Method for manufacturing by moulding elongate and hollow workpieces made from composite material

Description

[0001] The present invention relates to the field of elongated and hollow composite parts, and more particularly to long parts such as, but not limited to, drive shafts, connecting rods, and cardan shafts, and it relates to a method for manufacturing these parts by molding.

[0002] Such composite structures are essentially made by creating a tube by filament winding onto which metallic appendages or inserts are attached, particularly at the ends, for the purpose of creating an interface with another element.

[0003] Such hybrid parts, consisting of a tube with metallic ends, are described in numerous documents, for example, document FR 2970533, where a metallic element is held in place by filament winding around radial pins attached to said metallic element. Besides the fact that such a technique is not optimal for holding elements and inserts, the tubular part itself remains the weak point of these hybrid parts.

[0004] Indeed, the manufacture of composite tubes is generally carried out using conventional filament winding, which involves depositing pre-impregnated reinforcing fibers onto a mandrel that rotates around its axis. The deposition is performed by at least one filament carriage that moves along the mandrel. The robot's speed and the mandrel's rotation speed determine the deposition angle. The fibers are placed by alternating and longitudinal sweeping of the carriage, arranging them helically on the rotating mandrel, with the fibers crossing with each sweep.

[0005] However, it is known that the length of a finished fabric is always less than the warp length because the crossing of the warp and weft threads consumes some of the length. This difference is called weaving. As a result, the warp threads undulate between the weft threads and therefore lack straightness, which reduces the strength and rigidity of the finished piece.

[0006] Fiber brittleness is therefore an important factor to consider when designing a composite part. In the case of manufacturing a tube by winding fibers or fiber webs, the crossing of the helices induces fiber brittleness, which limits the final performance of the resulting tube. Conventional filament winding leads to fiber crossing, causing brittleness in the preform produced. This brittleness results in a loss of performance in the tube manufactured using this type of preform.

[0007] However, the smudging is not the only drawback of the classic winding technique.

[0008] Another drawback is that the filament winding of the preform necessarily results in fiber expansion. This expansion leads to an outer diameter of the preform that is larger than that of the final part. To achieve the correct fiber volume ratio, this expansion must be neutralized by compressing the outer surface of the tube during polymerization. This leads to a reduction in the outer diameter, inevitably causing the fibers to wrinkle. Therefore, proper compression under external pressure is incompatible with maintaining good local fiber straightness, which limits the performance of the tube material.

[0009] Another drawback of this technique arises when manufacturing very long parts. It is impossible to hold the mandrel with its preform in a horizontal position without deflection due to gravity. Polymerization in this position therefore results in a lack of straightness in these long shafts produced in this way. An alternative is to position the mandrel with its preform vertically, which complicates the process, particularly for very long parts.

[0010] On the other hand, the production of a part wrapped around a mandrel limits the shapes of the part by the need to have to remove the mandrel from the mold.

[0011] Finally, the production of a drive shaft requires the addition of end pieces via fastening systems which can be costly, heavy or delicate in terms of justification or certification.

[0012] Furthermore, a molding process is known which consists of inflating, under pressure, a bladder placed inside a preform made of composite material. The bladder externally presents the shape, in reduced dimensions, of the molding impression; its inflation inside the preform gives the latter the external shape of said impression.

[0013] Such a process is described in document WO2005 / 105417. It should be noted, however, that the process described therein is not suitable for molding long objects, as it is difficult to control the shape and placement of the bladder over a long length.

[0014] This process therefore does not solve all the problems previously mentioned, particularly with regard to the great length.

[0015] It should also be noted that a process is known for strengthening a cylindrical part by helically winding pairs of filament layers around it under a predetermined tension and reversing the winding direction after each layer. This process, described in document FR 2 589 962, does not allow for the production of a preform intended for expansion molding.

[0016] US document 5,071,506 A also describes the fabrication of a tubular element by applying pressure to a tubular membrane pinched at the ends of the support mandrel.

[0017] The present invention aims to remedy the various technical problems mentioned above, in order to enable the production of a long, hollow composite part, exhibiting remarkable geometric precision, particularly its straightness, which can have a non-demolding internal shape, having an excellent fiber volume ratio while avoiding fiber wrinkling, allowing the production in the same manufacturing phase of the particular shapes materializing the ends of the part.

[0018] The present invention consists of a method for producing a tubular composite part, which is characterized in that it comprises the successive steps as defined in claim 1.

[0019] The application of pressure puts the helical fibers under tension, a tension which is maintained during polymerization.

[0020] According to an additional feature of the process according to the invention, after the step of cutting the initial preform to the correct length and before placing it in the mold, several intermediate steps are carried out successively, namely: production of one or more secondary preforms on a circular or ovoid mandrel, adapted to be able to be arranged around and / or inside the initial preform, cutting to suitable lengths of said secondary preforms, assembly of said secondary preforms on and / or in said initial preform.

