic material, e.g. poly- -~
urethane of the socalled integral-skin type, where the foamed
plastic has a firmly adhering, hard and tight surface on all
sides. The production of a foamed plastic of this type belongs ~`
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to the known technique and is, therefore, not described here.
The core material units meet one another edge against
edge in a plate-like structure with surface layers 7 and 8,
which may be similar or different to the two sides.
The surface layers can be prefabricated sheets of any
suitable materiale, for example metal, plastic or plywood,
which is glued to the core material units by means of a suit-
able adhesive layer 9.
The surface layers can also be produced by a selfcuring
plastic which, if desired, can be reinforced with fibres 10,
for example glass wool fibres, being sprayed directly onto a
moulding surface, and the core material units can be pressed
into the plastic before it is fully cured. The core material
units can also have reinforcement, e.g. be fibre-reinforced,
as suggested in the unit 6.
The construction of the sandwich structures is greatly
facilitated when the small core material units 5 (or 6) are
fastened to a carrier material, for example a coarse woven
material 11, as shown in Fig. 2. In this way,a large number
of core material units at a time can be put where they be-
long, and also transport and storing of the units is facili-
tated, since the woven material with the fastened units is
easily rolled or folded together.
The fastening can be performed, for example, by gluing
the units to the woven material, and by a suitable choice of ~-
the adhesive, the fastening to the web can be made temporary,
e.g. when the web is intended to be torn off after the core
material units have been permanently fastened to one of the
surface layers 7 and 8 of the sandwich structure, or the web
may serve as a reinforcement of the structure.
As shown in Fig. 3, the units can also be made as paral-
lellepipedes which are no~ right-angled in one direction. In
this way, a sort of bracing of the sandwich structure is at-
tained, and also that the fissures be-tween the individual
units do not open excessively in curved sandwich structures,
since instead a mutual displacement of neighbouring units
takes place.
As shown in Fig 4, a unit can also be of such shape that
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one of opposite sides forms a concave cylindrical surface 12
and the other a correspondingly convex cylindrical surface
13, allowing for the units to abut tightly also in curved
structures. This is advantageous in respect of strength,
since it makes the sandwich structure more compact, and at
the same time adhesive is saved, since the fissures between
the units do not open by laying out the units along an ar-
cuated surface.
Another embodiment of a unit is shown in Fig. 5. Here,
the corners 14 of the unit 5 are cylindrically recessed to
form a through-going opening when four units are placed in
a quadrangle.
Fig. 5 also illustrates that a central opening 15 can
be provided across the unit 5 with gully-shaped recesses 16
extending from the opening 15 to the borders of the unit.
This embodiment makes it possible for the units to be
fastened by injection of the adhesive, e.g. an adhesive in
the shape of, or based upon a polyester or an epoxy compound.
The adhesive is then injected through some of the transverse
openings and chases the air forward to and out through others
of the openings so that a close connection is ensured between
the core material units and the surface layers in the sandwich
structure to give it optimal strength. During the injection
of the adhesive, a vacuum can also be created to remove air
pockets behind the core material units.
The transverse, gully-shaped recesses 16 can serve to
facilitate the distribution of the adhesive, and to promote ;
the gluing if it is desired to give the sandwich structure
a thicker core by placing more core material units atop of
one another.
When thus two units are placed with the surfaces with
the gully-shaped recesses 16 abutting, a thicker unit is ob- ;~
tained with distribution channels, which are parallel to the
lengthwise direction of the sandwich structure and may serve
for injection of an adhesive.
Fig. 6 illustrates the making of a curved sandwich struc-
ture by means of core material units 5 (or 6). Here units are
used having the form of re~tangular parallelepipeds with a
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central venting or injection opening 15, the curved laying
out creating larger spaces below and between the units so
that it is important that the possibility exists of venting,
possibly supported by suction (evacuation).
