FR2999469A1 - Manufacturing composite material parts with open cell core, comprises placing mold in core provided on face of sealing layer, preforming sealing layer; bonding layer with core, and introducing structural resin between mold and layer - Google Patents

Abstract

The method comprises: placing a mold in a core (13) provided on a face of a sealing layer to make sealed connection with walls of the cells of the core, where the cells are surmounted by a layer of fibrous material (15); preforming the sealing layer; bonding the sealing layer with the core; introducing a structural resin (14) between the mold and the sealing layer, by impregnating the fibrous material layer that comes into contact with the sealing layer and the mold after polymerization to form an outer wall (20) of a workpiece. The method comprises: placing a mold in a core (13) provided on a face of a sealing layer to make sealed connection with walls of the cells of the core, where the cells are surmounted by a layer of fibrous material (15); preforming the sealing layer; bonding the sealing layer with the core; and introducing a structural resin (14) between the mold and the sealing layer, by impregnating the fibrous material layer that comes into contact with the sealing layer and the mold after polymerization to form an outer wall (20) of a workpiece. The sealing layer comprises a prefabricated complex shutter associating an adhesive film (11) that is placed in contact with the core, where a set of unidirectional fibers is partially embedded in the resin of the adhesive film so as to have a portion of the fiber emerging from the adhesive film so that the resin introduced into the mold simultaneously impregnates the layer of fibrous material and the protruding portion of the unidirectional fibers. The structural resin is introduced into the mold under pressure. The complex shutter is formed in the mold during the preforming step. The adhesive film and the unidirectional fibers are determined so that, during preforming, the adhesive film is softened in tight connection with the glued core and the unidirectional fibers simultaneously to partially fit into the softened adhesive film, leaving, after polymerization of the film, a portion of the unidirectional fibers emerging from the adhesive film. The unidirectional fibers are a thin sheet of 0.1 mm thickness and density of 150g/m 2>, where the fibers include plated wicks that are maintained together by heat setting method. The introducing step is performed by keeping the mold in a closed state.

Description

Process for manufacturing parts made of composite material with an open cell core The present invention relates to a new method for manufacturing parts made of composite material with an open-cell core, typically made of honeycomb. Such parts are also called honeycomb sandwich composite structures, and typically comprise a honeycomb honeycomb core covered by a composite material wall consisting essentially of a resin reinforced with fibers, this wall being secured to the core. during the production of the piece.

These parts or structures are particularly appreciated for their mass / performance ratio, taking advantage of the high strength of the reinforced resin wall, and the lightness of the alveolar core. However, traditional implementations of the "prepreg-autoclave" type represent increasingly high technical and economic constraints. STATE OF THE ART In the general field of manufacturing composite parts, several molding processes known by fiber impregnation with resin can be used to make these parts and, in particular, molding processes using closed molds. Typically, the RTM (resin transfer molding) process generally consists in injecting the resin under pressure into a closed mold in which reinforcing fibers such as glass fibers, resin fibers or carbon or aramid have been previously placed. Then a clean heating is carried out to polymerize the resin.

This gives a rigid piece of composite material formed of fibers and polymerized resin. Furthermore, the resin infusion molding process, or LRI (Liquid Resin Infusion), is also known in which, instead of injecting the resin under pressure, it is made to fill the mold under the effect of a depression generated in it. In this case, generally, only one of the walls of the mold, called matrix, is rigid and shaped according to the shape of the part to be obtained, and the other wall is a flexible waterproof sheet which is placed above the assembly of the components of the part to be manufactured, including in particular the fibrous reinforcement. The tarpaulin is sealed with the die on its periphery. Depression is then created between the matrix and said tarpaulin, which causes the resin to infuse inside the fibrous reinforcing elements. Then the resin is polymerized to obtain the desired rigid part.