[0021] According to an additional feature of the process according to the invention, before the step of placing the initial preform in the mold, one or more auxiliary pieces having a central axial opening are placed in the mold, into which said initial preform is introduced.

[0022] In order to implement the process according to the invention, it was necessary to design a new mold device which is characterized in that it comprises: a body consisting of at least two longitudinally separable parts, allowing after polymerization its opening and the demolding of the molded part, and comprising internally in a hollow the shape to be molded, a tubular membrane suitable for being introduced inside the initial preform, two sleeves, each disposed at one end of said body, having a central opening allowing the passage of said membrane, and each comprising means for retaining in a watertight manner the edge of one end of said tubular membrane, two end caps, closing said sleeves, and adapted for achieving the seal, and each equipped with means for connection to external means for pressurizing the inside of said tubular membrane.

[0023] According to an additional feature of the mold device, the means for retaining the edge of one end of the tubular membrane in a watertight manner consist of an axial annular lip extending around the periphery of the orifice of the central opening, and on the external side, and on which the extreme edge of the membrane is folded.

[0024] According to another additional feature of the mold device, the mold is associated with a rigid frame which supports it through insulating spacers, and it has along its length geometric control targets allowing the detection of any straightness defect, while said spacers cooperate with adjustment elements suitable for correcting said defect.

[0025] In practice, the preform is placed in at least two longitudinally separable sections of the mold, which is then closed. Preferably, drainage accessories such as micro-perforated fabric and felt will have been previously inserted between the preform and the membrane.

[0026] The sleeves are then sealed together, each to one end of the at least two longitudinally assembled separable parts, and each end of the membrane is fixed to a sleeve, then the plugs close the ends of the mold thus formed.

[0027] The process according to the invention offers new possibilities for manufacturing transmission shafts from composite material. Thus, without limitation, one can consider shafts comprising at least one of their ends a bonding flange, or comprising at least one of their ends a shape adapted to cooperation with the cross member of a Cardan joint, or even comprising auxiliary parts, namely one or more bearing rings and / or one or more levers and / or one or more gears, and which have different internal diameters.

[0028] The advantages and characteristics of the process according to the invention will become clearer from the following description, which relates to the attached drawing, which represents a non-limiting embodiment thereof.

[0029] In the attached drawing: there figure 1 represents a partial schematic view illustrating a step of the process according to the invention, the figure 2 represents a schematic cross-sectional view along a longitudinal plane of a mold, during another step of the process according to the invention, the figure 3 represents the same schematic cross-sectional view of a mold, during another step of the process according to the invention, the figure 4 represents a perspective view of the molding device, the figure 5 represents a schematic cross-section of the same mold device, the figure 6 represents a perspective view of a variant of the same mold device, the figure 7 represents a perspective view of an elongated tubular part that can be obtained by the process according to the invention, the figure 8 represents a cross-sectional view along a longitudinal plane of a portion of the mold assembly, during a step in the process for manufacturing the part of the figure 7 , there figure 9 represents a view of the mold device, during another step in the process for manufacturing the part of the figure 8 , there figure 10 represents a perspective view of a variant of the elongated tubular part that can be obtained by the process according to the invention, the figure 11 A represents a perspective view of the end of a tubular part obtained by the process according to the invention, the figure 11 B represents a perspective and cross-sectional view of the same end, the figure 12 A represents a perspective view of the end of a universal joint obtained after machining the part of the figure 11 , there figure 12 B represents a perspective and cross-sectional view of the same end of the gimbal, the figure 13 represents a perspective view of a complex part obtained by the process according to the invention, the figure 14 represents a schematic cross-sectional view along a longitudinal plane of a mold during a step in the manufacturing process of the complex part of the figure 13 , there figure 15 represents the same schematic view, during another stage of the manufacturing process of the complex part of the figure 13 .

[0030] With reference to the figure 1 , we can see the first step of the process of making a tubular composite part according to the invention, namely the making of a preform 1, obtained by filament winding on a mandrel M.

[0031] According to the process, it is necessary to overcome the problem of enfouling, which is achieved by superimposing several distinct layers, each layer being entirely made up of either a winding of fibers arranged in parallel helicals, or of pre-impregnated unidirectional fiber sheets.

[0032] In this case, on the figure 1 The preform 1 comprises two layers C1 and C2 of N-shaped pre-impregnated unidirectional fiber webs, deposited in helicals, all parallel within the same layer, and with opposite directions from one layer to the other. Thus, there is no overlapping of N-shaped webs within the same layer, and therefore no smearing.

[0033] Now referring to the figure 2 , we can see a mold 2, allowing the process according to the invention to be implemented.

[0034] This mold 2 comes from the assembly of several elements, including in this case a main body 3, two sleeves 4, two plugs 5, and a tubular membrane 6.

