Patent Publication Number: US-6669519-B2

Title: Methods for manufacturing a structural sub-assembly and a gliding board; structural sub-assembly and gliding board made by such methods

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon French Patent Application No. 01.16965, filed Dec. 19, 2001, the disclosure of which is hereby incorporated by reference thereto in its entirety, and the priority of which is hereby claimed under 35 U.S.C. §119. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a sub-assembly provided to make a gliding board, particularly a board for gliding on water, such as a surfboard float. 
     The invention also relates to a method for manufacturing such sub-assembly, and a gliding board made from such sub-assembly. 
     2. Description of Background and Relevant Information 
     Conventionally, a surfboard float is made from a foam blank, particularly a polyurethane foam, which is formed in a mold. The foam blank is machined by planing and sanding over a low thickness to locally customize its shape, then it is coated with an envelope made of resin-impregnated fiberglass that forms an outer reinforcement shell and gives the float its final form. Decorating and glassing give the float its final aspect. 
     In certain cases, the foam blank is longitudinally cut in two sections that are then glued against a wooden stringer that reinforces its structure and imparts to it a predetermined longitudinal camber. 
     The drawback to such a constructional technique is the weight of the final float. Indeed, the foam is relatively dense, its typical density is 50 kg/m3. And theoretically, it is not possible to decrease the density of the foam without negatively affecting the mechanical properties of the float. 
     According to another constructional technique originating from windsurfing, one starts with a foam blank having a relatively low density (for instance, 18 kg/m3) which is machined to shape. This blank is covered with a skin made of resin-impregnated fiberglass. An envelope of foam having a higher density is attached around this sub-assembly. Then, one applies webs of resin-impregnated fiberglass in order to form the outer shell. 
     Such a constructional method allows for a weight reduction of approximately 20% or more while maintaining a good rigidity underfoot. However, its implementation is relatively complex. In addition, the central foam blank is usually made of expanded polystyrene foam. This material has the defect of taking in water. During its lifetime, the float may be thrown against a reef or a rock. If the outer shell is damaged, one runs the risk of water infiltrations, the water weighing down the float and being rather difficult to evacuate. 
     Lastly, it is known to make hollow floats having sandwich skins. Either one makes two half-shells that are then assembled together, or one makes the assembly in a closed mold with an inner bladder that is inflated in order to push and press the sandwich skins against the walls of the mold. In any case, these types of floats have walls of constant thickness. 
     This manufacturing technique provides results in the production of light-weight boards. However, it is not possible to customize the form of the float. In this case, the form of the outer shell depends exclusively on the form of the mold. 
     SUMMARY OF THE INVENTION 
     According to the invention, an improved sub-assembly is provided which enables the manufacture of lighter-weight gliding boards, ones that have a greater volume for an equal weight, while maintaining a form that can be customized. 
     More particularly, according to a method of the invention, a structural sub-assembly for a gliding board is manufactured by a method that includes the following: 
     forming two half-shells; 
     assembling the two half-shells by means of an adhesive resin foam. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood from the following description, with references to the attached drawings related thereto, and in which: 
     FIGS. 1 and 2 schematically show, in a transverse cross-section, an embodiment of a structural sub-assembly having two half-shells assembled according to the invention, respectively before and after gluing; 
     FIGS. 3 and 4 are schematic views showing a detail of FIG. 2, respectively before and after machining the lateral rails of the sub-assembly; 
     FIGS. 5 and 6 are views similar to those of FIGS. 3 and 4 showing an alternative embodiment of the invention in which the foam glue is spread so as to form an excess thickness on the exterior of the sub-assembly. 
     FIGS. 7 and 8 are views similar to those of FIGS. 3 and 4 showing an alternative embodiment of the invention in which the foam glue is spread so as to form an excess thickness on the interior of the sub-assembly that, at least in this embodiment, is hollow; 
     FIGS. 9-11 are views similar to those of FIGS. 2-4 showing an alternative embodiment of the invention; 
     FIGS. 12 and 13 are views similar to those of FIGS. 1 and 3 showing yet another embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As known, a surfboard float is an elongated board having a central portion, a tapered and slightly raised front tip, and a slightly raised rear tail that has a decreased width. 
     FIGS. 1-4 show a first embodiment of the invention in which a structural sub-assembly is made from two assembled half-shells. In the example shown, the sub-assembly is a hollow sub-assembly constructed according to a sandwich technology. Such a particularly light sub-assembly is used advantageously in manufacturing a surfboard float. For other types of gliding boards, the invention can be implemented with other embodiments of the sub-assembly. 
