Blow moulding

How blow moulding works

Blow moulding is a process used together with either extrusion or injection moulding. Blow moulding is accomplished in two steps:

  • Creating a tube of molten plastic, called a parison (the same term as used in glass-blowing)
  • Inflating the tube to the desired final shape.

Forming the parison can be done by either extrusion (used for thicker plastic bottles) or injection moulding (as in the case for the thin see-through softdrink bottles).

Blow moulding only uses thermoplastics. Stiffer and stronger materials are often used so that the walls of the product can be thinner.

Extrusion blow moulding step by step

In most cases of extrusion blow moulding, the process is organised for very high production operation for making plastic bottles. The sequence is automated and usually integrated with downstream operations such as bottle filling and labelling. It is usually a requirement that the blown container be rigid, and rigidity depends on wall thickness among other factors.

Extrusion blow moulding - Step 1 - Plastics manufacturing - DemaPlasTech

Step 1

A parison is created through extrusion moulding and fed between 2 parts of a mould.

Extrusion blow moulding - Step 2 - Plastics manufacturing - DemaPlasTech

Step 2

The mould is closed around the parison sealing off its ends. The one end is also sealed off by a blow pin.

Extrusion blow moulding - Step 3 - Plastics manufacturing - DemaPlasTech

Step 3

Air is blown through the blow pin into the sealed parison forcing it into the shape of the mould.

Extrusion blow moulding - Step 4 - Plastics manufacturing - DemaPlasTech

Step 4

The mould is separated and the blow pin withdrawn, leaving the new plastic blown product behind.

Injection blow moulding step by step

A variation of this process – called stretch blow moulding – is often used to make soft drink bottles.

Injection blow moulding - Step 1 - Plastics manufacturing - DemaPlasTech

Step 1

A parison is injection moulded around a blowing rod; Sometimes this parison is called a preform.

Injection blow moulding - Step 2 - Plastics manufacturing - DemaPlasTech

Step 2

The injection mould is opened and the preform is transferred to a blow mould.

Injection blow moulding - Step 3 - Plastics manufacturing - DemaPlasTech

Step 3

The soft plastic is inflated to conform to a chilled blow mould

Injection blow moulding - Step 4 - Plastics manufacturing - DemaPlasTech

Step 4

The blow mould is opened and the blown product is removed.

Uses for blow moulding

Mostly, blow moulding is used to produce disposable containers for packaging and liquids.

That said, it is also used to manufacture:

  • large shipping drums for liquids and powders
  • large storage tanks
  • car gasoline tanks
  • toys
  • hulls for sail boards and small boats.
Blow moulding - Plastics manufacturing - DemaPlasTech
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