Method for foam injection molding of thermoplastic resin

A method for injection foaming molding of a thermoplastic resin to foam-mold a foaming-agent-containing plasticized resin, in which a foaming agent is blended into a plasticized thermoplastic resin, by using an injection apparatus for injecting the foaming-agent-containing plasticized resin and a mold, in which the foaming-agent-containing plasticized resin injected from the injection apparatus is filled into a mold cavity with a variable volume and by expanding the mold cavity filled with the foaming-agent-containing plasticized resin,

• the method comprising the following steps (1) to (5):
• (1) injecting the foaming-agent-containing plasticized resin from the injection apparatus to fill the mold cavity in a mold-clamped state, the foaming-agent-containing plasticized resin contains a bubble-core forming agent and a foaming gas, with the foaming gas being a member selected from the group consisting of air, carbon dioxide gas, nitrogen gas and mixtures thereof;
• (2) releasing a clamping force of the mold immediately after the foaming-agent-containing plasticized resin is injected and filled into the mold cavity, so as to release the pressure of the foaming-agent-containing plasticized resin in the mold cavity;
• (3) maintaining the pressure-released state of the foaming-agent-containing plasticized resin in the mold cavity for a predetermined period of time;
• (4) expanding the mold cavity to foam the foaming-agent-containing plasticized resin;
• and
• (5) cooling and maintaining the foaming-agent-containing plasticized resin while the mold cavity is in the expanded state.