U.S. patent application number 10/760027 was filed with the patent office on 2004-07-29 for method of manufacturing a sandwich panel, made of composite material, and a panel thereby obtained.
Invention is credited to Duret, Nathalie, Fournier, Alain.
Application Number | 20040146687 10/760027 |
Document ID | / |
Family ID | 9550062 |
Filed Date | 2004-07-29 |
United States Patent
Application |
20040146687 |
Kind Code |
A1 |
Fournier, Alain ; et
al. |
July 29, 2004 |
Method of manufacturing a sandwich panel, made of composite
material, and a panel thereby obtained
Abstract
In order to manufacture a sandwich panel using the RTM
technique, a stack comprising a core (10) with open cells, a film
(12) of intumescent material covering each of the faces of the core
(10), a dry barrier fabric (14) covering each of the films (12) and
an overlay of dry fibers (16) covering each of the barrier fabrics
is placed in a mold. During polymerization of the films (12),
pressurization of the mold and the presence of the barrier fabrics
(14) prevent penetration of the foam into the overlays (16). The
resin is then injected into the mold and then polymerized, without
danger of penetration into the cells of the core (10).
Inventors: |
Fournier, Alain; (Le
Parayre, FR) ; Duret, Nathalie; (Blagnac,
FR) |
Correspondence
Address: |
Robert E. Krebs
Thelen Reid & Priest LLP
P.O. Box 640640
San Jose
CA
95164-0640
US
|
Family ID: |
9550062 |
Appl. No.: |
10/760027 |
Filed: |
January 16, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10760027 |
Jan 16, 2004 |
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09642650 |
Aug 22, 2000 |
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6679969 |
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Current U.S.
Class: |
428/71 ; 428/117;
428/73 |
Current CPC
Class: |
B29D 24/005 20130101;
Y10T 428/24157 20150115; Y10T 428/236 20150115; Y10T 428/233
20150115; B29C 70/088 20130101; B29C 70/48 20130101 |
Class at
Publication: |
428/071 ;
428/073; 428/117 |
International
Class: |
B32B 003/12; B32B
005/20 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 21, 1999 |
FR |
99 11766 |
Claims
1. Method of manufacturing a sandwich panel made of composite
material, comprising the following steps: placing, in a mold, an
assembly comprising an open cell core, a film of intumescent
material covering each of the faces of the core, a dry barrier
fabric covering each of the films and an overlay of dry fibers
covering each of the barrier fabrics, said barrier fabrics being
sealed against a foam capable of being provided during the
polymerization of said films and which can be wetted by a resin
capable of being injected into the mold; closing the mold;
pressurizing and heating the mold according to a cycle for the
expansion and polymerization of the intumescent material, in a way
that forms said foam, closing off the cells of the core on each of
its faces, without impregnating the dry barrier fabrics; evacuating
the mold and injecting said resin into it in a manner that
impregnates the overlays of dry fibers and the dry barrier fabrics;
carrying out a cycle for polymerization of the resin; stripping the
panel obtained from the mold.
2. Method according to claim 1, in which films are used of a
thickness such that after polymerization, said foam only fills the
parts of the cells of the core which are close to the faces of the
core.
3. Method according to claim 2, in which films are used which have
a thickness of about 2.5 mm.
4. Method according to claim 1, in which barrier fabrics made of
calendered polyamide are used.
5. Sandwich panel made of a composite material comprising an open
cell core and skins covering both faces of the core, said skins
being formed of fibers and resin, a panel in which the cells of the
core are closed off by a foam on each of said faces, and in which
each of the skins comprises, starting from the core of the panel, a
barrier fabric and a fiber overlay, both of which are impregnated
with one and the same resin polymerized and stuck onto the core of
the panel.
6. Panel according to claim 5, in which said foam only fills the
parts of the cells of the core which are close to the faces of the
core.
7. Panel according to claim 5, in which the barrier fabrics are
produced in calendered polyamide.
Description
TECHNOLOGICAL FIELD
[0001] The invention relates to a method of manufacturing a
sandwich panel made of composite material comprising an open cell
core, for example in the form of a honeycomb, using the Resin
Transfer Molding (RTM) technique.
[0002] Another subject of the invention is a sandwich panel made of
composite material manufactured according to this method.
