FR3068371B1 - PROCESS FOR MANUFACTURING A UNIDIRECTIONAL SHORT FIBER TABLE - Google Patents

Abstract

The method comprises a step of producing an intermediate layer of short fibers (12b), elongate in a longitudinal direction (X), by layering with intercrossing of the fibers; a step of passing the intermediate web (12b), along the longitudinal direction (X), through a looping device (18) so as to generate undulations in a transverse direction (Y); a step of accumulation of the corrugations against an abutment member (24), so as to obtain a corrugated sheet (12c) and; following the accumulation step, a step of stretching the corrugated web (12c) in the transverse direction (Y) to flatten the corrugations and thus obtain a unidirectional short fiber ply (12).

Description

Method of manufacturing a unidirectional short fiber web

The present invention relates to the manufacture of a unidirectional short fiber web.

A unidirectional fiber web is a web whose fibers are all parallel to each other.

Unidirectional fiber webs are generally used for the manufacture of composites. In particular, the parallelism between the fibers makes it possible to achieve a low level of porosity in the final composite. Indeed, it is possible to strongly compress a stack of unidirectional fiber plies so as to achieve an optimal density, for which the matrix of the composite occupies the entirety of inter-fiber spaces. The performances of a composite made from unidirectional fiber plies are thus particularly satisfactory.

Already known in the state of the art, in particular according to FR 2 982 283, a method of manufacturing unidirectional fiber plies. This process is particularly suitable for very long fibers, especially greater than 80 cm. Although this method may optionally be adapted to short fibers, it requires a large number of steps and is particularly expensive to implement. The object of the invention is in particular to remedy these drawbacks, by proposing a process for manufacturing unidirectional fiber sheets, adapted to short fibers, and that is simple and inexpensive to implement. For this purpose, the subject of the invention is in particular a method for manufacturing a sheet of unidirectional short fibers, comprising: a step of producing an intermediate sheet of short fibers, elongated in a longitudinal direction, by layering with intercrossing fibers, - a step of passing the intermediate web, along the longitudinal direction, through a loopback device comprising a set of rotating disks and fixed loop elements, so as to generate corrugations in a transverse direction perpendicular to the longitudinal direction, a step of accumulation of the corrugations against an abutment member, so as to obtain a corrugated sheet, characterized in that it comprises, following the accumulation step, a stretching step of the corrugated web in the transverse direction to smooth the corrugations and thereby obtain a unidirectional short fiber web s.

The stages of passage and accumulation allow perfectly parallelize the fibers of the web. Thus, the stretching step makes it possible to obtain a substantially flat sheet from the corrugated sheet whose fibers are parallel.

The method according to the invention thus makes it possible to obtain, in a simple and economical manner, a substantially flat sheet of unidirectional fibers.

A method according to the invention may further comprise one or more of the following characteristics, taken alone or in any technically feasible combination. - The loopback device is configured so that the undulations are formed by loops each having a width G predefined in the transverse direction, and a height H predefined in an elevation direction perpendicular to the longitudinal and transverse directions, the topping step with intercrossing being performed so as to orient the fibers parallel to a general direction forming an angle α with the longitudinal direction, given by the relation sina = G / 2H to +/- 5 °. - The method comprises a preliminary step of feeding a carding device with short fibers in order to obtain a card web, the step of forming intermediate web being performed by lapping the card web with intersecting. Short fibers are natural fibers, for example flax, hemp, jute or sisal. - The corrugated sheet having following the accumulation step a surface weight W, the stretching step is performed so as to obtain a unidirectional short fiber web having a basis weight v = 2.W.G / H. - The short fibers consist in part of fusible fibers, the process comprising, following the accumulation step, and prior to the stretching step, a step of heating the base of the corrugations to melt the fusible fibers . - The method comprises, following the stretching step, a calendering step of the unidirectional fiber web. The fibers have a length of less than 150 mm. The invention also relates to a device for manufacturing a sheet of unidirectional short fibers, comprising: - a lapping device with interwoven fibers, suitable for producing an intermediate sheet of short fibers, elongated in a longitudinal direction; loopback comprising a set of rotating disks and fixed loop elements, capable of generating corrugations in a transverse direction perpendicular to the longitudinal direction, - an abutment member arranged at the exit of the loopback device, suitable for making an accumulation of the corrugations , to form a corrugated sheet, characterized in that it comprises a device for stretching the corrugated sheet in the transverse direction, adapted to smooth the corrugations and thus obtain a layer of unidirectional short fibers.

