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| Author | Topic: Stop full or empty ? |
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poly Senior Member Posts: 23 |
posted November 22, 2005 05:11 PM
   
To prevent contamination when you start up again, is it better to stop an extruder full of polymer then close the heat, or is it better to stop it completely empty then close the heat ? Thank you IP: Logged |
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Tom C Moderator Posts: 650 |
posted November 23, 2005 09:20 AM
Stopping the extruder full and shutting down may cause some of the polymer to expereince very long times as at temperature. Polymers are poor conductors so some of it will cool very slowly. Thermally sensitive polymers may degrade. This is often a problem in dies which are full of resin when the extruder is stopped. I would purge the extruder and die with a thermally stable polymer formulation of higher viscosity, run the screw dry, and then remove as much polymer as possible from the adaptor and die area. Turn the cooling on full to cool the extruder before turning the power off. ------------------
Tom Cunningham
Extrusion Technical Services www.ExtrusionTechnicalServices.com IP: Logged |
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poly Senior Member Posts: 23 |
posted November 23, 2005 10:49 AM
Thank you for your reply. We already shut everything down the way you do. Unfortunately, we have a gel problem that always reoccures. I have made pressures to install filter packs that seem to work good (80-250-250-250-250-10µm-250-250-250-250-80 mesh). I don't know why, but gels still manage to get through once in a while. I have suggested they may come from downstream (after the filter) or from the screeen changer itself, but we can't really be sure. We are still not purging the extruder with an another product because we are afraid of the possible reaction with our polymer (PEO). So that's why I was asking your opinion. Thank you IP: Logged |
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Tom C Moderator Posts: 650 |
posted November 23, 2005 12:25 PM
First thing to do is look at your gels under a microscope or glass. What color, what shape, what size? ------------------
Tom Cunningham
Extrusion Technical Services www.ExtrusionTechnicalServices.com IP: Logged |
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poly Senior Member Posts: 23 |
posted November 23, 2005 04:41 PM
The gels are of many types. Difficult to say what is causing them. I think the most probable cause comes from dead spots in the extruder, elbow or screen changer. Once in a while, degraded polymer just breaks loose and gets in the flow. Sometimes we also get gel showers (maybe unmelts) that can last a few minutes at the time. Is it normal for those kinds of gels to get through a large quantity of such fine screens ? My guess is that it must come from downstream... IP: Logged |
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Tom C Moderator Posts: 650 |
posted November 24, 2005 09:28 AM
It is impossible for large particles to pass through small screens except in these conditions (which do occur) 1) The operator installs the wrong screens If none of the above conditions occur, and particles larger than the smallest screen mesh show up in the product, then it can be concluded that the gels are coming from downstream. To solve the downstream problem an analysis of the flow should be done through all the adaptors and die components to see if there is enough force from the flow to keep all the surfaces clean. I can provide such an analysis if you are interested. If it is found that there are insufficient clenaing forces the adaptor and die can be redesigned to improve the flow. ------------------
Tom Cunningham
Extrusion Technical Services [This message has been edited by Tom C (edited November 24, 2005).] IP: Logged |
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poly Senior Member Posts: 23 |
posted November 24, 2005 07:58 PM
Thank you very much for this information. I think you may be right about the insufficient force downstream. Maybe we should run with the die's lips less openned to increase pressure. I will have to talk about this to my team. As for the true gels, can you remind me what they are exactly? Unfortunately, I'm not in a position to be able to hire your services, even if I know we would need your expertise. Thank you IP: Logged |
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Tom C Moderator Posts: 650 |
posted November 24, 2005 09:56 PM
I have a paper describing Gels on my site; www.ExtrusionTechnicalServices.com on the downloads page. You could always mention my services to your superiors. ------------------
Tom Cunningham
Extrusion Technical Services www.ExtrusionTechnicalServices.com IP: Logged |
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poly Senior Member Posts: 23 |
posted November 25, 2005 11:27 AM
I will talk about you to my superiors. Do you offer your services in Canada ? In Quebec to be more precise ? Best regards IP: Logged |
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Tom C Moderator Posts: 650 |
posted November 25, 2005 11:38 AM
Our services are offered Worldwide. I do have an associate located near Toronto who is more than qualified. He may not be much closer than myself, depending on your location. We use the very well develop set of software tools developed by Compuplast; www.compuplast.com Compuplast also offer extrusion training near Toronto. That information can be found at; You can find my email by looking up my personal information.
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Tom Cunningham
Extrusion Technical Services www.ExtrusionTechnicalServices.com IP: Logged |
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