The stiffness of a tube, plate or any other material is proportional to the thickness thereof to the power of three. However, thickness generally results in mass and this may be undesirable for particular applications. In order to now enable making of lighter materials, two layers of strengthening material are fixed to each other with interposing of an intermediate layer (core layer) which is embodied in light material. This entity is called a sandwich structure.

Often used heretofore as intermediate layer have been foams such as phenol foam, polyether foam, polyester foam, polyurethane foam etc. The intermediate layer is glued between the two outer sheets, injected therebetween or attached thereto in mechanical manner.

Intermediate layers are also known with a honeycomb structure. Such a honeycomb structure generally consists of impregnated paper, aluminium, aramid fibre etc. Here also the outer layers are fixed to the core layer for instance by means of glueing.

Double woven fabrics are further known in glass fibre, the pile threads of which consist partially of thermoplastic fibres. The pile threads provide the dis- tance (thickness) between the two outer layers formed by the woven fabric. Such a double woven fabric is subse- quently impregnated with a synthetic resin and a sandwich structure is obtained after setting thereof.

A three-dimensional knit is also known which like- wise consists partially of glass fibre and the pile threads of which consist partially of thermoplastics. The pile threads here also form the intermediate layer.

It is the object of the present invention to provide a method with which a new type of sandwich structure can be obtained in simple manner. This object is achieved according to the invention by means of a method for manufacturing a sandwich structure which consists of at least one outer layer and at least one core layer, which core layer is built up of a number of tubes running substantially parallel to the outer layer (s), which method comprises of providing a tube structure consisting of a number of tubes which lie parallel adjacently of each other and which are manufactured from a fibre mate- rial such as a woven fabric, knit, web or membrane, impregnating the tube structure with a plastic, placing a solid core in the tubes, fixing the form of the tubes by optionally causing shrinkage of the fibres of the fibre material and setting the plastic round the solid core, removing the solid core to obtain the core layer and connecting the core layer to one or more outer layer (s).

The result of such a method is a sandwich structure comprising two outer layers with a core layer received therebetween which is built up of a number of tubes running substantially parallel to the outer layers and impregnated with a set plastic. This sandwich structure likewise forms part of the present invention. It is noted that a sandwich structure consisting of a core layer of a number of tubes running practically parallel and two outer layers is also new per se. Such a structure, but then manufactured in a manner other than described here, therefore also falls within the scope of the invention described herein.

Fibre material is understood to mean any material which is manufactured from fibres. These may be long fibres, such as threads, but also short fibres. The fibre material is for instance a woven fabric, knit, membrane or web. The fibre material is preferably manufactured from shrinkable material so that the shape of the rods inserted into the individual tubes can be transferred to the tubes.

The tube structure can be provided in different ways. Use can thus be made of two woven fabrics, knits, webs, membranes or combinations thereof which are laid on top of one another and in which parallel tunnels or tubes are created by stitching, knitting, welding, glueing etc.

A tube shape can also be woven or knitted directly. Two parallel woven fabrics, knits, webs, membranes or combi- nations thereof can further be connected by a number of partitions running substantially parallel, thereby also resulting in tubes.

The plastic can be a random thermosetting or thermo- plastic plastic (resin). A thermosetting plastic is preferably chosen from polyester resin, epoxy resin or phenol resin.

After the tube structure has been provided, it can be pre-saturated with the thermoplastic or thermosetting plastic and then either dried, cured or brought into B- stage."B-stage"is understood to mean that the resin is already partially reticulated and has thereby acquired a rubber-like structure. The optionally present copolymer- izing monomer is already bound by this treatment. By increasing the temperature or the pressure this resin brought into B-stage once again becomes fluid, whereafter it polymerizes further and sets fully.

However, before the used resin is set, a rod, for instance of chrome-plated metal, is inserted into each of the tubes of the double woven fabric, knit, web or mem- brane, whether pre-impregnated or not, whereafter the whole is heated or irradiated. The heating can for in- stance be effected via the metal rods, but heating or irradiation from outside is also suitable.

The treatment with heat or radiation preferably has two effects. The synthetic fibres will thus shrink, whereby the woven fabric, knit, web or membrane will form around the rods. Through setting of the plastic the tubes are also fixed in the shape of the rods. This shape can be round, square, triangular, oval or any other shape.

When a thermosetting plastic is used as plastic, such as

polyester resin, epoxy resin or phenol resin, it can be polymerized or polycondensated by means of heating.

With the method of the present invention, sandwich structures of relatively great length can be manufactured in a continuous process, without loss of core material.

