posted November 01, 2006 05:54 PM               

Principles of Understanding

The keys to successful compounding are:
• The creation of multiple elongational flow fields. (Well known in twin screw extruders).
• Upstream elongational flow fields to develop high critical shear stress for dispersive mixing.
• Downstream elongational flow fields for distributive mixing.
• Low temperature rise during mixing.
• Reverse Axial Flow for uniform distribution

The keys to successful melting are:
• Elongating flow fields to create a melting mechanism superior to the classic high pressure solid bed/barrel melting model of single screw extruder.
• Low pressure melting to keep melt temperature low.

The key to uniform product extrusion is stable pressure.

The keys to vented extrusion are:
• Multiple vents.
• Large surface area to expose the entrapped gases to vacuum.

Recirculator Screw

Randcastle has developed a patented screw design that brings all these advantages into a series of mixing elements on a single screw extruder. These compounding/melting/high-production/venting/surge suppressing elements consume as little as 2 L/D's.

posted November 01, 2006 09:13 PM               

Amoung the many things I do is design screws and sell Randcastle equipment. I don't design the Randcastle recirculator screws however.

Initially I told Keith Luker, Randcastles owner and the inventor of the recirculator, that the screw would not work as advertised. Since then I've had to eat my words. Curious as to how it worked so well, I then spent a lot of time studying the screw concept to see why it gave such unique results in a variety of tests.

Over the course of time I believe I have a better understanding of the mechanisms involved, some that even Mr. Luker was not aware of at the time.

One of the most important principles is that most of the screw runs at essentially no pressure for melting and mixing. This is vastly different from other extrusion screws where melting and mixing is done under pressure. Melting under pressure is a primary source for output instability as the solid particles and melted material fight for position in the channel. Low pressure also and allows for large extensional flows unlike other screws. Extensional flows in twin screws are much less than the twin screw people would have you believe.

The results so far have been;

Long term pressure stability using pressure feedback as good as +/- 5psi. This is less than the drift of most pressure transducers.

Fantastic distribution of additives and modifiers, even nano materials. Low pressure processing does not produce agglomerates and extensional flows seperates the particles for efficient wetting with polymer.

What is typically extruded could be described as a "Low Stress" melt. It has a very smooth look to it. We believe it is the result of good mixing and molecular disentangement that occurs with extension. This seems to help produce very good looking film samples.

I think this screw technology is truly something new under the sun. Since it is new it is still being studied to determine it's full potential. For miximg on micro-single-screws the results have been revolutionary. How it could help other processes is not fully developed yet. I have several applications under testing at the present.

If you would like more information, please email me.

------------------
Best Regards,

Tom Cunningham

posted November 02, 2006 10:30 AM               

This screw described sounds really unique!
If it reduces polymer shear due to pressure,and reduces the inherent surge common to most single screws, could this screw work well with high melt viscosity rigid PVC---as compared to twin screws?
Or better yet, could this screw design be applied to counter rotating conical or parallel twins to get the best of both??
(Is showing a diagram or picture out of the question??)

posted November 02, 2006 11:54 AM               

I don't know of any pictures posted on the net yet to link to. Something I may work on.

As far as PVC I'm not sure if any tests have been run.

Conceptually the mixer could be modified to run counter but not corotating in twins. I believe the self wiping feature would be lost.

------------------
Best Regards,

Tom Cunningham

posted November 06, 2006 07:40 PM               

(I didn't see much explained about the "recirculator" in those links to understand the concept)

posted December 07, 2006 07:58 AM               

The screw is conventional at the die, so it does produce pressure for output. The no-pressure aspect is important for initial mixing of particulate additives and fillers, a task that would be completed by the discharge end of the screw.

The Recirculation part of the screw was designed for increased back-mixing. This is important in Micro-Extruders because pelletized materials tend to have a plug flow characteristic in the Micro Extruders. Basically there is a space between the end of the mixer and following screw flights where the melt can either go forward or back into the mixer, hence recirculation.

------------------
Best Regards,

Tom Cunningham