TECHNICAL FIELD The present invention relates to a method of forming products, especially prototype pieces, of thermoplastic materials.

BACKGROUND ART Thermoplastic material, such as reinforced plastics material sheets, can be formed into products by heating it first to a softening temperature and transferring it thereafter to a mould in which it is pressed and allowed to set. The moulds are usually machined or cast metal moulds. The manufacture of prototype pieces in particular has therefore been difficult, expensive and time-consuming.

DISCLOSURE OF INVENTION

The object of this invention is to provide a method by which the manufacture of products formed of a thermoplastic material, and in particular the production of prototype pieces, is faster and less expensive than before.

According to the method of the present invention, a sheet of thermoplastic material is heated and transferred to a mould against which it is formed by pressing; and it is a characteristic feature of the invention that the mould is a model backed against a planar mould table by a backing material.

When the present invention is employed there is no need to produce a conventional mould. A model of the desired shape can be used as the mould. The lower surface of the model, i.e. the surface facing the mould table, need not be planar. The model can, for example, have the shape of a dish turned upside down and it can be made of sheet metal. According to the invention it is, prior to placing

on the mould table, filled with for example polyurethane foam which gives a planar lower surface.

A material the surface of which is easy to form to a plane is suitable to be used as the backing material. Materials of this kind are for example foamy settable substances, such as polyurethane, or casting materials, such as gypsum. Prior to placing the model on the mould table, the backing material is cast to the lower surface of the model in a way to produce a planar surface to be placed against the mould table.

According to the method of the present invention, it is possible to produce fast and with low costs a prototype piece with the corresponding properties, for example appearance, insulation ability, ridigity, etc., as the products manufactured with known methods. The methods allows the use of models of a large variety of shapes. It does not cause high tool costs as the same planar mould table can be used for various shapes of models. The model can be made of different materials such as for example wood, plywood, sheet metal or a sheet of polyester. It is a prereguisite of the material of the model that it will not be deformed by the hot thermoplastic sheet.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be described further, by way of example, with reference to the accompanying drawing which is a schematical sectional view illustrating manufacture of a prototype piece by the method according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

In the drawing, a model 2 is supported by a backing material 1 and placed on a mould table 6 having a planar top surface. A thoroughly soft and pliable sheet 3 of a thermoplastic material heated in an oven is transferred to the mould (1 and 2). An elastic diaphragm 4 of a pressure

bell 5 is applied and presses the thermoplastic material tightly againts the mould by means of the pressure prevailing in the pressure bell. At the same time, air is exhausted from the region of the sheet to be formed via passages 7. When the sheet 3 has set following cooling it is removed from the mould.

A pressure of less than 0.1 MPa is in most cases adeguate for the pressing. However, higher pressures can be used, particularly if the density of the backing material is increased.

INDUSTRIAL APPLICABILITY

The present method is very well applicable in forming prototype pieces of porous sheet-like fiber-reinforced non- compressed particle-form plastics material, the fiber and plastics components of which are bound together. Naturally, it is also possible to form according to the method also, for example reinforced plastics blancs coated with textile, or blancs composed of superimposed layers, the layers having different fiber content.