Note: Descriptions are shown in the official language in which they were submitted.
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Description
Method For Making Honeycomb Core Composite Articles
Technical Field
The present invention relates to manufacturing honeycomb core
composite articles, and more particularly, to an improved method of
manufacturing such composite articles wherein the honeycomb core
thereof is accurately located and substantially distortion-free.
Background Of The Invention
Composite articles incorporating honeycomb cores are
commonly utilized for fabricating aerospace structures due to their
advantageous strength to weight ratio. Honeycomb core (HC)
composite articles are typically comprised of upper and lower
composite skins, i.e., fiber reinforced resin matrix laminates, that are
separated and stabilized by the honeycomb core. Due to the high
bending stiffness and compressive strength properties of HC composite
articles, i.e., the honeycomb core functions as a shear web and spaces
the composite skins from the bending neutral axis, HC composite
articles have particular utility in aerospace applications such as aircraft
fuselage panels and door structures. The high strength and low weight of
such construction results in lower overall aircraft system weight.
HC composite articles may be fabricated utilizing various
composite forming methods. The most commonly employed technique
involves the use of a vacuum bag molding assembly wherein an
impervious membrane or "vacuum bag" is employed for consolidating
the composite skins and ensuring proper adhesion thereof to the
centrally disposed honeycomb core. More specifically, the lower
composite skin, honeycomb core, and upper composite skin are
sequentially laid in a rigid mold member so that the honeycomb core is
overlaid by the upper and lower composite skins. The upper and lower
composite skins are formed from uncured "prepreg" or "B-stage"
laminates comprised of a fiber reinforcement such as graphite, aramide
or fiberglass fibers disposed in a binding matrix such as epoxy, phenolic
or other similar organic resinous material. Film adhesive, which is
applied to the honeycomb core prior to the lay-up, forms the bonds
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between the upper and lower composite laminates and the honeycomb
core. The vacuum bag is disposed over the rigid mold member and
sealed thereto so as to form a mold cavity which is occupied by the
uncured composite lay-up. The mold cavity is then evacuated and
additional pressure and temperature are applied via an autoclave oven
to cure the lay-up. The combination of vacuum and external pressure
functions to consolidate the composite skins, remove air and volatiles
from the resin binder, and apply the necessary compaction pressure to
ensure full and uniform adhesion of the lay-up.
Difficulties commonly encountered during the fabrication of HC
composite articles relate to shifting and/or distortion of the honeycomb
core under compaction pressure. While the honeycomb core is
relatively stable in the direction of the individual cells, i.e., the cells
provide significant buckling stability, it will be appreciated that pressure
applied transversely of the cells may cause distortion and/or shifting,
e.g., accordioning, of the honeycomb core due to the inadequate
strength thereof in a lateral direction. This is more clearly understood
by reference to Fig. 1 a wherein a lay-up of upper and lower composite
skins 100,102, and a honeycomb core 104 is disposed in a vacuum bag
molding assembly 108. The vacuum bag 110 is shown applying a
lateral component of pressure P along the ramped edges of the
honeycomb core, which lateral pressure component causes the local
collapse and distortion of the honeycomb core edges. Fig. 1b shows a
top view of the cured HC composite article wherein the distortion,
indicated by dashed lines 112, is exaggerated for illustrative purposes .
Attempts to overcome problems of distortion and shifting have
included stabilization techniques wherein the edges of the honeycomb
core, i.e., several rows of honeycomb cells about the entire periphery,
are stabilized by the application of film adhesive or filled with a low
density syntactic foam. Once cured, the film adhesive and/or the foam-
filled cells serve to retard the accordioning of the honeycomb core.
U .S. Patents 4,680,216 and 5,354,195 discuss honeycomb core
stabilization techniques and various materials useful therefor. While
these techniques have been marginally successful in limiting distortion
of the honeycomb core (on the order of about .64 cm to .95 cm (.25 in
to .375 in), such materials are substantially parasitic and are not
practical for applications wherein minimization of overall aircraft weight
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is a critical design criterion. Furthermore, these stabilization options are
not acceptable for applications wherein accurate and distortion-free core
location is highly critical. For example, applications requiring the use of
~ radar absorbent (i.e., carbon-loaded) honeycomb core to defeat enemy
radar require far more exacting manufacturing tolerances than those
which can be produced by prior art stabilization techniques. If shifting
of the radar absorbent honeycomb core should occur during the
manufacturing process, radar coverage on the aircraft could be
compromised.
