Patent Application
20230090336
The invention relates to a method for producing a shapable core for producing composite material products. The invention also covers the shapable core obtained.
Today, and increasingly, a very large number of applications require the use of composite materials. Initially the reserve of the aeronautics, nautical and automotive fields, the use of composite materials is ever more frequent, and is found in almost all of industry, as well as the nautical, automotive and aeronautics industries.
The manufacturing techniques using composite materials have resulted in the emergence of very varied assemblies, which mainly associated a reinforcement element and a matrix. The reinforcement is mainly a fibrous element, such as a fabric made of mineral or organic fibers, for example glass or carbon fibers, or thermoplastic fibers. The matrix is mainly based on at least one epoxy or polyester resin for example. The reinforcement may be referred to as matte and, following application of the resin and polymerization, forms a composite material. In order to increase the resistance of the product made of composite materials, it is known to add a core which, positioned between at least two skins based on woven or non-woven fibers, allows for a significant improvement in the mechanical resistance, by increasing the inertia of the composite product thus obtained, following polymerization of the matrix. The thickness is significantly increased, but the amount of material and the weight of the product increase in a very moderate manner.
This manufacturing method, and the product thus obtained, often referred to as sandwich, can be implemented using fibers, resins or cores of very varied materials, providing very varied mechanical properties. For example, polyurethane foams, or polyethylene terephthalate foams, or any other synthetic foam, are often used as the core. More natural materials such as balsa or cork are also used, which have different technical characteristics but are often more expensive than the synthetic materials.
A method exists for producing a core which is particularly attractive from the perspective of the mechanical resistance, costs, and ease of implementation, and which was the subject of the French patent application FR 2 948 693, in the name of the same applicant as that of the present invention. Said method for producing a core comprising integrated bridging fibers for composite material panels consists in depositing a surplus of fibers on at least one of the two faces of the core, then needle punching some of said fibers in order to cause them to pass through the core, and then removing the fibers that have not been needle punched. Said bridging fibers, projecting from the faces, are intended to subsequently create a mechanical connection between the skins of the two faces of the core. The two faces of a core receive at least one skin, generally two skins, based on non-woven fibers of fabrics. The bridging fibers and the fibers of the skins/fabrics can be of different types and characteristics, which infinitely increases the combinations. In the case of forming pieces of composite material structures, the shapes to be obtained are rarely entirely planar. Before adding the resin and polymerizing, by whatever method, these panels must be able to be shaped so as to match given profiles, before polymerization and acquisition of mechanical resistance properties. Another aim is to be able to shape these panels without said panel breaking, because, before receiving the skins and the resin, the core remains fragile, especially in large dimensions. Yet another aim is to be able to obtain a finished piece having continuous surface state, without any projecting edge, without any step, without bumps, without waves, i.e. as perfect as possible, even before the final surface treatment, so as to limit the reworking, i.e. sanding, coatings, etc.
The method for producing a shapable core for panels in a general manner, according to the present invention, must allow for implementation of said method for producing a core with bridging fibers, and thus allow the shaping of panels comprising needle punching of fibers through the core material.
Whatever the material selected for the core, and irrespective of the resistance obtained after polymerization of the matrix of the sandwich, there are a number of restrictions for the implementation of the set of elements forming the composite material, before application and polymerization of the resin.
Indeed, in order to form large surfaces, for example a boat bridge, the type of materials of the rigid panel used for the core and specified above often offers only limited mechanical parameters prior to application and polymerization of the resin. This resistance limit relates more specifically to the limit of the compression resistance, more exactly the limit of the contact pressure resistance. Indeed, as long as the resin of the composite material is not applied and polymerized, the resistance of the shapable core to the contact pressure often remains relatively low.
Said limited resistance to the contact pressure poses problems during use and placement of the core and skins, especially on a non-planar shape. In practice, and in order to form large surfaces, it is necessary for the operator to move directly on said surfaces during assembly. During such movements, it is essential to be able to make use of a contact pressure resistance that is sufficiently high as to allow the operator to walk, but also to kneel or to rest on the surfaces during assembly, because the operator often has to rest on the surface of the composite material sandwich with his knees or his elbows. However, this punching pressure of resting on the elbows and/or knees has a higher impact, with the result that the movements and the work of the operators can thus damage the core and create dips in the surface of the material forming said core, which may be permanent. The foams have little mechanical resistance, but are also often not very resilient, and thus do not return to their initial shape after deformation. It could be conceivable, in order to increase the contact surface and reduce the contact pressure, to provide plates for the operators to rest, reducing the punching effect, distributing the weight over a larger surface, in the manner of snowshoes, but the practical and gestural aspects for the operators are of course worsened.
