Foamed thermoplastic resin cork having a natural cork-like appearance and a method of injection molding the cork

A method for making a closure made of a thermoplastic resinous material for liquid product containers comprising the steps of:

• providing a hot runner injection mold assembly having a plurality of mold cavities, wherein each cavity includes walls defining the shape of said closure and a cavity entrance orifice communicating with each of said cavities, the wall of each mold cavity opposite the entrance orifice being sufficiently spaced from said entrance orifice to permit the formation of a strand of thermoplastic in said cavities,
• rapidly injecting, at high pressure, an expandable thermoplastic resinous material containing a blowing agent therein in an amount less than that required to fill the mold cavity and in the form of a strand, directly into each of said mold cavities through said entrance orifice, said strands collecting in each of said cavities as randomly-oriented coils, the pressure produced in said mold cavity by said injection temporarily pressurizing said cavity, thereby retarding the expansion of the plastic material sufficiently to allow completion of the injection of said thermoplastic resinous material, while maintaining the integrity of the strand configuration,
• sealing said entrance orifice immediately after the thermoplastic resinous material has been injected into said cavity to stop any further flow of said resinous material into said cavity and keeping said entrance orifice sealed until said resinous material subsequently expands and solidifies to form said closure;
• simultaneously with said temporary pressurization, gradually removing a portion of the air displaced by said injected thermoplastic resinous material through controlled minimum venting of said mold cavity, thereby expanding the coiled resinous material in a controlled manner through a continuation of said minimum venting to fill said mold cavity, the walls of said mold cavity being maintained at a temperature sufficiently cool to cause the resinous material to solidify into a rigid, outer skin when it contacts said walls, whereby the coalesence of said coils during expansion creates randomly-oriented grain lines in the surface of said closure which resembles the appearance of natural cork, and wherein the closure thus formed has a liquid impervious outer skin and a foamed inner core with no appreciable void spaces or separations between said outer skin and said foamed inner core into which said outer skin could collapse when compressed.