GB1369744A - Injection moulding - Google Patents

Abstract

1369744 Injection moulding IMPERIAL CHEMICAL INDUSTRIES Ltd 3 May 1973 [8 May 1972] 21353/72 Heading B5A An injection unit comprises two separate sources 11, 21 of plastics materials, a valve body having a pair of channels 10, 20 therethrough, each communicating with one of the sources and with a mould cavity (not shown), a pair of members 13, 23, one associated with each channel, each member being mounted in a bore extending from the associated channel and slidable in the bore from a position wherein it closes the channel to a position wherein the channel is open, the arrangement being such that, when the member is in the position closing the channel, material in the channel upstream of the member can exert a force on the member in the direction of moving the member to the open position, and means 31, 32 to apply pressure to the other ends of the members 13, 23. The means 31, 32 comprises a ram 31 to which pressure is applied by an hydraulic cylinder 32. The members 13, 23 comprise shuttles whose sliding movement is limited by stops 17, 27 (Fig. 8) the shuttles being provided with chamfered ends whereby pressure of moulding material can move them to open position against the pressure applied by ram 31. Injection cylinders 11, 21 contain screw rams 18, 28 which can be reciprocated by hydraulic cylinders 19, 29. The cylinders 19, 29 and 32 are connected through valves 100, 200, 300 to a constant volume pump 3. The hydraulic circuit also includes a pressure release valve 5, on/off valves 101, 102 and valves 102, 202 which open at different predetermined pressures. In operation, material from one source is injected first then material from both sources, and then material from the other source. A further injection from the first source may follow. The invention may be applied to a sandwich moulding process in which the core material may be foamable. The two injection channels 10, 20 may converge into a single sprue channel 2 (Fig. 8) or may communicate separately with the mould cavity. Figure 9 shows a modified construction which enables all the material downstream of the shuttles 13, 23 to be removed as part of the sprue of the moulding, thereby reducing contamination.