FR3075086A1 - METHOD FOR MANUFACTURING A STRUCTURAL PART FOR A MOTOR VEHICLE AND STRUCTURAL PART THEREFROM - Google Patents

Abstract

 The present invention relates to a method for manufacturing a structural part for a motor vehicle, comprising the following steps:  - Stacking in a mold (26) at least a first mat (12) and a spacer (16) in foam, the first mat (12) comprising at least one veil of fibers and a resin containing a liquid impregnating the fibers, and  - Heating the stack to solidify the resin.  Before the heating of the stack, the mold (26) is hermetically closed. At least part of the resin evaporates to generate pressure within the foam spacer (16) when the mold (26) is hermetically closed.

Description

Method of manufacturing a structural part for a motor vehicle and structural part resulting from such a method

The present invention relates to a method of manufacturing a structural part for a motor vehicle comprising the following steps:

stacking in a mold at least a first mat and a foam spacer, the first mat comprising at least one veil of fibers and a resin containing a liquid impregnating the fibers, and

- heating of the stack to solidify the resin.

For example, the structural part forms a panel of a motor vehicle, in particular a rear trunk masking shelf, a false interior or trunk floor, an under-engine or a trunk loading floor.

The specifications generally impose strict specifications for such a structural part. In particular, the structural part must have the lowest possible mass, good mechanical characteristics, mainly good bending and creep behavior at high temperature (in particular at 85 ° C.). The part must be suitable for presenting three-dimensional shapes, for example concave shapes allowing the inclusion of complementary parts such as handles, and / or for presenting edges having particular profiles, in particular rounded or curved. In addition, they must maintain good behavior in the face of different climatic conditions, especially in wet conditions.

Finally, such a structural part must remain economical, so that its manufacturing process must be particularly simple and require little investment. Ideally, the process should have only one pressing step requiring only one mold.

Structural pieces of the “sandwich” type produced in a single pressing step, comprising a spacer, generally made of honeycomb essentially of cardboard, and skins based on natural fiber mats (wood, hemp, flax tow, kénaf) and resin, generally do not have satisfactory mechanical characteristics, in particular as regards the behavior in bending and under certain climatic conditions, in particular in wet conditions.

The use of a foam as a spacer is particularly advantageous compared to cardboard spacers because it makes it possible to obtain low thicknesses and densities while maintaining good mechanical strength.

However, to make a sandwich type panel having a foam as a spacer, it is necessary to exert significant pressure when the mold is closed to densify the skins and associate them perfectly with the spacer.

This pressure often leads to the crushing of the foam, even when a so-called rigid foam is used.

For all these reasons, the part comprising a foam spacer is not produced in a single step and repeats must be carried out. This generates significant costs as well as a significant lengthening of cycle times.

In patent application US 6,287,678 B1, a composite panel comprising a thermoplastic foam covered on each side by natural fibers impregnated with a thermoplastic material is produced by molding during a two-step process.

However, such a method is not entirely satisfactory. Thus, several successive stages of training are necessary, which involves logistics and significant costs.

An object of the invention is to provide a method of manufacturing a structural part comprising a foam spacer which offers a reduced manufacturing cost, while respecting the characteristics imposed by the specifications, in particular in terms of thickness, mass and mechanical properties.

To this end, the subject of the invention is a method of manufacturing a structural part, in particular for a motor vehicle, characterized by the following steps:

- before heating of the stack, hermetically closing the mold, and

- evaporation of at least part of the resin to generate pressure within the foam spacer when the mold is closed hermetically.

The method according to the invention may include one or more of the following characteristics, taken alone or in any technically possible combination:

- Before closing the mold, the stacking in the mold of a second mat on the spacer, the spacer being disposed between the first mat and the second mat;

- It includes the following preliminary steps: supply of a veil of fibers, and impregnation of the veil with liquid resin to form a mat impregnated with the resin;

the resin contains a liquid, in particular between 30% by mass and 60% by mass of liquid, the evaporation of at least part of the resin comprising the formation of gas from the liquid contained in the resin;

- the liquid contained in the resin is water;

the liquid resin is an acrylic resin;

- the spacer is made from PET foam;

the fibers include natural fibers taken from flax, sisal, jute, hemp, kenaf fibers, and their mixtures;

- The stack is heated to a temperature above 150 ° C and advantageously between 160 ° C and 200 ° C;

- the pressure exerted on the stack when the mold is closed is less than 0.5 MPa;

- The thickness of the stack before closing the mold is strictly less than or equal to the thickness of the mold cavity of said mold, the foam spacer undergoing expansion during the evaporation of at least partial resin; and

- the retractor has no blowing agents.