Other attempts to yield a distortion-free core have included the
use of restraint devices formed or assembled about the periphery of the
molding assembly. Fig. 2a depicts a vacuurn bag molding assembly
wherein rows of vertically protruding pins 120 are affixed to a rigid
mold member 122 and disposed in adjacent relation to the HC
composite article 124 to be formed. As the upper composite skin 126 is
laid over the honeycomb core 128, the pins 120 are caused to engage a
peripheral portion 130 of the upper composite skin 126, i.e., pierce the
composite fabric, to prevent lateral displacement thereof during the
molding/compaction process. A bridging effect is thereby created in the
upper composite skin 126, i.e., between the uppermost corner 132 of
the honeycomb core 128 and the mating surface 134 of the lower
composite skin 136, to react lateral compaction pressure and,
consequently, prevent distortion of the honeycomb core 128. While
this technique is suitable for high tolerance applications, e.g., LO
applications, the protruding pins 120 are a source of high maintenance,
i.e., requiring periodic cleaning and repair, pose a hazard to the
operator, and create difficulties when sealing the vacuum bag 138 to the
rigid mold member 122. Regarding the latter, the vacuum bag 138 must
be sealed outboard of the protruding pins 120, thus requiring the
additional step of disposing a protective elastomer strip 139 over the
protruding pins 120 to prevent damage to the vacuum bag 138.
A similar approach is shown in Fig. 2b wherein a perforated or
apertured metal strip 140 is substituted for the protruding pins 120. The
peripheral portion 130 of the upper composite skin 126 is laid over the
apertured metal strip 140 such that under compacting pressure the
apertures 142 thereof capture or grip the peripheral portion 130 to
prevent lateral displacement of the upper cornposite skin 126. This
CA 02211~6 1997-07-2~
approach yields similar results to the above-described pinned con~iguration.
however. Iaborious cleanin(J is required tO remove e~cess resin from the
aperlUreS 1~7 prior ~o initialing the ne.Yt cure cycle.
Yet another method ~'or yielding a distortion-free HC composile arlicle is
5 described in Brayden et al. ~.S. Pa~em ~,7~7~651. The metho~:i hl~ol~es Ihe
introduction of pressurized air or gas~ such as that produced b- an auloclave~ on the
back-side of the upper t'ace plies of the HC composite article. and parlicularly on ~he
face plies overlaying a ramped porlion of the honeycomb core. The back-pressure
effects an equalizing or reacting force ~vhich prevents lateral movement and
I () distortion of the hone comb core. The moid assembly IS equipped ~ith a press~lre
port which introduces pressurized air or gas ~o the volume or cav it- occupied b~ the
honeycomb core and belween the face plies. The honeycomb core inc!udes
perforations in the cell ~valls to permit communication of pressure fi-om one ~ell lo
an adjacen~ cell and ultimately ~o ~he entire back-side of the upper face ~lies. While
l 5 this technique offers a unique soluIion, ~he COSt associated with mold modifica~ions
and the laborious set-up associated with creating an closed internal system for
internally pressurizing the HC composite article is fiscally disadvantageous.
A need therefore e~cists to provide an improved method of manufacluring HC
composite articles which provides accurate and distortion-free core location and70 minimizes repair and/or maintenance of the molding assembly.
Disclosure Of Invention
It is an object of the present invention to provide a method for manufacturing
HC composite articles wherein the honeycomb core thereof is accurately located and
~5 substantially distortion-free.
lt is another object of the present invention to provide a method for
manufacturing HC composite articles wherein modifications to and maintenance of
the molding assembly are minimi7.od.
