Patent ID: 7077987

Claim:
A method for foam-injection molding a thermoplastic resin, comprising: (1) providing a mold comprising a fixed mold half and a movable mold half mating with said fixed mold half to thereby form a mold cavity defined by an inner wall of the fixed mold half and an inner wall of the movable mold half, said mold cavity having an inner wall and communicating with a resin inlet, said inner wall of said mold cavity having a molten resin releasing means, (2) injecting a foamable thermoplastic resin in a molten form, under preselected injection temperature and pressure conditions, into said mold cavity of said mold through said resin inlet in an amount which is in the range of from 95 to 110% of the weight of the foamable molten resin having a resin volume equal to the volume of said mold cavity, wherein the resin volume is as measured under said preselected injection temperature and pressure conditions, thereby forming a foamable molten resin mass in said mold cavity, (3) applying a pressure to said foamable molten resin mass in said mold cavity to press the surface of said foamable molten resin mass against said inner wall of said mold cavity, thereby allowing a surface portion of said foamable molten resin mass to be solidified to form a surface skin layer of said foamable molten resin mass, and (4) releasing a portion of said foamable molten resin mass under the foaming pressure exerted by said foamable molten resin mass to the outside of said mold cavity through said molten resin releasing means, thereby lowering the pressure exerted on the foamable molten resin mass, so that foaming of the foamable molten resin mass in the mold cavity is caused to occur to form a foamed resin mass having said surface skin layer remaining substantially non-foamed, wherein said molten resin releasing means of said mold cavity is an openable/closable valve, and wherein said mold has a molten resin release cavity which communicates with said mold cavity through said openable/closable valve, the volume of said molten resin release cavity being larger than the volume of said portion of the foamable molten resin mass which is released through said openable/closable valve to the outside of said mold cavity in step (4), and wherein said molten resin release cavity has a gas passage communicating with the outside of said mold.