TECHNICAL FIELD

[0001] The present disclosure relates to a composite panel and a method for manufacturing such a composite panel.

[0002] Such a composite panel or method is particularly useful to provide light but strong parts able to withstand high bending loads. Such a composite panel or method can be used in any technical field for forming parts of any type. For example, it can be used in the automotive industry for manufacturing automobile parts.

TECHNOLOGICAL BACKGROUND

[0003] Composite sandwich panels are known panels comprising two face sheets separated by a low density core, such as a honeycomb core. These panels show high bending stiffness, as the core separates the face sheets from the bending neutral axis, while having a low weight thanks to the low density core.

[0004] However, when such a core includes open cells, while molding the face sheets on the core, some material enters the core and partly fills the core cells, thus leading to a weight increase and, of course, to an increase in consumption of material and related costs.

[0005] As a result, in order to avoid this situation, it has been suggested to bond a closing film on the main faces of the core so as to close the cells, thus preventing material to enter the cells during the molding of the face sheets.

[0006] However, the adhesion of the face sheet to the closing film is rather poor and constitutes therefore the weakest link in edgewise loading of such a composite sandwich panel. Consequently, when loading such a composite panel equipped with a closing film under edgewise in-plane loading, the face sheets separate from the core prematurely, thus not achieving the predicted edgewise loading strength of the composite sandwich panel.

[0007] A proposal to increase the adhesion of face sheets to the core was to select materials for the closing film and the face sheets able to chemically react so as to create chemical links between the closing film and the face sheets. However, such a solution brings important constraints in the material selection, thus reducing the design freedom of the panel.

[0008] Another proposal was to embed short fibers in the closing films so as interlock with the material of the face sheets. However, embedding fibers to the closing films complicate the manufacturing method for limited results

[0009] As a result, there is a real need for a composite panel and a method for manufacturing such a composite panel which are, at least partly, devoid of the drawbacks of the above mentioned known methods. SUMMARY OF THE INVENTION

[0010] The present disclosure relates to a composite panel, comprising a core, having cells open to at least a first face of the core, a closing film, bonded to the first face of the core, a face sheet, bonded to the closing film, wherein the closing film comprises holes, and wherein penetrating portions of the face sheet penetrate at least partially into at least some holes of the closing film to interlock the face sheet with the closing film.

[0011] Thanks to this configuration, it is possible to close the cells of the core, thus getting the benefits mentioned above, while ensuring a strong adhesion of the face sheet to the core, thus reducing the risk of a debonding of the face sheet under edgewise loading for example.

[0012] Particularly, this configuration provides a mechanical interlocking ensuring a strong cohesion whatever the materials of the closing film or the face sheet.

[0013] In addition, perforating the closing film, or providing a closing film already perforated, is easy so that manufacturing such a composite panel remains simple. Furthermore, the penetration of the face sheet material into some holes of the closing film can happen naturally thanks to capillarity without the need of adapting the molding step of the face sheet.

[0014] Accordingly, this composite panel is light, strong, and easy to manufacture.

[0015] In some embodiments, the cells of the core are open columnar cells extending along a common cell direction. Such columnar cells bring a high stiffness in their cell direction while having a very low density. In addition, the present invention is all the more useful for such cores. [0016] In some embodiments, the common cell direction is normal to the first face of the core. This brings an increased stiffness.

[0017] In some embodiments, all the cells of the core are identical.

[0018] In some embodiments, the cells of the core are organized according to a honeycomb structure. Such a structure brings a very high stiffness and reliability. However, in other embodiments, the cells could have square or triangle sections, for example.

[0019] In some embodiments, the core is a continuous foam.

[0020] In some embodiments, the diameter of the cells is comprised between 5 and 50 mm. In the present disclosure, the use of "diameter" is not intended to be restricted to circular shapes: indeed, more generally, the diameter is defined to be the largest distance that can be formed between two opposite parallel lines tangent to the boundary of the considered shape, i.e. the cell section in the present case. [0021] In some embodiments, the core comprises at least one of a metal, a polymer or a composite material. More specifically, the core can be completely made in one of these materials.

[0022] In some embodiments, the first face of the core is flat. Preferably, both main faces of the core are flat. [0023] In some embodiments, the closing film comprises at least one of a polymer, a metal or a composite material. More specifically, the closing film can be completely made in one of these materials.

[0024] In some embodiments, the face sheet comprises, and is preferably made in, a polymer, preferably a resin. [0025] In some embodiments, the closing film comprises at least three holes, arranged in a non-linear configuration. This configuration is enough to ensure a proper adhesion of the face sheet in the whole area located between these three holes.

