BACK-PRESSURE CONTROLLING SYSTEM FOR INJECTION MOLDER BY NUMERICAL CONTROL DEVICE

PURPOSE:To perform optimum kneading by a method wherein a torque limit, which corresponds to the value of frictional force minus set back pressure, is applied so as to output the offset voltage possessed by a numerical control device in order to drive a servo motor in such a direction as to retreat a screw, when the set back pressure is smaller than the frictional force of a transmission mechanism. CONSTITUTION:When a set back pressure is smaller than frictional force, an offset voltage (minus) in the direction to retreat a screw is outputted from an arithmetic circuit 2 and, in addition, a torque limit value corresponding to the value of frictional force minus a set pressure is outputted as the torque limit value from the arithmetic circuit 2. After that, when the screw is rotated to start metering and kneading, a driving current command to drive a screw motor M so as to retreat the screw is issued through an error amplifier 6 by the offset voltage acting in the direction for retreating the screw. However, owing to a torque limiting means 8, only the driving current command, the value of which corresponds to the set value of the frictional force minus the set back pressure, is issued. But, because the frictional force of the transmission mechanism exists, the set back pressure is actually applied to the screw.