EXISTING PET TECHNOLOGIES

To produce PET containers it is necessary first to injection mould 'test-tube' shaped preforms, which must then be brought to the right temperature for being stretched and blow moulded to form the finished containers.

Machines and moulds for the production of PET preforms and containers are commecrially available, using various technologies that can be classified into three categories:

The 2-stage process: In the "two-stage process", an injection moulding machine is used to mould preforms, which are cooled to ambient temperature and stored for later use. Independently a stretch blow moulding machine is used to reheat preforms to a stretch-blow temperature and then stretch and blow mould them to the shape of the required container.

Machines and moulds with many cavities have been developed for high production rates with this "two-stage process", making it the process of choice for high output applications such as carbonated soft drinks and mineral water bottling. Such machines however are prohibitively expensive for processors requiring low to medium outputs. Also this "two-stage process" is not well suited to containers with asymmetrical shapes (e.g. oval bottles) or with wide neck openings (jars).

The 1-stage process: In the "one-stage process" the two steps of injection moulding the preforms and stretch-blowing them into containers are achieved with one machine using machine-specific moulds. The preforms are moulded in an injection mould, cooled in this mould to their average stretch-blow temperature and then tranferred to a stretch-blow station where they are stretched and blow moulded into the final container shape. In some cases the moulded preforms are transferred to a conditioning station for further temperature conditioning, before being taken for stretch blowing. The formed containers are then taken to another station where they are released or ejected.

A one-stage machine typically has three or four stations (injection, {conditioning,} stretch-blow, container ejection) with a rotary indexing table for thansferring the preforms between stations. There are separate clamping mechanisms for the opening and closing of the injection moulds (vertical clamp) and of the blow moulds (horizontal clamp). The mould sets used in one-stage mechines are costly, partly because some mould components like neck formers must be duplicated several times, as several sets are needed to carry preforms from station to station.

One-stage machines are well suited to low production rated and to containers with asymmetrical shapes or with wide neck openings. The one-stage process is less energy intensive than the two-stage process because it eliminates the need for complete cooling of the preforms and later their reheating for stretch-blowing.

The 1½-stage process: Sometimes also called the integrated process, this is a variant of the one-stage process, where one machine is used for injection of preforms and stretch-blowing of containers, but the preforms are not tranferred directly from the injection to the blowing station. Instead they are removed from the injection mould and placed on a tranfer chain, which passes them first throught an oven for reheating and then to the blow moulds for stretch blowing. In this integrated process, the number of blow mould cavities is smaller than the number of injection mould cavities, taking advantage of the faster blowing cycle compared to the injection cycle (unlike the 1-stage process where the number of injection and blow cavities are the same).