Plastics Technology Practice - The Hong Kong Polytechnic University

Plastics Technology Practice
• Specification of Moulding Machines
Shot size and clamping force are usually used to describe a machine. We need
to consider both shot size and clamp tonnage when choosing a machine.
i) Shot size is the maximum amount of material the machine will inject
per cycle (single shot) and the unit is ounces (oz) or grams (g). The
standard for shot size measurement is general purpose polystyrene
moulding in single shot.
ii) Clamping Force is the maximum force a machine can apply to a
mould. The unit of clamping force is tons.
3.1.3 Part Design for injection moulding
Part design is a very important in injection moulding, good part design can reduce
the manufacturing cost and reduce the defects during manufacturing.
• Uniform Wall Thickness
Uniform wall thickness should be the primary consideration in part design
because different wall thickness causes different shrinkage which increases the
difficulties dimension control and cause serious warpage in the injection
moulded products.
• Draft Angles
Draft Angles are added in the internal
and external walls for the mould part to
be ejected from the mould. Draft angle
requirement are smaller in external walls
than internal walls.
• Radii/Fillet
Internal sharp corners and notches are the leading cause of failure in injection
moulded thermoplastic parts. To avoid the problem occurred, radii / fillet is
commonly employed to all “sharp” feature.
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