Injection moulding of preforms is mostly used to enhance the moulding of PET. This is commonly known as the two-step process.
In step one being the preform shape is injection moulded. In step two the preform is reheated and blow moulded. This two-step process is popular in the mass production of preforms and bottles.
An important part of the injection moulding system is the dehumidifying heater dryer. This removes water from the resin pellets, necessary because molten PET is sensitive to water at high temperature and its presence affects the molecular weight of the PET and causes discolouration and burning of the product.
Therefore the temperature these are set to is very critical. Dryers will need to be set at 170c for around 3-4 hours before the pellets can be introduced into the barrel and plasticised. Barrel temperatures should be set at around 240-280c to facilitate the melting of the PET Pellets.
Accurate temperature control of the barrel, injection nozzle and mould are paramount to maintain flow and accurate moulding. Injection speed need to be relatively fast, but not too fast as too cause shear and cause the material to become fragile. Injection pressure will typically be in the range of 80 – 140 MPa, depending on product weight, and back pressure will typically need to no more than 100 bar to prevent wear.