[0035] The main body 3 is divided longitudinally into, but not limited to, two parts 30 and 31, internally comprising an impression, respectively 32 and 33, allowing the form to be reconstructed in relief.

[0036] The two sleeves 4 are intended to be joined in a watertight manner, each to one end of the main body, they are annular in shape and have at their central opening 40, means 41 for attaching an extreme edge 60 of the tubular membrane 6.

[0037] In the embodiment shown, each of the sleeves 4 on the outside periphery of the opening 40 has an annular lip 41 on which the extreme edge 60 of the membrane 6 can be turned over, so as to achieve sealing, with locking means, not shown, ensuring retention.

[0038] The plugs 5 are intended to be securely attached each to a sleeve 4, and they are pierced with a hole 50 allowing the mold 2 to be pressurized.

[0039] Membrane 6 is made of an elastomer-type material with high deformation capacity and adapted to the polymerization temperature used.

[0040] According to the process, after the length of the preform 1 has been rectified, the tubular membrane 6 is inserted axially into it, preferably after the interposition of drainage elements 61 such as micro-perforated fabric and felt. The assembly is then placed between parts 30 and 31 of the body 3, which is subsequently closed. The sleeves 4 are fixed to the body 3, the extreme edges 60 of the membrane are secured to it, and the plugs 5 close the mold 2.

[0041] As can be seen on the figure 3 The mold 2 is brought to the softening temperature of the impregnating resin and under pressure P, from both ends so that this pressure is well balanced, which causes the membrane 6 to swell and the preform 1 to deform, which comes into contact with the cavities 32 and 33. Then an increase in temperature and pressure according to the conditions recommended for the polymerization of the resin leads to the consolidation of the composite part.

[0042] Now, referring to figures 4, 5 et 6 , we can see the external characteristics of mold 2, necessary to respect the geometry of the part to be manufactured when the latter is of great length.

[0043] It is known that the geometry of the part depends on the geometry of the mandrel, which is difficult to control in the case of long lengths. Because the mandrel is slender, it is inherently flexible. It is therefore subject to the effects of gravity as well as the stresses generated by the relative thermal expansion between the composite part and the mandrel.

[0044] The mold device allows for the production of hollow parts by conforming the external shape of the parts to the internal shape of the mold. This feature allows for control of the part's geometry during its polymerization phase.

[0045] As can be seen in these figures, the mold 2 is supported by a rigid frame 7, by means of insulating spacers 70, for example, but not limited to, made of titanium alloy or ceramic, which cooperate with adjustment elements 71 linked to the frame 7, while geometric control targets 72 are attached to the mold 2.

[0046] As can be seen on the figure 6 , the mold is wrapped in thermal insulation 73, while leaving the geometric targets 72 and the adjustment elements 71 accessible.

[0047] In practice, mold 2 incorporates heating elements and thermocouples which ensure thermal management, while pressurization is achieved by compressed gas.

[0048] Under the influence of gravity and during the pressure and temperature phases, mold 2 deforms. External measurement of the target positions then allows for continuous readjustment of the overall straightness of mold 2 by adjusting the control elements 71.

[0049] It should be noted that the means of measuring the geometry of the mold can be of all kinds, mechanical, optical, by Hall effect, etc...

[0050] The adjustment elements 71 can be directly actuated by operators during polymerization, or by any type of position control allowing realignment of the mold 2.

[0051] Now, referring to figures 7, 8 et 9 , we can see a variant of the process according to the invention, and in particular on the figure 7 , a tree 8 fitted at each of its ends with a flange 80, and on the figures 8 et 9 , the molding operation to obtain these 80 flanges.

[0052] These flanged shafts are of great interest because they eliminate the need for metal connecting parts, the connection of which with the composite part is always delicate.

[0053] Thus on the figure 8 , we can see that to make a flange 80, we associate with the initial preform 1 two secondary preforms 10 and 11, which are in the form of tube segments, one 10 engaged inside the initial preform 1, and the other 11 arranged around the initial preform 1, so as to frame the extreme part of the initial preform 1.

[0054] The stacking of secondary preforms 10 and 11, inside or around the initial preform 1, allows a local increase in thickness which, in association with choices of inclination of the fiber of the secondary preforms 10 and 11, makes it possible to optimize the resistance of the flanges 80.

[0055] On the other hand, the length of the initial preform 1, and the location of the stack of secondary preforms 10 and 11, is in accordance with the mold cavity 2, so that this stack partially exceeds the tubular part of this cavity, and that the membrane 6, under the effect of pressure, pushes the preform outwards to form the flange 80. For this purpose, the mold 2 has an adapted cavity, namely clearances between the body 3 and the sleeves 4, allowing a judicious distribution of the increase in thickness outwards as well as inwards in order to optimize the resistance of the end thus formed.

