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| Author | Topic: Barrel/Screw Wear |
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andrewm Member Posts: 10 |
posted June 12, 2002 10:44 AM
   
Extrusion theory dictates that you want maximum surface friction at the barrel and minimal at the screw. A brand new chrome plated screw (2.25" 30:1 2-stage) running in an older barrel (feed section wear known) does not fully breakdown and homogenize colourants in FPVC - streaking. An older - worn screw does (in same machine). Could the material be sliping along the screw like an auger instead of being sheared? Any suggestions? [This message has been edited by andrewm (edited 06-12-2002).] [This message has been edited by andrewm (edited 06-12-2002).] IP: Logged |
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Tom C Moderator Posts: 737 |
posted June 14, 2002 04:53 PM
[QUOTE]Originally posted by andrewm: [B]Extrusion theory dictates that you want maximum surface friction at the barrel and minimal at the screw. What you say above mainly has to do with solids feeding.
Likely reasons for the above observation: 1) The worn screw works the material more in the screw tip - barrel gap. 2) The new screw probably has higher output. Melting may finish further down the screw, reducing mixing. 3) The new screw probably develops higher pressure along the screw length. High pressure is known to agglomerate pigments. Tom C IP: Logged |
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griffex Senior Member Posts: 38 |
posted June 16, 2002 01:59 AM
Dear Andrew The friction on barrel/screw root theory is, as Tom says, to maximize inpush in feed zone. Remember that you may not want maximum inpush, if it causes overheating at entry to metering zone. As for wear contributing to dispersive mixing, I'd need to know channel depths in the two screws, especially in metering zones. Without these numbers, you can't assume differences are due to wear or anything else. Compare at equal throughput, not equal rpm, and do you still get a difference? Sometimes leakage over flights helps this mixing. Allan Griff IP: Logged |
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andrewm Member Posts: 10 |
posted June 17, 2002 02:11 PM
Thanks for your input Tom and Allan! The screws, new and old are the same design. Old first stage feed section wear (2nd to 5th flight from hub) is about 40-50 thou on OD. First stage C.R. is 3.25 with feed F.D. at 0.26" and 2.69 with feed F.D. of 0.35" in the second stage. Using a reverse hill heat profile has helped reduce the streaking (hot feed zone - normal metering zone) with the new screw and higher rpm. A new barrel is on the way to hopefully save the day. IP: Logged |
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Steve H Moderator Posts: 386 |
posted June 17, 2002 04:00 PM
Hi Andrew What's happening with melt pressure with the two screws? G'day Allan - good to see you here
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andrewm Member Posts: 10 |
posted June 17, 2002 04:08 PM
Hi Steve, The new screw has a consistent melt pressure at 3000 psi (reverse hill heat profile), the older screw needs a hill profile (cold in feed and normal in metering zones) to maintain about 2700 psi. A flat heat profile yeilds 1000 psi fluctuations on the older screw. IP: Logged |
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Steve H Moderator Posts: 386 |
posted June 18, 2002 01:07 AM
Hi Andrew Tom's comment regarding mixing taking place through material passing over the flight on the old screw is on the money- I had this happen to me. I'm not convinced that a new barrel is going to help you, unless you are getting a longer one, and another screw- can you induce a little more back pressure on the new screw and slow the screw RPM's down a little (being careful not to "cook" the material. I think I'd be inclined to try mixing pins in the metering section of the screw, or maybe having a replaceable tip with a mixing section. Steve H IP: Logged |
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Peter P Senior Member Posts: 61 |
posted June 25, 2002 03:10 PM
Bit late to jump in I know but by FPVC I take it you mean flexible PVC. If so, you are probably overlubricating your material. To continue your general extrusion theory, if your velocity at the barrel wall is zero (maximum surface friction) then the residence time of the material at this point will be infinity. With PVC, which is very heat sensitive, you need to increase the wall velocity, i.e. make it slip at the wall. Of course it will slip more with the new, shiny screw than it will with the old rough screw. I hope this helps. Regards, Peter P IP: Logged |
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andrewm Member Posts: 10 |
posted July 04, 2002 09:47 AM
Well it looks like you were right Steve, the new barrel has not saved the day. Streaking is still occuring in the same colours as before switching. Currently I am using what I call a "cliff" profile. Very hot first stage feed and metering, and normal to colder second stage feed and metering. This has reduced the in-push and increased the second stage melt pressure; the streaking has stopped. Our screw manufacturer had also suggested mixing pins. Is there a standard pattern for PVC? The second stage metering section is 5 diameters long. IP: Logged |
