a method of forming a preform having crystallized thermoplastic material from a blank of orientable thermoplastic material , such as polyethylene terephthalate , which comprises laterally displacing first and second regions of a blank , preferably tubular , of orientable thermoplastic material to form an intermediate region between the displaced regions having outer surfaces facing in opposite directions . pressure is applied to the outer surfaces of the intermediate region to relatively displace these outer surfaces longitudinally of the blank while concurrently reducing the thickness of one of the first and second regions to the value of the thickness that the material would obtain if freely stretched to flow to achieve crystallization of the material in the aforesaid one region . the material in the one region , during the reduction of thickness thereof flowing from the orientable material thereof to a transition zone to form the one region which is reduced in thickness and is elongated in length .