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Screw Cooling Systems :

Screw Cooling -
1. Larger screws are bored though for passage of heat transfer medium. However, care must be used not to over-cool the screw.
2. The cooling may be freeze a layer of plastic on the screw root, reducing the channel depth thus producing more shear at a cost throughput.
3. This may  also reverse the required relationships between the friction coefficient (low friction coefficient on the screw, high on the barrel),  further reducing the drag flow.
4. Furthermore,  there is a danger that the material staying a long time near the screw root will degrade,  contaminating the product.
5. It is important to remember that the conveying ability of the screw is controlled by the friction coefficient ratio : f(barrel)/ f(screw) it is important to maximize this ratio.
6. Under normal circumstances the minimum value of the ratio that guarantees conveying is 1.4
7. Screw cooling may be recommended to prevent decomposition of heat sensitive materials.
8. However, it should be carried out using the cooling fluid at the temperature above the softening point of the principal polymeric component.
Example :
During extrusion of PVC the extruder screw is cooled using circulating oil at 120 - 140°C,
i. e.,  at temperature 60 to 80°C above the glass transition  temperature of the resin no freezing will be found here.

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