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Extrusion, Single Screw

1" extruder running either Pebax or nylon with screw bridging

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Author	Topic: 1" extruder running either Pebax or nylon with screw bridging
peterturk Senior Member Posts: 6 From:Windsor, CA,USA Registered: Sep 2008	posted December 01, 2009 06:59 PM We are having issues on a 1" extruder running either Pebax or nylon with screw bridging (material sticking to the screw in the feed section). We have checked barrel dimensions to insure no wear in the feed section. We have evaluated both nitrided and chrome plated screws. We have also dropped the 1st barrel zone to around 330F. We still get spotty results with some runs staring OK and some not. Any ideas? Could age of screws be an issue, especially if we are seeing wear of over 0.005" 3-7 flites down? Is screw root coating a big contributor? Is screw cleaning critical? We use a burn off oven (@1200F) to clean dirty screws and subsequently wire wheel and scotch brite polish the screws. IP: Logged
Tom C Moderator Posts: 977 From:Brodheadsville, PA USA Registered: Jun 2001	posted December 01, 2009 07:13 PM Peter, Could be the heat from the barrel is traveling back in the screw during warmup or running. Do you have throat cooling on and confirmed running through the throat? I have customers who have installed throat water cooling on 1" machines and it solved many feeding and bridging problems. ------------------ Best Regards, Tom Cunningham www.ExtrusionTechnicalServices.com IP: Logged
peterturk Senior Member Posts: 6 From:Windsor, CA,USA Registered: Sep 2008	posted December 02, 2009 09:26 AM quote: Originally posted by Tom C: Peter, Could be the heat from the barrel is traveling back in the screw during warmup or running. Do you have throat cooling on and confirmed running through the throat? I have customers who have installed throat water cooling on 1" machines and it solved many feeding and bridging problems. IP: Logged
peterturk Senior Member Posts: 6 From:Windsor, CA,USA Registered: Sep 2008	posted December 02, 2009 09:30 AM Tom, This is an area that we have investigated. We do have an RTD in the feed zone and we do see variations in feed throat tempertaures from 80-100F, but the variations do not tend to follow the feeding issue. We did actually try hooking up a small chiller to isolate one of the extruders from the chilled water system with minimal improvement. We do not currently monitor water flow rate through the feed throat. I have had someone mention the option of screw cooling. quote: Originally posted by Tom C: Peter, Could be the heat from the barrel is traveling back in the screw during warmup or running. Do you have throat cooling on and confirmed running through the throat? I have customers who have installed throat water cooling on 1" machines and it solved many feeding and bridging problems. IP: Logged
Tom C Moderator Posts: 977 From:Brodheadsville, PA USA Registered: Jun 2001	posted December 02, 2009 02:54 PM Try scew cooling or blowing compressed air (slowly) on the screw root in the feed while it is not running. ------------------ Best Regards, Tom Cunningham www.ExtrusionTechnicalServices.com IP: Logged
louis33 Senior Member Posts: 177 From:Allentown, PA - USA Registered: Jun 2001	posted December 10, 2009 04:28 PM Two "crazy" ideas that have both worked for us in medical extrusion of those materials on small (1' and 1 1/4") machines. Make sure that the material is not over dried. We actually added some moisture to over dried resin to get it to feed. Over dried is defined as a very low moisture content as registered on an Arizona Instruments Moisture Analyzer. Second trick was to rough up the throat section using some emery paper. I know everyone tries to get the screws as slippery as possible, but if the throat section/feed zone is like glass, you may be fighting a losing battle. IP: Logged
peterturk Senior Member Posts: 6 From:Windsor, CA,USA Registered: Sep 2008	posted December 10, 2009 04:38 PM What kind of moisture content (%) do you consider too low? We use a similar moisture analyzer. IP: Logged
louis33 Senior Member Posts: 177 From:Allentown, PA - USA Registered: Jun 2001	posted December 11, 2009 03:19 PM Generally, when the material was left in a desiccant drier too long ( a couple of days) and the readings got down to the lowest that the analyzer was capable of, we "added" moisture. That was done by placing the material in an open tray and "squirting" it with a spray bottle. I know it sounds crazy, but it worked for the over dried material. IP: Logged
stevet Member Posts: 2 From:Newport gwent UK Registered: Feb 2010	posted February 10, 2010 11:09 AM Hi had similar problems- 1. screw not going fully home due to debris logged in the screw housing. 2. temperaure may be indicating lower in barrel zone 1 than it actually is due to thermocouple location or zone heating not working correctly- check the zone amps are not low. 3.Pellet diameter too large- try smaller pellet size. NB burning off screws and wire brushing damages screw surface finish. Hope this helps IP: Logged

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