Method for producing a yarn layer by means of stitches, in particular for producing composite material parts

Description

[0001] The present invention relates to a method for manufacturing a sheet of threads by stitching, in particular for the production of technical parts, more particularly parts made of composite materials.

[0002] Machines are known, more particularly embroidery machines, capable of producing stitches on a base, for example a sheet of woven threads.

[0003] DE 10 2013 102489 A1 discloses an example of the prior art.

[0004] These embroidery machines can follow any type of programmed shape.

[0005] Thus, on a base, said base being integral with a defined support which stretches said base, wicks of another material for example are deposited.

[0006] For the remainder of the description, a "wick" is defined as a set of unidirectional threads or fibers, covered or not, twisted or not, said threads or fibers forming a bundle, these threads being able to be made of materials of different natures, possibly impregnated. It can even be a woven ribbon with an integrated matrix.

[0007] More generally, any filiform product can be treated by implementing the method according to the invention, such as ribbon, tube, optical fiber.

[0008] This definition of "wick" is used to avoid confusion of vocabulary with "sewing thread".

[0009] This wick is distributed by a head on the base following a programmed shape which can be rectilinear, curvilinear or a combination of the two to follow a complex geometric shape.

[0010] This wick thus distributed on the said base must be fixed there by a sewing thread in order to preserve the geometric shapes of the deposit.

[0011] These geometric shapes can for example be used to arrange reinforcements on a base in perfectly circumscribed areas to produce dry preforms with a view to producing technical parts, more particularly in composite materials by embedding this preform in a resin matrix for example, whether the process uses a mold with resin injection, or a bladder for a vacuum infusion of resin. The technique chosen can also provide for pre-impregnated material introduced into a mold, the final implementation technique having no impact on the process of the present invention which applies precisely to all these techniques.

[0012] This process allows parts to be produced with very precise reinforcements, thus limiting the consumption of raw materials, limiting the weight, and positioning the reinforcements exactly.

[0013] This process allows productions, particularly in small and medium series, at limited costs due to a necessarily limited workforce since the fibers are deposited automatically, changes in the type of wicks are also made possible, like colored threads on an embroidery machine.

[0014] The aim of the method according to the present invention is to increase the productivity of wick laying with an identical sewing speed and therefore an identical number of stitches and an identical stitch length. This aim is achieved by limiting changes of direction which are penalizing for the machines because it is necessary to overcome the inertia linked to the movement of the base support plate, this plate being movable under the head for laying wicks and sewing said wicks.

[0015] In fact, whatever the mode and mechanical means of maneuvering in translation, both horizontal and vertical, of the support plate, inertia remains a problem that must necessarily be overcome.

[0016] When it comes to micromotors, the problem also arises.

[0017] The method according to the present invention is now described in detail according to a particular, non-limiting embodiment, with reference to the appended figures, figures which represent: Figure 1 : a view of a removal of a single wick illustrating the removal of the prior art, Figure 2 : a view of a removal of at least two wicks according to the method of the present invention, with sewing of each of the at least two wicks, Figures 3A à 3C : schematic of removal with a rotating head, Figure 4 : a view of a removal of at least two wicks according to the method of the present invention, with a stitching of all of the at least two wicks, Figure 5 : a view of a removal with a half-turn of the wicks in continuity, Figure 6 : a view of a deposit of several wicks, with variable width.

[0018] On the figure 1 , a single wick 1 is shown, placed on a base 3, this wick being secured by a seam 5 comprising continuous stitches 5-1, 5-2, 5-3 of the zigzag type.

[0019] The support plate of the embroidery machine receives the base 3 on which is placed the at least one wick 1 from a removal and sewing head, not shown, said device being commercially available.

[0020] This sewing and depositing head comprises two juxtaposed parts, a depositing head and a sewing head. The depositing head ensures the depositing of the at least one wick on the base and this depositing head is movable in rotation while the sewing head has a needle fed by a sewing thread, fixed in rotation and movable in vertical translation, this in a known manner.

[0021] A single strand is laid and the stitches are on either side of said single strand, alternately, when this type of zigzag stitch is provided, the sewing providing for a change of direction.

[0022] It is understood that the sewing has the sole purpose of mechanically maintaining said wick on said base in order to retain this wick in the chosen and necessary position on said base depending on the technical part to be produced subsequently from said base supporting its wicks, following a given geometric distribution.

[0023] This stitching does not necessarily mechanically affect the intrinsic qualities of the product since the weft thus produced is then embedded in a resin matrix or molded using pre-impregnated wicks, for example, and more generally whatever the method chosen.

[0024] The positioning and monitoring of the chosen removal geometry are essential so that the finished product has the wicks with the necessary positioning for the absorption of forces, in particular, in the chosen directions.

[0025] On the figure 2 , the method according to the present invention proposes sewing a set of wicks, comprising at least two wicks, in this case three wicks M1, M2 and M3 with stitches S1, S2 and S3, i.e. four stitches p1 to p4, in alignment for each of the wicks, the change of direction being carried out only on the outer edges of the set of wicks, therefore on the outer edges of the at least two wicks deposited in this case of the three wicks.

[0026] So, on the figure 2 , three points S1, S2 and S3 are provided for three wicks, each wick being held between two stitches, M1 between p1 / p2, M2 between p2 / p3 and M3 between p3 / p4.

[0027] It can be seen that the change in direction of the support plate of the base 3 in relation to the laying and sewing head, in this case the direction of rotation of the rotary laying head, is carried out at stitches p1 and p4, the three points S1, S2 and S3 being aligned in the same direction.

