Process Encyclopaedia

Overview

Essentially, injection moulding is die-casting for polymers. It is normally automated and used for high speed, high volume production where component quality does not need to be high - over 50% of polymer parts are produced this way.

The mark left by the ejector pin can often be seen on cheap mouldings. To increase the shapes possible, sophisticated dies with moving and unscrewing inserts are used.

Injection moulding machine

Injection moulding die

[Pictures courtesy of Withersdale Plastics Ltd.]

Process details

Materials and shapes

• Thermoplastics dominate, but can also be used for thermosets, rubbers, polymer foams and short-fibre composites.
• Can make intricate shapes, though not suitable for thick sections.
• Typical part sizes are 100-600g, although parts up to 25kg can be made at great expense.
• Parts generally do not require finishing, although parts for feeders etc. may require removal.
• A wide variety of surface finishes and embosses can easily be incorporated into the die design.
• Thermoplastic scrap is easily recycled, but other materials must be disposed of carefully.

Economics

• To reduce costs, several parts are often moulded together on a "tree-like" structure; parts can then be separated after moulding.
• The cycle time is limited by solidification time and time to open and close the mould. Production rates from 1-20 parts/minute are readily achievable.
• Capital cost for machines are from £10,000 - £100,000 and dies can cost between £1,000 and £10,000.
• Injection moulding is only economic for batches of 10,000 - 100,000 or more and so is usually automated.

Typical products

• toys,
• model-making kits,
• handles,
• food containers,
• cups,
• electrical and plumbing fittings

Links

• The Science Museum have an animation.
• Specific Injection Molding Processes
  • Air Injection Molding
  • Insert Injection Molding
  • Outsert Injection Molding
  • Push-Pull Injection Molding
  • Sandwich Injection Molding