The present invention relates to the field of compression moulding of hollow bodies of thermoplastics material and is intended to be applied specifically to a compression moulding process by means of which two superposed layers of thermoplastics material are moulded simultaneously in a single assembly and moulding operation.

Such a moulding process is known from Italian Patent Application No. 67598-A/90 which describes a method in which compressed air is supplied between two superposed layers of thermoplastics material at a pressure sufficient to mould the two layers into the dye cavities of the mould.

More precisely, the present invention relates to a method for depositing two superpos.ed layers of thermoplastics material on a lower dye of compression moulding equipment.

Automatic equipment has already been proposed for enabling a thermoplastics sheet to be conveyed from the zone, for example an oven, in which it is heated to the moulding zone and which enables sheets to be deposited on the lower dye of the moulding equipment. One of these

known devices includes a carriage movable between the heating station and the moulding station and carrying a belt conveyor-

The equipment currently used enables the sheets to be deposited on the dye during the forward travel of the carriage only or during the return travel only, the term forward travel being understood as the travel in which the deposition device inserts itself between the two dyes and the term return travel being the phase in which this device moves away from the moulding zone. Therefore with the known equipment, two successive deposition cycles are normally required for the deposition of two superposed layers since a forward travel and a return travel of the carriage are required for the deposition of each sheet. The object of the present invention is to provide a method of depositing two superposed layers of thermoplastics material which enables the time taken for the deposition cycle to be reduced compared with known solutions.

According to the present invention, this object is achieved by virtue of the fact that the deposition device effects the deposition of a first layer of thermoplastics material during a forward travel of the movable carriage by actuation of the conveyor in a sense concordant with the direction of movement of the carriage and effects the deposition of a second layer of thermoplastics material

during a return travel of the movable carriage by actuation of the conveyor in the sense discordant with the direction of movement of the carriage.

The two layers of thermoplastics material may be two separate sheets of different thicknesses or may be obtained by the folding of a single sheet on itself, the folding being achieved by the reversal of the direction of movement of the carriage at the end of the forward movement.

Further characteristics and advantages of the invention will become apparent during the course of the detailed description which follows, given purely by way of non- limitative example, with reference to the appended drawings, in which:

Figures 1 to 3 are schematic views illustrating the sequence of deposition of two separate sheets,

Figure 4 is a schematic view showing the moulding phase, and

Figure 5 is a schematic view similar to Figures 1 to 3 illustrating the deposition of a single sheet folded on itself.

With reference first to Figure 1, the lower dye and the upper dye of equipment for the compression moulding of thermoplastics sheets are schematically indicated 10 and 12 respectively.

Alongside the moulding zone defined between the two dyes 10, 12 is a device generally indicated 14 for conveying and depositing the sheets. The device 14 is constituted essentially by a movable carriage 16 carrying a conveyor belt 18. In the description below and in the claims, the term "conveyor belt" is intended to mean a conveyor having a flexible member which passes over a pair of pulleys, one of which is motor-driven. The flexible member is preferably constituted by a flexible metal mesh which can withstand the temperatures, of the order of about a hundred degrees centigrade, to which the sheets of thermoplastics material are heated without appreciable yielding or expansion.

The movable carriage 16 engages a pair of lateral guides not illustrated and is driven by a .rack and pinion mechanism which drives its translational movement between a heating station and a moulding station. The conveyor 18 has a motor-driven pulley 20 which can be driven in both senses of rotation and an idle return pulley 22, both carried by the movable carriage 16. The movable carriage 16 also carries a metal plate 24 linked to the horizontal upper pass of the conveyor 18 and forming a ramp for the deposition of the sheets on the lower dye 10.

Figures 1 to 3 illustrate the sequence of operation of the device 14 for depositing two thermoplastics sheets

26, 28, which are generally of different thicknesses, one on top of the other. The two sheets 26, 28 may have a film of plastics material which facilitates their adhesion to a covering textile layer which is applied, in known manner, to the outer faces of the sheets 26, 28 during moulding. The film which facilitates the adhesion of the textile layer faces upwardly in the condition in which the two sheets 26, 28 rest on the horizontal upper pass of the conveyor 18 to avoid any contact with the surface of the conveyor causing deterioration of the adhesiveness of the film.

With reference to Figure 1, the movable carriage 16 is brought to a position in which the free end 30 of the inclined ramp plate 24 lies approximately over an edge of the lower dye 10. The first sheet 26 is advanced by actuation of the drive pulley 20 of the conveyor 18 until an edge of the sheet is located in correspondence with the free end 30 of the inclined plate 24.

Starting from the configuration of Figure 1, the movable carriage 16 is advanced in the direction indicated by the arrow 32 in Figure 2 at a constant velocity and the conveyor 18 is actuated in a sense concordant with the direction of advance 32 of the carriage 16. The absolute value of the conveyor belt velocity is equal to the translational velocity of the carriage 16. Thus the sheet 26 is deposited on the lower dye 10 (Figure 2)

without the sheet being stretched. Naturally, before the sheets 26, 28 are deposited, they are preheated so that they are in a plastic or ductile state which enables the deposition to be likened to the "spreading" of the sheet on the lower dye 10.

After the first sheet 26 has been deposited, the device 14 is in the configuration illustrated in Figure 3. Starting from this configuration, the conveyor 18 continues to move in the same sense (in the sense which, in the embodiment illustrated in the drawings, corresponds to actuation of the drive pulley 20 in the anticlockwise sense) and the translational movement of the carriage 16 is reversed which now moves in the direction indicated by the arrow 34 in Figure 3. Hence, in this phase, the conveyor 18 is actuated in the discordant sense with respect to the direction of movement of the carriage 16.

During the phase in which the second sheet 28 is deposited, the end portion of a compressed air duct is inserted between the two sheets 26, 28 in a manner similar to that described in detail in patent document No. 67598-A/90. The compressed air flow keeps the sheets 26, 28 apart whereas otherwise they could adhere to each other since they are in a plastic or ductile state.

It will be noted that the sequence of deposition of the

sheets 26, 28 means that the film of plastics material which facilitates the adhesion of a covering textile layer is located on the downwardly-facing surface of the lower sheet 26 and on the upwardly-facing surface of the upper sheet 28.

After the deposition of the layers 26, 28 on the lower dye 10, the two dyes 10, 12 are closed, as illustrated in Figure 4, and the pressure of the air supplied between the two sheets is increased so that these are moulded into in the dye cavities of the mould.

If the outer surfaces of the finished piece are to have an aesthetic cover, the covering layers (for example textile) are stretched on positioning frames which are inserted between the two dyes before, or simultaneously with, the deposition of the sheets 26, 28.

The method of depositing the layers 26, 28 described above is particularly suitable for two sheets 26, 28 of different thicknesses.

When the walls of the finished sheet are of the same thickness, the two superposed layers deposited on the lower dye 10 may be obtained from a single sheet 40 folded on itself, as illustrated in Figure 5. Again in this case, during the forward travel of the carriage 16, the conveyor 18 is driven in the sense concordant with

the direction of advance of the carriage 16 to deposit a first layer 42. At the end of the forward travel of the carriage 16, the direction of movement of the carriage is reversed while the sense in which the conveyor 18 is driven is kept the same and thus the folding of the sheet 40 and the deposition of a second layer 44 is achieved. Again in this case, compressed air is blown between the two layers 42, 44 at the same time as the second layer 44 is deposited. The moulding of the two layers 42, 44 is achieved by supplying air at high pressure following the closure of the two dyes 10, 12.