Consequently, these dips caused during the placement of the panels thus creates hollow deformations on the surface of said panels. Said deformations thus bring about deformation of the surface of the sandwich composite material, which remains after polymerization of the resin, and a surplus of resin is consumed if said volume of resin, linked to said deformation, is compensated in order to keep the same plane, which is naturally the case.
However, such surplus of resin, locally, is not satisfactory, since too great a thickness of resin, which is not reinforced, offers a limited resistance, which leads to an over-consumption of resin due to the natural filling of the dips, and thus an increase in costs; even the weight is penalized, without being able to gain any advantage from this. Generally, the shrinkages, depending on the thicknesses, are different, and visible deformations remain before finishing, and even rather after finishing, in particular after painting, which causes said faults to reappear.
In order to prevent the dips on the core during manufacture, a simple solution is to increase the density of the material forming the core, in order to thus increase the compression resistance of said core. However, the increase in the density leads to an increase in the rigidity, and thus a reduction in the shaping ability.
In the case of the formation of curved and/or rounded, indeed warped, surfaces, it is necessary to be able to fold, or more precisely to be able to curve, the panels. After a certain angle of curvature, in order to prevent rupture, one solution consists in cutting the core into units of smaller dimensions, so as to allow a shaping ability, and thus a gap between the units of material thus cut. A cutting technique is known consisting in cutting cubes of the same dimensions, in the core. The core cut in the form of cubes of the same dimensions is thus able to be curved and to fit to a curved surface. The major disadvantage is associated with the fact that such cutting into cubes cannot be kept in shape before use, because the cubes are dissociated. It is thus difficult to form large surfaces with ease.
One solution consists in adhesively bonding a flexible sheet, for example stitched, over the entirety of the upper or lower face of all the cubes, in order to interconnect them and preserve an integral entity which can be handled and easily transported. However, said stitched element requires the provision of additional material which, moreover, has to be adhesively bonded, and it is necessary to select from the materials which are compatible with the resin of the matrix.
It is also sometimes essential to connect said cut core elements when the method for manufacturing the composite material uses needle punching intended to cause bridging fibers, positioned on one face of a core, to pass to the opposite face of said core. Indeed, during the needle punching phase, it is essential to retain the material forming the core as though it were integral, in order that it can be needle punched without the risk of the needle moving or carrying away a piece of material forming the core, in the case mentioned above of cutting into cubes. Very often, the core has to be retained mechanically between two plates.
A known solution, for obtaining a shapable core, consists in forming partial cuts in the form of notches in the direction of the height of the core, but over a height that is smaller than the total height of said core.
The American patent application US 2011081514 proposes numerous solutions for shaping cores cut into cubes. Variants provide for machining of the panels according to the curvature, in particular
These notches thus increase the flexibility and the possibilities of curvature of the core. Nonetheless, the use of this method all the same makes shaping difficult, or indeed impossible, when the density of the core is significant, it being possible for any pronounced curvature to lead to ruptures adjacent to the notches. Indeed, in order to prevent dips during movements of workers on the core element, the density of the core must be sufficiently high, but the corollary is an increase in the rigidity. However, on account of the increased stiffness, the stresses are concentrated in particular in the thinner regions, which can thus cause breakages in the event of bending stresses that are too great, risking complete breakage of the core during shaping, and making the straight notches even more easily susceptible to rupture.
A product marketed under the designation ROHACELL® is also known, which can be cut and machined easily, but these are merely panels, without any reinforcement, and thus unsuitable for producing composite parts having high mechanical resistance, no through bridging fiber thus being provided. Said panels can be machined in order to be connected for example by a mortise and tenon joint, but such panels are unsuitable for the of composite material parts application according to the present invention.
Another problem is also that of limiting the mechanical resistance rupture regions, and the in-line cuts, such as those of the cubes, mentioned above, which causes lines of mechanical weakening. This is all the more noticeable when the cuts are straight and the panels do not have bridging fibers.
A problem to be overcome is that of the shaping ability. When a core has to be positioned on a given shape, for example a mold of a part of a boat, bridge, roof, it is noted that the shapable cores of the prior art, formed by cutting into cubes, exhibit some degree of resilience and still tend to return to a planar shape. This resilient effect disrupts the close-fitting shaping.
The present invention aims to overcome these problems by proposing panels which are highly mechanically resistant, solving the problems of punching, ensuring shaping ability, while remaining manipulable and offering all the combinations of materials for cores/skins/bridging fibers. The present invention proposes a method for manufacturing a shapable core for manufacturing a composite material product which offers possibilities for shaping on curved surfaces.
