What should be the unwinding tension for 12 micron PET film and for 30 micron Polyethylene film. Please give me the formula for calculating the rewinder tension for the laminate.
I will use page 5.10 from the 2010 version of the Web101 notes. I apologize for the mixed units, but we will get the answer anyway.
for Polyester (Mylar)
12 microns is 0.47 mils. Looking at the table for polyester we have 0.75 PLI per mil or 0.35 PLI or lb/in. In metric units this would be multiplied by the conversion factor of 0.175 to get 0.061 kN/m or 60.6 Newtons per meter of web width.
Similarly for polypropylene
40 microns is 1.57 mils. The table suggests 0.25 PLI per mil 0.39 PLI or lb/in. In metric units this is multiplied by 0.175 to get 0.0687 kN/m or 68.7 Newtons per meter of web width.
So, on the unwinds we would initially try about 60 and 70 Newtons per meter of web width respectively for the Polyester and Polypropylene. Then we would observe how the web behaved (such as curl) and adjust accordingly.
The bigger problem is trying to calculate the tension on the laminate. You can't simply add 60 + 70 together because the Polyester is much stiffer. The laminate will probably only want about 80 Newtons per meter.
A better way to handle the laminate and possibly the individual plies is to do a tensile test and design the process to tension the web 10-25% of the strength.
Rules for tensioning of webs and many other must-know subjects are detailed in my Web Handling and Converting shortcourse March 27-28 in Chicago.