Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS FOR THE PRODUCTION OF A COMPOSITE MATERIAL
PANEL iiTH RESIN TRANSFER MOULDING
TL'C!'DTDTTlIIt
TECHNICAL FIELD
The invention relates to a process for the production of a composite material
sandwich panel having a core formed by an open cell structure, such as an
open cell foam or honeycomb structure and at least one skin formed from
fibres and resin. More specifically, the invention relates to a process for
the production of such a panel, in which the skin is formed on the open cell
core using the resin transfer moulding method.
A particular application of the invention relates to the production of
partitioning and cladding panels in aircraft. More generally, the invention
can be used for producing all panels having to have a good mechanical behav-
iour and a very good surface appearance for a particularly low weight.
PRIOR ART
Composite material panels having a sandwich structure are conventionally
produced by draping using methods currently used for the production of
composite material parts. These production methods consist of draping resin
impregnated fibres on at least one of the faces of a cellular material core,
such as a foam or a honeycomb structure. When draping is finished, the
assembly is placed in an oven or autoclave in order to polymerize the resin.
In certain industries, such as the aeronautical industry, this traditional
production procedure for composite material parts has tended to be replaced
by resin transfer moulding or RTM when the parts to be produced have a mono-
lithic structure. According to this procedure, different layers of fibres,
which are not impregnated with resin, are placed in a mould having the shape
of the part to be produced. As a function of the nature of the part, the
fibres may or may not be woven. As the mould is heated to a relatively high
temperature, a very low viscosity resin is injected under pressure into the
empty mould, so as to completely fill the mould and impregnate the fibres,
whilst filling the spaces separating them. When resin injection is ended,
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the part is subject to a polymerization cycle prior to demoulding.
Compared with the conventional method for the production of composite mater-
ial parts by draping, this resin transfer moulding method offers significant
implementation advantages explaining its even-increasing popularity. It also
ensures a perfect reproducibility of the parts and a significantly improved
surface state. However, up to now this resin transfer moulding method has
not been usable for the production of panels having a sandwich structure,
particularly when the cellular structure forming the panel core has cells
issuing onto its two faces, as is the case When the panel core is constituted
by a honeycomb structure or an open cell foam. Thus, the use of resin
transfer moulding for the production of such panels would lead to the filling
of the cells with resin, bearing in mind the very low viscosity of the resins
used, as well as the relatively high pressures and temperatures inherent in
this procedure. Even if the filling of the cells of the honeycomb core of
the thus produced panel was acceptable from the mechanical behaviour stand-
point with respect to the panel, it would lead to an unacceptable weight
increase in industry such as the aeronautical industry.
In order to obviate this disadvantage, consideration has been given to
filling with foam the cells of the honeycomb core. However, although this
procedure is acceptable from the standpoint of the weight of the resulting
panel, which is only insignificantly increased, it is unusable on non-planar
panels. Thus, the shaping of the panel, which must precede the injection of
the resin, inevitably creates gaps between the foam and the walls of the
cells. These gaps are filled with resin during the injection of the latter,
which once again leads to an unacceptable panel weight increase.
Another procedure envisaged for avoiding the penetration of resin into the
cells of the honeycomb core consists of using intumescent or swelling adhe-
sives, whose expansion coefficient of approximately 300% has the effect of
filling the cells of the honeycomb core. However, this procedure is also
very disadvantageous with respect to the weight of the panel obtained.
It is also known from US-A-5 141 804 to produce a sandwich structure using
the conventional draping method, by interposing between adjacent layers of
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fibres preimpregnated with resin, an intermediate thermoplastic film coated
on its two faces with an adhesive, in order to improve the cohesion and
stability of the structure obtained. In the case where the sandwich struc-
ture has a honeycomb core, adhesive-coated intermediate films are interposed,
S with the same objective, between the honeycomb core and the layers of resin-
preimpregnated fibres adjacent thereto. The polymerization of the resin and
the adhesion of the different layers are carried out simultaneously by the
application of an appropriate temperature and pressure cycle.
DESCRIPTION OF THE INVENTION
The main object of the invention is a process for the production of a comp-
osite material panel having an open cell core, permitting the use of resin
transfer moulding and avoiding the penetration by the resin or any other
material into the cavities of the open cell core, so as to limit the weight
of the panel to a level acceptable in aeronautics, no matter whether said
panel is planar or not.
The invention also relates to a process for producing a sandwich panel of
composite material, whose implementation time only slightly exceeds the
duration of resin transfer moulding and which requires no tools other than
those conventionally used for performing this moulding procedure.
According to the invention, these results are obtained by means of a process
for the production of a composite material panel having an open cell core,
at least one skin formed from fibres and resin, and a tight membrane inter-
posed between the core and the skin, said process being characterized in that
it comprises the following stages:
- the putting into place of the core, an unpolymerized adhesive film, the
tight membrane and dry fibres, in this order, in a mould,
- closing the mould,
- polymerization of the adhesive in the closed mould, so as to adhere the
tight membrane to the open cell core,
- injection of resin into the mould, without opening the latter, so as to
impregnate the fibres,
- polymerization of the resin in the mould, without opening the latter, so as
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to form the skin and
- demoulding the thus obtained panel.
