Note: Descriptions are shown in the official language in which they were submitted.
BACKGROUND OF THE IN~ENTION: ~2~34
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Field of the Invention
The present invention re7ates to a novel process providing molds for
integral co-curing of a composite assembly made of a skin and stiffeners.
Description of Prior Art
~ here is a trend to increase the use of so-called composite
material in the manufacture of aircraft parts. Such composite material is
usually constituted by interwoven fabrics formed of reinforcing fibers,
and of un;directional fiber materials, such as fiberglass, graphite,
boron, etc., and impregnated with epoxy resins. An advantage in the use of
such materials is that they eliminate a great many mechanical fasteners
such as rivets utilized in attaching the usual metal panels to the
framework while providing a strong and light structure. A typical example
of the use of such skins is disclosed in U.S. Patent 4,395,450 issued on
July 26, 1983 to the Boeing Company. However, this patent is concerned
with the construction of structural elements having reinforcing cores
disposed between cross-skin panels.
SUMMARY OF INVENTION:
The present invention is concerned with a process and related molds
for the fabrication of a single skin type structure, such as a panel,
having stiffeners integrally cured therewith. These stiffeners are usually
formed with lateral flanges to stiffen the skin and for securing the skin
to support framework. Because of the disposition of these lateral flanges,
it has not been heretofore easy to mold the skin and the stiffeners
integrally as the composite molded part could not be extracted from
conventional molds. Therefore, it is the practice to fabricate the skin
and stiffeners separately and then to secure the stiffeners to the skin by
the use of adhesive generally under heat and pressure. Fasteners are also
utilized to secure these stiffeners to the skin.
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Another fabrication method is to use expensive composite molds to
retain the skin and stiffeners in position in an autoclave for co-curing
the stiffeners and skin together. However, such molds have been found to
be quite expensive and bulky and difficult to handle and further require
special autoclaves capable of housing such molds.
It is a feature of the present invention to provide a novel process
using molds for co-curing a composite skin stiffener assembly with the
skin and stiffeners made of reinforced fibrous material impregnated in a
resinous substance.
Another feature of the present invention is to provide a novel
process for integral co-curing of a composite skin-stiffener assembly made
of fibrous material and resin that allows improved heating and co-curing
without an autoclave.
Another feature of the present invention is to provide a process and
a molding apparatus to integrally cure a skin and siffeners together and
to thereby avoid bonding and mechanical fasteners to join the same.
Another feature of the present invention is to provide a process and
a molding apparatus to integrally co-cure a skin to structural components
of open cross-section with interspacing gaps between them, thus allowing
access to the structural components for easy inspection and repair.
Another feature of the present invention is that the novel process
and molding apparatus produce smooth internal surfaces as well as smooth
external surfaces by laying and curing all the composite material in
pressed engagement against a corresponding curing face of the mold part.
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Another feature of the present invention is to provide a process
using molds for curing a composite skin-stiffeners assembly of single-skin
type wherein the pieces of composite material are held in position under
pressure by inflatable molding devices disposed ins;de mold enc10sures in
registry with interspacing gaps between the stiffener forming material,
thereby allowing removal of the inflatable molding devices by lifting same
away through the interspacing gaps.
According to the above features, from a broad aspect, the present
invention provides a molding process for curing a composite skin-
stiffeners assembly of single-skin type comprising the steps of providing
a stiffener mold part having at least one stiffener shaping ridge and a
valley formed there against on a corrugated curing face of the mold.
Composite material laminae are laid over the corrugated curing face in
stiffener shape of open cross section. An expandable molding device is
provided in the valley adjacent the ridge of the stiffener mold part
having the laminae of composite material laid thereon. A skin of
composite material is also laid over the curing face of the skin mold
part. The curing face of the skin mold part and stiffener mold part are
then juxtaposed defining one or more mold enclosures with at least a
portion of the skin of the skin mold part in contact with at least a
portion of the composite material over the ridge. The expandable molding
device is then expanded to apply pressure against the skin and the
composite material on the ridge and retains them in position. The
composite material of the skin and stiffeners is then co-cured under
appropriate heat and pressure. The expandable molding device is removed
after curing by lifting thereof through a corresponding interspacing gap
provided between the pieces of composite material over the adjacent
ridges.
