#magnified view
External surface (Left) and Internal surface (Right) of blow-moulded HDPE
System: High density polyethylene (HDPE)
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Processing: A ‘parison’ (tube) is continuously extruded into a mould which periodically closes together with a knife, to cut suitable lengths which are then expanded to fill the mould
Applications: Blow moulding is a continuous process used in the manufacture of bottles as well as sheets, bags, and tubes.
Sample preparation: The specimen has been sputter-coated with gold, to give a conducting surface
Technique: Scanning electron microscopy (SEM)
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Further information [External surface]: The surface shows periodic ridges at right angles to the flow direction. These are indicative of flow instability and a periodic slipping of the melt at the die wall. It also shows surface scratches which are typical of polymer surfaces. Hard abrasive particles, dragged across the surface, have displaced material to form grooves and ridges.
Further information [Internal surface]: The surface shows ~5 micron periodic ridges which are indicative of flow instability and a periodic slipping of the melt at the die wall. On a finer scale are catspaws of submicron polymer, drawn up by the 'wind’ in the blowing process.
Contributor: J A Curran
Organisation: Department of Materials Science and Metallurgy, University of Cambridge
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