Blow Moulding

Blow moulding steps

Blow moulding is a permanent mould process used for the manufacturing of hollow plastic parts. The process can be divided into two main types: extrusion blow moulding and injection blow moulding. Extrusion blow moulding requires less costly tooling, but injection blow moulding offers greater control over part parameters like wall thickness and weight.

This process consists of a few standard steps:

  • A parison (a preheated hollow tube made of plastic) is fitted onto the blowing head of the machine and then placed inside of the mould. A mould consists of two halves and seals the parison from top and bottom.
  • Hot air is blown through the blowing head into the parison, expanding it until it takes the shape of the mould.
  • The hot part is then left for few seconds in the mould for cooling before ejection.
  • Any extra material on both ends of the part is then removed to obtain the final finished part.
Row of hot orange glass bottles in factory

Blow moulded glass bottles

Typical Applications

Blow moulding is used for the production of thin wall hollow plastics parts including:

  • Water or soda bottles
  • Ducting
  • Bumpers

Materials for Blow Moulding

Most of thermoplastics including PET, PS and PC can be processed by blow moulding, as well as glass.

Design Considerations

  • Part wall thicknesses should be between 0.25 mm to 6 mm. Depending on the thickness of the part, a cooling aid may be required.
  • Maximum volume of 3 cubic meters with the maximum length of 12 mm is allowed.
  • All corners should have a minimum radius of 3 mm.
  • Threads, lettering, boss inserts, ribs and undercuts are possible in blow moulding.

Process Variations

  1. For the production of asymmetric parts, extrusion blow moulding is often used. This process can also produce parts with integrated handles. In this variation, the injection moulding is performed on the parison, then the part moves to the blow moulding machine in injection blow moulding process.
  2. Multiple parisons can be used to create multi-layered products

Economic Considerations

  • Depending on the size of the product, a few hundred to a few thousand pieces can be produced per hour
  • Tooling costs are quite high as expandable molds are required for this process. For this reason, it is only economical for high volume production.
  • Equipment costs increase with an increase in the level of automation
  • A single worker can control several fully automated machines, resulting in low labor costs.

Quality Considerations

  • Good surface finishes are possible with high pressure, but this reduces control over wall thickness.
  • Allowances should be added to the product dimensions across the parting line.

Advantages

  • High production rate
  • Low labor cost
  • Moderate labor skills required
  • Very economical for large production volume
  • Little to no waste material is produced
  • High level of automation is possible

Disadvantages

  • High tooling cost
  • High equipment cost
  • High lead time for startup
  • Not economical for low volume production
  • Low flexibility because of dedicated molds

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