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| Author | Topic: Purge Resins |
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Tom C Moderator Posts: 270 |
posted January 04, 2004 01:34 PM
   
When shutting down an extruder for a while some resins are not very stable and can cause problems when restarting the extruder. THe problems usually appear as black carbon specs, but gas generation, corrosion, and locked up screws can sometimes result if the offending resin is not pruged out when stopping. The resins that come to mind with stability problems are: PVC Please add others you can think of. The question is what to purge the extruder with in each case considering compatibility, effectivness, cost, and cross contamination For example I have purged PET extruders with PBT resin effectivly. PET and PBT are compatible and PBT is more stable than PET. FR formulations can be purged with the same resin without the FR package without fear of contaminatton. One of the post popular resins to use as purge is HDPE. However I seen many cases where a skin of HDPE can be seen on the extrudate for quite a while until all the HDPE clears. Machine operators don't often notice this incompatibility problem and the product goes down the line as first quality. I'm looking for better methods. If anyone has good purging recommendations for unstable systems under short term shutdown, please comment. Thanks, Tom C IP: Logged |
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zabielski Senior Member Posts: 261 |
posted January 05, 2004 05:53 AM
I still prefer a HDPE with a very low MI. Then I would purge with a high MI HDPE. It will take awhile for the purge to clear, as it simply takes time to push out the low MI. However, the low MI HDPE does a fantastic job in clearing out all other resins. IP: Logged |
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J Inman Member Posts: 2 |
posted January 08, 2004 09:04 PM
I am firm believer in commercial purges. NO, I don't sell them! I run rigid and semi-rigid PVC and I tried HDPE purging, after picking up more unusable scrap than I care to remember. I sent off for some purge samples, I was sold. Overall less production losses particularly in color and die changes.I've heard good and bad about both of the products I decided to use, but follow directions and they work.(Generally I use a Lot lESS THAN THEY RECCOMMEND AND FLUSH WITH REGRIND). I went through some sticker shock at first. IP: Logged |
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2beeps Moderator Posts: 68 |
posted January 09, 2004 11:01 PM
For rigid PVC (assuming you have the ability to blend compound)here is a purge that has worked very well in my extrusion experience: PVC (K=to K value of your resin)-----100parts All the above blended to a powderblend in When this is run through an extruder/die setup, until the screws are empty, the machine can then be shut down with purge in the die----and a slow reheat of the die prior to restart will allow the purge to be pushed out. Screws,barrels, and dies are kept clean I am convinced the same basic principal can be used to make similar purge compounds for flexible PVC, and other resins-----basically Skip IP: Logged |
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