Weighing method in prepla type injection molding machine

A weighing method for a prepla type injection molding machine, comprising both a plasticating cylinder having an internal screw and an injection cylinder having an internal plunger, a forward portion of the injection cylinder supplied with plastic resin through a resin passage from the plasticating cylinder by revolution or advancement of the internal screw, and the plastic resin injected by advancement of the internal plunger after the resin passage has been blocked, comprising the steps of:

• sequentially setting a supply start position of plastic resin and a forced retraction release position along a retraction stroke of the internal plunger, the retraction stroke having a weighing start position and a weighing stop position, the supply start position and the forced retraction release position set by reference to the weighing start position;
• after injecting and pressure-keeping plastic resin into a mold, beginning to move said internal plunger from the weighing start position toward the weighing stop position;
• during said movement of said internal plunger, beginning to supply the forward portion of the injection cylinder with plastic resin when the internal plunger attains the supply start position;
• during said movement of said internal plunger, forcedly retracting said internal plunger by using a speed control from the weighing start position to the forced retraction release position;
• and
• stabilizing the weighing by changing the speed control to a pressure control once the internal plunger attains the forced retraction release position.