CH625465A5 - - Google Patents

Description

The present invention relates to a laminated sandwich structure having a core of cellular plastic material caught between two layers of external coatings of a hard material capable of withstanding pressure, tension and wear.

Sandwich structures are used in various fields, in particular for the manufacture of hulls of boats of different sizes, watertight bulkheads, open or closed containers, and in the automotive industry for the manufacture of platforms and vehicle bodies such than refrigerated trucks.

In general, sandwich structures are used in all applications which require a light, resistant, particularly flexural, and / or thermal insulating material.

A known type of sandwich structure uses pieces of balsa cut crosswise, that is to say the fibers of which are substantially perpendicular to the surfaces of the structure. The balsa is cut into small flat blocks which are juxtaposed edge to edge in the structure where they can be temporarily maintained by glued wires or sheets. This kind of core made up of small juxtaposed blocks makes it possible to manufacture more or less curved sandwich structures.

Balsa is, however, a relatively expensive and difficult to obtain wood, which justifies the search for an alternative material. For this purpose, different cellular plastics have been used, prepared in the form of large blocks which are then cut into slices which may be cut into smaller blocks.

However, this solution has a number of drawbacks. First, the compressive strength is not uniform and can vary by 50% on the surface of a wafer cut from a large block. Second, the surface of the wafer has many small cavities, due to the cut cells, which are partially responsible for the poor compressive strength and additional consumption of adhesive during assembly of the sandwich structure. It is indeed necessary to fill all the cavities with adhesive to obtain good adhesion of the external coatings.

The present invention therefore relates to a sandwich structure with a core of cellular plastic material reducing the various drawbacks of the prior art mentioned above.

To this end, the structure according to the invention is characterized in that the core consists of blocks of cellular plastic material, all the faces of which are impermeable to moisture, two opposite faces of each block being flat and parallel, the blocks having a configuration such that, when placed edge to edge, they form a continuous surface. The blocks can be mass produced in individual molds, so that each block has hard and smooth faces which form a kind of box offering good rigidity along its edges and on its corners.

In addition to their contribution to the overall robustness of the sandwich structure, the hard and smooth faces of the blocks have a damping and distribution effect of localized compression forces which makes the structure much more resistant to this kind of stress. The amount of adhesive required to assemble the sandwich structure is minimized by the presence of smooth and impermeable surfaces. Finally, these impermeable surfaces prevent the absorption of moisture in the blocks and, even if the surfaces of some blocks are damaged, moisture can only penetrate into these blocks and not into the adjacent blocks which are intact.

To simplify and facilitate the production of sandwich structures, the core elements can be parallelepipedic blocks. For certain applications, these blocks can be rectangular parallelepipeds of 30 x 30 x 10 mm.

In a variant, the parallelepipedic blocks have two opposite sides in the form of parallelograms, so that each block serves as a spacer reinforcing the sandwich structure.

It may be advantageous to reinforce the core elements so as to substantially increase their resistance and their rigidity, particularly in the case of sandwich structures whose thickness exceeds 30 mm. The sandwich structures must be able to withstand not only transverse forces, but also forces parallel to their plane, in which case it is the material of the core which collects most of the forces.

For example, cell blocks can be reinforced with fibers, particularly glass fibers, which are incorporated into the plastic before foam is formed. It is also possible to use other reinforcing materials, such as papers, fabrics, ribbons or plastic sheets which are placed in the molds before the formation and solidification of the foam. Note that the reinforcing material can extend from one mold to another to ensure continuity between the blocks which thus form large sheets.

In particular, for the manufacture of large sandwich structures, it is often convenient for the core elements to be pre-assembled, that is to say connected by a thin support such as a sheet or a plastic film, or else drowned sons. This kind of support is easily obtained at the block manufacturing stage and the resulting product is easy to handle and can be rolled up for transport. You can then use it in sheets or sheets as large as you want. The sandwich structures are most often constructed on site, that is to say that the first external coating is first placed in the mold, possibly reinforced with

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fibers or sheets, then the core which is glued to the first layer in pieces as large as you want, and finally the second external coating.

