PROCESS FOR MOLDING OF COMPOSITE PIECES

TECHNICAL FIELD

The present invention relates to a process for molding of composite pieces (or products) based on organic materials and fibrous reinforcements, and relates to a device for implementation of the process as well as to the products which are obtained.

BACKGROUND ART

In the molding processes of the compression molding type, the materials to be molded are positioned between a mold and a countermold, and these molds upon closing compress these materials leading to their distribution in the mold, particularly by flow of the organic material. A number of variants exist, depending in particular on the type of material loaded in the mold and the type of pieces which are to be obtained. Up to now, the materials most commonly used are composites based on thermosetting organic material (with a low melt viscosity allowing suitable flow and filling of the mold) and based on fibrous reinforcements (such as glass fibers), these composites generally being present in the form of prepregs in the form of sheets ready for use commonly called SMCs, the pieces produced being car body pieces in particular.

The qualities sought for these types of products are generally good mechanical properties as well as a nice surface appearance. Even if long practice in the domain of thermosetting composites has resulted in specific processes which satisfactorily meet these needs, the use of new generations of composites based on thermoplastic materials with different working characteristics (different melt viscosity, different surface tensions...), in particular of composites based on co-mixed strands (in which the thermoplastic organic material is present in the form of filaments intimately mixed with glass filaments) such as those marketed under the commercial name Twintex® by the company Saint-Gobain Vetrotex France, has instigated the development of new more specific processes.

DISCLOSURE OF INVENTION The main problem that the present invention aims to solve is to eliminate the surface defects that can appear during molding, particularly by compression, of this type of thermoplastic composites (possibly less flowable, more difficult to paint, with a less suitable surface appearance and/or a less homogeneous surface appearance, etc., than the

thermosetting composites), without damaging the other desired properties (particularly the mechanical properties). In particular, the present invention seeks to directly obtain painted pieces made of thermoplastic composites with a satisfactory appearance (particularly without going through a repair operation on the molded product which requires application of a primer).

This problem is solved by the process for molding of composite products according to the invention which includes the following steps:

- at least one composite structure is introduced into a mold,

- the mold is closed, and the structure is shaped hot and under pressure, - a flexible layer and then a paint are successively injected,

- the composite product obtained is removed from the mold.

In the process according to the invention, the injected flexible layer is compressed at the time of the injection of the paint in order to allow passage of the paint, and when this injection is finished, it regains its shape, thus laying the paint on the parts facing the mold, and consequently making it possible to obtain a smooth surface at the site of the paint, the flexible layer simultaneously absorbing the undulations and porosities of the composite structure(s) forming the core of the product which is obtained. The process according to the invention thus enables one to obtain a new line of molded articles of different sizes and varied shapes of varying complexity with a particularly satisfactory surface appearance.

The present invention also relates to a device for manufacturing of a molded product, suitable for implementation of the process according to the invention, which includes:

- at least one mold formed by a mold (or male mold part) and a countermold (or cavity or female mold part);

- at least one component for injection of a flexible layer (coming, for example, from a low-pressure or high-pressure bi-injection machine);

- at least one component for injection of a paint (which can be combined with or identical or not to the preceding injection component). The present invention also relates to the molded products (composite products) obtained which are formed by at least one composite core covered at least in part (generally on one side, for example, on the side exposed during opening of the mold) by at least one flexible layer which is itself covered by at least one paint.

DETAILED DESCRIPTION AND PREFERRED EMBODIMENTS OF THE INVENTION

The aforementioned composite structure, composite core and composite product (or piece or article) according to the invention are more particularly thermoplastic composites, formed by at least one thermoplastic organic material (chosen, for example, from the polyolefins, particularly polyethylene (PE) or polypropylene (PP), the thermoplastic polyesters, particularly polyethylene terephthalate or polybutylene terephthalate, the elastomers, particularly an ethylene-propylene polymer (EPDM), polyvinyl chloride (PVC), the polyamides, polyphenylene sulfide, etc.) and by at least one material for reinforcement of this thermoplastic organic material (generally chosen from the known natural or synthetic, mineral or plant, reinforcing materials, in particular from materials such as glass, carbon, aramid, etc.), this reinforcing material preferably being glass.