     The sub-assembly  10  can thus be formed of an upper half-shell  22  that will form the final float deck and of a lower half-shell  24  that will form the hull. Each half-shell is formed of a foam plate  26 ,  28  that is shaped first. Depending on the type of foam used, this shaping can use various techniques. The foam used, for example, can be an extruded polystyrene foam that is generally in the form of a flat plate and that can be shaped by thermoforming to become a shaped plate, as illustrated in the figures. In the example shown, this shaped plate  26  is covered, on an inner surface  30 ,  32 , with at least one inner envelope layer (for example, a fabric made of fiberglass, carbon fibers or another fibrous material impregnated with a polyester or epoxy resin, or the like). In the example shown, the operation for laminating the inner surface  30 ,  32  of the half-shells  22 ,  24  can be advantageously performed under vacuum while the previously thermoformed foam plate  26 ,  28  is still in the thermoforming mold, so that the resin-coated layer of fabric can harden on the thermoformed plate while the latter is still pressed against the mold. The form of the half-shell is thus better guaranteed before assembly. 
     When the two half-shells  22 ,  24  are assembled together, for example, by gluing, one directly obtains a hollow rigid inner shell that is formed of resin-coated fabric layers arranged on the inner surfaces of the half-shells, on the one hand, and by an external foam envelope capable of being machined, on the other hand. The foams used, for example, can be extruded polystyrene foam plates having a density on the order of 30-50 kg/m 3 . The sub-assembly achieved, therefore, has at least one hollow inner portion  50  that gives it a substantial lightness without sacrificing its rigidity. 
     In order to implement this embodiment of the invention, it can be advantageous to provide for one of the half-shells, for example, the lower half-shell  24 , to be also laminated on its outer surface  34  before assembling the two half-shells. The half-shell thus laminated on its two surfaces  32 ,  34  is then particularly rigid during assembly with the other half-shell, which allows better control of the precision of the assembly and, therefore, a better control of the precision of the form of the sub-assembly. The foam envelope covering the shell is then no longer capable of being machined over its entire area. Indeed, since one of the surfaces is already laminated at the time of assembly, the geometry of this surface can no longer be substantially modified. Nevertheless, it has been noted that, in order to substantially modify the final behavior of the gliding float, it is often sufficient to modify the geometry of the lateral edges of the float (generally called the rails of the float). However, this geometry can be modified even if one of the outer surfaces of the float (for example, the lower surface), is already laminated. 
     In the example shown in FIGS. 1-4, one can see that the two half-shells are not symmetrical. Indeed, one can see that the lower half-shell  24  does not have lateral edges. During its shaping, the foam plate  28  is curved in the longitudinal direction (which is not shown in the drawings) to follow the longitudinal camber curve (sometimes called a “rocker” or a “scoop” curve). It could also be curved in the transverse direction, for example, to form a V-shaped hull that is concave or double concave. However, in the example shown, the lower half-shell does not have a transverse curve. In this case, because the deformation of the foam plate as compared to its initial planar state is relatively small, the shaping of the plate can occur without thermoforming, by simply pressing the plate against the mold by depression at the time of lamination. After the resin hardens, the rigidity of the resin-coated fabric  32  suffices to maintain the plate to the desired shape of the half-shell. 
     Conversely, the upper half-shell  22  is thermoformed so as to be curved longitudinally but also transversely to form lateral edges  36  that are curved downwardly. According to the invention, the inner surfaces (i.e., the lower surface  30  of the upper half-shell  22  and the upper surface  32  of the lower half-shell  24 ) are laminated with one or several fabric layers made of a fibers impregnated with a thermosetting resin. As can be seen in FIG. 1, the lower surface  34  of the lower half-shell  24  is also laminated before assembling the two half-shells. 
     As can be seen in FIG. 2, the assembly of the two half-shells is obtained by arranging glue  42  in the interface zone constituted of the lower edge of the lateral edges  36  of the upper half-shell  22 , on the one hand, and of the corresponding peripheral portion of the upper surface  32  of the lower half-shell  24 , on the other hand. 
