Thermoforming, or vacuum forming as it is also sometimes called, is a method for manufacturing shaped goods, whereby the material, in the form of semi-finished thermoplastic slabs, sheet, foils or similar, is post-formed to the desired shape by renewed heating or by using part of the heat taken up by the material during manufacture to the semi-finished item for subsequent shaping immediately afterwards. A number of different principles or methods for thermoforming are known and a common aspect of all is that one starts from a material that is in sheet or foil form that is heated so that it becomes plastic or, more specifically, assumes the so- called rubberised state. The material is subjected to pressure in this plastic condition so that it stretches and takes up the shape of the template or form used and that is held there until the material has cooled down under the vitrification temperature so that it maintains its new shape.

To avoid problems of enclosed air between the form part and the plastic material during the shaping process, continuous venting takes place between the said parts by means of a number of openings that are under negative pressure, so-called evacuation holes, arranged on the outer side of the form part. In this respect, form parts known so far include a negative pressure central enclosure or an inner space into which the evacuation holes open.

A disadvantage of this is that the form parts must be manufactured of relatively resistant material to withstand the large negative pressures to which the form parts can be subjected. For this reason, the form parts are commonly manufactured of an appropriate high resistant steel material, which not only makes their manufacture difficult and expensive, but also greatly contributes to the form parts being heavy and difficult to handle.

For some considerable time now there has been a desire to achieve form parts that are significantly cheaper and lighter than previously known form parts for thermoforming of thermoplastic materials, and the object of this invention is also to achieve a form part that makes this possible. More specifically, a form part is sought that has a design, construction and weight in principle equivalent to the significantly cheaper and thin-walled shell-like form parts that are used within the hardened plastics industry and where form parts of wood or glass-fibre reinforced polyester are commonly found.

A shell-like form part whose active forming surface is provided with a number of negative pressure openings is known from US 4 123 215. For the purpose of relieving the load

on the form part, the currently occurring central enclosure in the form, i. e. the relatively large central inner space with which the evacuation holes communicate, is replaced by a shell shaped as a double wall so that a number of channel-like, relatively low volume small spaces are demarcated in the walls. It should be realised that the forces that will thus act on the limiting surfaces of the form part will be relatively small due to the relatively small volumes of the channels.

The said channel-like space is put under negative pressure in approximately the same way as described above, whereby the openings located on the outer sides of the form parts, or the so-called evacuation holes, are on direct communication with the channel-like space. Even if this technique makes it possible to manufacture form parts of low weight, the arrangement of the double walls means that the design of the form part is relatively complicated and that it is thus also expensive to manufacture. Furthermore, the possibility to be able to utilise a gas permeable material as the shell is restricted since the shell itself should also form channels for discharging the air enclosed between the form part and the plastic material.

An objective of the present invention is thus to achieve a significantly simplified form part so designed that it can be manufactured of very light and cheap material. More specifically, a form part is sought that can be manufactured of very lightweight material that is also a permeable material such as cellulose plastic, for example.

This objective is achieved through the form part according to the present invention having the characteristics specified in the claims.

The invention is described in more detail below with reference to the following drawings where Fig. 1 and Fig. 2 show a cross-section of a form part according to the invention in different stages during the thermoforming of a plastic material for forming a boat hull.

In these figures, 1 generally designates a tentering frame in which a sheet 2 of thermoplastic is accommodated and held by its peripheral edges. Tentering frame 1 is moveable up and down in a vertical manner like a lift so that the sheet of thermoplastic can be manoeuvred between an upper position in which it can be heated to its shaping temperature by a heating unit (not shown in the figure), and a lower position in which the heated sheet can be draped or freely shaped against the outside of a body generally designated 3 and constituting a form part 3.

On its surface 4 that is active in shaping or on its outer side, the form part 3 is provided with a number of evacuation holes 5 whose task is to achieve an effective venting between the outer side 4 of the form part 3 and of the plastic sheet 2 in connection with this being draped over the form part. At its peripheral edges, the form part 3 is also provided with a sealing strip 6 whose function will become evident below. In the known manner, the evacuation

holes 5 are primarily located in the areas of the different corners and sharp edges of the form part, as well as in its recesses. The number of evacuation holes 5 and their placement relative to one another as well as their size is determined, also in the known manner, by the plastic material and design chosen.

The evacuation holes 5 are connected via a branched duct 7 and a main duct 8 with a source (not shown in the figure) of generating negative pressure. Using a separate duct 7 to each evacuation hole 5 obtains a significant advantage compared with the type of form part that uses one or more central inner connections in the form part set at negative pressure, namely that the form part can be made of significantly thinner material, whereby the forms will also be cheaper and significantly lighter.

When the suspended plastic material has attained the predetermined temperature, the tentering frame 1 moves downwards towards the form part 3, whereby the suspended plastic material 2 is draped over it. In connection with the shaping, the source that creates the negative pressure is activated so that the air that may possibly have been enclosed between the plastic sheet 2 and the outer side 4 of the form part 3 is led out through the evacuation holes 5. The shaping step is completed by enclosed air that might possibly remain being led out through the evacuation holes located on the outer periphery of the form part 3 when the plastic material comes into contact with the sealing strip 6 that runs around the form part.

The present invention is not limited to that described above and shown in the drawings but can be changed and modified in a number of ways within the scope of the concept of the invention as stated in the following claims.