AU709483B2

AU709483B2 - Process for the production of a composite material panel with resin transfer moulding

Info

Publication number: AU709483B2
Application number: AU70335/96A
Authority: AU
Inventors: Dominique Guittard; Daniel Soro
Original assignee: Airbus Group SAS
Current assignee: Airbus Group SAS
Priority date: 1995-10-27
Filing date: 1996-10-22
Publication date: 1999-08-26
Anticipated expiration: 2016-10-22
Also published as: CA2188629C; DE69605884T2; ES2143161T3; FR2740383B1; FR2740383A1; AU7033596A; EP0770472A1; DE69605884D1; EP0770472B1; CA2188629A1

Description

DESCRIPTION

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 behaviour 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 25 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 monolithic 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, 2 the part is subject to a polymerization cycle prior to demoulding.

Compared with the conventional method for the production of composite material parts by draping, this resin transfer moulding method offers significant implementation advantages explaining its ever-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 standpoint with respect to the panel, it would lead to an unacceptable weight increase in industry such as the aeronautical industry.

:e S. 20 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 adhesives, 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 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 structure has a honeycomb core, adhesive-coated intermediate films are interposed, with the same objective, between the honeycomb core and the layers of resinpreimpregnated 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 According to a first aspect, the present invention consists in 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 interposed between the core and the skin, said process comprising the following stages: 15 putting in a mould a stack formed by enveloping the core in a three- S. 15 layered structure wherein an unpolymerized adhesive film forms a first layer adjacent the core, a fluid tight membrane forms a second layer adjacent the adhesive film, and a layer of dry fibres forms a third layer adjacent the fluid tight membrane; closing the mould; 20 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 to form the skin of a composite material panel; and demoulding said panel.

In preferred embodiments, the present invention discloses a process for the production of a composite 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.

Preferably, the polymerization of the adhesive takes place at a first temperature and the injection of the resin at a second temperature which, at the most, slightly exceeds the first temperature.

4 In one preferred embodiment of the invention, the first and second temperatures are equal. The resin is then injected immediately after the polymerization of the adhesive.

However, in another preferred embodiment of the process, the first and second temperatures 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, in still further preferred embodiments, 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.

S: 15 The invention also relates to a process for producing a sandwich panel of composite material, whose implementation time only slightly exceeds the 0duration of resin transfer moulding and which requires no tools other than those conventionally used for performing this moulding procedure.

0. When it is wished to produce a panel having two skins, further 20 embodiments disclose that in the mould is place 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- 5 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 invention, 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 15 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 S 20 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. I, 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 resin 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 6 of said cavity.

Initially the mould 10 is open and within the 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 function 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.

15 According to the invention, the elements placed in the cavity 12 prior to the S" closing of the mould 10 also comprise a tight membrane 22 interposed between each of the fibre layers 20 and the honeycomb core 18, as well as an 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.

25 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 treatment 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 7 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 15 (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 mould 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 purpose 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 8 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 Ti, or adapted to the resin injection temperature T1, if said two temperatures differ. In general terms, it should be noted that the resin injection temperature T2 is at the most slightly higher than 15 the adhesive polymerization temperature Ti.

a .00. Fig. 2 illustrates the case where the resin injection temperature T2 slightly exceeds the adhesive polymerization temperature Ti. In this case, resin injection is preceded by a stage of adapting the temperature of the mould S..o 20 making it possible to bring the latter to the resin injection temperature T2.

The duration t2 of this temperature adaptation stage depends on the difference between the temperatures T2 and T1.

oo In the not shown case where the temperature T2 is lower than the temperature Ti, 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 TI, the heating of the mould 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.

9 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 15 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 approximately 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 cont: 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.

9A It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

9 a 9# THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS: 1. 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 interposed between the core and the skin, said process comprising the following stages: putting in a mould a stack formed by enveloping the core in a threelayered structure wherein an unpolymerized adhesive film forms a first layer adjacent the core, a fluid tight membrane forms a second layer adjacent the adhesive film, and a layer of dry fibres forms a third layer adjacent the fluid tight membrane; 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 to form the skin of a composite material panel; and demoulding said panel.

2. Process according to claim 1, wherein the polymerization of the 20 adhesive takes place at a first temperature and the injection of the resin at a second temperature, which at the most slightly exceeds the first temperature.

3. Process according to claim 2, wherein the first and second temperatures are equal, resin injection taking place immediately after the polymerization of the adhesive.

4. Process according to claim 2, wherein the first and second temperatures differ, the polymerization of the adhesive being followed by a stage of adapting the temperature of the mould prior to the injection of the resin.

Process according to any one of the preceding claims, wherein use is made of an unsupported adhesive film.

6. Process according to any one of claims 1 4, wherein use is made of an adhesive film supported by a light fabric.

7. Process according to any one of the preceding claims, wherein prior to the closing of the mould, within the latter are also placed protective plates encircling the peripheral borders of the open cell core.

Claims

8. Process according to any one of the preceding claims, wherein in the mould is placed an unpolymerized adhesive film, a tight membrane and fibres on each face of the open cell core, so as to produce a panel having two skins.
9. Process according to any one of the preceding claims, wherein use is made of an open cell core with a honeycomb structure. Process according to any one of claims 1 8, wherein use is made of a core constituted by an open cell foam.
11. 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 interposed between the core and the skin, substantially as hereinbefore described with reference to the accompanying drawings. .Dated this first day of July 1999 Aerospatiale Societe Nationale Industrielle Patent Attorneys for the Applicant: B RICE CO DESCRIPTIVE ABSTRACT For the production of a composite material sandwich panel having an open cell core (18) and at least one skin formed from resin impregnated fibres the core (18) and unimpregnated fibres (20) are placed in a mould (10) and a tight membrane (22) and an adhesive film (24) are interposed in this order between the fibres and the core. After closing the mould a single heat cycle makes it possible to polymerize the adhesive in order to seal the cavities of the open cell core (18) with the membrane, followed by the injec- tion of the resin under pressure into the fibres (20) and finally polymerize said resin prior to the removal of the panel from the mould. (Fig. 1) e f *4 4* 4 4 0 4 4 Oo•• oo• 1