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Injection-moulded polystyrene ruler
injection moulding images
Image by CORE-Components
DoITPoMS, University of Cambridge

The colours in the image are the outcome of birefringence and relate to the residual strain in the polystyrene. The pattern of strain is indicative of the flow of material for the duration of the injection procedure and it highlights the injection point (identified as a ‘gate’) at the finish of the ruler the molecular alignment is greatest near this point. Towards the edges of the ruler and along its length, the material becomes much more relaxed and as the molecular alignment falls, the retardation of light is much less. If heated above the glass transition temperature of polystyrene (about 100 deg C), the material will tend to unwind, particularly along the centreline, near the gate. This will outcome in a wrinkled kind.

Polystyrene (PS)

Not specified


Polystyrene was injected into a mould by way of an ‘edge gate’ at the finish of the ruler

Polystyrene is hard and affordable and its use is extremely widespread. CD situations and clear plastic cups are frequent examples. Domestic appliance casings are also typically made from PS

Sample preparation

Cross-polarised light microscopy

J A Curran

Division of Components Science and Metallurgy, University of Cambridge

View micrograph in DoITPoMS web site