The reinforcing material is generally present at least in part in the form of fibers (strands and/or filaments, the filaments being of very small diameter, for example, on the order of 5-24 microns in diameter in the case of glass, and the strands being generally formed by a number of filaments assembled together), with it possible for these fibers to be chopped or continuous, dispersed or assembled (in particular, in the case of the continuous fibers, assembled in the form of parallel strips or networks of interlaced fibers). Possibly, the aforementioned composite structure, core and/or product can contain additional reinforcements in other forms (such as glass spheres) and/or additional loads (generally mineral loads) (such as calcium carbonate, talc, kaolin, micas, aluminum hydrate, etc.). Preferably, the percentage of reinforcements in the structure or the composite core is on the order of 50-80 wt% of the structure or of the core, and the percentage of loads (used for improving the dimensional stability, for example) is on the order of 0-20 wt% (of the structure or of the core, respectively), the overall percentage of reinforcements and loads not exceeding 80 wt%.

Preferably, the composite structure used contains at least some continuous reinforcing fibers (in particular, continuous reinforcing strands and/or continuous composite strands as mentioned hereafter) and at least one material capable of flow (for example, a thermoplastic organic material, possibly loaded with discontinuous reinforcements or loads, for example, made of chopped reinforcing strands), the material capable of flow being combined with and/or incorporating (or serving as matrix for) said

continuous reinforcing fibers (and/or other fibers and/or other reinforcements and/or other loads if necessary).

According to an advantageous embodiment, the composite structure is formed by or by using mixed strands obtained by simultaneous joining and winding of organic fibers and reinforcing fibers, and preferably said co-mixed or composite strands, composed of organic filaments and reinforcing filaments mixed within the strands, these strands being generally obtained by assembling of filaments directly during their manufacturing (as described, for example, in the patents EP 0 599 695, EP 0 616 055, EP 0 367 661 and WO 98/01751 in which, during drawing of the glass filaments, the organic filaments are simultaneously extruded and carried along, the paths followed by the filaments converging before said filaments are assembled together in the form of at least one composite strand). The most common composite strands consisting of intimately mixed glass filaments (in percentages of 60-75 wt%) and filaments of thermoplastic organic material are marketed under the commercial name Twintex® by the company Saint-Gobain Vetrolex France. The melting of the organic material is facilitated by the use of these co-mixed strands, making possible a gain in productivity, with it possible for the products obtained to be more homogeneous and to have improved mechanical properties in comparison with the traditional products, and for the orientation of the reinforcing strands in the finished product also to be better controlled. The composite structure can be continuous or not, homogeneous or not, formed by one or more parts or layers which are, if necessary, initially separated, etc. It can in particular contain or be present in the form of one or more networks of strands, of the type of fabric(s) or knit(s) or non-woven(s) (chopped strand or continuous strand mat(s), strips of parallel strands crossed and/or connected together by connecting strands and/or filaments made of glass or polyester or polyamide, etc.), with it possible for this network or these networks if necessary to have a preferential direction of orientation of the strands or of the reinforcing material, the strands being generally at least in part reinforcing strands, possibly impregnated or composite. This structure can also contain or be present in the form of one or more continuous or solid or consolidated layers (layers in which, in general, the organic material has been melted in order to form a matrix in which the reinforcing fibers are buried and/or layers which are assembled, for example, hot and under pressure), for example, in the form of slab(s) or sheet(s), stack(s) of sheet(s) or sandwich(es).

