Molded thermoplastic article and method of making same

A method of making a completely closed, thin-walled, hollow article of fusible, or thermoplastic resin (such as polyethylene) without seams or a vent hole, comprising the steps of: rotationally molding the article in a mold heated to a temperature above the fusion point of the resin; and rapidly chilling the molded part from a temperature above the heat distortion point of the resin to a temperature well below the heat distortion point; causing the plastic to stiffen and become rigid before it is collapsed by the partial vacuum inside the part due to contraction of the trapped air as it cools. In one embodiment of the invention, the mold itself is rapidly chilled by cold air or water sprays, causing the molded part to chill rapidly before the air can cool. In another embodiment, the part is removed from the mold and allowed to cool and partially collapse; after which the part is reheated slowly up to a temperature slightly above the heat distortion point. Air trapped inside the part expands, and elastic memory of the resin causes the heat-softened part to resume its original molded shape. The part is then chilled rapidly, to stiffen the resin in the undistorted shape before the air inside has a chance to cool and contract.