Injection foam molding process

Claim 1

In an injection foam molding process including the steps of forming a molten mixture of heat fusible synthetic polymeric resin and a compound chemically decomposable by heat to form gas as blowing agent, injecting said mixture into the cavity of a mold under conditions that the mold cavity is filled with the mixture at foaming temperature, said mold having a first section which protrudes into the mold cavity at the time of injection and which has a surface portion and side edges, said side edges having portions initially exposed within the mold cavity and which are in sliding engagement with side edges of another mold section to form a tight joint, said first mold section being movable to bring said surface portion into registry with a mold surface portion of said other mold section to increase the volume of the mold cavity, moving said first mold section to increase the volume of the mold cavity to permit the mixture to foam and solidifying the mixture, the improvement which comprises sealing the mold, supplying gas to the mold cavity to develop a gas pressure of at least about 10 Kg/cm 2 to resist foaming of the molten mixture injected into the mold, injecting the molten mixture into the mold under conditions which fill the mold cavity with the mixture in substantially unfoamed condition, discharging gas from said cavity during said injection at a rate to avoid burning of said resin by gas compression temperature increase but not to lower the pressure below said foam resisting pressure, reducing the gas pressure substantially to atmospheric pressure substantially immediately when said mold cavity is filled and thereafter moving said first mold section to increase the volume of the mold cavity during the time interval in which A. thickened resin on marginal portions of the protruding mold surface portion adjacent the joint between the mold sections remains soft, deformable and extensible by the expansion force of the blowing agent, and B. the expansion force of said blowing agent has reached a value to force said resin against the moving wall and to extend portions of thickened resin on said marginal portions of said movable mold surface adjacent said joint between mold sections into close proximity to resin cooled on said side edges to form a narrow crease line.