Dissemination of IT for the Promotion of Materials Science (DoITPoMS)

DoITPoMS Micrograph Library Full Record for Micrograph 574

Full Record for Micrograph 574

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Micrograph no
Brief description
Injection-moulded polystyrene (PS) case
alignment, birefringence Link to MATTER Glossary entry for birefringence, injection moulding Link to MATTER Glossary entry for injection moulding, polymer Link to MATTER Glossary entry for polymer, polystyrene (PS), sprue
Polystyrene (PS)
Not specified
Standard codes
Polystyrene was injected into a mould via a noticeable 'sprue'
Polystyrene is hard and inexpensive and its use is very widespread. CD cases and clear plastic cups are common examples. Domestic appliance casings are also typically made from PS
Sample preparation
Cross-polarised light microscopy
Length bar
14 mm
Further information
The colours in the image are the result of birefringence and relate to the residual strain in the polystyrene. The pattern of strain is indicative of the flow of material during the injection process and it highlights the injection point (known as a 'sprue') which can also be identified by a small lump on the surface; the molecular alignment is greatest near this point. Towards the edges of the ruler and along its length, the material becomes more relaxed and as the molecular alignment falls, the retardation of light is less. If heated above the glass transition temperature of polystyrene (about 100 deg C), the material will tend to relax, particularly near the sprue. This will result in a wrinkling of the component.
J A Curran
Department of Materials Science and Metallurgy, University of Cambridge
Licence for re-use
Attribution-Non-Commercial-Share Alike 2.0 UK: England & Wales