posted March 07, 2003 04:29 AM            

What you describe is something we found with a client extruding nylon to spin nylon fibres, in which the melt pump was controlled by the barrel pressure. Output was very eratic.

posted March 07, 2003 02:18 PM               

Steve H

posted March 08, 2003 09:14 PM            

quote:

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Originally posted by pawelkar:
. . . The strange thing is that we had been extruding this tube on the PFEX line up until this year it was good. We did replace the barrel and feed throat and it is now made of something different could this affect it? Any suggs. on the screw design? [/B]

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If the old screw design worked, why switch? You mention, if I read it correctly, that the same machine ran the material up until you replaced the screw and barrel.

So, I will assume that you replaced the screw with one of the same design.

Be aware that there are differences in surface finish on feed screws and barrels. The problem that you describe sounds a lot like material slipping than grabbing, then slipping again, in your feed section. For a feed screw to work, the material must slip on the screw and catch on the barrel. Though I've often felt that if the opposite were true, it would still feed.

What you appear to have is intermittent slippage on both.

I would examine what worked, compare it to what is not and note the differences.

Then I would contact a screw manufacturer that would design a "material specific" feed screw that came with a guarantee.

Hope that helps some.

posted March 12, 2003 06:14 PM               

quote:

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Originally posted by pawelkar:
Hi, I am the operator of a PFEX air mandril extrusion line with RDN downstream puller/cutter and vacuum sizer. I have been going crazy trying to determine why I have had a difficult time extruding a nylon 11 or Besno type resin. The barrel psi goes crazy by bouncing from 1500 psi to 5500 psi(5000is normal). It is a 1 1/4 in barrel. Melt temp of 470 in barrel. I do not know if its feed trouble, temp trouble, raw material problems. I've checked dryer, replaced barrel and feedthroat, checked rpm's,temps, ensure good moisture samples, use a screenpack of 12x64/50x250. My average MFR is 1.0 10gr./min. And my tolerences are +/- .001, so you can see why psi hurts. We have checked all obvious process problems.
Are there any sugg. I am ready to try just about anything.

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Your drying may be too efficient. Many nylons run better with some moisture (usually up to .1%, depending on line speeds). The moisture acts as a lubricant. Sometimes you can use a sterate additive.

If it is too dry, it can depend on the time of year and location of the manufacture of the nylon. It may be dry enough out of the bag to use. Vacuum seal what is left in small batches, and you shouldn't have to dry it at all.

As well, the transition (I think it was 9 flights) may be too long. 3 TO 5 turns of transition should be efficient. Barrier designs work well for many nylons as well.

We currently run a nylon 6/6, and if it we dry it down to >.002% moisture, it will run at 70-100% motor load, with a lot of variability and trouble with shutting down the machine on torque overload.
Right out of the bag is about .06-.07%, and under the same conditions, runs at about 10% motor load with CpK of 2.6 (.070 OD ± .001). Of course, faster is better (we run at 250 FPM on a 1").