posted January 19, 2004 01:02 PM               

Are there any rules of thumb regarding density, polymer type, or particle size?

Do tests on smaller extruders relate to powder feeding performance on larger extruders?

Any thoughts would be helpful.

Thanks,

posted January 21, 2004 03:12 PM               

1) The bulk density is < X
2) The compressibility is > Y
3) Then angle of repose is > Z
4) The coefficient of friction at low pressure is < A and improves with pressure
5) The particle size is < B

If anyone has suggestions or expereince with X,Y.Z.A or B please share.

Thanks,

posted January 23, 2004 12:43 PM               

This is the deal. A customer has asked me to redesign a single screw so extrusion performance with powders is better. As usually they want a "Universal" powder screw. My experience is that some powders require a crammer feeder, while others don't. Presently some powder just get crammers automatically.

I have a whole lot of technology available to me where I can engineer the best possible feeding characterisitics in a screw for a powder without a crammer. However I believe there is a dividing line where I can't depend on an optimized screw alone, and will require a crammer for good output.

I now believe that working with tests as above I will be able to determine which way to go early in the design process, crammer or no crammer required.

posted January 23, 2004 01:13 PM               

Have you talked to Skip Thacker, he's got a wealth of experiance on handling PVC powder and might be able to offer an insight (or two)

posted January 25, 2004 10:33 PM               

Other than fluff from film scrap, I'm not familiar with other powdered extrusion materials, but I feel a crammer would be helpful in all cases, regardless of screw design, as a way to insure full screws.

Sorry for the length of this---wasn't sure where to cut it off!!

Regards, Skip

EXTRUSION: ATTRIBUTES OF SINGLE & TWIN SCREW MACHINES

Extrusion - The Machines

1. Single Screw Extrusion - Successful single screw extrusion of rigid PVC powder is dependent upon developing shear, at high RPM (25-80) and high compression (2.8 -3.5:1) in order to generate sufficient work to insure adequate fusion, a uniform melt and full achievement of physical properties.

At these high RPMs the material's radial velocity in the screw flight differs substantially when comparing material next to the screw with material next to the barrel. This contributes to shear and frictional heat buildup, as well as non-uniform melt viscosity of material entering the die.

A typical single screw extruder capable of handling rigid PVC powder blends has an L/D ratio of at least 24:1, is vented for removal of volatiles, and has a two stage screw with a decompression zone at the vent. The screw is also cored for oil temperature control.

In a standard screw , material transport depends on the helix angle of the screw flights as well as flight depth. As the helix angle decreases, material transport increases and frictional shear (slippage) decreases.

It would seem, then, that efficient material transport leading to high output rates and low shear can be readily achieved by reducing the helix angle. However, the need to achieve a uniform, homogeneous melt requires certain constraints on screw geometry. Rigid PVC melting starts on the leading edge of the screw flight. As an uneven melt temperature exists throughout the flight, the coolest (last to fuse) compoundis in the center of the flight.

Therefore, the helix angle must be large enough to provide a certain amount of slippage (shear) of the material to hasten melt uniformity in the flight, but not too large to hinder material transport (output). Higher RPMs are used to help develop the shear which overcomes unequal melt temperatures in the screw flights.

Also, the use of vertical hopper "crammer-feeders" helps to maintain output rates by providing a positive force feeding of powdered PVC to the feed zone of the screw,which also helps to minimize the typical "surging" of single screw extruders.
Some of the new single screw machines are considerably longer (30:1 to 32:1 LD), which permits the use of somewhat less rigorous processing conditions due to longer residence time in the barrel to achieve a more uniform melt.

The screw design of many single screw machines now includes mixing pins,secondary flow channels, or other features to aid in developing a homo-geneous melt.