When the structure is formed of several layers (connected or separated at the time of their insertion in the mold), different types of layers can be mixed, for example, composite layers and/or layers of reinforcing strands (with it possible for the strands to be continuous and/or chopped depending on the layers), and layers of organic material such as polypropylene films, etc., with it possible for the composite structure to be deposited in one or more parts (a single layer or multi-layers of each), which are flexible (fabrics, etc.) or rigid (pre-forms, slabs, based, if necessary, on consolidated fabrics), for example, in the form of strips, mats, slabs, pre-forms, sheets, etc., with it possible for the parts to be juxtaposed and/or superposed, of identical or non-identical dimensions, shape and/or composition (with it possible in particular to have different percentages of reinforcement, etc.), the number, dimensions, cutting (simple or complex) and the arrangement of these parts depending on the characteristics one wishes to obtain, differentiated or not according to the zones, in combination if necessary with other elements such as inserts (for example, metallic inserts, etc.). If necessary, depending on the complexity of the piece to be produced, a step of pre-forming of the composite structure (or of a part of the structure) can be performed before packing of the mold (followed, if necessary, by additional heating of the pre-form obtained, before introduction into the mold), by drawing, for example, with it possible for the pre-forming to be total or localized. In the process according to the present invention, the composite structure (in one or more parts, and generally in the form of at least one slab or sheet or pre-form, said structure having one or more cuts if necessary, suited to the shape that one wishes to obtain) is arranged on the molding part of the mold (for example, on the male mold part), which it covers completely or partly and from which it can project if necessary, depending on the type of structure used (greater coverage in the case of fabrics of composite strands, less coverage in the case of a structure allowing more flow, etc.), in particular, depending on its ability to drape itself and to flow, and depending on the piece which is to be obtained (reinforced uniformly or in a localized manner, complex or simple, etc.).

In one embodiment of the invention, the step of packing of the mold is preceded by a step of preliminary heating of the composite structure (for example, to the temperature of softening of the organic material) particularly so as to give said structure a certain flexibility and to make it more capable of draping the mold, and so as to facilitate the later molding. For example, in the case of a structure based on polypropylene and glass fibers,

the composite structure can be heated beforehand so that it has a temperature of approximately 180-280 ⁰ C (356-536 ⁰ F) at the time of its introduction into the mold and/or at the time of closing of the mold.

If necessary, at least the parts of the mold intended for receiving the composite structure can be treated beforehand, before positioning or draping of said structure, with at least one product capable of aiding in the later removal of the molded piece from the mold (in particular, the parts on the opposite side of the piece from that which receives the paint).

The mold in which the molding is to take place reproduces the desired shape and is traditionally formed by at least two parts (mold and countermold, or male and female mold part, or shells), which are generally metallic (made of aluminum, steel, alloy, etc.), these parts being separated and/or articulated (for example, around hinges) and traditionally mounted on a press. The mold is generally heated and/or advantageously temperature regulated so as to allow the shaping of the piece while at the same time avoiding sudden cooling of the material (temperature too low) and sticking during removal from the mold (temperature too high), the mold, for example, being regulated at a temperature on the order of 60-100 ⁰ C (140-212 ⁰ F) for a composite structure based on polypropylene and glass strands.

Before and/or after positioning of the composite structure (which is generally pre- heated and introduced into the mold while its temperature is still higher than that of the mold, in particular at least 100 ⁰ C (212 ⁰ F) higher) in the mold, and before the actual molding operation, it is possible as necessary to add additional equipment and/or materials, for example, inserts (made of metal, alloy, composite material, etc.), and/or at least one flowable material (in particular made of thermoplastic, possibly loaded with discontinuous reinforcements or loads, for example, chopped reinforcing strands, this flowable material coming, for example, from granules such as long fiber granules, granules concentrated and/or loaded with additives, etc.) and/or one or more additives (pigments, lubricants, plasticizer, etc.), this material or these additives for example coming particularly from an extruder and/or being added by injection before closing and/or after closing of the mold.

After packing of the mold with at least the composite structure and possibly the aforementioned additional equipment and/or materials, the actual molding operation is carried out, generally hot (with regulated temperature of heating of the mold as mentioned

in the preceding), and under pressure, generally after closing of the mold, for example, according to one of the following methods:

- molding of (thermo)compression or by extrusion-compression type, at pressures on the order of 50-300 bar, for example; - molding of (co-)injection type, at pressures on the order of 150-350 bar, for example;

- molding of co-injection/compression type, the injection taking place if necessary before closing of the mold at pressures on the order of 100-150 bar, and the molding after closing of the mold carried out, for example, at pressures on the order of 200-300 bar. On a non-limiting basis, molding by (thermo)compression is particularly suitable when using a composite structure which is rather rigid and/or complex (formed from several types of different associated layers), for example, in the form of a slab, with a thickness between 2 and 5 mm, obtained by heating and rolling of one or more networks (such as a fabric or a mat) of continuous composite strands (for example, glass/polypropylene) and/or of continuous reinforcing strands impregnated with thermoplastic material, combined if necessary with one or more thermoplastic films and/or with one or more layers containing reinforcements in the form of chopped strands (for example, a chopped strand mat), etc. Molding by extrusion-compression is particularly suitable for the manufacturing of large pieces from a rather rigid but not complex composite structure, for example, a composite structure in the form of a slab obtained by heating and rolling of one or more networks as mentioned in the preceding, a material capable of flow loaded and/or provided with additive(s) being then added by extrusion before closing of the mold and/or carrying out of the compression molding.

