RELATED CASES

The present application is a National Phase entry of PCT Application No. PCT/FR2017/000176, filed Sep. 22, 2017, which claims priority from FR Patent Application No. 16 70556, filed Sep. 27, 2016, which applications are hereby incorporated by reference in their entireties.

TECHNICAL FIELD

The present invention relates to a method for producing parts made of organic matrix composite material, also called polymer composite matrix material, comprising continuous unidirectional fibers and a polymer matrix.

BACKGROUND ART

There are known methods of producing a part of polymer matrix composite material, comprising a step of producing an initial dry preform, and a step of impregnating the dry preform with an impregnation polymer, for example by injection and/or infusion, to form a composite material part, the impregnation polymer constituting the polymer matrix of the composite material part.

The dry preforms are conventionally made manually from fabrics formed by weaving continuous unidirectional dry fibers, or from multiaxial fabrics or NCF (Non Crimp Fabric) fabrics comprising several superimposed plies of continuous unidirectional fibers held together by stitching. These preforms have a high manufacturing cost, and the wastage rates during the production of parts from such prefabricated fabrics are significant.

In order to automate the operation and to limit the wastage rate, it has been proposed to produce dry preforms by automatic lay up of continuous unidirectional fibers, the resulting preforms comprising several superimposed plies of continuous unidirectional fibers. The fibers can for example be applied contact application, by means of a roller, by a fiber placement process, each ply being formed by laying up on a mold of one or more joined bands, each formed of one or more flat fibers, ribbon type, such as carbon fibers consisting of a multitude of carbon threads or filaments. The dry preforms comprise a small amount of binder, generally less than 5%, to maintain the cohesion of the preform. However, this type of preform obtained by laying up superimposed plies may have insufficient permeability and cohesion between fibers to allow rapid and satisfactory impregnation, especially in the case of impregnation by RTM (Resin Transfer Molding) injection at high pressure and high temperature.

SUMMARY OF THE INVENTION

The purpose of embodiments of the present invention is to provide a method of manufacturing composite material parts that overcomes at least one of the aforementioned drawbacks.

To this end, the object of embodiments of the present invention is a process for producing a polymer matrix composite material part comprising a step of producing an initial dry preform, formed from continuous unidirectional dry fibers, preferably comprising a plurality of superposed plies, each formed from continuous unidirectional dry fibers, and a step of impregnating the dry preform with a first impregnation polymer, for example by injection and/or infusion, to form a part made of composite material, the impregnation polymer constituting the matrix of the composite material part, characterized in that it further comprises a step of applying non-woven fibers or filaments to a first main face of the dry preform, and a needling step of the filaments by means of a needling device comprising a plurality of needles, each equipped with at least one notch, so that the filaments are driven by the needles and arranged in a direction substantially perpendicular to the continuous fibers of the dry preform, the reinforced dry preform obtained after the needling step being subjected to the impregnation step.

The needles are driven in a back and forth motion by a driving system of the needling device, to cross the preform, preferably from one side to the other. During the movement of the needles in the direction of the preform, at least a portion of these filaments are positioned in the notches of the needles and are driven by the needles through the preform. These needled filaments positioned in the Z direction, in the thickness of the preform, stabilize the dry preform and make it possible to increase the cohesion of the dry preform for the subsequent impregnation step, and thus to avoid delamination of the preform during impregnation, especially in the case of injection of impregnation polymer. In addition, the perforations created by the needles and needled filaments positioned in Z increase the permeability in Z of the dry preform, in the thickness of the preform.

The impregnation is thus facilitated and can be carried out at high rates, without dry areas, and without deterioration of the preform. At the end of the impregnation step, the composite material part preferably comprises at least 30% by weight of impregnation polymer, and/or preferably at least 35% by volume of impregnation polymer, the impregnation polymer constituting the matrix of the composite material part.

Preferably, the composite material part according to the invention comprises at least 40% by volume of continuous unidirectional fibers, preferably at least 50% by volume, more preferably at least 55% by volume of continuous unidirectional fibers.

During the needling, the notches of the needles are filled by the filaments so that the unidirectional fibers are not or only slightly driven or damaged by the needles. In the case of preforms having continuous fibers with a principal orientation, the needles are arranged so that their notches cannot carry the continuous fibers of the preform, the notches being preferably arranged at 90° of the principal orientation of the fibers.

The continuous unidirectional fibers, preferably applied by fiber placement, are for example carbon fibers, glass fibers, synthetic fibers such as aramid fibers, polyethylene fibers, and/or natural fibers, as such for example flax fibers.