According to a further broad aspect of the present invention, the
mold parts are provided with heating medium embedded therein whereby to
supply the appropriate heat to the composite material of the skin and
stiffeners.
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According to another feature of the present invention, there is
provided a mold for co-curing a composite stiffeners assembly. The mold
comprises a skin mold part having a skin receiving curing face. A
stiffener mold part is also provided and has a curing face including at
least one stiffener shaping ridge and valleys on the opposite sides of
each ridge. The mold parts are operatively held with the curing faces
juxtaposed. An expandable molding device is disposed in each valley
whereby to press the composite material of the skin against the curing
face of the skin mold part and against the ridge of the stiffener mold
part during a heat curing cycle.
BRIEF DESCRIPTION OF DRAWINGS:
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A preferred embodiment of the present invention will now bedescribed with reference to the examples thereof as illustrated in the
accompanying drawings, in which:
FIGURE 1 is a cross-section view illustrating the construction of a
mold for co-curing a composite skin-stiffeners assembly in accordance with
the process of the present invention;
FIGURE 2 is a further enlarged cross-section view showing the
composite laminae laid on the mold parts;
FIGURE 3 is a plan view of a section of an aircraft wing constructed
with the composite skin-stiffeners assemb1y according to the present
invention;
FIGURE 4 is a section view along section lines 4-4 of Figure 3;
FIG~RE 5 is a fragmented section view of a composite skin-stiffeners
assembly showing the passage of a cross-rib over a stiffener in an
appropriate molding apparatus.
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FIGURE 6A is a fragmented section view of a cylindrical mold
utilized for the molding of the composite skin-stiffeners assembly for use
in the construct;on o~ a ~uselage; and
FIGURE 6B is an enlarged view of a portion of figure 6A.
DESCRIPtION OF PREFERRED ~MBODIMENTS:
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Referring now to the drawings and more particularly to Figures 1 and2, there is shown generally at 10, the construction of a molding apparatus
in accordance with the present invention for co-curing a composite
skin-stiffeners assembly such as shown at 9 in Figure 4. The molding
apparatus comprises essentially a skin mold part 11 and a stiffener mold
part 12. Both of these mold parts are provided with curing faces 11' and
12'. The mold parts may be secured to one another by conventional
attachment means such as hinges, etc. (not shown) whereby to juxtapose the
curing faces 11' and 12'.
As shown more clearly in Figure 1, the skin mold part 11 has a skin
receiving cavity 13 formed in the curing face 11' thereof. This skin
receiving cavity is a shallow elongated flat cavity to retain laminae of
fibrous material impregnated with a resin thereon.
The stiffener mold part 12 is provided in its curing face 12' with
stiffener shaping ridges 14 and 15 machined or otherwise formed in the
stiffener mold curing face. The ridges 14 and 15 have a flat top face 23
and side walls 23' which are angulated toward the top wall 23 to define a
substantially inverted trough-shaped sect;on whereby to facilitate the
extraction of the cured composite skin-stiffeners assembly. A cavity 16 is
defined between the ridges 14, 15 for receiving therein an expandable
molding device herein in the form of an inflatable membrane 17. This
membrane is usually shaped to expand to press against each of the
surrounding walls of the valley 16 and the opposed skin portion
thereagainst when the mold parts are closed.
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In order to apply heat to specified surrounding areas of the mold
cavities and to the skins, there is disposed within each of the mold parts
ll and 12 heating tubes or elements 18. The heating medium may be
constituted by embedded electrical resistive elements or else by conduits
through which a heating fluid is circulated. These elements or tubes are
disposed in areas where the fiber resin-impregnated mat~rial is to be
co-cured. That could be provided by heating the entire curing surface of
the mold.
