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| Author | Topic: Prevent waviness |
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walterzj Senior Member Posts: 5 |
posted September 08, 2005 02:15 PM
   
Hi, we have a problem, some pallets of acylic sheets were rejected cuase present one wave(1-2mm) on the edge. What could cause this? Which parameters are realted, the sheet is totally flat when it come out the process line. But it becomes wavy after 24 hours, but not all, just 500 PPM. Thanks for your help. IP: Logged |
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Tom C Moderator Posts: 683 |
posted September 08, 2005 03:21 PM
Wavyness and warping are signs of stress in the polymer put there during production. Often the product is straight when produced, as it has been forced to be straight As time goes by, and sometimes with the addition of heat, internal stress causes the product to warp. Typically excessive stress is due to cooling too fast. Sometimes as product is stacked up, the inside of the pallet is hot, and under its own weight the stress will relieve itself and stay flat. The edges cool much faster due to air exposure so the molecules don't have mobility due to heat to rearrange themselves into a low stress position. It would be best to see if you can cool the sheet more slowly over a longer time. ------------------
Tom Cunningham
Extrusion Technical Services www.ExtrusionTechnicalServices.com IP: Logged |
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walterzj Senior Member Posts: 5 |
posted October 04, 2005 05:48 PM
Thanks Tom , for your orientation. We have fans in order to cool the pieces, but I dont know if our rate of cooling is fine, in order to prevent waviness. Whitin 9 meters (1 minute), the pieces goes from 160°F->144->131->126->121->108->104->98 final product. After that, the stacked products (250 sheets) are packed with dissicants and bags. After 1 or 2 days our customer use it , at 81°F (warehouse). But at night they turn off the air condition , temp reach 90 or more and some products turn wavy overnight, but other not. I dont know if humidity or temp are the cause. but if the cooling rate may be resposible , does this rate is to faster or not? Thanks Walter IP: Logged |
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Tom C Moderator Posts: 683 |
posted October 05, 2005 09:39 PM
Thin acrylic sheets will warp if there is a difference in moisture content from one side (or area) to the other. This is due to water absorbtion and swelling. Swelling from water in acrylic is slight, but can cause problems in some applications like fish tanks. Given the conditions in your plant it seem the sheets stay dry in the center of the stack, while the edges absorb moisture. This casues a difference in swelling, and susequent warping. If the sheets were to be set out individually the warping would likely go away due to the whole sheet swelling equally. If the pallets were closed up overnight the wavyness might not appear. Give it a try. The possibility of stress relaxation also exists, but given the variable moisture levels in the plant swelling is more likely. ------------------
Tom Cunningham
Extrusion Technical Services www.ExtrusionTechnicalServices.com IP: Logged |
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walterzj Senior Member Posts: 5 |
posted October 06, 2005 11:17 AM
Thanks Tom, Yes, we tested to set up the sheets individually, and it worked, but this method is un-practical. The pallets are stacked just after the end of line 105°F, they wait 2 hours to quality check, then we introduce 2 dissicants, edge protectors we close the bags and then overpackage with film.This is always, but sometimes they stay unwrapped more time. Excuse me , but I dont understand "to close up overnight", we normally close it up and anyway some get wavy, also sometimes ,maybe, the bag could be damaged. Other relationship we saw is that our customer frecuently reject at morning , they left open partial pallets overnight , no air conditioning at night (means 90°F) , but at morning 81 °F since 6:30 AM. If water irregular absorption could be the cause. Would it better to : * left it open overnight at ambient 87°F in order to allow to hidrate easly than wrapped? *Or even force air trough pallets, maybe with some moisture. * avoid the use of dissicants Or by the other hand avoid hidration using more dissicants and more resistant bag and close them inmediatly ? Thank for your comments. Thanks Walter Zetzsche IP: Logged |
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louis33 Senior Member Posts: 142 |
posted October 06, 2005 12:18 PM
I am curious as to HOW they warp. Are the sheets warping (Curling) towards one side, do they have a uniform wave pattern, are they belled in towards the center or is it a totally random pattern. As has been suggested, proper cooling (slow) at the initial forming stage is critical to producing a stress free sheet. IP: Logged |
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Tom C Moderator Posts: 683 |
posted October 06, 2005 09:09 PM
Some acrylic sheet is made using impact modified acrylic. The impact modifiers are more sensitive to moisture absorbtion than the base acrylic. There may be an interaction between stress relief and absorbtion of mositure by the modifier. The moisture may relieve the stress by softening the modifier enough to allow movement and warpage. This is pure speculation. The numbers I have seen for moisture absorbtion in PMMA is up to 0.5% This is a significant amount compared to the 0% as it comes out of the extruder and then through the roll stack. Since sheet is produced "dry" , perhaps a light water spray prior to stacking might help the sheet acheive an even moisture content while being stored. I'm suggesting only spraying a small amount, not enogh to be obvious in the stacked sheet. I have good relations with a couple of acrylic makers. If you email me the resin supplier name I might be able to reach the correct technical resource for this issue. ------------------
Tom Cunningham
Extrusion Technical Services www.ExtrusionTechnicalServices.com IP: Logged |
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walterzj Senior Member Posts: 5 |
posted October 07, 2005 06:12 PM
Hi, answering Louis , the sheet presents one or two waves(1-3mm), mainly in the operator side. Its not warping. Just waves. Answering Tom, some times we receive high impact Acrylics , but its evident , not frecuent and its not related. This idea to apply water (steam or wet air or sprayed)sounds gut, once I thought that but now maybe I will really try to run some tests. IP: Logged |
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griffex Senior Member Posts: 36 |
posted January 30, 2006 12:43 AM
The key evidence is your statement that individual cooling works. I've worked with big fans to cool sheets after extrusion. Stack the sheets vertically with spaces and blow through. It's more handling but maybe it is worth it. Saludos Allan Griff algriff@griffex.com IP: Logged |
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