Method of controlling electric injection unit of injection molding machine

A method of controlling an electric injection unit of an injection molding machine during an injection process having an injection phase for filling a cavity, formed in a mold, with a molten resin and a dwelling phase for maintaining pressure in the cavity, said injection unit having an injection screw driven by an electric motor, said injection molding machine being provided with an injection pressure detector and at least one of a screw position detector capable of detecting a position of the screw and a timer capable of measuring a time elapsed from a start of the injection process, said method comprising the steps of:

• dividing the injection molding process into a plurality of injection stages to be executed sequentially, wherein injection speeds in the adjacent injection stages are different from each other;
• executing test injection molding operations;
• determining an optimum processing condition for the injection process, under which a molding product of satisfactory quality can be obtained, based on a result of the test injection molding operations, the optimum processing condition being specified by optimum injection pressure characteristics that represent an injection pressure in each of the injection stages and optimum injection speed characteristics that represent an injection speed in each of the injection stages;
• calculating a reference value of a parameter PS or a parameter PT for each of the injection stages based on the optimum processing condition, the parameter PS being an integral of the injection pressure with respect to the screw position, the parameter PT being an integral of the injection pressure with respect to the time elapsed from the start of the injection process;
• storing, in a storage unit of the injection molding machine, the optimum injection pressure characteristics, the optimum injection speed characteristics, and the reference value of the parameter PS or the parameter PT for each of the injection stages;
• and
• executing a productive injection molding operation by carrying out the injection stages based on the optimum injection pressure characteristics and the optimum injection speed characteristics stored in the storage unit, each of the injection stages including the steps of:
• moving the screw at a speed specified by the optimum injection speed characteristics;
• monitoring the injection pressure and the screw position or the time elapsed from the start of the injection process throughout the injection stage in a real-time mode, and calculating an actual value of the parameter PS or the parameter PT in a real-time mode;
• comparing, in a real-time mode, the actual value of the parameter PS or the parameter PT with the reference value of the parameter PS or the parameter PT assigned to the injection stage;
• and
• entering into a next injection stage, when the actual value of the parameter PS or the parameter PT reaches or approaches close to the reference value of the parameter PS or the parameter PT.