What are the sprue, runners and gates in an injection mould?

The sprue carries melt from the nozzle into the mould, and runners distribute it to each cavity. A gate is the narrow entry into each cavity.

Why should an injection-moulded part have an even wall thickness?

Thick sections cool and shrink more slowly than thin ones. Uneven walls cause sink marks, warping and longer cycle times.

Explain why draft angles are designed into injection-moulded products.

A draft angle is a slight taper on the faces that run in the direction the mould opens. The taper lets the part release from the mould without scraping or sticking.

CCEA A-Level Technology and Design 8900

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Processing Materials

Injection and blow moulding

3 things to know
AS 1 Paper 1

Overview

The injection moulding cycle, sprues, runners and gates, wall thickness, draft angles and ribs, a mould cost calculation, and extrusion and stretch blow moulding.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Describe how the polymer is melted and injected in injection moulding.

Key term

What happens in injection moulding after the mould cavity has been filled?

Key term

Why is an injection mould water-cooled?

Questions from this subtopic

Each one with the model answer underneath.

1
Describe how the polymer is melted and injected in injection moulding.
Model answer
Polymer granules are melted by heater bands and a rotating screw inside a barrel. The screw then rams the melt under high pressure into a closed, cooled mould.
2
What happens in injection moulding after the mould cavity has been filled?
Model answer
Pressure is held while the part cools, packing in extra melt as the polymer shrinks. The mould then opens and ejector pins push the solid part out.
3
Why is an injection mould water-cooled?
Model answer
Water channels in the mould remove heat so the part solidifies quickly. A shorter cooling time gives a shorter cycle and a higher output.
4
What are the sprue, runners and gates in an injection mould?
Model answer
The sprue carries melt from the nozzle into the mould, and runners distribute it to each cavity. A gate is the narrow entry into each cavity.
5
Why should an injection-moulded part have an even wall thickness?
Model answer
Thick sections cool and shrink more slowly than thin ones. Uneven walls cause sink marks, warping and longer cycle times.
6
Explain why draft angles are designed into injection-moulded products.
Model answer
A draft angle is a slight taper on the faces that run in the direction the mould opens. The taper lets the part release from the mould without scraping or sticking.

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