| next newest topic | next oldest topic |
| Author | Topic: Air bubbles |
|
poly Senior Member Posts: 28 |
posted January 29, 2005 09:23 PM
  
First, I will try to briefly describe my type of extrusion. I extrude a polymer with two additives in order to form a thin 20 µm film that will be applied on an other type of film running under a vertical die (150 deg C), on a stacker roll (cooled at -10 deg C). I cannot give you more information because of a confidentiality agreement. Lately, we increased production by increasing the flow in the extruder and the speed of the web running on the stacker roll. The thickness is still the same at 20 µm. The film always comes out of the die looking really clear and without any air bubbles. Behind the die we have a vacuum box used to make the film stick to the web as soon as it's applied, eliminating the air trapped between the layers at the same time. It had always worked perfectly at the previous flow but now with this new increased production rate it won't work at all. The film will stay full of bubbles and orange peel even if we increase the vacuum pressure. It will then be impossible to measure the thickness with a micrometer. Again I must remind you that the film coming out of the die is perfectly clear before being applied on the web. Why this rough texture when applied on the web? Do you have any suggestion on how to eliminate the air bubbles or the orange peel texture? Orange peel can be caused by more friction on the die's lips when the flow is increased and the thickness is the same, I know. I tried increasing the die temperature significantly but it didn't make any difference. Any ideas? Thank you IP: Logged |
|
Tom C Moderator Posts: 977 |
posted January 30, 2005 07:17 PM
Poly, You could be suffering from shark skin caused by excessive shear stress at the die exit. Shear stress will increase with increased line speed. A common solution is using a fluoropolymer processing aid. Tom C [This message has been edited by Tom C (edited January 31, 2005).] IP: Logged |
|
poly Senior Member Posts: 28 |
posted January 30, 2005 10:00 PM
Can it be used for all types of polymer? How does it work? Our polymer is PEO (polyethylene oxide). Thank you [This message has been edited by poly (edited January 30, 2005).] IP: Logged |
|
Tom C Moderator Posts: 977 |
posted January 31, 2005 08:31 AM
These additives are called a "Polymer Processing Aid" or PPA for short. They work by not being compatible with the base polymer, and depositing on the die surfaces. The PPA deposited on the die surfaces allow the base material to slip against the die surface at the high shear stresses that normally casue shark skin. Some materials can't be processed without PPA. Your material is likely as incompatible as any other material. Low levels are used, so there is usually no influence on the final product. Sources are likely to be Dupont and Daikin. Tom C IP: Logged |
|
poly Senior Member Posts: 28 |
posted January 31, 2005 09:07 AM
Thank you for this information. I will again try to convince my superiors to look into this. Unfortunately, we use extrusion as one of our different processes. We only use it to coat the web I was talking about. It's a small part of our finish product. This is why we do not have any extrusion specialist. It's a good thing for them that we have good experienced extrusion operators to begin with. IP: Logged |
|
louis33 Senior Member Posts: 177 |
posted January 31, 2005 09:46 AM
I'm thinking that if you don't have the "shark Skin" as the melt exits the die, that is not your problem. Not seeing the process, I'd have to guess that the increased rate has something to do with it. You mention that you are putting the film on a substrate. Is there a chance that the substrate is pulling more heat out of the process? Surely the cooling rate has changed significantly. Try preheating it with a small hot air gun. If you see an improvement, rig up Just thinking outside of the box here. IP: Logged |
|
poly Senior Member Posts: 28 |
posted January 31, 2005 10:06 AM
You might have something there! We do cool the stacker roll (where the substrate lays) down to -10 deg C. We did the same with the old production rate. Maybe too much heat as you say is pulled out, thus preventing the vacuum from really extracting the air bubbles. It's worth the try. I'll keep you informed. Thanks a lot to you both [This message has been edited by poly (edited January 31, 2005).] [This message has been edited by poly (edited January 31, 2005).] IP: Logged |
|
stevet Senior Member Posts: 6 |
posted February 10, 2010 06:41 PM
Hi increasing the process rate normally gives higher melt temperature and more prone to gas - too much for the barrel vacuum system to remove. The gas is trapped in and comes out as a rough surface finish untill the substrate cools below the softening point. Investigate possible methods of reducing melt temp or increase cooling rate.Obviously any gas trapped in the sheet may give problems later in its life. A good test to see if there is trapped gas is to complete a heat stability test where a test sample of the sheet is slowly heated to just above its softening temp and any trapped gas would show up as blisters. Trust this gives you some ideas. Stevet IP: Logged |
(OK Build)a permanant solution. All times are ET (US) | next newest topic | next oldest topic |
  |
|
Contact Us | Feed Screw Designs
Powered by Infopop www.infopop.com © 2000
Ultimate Bulletin Board 5.45a
