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Author
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Topic: black specs
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felix
Senior Member Posts: 47
From:
Registered: Jul 2001
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posted August 10, 2005 11:46 AM
   
Hello, we have a film line running 85-90% MPDE, with 10-15% octene, that runs with no problems. When I switch to 85% HDPE, 15% octene and we get very small black gels, all of uniform size. This only happens when we make thick films, +4 mil, thinner films don't generate the small gels. We tried running on a diff line and same thing happened.
Any ideas on what could be causing this, and if it is degradation? Could it be that the line speed is too low because of the film thickness? The MDPE does not generate this condition no matter what the film thickness is. IP: Logged |
Tom C
Moderator Posts: 572
From:Brodheadsville, PA USA
Registered: Jun 2001
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posted August 10, 2005 10:29 PM
A couple of questions;Do you use a screen pack? Has it been inspected to see if the same contamination is caught up on the screens? Are the black specs smaller or larger than the mesh opening in the screens? When was the last time the equipment was taken apart and cleaned? What is your melt temperature (not machine temperatures) running both formulations? ------------------
Best Regards, Tom Cunningham Extrusion Technical Services www.ExtrusionTechnicalServices.com IP: Logged |
felix
Senior Member Posts: 47
From:
Registered: Jul 2001
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posted August 11, 2005 10:40 PM
Hello, Tom,
Thanks for your response.
The extruder does run with a screen pack. The black specs do not get caught in the mesh. When a tighter screen is used, they still don't get caught. This tells me they are somehow forming after the screen, in the block, or the die. When it happened, just the die was stripped and cleaned, nothing out of the ordinary was found. I am not sure what the MT was as the probe was broken, but I will try to get a reading when it is fixed. What I can't understand is why the black specs appear when the film thickness increases. It's a brain teaser. quote:
Originally posted by Tom C:
A couple of questions;Do you use a screen pack? Has it been inspected to see if the same contamination is caught up on the screens? Are the black specs smaller or larger than the mesh opening in the screens? When was the last time the equipment was taken apart and cleaned? What is your melt temperature (not machine temperatures) running both formulations?
IP: Logged |
Tom C
Moderator Posts: 572
From:Brodheadsville, PA USA
Registered: Jun 2001
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posted August 12, 2005 09:43 AM
Felix,Because of the lack of information, I'll have to guess at the source. Sheet and film dies typically have a coathanger style manifold. Changes in resin and process conditions (sheet thickness) will change where dead areas occur in the die, and whether the die will be self cleaning. These problems can be seen as fast or slow flow sections of the die, needing to radically change the die lip opening, or specs and stripes occuring in fixed areas of the extrudate. Die deckles can be a significant source of degradation. If the black specs occur uniformlly across the sheet I would suspect contamination before the die. You should check the spec size verses the screen meshing opening size microscopically to determine if the specs are getting through the screens. When running sheet the extruder throughput and pressure profile in the screw can be dramatically different then when running film. If there is significant wear on the barrel, or screw, sometimes the gap is filled with carbonized resin on the barrel. When the process conditions change, the carbon breaks off the barrel, is broken up by the screw, and is included in your product. Please contact me directly if you feel you need additional technical resources to resolve this issue.
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Best Regards, Tom Cunningham Extrusion Technical Services www.ExtrusionTechnicalServices.com IP: Logged |
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