The present invention relates to a method of manufacturing reinforced plastics material products from a thermoplastic sheet by pressing.

Reinforced plastics material products can be manufactured from a sheet by first heating it to a softening temperature and thereafter transferring it into a mould to be first pressed and then let to set therein. A high moulding pressure is required in the forming of a reinforced sheet which calls for powerful presses and solid moulds. The manufacture of large pieces of reinforced plastics material involves especially big investments and, consequently, requires extensive serial production in order to avoid excessive manufacturing costs per piece.

Non-fibrous plastics sheets can be formed at a low moulding pressure by, for example, a vacuum forming technique, but the strength of the products made from such sheets is low.

The object of the invention is to provide a method of forming reinforced plastics material products from a sheet at a moulding pressure lower than before. Thus, the costs of tools will be reduced and test moulds sometimes needed can be manufactured quickly at a low cost.

In accordance with the invention, there is formed a reinforced plastics material sheet, comprising 60% to 90% by weight of thermoplast and 10% to 40%, preferably 20% to 30% by weight of reinforcing fibers having a diameter of 13 μm and being 6 to 16 mm long, said reinforcing fibers having been dispersed in said reinforced plastics sheet, whereafter the sheet is transferred into a mould to be

formed/shaped therein at a moulding pressure lower than 1 MPa and preferably lower than 0.6 MPa.

The sheet is preferably formed in a porous mould at an underpressure. The method of the invention provides the possibility of manufacturing, at a reasonable cost, large pieces being thin in some places and thick in others and of which is demanded, besides a high degree of smoothness of surface, also great strength and rigidity.

The fiber reinforced sheet used in the method of the invention is preferably formed according to the method disclosed in GB Patents 1129757 and 1329409, which method provides a very uniform dispersion of the fibers. However, it is also possible to manufacture such a sheet with other prior wet or dry forming methods.

Irrespective of the sheet manufacturing method, the fiber material has to be dispersed so as to make it appear in the form of single, separate fibers in the sheet.

For example, mineral and carbon fibers are appropriate for the purpose.

The individual fibers should not be shorter than 6 mm because shorter fibers do not provide sufficient reinforcement in the product. Nor should they be longer than 16 mm because longer fibers do not disperse adequately, thereby not reinforcing the product either.

The fibers should not be less than 7 μm in diameter because fibers of diameters less than 7 μm do not provide adequate reinforcement in the moulded product. Nor should they be greater in diameter than 13 μm because greater fibers do not provide a sufficiently smooth surface.

Glass fibers of the average length of 12 mm and diameter of 11 μm are preferably used.

The proportion of plastics material should not be less than 60% by weight because a smaller proportion of plastics material does not sufficiently fill the interstices between the fibers, but results in a porous structure. Nor should the proportion of plastics material be greater than 90% by weight because a greater proportion does not contain a sufficient amount of reinforcing fibers.

Appropriate plastics materials are, for example, polyethylene, polypropylene, polyvinyl acetate, polyvinyl chloride or some other thermoplastic material.

The sheet structure may be unconsolidated and permeable and thus being capable of being heated by hot air flowing through the strucure, or it may be partly or totally consolidated, in which case it is heated by, for example, an infrared heating method.

The invention will be described more in detail below, by way of example, with reference to the accompanying drawings, in which

Figures 1 a - 1 d are schematic cross sectional views of equipment for a preferred implementation of the method according to the invention, Figure 2 is cross-sectional illustration of an alternative sheet shaping apparatus to that of Fig . 1 d, Figure 3 i s s chem a t i c c r o s s - s e c t i on a l illustration of a second alternative of sheet shaping apparatus to that of

Fig. 1 d, and

Figure 4 is cross-sectional illustration of a third alternative of sheet shaping apparatus to that of Fig. 1 d.

Fig. 1 a illustrates a sheet 1, comprising thermoplast with glass fibers dispersed therein and being consolidated by means of heat and pressure in a manner known per se.

Sheet 1 is transferred into an infrared oven 2, in which it is heated to a moulding temperature by means of radiation devices or radiators 3 and is expanded, due to the internal stresses of the glass fibers, to such an extent that it grows in diameter (Fig. l b).

Fig. 1 c illustrates application of a barrier layer 4 onto sheet 1 to cover said sheet.

The completely soft and elastic sheet with the barrier layer covering such is next taken to a vacuum mould 5, in which it is caused to be pressed tightl against the surface of the mould by means of a suction effect brought about through ducts 6 which extend through the mould (Fig. 1 d). The barrier layer prevents air from being drawn through the sheet, thereby facilitating the maintenance of a desired pressure differential. When set, the sheet is removed from the mould.

In an alternative embodiment of the invention illustrated in Fig. 2, the sheet 1 having been heated to a moulding temperature is pressed in a mould having an upper section 8 and a lower section 5. The moulding pressure is brought about by a suction effect through the ducts 6 in the lower section of the mould, the upper section of the mould thereby preventing air from being drawn through the sheet. A moulding pressure higher than the underpressure is formed by pressing the upper section of the mould against the

lower section thereof by a pressing force of a certain degree.

In the embodiment illustrated in Fig. 3, the sheet 1, on which is disposed a barrier layer or an elastic layer 4 of a pressure bell 10, is pressed with said barrier layer or elastic layer against the lower section 5 of the mould by means of overpressure prevailing in the pressure bell.

In an alternative embodiment illustrated in Fig. 4, the sheet 1, on which is disposed a barrier layer or an elastic layer 4 of the pressure bell 10, is pressed with the barrier layer against the "breathing" lower section 5 of the mould by means of overpressure of, for example, 0.5 MPa prevailing in the pressure bell' and by means of underpressure of, for example, 0.07 MPa prevailing in a suction dome 11 below the lower section of the mould. The mould 5 is made by a spray method from metal, for example aluminium, so as to make it porous. Because of the porosity of the mould, an extremely uniform suction effect is achieved.

In the embodiments shown in Figs 2 and 3, the forming of the sheet can be accomplished merely by the upper side moulding force, • or by overpressure or, in the embodiment of Fig. 4, without the pressure bell and merely by underpressure.

It will be appreciated that the mould is appropriate for forming, for example, reinforced plastics material blanks covered with fabric or blanks composed of layers one on top of each other, said layers having different fiber contents.

Example

A blank was manufactured of thermoplastic, reinforced sheet, the composition of which was

E-grade glass fiber (12 mm long, 11 μm in diameter), 20% by weight and

polypropylene, 80% by weight.

The sheet thickness was 3 mm and density 1.12 g/cm ³ . ^■

The blank was heated to 200°C, causing it to expand to a thickness of 4.5 mm.

The blank, with a 0.2 mm thick polyethylene layer fitted thereon, was formed in an apparatus according to Fig. 3 at a pressure of 0.07 MPa, thereby producing a 4 mm thick formed piece. The polyethylene layer adhered to the piece thus forming a completely smooth surface. The tensile strength of the structure was 70 N/mm ² .