INJECTING MOLDING METHOD INVOLVING PRESSING.

摘要

Injection moulding method involves pressing for producing a moulded article by injecting a plastised and measured thermoplastic resin into cavities through a sprue-runner-gate and cooling it with a mould clamping force applied, the present invention provides a method for preventing the occurrence of weld marks, non-uniformity of injected resin quantities among cavities, photoelastic strain, flow marks, etc. A cavity formed inside the mould is in advance expanded to a volume sufficient to prevent a branch flow of inflowing resin and is kept at temp. sufficient for the resin to keep a fluidisation state. The thermoplastic resin which is heated and is under a molten state is preparatorily measured and injected into this expanded cavity in quantity greater than necessary for obtaining a moulded article. Before the full quantity of this thermoplastic resin measured in advance is injected into the cavity, the cavity is diminished. At the point of time when the cavity is diminished to a sum of the volume of the moulded article and the mould shrinkage quantity of the thermoplastic resin due to cooling, this diminishing operation is controlled in such a manner as not to leave any uncharged portions inside the cavity which would cause flow marks. At the time of its diminution, the injection pressure is lowered to a pressure at which the backflow of the thermoplastic resin, which might have been over-changed into the cavity, from the cavity due to the diminution of the cavity is not prevented and the resin can be injected to the uncharged portions where the resin might remain inside the cavity. The sprue is cut off when the behaviour of the resin inside the cavity goes stable, and the resin injected into the cavity is cooled and hardened under pressure to a withdrawing temp.