Device for controlling the clamping force of a motor-driven injection molding machine

A control device for a motor-driven injection molding machine that comprises a mold clamping system having a servo-motor and a ball screw mechanism in which the ball screw mechanism comprises a screw shaft and a nut for use in converting rotation of the servo-motor into rectilinear motion, wherein:

• the control device comprises a pattern generator for generating a set pattern for the clamping force, said pattern generator calculating, in response to application of the set clamping force, a travel distance S0 of a predetermined movable member in the mold clamping system according to the set clamping force, generating the set pattern having an allowable range of deviation 2.DELTA.S that is defined by an upper limit (S0+.DELTA.S) and a lower limit (S0-.DELTA.S) relative to the travel distance S0 calculated, said mold clamping system continuously decreasing, by rotating the screw shaft in the reverse direction during a predetermined clamping time interval Tf, such that the clamping force is within the range defined by the allowable range of deviation 2.DELTA.S, after the clamping force reaches the value defined by the upper limit (S0+.DELTA.S).