Preferably, the polymerization of the adhesive takes place at a first temper-
s ature and the injection of the resin at a second temperature at the most
slightly exceeding the first temperature.
In a first embodiment of the process, the first and second temperatures are
equal. The resin is then injected immediately after the polymerization of
the adhesive.
However, in a second embodiment of the process, the first and second temper-
atures differ. The polymerization of the adhesive is then followed by a
stage of adapting the temperature of the mould prior to the injection of the
resin.
As a function of the particular case, it is possible to use either an
unsupported adhesive film, or an adhesive film supported by a light fabric.
Advantageously, the closing of the mould is preceded by the putting into
place within the same of protective plates encircling the peripheral borders
of the open cell core. These protective plates ensure that the resin under
pressure does not damage the walls of the peripheral cavities during the
injection of the resin.
When it is wished to produce a panel having two skins, in the mould is placed
an unpolymerized adhesive film, a tight membrane and fibres on each face of
the open cell core.
The process according to the invention can be used both when the open cell
core is constituted by a honeycomb structure and also when it is an open cell
foam.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is described in greater detail hereinafter relative to a non-
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limitative embodiment and the attached drawings, Wherein show:
Fig. 1 A perspective view with sectional breaking away of a closed mould
in which have been placed the different components of a composite
material panel, whose production includes, according to the inven-
tion, the resin transfer moulding procedure.
Fig. 2 In continuous line form, the evolution of the temperature T as a
function of time t and in broken line form, the evolution of the
pressure P as a function of time t during the performance of the
process according to the invention.
DETAILED DESCRIPTION OF AN EMBODIMENT
The production process according to the invention will now be described in
its application to the production of a planar panel having a honeycomb core,
whereof the two faces are covered with a skin formed from fibres and resin.
However, this process can also be applied to the production of a panel having
a certain curvature, as well as to the production of a panel having a skin
only covering one of the faces of the honeycomb core and to the production of
a panel, whose core is constituted by an open cell foam.
In fig. 1, reference 10 designates in general terms a multipart mould, which
internally defines a cavity 12, whose shape is complimentary of that of the
panel to be produced.
As is very diagrammatically illustrated in fig. 1, the mould 10 is equipped
with means making it possible to perform the resin transfer moulding method.
These means comprise in particular at least one resin injection passage 14,
which preferably issues into the cavity 12 in the vicinity of each of the
faces facing said cavity intended to form the faces of the panel to be
produced. The means for performing the zesin transfer moulding method also
comprise at least one passage 16 for placing the cavity 12 under a vacuum
and for discharging the excess injected resin. This passage 16 is located on
one side of the mould 10 opposite to that where the passage 14 issues and it
also issues into the cavity 12 in the immediate vicinity of the facing faces
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of said cavity.
Initially the mould 10 is open and within tie cavity 12 are placed the
different elements to form the panel to be produced.
In the embodiment described and which relates to the production of a panel
with two skins, said elements consist of a honeycomb core 18 interposed
between two fibre layers 20 for forming the panel skins. It should be noted
that the fibres 20 are dry, i.e. not resin impregnated and that, as a func-
lion of the particular case, can be constituted by one or more thicknesses
of woven or unwoven fibres. The nature of the fibres will obviously depend
on the part to be produced. They can e.g. be carbon fibres, but can be any
type of fibre without passing beyond the scope of the invention.
According to the invention, the elements placed in the cavity 12 prior to the
closing of the mould 10 also comprise a Light membrane 22 interposed between
each of the fibre layers 20 and the-honeycomb core 18, as well as as adhesive
film 24 interposed between each tight membrane 22 and the face facing the
honeycomb core 18.
Each of the tight membranes 22 is constituted by a tight film of very limited
thickness, whose function is to prevent resin, which will be subsequently
injected under pressure through the passage 14 during the resin transfer
moulding, from penetrating the cavities or cells of the honeycomb core 18.
The tight membranes 22 can be made from any material able to retain the
necessary sealing action under relatively severe temperature and pressure
conditions characterizing the injection of the resin. The thickness of the
membranes also depends on the dimensions of the cells of the honeycomb core
18. Moreover, the tight membranes 22 must be able to withstand a relatively
significant elongation and must be effectively fastened to the adhesive
films 24, as well as to the resin subsequently injected through the passage
14. In a non-limitative example, the tight membranes 22 can be made from
polyamide 6.6 or polyether-ether-ketone (PEEK). The membrane surface treat-
ment must be such that they adhere on their two faces on the one hand to the
adhesive film 24 and on the other to the resin which will be injected into
the fibres 20.
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The adhesive films 24 are intended to fix the tight membranes 22 to the two
faces of the honeycomb core 18 before the resin is injected through the
passage 14 into the cavity 12. They can be~formed from unsupported adhesive
or adhesive supported by a light fabric, so as to optimize the weight of the
panel obtained. The mechanical characteristics of the adhesive films 24 are
chosen so as to comply with the requirements of a satisfactory behaviour of
the structure of the panel when it has been completed.
