Injection Moulding

Design Guidelines:

Molten plastic is injected into a cavity this will then be moulded to the shape of the mould before being ejected out with the necessary part. The mould is usually difficult to remove and it therefore is occasionally difficult to remove from the mould, this therefore leaves ejector pin marks.

Mould Temperature & Effect on Flow

  • If it is cooled too quickly it may cause stress marks.
  • Avoid having a frozen skin (could peel from the plastic)
  • Heat the mould to avoid stretch marks and frozen skin.
  • Pump cold water to speed up the cooling down (setting) time.


Weld Line

  • Shows that the mould hasn’t properly set before cooling and ejecting
  • Ensure that you minimise the distance from the injecting point to avoid unwanted cooling

Shrinking & Warpage

  • Practice a consistent wall thickness throughout the product.
  • Due to different thicknesses and cooling times sink marks may occur.

Design Practices: 

  • Uniform Wall Thickness
  • Reduce all sharp Edges
  • Smooth Wall transition
  • Fillets & Radii
  • Bosses
  • Ribs
  • Hinges

Materials with Live Hinge

  1. Nylon
  2. Polypropylene
  3. Polyethylene

*Avoid 90 degree bends this slows the plastic down and makes it cool quicker which would cause problems.

This process is extremely expensive and the outcomes are fairly cheap and have an average low sell on cost… is it really worth it? I find knowing that the option is there valuable but at the end of the day I will definitely attempt to stay clear of this process!

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