Screw injection moulding of plastics, esp. fast=curing gp. - has injection pressure controlled during holding=pressure phase to limit material flow for improved moulding quality near gate

摘要

Process for screw injection moulding of plastics, esp. fast-curing glass-fibre reinforced polyester resin, in which the hydraulic pressure applied to the injection cylinder (17) is controlled to limit flow of material to or from the mould (23) during the holding-pressure phase. ADVANTAGE - Compared with constant holding pressure, this invention, by limiting flow, avoids cracking and surface defects in the moulding in the gate region. Compared with mechanical closing of the mould after injection, this invention is mechanically simple, requires no attachments to the mould (or multiple moulds in carousel arrangements) and avoids wear of the mechanisms, esp. with glass-fibre reinforced materials. The simple moulds also make more economic short runs of differing prods. Relatively simple control of hydraulic pressure. The moulding shrinkage can be compensated by metering the moulding mass from the cushion material