Some processes combine mold vacuum and pressure injection. In the particular case of the realization of honeycomb core composite parts, the RTM process is not currently used, in particular because the injection of resin under pressure naturally tends to fill the cells, which is contrary to the general purpose of these pieces which is to lighten them. For these types of parts, the method commonly used is the "prepreg-autoclave" method previously mentioned, wherein the wall is formed from a resin prepreg fiber fabric, called "prepreg". This process is illustrated in FIG. 1. A first layer 2 of this pre-impregnated fabric is placed on the mold 1, then the honeycomb core 3 coated with a film of glue 4 is placed thereon. then placing a second layer of pre-impregnated fabric, whose edges are brought into contact with the first layer 2 so as to completely envelop the core. A tear-off fabric 6 or a surface film that can be removed once the piece is finished is then successively placed, then a separating film 7 and a drainage fabric 8. The whole is covered by a waterproof tarpaulin 9, which is secured by its periphery with the mold 1 sealingly by means of a sealant 9 '. The assembly is then placed in a pressurized and thermostatically controlled enclosure, so that the first layer of preimpregnated fabric conforms exactly to the mold, the compaction of the preimpregnated fabric also leading it to bind with the soul thanks to the film of glue, before curing and hardening to form the wall of the room. But as indicated above, this method has several disadvantages: the pre-impregnated fiber fabric is expensive; it is not easy to conform to parts of complicated geometry.

It has therefore already been envisaged to replace this method with others allowing to eliminate or at least reduce these disadvantages. Thus, EP1897680 or US2009 / 0252921 disclose the manufacture of such open core core composite parts by an infusion type process. The introductory part of these patent documents, to which reference may be made, also describes various previously known types of open cell core composite material manufacturing processes, and in particular the use of the RTM technique. According to this technique of the RTM, unlike the infusion process, the resin is injected under pressure into a sealed closed mold in which the core and the dry fiber layers have been previously placed. To prevent the resin under pressure from infiltrating the cells of the core, it is necessary to prepare the latter, by closing the cells beforehand with a layer of waterproof material sufficiently resistant to then prevent the resin from entering the cells. .

A problem is then that it is no longer possible to provide a direct connection of the outer composite layers, formed by the resin infiltrated in the fiber mat, with the core, since the sealed layer prevents the resin from coming into contact of soul. A known solution to this problem, described for example in EP 0770472, is to use a thin membrane, for example polyamide or polyether-etherketone, to form the waterproof layer. The core of the part is prepared by placing a glue film under the sealing layer, to ensure the connection between the latter and the core. Then this pre-assembly is heated to ensure the polymerization and hardening of the sealed layer, and consequently its tight connection with the core, thus ensuring the closure of the cells of the core. The RTM composite layer is then produced in conventional manner. But then the assembly of the sealing layer by bonding to the core must be carried out in a first step, and the injection of resin in a second step, which greatly complicates the process. In addition, it is necessary to carry out a specific surface treatment of the selected membranes to allow good bonding thereof on the one hand with the core, and on the other hand with the covering layer. EP 0786330 describes a similar process, using a waterproof film made of a material chosen from a fluorinated polymer, a polyimide, a polyetherimide or polyetheretherketone, or mixtures thereof, and more particularly polyvinyl fluoride for example. EP0722825 describes a variant of these processes, in which the layer intended to seal before resin injection is itself a prepreg layer. A first step is to prepare the workpiece by successively placing on each side of the core, glue films, prepreg layers, and dry fiber layers to subsequently form the outer cover layer. The piece thus prepared is placed in the mold, and sufficient heating thereof is made to ensure the connection between the prepreg and the core. Cooling then makes it possible to harden the sealed layer thus formed, and to bring the temperature to a temperature suitable for resin injection. Then the resin is injected under pressure into the mold to spread between the wall of the mold and the previously formed sealing layer W, by infiltrating the dry fibers, to finally conform to the shape of the mold and form the final covering layer , in a manner known per se for the RTM process. A disadvantage of this method is that it requires the use of a prepreg layer, which can increase the cost of the process and whose implementation is subject to known constraints in the use of prepregs. Another disadvantage is the weakness of the bond between the hardened sealing layer and the covering layer. OBJECT OF THE INVENTION The object of the present invention is to solve the problems mentioned above, and aims in particular at enabling the effective manufacture of open cell core composite parts by the process of transfer of liquid resin under pressure. . It thus aims in particular to allow a sealing of the cells of the core sufficient to withstand resin injection pressures of up to 8 bar. It is also intended to allow industrialization of the manufacturing process, including for the manufacture of parts having a complex structure, for example having specific structuring elements, such as composite reinforcements passing through the core.

