Image icon  Injection-moulded polystyrene ruler

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Description: 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 °C), the material will tend to relax, particularly along the centreline, near the gate. This will result in a wrinkled form.
Keywords: alignment • birefringence • injection moulding • polymer • polystyrene (PS) • ruler • sprue • strain
Categories: Science approaches > Microstructure
Materials > Polymers & elastomers > Plastics > Thermoplastics > Polystyrene (PS)
Processes > Shaping > Moulding
Testing, analysis & experimentation > Metallography
Scale > Micro
Created by: J A Curran, Department of Materials Science and Metallurgy, University of Cambridge
DoITPoMS, University of Cambridge
Published by: DoITPoMS, University of Cambridge
License: This resource is released under the Creative Commons Attribution Non-Commercial Share Alike license (2.0 UK: England & Wales).
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Date created: 03 October 2002
Date added: 21 August 2009
Resource ID: 1602