The invention also relates to a structural part capable of being obtained by a process as described above.

The invention will be better understood on reading the description which follows, given solely by way of example and made with reference to the appended drawings in which:

- Figure 1 is a sectional view of a structural part manufactured according to an embodiment of the invention;

- Figure 2 is a schematic sectional view of the stacking step according to an embodiment of the invention;

- Figure 3 is a schematic sectional view of a first mat according to the invention.

FIG. 1 illustrates a composite part 10 of a motor vehicle. The part 10 is a structuring part of the motor vehicle, for example a panel, such as a rear trunk masking shelf, a false cabin or trunk floor, an under-engine or a trunk loading floor.

The part 10 is a “sandwich” type panel which comprises at least a first mat 12 and a spacer 16. In a preferred embodiment, the part 10 comprises a first mat 12, a second mat 14 and a spacer 16 interposed between the two mats 12, 14. More particularly, the first mat 12 and the second mat 14 are fixed on opposite faces 18A, 18B of the spacer 16.

At least one of the first and second masts 12,14 comprises at least one veil 20 of fibers 22, said veil 20 comprising a plurality of parallel fibers 22 linked together by a solidified resin 24.

At least a portion of the fibers 22 of this veil 20 advantageously has a length greater than 20 centimeters, in particular 50 centimeters, for example between 50 and 80 centimeters.

More particularly, the first mat 12 and the second mat 14 each comprise a plurality of webs 20 of fibers 22, these webs 20 being stacked on each other. For example, each of the masts 12, 14 comprises between three and eight sails 20 stacked.

Note that the first mast 12 and the second mast 14 may have the same number of sails 20 stacked, or alternatively different numbers of sails 20 stacked. Indeed, according to the destination function of the structural part 10, the two faces of this structural part 10 are not subjected for example to the same constraints, so that it is not necessary that these two faces have the same characteristics structural. Thus, a face of the structural part 10 which is subjected to few constraints could have only a small number of sails 20. The reduction in the number of sails 20 on one face makes it possible to reduce the mass of the structural part 10.

Preferably, at least a portion of the parallel fibers 22 of each web 20 are natural fibers 22. In one embodiment, all of the fibers 22 of at least one of the webs 20 are made of long natural fibers 22. As a variant, part of the fibers 22 of at least one of the webs 20 is formed by artificial or synthetic fibers 22, distinct from the natural long fibers 22, or by a mixture of these fibers 22.

Advantageously, natural fibers are fibers extracted from plants. Preferably, the natural fibers are chosen from flax, sisal, jute, hemp, kenaf fibers, and their mixtures. Advantageously, the fibers are flax fibers.

The artificial fibers are for example chosen from regenerated cellulose fibers, such as viscose.

Synthetic fibers are formed from petroleum derivatives or molecules from green chemistry (for example ethylene from bio ethanol). Mention may be made, among the fibers suitable for the present process, of polyolefin fibers such as polyethylene or / and polypropylene, polyester, polyamide, polyimide fibers, and their mixtures. They can also be bicomponent fibers formed from a polymer and a copolymer, the polymer and its copolymer having different melting points. The synthetic fibers chosen are preferably based on thermoplastic polymers, which makes it possible, during a thermoforming step at the melting point of the polymer, to bind the flax fibers.

Advantageously, the mass proportion of long fibers of natural origin in each veil 20 is greater than 50% of the total mass of the fibers 22 of this veil

20.

Advantageously, the fibers 22 of the web 20 are parallel to a longitudinal direction. Preferably, the veil 20 used in the process of the invention comprises fibers 22 perfectly parallelized (the parallelism being further accentuated by the fact that the natural fibers 22 such as flax or hemp have very little crimp). The transverse cohesion of this type of veil 20 is possible thanks to the impregnation of a resin 24.

