Method for form-pressing wood fibre panels and form pressed panels, for example door skins

Abstract: Method for manufacturing form-pressed wood fibre panels, for example door skins, and panels manufactured according to the method. The process starts from a workable, mass produced panel displaying high thermal conductance, for example a so-­ called MDF-board or chip board. The panel is preheated to a condition where the wood fib­ res and the binding agent which binds the wood fibres form a pliable or stretchable composition though still in continous panel-like condition. This preheated panel is fed to a form-pressing device where the pressure is relatively slowly increased from 0 to approximately 30 kg/cm² during continuing heat supply to a temperature of around 150-200°C. Usually this is attained in connection with door skins after a period of approximately 1,5 minutes. Finally, a form-stable, form-pressed panel is removed from the press.

Description: The invention relates to a method for form-pressing wood fibre panels of board type or MDF (medium density board) according to the preamble of claim 1. The invention also enables form-pressing of mass produced thin chip boards through the particular choice of binding agent. For a long time there has existed the need for cheaper, outer covers of building elements such as doors, shutters, etc, and this need has increased particularly since the price of solid timber and labour costs have risen. Historically a first method exists which is based on wood fibre panels or semi-finished articles according to the so-called HB-method, where the letters HB stand for hard board. According to the method which is described for example in Danish patent no. 153 640, a heat setting binding agent is employed in an initial panel which is either of wood fibre type (board or MDF) or of wood chips (chip board). However, in order that the panel will be deep formable, the known process requires a so-called outer coating of paper on veneer. Mention has also been made of using a laminate structure which is either laminated on site or is supplied in a laminated condition. With these techniques, however, optional mechanical working on the panel afterwards cannot be carried out, at least if the coating is of paper. A veneer coating allows certain slight mechanical working. Likewise, the comparatively low density, in the order of 400-600 kg/m³, leads to a relatively slow thermal conductance, meaning a long press time in the form press device. In addition, if the HB-technique is to be used, large investment for carrying out the process is required since, for example, only natural glue is used as the binding agent. A next step in the development in this field is described in US Patent No. 4 844 968 (Swedoor's door skin patent). Here wood particles or so-called chips are used which, with the help of a resin impregnated outer coating serving also as a carrier layer during the process, gives a high quality formable product, for example in the form of a door skin with the additional possibility to give the surface a certain structure, for example a wood grain-like finish. Here, however, one works with the starting point in a "semi-product", i.e. chips, which means that considerable manufacturing costs are built into the manufacturing process of the door skin itself, these costs being necessarily reflected in the end product. GB-A-2 184 139, upon which the preamble of claim 1 is based, describes a fibreboard, particularly a medium density fibreboard, comprising particles of fibrous material secured together by an adhesive binder. The fibreboard may be reshaped by the application of heat and energy. It is therefore an object of the present invention to provide a method of form-pressing wood fibre panels, which method is particularly suitable for form pressing thin panels to a considerable depth. Since in the process according to the invention a starting material can be used which is not only mass-producible, but which also has a higher density than chip material as given in DK 153 640, this means that a better thermal conductance is obtainable which implies a shorter form-pressing time. Since, according to the invention, a preheating step is also included, this means that this step can also be shorter than the corresponding step in connection with the technique according to DK 153 640. The invention also provides a method which offer advantages of scale, workability and faster thermal conductance. More specifically the invention provides a method for form-pressing wood fibre panels or chip board. The latter is fully possible with a suitable glue compound and chip size in a thin, finished chip board. The method is characterized in that the panel is preheated so that the wood fibres and the binding agent which binds the wood fibre form a pliable or stretchable composition, in that this composition, while still in continuous panel-like condition, is form-pressed via application of an increasing pressure during continuing heat supply, and in that the pressure and the heat supply are interrupted before the elastic limit of the panel-like stretchable composition is attained. The definition "binding agent which displays thermoplastic properties during heating ", means that this does not relate to what is commonly known as thermoplastic binding agents in panels. What is interesting is that despite the hardening binding agent, the panel has thermoplastic properties during heating. If required, as a final step the panel can also be cooled though it is normally handleable directly after the pressing. In a preferred embodiment a panel with a density between 700-900 kg/m³ is selected as the wood fibre panel. For manufacturing of door skins it has been shown suitable to start with a panel which has a uniform initial thickness of 2-6 mm, whereby the process is such that the thickness, considered as product thickness, after the form-pressing is somewhat reduced, though still substantially constant. Panels of this size are mass produced as so-called MDF-boards. In one embodiment the form-pressing is carried out to a depth of approximately 6 mm at maximum. In a preferred embodiment the total cycle time for manufacturing of door skins, including preheating, pressing and cooling, is selected to fall between 2-4 minutes. Preheating is suitably carried out by a combined supply of contact heat and radiation. The final temperature during form-pressing of the door skin is selected to lie between 115-200°C and during the form-pressing step the pressure in the form is slowly increased from 0 to 30 kg/cm². The invention also provides a panel-like product, for example a door skin. The product is characterized in that it is form pressed to a depth of approximately 6 mm maximum and that the product comprises a wood fibre panel with thermoplastic properties. In a preferred embodiment the density of the panel is between 700-750 kg/m³, and the quantity of the binding agent is between 5-15 percent weight. A number of various trial runs under production conditions relating to verification of the grounds for the principles underlying the invention have been performed. Accordingly, mass produced so-called MDF-boards with a thickness of 3 mm were used. This board was preheated through radiation to a temperature of at least approximately 50°C on the surface. Thereafter the board was loaded into a press tool comprising male and female type active zones. The male type zones were provided with a coating of "TEFLON". During a time period of approximately 90 seconds the pressure on the preheated board was continuously increased from 0 to 30 kg/cm², meaning a relatively slow pressure increase. Simultaneously therewith the temperature was increased to an end level of approximately 150-200°C. Since these parameters were reached from an initial condition where the wood fibres and the binding agent after preheating, according to earlier terminology, form a pliable or stretchable composition in continuous panel-like condition, a fully form stable pressed panel with deep formed grooves and ridges of maximum approximately 6 mm depth therein is obtained after a holding time of approximately 30 seconds at final pressure. The method, respectively the product, is accordingly built up of known components though components which are used for the first time in a completely new method and a new combination.

