Method for producing rotationally molded parts from semi-crystalline materials

A method is provided for producing dimensionally stable rotationally molded parts from semi-crystalline materials. A test mold is constructed having dimensions closely approximating the desired dimensions of the final part. A test part is then molded in the test mold using conventional rotational molding techniques. After the test part has partially cooled, it is removed from the mold, allowed to cool to ambient temperature, measured and annealed in an oven. During annealing, the part is heated to the crystallization temperature, midway between the glass transition temperature and the crystalline melting temperature. After annealing, the part is allowed to cool to ambient temperature and measured again. The post annealing dimensions are then compared with the dimensions determined before annealing to determine the amount of shrinkage. A production mold is then constructed to take into account the amount of shrinkage calculated. Production parts may then be molded in the production mold and annealed at the crystallization temperature to attain parts having a small dimensional tolerance and high tensile strength.