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April 05, 2006
What to control: tension or strain?
Here's recent question:
Is it better to control tension and let strain fall where it may or control strain at whatever tension is necessary?
TJW Answer:
This is a big question, but let's try to answer it simply.
Most webs are elastic, so controlling tension or strain are really the same thing. If you have a web with an elastic modulus of 500,000 psi, then 0.5 lbs/ in / mil will be 500 psi stress or 0.1 percent elongation. It doesn't matter whether you run at 0.5 PLI per mil or 0.1 percent strain.
(All webs should be run at an average stress or strain that is 10-20 percent or less of their yield or break point.)
However, sometime things are not elastic.
For crepe paper, such as used to make masking tape, you can pull with too much strain and pull out the crepe, never to be recovered. This is a bad thing since the masking tape user is paying to get that strain for their conforming tape application.
In making nonwovens, before the binder is applied and cured or dried, the fibers can easily be pull out changing the product thickness, porosity, or other property. Overstraining the material is a bad thing.
Some webs are viscoelastic. If you pull on vinyl tape, it will stretch more every second or minute you keep the load on it. To avoid this, control strain, not tension. I was recently asked if web handling applied to cookie dough. Yes, but you better thing about the time - tension material property, the viscoelasticity of the dough.
Bottomline answer:
For most product, controlling tension, the easier to measure property is normal and let strain fall where it may.
It's still good to know what your typical web strain, because as I wrote my recent article in PFFC, strain is the secret to web handling. Strain determine specs for roller alignment, diameter variations, speed variations, etc.
For those special products that yield easily or are highly viscoelastic, look at controlling strain.
Posted by Tim Walker at April 5, 2006 10:57 AM
