Posts: 10
From:Waterlo, Ontario, Canada
Registered: Jun 2002

posted November 14, 2002 09:29 AM               

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This question is for anyone with experience processing flexible PVC. Is there a standard for screen pack mesh weights? We currently run a 20-60-20 screen pack with a 2-stage powder fed screw in a 2.25", 30:1, vented, extruder. We intermitently experience a phenomenon dubbed, chicken tracks. Origionally I thought this was post-vent degassing of unmelted material but have found that under a microscope, most of the elongated bubbles (shaped like a comet trail) have tiny particles in them. They look like colourants, most likely TiO2 (because they are white and hard). We are going to try heavier weight screen packs, but are worried about pressure and degradation increases. Any other ideas or suggestions are welcome!

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Posts: 386
From:McHenry, IL USA
Registered: Nov 2002

posted November 17, 2002 08:39 AM               

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andrewm: Your "chicken tracks" are most likely the particles of TiO2. However, if you screen them out and retard them, you'll try to add more TiO2 to offset the color change, and the vicious cycle revoles and revolves.

Look into the particle size distribution of the TiO2 your purchasing first, then move to a smaller particle size. Now then, you'll have to do a cost study to deterimine if the slightly higher price off-sets the cost of producing scrap.

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