The resin 24 impregnating the fibers 22 initially comprises, before solidification, a liquid which is advantageously water. Preferably, the resin 24 comprises between 30% by mass and 60% by mass of liquid. Advantageously, it comprises between 30% by mass and 60% by mass of water.

Preferably, the resin 24 is chosen from acrylic resins, that is to say resins in aqueous solution, in particular resins comprising an acrylic polymer such as a polymer or a copolymer of acrylate or methacrylate.

This type of resins generates water during its crosslinking.

In one embodiment, at least the first mat 12 is produced by supplying a veil 20 of fibers 22 and impregnating said veil 20 with liquid resin 24.

Said impregnation step can be carried out in different ways, such as by spraying the resin 24 on the web 20 or by coating by contact the resin 24 on the web 20.

The spacer 16 is made of porous foam. The foam is preferably a foam comprising open cells. It advantageously comprises a porosity of between 0.88 and 0.92, the porosity being measured according to the method described in the article Pressure / mass method to measure open porosity of porous solids. Y Salissou and R Panneton. Journal of Applied Physics 101, 124913 (2007).

Preferably, the foam has a Young's modulus in the thickness direction between 15 MPa and 25 MPa, preferably between 17 and 24 MPa, advantageously between 20 MPa and 22 MPa, the Young's modulus being measured according to ISO 18437-4: 2008.

Advantageously, the foam is chosen from polyester foams, preferably from foams based on semi-aromatic co-polyesters. Advantageously, the foam is a foam based on poly (ethylene terephthalate), also called PET foam.

Preferably, the spacer 16 is devoid of additional additives, such as blowing agents. It is, for example, free of halogenated hydrocarbons such as trichlorofluoromethane (Freon 11).

Furthermore, at least one of the faces of the structural part 10 can optionally be provided with a facing (not shown here) giving its aesthetic appearance to the structural part 10.

A method of manufacturing the structural part 10 is implemented in a mold 26, illustrated diagrammatically in FIG. 2.

The mold 26 comprises two half-molds 28A, 28B movable relative to one another between an open position and a closed position.

The two half-molds 28A, 28B define a molding cavity 30. Advantageously, the molding cavity 30 in the closed position is combined with the shape of the structural part 10 at the end of the manufacturing process.

The mold 26 has at least one closing mechanism (not shown here) capable of passing the mold into the closed position and of keeping it closed hermetically.

Advantageously, the molding cavity 30 has a thickness equal to or greater than the thickness of the stack comprising the first mat 12, the spacer 16 and possibly the second mat 14 when the stack comprises two mats.

The manufacturing process will now be described. This method comprises a step of stacking in the mold 26 at least a first mat 12 and a spacer 16. Advantageously, a second mat 14 is disposed on the free face of the spacer 16, the spacer 16 being thus arranged between the first mat 12 and the second mat 14.

Then the mold 26 is passed into its closed position. When the mold 26 is closed, the pressure exerted on the stack by the walls of the mold 26 is less than 0.5 MPa (Megapascals).

The method then includes a step of heating the stack. This step takes place after the mold 26 is closed hermetically. The heating of the stack in the mold 26 is carried out using a thermal fluid.

Heating the stack allows the resin 24 to solidify and crosslink. The heating of the stack also causes the evaporation of at least part of the liquid included in the resin 24 and generated by the crosslinking. Advantageously, all of the liquid present in the resin 24 after crosslinking evaporates.

Preferably, the stack is heated to a temperature above 150 ° C and advantageously between 160 ° C and 1200 ° C.

The gas generated by heating the stack is unable to leave the hermetically sealed mold 26 and therefore migrates inside the foam spacer 16, in particular in the pores of the foam. The foam spacer 16 then expands.

The pressure in the mold is then greater than 0.5 MPa and is in particular between 1.0 MPa and 1.5 MPa.

