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| Author | Topic: Trouble shooting MD orientation |
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awan Member Posts: 3 |
posted May 29, 2009 01:30 PM
  
He'res my problem, too much orientation MD, extruding PS/SBC blend with 3% hips. 0.060" gauge to make drink cups (51/2 inch draw.) what should I look for interms of trying to reduce orientation, the resultant is cups sidewalls are cracking too easily. IP: Logged |
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Tom C Moderator Posts: 933 |
posted May 29, 2009 02:03 PM
What does the extruder downstream look like? What is the draw down ratio? ------------------ Tom Cunningham www.ExtrusionTechnicalServices.com IP: Logged |
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awan Member Posts: 3 |
posted May 29, 2009 02:15 PM
DRaw ratio per the operator is around 50:1.but let me ask this by draw ratio you mean the die gap v/s the chill roll gap? Or the die gap v/s the finished sheet thickness? And I am not sure I understand the other question. IP: Logged |
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Tom C Moderator Posts: 933 |
posted May 29, 2009 03:18 PM
I would first look at chill roll surface velocity divided by die exit velocity to figure out the draw down ratio. If there is too much draw at the die causing orientation. To minimize orientation you probably want number like 1.1:1 .
That would mean if you were making 0.050" thick sheet the die opening would be 2.5". Regarding line configuration, there are various ways to handle the sheet after the melt exits the extruder. How that equipment is operated can affect orientation. ------------------ Tom Cunningham www.ExtrusionTechnicalServices.com IP: Logged |
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awan Member Posts: 3 |
posted May 29, 2009 03:46 PM
You are right 50: is wrong. So I will check chill roll surface velocity but how do you check the melt exit velocity? it's molten plastic. IP: Logged |
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Tom C Moderator Posts: 933 |
posted May 29, 2009 04:04 PM
You need to know your output rate. If you don't know it you can get an estimate from the calculator found at: www.ExtrusionTecnicalServices.com Convert Rate (lbs/hr) into volumetric output by dividing by the melt density in lbs/cu.ft. For PS it is about 53.7 lbs/cu.ft. Gives cu.ft/hr. Multiply that by 1728 to give cu.inches/hr. Multiply the die exit width by the gap in inches. Gives the die output cross section in square inches. Divide the volumetric output (cu.inches/hr) by the die cross sectional area (sq.inches) to gets the average exit velocity in inches per hour. Divide by 3600 to get inches per second. Divide that number by the inches/second velocity of the roll surface to get the draw ratio.
------------------ Tom Cunningham www.ExtrusionTechnicalServices.com IP: Logged |
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