AU691979B2 - Forming process for fibreboard - Google Patents

Description

"FORMING PROCESS FOR FIBR3BOARD" The present invention relates to a process for forming medium-density fibreboard. As used herein, the term 'fibreboard' means a rigid board formed by binding wood fibres with a resin binder. Fibreboard is sold under a number of trade marks, such as 'Customwood' and 'Liteboard'.

Fibreboard is used in a range of applications, but hitherto mostly has been used as a flat board. There are two known processes for bending or curving fibreboard, one involving lamination, the other a chemical treatment, but both are complicated and expensive to use.

It is an object of the present invention to provide a process for forming fibreboard into a curved shape, which process is rapid, easily used, and inexpensive.

Medium-density fibreboard is usually produced in thicknesses 12, 16, and 18 mm and has the following properties:- Property Mean Value Density 600 Kg/m 3 S 15 Internal bond 700 Kpa Modulus of rupture 36 Mpa Modulus of elasticity 2600 Mpa 1 hour thickness swell 18mm 12 mm 1 hour water absorption 18 mm 12 mm Surface Soundness 2.0 KN Moisture Content Thickness Tolerance +0.15 mm The present invention provides a process for forming medium-density fibreboard comprising: raising a sheet of fibreboard to be formed to a predetermined temperature, and subjecting the board at said temperature to a predetermined pressure for a predetermined time; said pressure being applied by one or more platens shaped to the required final curvature of the fibreboard.

Preferably, the or each platen is heated to said predetermined temperature.

Preferably said predetermined temperature is in the range 75"C 900 C, (optimally 80" said predetermined pressure is in the range 60-80 tons (optimally 65 tons) and said predetermined time is in the range 2-4 minutes (optimally 2-2.5 minutes).

Temperatures over 90 CC tend to over dry the fibreboard, making the surface brittle and rough. If pressing time is extended much beyond 4 minutes, too much moisture is removed from the fibreboard, causing the platen temperatures to rise rapidly, with resulting burning and damage to the board surface.

*0 By way of example only, a preferred embodiment of the present invention is described in detail with reference to the accompanying drawing, which shows in diagrammatic form the apparatus used for the process.

Referring to the drawing, a press 2 (shown only in part) is of known type 15 and is capable of operating in the range 5 80 tons. Platens 3, 4, are mounted respectively upon the lower and upper faces of the press. Both platens can be heated electrically.

The lower platen 3 is upwardly convex and the upper platen 4 is concave, but the convex and concave curves do not match:- the concave surface is curved in a shallower curve than the convex curve. The curvature of the convex curve matches the desired curve of the final product.

To form a flat board 5 to the desired shape for e.g. a chair seat, the board is preheated in an oven to a preselected temperature 1700C) for about 2 minutes, and the platens 3 and 4 also are preheated to 1500 C and 1100 C respectively. If the board is veneered, the board is arranged in the press with the veneer adjacent the platen 4, since platen 4 is heated to a lower temperature than platen 3, to avoid scorching the veneer. The board 5, which has been cut to the correct dimensions for the chair seat, is then placed upon the platen 3, and the platen 4 is pressed downwards in the direction of arrow A, to exert a pressure of 80 tons, (preferably 65 tons) on the board 5 for approximately 2 2.5 minutes.

Typically, a chair seat measures 16 square inches, so an applied pressure of 65 tons gives a pressure of about 570 psi, The combined heat and pressure are believed to convert to steam some of the residual moisture in the wood fibres of the fibreboard, thus partially collapsing some of the cells in the fibres and permitting the fibreboard to reform to the desired curved shape, and to set in this shape as the board cools.

Temperature measurements made at the end of the pressing cycle show temperatures: platen 3 120" C platen 4 1000 C upper surface of board 5 800 C lower surface of board 5 900 C The relatively high temperature at the lower surface of the board may result .o••oi o in some surface scorching, but this generally is acceptable on the underside of the article.

One or both of the platens must be vented to allow steam to escape from the board as it is heated and pressed. If the steam cannot escape during forming, it builds up in the board and escapes suddenly when the pressure is released, damaging the board's surface.

It is believed that the process could work satisfactorily with a flat, rather than convex, platen 3.

If the board is not preheated before pressing, it is necessary to extend the pressing time.

The above-described process has been found to work well with mediumdensity fibreboard, ply-veneered medium-density fibreboard, and melamine-surfaced medium-density fibreboard, all in a range of thicknesses. However, the process of the present invention is not suitable for forming moisture-resistant grades of medium-density fibreboard.

Tests carried out on the formed board indicated that the surface density of the board was slightly decreased, but there was a marked increase in core density after forming.

Claims (8)

1. A process for forming medium-density fibreboard (other than moisture- resistant fibreboard) comprising: raising a sheet of fibreboard to be formed to a predetermined temperature, and subjecting the board at said temperature to a predetermined pressure for a predetermined time; said pressure being applied by one or more platens shaped to the required final curvature of the fibreboard.

2. The process as claimed in claim 1, wherein said pre-determined temperature is in the range 75 C

3. The process as claimed in any preceding claim wherein said pre-determined pressure is in the range 60-80 tons.

4. The process as claimed in any preceding claim wherein said predetermined 15 time is 2 4 minutes. *o The process as claimed in any preceding claim wherein said platens also are pre-heated to a temperature in the range 1000 C 120 C

6. The process as claimed in any preceding claim wherein two platens are used, one platen being concave and the other convex, with said convex curve matching the desired final curve of the fibreboard and the concave curve being shallower than said convex curve.

7. The process as claimed in any preceding claim wherein the or each of said one or more platens is vented.

8. The process as claimed in any preceding claim wherein said medium-density fibreboard is veneered with ply on one or both faces.

9. The process as claimed in any preceding claim wherein said medium-density fibreboard is melamine surfaced on one or both faces. A process for forming medium-density fibreboard (other than moisture- resistant fibreboard) substantially as hereinbefore described with reference to and as shown in the accompanying drawing. DATED THIS Sixth DAY OF November, 1995. ZFRAMECRAFT PRADT TDNT ATOcRY BY PIZZEY COMPANY PATENT ATTORNEYS PIZZEY COMPANY PATENT ATTORNEYS Abstract A process for forming medium-density fibreboard other than moisture-resistant fibreboard consisting of raising the fibreboard to a pre-determined temperature and then subjecting it to a predetermined pressure for a predetermined time; the pressure is applied by one or more platens shaped to the required final curvature of the fibreboard. e6 ooe *eoo