In a particular embodiment the insulation value and stiffness of the tubes can be increased by injecting them with a material such as a foam-forming resin. This latter has the advantage of being relatively light. A solid resin can however also be used.

The shape of the tubes is fixed by introducing the rods into the tubes. In addition, it is possible to give the whole core layer itself a determined shape, such as round, square, triangular etc.

An outer layer is attached on either side of the core layer. Such an outer layer can be of any desired material, for instance on a basis of wood, metal, plas- tic, glass fibre, carbon, aramid and the like. The layers can be mutually attached by different, per se known connecting techniques, for instance stitching, knitting, glueing, needle punching and the like.

The sandwich material according to the invention can be used for instance in a double-walled container which is laminated round a telescopic core. The inner tube consists of a glass fibre-reinforced laminate onto which a core layer according to the invention is laid, whereaf- ter an outer layer of glass fibre-reinforced plastic is laminated thereon. The sandwich formed in this way and consisting of three parts can then be laminated in one operation. Other applications for the sandwich structure according to the invention are for instance doors and walls.

The invention further relates to a device for per- forming the method. Such a device comprises means for supplying a tube structure optionally already impregnated with synthetic resin to an assembly of core rods lying substantially parallel, means for sliding the separate tubes of the tube structure over the individual core

rods, means for setting synthetic resin and thereby fixing the tube structure, means for discharging the set and fixed tube structure and means for making to size the set and fixed tube structure.

Further means may optionally be provided for arrang- ing synthetic resin in, on or round the tube structure prior to the setting.

The device according to the invention can be applied together with means for arranging one or more layers on one or both sides of the set and fixed core layer in order to obtain a sandwich structure.

In an advantageous embodiment of a device according to the invention, the assembly of core rods lying sub- stantially parallel consists of a number of rods bent on one side at substantially a right angle which make con- tact with a conveyor roller for the tube structure posi- tioned in the angle. The advantage hereof is that tubes of unlimited length can be produced.

In the case of a tube structure with a substantially round cross-section of the individual tubes, the means for discharging the set and fixed tube structure advanta- geously consist of a number of pressure rollers or drums, at least one of which is provided with a profile corre- sponding with the outer profile of the tube structure for discharging.

The present invention will be further elucidated with reference to the accompanying drawings, in which: fig. 1 shows the core layer of a sandwich structure according to the invention in a first embodiment; fig. 2 shows the core layer of a sandwich structure according to the invention in a second embodiment; fig. 3 shows a device for performing the method according to the invention in a first embodiment; fig. 4 shows a device for performing the method according to the invention in a second embodiment; fig. 5 shows a sandwich structure in which the core layer according to fig. 1 is used; and

fig. 6 shows a sandwich structure in which the core layer according to fig. 2 is used.

Fig. 1 shows two layers of fibre material 1,2 which are stitched to each other at fixed distances in order to obtain a number of tubes 3. Fig. 2 shows an alternative in which partitions 6 are arranged between the two fibre layers 4,5. These partitions 6 divide the space between fibre layers 4 and 5 into a number of tunnels 7.

Fig. 3 shows a feed roller 8 having thereon a tube structure 9 consisting of a double woven fabric with lengthwise stitching, whereby tunnels are formed in the double woven fabric. Via a transport roller 10 this tube structure 9 is guided over an assembly of round core rods 11 bent over a transport roller 12. Tube structure 9 is subsequently guided over rods 11 through a furnace 13 where the synthetic resin with which the tube structure is pre-impregnated is set. The tubes herein also shrink and are fixed round the core rods. They will in that case take on the shape of the core rods. In an alternative case, impregnation with synthetic resin takes place, after sliding onto the rods and before setting, in a separate device (not shown) or in a part of the device designated with numeral 13.

On the other side of the furnace the now set and fixed tube structure is now carried to the outer end 17 of the assembly of rods via profiled rollers or transport rollers 14 and pressure rollers 15. The tube structure is there made to size in order to obtain a core layer 16.

An alternative embodiment is shown in fig. 4 in which tube structure 19 is the same as shown in fig. 2 and core rods 18 have a square section. The transport or pressure rollers 20 are flat. A core layer 22 with even top and bottom is obtained at the outer end 21 of the core rods by cutting or trimming.

Fig. 5 shows the use of a core layer according to fig. 1 in a sandwich structure, which has outer layers 23 in addition to core layer 1. Fig. 6 shows a similar sandwich, though now with the core layer of fig. 2 and outer layers 24.