a method and apparatus are disclosed for developing a strain crystallized morphology in blowbottles of thermoplastic material by a heat treatment process subsequent to a blow molding operation . in a first embodiment , the blow mold itself includes means for differentially heating the blown article along its length so that only those portions which have been significantly molecularly oriented are heat treated . in a second embodiment , separate mold - shaped members are mounted adjacent the blow mold to receive the blown articles to apply the desired heat treatment process . in the method , an essentially amorphous thermoplastic parison is expanded in a blow molding operation at a temperature conducive to molecular orientation and strain induced crystallization . then , the highly molecularly oriented bottle portions are heat treated to develop crystalline growth , while other non - molecularly oriented bottle portions may be simultaneously cooled .