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.
Another subject of the invention is a sandwich panel made of composite material manufactured according to this method.
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.
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.
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.
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.
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.
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 cure 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.
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/m2 (this value is multiplied by two in order to take into account both sides of the panel). Finally manufacture takes a long time.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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:
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.
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.
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.
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.
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/m2 (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/m2 (also to be multiplied by two) obtained by using the technique described in document EP-A-0 722 825.
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.
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.
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.