This device allows the implementation of the method as defined above. The invention finally relates to a sheet 1 of unidirectional short fibers), characterized in that it is carried out during a manufacturing process as defined above. The invention will be better understood on reading the description which follows, given solely by way of example and with reference to the appended figures, in which: FIG. 1 schematically represents a device for manufacturing a web of unidirectional short fibers according to an exemplary embodiment of the invention; FIG. 2 schematically represents, in a view from above, fibers of a sheet of fibers passing through the manufacturing device of FIG. 1, before passing through a closure device fitted to this manufacturing device; - Figure 3 shows schematically the evolution of a fiber passing in the loopback device; FIG. 4 schematically represents the steps of a method for manufacturing a unidirectional short fiber sheet, using the device of FIG. 1.

FIG. 1 shows a device 10 for manufacturing a sheet 12 of unidirectional short fibers.

Usually, a fiber is said to be short when its length is less than 500 mm. Preferably, the ply 12 comprises short fibers less than 300 mm long, for example less than 150 mm.

The fibers of the ply 12 are preferably natural fibers, for example linen, hemp, jute or sisal, or a mixture of natural fibers of different natures. Alternatively, the fibers of the web 12 may also comprise, at least in part, synthetic fibers.

Advantageously, the fibers of the ply 12 are partly made of fusible fibers, that is to say those capable of breaking when they are subjected to a predefined melting temperature. For example, the web 12 has between 2 and 70% fusible fibers. In particular, a proportion of 50% to 70% of fusible fibers is envisaged when the sheet 12 is intended for the manufacture of a thermoplastic matrix composite.

The manufacturing device 10 comprises a carding device 14 of conventional type.

The carding device 14 is fed at the input with short fibers, as defined above, in order to obtain at the output a card web 12a.

The carding device 14 is followed by a device 16 for laying with interlacing of fibers, also of conventional type. The card web 12a is provided at the entrance of the lapping device 16. The lapping device 16 then produces, as an output, an intermediate sheet of short fibers 12b extending in a longitudinal direction X, which is the direction of advancing the intermediate ply 12b in the layering device 16.

The topping device 16 is of conventional type, and will not be described in more detail. The peculiarity of the use of this device in the context of the invention is that only one fold is made, so that the fibers intersecting with the layering device extend parallel to a direction forming an angle mean a of about 60 ° with the longitudinal direction X, in the case of course where the width of the web corresponds to the carding width.

The intermediate ply 12b, at this stage of the process, therefore consists of two superimposed webs having their fibers mainly oriented with an average angle of 60 ° symmetrically with respect to the longitudinal axis, materializing the machine direction.

As shown in FIG. 2, the fibers, denoted by the general reference 15, are each substantially parallel to a general direction A, B forming a predetermined mean angle α with the longitudinal direction X. The fibers being intersecting, some are aligned. parallel to a first general direction A forming an angle α with the longitudinal direction X in a clockwise direction, and others are aligned parallel to a second general direction B forming an angle α with the longitudinal direction X in a trigonometric direction.

The manufacturing device 10 comprises, at the output of the layering device 16, a looping device 18 known per se, for example in part similar to that described in EP 0 859 077. Such a loopback device 18 is intended to loop the fibers 15 of the intermediate web 12b by verticalizing them, thus forming undulations.

The loopback device 18 comprises a set of rotating discs 20 carried on a transverse common axis, extending in a transverse direction Y perpendicular to the longitudinal direction X. The rotating discs 20 are driven in continuous rotation at a preferably equal peripheral speed. at the input speed of the intermediate web 12b in this loopback device 18.

The rotating discs 20 are preferably each provided on its periphery with a toothing for driving the intermediate web 12b.

The loopback device 18 also comprises looper fingers 22, each being disposed between two adjacent disks 20. The pouting fingers 22 extend to an end substantially tangentially with respect to the discs 20. Thus, each fiber 15 is pre-bent by being driven at each end by a respective disc 20, by overcoming the looper finger 22 corresponding disposed between these two disks 20.

The path of a fiber 15 between two adjacent disks 20 is shown diagrammatically in FIG.

The fiber 15 is at an angle α with the longitudinal direction X, which is also the advancing direction of the intermediate ply 12b in the loopback device 18.

A front portion of the fiber 15 is driven by one of the disks 20, and a rear portion of this fiber 15 is driven by the other disk 20. These two disks being integral with the same axis, their rotation speeds are identical.

The front portion of the fiber 15 arrives first end of stroke, against a stop member 24 which will be described later. While the front portion is in abutment, the rear portion continues to advance until it also abuts, thereby bending the fiber 15 which then forms a loop.

The fibers 15 passing through the looping device 18 all have the same behavior as described above, so that the set of looped fibers 15 form undulations over the width of the intermediate layer 12b, taken in the transverse direction Y The sheet resulting from the looping device 18 will be called corrugated sheet 12c.

