GB1425612A - Moulding of plastics foam - Google Patents

Abstract

1425612 Moulding foam blocks DUNLOP Ltd 23 May 1974 [29 May 1973] 25430/73 Heading B5A A process for the batch production of blocks of plastics foam, e.g. of polyurethane, in which the blocks produced have a flat upper surface, comprises depositing a foamable reaction mixture on to a support 10 which together with side walls 23 defines a mould cavity, and when the reaction mixture begins to foam lowering the support to effect relative movement between the support and the side walls so as to increase the volume of the cavity at such a speed that the zone of contact between the upper surface of the foam and the side walls does not move, the relative movement being stopped when the foam ceases to increase in depth. Fig. 1 shows apparatus suitable for performing the method, the support 10 being movable by a scissors type lowering device 10a operated by a motor 20. The side walls 23 of the mould are hinged to enable them to be swung outwardly for access to the moulded block. Fig. 3 illustrates a control arrangement for the lowering operation in which a control plate 25 resting on the surface of the foam 36 is connected via a linear variable differential transformer 26, microswitch 28 and thyristor controller 27 to the motor 20. Upward movement of the plate 25 triggers the microswitch 28 and starts the motor 20 and thus downward movement of the tray 10, this being continued until the control plate 25 returns to its equilibrium position. The speed at which the motor 20 runs is governed by the movement of the plate 25, detected by the LVDT 26 which feeds appropriate speed instructions to the thyristor controller 27. Instead of the control plate 25 a photo-electric cell could be used to detect movement of the upper surface of the foam. As an alternative to the control mechanism illustrated, the rise of foam may be judged visually and the tray 10 lowered manually, e.g. by a winding device.