Weighing method in prepla type injection molding machine

A weighing method for a prepla type injection molding machine, comprising both of a plasticating cylinder with an internal screw and an injection cylinder with an internal plunger, for supplying the forward portion of the injection cylinder with plastic resin by revolution or advance-movement of the screw, and injecting plastic resin by advance of the plunger after blocking a resin passage, comprising the steps of:

• setting sequentially a supply start position of plastic resin 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 said plunger from the weighing start position after injection and pressure-keeping to the 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 forcedly retracting the plunger by using a speed control up to the forced retraction release position thereof, and stabilizing the weighing by changing the speed control to a pressure control once the plunger attains the forced retraction release position.