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US 3,984,511 AGranted
Method of rotational molding
AssigneeDart Industries Inc.
InventorSidney G. Lammers
Life of this patent
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Abstract
The method of forming, by rotational molding, a foamed polystyrene article that has a continuous covering of thermoplastic material such as low density polyethylene. The method requires the utilization of polystyrene beads that contain substantially less than the maximum possible percentage of blowing or expanding agent. A high temperature, low residence time heating step is found to be adequate for the formation of the article.
Claims3
The method of forming an integral three dimensional product having an internal core of expanded resinous material and a continuous surface layer of resinous thermoplastic material comprising:
- a. introducing into a hollow mold a composition comprising a powdered resinous thermoplastic material and an unexpanded resinous polymeric particulate material impregnated with up to less than 3% by weight of an organic volatile liquid blowing agent which is such to just cause expansion of said resinous particulate material upon heating thereof, said powdered resinous thermoplastic material having a melting point temperature which will cause expansion of said particulate material;
- and
- b. heating the mold to a temperature which will cause expansion of said resinous particulate material, while rotating same whereby the powdered resinous thermoplastic material will be formed on the inside surface of the mold as a continuous surface layer and the expandable resinous particulate material will expand to form a cellular core of expanded resinous material.
Dependencies are inferred from the claim text.