[0026] In some embodiments, the closing film has a hole density of at least 0.01 hole/cm ² , preferably at least 0.1 hole/cm ² , still preferably at least 1 hole/cm ² . The higher the hole density, the stronger the adhesion of the face sheet to the closing film.

[0027] In some embodiments, at least 10%, preferably at least 30%, still preferably at least 90%, of the cells corresponds with a hole of the closing film. [0028] In some embodiments, some cells correspond with several holes of the closing film.

[0029] In some embodiments, the hole density of the closing film is variable in some areas of the closing film. Accordingly, it is possible to tailor the adhesion of the face sheet if an increased bending stiffness or impact strength is required in specific locations.

[0030] In some embodiments, the holes of the closing film are arranged according to a periodic pattern. This leads to steady adhesion of the face sheet all over the surface of the closing film. However, in other embodiments, the holes could be arranged according to no periodic pattern, possibly randomly.

[0031] In some embodiments, the holes of the closing film are arranged according to a uniform pattern.

[0032] In some embodiments, the diameter of at least 50%, preferably at least 90%, of the holes is comprised between 0.2 and 2 mm. The inventors have determined that this range is a good compromise with a diameter large enough to allow the penetration of material while remaining small enough to prevent too large a quantity of material from penetrating the holes. Particularly, the diameter of the holes can be tuned according to the viscosity of the material of the face sheet during the molding step. [0033] In some embodiments, the diameter of at least one penetrating portion is greater than the diameter of the corresponding hole penetrated by said penetrating portion. Preferably, this is the case for at least 50% of the penetrating portions, still preferably for 90% of them. Accordingly, the penetrating portions are locked beyond the holes so that the face sheet cannot be removed without breaking the penetrating portions and/or the closing film, which increases the adhesion of the face sheet to the closing film.

[0034] In some embodiments, the core comprises two opposite main faces, the first and a second face, and the cells of the core are open to the first and the second faces. [0035] In some embodiments, a second perforated closing film is bonded to the second face of the core.

[0036] In some embodiments, a face sheet is bonded to each perforated closing film. Accordingly, a sandwich composite panel can be formed, which highly increases the bending stiffness of the panel. [0037] The present disclosure also relates to a method for manufacturing a composite panel, comprising the steps of: providing a core having cells open to at least a first face of the core, bonding a closing film on the first face of the core, perforating the closing film so as to create holes in the closing film, coating the perforated closing film with a fluid surface material such that at least a portion of the fluid surface material penetrates at least some holes of the perforated closing film, and solidifying the fluid surface material such that a face sheet is formed interlocked on the perforated closing film. [0038] In some embodiments, the perforating step takes place after the bonding step.

[0039] However, in other embodiments, the perforating step could take place before the bonding step.

[0040] In some embodiments, the holes are created thanks to a reciprocating needle.

[0041] In some embodiments, the holes are created thanks to a plate equipped with several needles.

[0042] In some embodiments, the holes are created thanks to a rotating cylinder equipped with several needles. [0043] In some embodiments, the coating step is carried out in a mold.

Particularly, the coating step can be carried out through a resin transfer molding (TRM), a wet compression molding, or a vacuum infusion, for example.

[0044] The above mentioned features and advantages, and others, will become apparent when reading the following detailed description of exemplary embodiments of the presented composite panel and manufacturing method. This detailed description refers to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0045] The accompanying drawings are diagrammatic and seek above all to illustrate the principles of the invention.

FIG 1-5 illustrate succeeding steps of an exemplary method for manufacturing a composite panel according to the disclosure.

FIG 6 is an enlarged view of the area VI of FIG 4.

FIG 7A-C illustrate, in section views, different exemplary perforating steps. DETAILED DESCRIPTION OF EMBODIMENTS

[0046] In order to make the invention more concrete, exemplary embodiments of composite panels and exemplary manufacturing methods are described in detail below with reference to the accompanying drawings. It should be recalled that the invention is not limited to these examples.

[0047] FIG 1 illustrates a core 10 in perspective view. This core 10 is a honeycomb core composed of identical hexagonal columnar cells 11, partitioned by peripheral walls 12, extending next to each other along a common cell direction. The core 10 comprises two opposite main faces 13, flat and orthogonal to the cell direction; each cell 11 is open to both main faces 13.

[0048] In the present example, the core 10 is made in metal, such as aluminum, has a thickness of 6 mm and a cell diameter of 4 mm. In another example, the core 10 could be made in polymer, such as Polypropylene. [0049] FIG 2 illustrates a step of bonding a closing film 20 on each main face 13 of the core 10. The closing films 20, preferably identical, are bonded all over the main faces 13 so as to close every cell 11 of the core 10 at both ends.