To obtain maximum resistance of the sandwich structure, it is necessary to avoid that air pockets are trapped during the bonding of the core elements to the external layer, that the adhesive is applied separately or that the core elements are pressed in a later hardening adhesive layer. When using balsa blocks, this problem hardly arises, since the wood is relatively porous and absorbs the adhesive by capillary action, the air disappearing in the pores of the wood, so that the balsa blocks practically adhere of themselves.

With cellular plastic blocks, this effect does not exist and, to obtain optimal resistance of the structure, care must be taken to avoid the inclusion of air bubbles. In a particular application, the evacuation of air is ensured by openings formed through the composite core.

Thus, the blocks can be drilled with a central hole perpendicular to their large faces.

At the manufacturing stage, these holes can be obtained in the molds by means of conventional retractable pins, but it is obvious that this method complicates the machine. A simpler solution consists in transferring the openings to the corners of the blocks, for example by chamfering one or more of their edges. Thus, when four blocks are assembled edge to edge, they form a central through passage without one or more of their edges being cut. The circular or polygonal section of this passage is of no importance for the evacuation of the air trapped under the blocks.

The presence of such openings also makes it possible to fix the blocks by injecting the adhesive under pressure in certain passages. The injected adhesive expels the air and expels it through the other passages or through the interstices between the blocks. In addition, the appearance of the adhesive at these locations makes it possible to control its good penetration and can be used to determine the number and distribution of the injection points.

To facilitate lateral flow of the adhesive, the blocks of the present invention can also have grooves in one of their large faces, or in both. In addition, by superimposing two blocks, a thicker core is obtained having longitudinal and transverse passages which facilitate the injection of adhesive and allow greater resistance to be obtained. To facilitate the filling of voids in the structure, vacuum can be applied at the same time as the adhesive is injected.

To produce curved sandwich structures, for example in the case of a boat hull, it is preferable that the narrow faces of the core blocks are cylindrical surfaces respectively convex and concave with the same radius of curvature. It is thus possible to juxtapose blocks on a curved surface without leaving triangular voids between the adjacent blocks thanks to the nesting of the convex and concave faces. The structure obtained is therefore more compact and requires less adhesive to fill its interstices.

Each block can have two convex faces and two concave faces, or one block out of two can have only convex faces, the others having only concave faces.

The following description and the accompanying drawings illustrate by way of example an embodiment of the subject of the invention.

Fig. 1 is a section through the sandwich structure of the invention.

Fig. 2 is a perspective view of several blocks juxtaposed and fixed on a support canvas.

Fig. 3 shows a parallelepiped block with two inclined faces.

Fig. 4 shows a block with two convex faces and two concave faces.

Fig. 5 shows a special block forming injection passages.

Fig. 6 is a section illustrating the appearance of a curved sandwich structure.

The sandwich structure illustrated in the drawings includes a

core made of small parallelepipedic blocks 5,6 of cellular plastic material, for example of polyurethane foam with integral skin, all the faces of which form an adherent, hard and impermeable surface. These blocks of cellular plastic material can be prepared by conventional molding techniques which will not be described here.

The blocks 5, 6 are juxtaposed edge to edge between two external coatings 7, 8 which are layers of identical or different materials.

The external coatings can be sheets of different materials, for example metal, plastic or plywood, bonded to the cellular core with a suitable adhesive 9.

For surface coatings, it is also possible to use a self-hardening plastic material, optionally reinforced with fibers 10, such as wool or glass fibers, which is applied by spray gun directly to a molding surface, the blocks 5,6 then being pressed against the plastic layer before it hardens completely. The core elements can also be reinforced, for example by fibers, as illustrated for block 6 in FIG. 1.

The construction of the sandwich structures is greatly facilitated when the cell blocks 5 (or 6) are fixed to a flexible support, for example a coarse tissue 11, as in FIG. 2. This presentation makes it possible to properly position a large number of blocks at one time and also facilitates the transport and storage of the core material which can be easily wound or folded thanks to the flexibility of its textile support.