These and other objects are achieved by a method for making HC composite
30 articles including the steps of: forming a honeycomb core having a core edge,placing a first composite skin in combination with a rigid mold member having a
mold surface, mating the honeycomb core to the first composite skin thereby
forrning a mated subassembly, laying-up a second composite skin over the mated
subassembly, which second composite skin is an uncured composite skin and which
35 honeycomb core, first composite skin and second composite skin~ in combination,
define a composite lay-up, disposing a vacuum bag over the composite lay-up
thereby forming a mold cavity between the mold surface and the vacuum bag,
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evacua~ing the mold cavity to urge the vaCuum bag against the second composite
skin for compacting the same against the ma~ed subassembl~v. and curing the
composite la~-up to form the honeycomb core composite article. The method is
further characterized by the steps of .t'orming the first composite skin prior to the
. placing slep so ~hat the t'irst composite skin is a precured composite skin having a
restr~int edge~ disposing the second composite skin in combination with the mated
subassembl-i such that a peripheral portion of the second composite skin e.~l;tends
bevond the restraint edge bv a distance ~ and, furthermore. so that the peripheral
portion is in superposed abutting engagemen~ with the mold s-lrf.lce ot' the rigi :!
10 mold member~ and ~vherein the evacuating slep causes the v acuum bag to et'~ct
enga ~ement of the peripheral portion ~vith the restrain. edge and the rnold surface tor
preventing laleral displacement of the second composite sl~in upon curing ot' the
composite la!-up.
The t'oregoing and other objects. fealures and advantages or'the present
1~ invention ~ill become more apparent in light of the following
4/1
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detailed description of exemplary embodiments thereof, as illustrated in
the accompanying drawings.
Brief Description Of The Drawings
A more complete understanding of the present invention and the
attendant features and advantages thereof may be had by reference to
the following detailed description of the invention when considered in
conjunction with the following drawings wherein:
Figs. 1 a and 1 b depict a honeycomb core (HCj composite article
being formed in a vacuum bag molding assembly and the resultant
distortion of the honeycomb core under molding assembly pressure;
Figs. 2a and 2b depict prior art molding apparatus for preventing
distortion o~ a vacuum-formed HC composite article;
Fig. 3 depicts an exploded partial cross-sectional view of a HC
composite lay-up employing the teachings of the present invention;
Fig. 4 depicts the HC composite artic!e manufactured by the
method of the present invention, and more particularly, the
consolidation of the lay-up by a vacuum bag molding assembly.
Best Mode For carrying Out The Invention
Referring now to the drawings wherein like reference characters
identify corresponding or similar elements throughout the several views,
Fig. 3 depicts an exploded partial cross-sectional view of a honeycomb
core (HC) composite lay-up 10 disposed between a rigid mold
member 12 and a vacuum bag 14 which, in combination, form a
vacuum bag mold assembly 16. The composite lay-up 10 includes
precured and uncured composite skins 20, 22, and a honeycomb
core 24 disposed therebetween.
The honeycomb core 24 may be any of the conventionally used
open cellular honeycombs such as aluminum, phenolic or NOMEX (~
(NOMEX is a registered trademark of Hexcel Corp., Chatsworth, CA for
an aramid fiber or fabric). For applications wherein weight is a critical
design criterion, a lightweight honeycomb core having a density of
about 28.84 Kg/m3 (1.8 Ibs/ft3) is preferred. Generally, honeycomb
core is purchased in bulk and machined to the desired shape and size
using Numerically Controlled (NC) machining apparatus. While the
honeycomb core 24 is shown to have a ramped surface 26 to gradually
transfer shear loads to the precured and uncured composite
CA 02211~6 1997-07-2~
skins 20~ 22~ it will be understood that the ~eachings of the present in-ention are
equally applicable to honevcomb cores having a right-angled conficTuration
The precured composite skin ~0 is comprised of fiber reinforced resin
matri~ composite laminates having a fiber reinforcement such as graphite~ aramide
S or fiberglass fibers disposed in a binding matri~ such as epo.Yv. phenolic or other
similar organic resinous material. Preferably, two composite laminates havin( a
total thickness of at least about .038 cm (.015 in) are laid-up in a mold assembl~r
and cured to form a cured composite panel. Any conventional t'abric;ltiol1 metll()d.
such as vacuum forming. resin transfer molding, Ol matcllecl metal molding ma~
be used to form and cure the composite panel. The prel'erred melhod. however.
utilizes a v~cuum bag mold assembly wherein the same rigid mold member l ~
employed for forming the HC composite lay-up is used ~'or t'orming the precured
composite skin 20. Once cured, at least one end of the cured composite panel is
trimmed~ thus forming the precured first composite skin '0. The trimming
I S operation also produces ~ restraint edge 30 having a right-anglea confi(T~Ira~i()n
~0'. The utility of this construction will become eviden~ when discussing ~he
subsequent processing steps.