STATE OF THE TECHNOLOGY
[0003] The traditional technique for manufacturing sandwich panels
made of composite material consists of draping cloth or fiber
fabrics impregnated with resin onto each of the two surfaces of an
open cell core, most commonly constituted by a honeycomb structure.
The blank obtained is then placed in an autoclave or is subjected
to a polymerization cycle that enables one to ensure curing of the
resin.
[0004] This traditional technique has the particular disadvantages
of being expensive and difficult to implement. In effect, the
draping operations are carried out either manually or using very
complex machines. Furthermore, in practice, it is not possible to
obtain two perfectly identical items and the surface finish of
these items is rather poor.
[0005] Having regard to these disadvantages, the traditional
technique of draping is being replaced more and more frequently by
the RTM technique, in order to manufacture very diverse items
(items of large size, with complex geometry, functional integration
etc.) particularly in high technology industries such as the
aeronautic, automobile and naval industries.
[0006] The RTM technique consists of placing a preform of dry
fibers in a mold, evacuating it and injecting into it, at low
pressure, a resin that has a very low viscosity. Applying a
polymerization cycle allows curing of the resin, before the item is
stripped from the mold.
[0007] When the RTM technique is applied to the manufacture of a
sandwich panel having an open cell core, for example in the form of
a honeycomb, precautions must be taken to prevent the resin
injected into the mold filling up the cells of the core of the
item. Various solutions to this problem have already been
proposed.
[0008] A first known solution, described in document EP-A-0 722
825, consists of interposing successively an adhesive film and a
fold of fibers pre-impregnated with resin between each of the faces
of the open cell core and the overlay of dry fibers. After putting
this assembly into place in the mold and closing it, a first
polymerization cycle enables one to cute the resin contained in the
pre-impregnated fold of fibers and to stick it onto the
corresponding face of the open cell core. The very low viscosity
resin is subsequently injected into the mold in a way that fills
the dry fiber overlays. The mold is opened after polymerization of
the injected resin.
[0009] This technique enables one to avoid filling up the open
cells of the core of the component with the low viscosity resin
injected into the mold. However, it has the disadvantage of leading
to the manufacture of a component that includes three different
resins, which means that the skins are not homogeneous and this can
lead to problems of adherence of the overlays to the core of the
item. In addition, this technique requires the use of three
elements (counting the overlay of dry fibers) and a two phase
manufacturing cycle (polymerization of the resin contained in the
pre-impregnated fibers and then injection of the RTM resin and
finally polymerization of this resin). The pre-impregnated fibers
require the polymerization phase so they act as a barrier.
Furthermore, the presence of folds of fibers impregnated with resin
tends to make the component obtained heavier. In effect the mass
per unit surface area of such components is about 500 g/m.sup.2
(this value is multiplied by two in order to take into account both
sides of the panel). Finally manufacture takes a long time.
[0010] Another known solution, described in document FR-A-2 740
383, consists of interposing an adhesive film and a sealing
membrane between each of the faces of the open cell core and the
corresponding overlay of dry fibers. After closing the mold, the
adhesive is polymerized in such a way that each of the membranes is
stuck onto the open cell core and then the low viscosity resin is
injected into the mold and polymerized.
[0011] In comparison with the previous technique, this one
simplifies the polymerization cycle and enables one to reduce the
mass of the component. However, this technique is not easy to use
for the manufacture of panels with complex geometry. In effect, the
sealing membrane is difficult to deform and this leads to problems
of folds and creases when it is being placed over a core which is
not flat.
[0012] A third technique for manufacturing sandwich panels is
proposed in document EP-A-0 722 826. In this case, an intumescent
film is interposed between each of the faces of the open cell core
and the overlay of dry fibers. After closing the mold, a cycle of
expansion and polymerization of the intumescent films has the
effect of filling the open cells of the core with foam. The low
viscosity resin is subsequently injected into the mold and then
polymerized.
[0013] When it is implemented in a single phase, this technique
enables one to provide direct adhesion of the injection resin onto
the open cell core. However, it has the particular disadvantage
that the foam formed by the intumescent films during the expansion
and polymerization cycle of these films, is also propagated into
the dry fiber overlays which they partially fill up to the surface
of the panel. As a consequence, the resin subsequently injected
into the mold only fills a part of the dry fiber overlays. Under
these conditions, the component obtained does not have the desired
mechanical properties. In addition, the integral filling of the
cells of the core by the foam leads to an undesirable increase in
the mass of the panel finally obtained.
