Method of constructing mold for forming plastic foam

Abstract

A mold for forming plastic foam parts in which an injection-molded plastic member forms a molding surface of the foam molding cavity against which foam plastic is adapted to be molded. The plastic member is preferably injection molded in a metal molding die from either polyethylene or polypropylene so that its molding surface is the reverse of the exterior surface of the plastic part to be foam molded therein. Vent passages and sealing beads may be integrally formed with the injection-molded member.

I claim: 
     
       1. A method of constructing a mold having a cavity in which a plastic foam part is adapted to be molded, said mold including a pair of separable mold sections, each of which is adapted to receive an injection molded cavity-forming liner which, when the mold sections are closed with one another, cooperate to define said mold cavity, comprising the steps of forming two pair of metal dies, each pair having molding surfaces which correspond with a half section of the exterior surfaces of the foam part to be molded, utilizing each pair of said metal dies to form an injection molding cavity defined by the respective molding surfaces of said two metal dies; subjecting a thermoplastic synthetic resin material to sufficient heat and pressure to plasticize the same and injection molding said material in said injection molding cavity of each pair of said metal dies to form a pair of injection molded thermoplastic liners for said foam mold sections each having an exterior surface contour formed by the molding surface of its respective pair of metal dies which is the reverse of the external surface contour of the two opposite sides of the plastic foam part to be molded; removing said injection molded liners from said pairs of metal dies; and inserting the two injection molded liners in opposed relation, one in each of said separable mold sections such that when the mold sections are closed said exterior surface contours of the two liners cooperate to define a foam molding cavity which corresponds in size, shape and contour to the size, shape and contour of the foam part to be molded therein. 
     
     
       2. The method called for in claim 1 including the steps of forming a recess in at least one of said pair of metal dies at a selected location in said molding surface thereof and, during the step of injection molding, causing said thermoplastic material to flow into said recess so as to form an integral projection on the injection molded liner which extends therefrom in a direction opposite from said exterior surface contour thereof; forming an opening through one of said separable mold sections to register with said projection and through which said projection is adapted to extend to the exterior of the mold section when the injection molded liner is located therein; and forming a passageway through said projection to provide an integral vent passageway in said injection molded liner which vents the foam cavity defined by the two injection molded liners to atmosphere. 
     
     
       3. The method called for in claim 1 including the step of forming a groove in at least one of said pair of metal dies extending peripherally around the molding surface thereof, said groove lying in a flat plane, and, during the step of injection molding, causing said thermoplastic material to flow into said groove so as to form a continuous peripheral bead projecting from and extending in a flat plane around the exterior surface contour of the respective injection molded liner and forming, by injection molding, a peripheral surface on the other injection molded liner which lies in a flat plane and with which said bead is adapted to engage so as to form a seal around said mold cavity at the parting line between the two injection molded liners when the two mold sections are closed with one another. .Iadd. 4. A method of constructing a mold having a cavity in which a plastic foam part is adapted to be molded, said mold including a pair of separable mold sections, at least one of which is adapted to receive an injection molded cavity-forming liner which, when the mold sections are closed with one another, cooperates with the other mold section to define said mold cavity, comprising the steps of: forming a pair of metal dies having molding surfaces which correspond with one half section of the exterior surface of the foam part to be molded; utilizing said dies to form an injection molding cavity defined by the respective molding surfaces of said two metal dies; subjecting a thermoplastic synthetic resin material to sufficient heat and pressure to plasticize the same and injection molding said material in said injection molding cavity of said pair of metal dies to form an injection molded thermoplastic liner for said one foam mold section having an exterior surface contour formed by the molding surface of the metal dies which is the reverse of the external surface contour of one side of the plastic foam part to be molded; removing said injection molded liner from said dies; and inserting the injection molded liner in said one separable mold section such that when the mold sections are closed said exterior surface contour of the liner cooperates with the other mold section to define a foam molding cavity which corresponds in size, shape and contour to the size, shape and contour of the foam part to be molded therein. .Iaddend.