The precured first composite skin 20 is then returned to the rigid mold
member 12 and the honeycomb core 24 is mated therewith to form a mated
subassemblv 36 (see Fig. 4). Furthermore, the honeycomb core '~ is disposed in
superposed relation with the precured composite slcin ~0 so that the restraint
edge 30 e~tends beyond the edge 32 of the honeycomb core 24. The portion of
the precured composite skin 20 e~tending beyond the core edge 32 defines an
upwardly facing mating surface 38 which will, in subsequent steps~ be disposed in
combination with and bonded to the uncured composite skin 22. The mating step
additionally includes the application of a bonding adhesive 34a to one or both of
the mating surfaces, i.e., to either the honeycomb core 24 or the precured
composite skin 20, for bonding the core to the precured composite skin 20. In the
preferred embodiment, the bonding adhesive 34a is co-cured with the completed
composite lay-up l0, i.e., concurrent with the curing step (cure cycle) described
hereinbelow. However, it is also contemplated that the bonding operation may be
performed prior to subsequent processing steps, e.g., as a independent operationrequiring a unique cure cycle. The preferred bonding adhesive 34a is a film
adhesive such as EA 9690 manufactured by Hysol Corp located in Pittsburgh, CA.
The uncured composite skin 22 is comprised of one or more l~min~tes of
uncured preimpregnated fiber reinforced composite material having a fiber
reinforcement such as graphite, aramide or fiberglass fibers disposed in a binding
CA 02211~6 1997-07-2~
matrix such as epo~cy~ phenolic or other similar organic resinous material. In the
preferred embodiment a bonding adhesive '4b. such as Hysol 9690 film adhesive~
is first applied to the e~posed upper surfàce 40 or' the hone- comb core ~4 in
preparation for the lay-up of the uncured composite sLiin 22. The adhesive 3~b
S facilitates the bonding of the uncured composite sL;in " to the lloneycomb
core ~4, however~ it will be appreciated that the uncured binding matri~ i.e.~ resin
of the uncured composite skin ?7, m~y adequately wet the honeycomb core
surface during the cure cycle to et'fect a suitable bond~ thereby eliminatin~J the
need for the bonding adhesive 3~b. _
The uncured composite skin " is laid over the mated subass~mbl- 36 all(l
is precompacted using con~entional debulklng techniques to ensure intima~e
contact with the e~posed upper honeycomb core surface 40 and the mating
surface 3~ of the precured composite skin '0. Furthermore~ the uncured
composite skin 22 is appropriately sized so as to permit a peripheral portion 4~i S thereof to e~tend bevond the restraint edge 30 of the precured composite sliin ~0.
Moreover~ the peripheral portion 44 is in superposed abutting engagement with
the mold surface I 's of the rigid mold member 1 ' and e~tends a distance X
beyond the restraint edge 30. Preferably, the peripheral portion 44 e~ctends at least
1.9 cm (.75 in) beyond the edge 30 and, more preferably, about ~.5 cm (1 in)
'0 beyond the edge 30.
The vacuum bag 1~ is disposed over the completed composite lay-up 10
and sealed to the rigid mold member 12, or to itself~ 1~ti~ in~ a conventional
sealer strip 50 such as "Prestite" (Prestite is a brand name of a semi-adhesi~e
compliant material produced by 3M located in St. Paul, MN for a semi-adhesive
25 compliant). Optionally, a separator or release film 52 and a breather ply 54 may
be interposed between the uncured composite skin " and the vacuum bag 14.
The separator film 52 facilitates release of the vacuum bag 14 after curing the
composite lay-up 10 while the breather ply 54 facilitates the removal of air andvolatiles from the uncured
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composite skin 22 during the cure cycle. Representative materials are
specified in the below-described example. In the described
embodiment, the use of the release film 52 and breather ply 54 are
preferred, however, the composite lay-up 10 is minimally comprised of ~.
the honeycomb core 24, the precured composite skin 20, and the
uncured composite skin 22.
