Method and apparatus for forming thermoplastic resin foam

An apparatus for forming a thermoplastic resin foam comprising:

• a screw cylinder having a material supply hole arranged near a rear end portion of one end of the screw cylinder and an injection nozzle arranged on a top end portion of the other end of the screw cylinder;
• a screw, provided in said screw cylinder, rotatably driven in directions of plasticization and injection, said screw corresponding to said screw cylinder and classified as a first metering portion, a low-pressure portion, and second metering portion in that order from the rear end portion to the top end portion;
• a gas supply hole for injecting an inert gas, such as a carbon dioxide gas or a nitrogen gas having, at least in pressure, a pressure equal to or greater than a supercritical pressure or for injecting an inert gas under a supercritical state, said gas supply hole being disposed at a position corresponding to the low-pressure portion of said screw of said screw cylinder;
• drive means for driving said screw in the directions of plasticization and injection;
• and
• a controller provided with a setting device, wherein the setting device has a pre-set pressure value for the screw cylinder, and when a pressure in the screw cylinder has dropped below the pre-set value, the controller causes the drive means to drive the screw in the direction of plasticization and to stop the screw when the pressure becomes equal to or greater than the pre-set value in order to maintain the pressure at a supercritical pressure or more inside the screw cylinder.