In the embodiment shown in Figures 1 and 2, it can be seen that a
flat recess l9 is disposed in the valley 16 and in a portion adjacent each
of the ridges 14 whereby to mold a stiffener flange 20.
The operation of this molding apparatus or the process carried out
thereby consists in laying strips or laminae of composite material over
the stiffener shaping ridges 14 and into the recess l9. A skin of like
composite material is also disposed in the skin receiving cavity 13 of the
skin mold part ll and retained thereon by its tackiness or by gravity or
other suitable means such as suction, etc. The inflatable molding device
17 is then disposed in the valley 16 and thus within the mold enclosures
formed by juxtaposing the curing faces of the mold parts. Air pressure is
then appl;ed to the inflatable molding device by a pump (not shown). This
air pressure applies the inflatable membrane against the composite
material retaining them in position and under required pressure. Heat and
external pressure are then applied to the mold parts to maintain equal
pressure from all sides. During the co-curing of the stiffener ridges 21
and the skin 22, the portions where the composite material are in contact,
such as in the flat top face 23 of the ridges 15 and the adjacent face of
the skin mold part, will mold together and interconnect as a solid part
due to the curing of the resin under temperature and pressure in that
region. The heat and internal and external pressure are applied for a
predetermined time as specified by a cure cycle for the composite material
used, after which time the mold is cooled down, opened, and the reusable
inflatable membranes 17 are deflated and removed through the space
intermediate the free edges of the lateral stiffening attachment flanges
20.
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Figure 4 illustrates a product formed with a mold by a process of
the type described with reference to Figures 1 and 2. As herein shown,
the article is utilized in the construction of an aircraft wing 30 as
illustrated in Figure 3. Figure 4 is a sect;on view through section lines
4-4 of the wing of Figure 3. It can be seen that with this type of wing
construction, the outer surface of the wing is substantially free of
fasteners.
Referring now to Figure 5, there is shown a typical modificat70n
that can be made to the composite skin-stiffeners assembly. In this
particular embodiment, there is shown a stiffener 31 having formed therein
a passage 32 for the attachment of a cross-rib 33 extending transversely
thereacross. This transverse rib is herein shown as being an inverted
T-shaped rib also formed of composite material and having mold pressure
pads 34 on each side thereof. In this particular embodiment, it is also
seen that the stiffener ridges are formed of decreasing height at their
locations 35 in the passage area 32. Various other modifications are, of
course, possible.
Figures 6A and 6B illustrate another configuration of the mold parts
11 "' and 12"'. As hereinshown, the skin mold part 11"' i5 cylindrical
in shape and may be formed in sections. The stiffener mold part 12" ' is
formed of a plurality of mold sections having ridges 40 which are
laterally spaced apart. Inflatable molding devices are inserted in each of
the mold enclosures 42 defined between the ridges 40. Such molds are
utilized in the construct;on of a tubular aircraft fuselage.
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Although the mold parts 11 and 12 as herein described are provided
with heating means embedded therein, it is also possible to insert these
molds within an autoclave to effect the heating and the required pressure
for a cure cycle. However, it is much more economical to provide the
heating means in the manner described herein, as the curing can be done at
the same location where the fabric is laid within the mold. With the
particular process and related molds as above described, it can also be
seen that the stiffeners can be of different sizes, thicknesses or widths
and can be shaped as required. They do not have to extend parallel to one
another and thus allow substantial freedom of layout. No trimming of the
skin or stiffeners is required after the curing, and flat or contoured
structures can be molded. Cross-ribs could also be incorporated in the
co-curing and mechanical fasteners and secondary adhesive bonding is
dispensed with as a result of the integral co-curing of the skin and
stiffeners. It is also possible to manufacture complete bodies for the
construction of the various aircraft part or parts not necessarily
restricted to the construction of airsraft, such as floor structures,
walls, ceilings, etc.
It is within the ambit of the present invention to cover any obvious
modifications of the examples of the preferred embodiments described
herein provided such modifications fall within the scope of the appended
claims.
