Hot press including cam rod penetrating top platen

A hot press which is provided with a base platen, a top platen supported by a plurality of tie rods fixed uprightly on said base platen, a movable platen guided movably by said tie rods, at least one hydraulic cylinder for elevating or lowering said movable platen, at least two heating plates guided movably between said to platen and movable platen, a double action clamping cylinder including a primary and a secondary cylinder chamber for operating said movable platen and means for maintaining a predetermined space between each adjacent heating plate when said movable platen is positioned at a lowest limit, comprising:

• a top heating plate hung movably down from said top platen by support means;
• a cam rod moving in cooperation with movement of said top heating plate and penetrating through said top platen;
• means for detecting a start of elevation of said top heating plate by an electric means mounted on the top platen for detecting the motion of said cam rod and generating a signal for switching a control of said hydraulic cylinder from a speed control to a pressure control;
• a pressure control circuit for generating clamping pressure by difference in pressure between said primary and secondary cylinder chamber,
• wherein said pressure control circuit comprises:
• two pumps which deliver a pressurized hydraulic medium to the primary and secondary cylinder chambers of said double action clamping cylinder;
• a primary passage between said primary cylinder chamber and one of said two pumps;
• a secondary passage between said secondary cylinder chamber and the other of said two pumps;
• a relief valve and a directional valve which are both included as part of said primary passage;
• a second relief valve and a second directional valve which are both included as part of said secondary passage, wherein said primary and secondary passage are provided independently of each other and one of the pumps delivers the pressurized hydraulic medium to only the primary chamber and the other pump delivers the pressurized hydraulic medium to only the secondary chamber.