PET is transformed several times as it goes form PET pellet to preform. As a resin pellet, PET is thermally crystallized to a level of 50% to 70% and PET appears white. During injection process, these crystals are melted, resulting in an amorphous melt, which is injected into preform mold cavities. PET is clear and has a little strength or barrier properties. In the reheat stretch blow machine the material is forced by stretch rod and blow air to orient in axial and hoop direction forming small, strain-induced crystals. The bottle turns out clear and has higher strength or barrier properties.
There are two ways of producing PET preform/bottle: one-stage “hot preform” method and two-stage “cold preform” method.
The One-Step method -- from PET granule to finished bottle -- all processes are completed within one integrated machine. This means the injection-molded preform is taken off from the injection cavity while still hot enough to be stretch blown to form the bottle. No extra heating is required, and since preforms are freshly molded every time, there is no risk of surface damage during storage or transportation. Besides, there are many other advantages such as energy saving for reheating is not required; high productivity as cooling and reheating are not necessary; no pollution as human hand is out of reach. One-Step method is highly suited to small and medium scale production lines.
The Two-Stage method refers to the injection stretch blow molding process which is composed of an injection molding machine for making the preforms and a reheat blow molding machine to reheat the cooled preforms and blow the bottles. The preform is injection molded on the first, then reheated and blown on the second. The requirement for a preform heating system means the two-stage process has a lower thermal efficiency but a perfect neck finish and economical resin usage. This method is most suited for PET bottle production on medium & large-scale.