The invention also aims to allow a simplification of the manufacture, by a process without part recovery, all operations of preforming, sealing the core, impregnation and firing of the injected resin, being made in the same mold, without the need to change or even open during the process. GENERAL DESCRIPTION OF THE INVENTION With these objectives in view, the subject of the invention is a process for manufacturing parts made of composite materials with an open-cell core. Said parts comprise an outer wall and a core, or core, typically honeycomb, comprising a set of cells or cells oriented parallel to each other and transversely to the apparent surfaces of said parts, the cells being closed at least on one side by said outer wall formed from a cover layer of fibrous material embedded in a matrix resin. During the process, the core provided on said face with a sealing layer intended to make a tight connection with the walls of the cells of the core, and surmounted by a layer of fibrous material, is placed in a mold. then we proceed to a preforming step intended to form the sealing layer and bind with the core. Then introduced between the mold and the sealing layer a structural resin which, by impregnating the layer of fibrous material, comes into contact with the sealing layer and the mold to form after polymerization said outer wall of the workpiece. According to the invention, the method is characterized in that the sealing layer consists of a closure complex associating a structural adhesive resin adhesive film, disposed in contact with the core, and a fold of associated unidirectional fibers. audit adhesive film. The unidirectional fibers are partially embedded in the resin of the unpolymerized adhesive film, so as to have a portion of these unidirectional fibers emerging from the adhesive film on the opposite side to the core, so that the resin introduced into the mold simultaneously impregnates the layer of fibrous material and the emerging part of the unidirectional fibers The structural resin can be introduced into the mold under pressure, according to the RTM technique, or by infusion, or by combining a vacuum generated in the mold and a resin injection under pressure. The closure complex is composed of fibers whose manufacturing technique allows the removal or at least the minimization of local embossments. Local burrs are phenomena related to the impregnation of the resin. In particular, in the case of certain weaves, the resin penetrates preferentially to the crossings of the fibers. In the context of the invention, this is problematic, since it creates heterogeneities and non-watertight zones in the resin. According to a first embodiment, the closure complex is prefabricated. The sealing complex may be prepared by combining the unpolymerized adhesive film with the unidirectional fiber ply so that the fibers are already partially embedded in the adhesive film resin. This complex is thus a semi-intimate assemblage of materials between a fold of fibers and a structural adhesive thermosetting resin film. According to a second embodiment, the closure complex is formed in the mold during the preforming step, the adhesive film and the fold of unidirectional fibers being determined so that, during preforming, the softened adhesive film comes in bonding sealed with the core and simultaneously unidirectional fibers partially integrate into the softened adhesive film, leaving, after polymerization of this film, a portion of these unidirectional fibers emerging from the adhesive film. The adhesive resin associated with semi-impregnated unidirectional fibers in it forms an extremely resistant matrix-fiber composite sealing layer, in particular capable of withstanding pressures of up to 8 bar, which guarantees that it There will be no breakage of this sealing layer during the subsequent injection of structural resin under pressure. In addition, a particularly strong bonding of the core with the outer skin made of structural resin reinforced by the layer of fibrous material is ensured by the fact that the fibers of the unidirectional fiber ply are embedded in both the glue film. integral with the core and in said structural resin. The bond between the adhesive structural resin and the structural resin of the skins is not only an adhesive bond, but also a mechanical bond. This solves the problems posed by the use of the sealing films mentioned in the prior art as well as those posed by the use of a prepreg. The core may typically be a honeycomb material having hexagonal cells 3 to 5 mm or larger. The cells may also be rectangular or other shapes, with dimensions up to 10 mm or more. The core can be made of paper impregnated with resin, type NOMEX CD by aluminum, glass, etc. The adhesive resin or KEVLAR CD structural film is exemplary of a thermosetting polymerizable resin at about 180 ° C, having a thickness of about 0.1 mm and a basis weight of about 300 g / m 2. This layer may optionally be supported by a fiber frame. The unidirectional fiber ply is a thin ply, about 0.1 mm thick and about 150 g / m 2 in density, consisting of spread slivers held together by a conventional heat-setting method. The fibers of this sheet may be carbon fibers, glass, Kevlar CD, having a diameter of the order of 10 μm. The layer of fibrous material may consist of one or more plies of woven or non-woven fibers 1 to 20 mm thick. The volume content of fibers in the part is preferably greater than 50%, preferably greater than 57%. The injectable resin is a thermosetting or thermoplastic resin with a viscosity of less than 1000 centipoise, preferably between 50 and 500 centipoise. During the preforming step, the mold is heated to a temperature of 120 to 180 ° C for one to two hours to soften the adhesive film and, thereby reducing the viscosity of the glue, allow the glue to wetting the edges of the cell walls of the core to form a connecting meniscus between said film and said walls of the honeycomb. Then the temperature is lowered to about 60 ° C, before proceeding to the introduction of the structural resin into the mold. According to a preferred embodiment, the resin is introduced into the mold under pressure, typically at a pressure of 1 to 8 bar, more particularly at a pressure of about 3 bar. This pressure, which will be adapted in particular as a function of the desired thickness of the outer wall and therefore of the thickness of the fibrous reinforcing layer, must be sufficient for the injected resin to completely fill the mold by infiltrating and coating correctly the fibers of the layer of fibrous material. In addition, the pressure is not too high which ensures that the sealing layer maintains its integrity during injection. Practically, vents are formed in the mold, of dimensions and at predetermined locations remote from the resin injection point (s), and the injection is stopped when the resin reaches these vents, so that the mold is completely filled. Depending on the resin used, it is possible to post-cure for example at 120 ° C for 3 hours before proceeding to cooling and demolding.