The completed vacuum bag molding assembly 16 shown in
Fig. 4 forms a sealed mold cavity 60, i.e., between the rigid mold
member 12 and the vacuum bag 14, which is in fluid communication
with a vacuum pump 62. The vacuum pump 62 functions to evacuate
the mold cavity 60 of gaseous fluids, i.e., air and volitales, and to create
a pressure differential for urging the vacuurn bag 14 against the uncured
composite skin 22. Concomitantly, the peripheral portion 44 of the
uncured composite skin 22 is forced into engagement with the restraint
edge 30 of the precured composite skin 20 and against the mold
surface 12s.
The vacuum bag molding assembly 16 is then placed in an
autoclave oven (not shown) wherein the composite lay-up 10 is exposed
to additional pressure and temperature for curing. For the described
embodiment, the autoclave oven applies a pressure of about 3.72 x107-
4.46 x 107N/m2 (25 - 30 Ibs/in2) to the lay-up and elevates the
temperature thereof to about 121 - 232~C (250 - 450~ F) degrees for a
period of about 120 - 180 mins.
During the vacuum forming step, i.e., evacuation of the mold
cavity 60, and subsequent curing step, the peripheral portion 44 of the
uncured composite skin 22 engages the restraint edge 30 of the
precured composite skin 20 so as to provide a mechanical interlock
therebetween. The restraint edge 30 functions to inhibit the lateral
displacement of the uncured composite skin 22 and, consequently,
cause a bridging effect therein in the region overlaying the ramped
honeycomb surface 26. As discussed supra, the bridging effect
functions to react the lateral component of pressure P applied by the
vacuum bag 14. Concomitantly, the contacting interface Cl between the
peripheral portion.44 and the mold surface 12s, provides additional
restraint toward the objective of inhibiting lateral displacement. That is,
the frictional force along the contacting interface Cl augments the
restraint capacity of the restraint edge 30. Accordingly, by inhibiting
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lateral displacement and, consequently, the catenary displacement of
the uncured composite skin 22, shifting and/or distortion of the
honeycomb core 24 is minimized.
While the completed HC composite article may be used in the
configuration produced by the foregoing process steps, the cured
composite lay-up may be trimmed along line TL to net dimension to
form a horleycomb core composite article having a uniform thickness
peripheral flange 70. Preferably, the restraint edge 30 and the
peripheral portion 44 are removed during this operation. Accordingly,
the trimming operation removes the abrupt contour transition produced
by the restraint edge 30 which enhances the strength of peripheral
flange 70 for subsequent fastening and/or bonding operations.
The restraint edge 30 and the mold surface 12s are tooling
surfaces which are functionally equivalent to the prior art pinned and
perforated metal strip configurations, however, it will be appreciated
that the formation of these tooling surfaces is far less labor intensive.
The restraint edge 30 is formed in conjunction with the manufacturing
process and becomes and integral part of the finished HC composite
article. Furthermore, the only additional step involves the formation of
the precured composite skin 20. Insofar as it is advantageous to precure
a composite laminate for the purposes of improving laminate quality
and strength, the method of the present invention will have no adverse
cost effect on the manufacturing process when such improved quality
and strength are desired. For example, the precured composite skin 20
may be formed under a pressure of 1.1 1 X 1 o8 N/m2 (75 Ibs/in2) to
improve the fiber volume content, i.e., increase the fiber to resin ratio,
and, consequently, the strength of the composite laminate. Such
increased fiber volume is not achievable using conventional co-cure
methods, i.e., wherein both composite skins are uncured before entering
the autoclave oven, insofar as high compaction pressures cannot be
transferred via the low density honeycomb core.
With regard to fabricating the rigid mold member 12 to include
the mold surface 1 2s, which is slightly enlarged for superpositioning of
the peripheral por~ion 44, it will be appreciated that fabricating such an
additional area surface is inconsequential in terms of time and labor.
It will also be appreciated that the hazards and periodic
maintenance of the prior art tooling surfaces are entirely eliminated by
CA 02211~6 1997-07-2~
the method of the present invention. Thal is. the present method does not
introduce hazardous ;ooling surfaces e.g., pins~ or other tooling surraces. e.g.apertures w hich require periodic cleaning.
