Forming non-cylindrical articles from preforms of polyalkylene terephthalate

In the method of forming a molecularly oriented hollow article having a non-round cross section of substantially oval or substantially triangular or like cross section from a preform of thermoplastic material, which includes distending the preform in a mold while at molecular orientation temperature,

• the improvements wherein the thermoplastic material is moldable polyalkylene terephthalate having an inherent viscosity of at least about 0.55, said preform region forming said cross section is not treated before said distending for the purpose of establishing a circumferential temperature difference therein, and the distending comprises, in combination, the steps of:
• expanding first portions of said region against mold wall portions at the end of a minor axis of a cavity conforming to said non-round cross section to form relatively thick sections thereat while expanding other portions a greater extent than said first portions toward wall portions at the end of a major axis of said cavity thereby establishing a strain hardened pattern in the material wherein such expanded other portions are strain hardened greater than said thick sections;
• and
• drawing material out of said thick sections as such expanded other portions of greater strain hardened level continue to expand toward the mold wall portions at the end of the major axis;
• limiting the corner definition ratio to between about 3 to about 9 and the circular deviation ratio to no greater than about 2.4 at the cross section during said distending;
• and controlling said distending according to the relations:
• ##EQU5## wherein:
• A is between about 15 to about 100;
• and
• B is no greater than about 334;
• thereby forming said article having reduced wall thickness variability at the non-round cross section in comparison with an article having the same cross section formed of thermoplastic material which does not strain harden during distension.