Each loop has a height H, taken in a direction of elevation Z perpendicular to the longitudinal direction and in the transverse direction, and a width G taken in the transverse direction Y as shown in FIG. 3. It should be noted that the width G substantially corresponds to the gap between two adjacent disks.

It should be noted that the width G and the height H of the corrugations are predefined, the width G depending on the interval between two adjacent rotating discs 20, and the height depending in particular on this interval and on the initial length of the fibers 15.

Advantageously, the loopback device 26 is configured so that the width G and the height H of the loops formed correspond substantially to the relation sina = G / 2H. This makes it possible to optimize the formation of the loops and the parallelization of the fibers. The stop member 24 is of any type that can be envisaged. For example, the abutment member 24 is formed by a conveyor whose travel speed is less than the tangential speed of the discs 20. The stop member 24 is for example formed by a belt arranged between the movable discs 20 and in the plane of the looper fingers 18, and part of a stretching device 26 described below.

By accumulating at the output of the loopback device 18, the loops are parallelized, and in particular parallel to the transverse direction Y.

The manufacturing device 10 further comprises, following the looping device 18, a stretching device 26, able to transversely stretch the corrugated sheet 12c.

For example, the stretching is carried out according to the principle described in EP 2 998 424. The stretching device 26 then comprises a set of belts collecting the undulations from the loopback device 18, these belts diverging to flatten the corrugated sheet 12c.

Any other stretching device 26 can be envisaged, in particular having rollers of axes parallel to the longitudinal direction X.

Advantageously, as indicated above, the fibers 15 comprise fusible fibers. The manufacturing device 10 then preferably comprises a heating device 28 arranged at the exit of the looping device 18, and suitable for heating the base of the corrugations to the melting temperature, in order to melt the fusible fibers. This creates points of connection which avoid an extension of the corrugated sheet 12c in the longitudinal direction X, and allow to stretch the corrugations transversely without risk of tearing of the sheet.

It should be noted that when the angle of the fibers of the intermediate web 12b is of the order of 60 °, the transverse stretch is relatively small, of the order of 1.2. This stretching is therefore easily controllable, and does not require a complex stretching device.

Following transverse stretching, the unidirectional short fiber web 12 is obtained, in which the fibers 15 are perfectly aligned and oriented in the transverse direction.

The density of the web of unidirectional fibers 12 depends on the mass of the card web 12a, and the rate of accumulation of the corrugations. For example, the density of the web of unidirectional fibers 12 is between 50 and 200 g / m 2.

Advantageously, the manufacturing device 10 comprises a calendering device 30 at the outlet of the stretching device 26, intended to fix the structure of the web 12 for subsequent manipulation. Such calendering is possible when the fibers comprise at least between 2% and 5% of fusible fibers.

Alternatively, as shown in FIG. 5, the calendering device 30 is replaced by a double-band device 40, formed by two conveyors 40a and 40b separated by a distance D, and having a heating zone 41 followed by a cooling zone 42.

This dual band device 40 advantageously allows a second accumulation of the unidirectional fiber ply by the formation of folds with respect to the Y direction, the unidirectional fiber ply at the end of the device 26 being positioned for example in a zig-zag manner as shown Figure 5. This additional accumulation is useful when one seeks the weights of the unidirectional sheet out of the device 40 greater than about 200g / m2 (this value depends on the title of the fibers used). Indeed the structure of the device 18 tends to expand longitudinally randomly beyond a certain basis weight which leads to scramble fibers that lose their parallelism.

Moreover, the double-band device 40 makes it possible to fuse the fusible fibers and thus guarantee the cohesion of the unidirectional sheet in the same way as a shell.

This device 40 allows, by adjusting the distance D between bands, weights greater than 200 g / m2 and up to 1000 g / m2 or beyond.

As indicated above, a 50% proportion of fusible fibers is envisaged when the sheet 12 is intended for the manufacture of a thermoplastic matrix composite. Thus, the sheet 12 is easily calenderable and the composite can be easily thermoformed.

The manufacturing device 10 makes it possible to implement a manufacturing method which will now be described.

The manufacturing method comprises a preliminary step 100 of feeding the carding device 14 with short fibers 15 in order to obtain a card web 12a.

As indicated above, the short fibers have a length of less than 50 cm, preferably less than 30 cm, for example less than 15 cm.

The method then comprises a step 110 for producing an intermediate layer of short fibers 12b, elongated in the longitudinal direction X, by topping the card web 12a with the fibers interleaving. For this purpose, the card web 12a is passed through the lapping device 16.

