Abstract: An unstable weighing due to the plunger sliding resistance in a prepla type injection molding machine is prevented by dividing control of the plunger retraction movement for the weighing into speed control and pressure control. A plastic resin supply start position and a forced retraction release position are set along the way of the plunger retraction stroke by reference to a weighing start position. A plunger is moved from the weighing start position after injection and pressure-keeping to a weighing stop retraction position. Plastic resin is supplied to the forward portion of the injection cylinder when the plunger attains the supply start position from the weighing start position. The plunger is moved by forced retraction up to the forced retraction release position by speed control; the weighing is stabilized by changing the speed control to the pressure control once the plunger attains the forced retraction release position.

Abstract: The present invention has an object to prevent an unstable weighing due to the plunger sliding resistance, by controlling the plunger retraction movement for the weighing in the prepla type injection molding machine dividing into speed and pressure The method consists in setting sequentially a plastic resin supply start position and a forced retraction release position thereof in the way of the plunger retraction stroke referring to a weighing start position, for weighing plastic resin by moving a plunger 10 from a weighing start position after injection and pressure-keeping to a retraction position of weighing stop; starting to supply the forward portion of the injection cylinder with plastic resin, when the plunger attains the supply start position from the weighing start position, by forced retracting the plunger up to the forced retraction release position thereof by speed control; and stabilizing the weighing by changing the speed control to the pressure control once the plunger 10 attains the forced ret