[0028] In the case of a rotary and sewing dispensing head, as in the synoptic of the figure 3 , we note that, on the figure 3A , the first stitch p1 is made on the upstream edge in the direction of movement of the depositing and sewing head, that is to say on the outer edge of the first wick M1, external to be deposited, the first wick M1 is deposited and the second stitch p2 is made on the downstream edge of said wick M1, across due to the advancement of the support plate of the base and the depositing time of each wick in the time interval between two stitches.

[0029] This leads to a seam that is inclined relative to the direction of the strands.

[0030] This second p2 sting, figure 3B , is on the upstream edge of the wick M2 to be deposited. The wick M2 is deposited and the third puncture p3 is made on the downstream edge of the wick M2. The third puncture p3 is on the upstream edge of the wick M3 which is deposited in turn.

[0031] The fourth sting p4, figure 3C , is carried out on the downstream edge of the M3 drill bit, after its removal, therefore on the outer edge of the set of drill bits.

[0032] The change of direction is made at point p4 for a new series of removal of the three wicks M1, M2 and M3 from the set of wicks with new points and stitches p1', p2', p3' and p4' respectively.

[0033] Each of the wicks M1, M2 and M3 is therefore perfectly positioned and, for 6 stitches, the laying and sewing head only had to manage one change of direction on each outer edge of the set of wicks, instead of the 5 which would have been necessary in the prior art, without counting the starting point to return to the outer edge of the wick M1.

[0034] The seam is therefore of the zigzigzig / zagzag type.

[0035] On the figure 4 , a variant consists of arranging a set of wicks of at least two wicks, in this case three wicks, M1 to M3 by making a single stitch S1, from the outer edge of the first wick M1 with a stitch p1 to the outer edge of the at least second wick, in this case the third wick M3, with a stitch p2.

[0036] In fact, the three wicks M1, M2 and M3 are held by a single point and the change of direction occurs again only on the outer edges of the set of wicks.

[0037] On the figure 5 , the problem of thread seams is addressed when a reversal is performed to make the wicks turn around and lay them across the direction of advance of the base 3. The support plate receiving the base 3 ensures, in the embodiment chosen, a step-by-step advance, the step corresponding to the width of the set of wicks, for example, so that all the wicks are spaced by the same distance.

[0038] In fact, when the set of drill bits reaches the edge of base 3, it is necessary to make a half-turn of 180° to return to place the set of drill bits parallel to the first placement, this in a known manner.

[0039] If you want to deposit the same quantity of material and if you want perfect repeatability of successive deposits, it is necessary to carry out a suitable seam in the turns.

[0040] The aim is to respect the alternation M1, M2, M3 / M1, M2, M3 in both directions of installation, while limiting changes of direction.

[0041] Thus, the stitches p1 to p4 are made along almost straight lines in the turns, with stitches of almost identical length and with identical consumption and laying length of wicks M1, M2 and M3.

[0042] This allows for a homogeneity in the density of strands placed on the base and therefore a balanced density of fibers in the composite material product which will be produced with this sheet of threads.

[0043] Likewise, the radius of curvature is identical for all the bits, which in some types of bits is an important advantage, thus avoiding excessively small radii of curvature. This is even more important if we imagine a solution in which the bits in the same set are different. The length of the bits deposited is thus identical.

[0044] It is also possible, by laying sets of wicks, to vertically superimpose two or more wicks over a length L, from the same set when necessary, in order to create reinforcement or to create a zone without fibres which will be perforated, in order to maintain the continuity of the fibres of the different wicks.

[0045] We can even predict M1 inversions instead of M3 and M3 instead of M1.

[0046] This controlled laying of several strands simultaneously ensures faster work and an embroidery machine can be used to create such a sheet of threads. The laying has been described on the basis of the use of a rotary head but a comb could also be used.

[0047] Precision and computer control also make it possible to compete with other existing processes and devices.

[0048] The stitch mentioned is of the zigzag type but it could be of the three-sided zigzag type to form not triangles but rectangles.

[0049] The reliability of the frame production is also ensured by the precision of the embroidery head movements.

[0050] It is also possible to make series of stitches and to provide spacing between the series of stitches.

[0051] The description was made in relation to an embroidery machine head having a rotary head but a translational head would give exactly the same result.

Claims

1. Method for producing a layer of yarns grouped into rovings (M1, M2, M3), by stitches, on a base (3), in particular for producing technical parts from composite materials, characterized in that the method consists in carrying out the following steps: - depositing at least two rovings (M1, M2, M3) distributed by a deposition head on the base (3) according to a programmed shape which can be rectilinear, curvilinear or a combination of both to follow a complex geometric shape, - stitching the at least two rovings (M1, M2, M3) thus deposited on the base (3) with at least one stitch (S1-S3) comprising two stitch holes (p1-p4).

2. Method for producing a layer of yarns grouped into rovings (M1, M2, M3), according to claim 1, characterized in that the stitches (S1, S2, S3) are aligned, each roving (M1, M2, M3) accommodating a stitch hole (M1 / p1, p2; M2 / p2, p3; M3 / p3, p4) on either side.

3. Method for producing a layer of yarns grouped into rovings (M1, M2, M3), according to claim 2, characterized in that the stitch holes (p1-p4) are aligned.

4. Method for producing a layer of yarns grouped into rovings (M1, M2, M3), according to any of the preceding claims, characterized in that the seam is inclined with respect to the direction of these rovings.

5. Method for producing a layer of yarns grouped into rovings (M1, M2, M3), according to any of the preceding claims, characterized in that, when a reversal is carried out to turn the rovings around, the rovings are deposited with alternation (M1, M2, M3 / M1, M2, M3) and the lengths of rovings deposited are identical.

6. Method for producing a layer of yarns grouped into rovings (M1, M2, M3), according to any of the preceding claims, characterized in that at least two of the at least two rovings are deposited so as to be placed one on top of the other over a length L.