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Steve H Moderator Posts: 386 |
posted July 04, 2002 03:10 PM
Hi Andrew I'll hunt through my info and see if I can find anything on PVC screws. If the streaking problem is only present in some colours, and not in others, have you tried changing the colourant formulation. If you can get something that more closely matches the parent material, it may disperse more readily through it. Steve H IP: Logged |
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griffex Senior Member Posts: 38 |
posted July 07, 2002 10:21 PM
Interesting to see how this developed. Color dispersion doesn't have much to do with feed zone, and the profile won't matter too much either. Check out the viscosity of the color concentrate at extrusion speeds -- you may need a Brabender or Haake torque rheometer to do this, but watching amps on a small extruder may do. You want the concentrate to have much lower viscosity than the primary compound. Find out what is the plastic the colorant is dispersed in. Also look at pellets of concentrate hot-squeezed thin enough to be translucent, with a 30-50 power microscope, to see how fine the color particles are. Mixing pins might help, but will raise melt temperature and risk degradation, or else require more stabilizer. A Maddock section with a shallow barrier (0.5-0.6mm) may also work, but that means a new screw. What is particle size of primary compound? Often, smaller pellets will mix better. I've assumed pellet feed -- if it's a powder blend, you can add colorant as powder, too. Hope this helps. IP: Logged |
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Steve H Moderator Posts: 386 |
posted July 08, 2002 03:51 AM
Another posibility is the mixing section developed by Spirex Corp- the Pulsar. In this design the metering section is devided into constantly changing sections. These are either deeper or shallower than the average depth of the metering section. This results in a tumbling action with out the shear. Another posibility might be a Dulmage section. Steve H IP: Logged |
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andrewm Member Posts: 10 |
posted July 09, 2002 11:01 AM
Hello again, thanks for the reply's. Our materials are powder, compounded in a high intensity mixer. We have tried mixing sections in the past with success. Both tries though incorporated other design modifications that lead to processing problems. A maddock mixer with higher output (smaller root dia.) initially required a larger ratio gearbox to turn it (motor too small) and tended to get vent flow. A saxton section with j-welded feed flights worked well but for a short time only. The normal feed section wear (abrasive materials) simply moved down stream to both metering sections, eventually leading to pressure fluctuations and late degassing (after the vent). The major difference to date is that the last two screws have been built with chrome plating by manufacturers new to us. Our material suppliers have not changed, the viscosity of our colourants should not have changed either. I think the new screws simply convey material too fast with very little distributive mixing. The same screws in a worn out barrel (allowing spill-back) do not exhibit streaking. In a few weeks when the surface roughs up a little and the feed section begins to wear a litle, the streaking should stop. Until then heat profile modifications have helped by increasing residence time and increasing the melt temperature. Phew, blah, blah, blah! Which stage of the screw would you recommend locating a mixer? Both recent designs have them in the metering section of the first stage. IP: Logged |
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Tom C Moderator Posts: 737 |
posted July 09, 2002 02:00 PM
Andrew: I prefer to complete all processing before the vent. Processing after the vent can cause gas to be generated, defeating the purpose of the vent. It sounds as if you are using a crammer feeder to feed the powder mix. Is this correct? Tom C. IP: Logged |
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andrewm Member Posts: 10 |
posted July 09, 2002 02:53 PM
Hi Tom, The degassing after the vent only occured once the screw had worn in the first stage metering section to the point where some solid bed still existed after the vent. This is abnormal. This is just my theory for what had happened. I may be applying the wrong terminology to the situation. What physically manifested is what we call chicken scratches. A pattern shapped like chicken feet in the top layer extrudate. This is a co-extrusion process, laying a top sheet on a base layer profile. All of my information in this topic relates to the co-extruder. IP: Logged |
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andrewm Member Posts: 10 |
posted July 09, 2002 02:56 PM
PS- to answer your question Tom, the co-extruders are gravity fed. Under normal operating conditions, there is no degassing after the vent. Andrew. IP: Logged |
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Steve H Moderator Posts: 386 |
posted July 09, 2002 06:26 PM
Hi Andrew If you have a vent, the mixing section will work best in the first stage. As you have found locating it in the second stage will promote vent flow. And as Tom has said- any volatiles released as a result of the mixing Steve H IP: Logged |
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