The cell blocks are, for example, glued to the woven support and one can choose an adhesive ensuring temporary fixing, which makes it possible to tear off the support 11 when the blocks are permanently glued to one of the external coverings of the sandwich structure. Alternatively, the support can be kept and incorporated as reinforcement in the structure.

As illustrated in fig. 3, the blocks of cellular plastic material can be parallelepipeds which are not rectangular in one direction, which makes it possible to obtain a reinforcement of the sandwich structure and to avoid the formation of too large interstices in curved structures thanks to a relative displacement adjacent blocks.

Fig. 4 illustrates another block configuration which advantageously makes curved structures. In this case, the narrow faces of each block are modified to form mutually opposite concave 12 and convex 13 cylindrical surfaces. This configuration is advantageous in terms of resistance because the sandwich structure is more compact and, in the case of a curved structure, the amount of adhesive required is reduced because the contact surfaces slide against each other instead of step aside.

Fig. 5 illustrates another variant of core element 5 whose corners 14 are cut by partially cylindrical surfaces so as to form a round hole when four identical blocks are assembled in a quadrangle.

The block of fig. 5 also includes a central hole 15 to which end grooves 16 of rounded section which extend to the edges of the block.

This configuration is intended to facilitate the bonding of the blocks by the injection of an adhesive, for example based on polyester or epoxy resin. The adhesive is injected through some of the transverse holes in the blocks and expels the air which is expelled through the grooves 16 and the other holes to ensure maximum cohesion of the structure. During the injection of the adhesive, a vacuum can be applied to facilitate the evacuation of the air pockets trapped behind the blocks.

The transverse grooves 16 facilitate the distribution of the adhesive and improve the bonding if one wants to superimpose two or more blocks to obtain a thicker core.

By superimposing two blocks like the one in fig. 5 with their faces carrying the adjacent grooves 16, a thick block is obtained 5

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double sor having distribution passages in its median plane which can be used to inject an adhesive into the sandwich structure.

Fig. 6 shows in section a curved sandwich structure, the core of which is formed by rectangular parallelepipedic blocks 5 (or

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6). Some of these blocks have a central hole 15 for evacuating air or injecting adhesive. Due to the shape of the triangular interstices which exist between the blocks 5, it is important to provide an air evacuation, possibly supplemented by a

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Claims (9)

625465 2 1. Laminated sandwich structure comprising a core of cellular plastic material caught between two surface coatings of a hard and wear-resistant material, characterized in that the core consists of blocks of cellular plastic material of which all the faces are impermeable in humidity, two opposite faces of each block being planar and parallel, the blocks having a configuration such that, when placed edge to edge, they form a continuous surface. 2. Sandwich structure according to claim 1, characterized in that each block is reinforced by fibers incorporated in the mass of cellular plastic. 3. Sandwich structure according to claim 1, characterized in that the blocks constituting the core are parallelepiped-dic hexahedrons. 4. Sandwich structure according to claim 1, characterized in that two opposite faces of the blocks constituting the core are parallelograms, the others being rectangles. 5. Sandwich structure according to one of claims 1 to 4, characterized in that the blocks constituting the core are reinforced. 6. Sandwich structure according to one of claims 1 to 5, characterized in that the blocks constituting the core are assembled so as to form a substantially continuous plate by being fixed to a plastic film, to sheets or to ribbons of paper, fabric or backing threads. 7. Sandwich structure according to one of claims 1 to 6, characterized in that the blocks constituting the core have convex or concave faces, or in that each block has two convex faces and two concave faces. 8. Sandwich structure according to one of claims 1 to 7, characterized in that the escape of air through the composite core is ensured either by chamfers formed in the corners of the blocks, or by central holes drilled through the blocks, either through these two types of opening at the same time. 9. Sandwich structure according to claim 8, characterized in that grooves of rounded section are formed in one or both surfaces of the blocks which are to be applied against the surface coatings or against other layers of the sandwich structure .