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How To make PET preform/bottle?

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.

What are Steps for PET bottle production?

Drying PET
PET is kind of hygroscopic resin and absorbs moisture in the air. If the moisture is not removed before processing, the resin will hydrolyze and cause rejects. The simplest drying way is to use hopper dryer, however the drying effect is dependent on the air humidity and the drying is unstable and in any case could not reach the 50ppm requirement. While dehumidifying dryer recycles the air for reuse and attains the drying requirement of PET resin.

Plasticizing the PET
The dried PET resin are compressed and melt by a rotary screw.

Injection molding the PET preform
Molten PET is injected into the injection cavity and cooled rapidly to form a preform.

Heating the PET preform
The PET preform is heated up to a certain temperature for blowing.

Stretch blow molding the PET preform
The heated preform is simultaneously stretched and blown in a shaped blow mold to form a tough, lightweight container.

When PET is heated to a temperature, its chain-like molecules are sufficiently mobile to uncoil instead of breaking when extended so that stretching can orient it. In stretch blow molding, stretching from two directions at right angles gives biaxial orientation. Oriented PET contains closely packed chains aligned in the directions of stretch. The material becomes stronger because the molecules act together instead of individually. The tensile strength of oriented PET is several times that of the material without stretching, as well as the impact strength, barrier and chemical resistance are also significantly improved, so bottles can be lighter without sacrificing performance.

Ejecting PET container
The finished container is ejected from the molds.

During the heat processing, as PET melted temperature is too high, PET degrades to AcetAldehyde (abbreviated to AA).

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