The invention relates to a composite beam structure molded under compression and intended to be used as a blank for various light weight structures such as skis or the like. The structure includes upper and lower reinforcing layers having between them a support fabric surrounding a core member pro¬ duced from a synthetic material. The invention further relates to a method for producing such structures by compressing in a mold a synthetic material core placed between two essentially opposite surface layers and suitably intermeshed with support structures extending between said surface layers.

In the prior art beam structures for skis or the like struc¬ tures are known, said beams having at least a part of their cores formed of a synthetic material such as plastic. Gene¬ rally the main purpose of the core is to keep the rigidity lending surface and support layers of the beam in place, especially during the manufacture. Additionally the core sup¬ ports the surface and support layers in load conditions, wherefore said structures may be relatively thin without, however, buckling and crinkling due to a load. Additionally publications such as FI open-laid patent application 64743 and AT Patent 276 178 disclose structures wherein the core of the beam is essentially hollow, the surface and support structures being correspondingly thicker.

Generally the above mentioned structures provided with filled cores are manufactured according to two different principles.

According to one of the known manufacturing principles the core member is molded into a given form and subsequently the surface layers are attached thereto one or several at a time to form the final structure. AT Patents 299 022 and 334 795 disclose structures produced by surrounding especially wooden ribs with a cloth of glass fiber and by placing the thus obtained unit impregnated with a synthetic resin, into a mold to harden into a given form. Correspondingly AT Patent 254 748

discloses a structure wherein the core consists of expanded PVC plastic ribs which are glued together as the- production progresses, whereafter the unit is placed under pressure. Due to the conical form of the middlemost rib a lateral pressure is formed, the purpose of which is to fill the mold fairly completely. US Patent 3,918,731 discloses a method for the reinforcement of the surface of a foamed plastic core in such a way that the surface is made to resist stresses directed thereat. CH Patent 595 120 on the other hand teaches a method for producing a semi-fabricated core. In this connection may be mentioned also the honeycomb structure skis which, in accordance with the methods described e.g. in AT Patent 236 263 or DE open-laid patent application 2 049 074, are produced by coating a finished honeycomb structure with a suitable surface material in order to obtain a unit.

Another known production principle is based on placing an essentially hollow semi-fabricated product, consisting of surface structures and possibly of support structures, into a mold, injecting a foam plastic hardening in the mold to fill the cavities and to press the surface structures against the mold. As examples of this kind of structures there may be mentioned the solutions disclosed in DE open-laid patent app¬ lications 34 18 568 and 34 37 865.

It is typical for the known solutions that the production requires several different stages of operation such as for example in the application of the first .mentioned type of principles. The filling of a core cavity in a mold with a foamable and hardening plastic also requires several different stages of operation, especially the handling of the different layers and the placing thereof into the mold often involves great difficulties. Additionally it is difficult to control the quality and the characteristics of a core member produced in this way, nor is it possible to provide the core with a structure having different, controlled compressibility charac¬ teristics in different directions. The control of these characteristics is, however, very important in structures which are subject to repeated bending in one direction. A ski

is typically such a structure wherein the elastic resilience should be controlled in order to obtain suitable elasticity. Known methods may be useful for structures, where the beam cross-section is rectangular or almost rectangular. The manu¬ facture of structures with trapezium-shaped cross-sections is, however, difficult with known methods, and the quality is difficult to control.

In order to improve the above mentioned deficiencies and to provide a product well suited to mass production an improved beam structure and a method for the production thereof have been invented, the characteristic features of the invention being defined in the appended claims. Thus the structure of the invention is such that the core member consists of one preformed piece of a synthetic material or of several colla¬ teral preformed strips of a synthetic material, respectively, a support fabric suitably extending at least partly around the core and preferably between the strips so, that each strip is surrounded by a support fabric on at least three sides. Ac¬ cording to the invention the,-core blank is made of a synthetic material the compressibility of which, prior to the compres¬ sion, is smaller in one direction, preferably in the compres¬ sing direction of a mold cover, than the compressibility in a direction substantially perpendicular thereto. This difference in compressibility will result in that the main deformation will take place, in said substantially perpendicular direc¬ tion. The strips of synthetic material are compressed, due to the effects of compression and/or reaction pressure and suitably with a softening of the synthetic material, essen¬ tially in the last mentioned direction into the support fabric and/or into the finishing fabrics surrounding the structure so, that the blank will adopt a permanent cross-section defined by the mold used in the process. This cross-section is, in the manufacturing of skis, preferably trapezoidal.

According to the method of the invention the beam is compres¬ sed into its final shape. The core blank, prior to being placed into the mold, is preformed in one or several parts from at least partly hardened foam plastic. An upper and/or

bottom surface material, suitably consisting of glass fiber, carbon fiber or the like material and/or e.g. wooden or metal ribs, is positioned over the core and/or the core parts. The core blank and/or its parts and/or the surface material is optionally further surrounded by reinforcing, side surface and/or finishing layers as auxiliary layers, and the assembly thus formed is placed in the mold, the closed extension of which at least in one direction is slightly smaller than the corresponding extension of the assembly. Thereafter the mold is closed and compressed around the assembly, and the core blank material and/or a separate adhesive such as an epoxy resin or the like, which is optionally applied between the separate parts, is reacted to soften the foam plastic of the core in such a way that the assembly is shaped into the shape of the mold, whereby an integral connection is formed between the core, any auxiliary layers and the separate surface layers to provide a suitable rigid structure.

In the following the invention will be described in more detail with reference to a working example and the figures connected therewith, in which figures

Fig 1 shows a partial section of a finished beam structure, which in this embodiment is a ski blank;

Figs 2 to 5 show the different production phases of the struc¬ ture of Fig 1.

