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May 12, 2005
Don't be afraid of negative draw
Draw control has always been a favorite topic of mine. Why? Because it is widely used in converting equipment, it is so simple to design, but the web's reaction to draw is somehow counter-intuitive.
I was recently working on a multi-station printing press. Presses are commonly run in draw control to keep the printing cylinder in registration. If you had each print station changing speed in closed loop tension control, you would hit your mark on average, but be off registration in both directions.
The tension in a press is based on draw control. The tension into the first press station is controlled by a constant tension infeed section (I hope it is). The tension between all subsequent stations is the baseline web strain modified up or down by the draw (relative speed) of each station to the first station. As a widely used rule of thumb, the series of print stations should be set at equal speed or a slight increase in speed from station to station. These guidelines imply that shifting to a lower speed (a.k.a. negative draw) would be a bad thing. Is it?
Negative draw can be a bad thing, but if understood, it can also be the key to running webs with ultra low tension or stretch. Negative draw is bad for non-elastic products. If when you stretch your material it doesn't recover (or recovers slowly), then I can understand why slowing down the speed would lead to a web pile up.
However, most webs are quite elastic. They snap back when their stretch (strain) is reduced. This is where negative strain can be helpful. Want to lower tension? Figure out the stretch in your product at point 1, then set a progressive slower speed, say 100 fpm and 99.9 fpm in a negative draw, voila~, the web tension will drop corresponding to a 0.1% strain reduction.
For more on draw control, check out Drawing Conclusion, my May 2005 article for Paper, Film, and Foil Converter magazine.
Posted by Tim Walker at May 12, 2005 02:46 PM