In an embodiment where the initial molding cavity 30 is of the same thickness as the initial stack, the spacer 16 exerts pressure on the mats 12, 14 against the walls of the mold resulting in the densification of the mats 12, 14. In the case where the mats 12, 14 comprise an excess of resin 24, the excess resin migrates towards the spacer 16 which ensures better bonding.

Evaporation of the water present in the masts as well as of a part of the resin 24 in the hermetic mold 26 thus generates a back pressure within the spacer 16 which opposes the crushing force . It is therefore not necessary to exert significant pressure on the spacer 16 when the mold 26 is closed in order to associate the spacer 16 and the mats 12, 14 and manufacture the structural part 10.

Advantageously, the pressure exerted on the stack when the mold 26 is closed is less than 0.5 MPa.

In an alternative embodiment where the thickness of the initial molding cavity 30 is greater than the thickness of the initial stack, the spacer 16 undergoes a first phase during which it undergoes expansion until the stack is of the same thickness as the molding cavity 30, then it exerts pressure on each of the mats 12, 14 against the walls of the mold 26 causing the densification of the mats 12, 14.

For example, it is possible to obtain from a stack of a first mat 12, a spacer 16 and a second mat 14 a structural part 10 having a thickness at least 20% greater, in particular at minus 30% greater than the thickness of the initial stack.

Thanks to the present invention, it is possible to manufacture structural parts 10 for a vehicle in a single manufacturing step. It is not necessary to exert significant pressure on the stack to densify the mats 12, 14. This avoids the crushing of the foam spacer 16 which therefore retains its integrity and its mechanical strength.

It is not necessary to use potentially polluting blowing agents in the foam spacer 16, the expansion being generated directly by the evaporation of at least part of the resin 24.

The method according to the invention also makes it possible to manufacture structural parts 10 for motor vehicles of variable thickness from the same spacer 16 by choosing molds 26 of different molding cavities 30.

Claims (13)

1. - Method for manufacturing a structural part (10) for a motor vehicle comprising the following steps: - Stacking in a mold (26) at least one first mat (12) and a spacer (16) in foam, the first mat (12) comprising at least one veil (20) of fibers (22) and one resin (24) containing a liquid impregnating the fibers (22); - heating the stack to solidify the resin (24); characterized by the following stages: - before heating of the stack, hermetically closing the mold (26), and - Evaporation of at least part of the resin (24) to generate pressure within the foam spacer (16) when the mold (26) is hermetically closed. 2. - Method according to claim 1, comprising before closing the mold (26), stacking in the mold (26) a second mat (14) on the spacer (16), the spacer (16) being disposed between the first mat (12) and the second mat (14). 3. - Method according to any one of the preceding claims comprising the following preliminary steps: - supply of a veil (20) of fibers (22), and - impregnation of the web (20) with the liquid resin (24) to form a mat (12; 14) impregnated with the resin (24). 4. - Method according to any one of the preceding claims wherein the resin (24) contains a liquid, in particular between 30% by mass and 60% by mass of liquid, the evaporation of at least part of the resin ( 24) comprising the formation of gas from the liquid contained in the resin (24). 5. - Method according to claim 4, wherein the liquid contained in the resin (24) is water. 6. - Method according to claim 5, wherein the liquid resin (24) is an acrylic resin. 7. - Method according to any one of the preceding claims wherein the spacer (16) is based on PET foam. 8. - Method according to any one of the preceding claims wherein the fibers (22) comprise natural fibers taken from flax fibers, sisal, jute, hemp, kenaf, and mixtures thereof. 9. - Method according to any one of the preceding claims in which the stack is heated to a temperature above 150 ° C and advantageously between 160 ° C and 200 ° C. 10. - Method according to any one of the preceding claims wherein the pressure exerted on the stack during the closing of the mold (26) is less than 0.5 MPa. 11. - Method according to any one of the preceding claims, in which the thickness of the stack before the mold is closed (26) is strictly less than or equal to the thickness of the molding cavity (30) of said mold (26). ), the foam spacer (16) undergoing expansion during at least partial evaporation of the resin (24). 12. - Method according to any one of the preceding claims wherein the spacer (16) is devoid of blowing agents. 13. - Structural part (10) for a motor vehicle capable of being obtained by the method according to any one of the preceding claims.