The method according to embodiments of the invention may be advantageously used for the production of composite material parts, particularly in the automotive or aeronautics sector.

According to one embodiment, the nonwoven filaments applied during the application step are formed from at least one second polymer, the impregnation step being carried out at a temperature below the melting temperature, and preferably greater than the glass transition temperature of the second polymer, so that the filaments ensure good holding of the fibers of the preform during the impregnation step, in particular its mechanical resistance to delamination.

The needling density is defined in particular according to the desired permeability and stabilization, and the type of needle used. According to one embodiment, the needling density during the needling step is between 10 and 350 punches/cm2. In the case of a needling device equipped with forked needles, the needling density is preferably from 150 to 350 punches/cm2, more preferably from 200 to 300 punches/cm2. In the case of a needling device provided with barbed needles, each comprising a plurality of notches, the needling density is preferably from 10 to 100 punches/cm2, more preferably from 40 to 60 punches/cm2.

The working part of the needles preferably has a diameter of between 0.30 and 0.60 mm, preferably between 0.40 and 0.50 mm (gauges 38 to 42).

The width and/or depth of the notches, preferably the width and the depth of the notches, are between 0.03 and 0.1 mm, preferably between 0.04 and 0.06 mm.

The length of the filaments is defined according to the thickness of the dry preform, preferably in such a way that the needled filaments pass through the preform. Furthermore, the needled filaments should preferably be long enough to remain entangled on the first face of the preform, and thus ensure better stability of the preform.

According to one embodiment, the filaments applied on the preform have a length of between 10 and 100 mm, preferably between 40 and 60 mm, and a diameter of between 5 and 50 μm, preferably 10 to 35 μm.

According to one embodiment, the needling is carried out so that filaments transferred by the needles protrude from the second main face of the preform which is opposite to the first main face on which the filaments have been applied, over a length between 1 and 10 mm, preferably 2 to 6 mm, some of these filaments being able to form loops protruding from the second face of the preform.

According to another embodiment, particularly in the case of a preform of large thickness, the method according to the invention comprises a step of applying nonwoven filaments to the second main face of the dry preform, and a step of needling the filaments present on the second main face. Preferably, the needling of the filaments present on the second main face and the needling of the filaments present on the first main face are made simultaneously.

According to one embodiment, the needling is performed by means of forked needles, preferably with a needle penetration depth of between 1 and 10 mm, preferably between 2 and 6 mm. The use of forked needles makes it possible to obtain an efficient needling of the filaments without deterioration of the fibers of the preform.

According to one embodiment, the step of filament application comprises the application of a nonwoven veil or felt formed of the filaments. The felts used may be isotropic, with randomly oriented filaments, or oriented felts with filaments having a preferred orientation. In the case of oriented felts, the needles, and more particularly their notches, are arranged according to the orientation of the filaments so as to optimize the take up rate of filaments by the needles. In the case of preforms having continuous fibers with a principal orientation, the oriented felt is advantageously applied to the preform so that the filaments form a non-zero angle with the continuous fibers of the principal orientation of the preform, for example an angle of 90°.

According to one embodiment, the felt has a surface mass of 5 to 100 g/m2, and/or the preform provided with filaments comprises from 1 to 10% by weight of filament, preferably from 2 to 5% by weight of filament.

One part of the filaments of the felt is transferred through the preform, the non-transferred filaments present on the first main face may be removed by peeling the felt or not.

According to one embodiment, the remainder of the felt is held on the preform for the impregnation step, the nonwoven structure of the felts increases the permeability of the preform and thus makes it possible to promote the infusion and/or injection, the felts conferring a draining effect, more particularly in the case of an infusion. Felt may further increase the impact strength of the preform and/or improve the surface appearance of the preform.

The impregnation step may for example be carried out according to a wet impregnation process, a vacuum infusion process, an LCM process, an RTM, HP-RTM, Gap-RTM or VARTM process. According to one embodiment, the impregnation step consists of a step of injecting the dry preform placed in an injection mold, the injection being carried out at a pressure of at least 10 bars, for example in the order of 50 bars.

Preferably, the method comprises producing a nonwoven preform. According to one embodiment, the method comprises the production of an initial dry preform, comprising several superimposed plies, the plies being formed from continuous unidirectional dry fibers provided with a binder, the binder comprising a third polymer, the impregnation step then preferably being carried out at a temperature higher than the glass transition temperature, and preferably below the melting temperature of the third polymer forming the binder. The dry preforms are obtained by application of dry fibers provided with a binder and/or by application of dry fibers without a binder, and application of the binder, for example by spraying a liquid binder and/or spraying a binder in the form of powder, on the application surface and/or dry fibers previously laid. The dry preform comprises less than 10% by weight of binder, preferably less than 5% by weight of binder.