According to an alternative embodiment, the resin is introduced into the mold as a result of a depression generated therein, similarly to the infusion molding process. According to another preferred arrangement, the introduction of the resin into the mold is carried out in continuity with the preforming step, while maintaining the closed mold. Alternatively, the introduction of the resin can be done in an operation separate from the preforming step, after a possible mold change. This variant may in particular be preferred for producing parts of complex geometry, with, for example, relatively large localized variations in the thickness of the outer wall, where it may then be advantageous to perform the preforming in a mold having a specific shape adapted to form the sealing layer, and to perform the pressure injection, or the infusion, in a mold having the shape of the desired part. Brief description of the drawings Other features and advantages will become apparent in the description which will be made of a process according to the invention.

Reference is made to the accompanying drawings, in which: FIG. 1 is a representation of a method according to the prior art, already described in the introductory part of this specification; FIG. 2 is a schematic view of detail illustrating the complex; 3 is a diagrammatic detail view schematically showing the constitution of the wall 15 of a part formed according to the invention; FIG. 4 is a schematic sectional view of a Part made by the method according to the invention, - Figures 5 to 7 illustrate the different successive phases of the process, - Figure 8 is a representative graph of temperature and pressure variations in an exemplary implementation of the process. DESCRIPTION OF THE EXAMPLES AND ADDITIONAL INFORMATION FIG. 2 shows in section the closure complex 10 that can be used for the implementation of the invention. This complex comprises a fold 11 of polymerizable adhesive film associated with a fold of unidirectional fibers 12, for example carbon fibers of 10 μm in diameter, assembled in wicks spread to form a thin layer, typically of the order of 0, 1 mm. The fold 11 of the adhesive film has characteristics which enable it, on the one hand, to ensure, after polymerization and hardening, its bond with the honeycomb core and, on the other hand, to partially integrate the fibers. 12, as seen in Figure 2, during the softening of the constituent resin of said film when it is heated. The fold of unidirectional fibers thus has a portion 12a embedded in the fold of adhesive film 11, and a portion 12b emerging on the fold surface. Figure 3 illustrates the constitution of the wall 20 W finally obtained by the method according to the invention. It shows the adhesive film 11 polymerized and bonded to the honeycomb core 13, the structural resin skin 14 armed with two plies of woven or non-woven fibers 15. It should be noted in particular that the structural resin 14, in Infiltrating to the surface of the adhesive film, also impregnated in the portion 12b of unidirectional fibers 12 which was not embedded in the fold of adhesive film 11, and is also reinforced by these fibers . But the most important role of these fibers is to provide a mechanical bond between the adhesive film 11 and the structural resin 14, which completes particularly effectively the bonded bond from the single contact of these layers. The process according to the invention allows the production of a structured honeycomb core part, that is to say that the core includes one or more composite fiber resin / reinforcement skins. Thus, it is possible to take up significant local forces, for example via a metal insert, or to increase the rigidity of the part. FIG. 4 illustrates, by way of example, a structured part that can be produced thanks to the invention. This piece comprises three pavers 21, 22 each formed of a honeycomb core 23 coated with a sealing complex layer. The pavers are separated by stiffeners 25 made of fiber-resin composite. The central block 22 further comprises a metal insert 27, for example for a screw connection of the structured part with another part. A composite skin 26 covers the entire part and constitutes the outer wall. In the example shown, said wall is in fact made of a lower composite wall 26a, an upper composite wall 26b, and the side walls 26c are formed by extensions of the W composite walls forming the stiffeners 25. This example is of course not limiting, but illustrates the possibility of producing complex parts by the method according to the invention. In the previous examples, it was considered that the sealing complex 10 was prefabricated, and thus