The structure of the invention is described in the following by referring to the method for producing such a beam struc¬ ture. According to Fig 2 the production is started with cut¬ ting out rib-like core blank strips 2 from a foam plastic sheet 1. In accordance with the preferred embodiment of the invention the core blank material suitably used for producing skis comprises extruded polystyrene or the like semi-hard plastic sheet the compressibility of which preferably is smaller in the direction substantially perpendicular to the surface layer of the sheet than the compressibility in at least one direction substantially parallel to the surface

layer. As a suitable foam plastic sheet may be mentioned for example the extruded polystyrene sheet called Finnfoam (R), produced by the company Finnfoam Oy, Finland. The cutting is preferably performed so that the core blank settles in the mold 3, 4 in such a way that the smallest compressibility is disposed- towards the upper and lower layers 5, 6 of the struc¬ ture to be produced, and the largest compressibility corres¬ pondingly is disposed in a direction extending towards the sides 7 of the structure. In the described embodiment for the production of skis the fore and aft ends 8 of the ribs 2 are further chamfered to correspond with the thinned cross-section in the longitudinal direction of the final product.

The measures of the machined core blank strips 2 are chosen so that when they subsequently are used for producing an assembly with the different layers the extension A of the assembly at least in the lateral direction is slightly larger than the closed extension a of the mold 3, 4. With the term "closed extension" is herein meant the extension at which the mold wall in question settles when the mold is compressed.

The foregoing describes the production of a core blank from a sheet 1 by machining and by using an assembly formed of several collateral core blank strips 2. A major advantage of the invention is then that an approximate dimensioning is sufficient in the shaping of the core blank, since the final shaping takes place in the mold. The core blank according to the invention may, however, also be produced from one or several sheet parts which are produced in a special mold. According to one preferred embodiment the core blank or the parts of the core blank are produced by extruding directly into longitudinal strips, the cross-sections of which may be round, square or polygonal, suitably hexagonal.

According to the described preferred embodiment of the in¬ vention the glass fiber, carbon fiber or the like reinforcing or supporting fabric 9 is placed (Fig 3) around the separate collateral core blank ribs 2 in such a way that the fiber reinforcement extends over at least three sides of each rib 2,

whereby the separate blank parts are compiled into one core blank unity. For example in the case of a hexagonal rib the fiber reinforcement 9 extends suitably between the ribs 2 substantially vertically and/or horizontally depending on its position in the final beam. Over and under this unity there is further placed the material 10, 11, 12 of the upper and lower surface reinforcements and between and/or around these there is optionally further placed auxiliary reinforcing, side sur¬ face and finishing layers 13, 14 and 15. Thus an assembly is formed and is placed into the mold 3, 4 in the way shown by Fig 4. In the described embodiment the mold comprises a sub¬ stantially U-shaped piece 3 and a substantially flat cover 4, the sides 16 of the ϋ-shaped piece 3 being suitably chamfered to facilitate the detachment of the final piece. For the pur¬ pose of the invention the mold may, however, also be construc¬ ted in such a way that several of its sides are movable.

When the assembly has been placed in the mold, the parts 3, 4 of the mold are pressed together (Fig 5) and the temperature is raised to a value between about 75 and 100 oC, preferably to a temperature between about 80 to 90 oC, in which tempera¬ ture the structure is held for about 15 to 40 minutes, pre¬ ferably about 30 minutes. In this compression the plastic foam 2, which is at least partly softened due to the increased temperature, retaining its foam-like characteristic presses the auxiliary layers, such as the support fabric 9, the sur¬ face fabric 10, 12, the optional support rib 11, and the sur¬ face material 13, 14, 15, tightly against, the mold surfaces 16, 17, 18 filling the mold and any cavities 20 thereof. Due to the heat and the pressure the core material 2 settles in its new shape, which it retains also after the temperature has dropped. In order to improve the coherence of the obtained structure the core blank, the auxiliary layers and/or the assembly may, when so desired, further be wetted with a suitable adhesive agent which also hardens due to the heat treatment and/or due to a hardener added to the agent or by another reaction.

In the described example the vertical compression applied by the mold cover 4 forces the assembly into the mold piece 3, the lateral extension of which is slightly smaller than the lateral extension of the assembly, which causes a deformation of the foam plastic 2 especially in the lateral direction of the structure, which according to the invention is due to the asymmetrical compressibility of the used foam plastic. In that same connection the foam plastic is compressed and forced into the the glass fiber cloth 9 used as support ^" frame, to which it becomes attached, while at the same time filling the space between the individual core blank ribs 2 as well as that between the ribs and the mold 3, 4. In this way, according to the invention, the deformation of the core material, and at the same time the exact location of the support frame struc¬ ture, can be fully controlled during the whole compression procedure, by utilizing the non-symmetrical compressibility features of the core material.

The production material used for the core blank in the above described method is a suitably semi-hard extruded plastic sheet, but the invention is also applicable for using other types of plastic materials meeting the conditions set by the invention. Thus one may for example obtain specific elasticity characteristics in the beam structure by using a synthetic material comprising e.g. resilient closed cells.

Although the foregoing primarily describes a structure, wherein the upper and lower surface materials are depicted as separate parts 10, 11, 12, 13, 14 and 15, the scope of the invention includes likewise the embodiments wherein these said parts, as well as one or several of the support layers 9, consist of one and the same part, or they may respectively be formed of several separate parts and/or layers.

As a last stage the production of a ski from the beam ac¬ cording to the invention is performed by cutting away any ridges 19 which have possible penetrated outside the mold 3,

4, and by performing an optional final shaping, whereafter the surface of the structure is suitably matted for painting or another finishing treatment.

In the foregoing the invention has been described by reference to an exemplatory production of skis. It is, however, evident, that the invention may be applied to several other fields where durable light weight construction beams or planes are required.