Method of injection molding expanded thermoplastics and articles produced thereby
AssigneeThe General Tire & Rubber Company
InventorRobert A. Molbert
Life of this patent
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Abstract
Typically, injection molded expandable thermoplastics exhibit imperfect surfaces in the form of mottled areas, visible flow lines, and pin holes; these imperfections are caused by the quick chilling of the surface of the expanded thermoplastic as it contacts the mold surface. This invention is a method of injection molding expanded thermoplastics free of these defects by injecting a short-shot of the expandable thermoplastic into an elastic membrane positioned within a cooled mold cavity, containing the short-shot within the membrane until a skin forms on the thermoplastic in contact with the membrane, and then permitting expansion of the thermoplastic in the cavity to form a smooth skin or surface on a substantial portion of the article.
Claims9
A method of injection molding an expanded thermoplastic article to achieve a smooth skin over a substantial portion thereof, comprising the steps of:
- a. injecting through a sprue opening in a cavity formed by a pair of closed hard metal injection mold members that are maintained at a temperature below about 100.degree. F., there being attached about and over said sprue opening and interiorly of said cavity a smooth-surfaced, thin, nonporous cured elastic rubber membrane, a short-shot of non-expanded molten expandable thermoplastic into said cavity and against said membrane, while simultaneously increasing the cavity pressure to a level below the injection pressure and above the thermoplastic expansion pressure of said molten non-expanded expandable thermoplastic, so that said non-expanded molten expandable thermoplastic forms a pocket in said membrane and gradually cools and develops a smooth, thin skin adjacent said membrane, and
- b. releasing the cavity pressure to permit expansion of said thermoplastic and deformation of said membrane into full conformance with the interior surface of said mold cavity to form an expanded article having a smooth, thin skin over that portion in contact with said membrane, said membrane being non-adherent to and releasable from the skin of said article and from the interior surface of said mold cavity.
A method of injection molding an expanded thermoplastic article to achieve a smooth skin over a substantial portion thereof, comprising the steps of:
- a. placing a smooth-surfaced, thin, non-porous cured elastic rubber membrane in the cavity opening between a pair of opened hard metal injection mold members that are maintained at a temperature below about 100.degree. F.;
- b. closing said mold members to clamp said membrane therebetween to form a pair of compartments in said cavity;
- c. exhausting the air from the sprue-side containing compartment while gradually pressurizing the opposite compartment to deform said membrane into conformance with the surface of the sprue-containing mold member;
- d. injecting a short-shot of non-expanded molten expandable thermoplastic through said sprue into said cavity and against said membrane while simultaneously pressurizing said cavity opposite said membrane to a level below the injection pressure and above the thermoplastic expansion pressure so that the non-expanded expandable molten thermoplastic forms a pocket in said membrane and gradually cools and forms a smooth, thin skin adjacent said membrane;
- and,
- e. applying a vacuum to said pressurized cavity portion to permit expansion of said thermoplastic and deformation of said membrane into full conformance with said mold cavity to form an expanded thermoplastic article having a smooth, thin skin over that portion in contact with said membrane, said membrane being non adherent to and releasable from the skin of said article and from the interior surface of said mold cavity.
Dependencies are inferred from the claim text.