     According to the teachings of the invention, gluing is obtained by means of an adhesive resin foam, for example, a polyurethane foam. For example, a polyurethane foam such as MDI (Methyl-Diphenyl-Isocyanate) can be chosen, which has a polyol/isocyanate ratio (in weight) on the order of 100/140 or on the order of 100/110. The foam will have, for example, a free expansion density on the order of 25-200 kg/m 3 . The density of the resin in the area of the interface of the two half-shells can be more substantial due to the compression effect. According to a particular non-limiting embodiment, the resin can be applied while in liquid form so as to allow a perfect coupling of the two half-shells. The two half-shells would then be maintained pressed against each other while the resin is setting to avoid any risk of deformation due to swelling of the foamy adhesive resin during its expansion. 
     One can also use other types of adhesive resin foams, such as, for example, epoxy foams, having a slightly higher free expansion foam density. In any case, the adhesive resin foam can be colored, if desired. 
     With this construction, one can see in FIG. 3 (which shows in more detail the lateral edge of the sub-assembly right after assembly) that the top largest portion of the lateral ledge  38  of the structural assembly is formed of the lateral edges  36  of the upper half-shell whose outer surface  40  is constituted of foam. The lower portion of these lateral edges is constituted of the lateral side edge of the lower half-shell, the latter having a foam thickness  28  surrounded (on top and on bottom) by two thicknesses of resin-impregnated fabric layers  32 ,  34 . Since the thicknesses of the fabric layers  32 ,  34  are very low, they do not form an obstacle to the shaping by machining at the lateral edges. 
     Between the lower and upper portions of these lateral edges, one can see the glue line that is obtained by means of the adhesive resin foam  42 . Because, as seen in transverse cross section in FIG.  3 . e.g., the lower edges  36  of the foam  26  are exposed, a direct contact is provided between a surface of the foam  26  and the adhesive resin foam  42 . Due to the structure of this gluing material, i.e., adhesive resin foam. Which is very close to that of the foam which is to be machined, one avoids the presence of a particularly hard layer which would be due to the overlaying of a compact and rigid layer of glue, such as, for example, an epoxy glue, on the laminated upper surface  32  of the lower half-shell  24 . 
     Thus, in FIG. 4, one can see that the geometry of the lateral edge  38  of the structural sub-assembly has been modified over the entire height of the lateral edge  38 , for example, by planing and sanding. 
     However, in an alternative embodiment, one could make the peripheral portion of the upper surface  32  of the lower half-shell  24  without a lamination, such that the lateral edges  36  of the upper half-shell  22  are assembled by the foam glue line  42  directly against the foam  28 , in order to ensure a better continuity of the material forming the lateral edge  38  that, due to the invention which provides using a adhesive resin foam, is therefore only constituted of foam. Such a feature exists in the embodiment described hereinbelow in relation to FIGS. 12 and 13. 
     The lamination of one of the outer surfaces of the sub-assembly  10 , in this case the lower surface  34  of the lower half-shell  24 , can be complete (as shown). Also, it can relate to only a portion of the surface  34 , for example, the central portion, to further improve the machinability of the lateral edge  38 . 
     With this construction, the precise assembly of the two half-shells is facilitated by the great rigidity of the lower half-shell, and the sub-assembly remains capable of being machined over its entire upper surface and over its lateral edges, allowing a great capacity for customizing the sub-assembly. Once customized, the structural sub-assembly is covered with an outer envelope, for example, a layer of resin-impregnated fibers to form the gliding board. As the case may be, one can choose to also cover the outer surface  34  of the sub-assembly, that is already laminated, with this outer layer so as to increase the rigidity and the solidity of the float or, on the contrary, one can choose not to cover this surface  34 , that is already laminated, in order to limit the weight of the float. 
     In the case where one wishes to promote the possibility of customizing the float hull, one could provide for the half-shell that is laminated on its two surfaces to be the upper half-shell, the lower half-shell then being laminated only on its upper surface  32 . Optionally, a portion of this envelope (whose geometry one does not wish to modify, for instance, the upper surface of the upper half-shell or the lower surface of the lower half-shell) can be covered with a rigid external layer. 
     To improve its rigidity further, one can advantageously provide the structural sub-assembly of FIGS. 1-4 with a longitudinal central partition that vertically connects the two half-shells, such a partition being furthermore known to one skilled in the art as a stringer. Such a central partition is for example made of foam or of wood. The partition can be edged with two layers made of resin-impregnated fibers that eventually connect continuously to the wall of the shell. 
     The present description is given only by way of example, and one could adopt other embodiments thereof without leaving the scope of the present invention, for example, by using several-longitudinal, transverse or otherwise appropriately directed partitions, these partitions forming linkages between the deck and the hull of the float. These partitions can possibly create a partitioning of the inner shell into several waterproof compartments. 
