posted June 29, 2005 06:30 AM               

Located at;
http://www.plasticextrusion.info/

No screw geometry aside from the screw diameter is required.

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Best Regards,

Tom Cunningham

Extrusion Technical Services

posted July 03, 2005 01:08 PM               

Tip velocity should read "Flight Tip Velocity".

As you look into extruder performance and the theory behind that performance you will find that extruders need to operate within a certain performance range unless they;

are starve fed
are grooved
are worn
have high head pressure
employ non-traditional melting methods
have unusually high or low polymer viscosity

or some combination of the aforementioned.

As extruders increase in diameter the shear rate experienced in the solid-barrel melting interface increases at the same RPM. The increase in shear rate results in a lower melt viscosity in that interface. The lower melt viscosity results in lower stress in the melt. Lower stress means less energy is put into melting the polymer.
Based on experience when the flight tip velocity (an indirect measure of the shear rate) reaches about 30 inches/second the melting performance of the screw has decreased enough to make the screw block with solids and surge.

This is the primary reason that small diameter screws can run high RPMs and large diameter screws need to be run at low RPMs.

This situation could be compensated for by using very long L :D screws at large diameters, but this is not the typical case.

Hope this helps to explains the inclusion of the Flight Tip Velocity in the calculator.

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Best Regards,
Tom Cunningham
Extrusion Technical Services www.ExtrusionTechnicalServices.com

[This message has been edited by Tom C (edited July 03, 2005).]

posted July 06, 2005 07:32 PM               

Flight tip is just a measurement of the linear speed of the flight against the barrel.

Thanks...