The present invention relates to manufacturing relief patterned sheet-like products. Examples of such sheet-like products are doors, cupboard doors, patterned wooden panels , and so on. The products are produced from materials that have thermoplastic properties, preferably materials that contain wood-fibre based material.

Relief patterned sheet-like products, such as kitchen cupboard doors, patterned panels, doors, etc., are in great demand. Such products are typically produced from a wooden sheet into which the pattern is cut. Alternatively, pre-shaped wooden blanks or preforms can be fastened onto one surface ofthe sheet. It is also known to combine these two techniques.

The work involved in providing sheet-like products with a relief pattern, which is done manually to a large extent, is expensive, time consuming and requires a high degree of expertise. Consequently, there is a need of less complicated and cheaper methods of producing relief patterned sheet-like products.

When compression moulding particle board, for instance , in a mould that has steep transitions between raised and sunken parts, a poor result is obtained due to the ease in which the board cracks. The board may even break in some cases.

WO 82/03595 teaches an apparatus and a method for producing patterned wood-fibre board which is intended mainly for use as a sound-insulating wall covering. According to this method, a wet wood-fibre pulp is formed with the aid of a patterned roll and then dried. When practising this known method , the board is patterned in the stage of its manufacture. Consequently, it is necessary to make large investments and to apply a complicated process when wishing to use this technique.

EP 0 489 994 describes a door that is provided with one or layers of relief-patterned thermoplastic material. In this case, the actual wood has not been formed or shaped

and there is used a pre-formed sheet of another material. Different areas of the surface are produced from different material with this technique. Furthermore, the technique is not suitable for producing sheet-like products that are patterned with sunken surfaces.

EP 0 574 953 teaches a method of producing relief-patterned wood-composite sheets, wherein the sheets may be formed with various different patterns at the same time. The press time is relatively long (about 5 min.), which results in relatively high manufacturing costs. Furthermore, the patterned sheets have a relatively small surface size (20 x 20 inches).

There is thus a need of alternative methods for producing relief-patterned sheet-like products with which the aforesaid problems are resolved and which enable sheets having a surface area of at least 1.5— 2m ² to be pressed.

Summary of the Invention

It has now been found that a relief-patterned sheet-like product of good quality can be produced quickly and simply by compression-moulding a sheet-like pre-form that includes a mixture of fibrous material and thermoplastic glue in a press of the pressure cell type.

The invention will now be described in more detail with reference to the accompanying drawing, in which

Figure 1 illustrates an example of a board pattern that can be produced by the inventive method; and

Figure 2 is a cross- sectional view of the patterned board shown in Figure 2.

Detailed Description of the Invention

By using a pressure-cell type press, it is possible to achieve effects that cannot be achieved with a conventional piston-type press. In a pressure-cell press, pressure is transferred by an elastic membrane. The blank is therewith pressed in many directions simultaneously, which enables the blank to be shaped quickly and readily in accordance with the shape of an underlying moulding tool. This enables the compression moulded product to be provided with very complicated patterns. Suitable blanks can also be pressed at very high pressures, about 800 bars and preferably in excess of 1000 bars, therewith achieving very short press times, e.g. press times below two minutes, preferably below one minute.

SE-C-452 436 describes a pressure-cell type plant which is well-suited for use in conjunction with the method for producing relief-patterned sheet-like products in accordance with the invention. This press is designed for the manufacture of not- readily shaped sheet-metal elements. A hard shaping tool is placed in the bottom of a tray and the blank or preform to be pressed is placed on top of the tool. The tray is pushed into the press and the blank then pressed against the shaping tool by means of an elastic membrane, for instance an hydraulically operated membrane.

If a sheet of fibrous material, for instance particle board, that contains no thermoplastic glue is compression moulded at high pressure with the aid of a pressure-cell press and a hard shaping tool, the sheet will buckle or collapse. Because thermoplastic glue is a cold flowing glue, these problems do not occur as readily when compression moulding a sheet of fibrous material that includes such glue. The pressing properties of such a sheet can also be enhanced by heating the sheet. The person skilled in this art will be able to establish appropriate press temperatures for different glue sorts, with the aid of simple tests.

According to the invention, the shapability of the fibrous sheet can be further enhanced, by heating the sheet to a temperature of 40-70°C, preferably to 45-60°C, prior to pressing. The sheet may be heated conveniently in a microwave oven.

The fibrous material is preferably a wood-fibre based material, such as sawdust, wood chippings or wood powder, and so on. Suitable thermoplastic glues are polymer glues, for instance EVA (ethylene-vinyl acetate-copolymer), ethylene-acrylate-copolymer, and PVAC (polyvinyl acetate) and other olefin glues, etc. The aforesaid sheet, or board, containing fibrous material and thermoplastic glue is produced by methods known in the manufacture of particle board, board sheets, etc.

The invention will now be described with reference to the following Example. The Example is given by way of illustration only and is not intended to limit the scope of the invention.

Example

A particle board whose glue component comprised the thermoplastic polymer EVA was first heated to a temperature of 60°C in a microwave oven and then pressed in a pressure-cell press obtained from ABB, Vasteras, Sweden. The particle board was pressed on a shaping tool of the desired pattern. The tool was heated to 80°C and the board then pressed for one minute at a pressure of 1000 bars. A particle board having a pronounced relief pattern was obtained.