Blow Moulding

Blow moulding is a manufacturing process that is used to create hollow plastic parts by inflating a heated plastic tube until it fills a mould and forms the required shape. The raw material in this process is a thermoplastic in the form of small pellets, which are first melted and formed into a hollow tube, called the parison. The parison is then clamped between two mould halves and inflated with compressed air until it forms the inner shape of the mould cavity. After the part has cooled the mould halves are separated and the part ejected.

Parts made from blow moulding are hollow, such as bottles and containers in a variety of shapes and sizes. Small products may include bottles for water, liquid soaps, shampoo, motor oil and milk. Larger containers made by this process include drums, tubs and storage tanks.

Blow moulding parts can be formed from a variety of thermoplastics, including:

  • Low density Polyethylene (LDPE)
  • High Density Polyethylene (HDPE)
  • Polyethylene Terephtalate (PET)
  • Polypropylene (PP)
  • Polyvinyl Chloride (PVC)


Blow moulding can be divided into three different processes, these are:

Extrusion Blow Moulding* (EBM):  An extruder uses a rotating screw to force molten plastic through a die head that forms the parison. The parison is extruded vertically between two open halves of the mould. They close around the parison and a blow pin enters through the top of the mould. Compressed air flows through the blow pin to form the part. Heat transfer from the chilled mould is used to cool the blow mould part.

Injection Blow Moulding* (IBM):  This process is used for the production of hollow plastic objects in large quantities. In the IBM process, the polymer is injection moulded onto a core pin, the core pin is rotated to a blow moulding station to be inflated and cooled. This is the least used of the blow moulding processes and is typically used to make small medical bottles. This process is divided into three steps, injection, blowing and ejection.

Stretch Blow Moulding* (SBM): In this process the plastic is first moulded into a “preform” using the injection moulding process. These preforms are produced with the bottle threads on one end. The preforms are then fed into a reheat stretch blowing machine. In the SBM process, the preforms are heated and blown, using high pressure compressed air, into bottles using metal blow moulds.  During the process the preform is stretched with a core rod. The main applications for this process are bottles for carbonated drinks and mineral water.

* Animations from British Plastics Federation www.bpf.co.uk