Claims: Method for form-pressing a wood fibre panel which contains a binding agent displaying thermoplastic properties during heating, characterized in that the panel is preheated so that the wood fibres and the binding agent which binds the wood fibre form a pliable or stretchable composition, in that this composition, while still in continuous panel-like condition, is form-pressed via application of an increasing pressure during continuing heat supply, and in that the pressure and the heat supply are interrupted before the elastic limit of the panel-like stretchable composition is attained. Method according to claim 1, characterized in that a panel with a density between 700-900 kg/m³ is selected as the wood fibre panel. Method according to claim 2, characterized in that the panel has a uniform initial thickness of 2-6 mm and that the thickness, considered as product thickness, after the form-pressing is somewhat reduced, though still substantially constant. Method according to claim 3, characterized in that the form-pressing is performed to a depth of approximately 6 mm maximum. Method according to claim 4, characterized in that the total cycle time for preheating, pressing and cooling is selected to fall between 2-5 minutes. Method according to claim 4, characterized in that a mass produced MDF-board is chosen as the panel. Method according to claim 6, characterized in that preheating is performed via contact heating and radiation. Method according to claim 7, characterized in that the final temperature during the form-pressing is selected to lie between 115-200°C and that during the form-pressing step the pressure in the form is slowly increased from 0 to approximately 30 kg/cm². Method according to claim 1, characterized in that a mass produced chip board is chosen as the panel.

Inventors: PERSSON GOERAN, PERSSON, GOERAN, Persson, Göran 
Applicants: SWEDOOR AB
Application year: 1990
Main classification: B27N3
IPC codes: B27N7