[0056] There figure 10 shows another advantage of the present invention, namely to allow an increase in the central dimensions of a transmission shaft 82, in order to increase its torsional and especially bending rigidity, which allows a significant increase in its critical rotational speed.

[0057] THE figures 11A, 11B, 12A et 12B demonstrate the possibility, through the process according to the invention, of easily producing a transmission shaft 83 whose end has a shape adapted to cooperation with the cross member of a Cardan joint.

[0058] As with the production of a flange seen previously, a stacking of secondary preforms inside and / or around the extreme part of the initial preform allows a local increase in thickness.

[0059] It should be noted that the choice of inclinations of the fibering of the secondary preforms allows to optimize the resistance of the ends.

[0060] Pressurizing the membrane, combined with raising the resin's softening temperature and the internal shape of the mold, allows for the expansion of end 84 without reducing the wall thickness, as can be seen on the figures 12A et 12 B.

[0061] After polymerization, machining of the end 85 thus obtained allows the production of the fork 85 necessary for the materialization of the part of the Cardan joint.

[0062] It should be noted that, according to the same approach, the process makes it possible to produce end shapes of parts, conforming to clevises, thus enabling the production of connecting rods.

[0063] The present invention makes it possible to use the deformation capacity of the preform in the mold to produce composite parts that are generally long in shape and more or less of revolution, cooperating with auxiliary parts.

[0064] Without being exhaustive, the figure 13 represents a shaft 9 produced by the process according to the invention, having the particularity of presenting bearing rings 90, levers 91 and a pinion 92, and which are of different internal diameters.

[0065] THE figures 14 et 15represent the mold 2 enabling the production of the shaft 9, and whose cavities 32 and 33 are adapted to receive the bearing rings 90, the levers 91 and the pinion 92, and which for this purpose have cavities 34, 35 and 36 respectively.

[0066] In practice, the bearing rings 90, the levers 91 and the pinion 92 are placed in part 31 of the body 3, then the initial preform with its possible secondary preforms is threaded inside the bearing rings 90, the levers 91 and the pinion 92, and part 30 of the body 3 is placed on top.

[0067] It should be noted that the ancillary parts may have internal shapes that facilitate their assembly to the composite part, namely: shapes of revolution exhibiting a roughness large enough to create micro obstacles opposing the sliding of the composite fibers which will have been included during the pressure applied followed by the polymerization phase, non-shapes of revolution allowing to oppose any relative movement between the ancillary parts and the composite part.

Claims

1. Method for producing a tubular-shaped composite part, characterised in that it comprises the following successive steps: - producing an initial preform (1) on a circular or ovoid mandrel (M) without creating shrinking, by superimposing a plurality of distinct layers (C1, C2), each layer being entirely composed either of a winding of fibres arranged in parallel helices, or of rolling one or more plies (N) of unidirectional pre-impregnated fibres correctly oriented in a helical manner, while the helices of one layer (C1, C2) have a helix angle that is the inverse of that of the helices of the adjacent layer(s), - cutting said initial preform (1) to the correct length, - axially introducing into said preform (1) a tubular membrane (6), after interposing internal drainage means (61) between said membrane (6) and said preform (1), - positioning the assembly in a female mould (2), between the parts (30, 31) of the body (3) of said female mould (2), - securing the end edges (60) of the tubular membrane (6) to two sleeves (4) designed to be secured in a sealed manner, each at one end of the body (3) of said mould (2), - producing a seal between said end edges (60) and said female mould (2) by securely attaching, in a sealed manner, end caps (5) to each of the two sleeves (4), said end caps being pierced with a hole (50) allowing the mould (2) to be pressurised, - heating the impregnation resin to its softening temperature and applying pressure through swelling at both ends of said tubular membrane (6), until the preform assembly is deformed by a diametrical increase in its dimensions, conforming to the female shape of the mould (2), - heating and pressurising the impregnation resin for polymerisation to enable the part (8; 83; 9) to consolidate, with its outer shape conforming to the inner shape of the mould (2), - opening the mould (2) and removing the part (8; 83; 9), - machining the final shapes of the composite part (8; 83; 9).

2. Method according to Claim 1, characterised in that after the step of cutting the initial preform (1) to the correct length and before placing it in the mould, a plurality of intermediate steps are carried out succesively, namely: - producing one or more secondary preforms (10, 11) on a circular or ovoid mandrel, designed to be arranged around and / or inside the initial preform (1), - cutting said secondary preforms (10, 11) to appropriate lengths, - assembling said secondary preforms (10, 11) on and / or in said initial preform (1).

3. Method according to Claim 1 or claim 2, characterised in that, prior to the step of placing the initial preform (1) in the mould (2), one or more ancillary parts (90, 91, 92) with a central axial opening are arranged in the mould (2), into which said initial preform (1) is inserted.