PROCESS FOR PRODUCING PLYWOOD WITH A LAMINATED CORE The present invention relates to the field of woodworking, and more particular to the sector concerned with the production of plywood boards suitable for the construction, for example, of furniture or supporting or covering structures.

Of the different types of plywood known under different names to those skilled in the art, the one to which the invention specifically relates can be called the"orthogonal laminated core"type. This type of plywood consists of two or more outer layers of wood connected by gluing to an intermediate layer (called the"core") which is interposed between them, and is produced by using glue, staples or wires to secure and connect together thin strips which are placed orthogonally to the outer layers. These strips can, if necessary, be made from a type of wood having poorer physical and/or aesthetic qualities than those of the wood of which the said outer layers consist.

This type of board, which was only marketed on a small scale in the past and has been practically absent from the market for several years because of its high production cost, has a structure which normally gives it advantages of stability and strength (particularly in respect of the traction exerted by screws) over the other types of plywood boards available on the market: those known in the trade as "multi-ply" (consisting of different layers of wood with the fibres of one orientated transversely to those of the next) and"blockboard" (consisting of two or more layers of external wood and an inner core consisting of blocks of solid wood). The superiority of the laminated core board is particularly noticeable where the aforesaid types of board are made from very soft woods, in which case blockboards, for example, are completely unsuitable if acceptable levels of resistance to screws are to be maintained.

In the type of board whose stability and strength most resembles the laminated core type, namely blockboard, the intermediate layer consists of elements (blocks whose dimensions are much larger than those of strips) placed adjacent to each other and fixed immovably with glue or wires, and the dimensions of the said blocks are such that a screw. which passes perpendicularly through a board of which they form the intermediate layer engages almost exclusively with one of the blocks. This makes it necessary to use wood of good quality, with a strength and density at least equal to those of poplar, since with softer or lower- grade woods the joint between the screw and the board is unreliable and has a limited life, because, for example, after an initial screwing and a subsequent unscrewing, the hole formed by the screw tends to become enlarged as a result of the crumbling of the threads which are very thin and therefore have insufficient strength.

For the same kind of reason, these threads would also have a very limited resistance to forces tending to axially extract a screw connected to them.

Furthermore, in addition to being limited to wood of a certain density, a blockboard has to have a sufficiently compact and regular intermediate layer, and therefore the adjacent blocks which form it must have edges which can be placed tightly adjacent to each other (to prevent the creation of voids) and cannot have localized defects (voids, or spongy or damaged areas).

Consequently, these elements cannot be formed from the waste left by other processes or from batches of wood of low quality and price, unless all defective pieces are rejected, with high production costs.

The object of the present invention is to provide a process which can be used to produce plywood boards of the laminated core type described above, with production costs which are significantly lower than those of conventional processes and no higher than

those of blockboards; and which, even when very soft woods are used, has significant properties of mechanical strength which are equal to, if not greater than, those of blockboards made with harder woods.

The resulting laminated core board will also have the advantage, with respect to blockboards, of a significantly lower density, since the very soft woods which can be used are lighter than those used to make blockboards.

According to this process, the aforesaid intermediate layer between the two outer layers is made from a compact block in the shape of a parallelogram having large dimensions (in other words with its long edges measuring several tens of centimetres), formed from a plurality of thin flat sheets of wood superimposed on each other and fixed permanently together by gluing.

This block is pressed and then divided into a number of sections by parallel cuts made in planes perpendicular to its constituent sheets: the aforesaid sections are then glued between two or more outer layers of wood, in such a way that the parts of the said constituent sheets are orientated perpendicularly to the said outer layers. Generally, the fibres of the strips forming the core and those of the outer layers must be transverse with respect to each other: however, for some applications it may be necessary for some or all of the outer layers to have their fibres orientated in the same direction as those of the core.

The fact that the process is based on the use of a block of large dimensions yields a number of advantages: in the first place, because of its significant thickness in the direction of pressing, it is possible to use adequate pressures to create partial interpenetration between the constituent thin superimposed sheets, where voids or irregularities are present in them.

Each sheet can also consist of a plurality of flat pieces with their ends or sides laid adjacent to

each other, and it is also possible for these pieces to have edges which are not precisely aligned with those of the adjacent pieces.

Since, as described above, the pressure can easily produce interpenetration, partially as a result of the thinness of the sheets and the softness of the wood, any overlapping of the aforesaid pieces becomes unimportant, and this greatly accelerates the block forming process (since there is no need to spend excessive time on correcting the alignment of the pieces), thus decreasing the production cost.

Additionally, if relatively thin sheets (with a thickness in the range from 1 to 8 mm for sheets of the"veneer"type and from 8 to 20 mm for sheets consisting of wood battens) are used, and fixed permanently together with suitable glue or resins, this forms a plurality of ribs (consisting of layers of glue or resin), in which it is statistically highly probable that all or part of the thread of a fixing screw will scarf, of the plywood board whose intermediate layer is formed by the said sheets (It should be pointed out that sheets having thicknesses different from those mentioned above can also be used without problems).