The topping 110 is made in a single fold, so that the fibers crisscrossed by the layering device 16 extend parallel to a direction forming an average angle α of about 60 ° with the longitudinal direction X, in the case obviously where the width of the batting corresponds to the carding width.

More particularly, the interleaving step 110 is made so as to orient the fibers 15 parallel to a general direction A, B forming an angle α with the longitudinal direction X, given by the relation sina = G / 2H to + / - 5 ° close.

The method then comprises a step 120 for passing the intermediate ply 12b, along the longitudinal direction X, through the looping device 28, so as to generate corrugations in the transverse direction Y.

The method also comprises a step 130 for accumulating the corrugations against the abutment member 24.

A corrugated sheet 12c is thus obtained, as previously described.

The corrugated sheet 12c has, following the accumulation step 130, a surface density W.

Advantageously, the method comprises, following the accumulation step, a step 140 of heating the base of the corrugations, for melting the fusible fibers.

The method then comprises a step 150 of stretching the corrugated sheet 12c in the transverse direction Y to flatten the corrugations and thus obtain a layer of unidirectional short fibers 12. The stretching step 150 is performed so as to obtain a sheet unidirectional short fibers 12 having a basis weight v = 2.WG / H.

Optionally, the method then comprises a step 160 for calendering the unidirectional fiber ply 12.

The method finally comprises, optionally, a step 170 of winding the web 12 thus formed, for handling.

The method according to the invention makes it possible to obtain, continuously, a layer of unidirectional short fibers. Production costs are therefore relatively low.

Claims (10)

1. A method of manufacturing a unidirectional short fiber ply (12), comprising: - a step (110) for producing an intermediate layer of short fibers (12b), elongate in a longitudinal direction (X), by topping; with intercrossing of the fibers, - a step (120) of passage of the intermediate web (12b), along the longitudinal direction (X), through a looping device (18) comprising a set of rotating disks (20) and of fixed looping elements (22), so as to generate undulations in a transverse direction (Y) perpendicular to the longitudinal direction (X), - a step (130) of accumulation of the undulations against a stop member (24). ), so as to obtain a corrugated sheet (12c), characterized in that it comprises, following the accumulation step (130), a step (150) of stretching the corrugated sheet (12c) in the transverse direction (Y) to smooth out the ripples and get nsi a sheet of unidirectional short fibers (12). 2. Manufacturing method according to claim 1, wherein the looping device (18) is configured so that the corrugations are formed by loops each having a width G predefined in the transverse direction (Y), and a height H predefined in an elevation direction (Z) perpendicular to the longitudinal (X) and transverse (Y) directions, the interleaving step (110) being performed so as to orient the fibers (15) parallel to a general direction (A, B) forming an angle α with the longitudinal direction (X), given by the relation sina = G / 2H to +/- 5 °. 3. Manufacturing method according to claim 1 or 2, comprising a preliminary step (100) of feeding a carding device (14) with short fibers to obtain a card web (12a), the step (110) of embodiment of intermediate web being made by topping the card web (12a) with intercrossing. 4. The manufacturing method according to any of the preceding claims, wherein the short fibers (15) are natural fibers, for example flax, hemp, jute or sisal. 5. Manufacturing method according to any one of the preceding claims, wherein, the corrugated sheet (12c) having following the accumulation step a surface mass W, the stretching step is performed so as to obtain a web of unidirectional short fibers (12) having a basis weight v = 2.WG / H. 6. Manufacturing process according to any one of the preceding claims, wherein the short fibers (15) consist in part of fusible fibers, the process comprising, after the accumulation step (130), and prior to the stretching step (150), a step (140) of heating the base of the corrugations to melt the fusible fibers. 7. The manufacturing method according to claim 6, comprising, following the stretching step (150), a step (160) for calendering the unidirectional fiber ply (12). 8. Manufacturing process according to any one of the preceding claims, wherein the fibers (15) have a length less than 150 mm. 9. Device (10) for manufacturing a unidirectional short fiber web (12), comprising: - a device (16) for interlacing the fibers (15), adapted to produce an intermediate layer (12b) of short fibers , extended in a longitudinal direction (X), - a loopback device (18) comprising a set of rotating disks (20) and fixed loop elements (22), adapted to generate corrugations in a transverse direction (Y) perpendicular to the longitudinal direction (X), - an abutment member (24) arranged at the outlet of the looping device (18), capable of producing an accumulation of the corrugations, to form a corrugated sheet (12c), characterized in that it comprises a device (26) for stretching the corrugated sheet (12c) in the transverse direction, suitable for smoothing the corrugations and thus obtaining a sheet of unidirectional short fibers (12). 10. Unidirectional short fiber web (12), characterized in that it is carried out during a manufacturing process according to any one of claims 1 to 8.