The second and third polymers may be different or identical or of the same family.

According to one embodiment, the third polymer forming the binder and/or the second polymer forming the filaments is a thermoplastic polymer, preferably chosen from the group consisting of polyamides, especially aromatic polyamides (aramids), polyesters such as polyethylene terephthalate, polyethersulfones, polyetheretherketones, phenylene polysulfides, polyurethanes, epoxides, polyolefins, polylactic acid, polyacrylics, and mixtures thereof. In the case of a thermosetting polymer, the third thermosetting polymer and/or the second thermosetting polymer are for example selected from the group consisting of epoxides, polyesters, vinyl esters, phenolics, polyimides, bismaleimides, and mixtures thereof. According to other embodiments, the filaments applied to the preform are filaments of glass, carbon, or metal.

According to one embodiment, the dry preform is obtained by a fiber placement method, known per se. According to one embodiment, the production of the dry preform comprises the production of superimposed plies, by contact application, by means of an application roller, of continuous unidirectional fibers on a laying up tool, each ply being produced by application of one or more bands in one direction on the laying up tool or on bands of the previous ply, each band being formed of one or more fibers.

The fiber placement is advantageously automated by means of a fiber placement head, known per se, comprising a compacting roller intended to come into contact against the tool to apply a band formed of one or more continuous flat fibers, and a guiding system for guiding one or more fibers on the roller, by relative movement of the application head relative to the laying up surface along different trajectories. The preform comprises for example from 2 to 100 superimposed plies.

The continuous unidirectional fibers are preferably in the form of flat continuous unidirectional fibers, conventionally called tows, comprising a multitude of filaments. The fibers have for example widths of one-eighth of an inch, one-quarter inch or one-half inch (⅛″, ¼″ or ½″). As used herein, the term «fibers» also refers to fibers of greater width, greater than ½ inch, conventionally called tape in placement technology.

Alternatively, the fibers can be fixed to the mold, for example mechanically or by gluing, only at the beginning and at the end of the trajectory, the roller preferably being in contact at the beginning and at the end of the trajectory, and possibly at a distance from the surface on the rest of the trajectory.

According to another embodiment, the needling according to the invention is used as a replacement for the stitching operation performed during the manufacture of a multiaxial fabric NCF, the dry preform then comprises several superposed plies, without a binder, the needling of the filaments applied to the first main face of the preform is produced by mechanically holding the fibers of the plies of the preform, for example by a pin system around which the fibers pass.

According to one embodiment, the method comprises a step of thermoforming the reinforced dry preform obtained after the needling step, preferably between the male forming tool and the female forming tool of a press, to obtain a three-dimensional preform, the thermoforming being performed at a thermoforming temperature lower than the melting temperature of the second polymer, and preferably greater than the glass transition temperature of the second polymer, the three-dimensional preform obtained after thermoforming being subjected to the impregnation step. Alternatively, the forming is performed by means of a vacuum cover, after positioning the preform on a forming tool.

In the absence of needling, the deformation of the preform during forming is performed fiber by fiber. The needling operation of the preform according to the invention prior to the forming operation makes it possible to obtain a homogeneous deformation of the preform, due to the sliding of the needled filaments.

Furthermore, the filaments of the felt present on each main face of the preform allow the preform to slide in the press without adhesion. The outer filaments stabilize the outer surfaces of the preform by limiting, or even eliminating, local deformations of the preform and thus ensure a homogeneous deformation of the fibers of the preform. The method according to the invention thus makes it possible to promote the sliding mechanisms between the forming tools and the preform, and thus to limit, or even eliminate, the fiber defects of the outer plies of the preforms resulting from the friction between the preform and the forming tools in previous forming processes.

The forming is preferably carried out at a forming temperature lower than the melting temperature of the third polymer forming the binder, and preferably greater than the glass transition temperature of said third polymer. The forming is carried out with heat, the initial preform being heated to a forming temperature before and/or during forming in order to increase the formability of the preform. The initial preform may be preheated prior to forming by passing through an oven or tunnel and/or the preform may be heated during forming by heating the male forming tool and/or the female forming tool. Preferably, the preform is only preheated by passage in an oven or tunnel, the tools of the press not being heated, thus simplifying the tools of the press.

The initial preform can be two-dimensional, obtained by laying up on the flat surface of a tool, or three-dimensional, the forming can then be carried out to obtain three-dimensional parts of a complex form that cannot be obtained by laying up, particularly by fiber placement.

