#doitpoms

Processing: Sand cast

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Sample preparation: Etched in sodium hydroxide solution

Technique: Reflected light microscopy

Length bar: 40 μm

Further information: The micrograph shows primary Al dendrite arms (white). The dendrite trunk has been intersected at an angle by the plane of polishing to give the observed morphology. Between the dendrites is the Al - CuAl₂ eutectic. Initially dendrites would have formed from the liquid, the regions between the dendrite arms known as the mushy zone transforming to a eutectic solid (L to Al + CuAl₂). These two phases form cooperatively as neighbouring lamellae with the lateral diffusion of material across the growing interface. The relative amounts of the two phases (Al and CuAl₂ ) within the eutectic are determined by applying the Lever Rule at the eutectic temperature.

Contributor: Prof T W Clyne

Organisation: Department of Materials Science and Metallurgy, University of Cambridge

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