[0014] When it is implemented in two phases, this technique becomes
more difficult. In effect, firstly one must apply a cycle for
expansion of the intumescent film and then proceed with the
injection of the resin and its polymerization which complicates the
operation.
[0015] In the embodiment described with reference to FIG. 5 in this
document EP-A-0 722 826, it is proposed to combine the technique
which has just been described with the technique disclosed in
document EP-A-0 722 825. In other words, it is suggested that an
adhesive film and a fold of pre-impregnated film are. interposed
between each of the intumescent films and the corresponding overlay
of dry fibers. Then, the expansion and the polymerization of the
intumescent films, the polymerization of the adhesive and the
polymerization of the resin which is impregnated into the
pre-impregnated folds of fibers are carried out simultaneously,
during one and the same polymerization cycle. As in the other
cases, the low viscosity resin is then injected into the mold and
then polymerized.
[0016] It may be thought that this latter technique enables one to
avoid the migration of the foam to the inside of the overlays of
dry fibers, because of the presence of the folds of fibers
pre-impregnated with resin, between the intumescent films and the
overlays of dry fibers.
[0017] However, the manufacturing time of the panel is
substantially increased because the first polymerization cycle must
ensure the expansion and the polymerization of the intumescent
films, the polymerization of the adhesive and the polymerization of
the resin contained in the pre-impregnated folds of fibers, all at
the same time. Furthermore, all the disadvantages of the technique
described in document EP-A-0 722 825 are again found in this case,
that is to say, in particular, an increase in the mass of the panel
and problems of heterogeneity of the skins and problems of
adherence between the various layers.
[0018] Furthermore, a sandwich panel is known from document EP-A-0
628 406, made of composite material manufactured in accordance with
the traditional draping technique and in which the life of the
panel is increased by interposing an intumescent film between one
of the skins and the honeycomb core. More precisely, the foam
formed by the polymerization of the intumescent film only fills the
honeycomb cells of the core close to the face corresponding to
it.
DESCRIPTION OF THE INVENTION
[0019] The main subject of the invention is a method of
manufacturing a sandwich panel made of a composite material using
the RTM technique which enables one to obtain, in a single phase,
that is to say, in a very short time, a component with a simple,
light structure, in which the injected low viscosity resin is
directly stuck onto the open cell core, without requiring a
manufacturing cycle that is too complicated.
[0020] Conforming to the invention, this result is obtained by
means of a method of manufacturing a sandwich panel made of
composite material, characterized in that it comprises the
following steps:
[0021] placing, in a mold, an assembly comprising an open cell
core, a film of intumescent material covering each of the faces of
the core, a dry barrier fabric covering each of the films and an
overlay of dry fibers covering each of the barrier fabrics, said
barrier fabrics being sealed against a foam capable of being
provided during the polymerization of said films and which can be
wetted by a resin capable of being injected into the mold;
[0022] closing the mold;
[0023] pressurizing and heating the mold according to a cycle for
the expansion and polymerization of the intumescent material, in a
way that forms said foam, closing off the cells of the core on each
of its faces, without impregnating the dry barrier fabrics;
[0024] evacuating the mold and injecting said resin into it in a
manner that impregnates the overlays of dry fibers and the dry
barrier fabrics;
[0025] carrying out a cycle for polymerization of the resin;
[0026] stripping the panel obtained from the mold.
[0027] In this method, the dry barrier fabrics combined with
pressurization of the mold prevent any penetration of the foam
formed by the intumescent films during their polymerization, into
the overlays of dry fibers. On the other hand, these dry barrier
fabrics are impregnated at the same time as the overlays of dry
fibers during the injection of the resin into the mold, in such a
way that this adheres directly to each of the faces of the open
cell core, without filling up said cells, because they have already
been blocked off by the foam.
[0028] It should also be noted that the only function of the
polymerization cycle prior to the injection of the resin into the
mold is to ensure the expansion and the polymerization of the
intumescent films. It is therefore a particularly simple operation
that is easy to carry out.