The nature of the adhesive used for forming the adhesive films 24 is mainly
chosen as a function of its polymerization temperature. Thus, in view of the
fact that said adhesive must be polymerized prior to injection and then the
polymerization of the resin, it must be able to withstand the temperature
conditions imposed by these two operations. To this end, numerous epoxy
resins can be used, e.g. the CIBA-GEIGY BSD 322 (180'C) and adhesive FM 123-2
(120'C).
Apart from the components of the panel which it is wished to produce, it is
recommended that in the cavity 12 of the oould 10 are placed protective
plates 26 for preventing the subsequently injected resin from deforming the
partitions of the peripheral cells of the honeycomb core 18. For this pur-
pose the protective plates 26 can be placed all around the honeycomb core 18,
so as to closely encircle the peripheral borders thereof, as illustrated in
fig. 1. These plates 26 are advantageously tightly connected to the edges of
the tight membranes 22, e.g. by means of a silicone paste, in such a way that
their positioning is maintained and the sealing is assured.
It should be noted that in a variant, the use of protective plates 26 can be
avoided, e.g. by giving the cavity 12 an adapted shape and by ensuring the
injection and extraction of the resin by means of passages 14 and 16 issuing
as closely as possible to the facing faces of the cavity 12.
When all the aforementioned elements have been placed in the cavity 12, the
mould 10 is closed (fig. 1) and the temperature is progressively raised to a
value T1 (fig. 2) corresponding to the polymerization temperature of the
adhesive of each of the adhesive films 24. This polymerization temperature
T1 is dependent on the nature of the adhesive used. In a non-limitative
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example, it can be approximately 120°C. The heating period tl at said
temperature T1 is chosen so as to guarantee the completion of the adhesive
polymerization cycle. Said period tl can, in a non-limitative
example, be between approximately 15 and approximately 30 min.
When the polymerization cycle for the adhesive films 24 is finished, the
tight membranes 22 are adhered to each of the faces of the honeycomb core 18,
so that the cells of said core 18 are tightly sealed.
At this stage, the mould 10 is kept closed and the temperature is either
unchanged if the resin injection temperature T2 is the same as the adhesive
polymerization temperature T1, or adapted to the resin injection temperature
T1, if said two temperatures differ. In general terms, 1t should be noted
that the resin injection temperature T2 is at the most slightly higher than
the adhesive polymerization temperature T1.
Fig. 2 illustrates the case where the resin injection temperature T2 slightly
exceeds the adhesive polymerization temperature T1. In this case, resin
injection is preceded by a stage of adapting the temperature of the mould 10,
making it possible to bring the latter to the resin injection temperature T2.
The duration t2 of this temperature adaptation stage depends on the differ-
ence between the temperatures T2 and T1.
In the not shown case where the temperature T2 is lower than the temperature
T1, the mould 10 can either be naturally cooled, or cooled by means of an
integrated cooling system. In the case illustrated in fig. 2 where the
temperature T2 slightly exceeds the temperature T1, the heating of the mould
10 is continued.
As soon as the resin injection temperature T2 is reached, said injection
takes place through the passage 14, under a pressure progressively increasing
to a value PI. As from the start of or slightly prior to the injection of
resin, a vacuum is created in the cavity 12 of the mould 10 through the
passage 16, connected for this purpose to a not shown vacuum circuit. In
exemplified manner, the value of said vacuum can be 1 hPa.
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The temperature and pressure characteristics relating to said resin injection
stage essentially depend on the nature of the injected resin. In exemplified
manner, said injection can take place at a temperature T2 of approximately
130°C and at a pressure PI of approximately 3 hPa. For information, the
viscosity of the resin used in the resin transfer moulding method is normally
between 10 and 30 mPa.s.
The resin injection time t3 is mainly dependent on the resin volume to be
injected into the mould 10, i.e. the dimensions of the panel to be produced.
For illustration, the injection stage can last about 10 min.
Still without opening the mould 10, this is followed by the polymerization of
the previously injected resin, whilst maintaining the pressure PI in the
mould. For this purpose the temperature of the mould is increased to a
temperature T3 corresponding to the resin polymerization temperature and is
maintained for a time T4 necessary for the completion of said polymerization.
In a non-limitative illustration, the temperature T3 can e.g. be approxim-
ately 160°C and the time t4 approximately 2 hours.
The resin injection and polymerization stages are conventional stages in the
resin transfer moulding method.
As a result of the prior adhesion of the tight membranes 22 to each of the
faces of the honeycomb core 18, only the fabrics 20 are impregnated with
resin during the injection thereof. This leads to a panel, whose cells con-
tain no material, which guarantees the lightness of this panel, whilst still
benefiting from the advantages inherent in the resin transfer moulding method.
Moreover, as a result of the adhesion of the tight membranes 22 prior to
resin transfer moulding and during the same heat cycle and without opening
the mould, the production period is only very slightly increased compared
with conventional production using the resin transfer moulding method.
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