placed on the core, at the time of manufacture of the workpiece. Figures 5 to 7 illustrate another embodiment, in which the closure complex 10 is in fact formed in the mold during preforming. In this case, as illustrated in FIG. 5, the part is prepared by placing the polymerizable adhesive film 11 on the honeycomb core 13. Then the fold of unidirectional fibers 12 is placed on top of it, and finally the fold or folds of Woven fibrous material 15. The assembly is placed in the mold whose closure generates the pressure required for compaction of the tissues and the closure complex. More specifically, the closing of the furnace causes the pressure application of the adhesive film 11 on the core 13, via the pressure, according to the arrow F, of the wall of the mold 101 on the folds of fibrous materials 15 which themselves press the fold of unidirectional fibers 12 onto the adhesive film 11. The subsequent rise of the oven temperature causes the softening of the adhesive film 11, leading it to adhere to the core 13 by wetting the edges of the walls 131 of the cells 132 and thereby forming meniscuses 111 which contribute to the strength of the bond between the adhesive film 11 and the core 13. Moreover, the softening of the adhesive film 11, while the fold of unidirectional fibers 12 is applied under pressure on this film due to the pressure resulting from the closure of the mold, allows the unidirectional fibers 12 to partially penetrate the resin of the adhesive film; leaving a portion 12b of these fibers protruding from said adhesive film, as shown in FIG. 6. The oven is maintained at a temperature, at about 160 ° C for a sufficient time, from 1 to 2 hours, to ensure the polymerization of the adhesive film resin and thereby forming the resistant sealing layer, then its temperature is lowered to about 60 ° C. The low-viscosity structural resin 14 is then introduced into the mold, between the wall 101 of the mold and the previously formed sealing layer 10, under a pressure of 2 to 3 bars, until the fibrous material 15 has been completely impregnated. the emergent portion 15 of the unidirectional fibers and filling the space between the wall 101 of the mold and the sealing layer 10, as shown in FIG. 7. The temperature of the oven is again increased towards 120 ° C. to complete the polymerization of the the injected resin 14 and constitute the fiber-resin composite forming the final outer wall 16 of the part. A typical cycle of manufacture, from the closing of the mold until demolding is shown in FIG. 8. The plot 3 represents the temperature variations and the plot 4 represents the pressure variations in the mold. The first part 3a corresponds to the heating intended for the softening of the adhesive film, followed by a phase 3b of complementary heating and holding at high temperature, intended to ensure the polymerization and hardening of the closure complex, before reducing the temperature (phase 3c). Then, during the injection of the resin 14, the oven is heated again (phase 3d), simultaneously with the increase in pressure 4a of the injected resin. As illustrated, this step can be carried out in two phases, with an intermediate stage. This results in maintaining the temperature 3rd and under pressure 4b. Then the oven is cooled while the pressure is reduced to the atmospheric pressure W, to allow the opening of the oven and demolding of the manufactured part. For standard part shapes, it is possible to perform all steps of the process in a single operation, that is to say without reopening the mold. The labor time is thus considerably reduced. As regards the mechanical performance of the interface between the outer wall and the core, tests have made it possible to validate that the resistance at the outer wall / core interface is very sufficient. It has indeed been found during the test that, under a tensile load perpendicular to the wall of up to 5.5 MPa, a rupture in the core has been observed, before a break in the connection between the core occurs. and outer wall. The fact of soul breaks first indicates this interface is at least as powerful as the intrinsic properties of the soul. The method according to the invention makes it possible to produce parts with a chamfer, that is to say portions of walls, in particular the side walls of a workpiece, oriented with a slope up to 35 900 relative to to the main faces of the part, that is to say the walls substantially parallel to the general orientation of the cells of the honeycomb core. Indeed, the presence of a rigid counter mold and the possible absence of vacuum during the cycle makes it possible to control the deformation of the core.