[0050] The closing films 20 can be bonded on the core 10 thanks to any adapted technique, and notably thermal welding. [0051] In the present example, each closing film 20 is a polymer film, for example in polypropylene, and has a thickness of 0.05 mm.

[0052] FIG 3 illustrates a step of perforating the closing films 20: holes 21 are perforated in each closing film 20.

[0053] In the present example, in each closing film 20, a hole 21 is perforated facing each cell 11, substantially in the center of the cell section; each hole 21 has a diameter of about 0.64 mm.

[0054] These holes 21 can be perforated thanks to different techniques. For example, as illustrated in FIG 7A, the core 10 closed by the closing films 20 can be driven in translation while a reciprocating needle 91 perforates the closing film 20 on a regular basis as a sewing machine. Particularly, the speeds of the core translation and of the needle movement can be tuned so as to set the desired hole distribution over the whole closing film 20.

[0055] In another example, illustrated in FIG 7B, the core 10 closed by the closing films 20 is driven in translation while a plate 92 equipped with several needles 92a reciprocates and creates several holes 21 at a time. The plate 92 can have a single row of needles 92a or a whole two-dimensional array of needles 92a.

[0056] In yet another example, illustrated in FIG 7C, the core 10 closed by the closing films 20 is driven in translation while a cylinder 93 equipped with several needles 93a is rotated at the edge of the closing film 20 so as to continuously perforate the closing film 20 from one end to the opposite end. Particularly, the cylinder 93 can extend over the whole width of the core 10 and be equipped with several rows of needles 93a, potentially offset from one row to the other. [0057] FIG 4 illustrates a step of coating the closing films 20 with a fluid surface material 30'.

[0058] This coating step can be carried out thanks to any adapted technique. For example, the core 10 closed by the closing films 20 can be introduced in a mold, leaving a gap between each closing film 20 and the walls of the mold, and the fluid surface material 30' can be injected in the mold so as to fill said gaps.

[0059] During this coating step, since the surface material 30' is fluid, it spreads over the whole surface of the closing film 20 and penetrates the holes 21 thanks to capillarity, forming droplets 3 extending beyond the holes 21 and thus jutting out from the closing film 20 into the inner space of the cells 11.

[0060] Considering the diameter of the holes 21, the fluid surface material 30' is fluid enough to penetrate the holes 21 but viscous enough to stick together so that the droplets 3 remain attached to the surface material lying on the outer surface of the closing film 21 without substantially filling the cells 11.

[0061] Particularly, as better visible on FIG 6, the coating step is continued until the droplets 3 reach a diameter greater than the diameter of the holes 21 but still lesser than twice the diameter of the holes 21.

[0062] In the present example, the surface material is a polymer resin, such as Epoxy, having a viscosity of 30 Pa.s at the temperature of the coating step (i.e. about 120°C).

[0063] Once the coating step is over, that is when the fluid surface material 30' has formed a substantially uniform layer together with droplets 3 penetrating the holes 21, the surface material is solidified during a solidifying step. Particularly, in the present example where the surface material is a thermosetting resin, the mold is heated in an oven at 120 °C during 1-2 min so as to cure the resin.

[0064] Once the solidifying step is completed, the sandwich composite panel 1 of FIG 5 is obtained. The panel 1 therefore comprises a central core 10, having a honeycomb structure, closing films 20 bonded to both main faces 13 of the core 10, and face sheets 30 bonded to each closing film 20, thus flanking both main faces 13 of the core 10. The face sheets 30 correspond to the solidified layer of surface material 30': once solidified, the droplets 3 form solid penetrating portions 31 of the face sheets 30 which penetrate the cells 11 of the core 10 through the holes 21 of the closing film 20.

[0065] Because the diameters of the penetrating portions 31 are greater than the diameter of the holes 21 of the closing films 20, the face sheets 30 are mechanically interlocked on the closing films 20.

[0066] In the present example, the face sheets 30 are formed by a solidified resin only; however, in other examples, the face sheets 30 could also comprise a reinforcement component, such as a fiber preform, embedded in a polymer matrix. In such a case, the penetrating portions are formed by droplets 3 of solidified matrix.

[0067] Even if the present invention has been described with regard to particular exemplary embodiments, it is clear that these examples may be modified without departing from the scope of the invention as defined by the claims. Particularly, individual features of different presented embodiments can be combined in additional embodiments. As a result, the description and the drawings shall be considered in an illustrative way rather than a limitative way. [0068] It is also clear that all the features described with reference to a method are transposable, individually or in combination, to a device, and vice versa.