     The sub-assembly according to this first embodiment can be machined in the same manner as a conventional foam blank, depending on what the shaper wishes, as long as the machining thickness remains distinctly smaller than the foam thickness. 
     The two embodiments shown in FIGS. 5-8 have the advantage of allowing a greater machining depth without risking a substantial decrease in the mechanical strength of the sub-assembly. 
     Indeed, in the second embodiment shown in FIGS. 5 and 6, one can see that the foamy adhesive resin  42  is not laid only on the interface of the two half-shells, but it is on the contrary laid so as to form an excess thickness  44  on the exterior of the sub-assembly along the entire length of the lateral edge  38  thereof. 
     The shape of this excess thickness can be defined with precision by means of a mold applied along the lateral edge  38 . This mold can be part of a tool ensuring the correct positioning of the two half-shells during assembly. The shape of the excess thickness can also be given by a depressurized flexible enclosure in which the subassembly is enclosed so as to exert the pressure necessary for gluing on the half-shells. In both cases, the foam that forms the excess thickness is therefore compressed. Conversely, one can also allow the resin foam to expand freely. In the example shown, the excess thickness extends first of all along the outer surface  40  of the upper half-shell (one can see that the thickness of the excess thickness of glue progressively decreases as one moves away from the interface zone), and secondly over a portion of the upper surface  32  of the lower half-shell  24  that, in this example, projects transversely toward the exterior in relation to the upper half-shell  22 . One could provide for this projection to be less pronounced or even non-existent, in which case the excess thickness would also extend along the side edge of the lower half-shell  24 . 
     Because the excess thickness  44  is constituted of foam, it is easily machined, and the increase in foam thickness that it forms allows greater freedom in the choice of the final geometry of the lateral edge  38  after machining, as the latter maintains a thickness that is substantially equivalent to that of the non-machined portions of the subassembly. Thus, the float is not embrittled. 
     Advantageously, the gluing of the two half-shells and the forming of the excess thickness will occur during a single operation. Nonetheless, one can choose in certain instances to execute these two operations successively. 
     Another advantage of making these excess thicknesses with the adhesive resin foam lies in the fact that one thus increases the gluing surface between the two half-shells and, therefore, the strength of the assembly. 
     In the embodiment shown in FIGS. 7 and 8, the excess thickness  44  is obtained on the inner side of the interface. It is naturally assumed then that the structural subassembly is hollow, i.e., the shell elements  22  demarcate at least one hollow inner portion  50 . In this embodiment, one can either let the excess thickness expand freely, or one can confine it by means of a strip  46  of resin-impregnated fabric, for instance, that connects the upper surface  32  of the lower half-shell  24  to the lower surface  30  of the upper half-shell. This strip  46  thus forms a confinement partition that prevents the foamy adhesive resin from extending too much toward the interior of the cavity of the hollow body. It also allows ensuring, through the pressure due to the expansion of the foamy resin, a complete contact between the excess thickness  44  and the two half-shells. When the excess thickness  44  of glue is in the hollow inner portion  50 , one can see in FIG. 8 that it is not affected by the final machining of the sub-assembly  10 . As a result, the excess thickness  44 , if it has a substantial volume and/or weight, can influence the mechanical behavior of the final board (particularly the flexional and torsional stiffness), but it can especially modify the distribution of weight and inertia of the board, thereby modifying its dynamic behavior. The size of the excess thickness can be made to vary along the periphery of the board so that, for example, it is non-existent in certain areas and conversely very substantial in others. 
     The embodiments shown and described hereinabove are constructed on the basis of a structural sub-assembly in which each half-shell is formed from a thermoformed sheet of foam having a substantially constant thickness. 
     In the alternative embodiment of FIGS. 9-11, the technique for constructing the upper half-shell  22  is slightly different in order to allow one to very easily obtain an upper half-shell  22  having, in the area of its lateral edges  38 , a foam thickness  26  that is greater than the foam thickness of the float deck, at least prior to machining the lateral edges  38 . Thus, one can choose to make the foam layer  26  of the upper half-shell  22  by molding, this molded layer  26  then being covered on its lower surface with a laminated inner layer  30 . This construction has a double advantage: on the one hand, it gives an increased foam thickness allowing more freedom at the time of machining to obtain the desired form and, on the other hand, it increases the area of the interface zone used for gluing, which reinforces the solidity of this gluing. 