[0029] It should be noted that a man skilled in the art who desires
to resolve the problem posed by the filling up of the honeycomb
structure, will quite naturally use a fold of fibers impregnated
with resin. The use of a polyamide fabric from the textile industry
would appear, on the contrary, to be a surprising solution for a
specialist from the aeronautics industry. Such a fabric does not
require any particular action to act as a barrier and provides a
seal for the honeycomb. Its simple application is sufficient. It
therefore permits the creation of the component in a single phase
(gain in manufacturing time a week instead of a month for a normal
manufacturing cycle), a benefit in mass and in cost.
[0030] So as to illustrate the mass benefit obtained by using a
fabric conforming to the invention, it should be noted that the
mass per unit surface area of the panel obtained is about 30 to 40
g/m.sup.2 (value to be multiplied by two to take account of the two
sides of the panel). This value should be compared with about 500
g/m.sup.2 (also to be multiplied by two) obtained by using the
technique described in document EP-A-0 722 825.
[0031] Advantageously, intumescent films are used that have a
thickness such that after polymerization, they only fill those
parts of the cells of the core close to the faces of the core. This
arrangement, which corresponds especially to the use of intumescent
films with a thickness of about 2.5 mm, enables one to limit the
mass of the panel obtained by reducing the volume of foam contained
in the cells of the core.
[0032] In addition, preferably, barrier fabrics made of calendered
polyamide are used, that is to say, made of polyamide which has
been subjected to a mechanical finishing treatment.
[0033] Another subject of the invention is a sandwich panel made of
a composite material comprising an open cell core and skins
covering both faces of the core, said skins being formed of fibers
and resin, a panel characterized in that the cells of the core are
closed off by a foam on each of said faces, and in that each of the
skins comprises, starting from the core of the panel, a barrier
fabric and, a fiber overlay, both of which are impregnated with one
and the same resin polymerized and stuck onto the core of the
panel.
BRIEF DESCRIPTION OF THE DRAWINGS
[0034] A preferred embodiment of the invention will now be
described by way of example and making reference to the appended
drawings, in which:
[0035] FIG. 1 is a section view which shows diagrammatically and in
an exploded fashion the different elements of an assembly capable
of being placed in a mold so as to manufacture a sandwich panel
made of composite material by the RTM technique and conforming to
the invention
[0036] FIG. 2 is a section view diagrammatically representing in an
exploded fashion a composite panel obtained from the assembly of
the elements illustrated in FIG. 1; and
[0037] FIG. 3 is a perspective view, with cutaways in section,
showing a closed mold in which the various elements making up the
panel have been placed, conforming to the invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
[0038] Like any method using the technique of molding by the
transfer of resin, the method according to the invention uses a
mold (not shown) that internally defines a cavity, the shape and
dimensions of which correspond to those of the panel to be
manufactured. It should be noted that this shape and these
dimensions can be of any kind, without departing from the scope of
the invention. In particular, the method conforming to the
invention enables one to manufacture both a flat panel or a panel
having a more complex shape.
[0039] As FIGS. 1 and 3 illustrate, an assembly is placed in the
mold cavity comprising an open cell core 10, a film 12 of
intumescent material, integrally and directly covering each of the
two faces of the core 10, a dry barrier fabric 14 integrally and
directly covering each of the films 12, and an overlay 16 of dry
fibers integrally and directly covering each of the barrier fabrics
14. When it is placed in the mold cavity, the stack formed by this
assembly integrally fills this cavity.
[0040] The open cell core 10 can be produced in any material with
open cells that can be used for the manufacture of a sandwich panel
made of composite material. In particular, this core may have a
honeycomb structure, defining cells 10a, for example cells of
hexagonal section, emerging on each of the two faces of the
structure. The nature of the material in which the core 10 is
formed, the thickness of it and the size of the cells 10a, depends
on the application envisaged.
[0041] Each of the two films 12 is produced in a material capable
of forming a closed cell foam when it has been subjected to a
suitable cycle of expansion and polymerization. This material is
chosen notably for its lightness and its ability to form a barrier
that opposes the entry into the cells 10a of the resin to be
injected into the mold. By way of an example, that is in no way
limitative, this material can notably be a reference film Synspand
9899 CF-100 mil, sold by the company DEXTER HYSOL.
[0042] Preferably, the thickness of each of the films 12 of
intumescent material is chosen so that the foam formed after the
expansion and polymerization cycle for this material, only fills
those parts of the cells 10a situated close to the two faces of the
core 10. To this end, advantageously films 12 of intumescent
material with a thickness of about 2.5 mm are used.