For this purpose, the method for producing a shapable core from a rigid panel, the plane of the panel being defined by the axes X and Y and the height H being oriented in the direction Z of an orthonormal reference frame, for producing composite material products, consists in cutting said panel to form core elements, the cuts being made along the axis Z, producing hooking means on each of the core elements cut in this way, so as to allow said core elements to be connected to each other and to produce a hinge connection with retention between said core elements in the plane XY. Each core element comprises hooking means in the form of protrusions and recesses, having mating profiles. Said hooking means are in the form of two protrusions and two recesses, having mating profiles and positioned on two opposing sides of each core element. In particular, the protrusions and the recesses are formed having the combined shape of a racket or mushroom, comprising a head and a thin connection.
The method consists, according to a variant, in making cuts in part according to the Z axis, at a height h that is less than the height H of the panel, creating a support base between the core elements. The different cuts are made using an oscillating blade, by milling, by laser, or using a punch.
According to another feature, bridging fibers FP are introduced into said shapable core, after the cutting of the core elements.
According to yet another feature, the method includes a step of surface treatment of applying a repositionable adhesive on at least one face of said shapable core.
The invention also covers the shapable core obtained by the implementation of the method.
The shapable core comprises bridging fibers FP which are oriented perpendicularly to the plane XY and/or are inclined. Said core is formed of a foam, in particular selected from the polyurethane foams. Said core advantageously comprises, on at least one face, repositionable adhesive.
The present invention will now be described according to a main embodiment and the variants thereof, with reference to the accompanying drawings, in which the various figures show:
In
According to a particularly advantageous arrangement of the invention, the core elements 16, forming the shapable core 10, are of an identical shape which is inscribed in a parallelepiped, and are positioned in the same plane XY. Said core elements 16 are provided with hooking means 17 for hooking with one another, formed of protrusions 18, in this case two protrusions 18, and recesses 20, in this case two recesses 20, having mating profiles for the protrusions 18 and developed in the plane XY. The two protrusions 18 and the two recesses 20 of each core element 16 are positioned, respectively, on two opposing sides of the core element 16, the recesses 20 being mating profiles, in geometric terms, capable of receiving the protrusions 18.
In the embodiment that is shown and retained, the protrusions 18 and the recesses 20 are formed having the combined shape of a racket or mushroom, comprising a head 18t, 20t and a thin connection 18m, 20m. Respectively, and as visible in the elevation view of two core elements 16 of
The geometry of the recesses 20 is virtually identical to that of the protrusions 18, to within a cut 14 thickness, i.e. to within the shaping space. Moreover, the width L18t is less than the width L20t, and the width of L18m is less than the width L20m, as specified in
The core elements 16 are also provided, on account of their geometry and their height H, with a vertical surface 24 which is deferred and is visible in
Said core 10 comprising bridging fibers FP is intended for receiving at least one composite skin 28, in this case two composite skins, placed above and below the core 10 and physically interconnected by said through bridging fibers FP. Said composite skins 28 may be formed in an entirely known manner, of a fabric of threads, or of a non-woven of fibers 30 and a resin 32. The bridging fibers FP are thus embedded in the resin 32, like the fibers or threads of the two composite skins 28, the resin also flowing along the bridging fibers FP, through the shapable core.
The bridging fibers FP contribute to interconnecting the core elements without preventing the shaping ability before the resin is put in place, because the through bridging fibers FP can slide through the material forming the shapable core 10 when the core elements are spaced apart, in particular on such small distances. Thus, as can be seen, the final product comprises a core, bridging fibers FP which connect the two skins in all directions providing the final product, comprising resin, with very high mechanical properties. The type and the characteristics of the fibers of the skins and the bridging fibers can be selected so as to be different or identical. There is a very great variety of combinations.
The production of the cut shapable core 10 according to the method of the present invention will now be described. The rigid panel 12 may be formed of a material different from a foam. Apart from the economic aspect, the material selected must have a certain capacity for resisting compression, in order to authorize movements of people without being deformed by contact by the operators. For a polyurethane foam, and in order to give an order of magnitude, this corresponds to a density of approximately 60 kg/m3 (i.e. approximately 0.5 to 0.6 MPa compression resistance).
The rigid panel 12 is shown rough, flat, as shown in
The patterns of the cuts 14 thus form core elements 16 that are identical in shape and geometry, and oriented differently in the plane XY. After the cuts 14 are formed, the protrusions 18 are directly positioned in the recesses 20 of mating shape, providing a mechanical connection of the core elements 16 in the plane XY.
