The blow moulding processis a low pressure process with typical blow air pressures around 25 to 150 psi. There are several different types of blow moulding. Injection Blow Moulding: This process if the least famous of all blow moulding processes and is used to make single serve and small medical bottles. The injection blow moulding process begins with injection moulding a polymer onto a core pin. This polymer is then rotated to a blow moulding station, where it is inflated and cooled. Extrusion Blow Moulding: This process begins with downward extrusion of a tube. As the desired length id reached, the mould is closed, holding the neck of the tube open, while the bottom end is pinched shut. A blow pin is then inserted from the neck of the hot tube and forms a threaded opening. This blow pin then inflates the tube inside the mould cavity itself. After the entire ensemble has cooled down, the bottle is ejected from the mould and the excess plastic from the neck and bottom is trimmed off. This process can be used with a number of polymers, including polypropylene, polyvinyl chloride, polyethylene, and others. Stretch Blow Moulding: In this process, plastic is first injection moulded into a 'preform' with the finished necks of the bottles at one end. Then the â€˜performâ€™ is heated till it reaches its glass transition temperature. High pressure air is then used to blow into the bottles using metal blow moulds. Meanwhile, the â€˜performâ€™ is also stretched with a core rod in order to fill the mould. In case of some polymers like Polyethylene Terephthalate also go through strain hardening during the stretching process, which allows them to resist deformation even under high pressures.
Stretch blow moulding produces items that are of excellent dimensional and visual quality as compared to products formed by extrusion blow moulding, making this process optimum for producing bottles and jars and other similar containers.