PET bottles are always made in two steps. First a suitable preform is injection molded. Then this preform is brought to the right temperature and blown into a bottle. These two steps can be combined in one machine and we call that the single-stage (or single-step) process. A variation of this approach is the integrated two-stage process (also called the 1.5 step process) that also produces both parts in a single machine but handles them differently. If an injection molding machine and a reheat stretch blow molding machine are used separately it is called the two-stage process.
There are many criteria to be used when deciding on which process to employ. Foremost, it is a volume question. A few years back yearly volumes over 20 million containers were usually considered to be mainly two-stage candidates. Recent advances in the integrated two-stage process have undermined those assumptions but we will not delve into this issue in this article. Instead, I will concentrate on those smaller volumes, i.e. below 20 MM/year, that were once considered single-stage territory.