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 Extrusion, Single Screw
 melt transport in extruder vent section
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| Author | Topic: melt transport in extruder vent section |
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g.weigerstorfer@erema.at Member Posts: 4 |
posted June 18, 2009 01:51 AM
  
We are a global player in the recycling business. Venting is allways one of our top topics. We used to believe that "foaming" of the melt in the vent-area is causing melt to come up the vent (it is definitely not back pressure). There is definitely enough volumen available in the flights. Now I am quite certain that it is the transport-ability of the melt that is causing problems. I know this from recycling of highly filled film (breathable) that picks up moisture. Now I think we have the same problem when running heavily printed PE-films. I think the melt slips in the barrel too much to get transported. Does anyone have experience with that sort of vent-problem? Could a grooved barrel in the vent-section help? Any answer would be highly appreciated. Thanks Georg IP: Logged |
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Tom C Moderator Posts: 938 |
posted June 18, 2009 07:28 AM
There are a number of issues that can cause venting. In the case of foaming materials if there is not enough length of screw before the vent hole to allow the foam time to collapse then you can get venting. If the discharge pressure is high, lack of screw pumping capacity can result in vent flow in foaming materials. There are a number of other fators to consider in vent diverter design. Slip is an unusual condition, and depends on the amount of lubrication. Grooves are not likely to help because grooves are normally designed to interact with solid pellets. Screw process analysis by computer software and vent diverter geometry checking are usually enough to resolve venting issues. ------------------ Tom Cunningham www.ExtrusionTechnicalServices.com IP: Logged |
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g.weigerstorfer@erema.at Member Posts: 4 |
posted June 18, 2009 07:50 AM
Hi Tom! Thanks for the quick reply. I hven been building and working for thee last 20 years, and I think to know what I am talking about. IP: Logged |
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Tom C Moderator Posts: 938 |
posted June 18, 2009 08:55 AM
Without looking at the screw design it is impossible to make an assesment. If both vent sections are designed the same you would have to conclude there is excessive foaming at the first vent, volume exceeding transport as you say. Some of the devolitilization occuring at the first vent is helping the second vent. I have seen some Erema screw designs which have been simple. Because of the force feeding specific output tends to be high which will make venting more of a challange. Having sucessfully redesigned several extrusion processes to eliminate venting, single screw and twin screw, I have my methods that work for me. ------------------ Tom Cunningham www.ExtrusionTechnicalServices.com IP: Logged |
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g.weigerstorfer@erema.at Member Posts: 4 |
posted June 22, 2009 04:23 AM
Hi Tom! We have tried "simple" degassing zone 1D pitch, single flight, deep cut and also 3 flights 2D pitch deep cut. Same result. When I understand your last message correctly, you offered to investigate in a degassing-re-design with us? If so, how could this co-operation look like? Best Regards IP: Logged |
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Steve H Moderator Posts: 416 |
posted July 16, 2009 10:15 PM
G'day Georg To try and improve the devolitizing action of the vent, you could try a Maddocks section at the end of the first stage of the screw, immediately preceding the vent. I had a screw cut to this configuration to address the issues in your first post and it worked well. Please give my best regards to Klaus. Steve Hodgson IP: Logged |
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