both a method and apparatus are disclosed for controlling a cyclically operated injection molding machine such that uniform , high quality molded articles are repeatedly produced from plasticized synthetic resinous material . the method and apparatus include integration of a sensed injection pressure with respect to time from the beginning of an injection stroke until plasticized material in a mold reaches a predetermined pressure . the integration defines a work index which is compared with a preset allowable range . if the work index lies outside the preset range , an adjustment is made to the back pressure acting on the plasticizing screw during the plasticizing portion of the subsequent molding cycle . the apparatus is permitted to make a predetermined number of adjustments in the back pressure after which a suitable indication is made that the molding apparatus is out of control itself . other viscosity - affecting parameters may be controlled including injection screw rotary speed and injection screw velocity , and barrel temperature of the plasticizing apparatus . a mold cavity pressure sensor indicates that the cavity is full , and thereupon stops the integration and may also reduce the injection pressure to a lower value , sufficient to supply additional material to compensate for shrinking due to cooling , yet not high enough to introduce strains , or cause flashing .