Molding by (co-)injection is particularly suitable for the manufacturing of small pieces from a composite structure which is generally not complex, which is, for example, introduced into the mold in the form of a pre-form obtained by pre-forming of composite strand fabric(s) and possibly heated before introduction into the mold, at least one flowable material which is loaded (coming, for example, from so-called long fiber granules) and/or provided with additives, which is then added by injection after closing of the mold. In molding by co-injection/compression, injection takes place in the partially open mold at lower pressure in order to avoid shifting of the pre-form, for example, the mold being re-closed for carrying out the compression molding.

The duration of heating and molding generally depends in particular on the thickness of the piece, on the composite structure used and on the flow necessary for forming details of the piece.

The injection of the flexible layer and of the paint advantageously occurring in this stage of the process according to the present invention can also take place according to different methods, in particular;

- in a first advantageous embodiment, at the end of the actual molding operation (shaping of the structure hot and under pressure), with the molded piece at the temperature (of heating) of the mold, with the mold still closed (but with relaxation of the closing force if necessary), the flexible layer and then the paint are successively injected at high pressure (pressure greater than 50 bar, for example, on the order of 150-300 bar at the site of mold);

- in another embodiment, at the end of the actual molding operation, the mold is partially re-opened (opening on the order of 1 to 3/10 mm), the flexible layer is injected at low pressure (on the order of 10-50 bar), the mold is re-closed in order to spread the flexible layer (with the low pressure maintained) and is then re-opened so as to inject the paint this time (still at low pressure on the order of 10-50 bar) before final closing in order in particular to complete the distribution and/or solidification of the paint (by compression and at the temperature of the mold and of the composite structure). The injection takes place using an injection needle through an opening made in the mold cavity at a site which is generally not visible or which is intended for being hidden on the final piece. The flexible layer is compressed at the time of injection of the paint and then in swelling coats the paint on the walls inside of the mold as explained in the preceding. The flexible (or compressible) layer (or underlayer or covering) is formed by a material which reacts (for example, which hardens or vulcanizes or polymerizes or sets or undergoes expansion, etc...) generally at the temperature of the mold, this layer being capable of being compressed during injection of the paint in order to allow passage of said paint, this layer having or keeping flexibility (or remaining compressible or keeping its elastic or elastomeric property) at the temperature of the mold (and if necessary after reaction, particularly in the solid state). Preferably, this flexible layer is characterized (in particular at the temperature of the mold and/or in the solid state) by a low Shore A hardness (measured according to the standard NF ISO 48), which is characterized in

particular at the temperature of the mold by a Shore A hardness less than 15 DIDC (hardness in international rubber hardness degrees), and at room temperature (solid state) by a Shore A hardness less than 50 DIDC, and in particular between 10 and 40 DIDC. The material of the layer is also preferably chosen so that it can adhere correctly (compatibility in terms of chemical adhesion in particular) to the composite structure after its addition to the heated mold. This material is generally injected in the pasty or liquid state and is spread (particularly by compression) in the form of a layer of small thickness (for example, on the order of 0.1-0.3 mm), and in general solidifies rapidly (practically instantaneously in particular, in the space of a few seconds, at the temperature of the mold, for example, 80-100 ⁰ C (176-212 ⁰ F)).

The material of the flexible layer can in particular be a polyvinyl chloride (PVC) (injected in emulsion at a temperature on the order of 20-30 ⁰ C (68-86 ⁰ F), for example) or an ethylene vinyl acetate or a polyurethane or an elastomer such as a thermoplastic elastomer (injected hot, for example, at a temperature on the order of 150-200 ⁰ C (302°- 392 ⁰ F), with it possible for this thermoplastic elastomer to be functionalized by addition of a peroxide, etc., for example), or another compressible material or a foam or foaming/expansible layer keeping flexibility after expansion, etc. As examples, it is possible to mention in particular the ethyl ene-vinyl alcohol copolymers or copolymers based on vinylidene chloride or the plasticols (PVC in emulsion) such as those marketed under the brand "Soarnol" by the company Nippon Gohsei, and/or under the brand "Ixan" by the company Solvin France and/or under the brand "Vinylcolors" or "vinylcolore" by the company Astracolorants (plastisols with a low Shore A hardness in particular between 10 and 30 DIDC), or the ionomer resins marketed under the brand "Surlyn" by the company Nippon Gohsei or under the brand "iotek" by the company Exxon Mobil chemical, etc.