[0043] The dry barrier fabrics 14 are fine light fabrics, which are
chosen mainly as a function of their porosity. More precisely, it
is essential that these fabrics form a seal against the foam formed
by the polymerization of the intumescent material constituting the
films 12, while being wettable by the resin of very low viscosity
subsequently injected into the mold. By way of a non-limitative
example, the dry barrier fabrics 14 can notably be produced in
calendered polyamide, that is to say that they are constituted by
polyamide fabrics that have been subjected to a mechanical
finishing treatment. The porosity of these fabrics has been
characterized by the measurement of the flow rate of air passing
through the barrier fabric on applying a pressure of from 200 Pa to
130 Pa. The value of 59 l/m.sup.2/s is satisfactory. Nevertheless,
these tests have given values ranging from 9 l/m.sup.2/s to 42
l/m.sup.2/s for a pressure of 130 Pa and ranging from 13
l/m.sup.2/s to 59 l/m.sup.2/s for a pressure of 200 Pa. These
values are given for information purposes and do not therefore have
any restrictive character. They are linked to the type of resin
used.
[0044] The dry fiber overlays 16 are prepared in accordance with
the usual techniques used to carry out the resin transfer molding
process, from long fibers chosen and orientated to satisfy the
mechanical properties required for the envisaged application.
[0045] When the stack constituting the assembly illustrated in
exploded fashion in FIG. 1 has been placed in the mold cavity (FIG.
3), this is closed and pressurized.
[0046] The mold is then heated according to an expansion and
polymerization cycle for the intumescent material used to form the
films 12. During this cycle, the duration and temperatures of which
depend on the nature of the chosen intumescent material, it is
converted into a foam 12a, which partially penetrates the cells
10a, as shown in FIG. 2. As has already been observed, this foam is
characterized by closed cells, which have the effect of integrally
closing off the cells 10a on the two faces of the core 10.
[0047] When the film of intumescent material 12 has a limited
thickness, for example, about 2.5 mm, the foam only penetrates into
those parts of the cells 10a close to the two faces of the core
10.
[0048] Conforming to the invention, during this expansion and
polymerization cycle for the intumescent material, the presence of
dry barrier fabrics 14 combined with pressurization of the mold
have the effect of preventing any expansion of the foam 12a towards
the outside and particularly to the interior of the dry fiber
overlays 16. At the end of the expansion and polymerization cycle
for the intumescent material, that is to say after complete curing
of the foam 12a, the two overlays of dry fibers 16 and the dry
barrier fabrics 14 are not impregnated with foam at all. On the
other hand, the core 10 is totally blocked off on both faces by the
foam 12a.
[0049] The pressure applied in the mold is then released and it is
evacuated and brought to temperature for injection of the very low
viscosity resin. This temperature is generally less than the
polymerization temperature of the intumescent material. It depends
on the nature of the resin that one wishes to inject.
[0050] The injection is then carried out in accordance with the
techniques used in traditional fashion in resin transfer molding
processes. The two dry fiber overlays 16 and the two barrier
fabrics 14 are then impregnated with resin over their entire
thickness and in the assembly of the panel to be manufactured. The
resin then comes directly into contact with the two faces of the
core 10 of the panel. Hence perfect adherence of the skins
constituted by the overlays impregnated with resin 16a and by the
barrier fabrics impregnated with resin 14a is achieved over both
faces of the core 10.
[0051] Next, in traditional fashion, a new polymerization cycle is
applied for the resin which has just been injected. The
temperatures and durations of the different steps of this cycle
depend on the nature of the resin used.
[0052] After cooling, the mold is opened and the sandwich panel
obtained is then extracted from it.
[0053] The description above shows that the panel obtained in this
way is free of resin on the inside of the open cells of the core
10, while the overlays 16a and the barrier fabrics 14a forming the
skins 18 of the panel are totally filled with resin and are free of
foam 12a. Furthermore, the structure of the panel is particularly
simple and the impregnation of the barrier fabrics 14 by the resin
enables one to guarantee perfect adherence of the skins onto the
core of the panel.
[0054] The description above also shows that implementation of the
method is particularly simple and rapid since the first
polymerization cycle is restricted to the expansion and to the
polymerization of the intumescent material in which the films 12
are formed.
* * * * *