It should be noted that, for evident mechanical and physical reasons, when the parallel flat strips which form the said block consist of a plurality of pieces lying adjacent to each other, it is preferable to arrange the said pieces in such a way that the joints between the pieces forming two contiguous parallel layers do not coincide.

To impart a greater rigidity to the plywood board which is to be constructed, it is also advisable, when forming the block, to arrange all the aforesaid thin layers with their fibres orientated in the same direction, in such a way that the strips have their fibres preferably perpendicular to those of the two outer layers to which they are glued to form the board.

The object of the present invention is therefore to provide a process for producing plywood

boards as described in the precharacterizing clause of the attached Claim 1, characterized by what is disclosed in the characterizing part of the said claim.

A preferred example of embodiment of the process according to the invention will now be described, with reference to the attached drawings, which show, in Figure 1, a perspective view of a plywood board of the block type according to the present state of the art; in Figure 2, a perspective view of the forming, by pressing, of a block of large dimensions, formed from a plurality of thin flat sheets superimposed parallel to each other and glued together, produced by the procedure according to the invention; in Figure 3, a perspective view of the block of Figure 2, which shows the planes along which the cuts which divide it into a plurality of parallel sections are made; in Figure 4, the perspective view of an individual section produced by cutting as shown in Figure 3; in Figure 5, a perspective view of a board made by the process according to the invention, by interposing one or more sections as shown in Figure 4 between two outer layers, and by gluing and pressing the assembly.

Considering Figure 1 initially, this shows the composition of a plywood board 11 of the block type according to the present state of the art: two outer layers 12 and 13 of wood are glued on opposite faces of an intermediate layer 15, consisting of a plurality of wood blocks 15p which are adjacent to each other and are fixed permanently together by joining with wood or wire (the latter possibility is not shown in the figure).

A board 11 made in this way, if constructed from very soft wood, has all the limitations and disadvantages listed above: because of the low strength

of the wood from which they are formed, the blocks 15p do not provide a reliable grip for the fixing screws, and the blocks must have regular shapes, with their edges preferably in contact overall and without significant defects or lack of precision.

This is manifested in an increase in the cost of the material to be used, both because of the proportion of defects which have to be rejected and because of the significant cost of the labour required to reject these defects, moreover, it is not possible or convenient to carry out this operation by means of automatic machinery; furthermore, the cost of this rejection operation is sufficient to make it inconvenient to use waste pieces obtained from other processes.

Figures 2 to 5 show the components of a plywood board 1 of the laminated core type during the essential stages of the process according to the invention.

Figure 2 shows how a plurality of flat sheets 5p, which are thin with respect to the thickness which is to be imparted to the board in production, are superimposed parallel to each other and subjected to pressing by suitable forces P after an adhesive, such as glue or synthetic resin, has been spread on them; after hardening, this adhesive will join them permanently together.

The joints at the ends or sides between the pieces constituting two adjacent sheets 5p should be suitably staggered, as stated above and as shown in Figure 2.

The combined action of the pressure exerted and the gluing produces a compact monolithic block 5 having large dimensions with respect to the thickness of the boards to be constructed.

This block 5, as shown in Figure 3, is then divided into a plurality of sections 5s which are parallel and adjacent to each other, by means of a series of cuts made in planes (a i) perpendicular to

the said sheets 5p. After these cuts, each section 5s, as shown in Figure 4, after being rotated through 90° from the vertical position which it had in Figure 3 to a horizontal position, consists of parallel parts (strips) of the said sheets 5p which are fixed permanently to each other and orientated vertically.

With reference now to Figure 5, this shows how one or more sections 5s, arranged as shown in Figure 4, are then inserted between two sheets of wood, of relatively higher quality, which form the outer layers 2 and 3 of the board 1 to be made, in such a way that the fibres of these outer layers are perpendicular to those of the thin sheets 5p forming each section.

A layer 10 of glue or synthetic resin is then applied to the opposite faces A and B of each section 5s which are contact with the said outer layers 2 and 3 respectively; in turn, the lateral joints 9 between adjacent sections 5s can be produced by gluing, by means of wires or staples, or simply by placing the sections next to each other (where the strength of the whole board is concerned, the strength provided by the layers of glue 10 are sufficient, provided that the joints 9 are tight and free of voids). A suitable pressure P1 is then exerted to form the board 1, enabling the outer layers to contact the inner core at all points, and making the board 1 monolithic and strong after the glue has hardened.

As stated above, it is preferable, in order to obtain better results in terms of the mechanical strength of a board 1 formed in this way, to arrange all the said thin flat sheets 5p with their fibres orientated in the same direction, which it is advisable to position perpendicularly to the outer layers 2 and 3 of the board 1.

Thus a board of low cost is produced, with excellent mechanical characteristics which also provide a reliable grip for fixing screws or the like.

Clearly, the process described above can be combined with other intermediate stages of adjustment or positioning, when the use of particular types of wood or a particular desired size of the board make this necessary.