Method and apparatus for regulating mold clamping force in an injection molding machine

An injection molding machine including an hydraulic clamp cylinder and piston for applying and maintaining a clamping force on a separable mold to maintain the mold in a closed condition, said apparatus comprising:

• (a) pressurizing means for providing hydraulic fluid under pressure to a clamp cylinder;
• (b) valve means positioned between the pressurizing means and the clamp cylinder, the valve means operable selectively between an open position to permit flow of pressurized hydraulic fluid from the pressurizing means to the clamp cylinder and a closed position to prevent flow of hydraulic fluid from the clamp cylinder to maintain hydraulic fluid under pressure within the clamp cylinder;
• (c) set point setting means for selecting and providing a predetermined set point clamping force value;
• (d) pressure transducer means in communication with the clamp cylinder for providing an output signal representative of the hydraulic fluid pressure within the clamp cylinder;
• (e) comparator means connected with the pressure transducer means and with the set point setting means for comparing the output signal of the pressure transducer means during a clamp force maintenance portion of a clamp operating cycle with the selected set point clamping force value to provide a correction signal representative of the actual pressure difference between the clamping force corresponding with the transducer output signal and the set point clamping force value;
• and
• (f) control means connected with the comparator means and with the valve means to provide a control signal for regulating the opening and closing of the valve means during a next succeeding clamp operating cycle to provide a clamping force during the clamp force maintenance portion of that next succeeding cycle that is closer to the set point clamping force value than was the clamping force in the immediately preceding clamp operating cycle.