S l~AM~
~ honevcomb core (HC) composite article was ~bricated using the
teachinus of the inven~ion as follows. A sheet ot 1.8 Ibs/ft3 NO~IEX~)
honevcomb core was machined tO a thickness dimension of 3.0 cm ~ l .18 in). alld ;
length and width dimension o~ 60.1 cm ( '~ in)~ respectivelv. rhe ed~es ot the
honeycomb core ~vere beveled to it'orm a 30~ ramp angle about the entire
periphery. The honevcomb core was then cleaned with alcohol alld dried in an
oven at 65.5~ C ( 1~0~ F) for a period of 1~0 minutes.
A firs~ composite skin was formed by cutting two plies of K~VLAR@~
~lber reinforced composite material (KEVLAR(~ is a regis~eted trademark of E.L.
l S du Pont de ~emours ~ Co. Iocated in Wilmington. DE for an aromatic poiyamideof high tensile strength) to a length and width dimension of 66.0~ cm (26 in).
respectivelv. The plies were sequentially laid in a rigid mold member of a
vacuum bag molding assembly. A Fluorinated Ethylene-Propylene (FEP)
separator film and a breather piy formed from a compliant porous material such as
N- 10 (the FEP film and N- 10 ply were obtained from Airtech International located
in Carson. CA) were respectively applied to the lay-up. A vacuum bag was
applied to the lay-up and sealed to the rigid mold member using Prestite adhesive.
The completed vacuum bag molding assembly was then placed in an autoclave
oven to cure the plies. In the autoclave oven, the composite plies were step-cured
for a period of 180 mins to a peak pressure and temperature of 1.1 1 X 1 o8 ~/m2(75 lbs/in2) and 1 82.2~C (360~ F), respectively. The lay-up resulted in a curedcomposite panel having a thickness dimension of .038 cm (.015 in). The cured
composite panel was then trimmed about its periphery to form a precured
composite skin having a restraint edge. The net size of the precured composite
skin was approximately 63.5 cm (25 in) square.
The precured composite skin was returned to the rigid mold member and a
first layer of Hysol 9690 film adhesive was applied to the entire exposed surface.
The prior-formed honeycomb core was then mated upon and centered over the
precured composite skin so that the
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restraint edge extended approximately 2.54 cm (1 in) beyond the edge
of the honeycomb core. A second layer of Hysol 9689 film adhesive
was then applied to the exposed upper surface of the honeycomb core.
An uncured composite skin comprising two plies of KEVLAR(~)
material was laid over the mated subassembly, i.e., the honeycomb core
and the precured composite skin, so that a peripheral portion of the
uncured composite skin extended 2.54 cm (1 in) beyond the restraint
edge of the precured composite skin. The uncured composite skin was
debulked using conventional vacuum debulking apparatus for 5
minutes. An FEP separator film and an N-10 breather ply were
sequentially laid over the uncured composite skin. A vacuum bag was
then placed over the uncured composite skin and sealed to the rigid
mold member, thereby forming a completed vacuum bag molding
assembly. The mold cavity was evacuated so as to apply atmospheric
pressure (full vacuum) via the vacuum bag to the underlying composite
lay-up. The peripheral portion of the uncured composite skin was
thereby simultaneously forced into engagement with the restraint edge
of the precured composite skin and the mold surface of the rigid mold
member, respectively.
The vacuum bag molding assembly was then placing in an
autoclave oven and step-cured as follows. The autoclave pressure was
raised to 1.49 X 107 N/m2 (10 Ibs/in2) and autoclave temperature was
elevated to 93.3~ C (200~F) for a first hold phase of 60 minutes.
Approximately 30 minutes into the first hold phase, the autoclave
pressure was gradually increased to 3.72 x 107 N/m2 (25 Ibs/in2).
Following the first hold phase, the temperature was gradually raised to
182.2~C (360 ~F) and held at a second hold phase (final cure phase) for
120 minutes.
The honeycomb core of the resultant composite article was found
to be substantially distortion-free and accurately located relative to the
composite skins.
The above described method may be used alone or in
combination with various core stabilization techniques such as those
described in U.S. Patents 4,680,216 and 5,354,195. Furthermore,
debulking operations may be performed between each of the above
described lay-up steps to improve the efficacy of the resultant bonds.
11
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Although the invention has been shown and described with
respect to exemplary embodiments thereof, it should be understood by
those skilled in the art that the foregoing and other changes, omissions
and additions may be made therein and thereto, without departing from
the spirit and scope of the present invention.
