Heat Stabilized PET

In broad terms there are two methods for producing heat stabilized bottles, the 
Single Blow and the Double Blow systems :

Single Blow

The Single  Blow  technique can be applied on a conventional   One-Step   ASB 
machine. The PET  container  is  blown inside  a heated blow mold. This  raises
the level of crystallinity of the PET, and  also  relaxes  some  of   the  stresses
induced in the PET by the stretching process so that the amount by which the 
bottles shrink when hot filled is reduced.

The  temperature  of  the  mold  is  limited by the need to ensure that  bottles 
on leaving the blow mold are rigid & will not be subject  to   deformation. Even
using special in-mold cooling systems, "single blow" methods   can  only   raise 
service temperatures to about 83°C (after storage).

Double Blow

Specially  developed  reheat  blow  molding  machines  are  required  to  make 
Double  Blow bottles. An oversized  bottle is blown which is then subjected  to
heat setting followed by a stress relieving process, before  being  blown  for a
second time to form the finished container. Double Blow  bottles  have  higher
levels of crystallinity plus very low levels of residual  stress,  and can  be  hot 
filled at temperatures up to 87°C (after storage).

(Note  that  heat resistance is lowered  if heat  stabilized  bottles  are  stored 
under conditions of  high temperature and humidity. This phenomenon   applies
to both the Single and Double Blow  methods.  The  hot   filling   temperatures  
quoted  above are for  bottles  that have been  stored   at  40°C and 90% RH 
for 24 hours).