Blow moulding strain crystallised polyethylene terephthalate! articles - by pre-stretching a heated parison to start strain crystallisation and blowing parison at molecular orientation temps

Polyethylene terephthalate articles housing a high degree of strain induced crystallinity are made by heating a parison to the molecular orientation temp. (75-110 degrees C) and stretching it to an areal strain of 2.0-2.75. The stretched parison is then cooled to a temp. = Tg (60-75 degrees C) and further stretched at that temp. to an areal strain of 2.4-3.0. The parison is then reheated to the molecular orientation temp. and blown. The pre-treatment of the parison brings the material to the threshold of, or just into, the region where strain crystallisation starts so that during the subsequent blowing, strain crystallisation as well as orientation is effected. Higher degrees of crystallisation are obtd. than in prior art processes using thermal crystallisation.