posted 06-20-2001 09:30 PM            

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In this plant every feed screw that goes out for rebuild or repair is always chrome plated no matter what material we are running, is it always necessary?

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posted 06-29-2001 11:38 PM               

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If your plant processes any polymer with corrosive degradation byproducts (rigid and flexible PVCs, acetals, etc.) chroming is advisable, in my opinion. If you only process the "candle-wax" polymers --PP,HDPE,LDPE-- chromed screws are not really necessary.

Regards,

Skip

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posted 06-30-2001 06:09 PM               

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Chormed screws are also easier to clean when you have to pull the screw. The polymer doesn't stick as well to the chrome as it does to bare metal. Maybe the difference is minute and can be ignored in some instances. With that in mind a chromed root should help the extrusion process because the molten polymer must stick to the barrel wall and slip on the screw surface, so that it is dragged along. Anything to help that condition will aide in providing higher output, closer to the ideal drag flow.

To calculate ideal drag flow and other properties for any screw geometry: Use ExtruSim 2000 downloadable at http://www.dualspiralsystems.com/blown_film_calculator.html

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posted 06-30-2001 07:33 PM               

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Hi Dualspiralsystems,
What you say concerning "drag flow" is true for single screw extruders, but not true for twin screw extruders--which provide a "positive displacement" push of the polymer down the barrel, at lower RPM and lower shear. This is why twins are preferred for rigid PVC applications.

Regards, Skip

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