Patent ID: 7166246

Claim:
A screw backpressure controlling method in an electric injection molding machine comprising: a sensor for detecting a screw backpressure as an electric signal; a servo motor for driving a screw forward and backward; a motor for rotating the screw to plasticize a material; and a controller for controlling said servo motor and said motor, wherein, in a plasticizing process in which said motor is rotated, said controller performs control by repeating a first process in which the rotation of said servo motor is controlled by a first retreating speed command value to make the screw retreat at a low speed when a screw backpressure signal is smaller than a backpressure set value and a second process in which the rotation of said servo motor is controlled by a second retreating speed command value to make the screw retreat at a high speed when the screw backpressure signal is larger than the backpressure set value; wherein, when said first process continues longer than a predetermined monitoring time, the first process is performed according to a first computed value that is obtained by subtracting an additional speed command value from said first retreating speed command value and the second process is performed according to a second computed value that is obtained by adding a speed difference to said first computed value, wherein the speed difference is, in turn, obtained by subtracting said first retreating speed command value from said second retreating speed command value.