posted December 01, 2002 10:13 AM               

I'm confident to say that yes, it will cost you quite a bit more. I can only guess that it's diamond dust polished.

TAYLOR HOBSON, as you may know, makes excellent Ra instruments, and I would suggest you ask that vendor who offers this screw finish smoothness the following questions:

1.) Is the 0.4 Ra measurement a result of taking the roughness average in the flat linear direction or the round direction. The travel of the measuring head of the Ra instrument can be very misleading if taken in the round direction. The instrument's head only travels a short distance.

2.) Based on this stated smootness, what is the number (n) average derived from? Is it based on 1 or 1,000 individual readings?

The benifits that I see are that it will be much easier to clean - provided that the polymer(s) used, are not burnt. I would be extremely careful in cleaning such a screw, as special cleaning methods would have to be employed - such as the use of a Pyrolisis Cleaner. To use "standard" brush's, scrapers, etc., may be harmful to this finish.

I can only assume as well that output may increase - but to what degree is anybody's guess. Due to drag flow, the barrel finish is also the "smoothness" to be concerned about as well.

Why have half a dollar when the whole dollar counts? This sounds to me like putting a polished cannon ball into the breech of galled barrel.

Such smmotness on the screw can also cause problems as well, such as if your running a very sticky polymer. Adhesion would be more as there is less overall surface area vs more overall surface area on the rougher screw finish.

I for one would like to see the test results of output rates comparing both. However, I'd also focus on the barrel interior as well.

Can you post a qoute of both costs of this screw finish?

Also, does the screw builder also have the capability of finishing the Inner Diameter of barrels as well?

posted December 01, 2002 01:02 PM               

Screw finish can be a problem. As part of my business I design single screw extrusion screws. On one occasion I optimised the design of a screw for one particular customer. He was so happy with the design he decided to have a second screw made for a similar machine. However the second screw didn't perform as well as the original. The only difference was that the second screw had a much finer finish than the first. The explanation we came up with was that the coefficient of friction between the pellets and the screw in the feed section, which is a prarameter we include in the screw simulation program, was too low, so the material didn't feed as well. The solution was to have a go at the screw in the feed section with some emery cloth to roughen up the surface a bit. It worked a treat and both screws perform very well now.

posted May 28, 2003 04:00 PM               

While the screw finish is important, it is more aptly appropriate to consider the differential of the coefficient of friction between the screw and barrel. This work was done in the 1950's by Darnell & Mol. In other words, a rough barrel finish can get away with a rough screw finish, while a smooth barrel with a rough screw can be disastrous.

posted May 28, 2003 06:50 PM               

quote:

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Originally posted by TomBlack:
Dave,

While the screw finish is important, it is more aptly appropriate to consider the differential of the coefficient of friction between the screw and barrel. This work was done in the 1950's by Darnell & Mol. In other words, a rough barrel finish can get away with a rough screw finish, while a smooth barrel with a rough screw can be disastrous.

Tom Black
PPS, Inc.
ppsincorp.com

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Thank you Tom, I was going to jumop in with that same comment.

Now that we solved that problem, riddle me thios.

Since the plastic can only be conveyed if it tends to stick to the barrel and slip on the screw, what if you exchanged that to make it slip on the barrel and stick to the screw? Does the plastic really know the difference . . . Can it tell which is moving, the screw or the barrel surface? What if the screw were stationary and the barrel rotated? Would you need to revers the surface finishes of each to make it work?

Food for thought - or questions to promote thinking outside of the box!

Jump in anyone - I'd love to hear your thoughts on this one.

posted May 28, 2003 08:59 PM               

Try this perspective;

Suppose just a handful of pellets were sitting at the bottom of the barrel, and a screw flight came by to move them. Then to the pellets the barrel would be still and the screw moving.

If the barrel were spinning then the pellets would be tumbling until they encountered a still screw flight. Then to the pellets the barrel is moving.

The above examples include the effects of gravity which dominates until other forces take over.

When the flight is full in the throat gravity also has an effect, but in a different way. The pressure of the material in the throat and hopper contacts all surfaces of the screw and throat and tends to lock the materail together so that pellets will not tumble when moved.

Frictional forces from the contact with the screw and barrel can then be transfered through the mass of material because it is (at least somewhat) locked together. Friction from the screw makes the locked mass want to spin with the screw. Friction from the barrel makes the material want to stay still with the barrel. If the net effect of all forces has the tendancy to stick with the barrel, then the helix of the flight interacts with the barrel sticking to produce a somewhat forward force vector, and material conveys. If the material would rather spin with the screw, then no conveying occurs.

To be continued.

posted June 03, 2003 12:25 PM               

Are you saying that the spinning barrel that is smooth along wiht a stationary screw that is rougher will work?

posted August 08, 2003 03:38 PM               

quote:

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Originally posted by louis33:
So Tom

Are you saying that the spinning barrel that is smooth along wiht a stationary screw that is rougher will work?

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So which would be easier, rotating the die and keeping the barrel in it's proper orientation, or rotating the barrel and keeping the die in it's proper orientation ? (For all of those cross orientation fans out there).

posted August 08, 2003 08:41 PM               

The answer to your question Louis is if there is enough pressure in the solids to make the pellets contact the barrel and the screw, comma, and the forces holding the mass of pellets onto the screw through friction exceed the forces holding the mass of pellets onto the barrel, comma, then the mass will not move. This situation does not occur often, but it does occur.

Pellets can transmit force through the mass pretty effectivly. That is why a couple of pellets stuck to the screw root in the feed can stop feeding. The same effect is true for the barrel. Grooves work by making pellets get stuck on the barrel surface.

Powder is a whole other story for another day.

I recall that long ago the whole spinning barrel, stationary screw thing was tried for fluids. To my recollection it resolved the issue and proved that pumping effects were only related to the velocity difference between the screw and barrel, and not whether it was the screw or barrel moving from our perspective.

To test the same thing for solids conveying I see as much more complex, but I would bet my money on velocity difference. (As long as the pellets don't fall out of the hopper.