Adhesive bonding and miscellaneous chemical manufacture – Methods – Surface bonding and/or assembly therefor
Reexamination Certificate
2000-08-22
2004-01-20
Ball, Michael W. (Department: 1733)
Adhesive bonding and miscellaneous chemical manufacture
Methods
Surface bonding and/or assembly therefor
C428S071000, C428S073000, C428S117000, C052S793100, C244S133000
Reexamination Certificate
active
06679969
ABSTRACT:
TECHNOLOGICAL FIELD
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.
STATE OF THE TECHNOLOGY
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/m
2
(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 fi
Duret Nathalie
Fournier Alain
Aerospatiale Matra Airbus
Ball Michael W.
Goff John L.
Krebs Robert E.
Thelen Reid & Priest LLP
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