The manufacturing method according to the invention is particularly intended to be implemented with a mold formed of two or more rigid parts. However, the preforming of the part, or one or more elements constituting it in the case of a piece of more complex geometry, can also be performed under vacuum on a rigid mold and a flexible mold against. Injection of structural resin may also be assisted by a vacuum, preferably less than 5 mbar, created in the mold. 20

Claims (10)

REVENDICATIONS1. Process for manufacturing parts made of composite materials with an open-cell core, said parts comprising an outer wall (20) and a core (13) whose cells (132) are closed at least on one side by said outer wall constituted by from a covering layer based on fibrous material embedded in a matrix resin, in which process the core (13) provided on said face with a sealing layer intended to produce a W sealing connection with the walls (131) of the cells of the core, and surmounted by a layer of fibrous material (15), a preforming step is performed to form the sealing layer and bind it with the core and, between the mold (101) and the sealing layer, a structural resin (14) is introduced which, by impregnating the layer of fibrous material (15), contacts the sealing layer and the mold to form after polymerization, said wall e outer part (20) of the part, characterized in that the sealing layer consists of a closure complex (10) associating an adhesive film (11) of structural adhesive resin, disposed in contact with the core, and a fold of unidirectional fibers (12) associated with said adhesive film, the unidirectional fibers being partially embedded in the resin of the unpolymerized adhesive film, so as to have a portion (12b) of these unidirectional fibers emerging from the adhesive film on the opposite side to the core, so that the resin (14) subsequently introduced into the mold simultaneously impregnates the fibrous material layer (15) and the emergent portion (12b) of the unidirectional fibers. 2. Method according to claim 1, characterized in that the structural resin (14) is introduced into the mold under pressure. 3. Method according to claim 1, characterized in that the closure complex (10) is prefabricated. 4. Method according to claim 1, characterized in that the closure complex (10) is formed in W the mold during the preforming step, the adhesive film (11) and the fold of unidirectional fibers (12) being determined in such a way that, during preforming, the softened adhesive film is bonded tightly with the core (13) and simultaneously the unidirectional fibers (12) partially integrate into the softened adhesive film, which remains, after polymerization of this film, a portion (12b) of these unidirectional fibers emerging from the adhesive film (11). 20 5. Method according to claim 1, characterized in that the adhesive film (11) is made of a polymerizable thermosetting resin at about 180 ° C, with a thickness of about 0.1 mm and a basis weight of about 300 g / m2. 25 6. Method according to claim 1, characterized in that the fold of unidirectional fibers (12) is a thin sheet, about 0.1 mm thick and with a density of about 150 g / m 2, consisting of stranded strands. maintained together by a conventional method of thermo fixing. 7. Method according to claim 6, characterized in that the unidirectional fibers (12) are selected from carbon fibers, glass, Kevlar CD, having a diameter of the order of 10 μm. 8. Process according to claim 1, characterized in that the injectable structural resin (14) is a thermosetting or thermoplastic resin having a viscosity of less than 1000 centipoise, preferably between 50 and 500 centipoise. 9. The method of claim 1, characterized in that the introduction of the resin (14) in the mold is W made in continuity with the preforming step, keeping the mold closed. 10. The method of claim 1, characterized in that the introduction of the resin into the mold is done in an operation separate from the preforming step.