     To increase the gluing area even further, one can see that the side edge of the upper half-shell  22  forming part of this interface is not flat. It has a recess  48  whose cross-section can be semi-elliptic as shown, but which can also have other profiles. The contact surface between the glue  42  and the upper half-shell  22  is therefore increased. Due to the fact that the glue is a foaming resin that expands, one is certain that the recess  48  is completely filled by the glue that comes in contact with the wall demarcating this recess. Gluing is therefore optimal. 
     After machining and as can be seen in FIG. 11, there is a thickness in the foam envelope  28  that is substantially equivalent, along the lateral edge  38 , to what it is on the rest of the half-shell  22 . Thus, machining does not create a weakening of the structural sub-assembly. In the case where a recess  48  of the interface has been provided, one can see that machining reveals a portion of the glue line  42  whose thickness is relatively substantial. The fact that the glue line is made by means of a foamy and therefore easily machinable material is particularly important here in order to make possible an easy shaping of the lateral edge  38 . 
     FIGS. 12 and 13 show an embodiment of the invention that has some of the features that have already been described. The upper half-shell  22  is formed of a foam plate  26  whose thickness is substantially constant and which is laminated on its inner surface  30 . The lower half-shell  24  is laminated on its two surfaces  32 ,  34 , but one can note that the peripheral edge  52  of the upper surface  32  is not laminated, such that the gluing of the two half-shells occurs foam-to-foam. The side edge of the upper half-shell, which is adapted to be opposite the peripheral edge  52  of the lower half-shell  24 , has a recess  48  similar to the one described in the previous embodiment. In this case, the recess  48  is formed only on the inner side of the side edge, i.e., on the side of the inner cavity  50 . In this embodiment, it is provided to make an excess thickness  44  on the inner side of the glue line  42 , i.e., in the inner cavity  50 . In order to keep the excess thickness from overly extending transversely toward the interior of the cavity, a confinement partition  54  forming a barrier along the contour of the board has been arranged, for example, on the inner surface  34  of the lower half-shell  24 . This confinement partition  54  can have a height that is lower than the height of the cavity  50  in the corresponding zone such that it does not come in contact with the upper half-shell  22  as seen in FIG.  13 . This alternative is advantageous in terms of ease of manufacture, since there is no specific height tolerance to be kept for the confinement partition. On the contrary, one could provide for this confinement partition to be adjusted in height to come barely into contact with the upper half-shell. If it is more restrictive in terms of manufacturing, this alternative has the advantage of conferring to the confinement partition an additional function of vertical reinforcement and stiffening in addition to its first function of confining the glue line. This confinement partition can be made from a block of foam that is cut to the desired shape, or from a cord of foam that is allowed to expand freely on the inner surface  34 . It can also be made in the form of a plate element made from a rigid material, for instance, wood, or a sandwich material. One can arrange a confinement partition along the entire interface between the two shell elements or, conversely, one can provide partition sections only along certain zones of the interface. In any case, the confinement partition demarcates, in the vicinity of the gluing interface, a predetermined volume that the glue foam fills when expanding, thus coming into contact with the inner surfaces  30 ,  32  of the two half-shells. 
     Once assembled, this embodiment shows the advantage that the contact surfaces of the glue foam with the two half-shells are very important. In particular, a good contact between the glue and the upper half-shell  22  is ensured due to the confinement partition  54 . Furthermore, the non-laminated peripheral edge  50  and the inwardly off-centered recess  48  make it very easy to machine the lateral edge  38  that is almost exclusively composed of foam. 
     Finally, once shaped, the structural sub-assembly with its machined foam layer is provided to be covered with an outer envelope. This outer envelope is preferably a web  9  (woven or non-woven) of fiberglass or the like covered with a resin, and it can receive the finishing operations in the same manner as a conventional float. One thus obtains a particularly efficient gliding board. Nevertheless, the outer envelope can be made differently, for example, by simple thermoforming of two sheets of thermoplastic material, as is known particularly in the field of windsurfing floats. 
     The invention has just been described for a sub-assembly formed solely of two shell elements, but it can be transposed effortlessly should the sub-assembly be composed of more than two shell elements. Likewise, a sub-assembly can have several of the aspects of the invention described hereinabove one by one, for instance, an excess thickness of glue both on the inner side as well as on the outer side, or an excess thickness of glue on the inner side combined with a recess of the side edge of the interface, etc. In addition, the invention could be applied to the construction of gliding boards other than surfboard floats, for example, for windsurfing floats, for floats adapted for swimming in waves and, in general, for any nautical activity in which the float functions primarily in the lift mode.