Indeed, the mating shapes of the protrusions 18 and of the hollows 20 prevent any significant movement and thus any separation of the core elements 16 in the plane XY of the shapable core 10. In addition, since the cuts 14 are of a very small thickness, measured in the plane XY of the core, of the order of a few tens of mm, the core elements 16 can thus be separated only by performing a vertical translation according to the axis Z, of one core element 16 relative to another core element 16, which makes it possible to cause the protrusions 18 to translate vertically with respect to the hollows 20, or vice versa, in order to form an interlocking connection and a hinge with retention.
If, for practical reasons or reasons of implementation, it is necessary to exactly retain the core elements 16 relative to one another, flat or in a curved manner, in order to prevent the relative vertical offset of the core elements 16, a non-through cut 34 may be made. Said non-through cut 34 is formed at a height h, in part, which is smaller than the height H of the shapable core 10. The formation of non-through cuts 34 thus creates a support base 36 which is developed according to the plane XY and which retains all the core elements 16 together in the same plane. The support base 36 makes it possible to facilitate the implementation of the shapable core 10 and prevents any offset of the core elements 16 relative to one another, and the low height provides shaping ability. The support base has another advantage during the production of the shapable core 10—when the core elements 16 undergo the addition of bridging fibers FP by needle punching, after the cuts 14 have been made, it is necessary to retain the core elements 16 relative to one another. Indeed, the needle punching of fibers through the core 10 is performed using needles which, on account of their hooking power, can possibly carry along a core elements 16 upon retraction of the needle and after the bridging fibers FP have passed into the material forming the core elements 16. Of course, it is not desirable for the core elements 16 to be taken away or carried along by the needle, and it is thus necessary to retain the core elements 16 by means of clamping, and thus prevent any movement of said core elements 16 according to the axis Z. The support base 36 also contributes to easily preventing the movement of the core elements 16 according to the axis Z, and makes it possible to retention the core elements 16 in their initial position, during the needle punching of the bridging fibers FP.
In the same way, when each shapable core 10 is cut in order to follow the contours of a geometric shape, the pieces of the core elements, thus cut, at the periphery are retained by the mechanical hinges 22 and by the continuity of the support base 36.
The stresses and the movements of the shapable core 10 in the plane XY are absorbed by the mechanical hinges 22. On account of their geometry, the mechanical hinges 22, formed by the protrusions 18 and the hollows 20, are able to absorb the stresses in the plane XY. Indeed, the width L18t of each of the heads 18t of the protrusions 18 is wider than the width L20m of each of the thin parts of the hollows 20. Separation of the protrusions 18 and the hollows 20 according to the plane XY is thus impossible, which thus prevents any separation of the core elements 16 according to the plane XY, in all directions of said plane, and thus allows for absorption of stresses in the plane XY, while allowing a hinge effect having a limited angulation but sufficient for shaping ability.
Except for the core elements 16 positioned at the periphery of a shapable core 10, each core element 16 is fitted together with four other core elements 16-1, 16-2, 16-3 and 16-4 which surround it, visible in
As is visible in these same figures, the mechanical hinges 22 created by the protrusions 18 and the recesses 20 also make it possible to retain the core elements 16, cut so as to form a transverse cut of the shaped core 10. The present invention thus allows any form of cut, without the risk of the core elements 16 separating from one another.
In the case of use of bridging fibers FP, the retention of the core elements 16 is also achieved by the bridging fibers FP themselves, which thus promote the handling and positioning of the shapable core 10 before composite skins or any other stratification element is applied.
According to a variant of the method according to the present invention, it is also possible for the shapable core according to the present invention to undergo a surface treatment. Said surface treatment consists in applying repositionable adhesive 38 to at least one of the faces of said shapable core 10. Said repositionable adhesive may be applied by spraying in the solvent phase, or hot if the adhesive is of the hot melt type, to cite just these examples. A detailed method of an implementation of this kind is found in the patent application FR 2.865.431, in the name of the same applicant. In this case, this method has an application from the perspective of the placement of the shapable core 10 according to the present invention, in a vertical mold for example, or on a slope, such as a boat hull or boat bridge mold. Said repositionable adhesive 18 has another significant advantage which has never been mentioned because the question had not arisen before the existence of the present invention.
This advantage is illustrated in
It will also be noted that the method according to the present invention makes it possible to produce cores of rigid material which are shapable, in order to form rolls, which is of some advantage with regard to handling during transport or on site, or indeed for the use of large surfaces.
| Number | Date | Country | Kind |
|---|---|---|---|
| FR2001306 | Feb 2020 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2021/052957 | 2/8/2021 | WO |
