The flexible, but short, (CH 2 ) 2 groups tend to leave the chains relatively stiff and PET is notes for its very slow crystallization. If cooled rapidly from the melt to a Temp below Tg, PET solidifies in amorphous form.
If PET is reheated above Tg, crystallizaiton takes place to up to 30%.
In many applications PET is first pre-shaped in amorphous state and then given a uniaxial (fibers or tapes) or biaxial (film or containers) crystalline orientation.
During Injection Molding PET can yield amorphous transparent objects (Cold mold) or crystalline opaques objects (hot mold)