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

DoITPoMS Micrograph Library Micrograph 569 and full record

Micrograph 569 and full record

Link to image file for micrograph 569
Micrograph no
Brief description
Injection-moulded polystyrene ruler
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), ruler, sprue, strain Link to MATTER Glossary entry for strain
Polystyrene (PS)
Not specified
Standard codes
Polystyrene was injected into a mould via an 'edge gate' at the end of the ruler
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
11 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 'gate') at the end of the ruler; 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 along the centreline, near the gate. This will result in a wrinkled form.
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

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