a method is disclosed for enhancing the yield strength and density of polyethylene terephthalate and other similar thermoplastic materials which are susceptible to strain hardening . in the method , the plastic material is thermally conditioned to a temperature within a range conducive to molecular orientation , and then is mechanically conditioned by stretching in a first direction to a length at least about 2 . 6 to 2 . 75 times the original length . thereafter , the plastic material is stretched in a second direction , with strain hardening and strain - induced crystallization increasing drastically substantially immediately upon the initiation of the second stretching operation due to the mechanical conditioning by elongation during the first stretch . the invention is particularly suitable for forming blow molded containers , where a blowable parison or preform is first stretched along its longitudinal axis to the critical length and is then extensively strain hardened while being blown to the configuration of the container .