The paint is chosen so as to react (harden, crosslink) at a temperature generally less than or equal to that of the mold and greater than room temperature (for example, a temperature on the order of 50-100 ⁰ C (122-212 ⁰ F)). This paint is generally injected (at a temperature, for example, on the order of 30-50 ⁰ C (86-122°F)in order to be pre-heated without initiating crosslinking) in the form of a solution or dispersion and is spread

(particularly by compression) in the form of a layer of very small thickness (for example, on the order of 5 to a few tens of microns, in particular 5-20 μm) and in general solidifies in a few seconds or tens of seconds (at the temperature of the mold, for example, between

80 and 100 ⁰ C (176-212 ⁰ F)). The paint can be rigid or flexible (that is, accepting slight deformations of the piece without cracking or fissuring) and can have particular properties, conferred if necessary by one or more additives or loads which it contains (for example, an additive improving the resistance to hydrocarbons or to ultraviolet, etc.). Advantageously, it can be a two-component paint (the two components reacting at the aforementioned temperature), but it can also be a single component paint, for example, with a catalyst (which is locked and can be unlocked with heat and possibly under pressure). As examples, the paint can be a water [based] paint or an acrylic or vinyl paint or a paint based on at least one polyolefm (polyethylene, polypropylene, etc.), generally in emulsion [form], or a modified paint or one based on an elastomer (for example, a flexible paint modified by a elastomer capable of crosslinking and/or based on a modified polypropylene styrene-ethylene-butadiene-styrene, etc.) or based on a polyvinyl chloride (for example, a plastisol), or based on at least one other thermoplastic or thermosetting organic material, generally in emulsion, etc. Preferably, it is a two-component paint of polyurethane or acrylic or polyester type.

The paint is preferably injected onto the flexible layer while it is sufficiently hardened so as not to be shifted or flushed out by the injection of the paint while remaining sufficiently adherent or adhesive on the surface to allow suitable bonding of the paint (the time between the injection of the flexible layer and the paint being generally on the order of a few seconds), just as the flexible layer is generally injected onto the molded piece partly stiffened but still tacky (adhesive) on the surface in the mold so as to allow better adhesion of the different parts to one another.

After injection of the paint and closing of the mold if necessary in order to complete the molding, the composite product obtained can be removed from the mold, generally hot (in particular, at the temperature of the mold), after re-opening of the mold, and is trimmed as needed (in particular in the case of molding operations by co- injection/compression or extrusion/compression) and/or cut to its final dimensions and/or is subjected to other processing operations such as perforation(s), addition of insert(s), etc. Generally, the molded or composite product obtained according to the invention contains a percentage of reinforcing material(s) (or reinforcements) between 20 and 75 wt%, with it possible for the concentration in terms of reinforcements to vary depending on the sites of the composite.

Implementation of the process according to the invention is easy and fast; it enables one to directly obtain reinforced painted pieces with shapes of varying complexity and of varying dimensions in a single molding operation. The composite which is formed has a suitable cohesiveness between the different elements composing it and suitable mechanical strength properties, as well as an appearance without surface defects and which is suitably resistant to scratching (with the resistance to scaling of the paint also improved owing in particular to the addition of the flexible layer). The flexible layer in particular absorbs the possible undulations of the composite structure (fabric meshwork, etc.), the average roughness of the finished product being advantageously less than 10 μm, and in particular less than 5 μm (suitable roughness in particular for application and automobile class products).

The composite piece obtained by the process according to the invention can constitute the finished product or form just a part of it, and can undergo other processing operations for the purpose of obtaining the final product (cutting, etc.) The products obtained according to the invention can be different types of products such as seats, machine tool casings, car bodies, etc.