Is There a Doorman at Your Winder or Do You Let Just Any Web Get In?

I was just reviewing a lab coater line the other day. It had a good plan for tensioning, rollers, and guiding, but the winder was a simple driven spindle.

Is a driven spindle a rewinder? Yes, if all you need from a winder is the ability to wind (i.e., accumulate through twisting).

Will it create a functional wound roll? Maybe? A functional wound roll should be wind-able (it doesn’t fall apart before it is finished, transport-able (it doesn’t fall apart when you try to move it), unwind-able (you can get the product back out of the roll without blocking, telescoping, and it should not look like an abused trash bag.)

Will a simple spindle do that? In many cases, no. If your product is stiff enough to not wrinkle, rough enough to not air lubricate, flat or soft enough not be walk laterally atop the uneven shape of the winding roll, then maybe the simple driven spindle is for you. There is probably a huge base of converters that can say yes to all of this, but they never seem to call me.

I think a winder needs a doorman. Don’t let just any rumpled, staggering, all lubed up web into your roll. A winding nip roller keeps the entrance to your winding roll in order. Here are the benefits of a winding nip (and a few potential detriments).

A winding nip roller has five effects, mostly beneficial:

  1. More Tension–The winding nip adds tension, creating more wound-on tension than center torque alone.
  2. Less Wrinkles–With proper wrinkle prevention or spreading into the nip roller, the nip will lay the web down wrinkle-free on to the winding roll, preventing the roll variations (e.g., TD diameter variations) from gathering or shifting a web so severely it wrinkles.
  3. Properly Aligned–With proper guiding or a short web path from slitting to winding, the nip will lay the web down with good alignment, preventing the roll variations (e.g. TD diameter variations) or uneven escape of air to shift the web.
  4. Squeegee Air–The winding nip reduces the air that enters the winding roll, preventing roll stress and structural change from excess air escaping over time.
  5. More Cylindrical–A winding nip may promote better cylindricity, especially in products with low stack modulus.

Things that go wrong with nips:

  1. Deflection–Don’t make your winding nip out of a noodle, and don’t have it overhang the winding roll too much. If you have a big width change, that second one is hard to do. Either way, try to keep deflections down to less than 5 or 10 mils, which is something your soft wound roll or rubber roller covering can forgive.
  2. Wrinkling–Nipped processes are always sensitive to baggy and skewed webs, especially when tension isn’t high enough to pull out the extra length. Spreading before winding can help.
  3. Damage–Some materials are sensitive to pressure and just can’t be nipped. Luckily, many of these products are rough or porous and can get by without the air squeegee benefit.
  4. Nips and Hardbands–The nips will press hardest on the large diameter portions of the winding roll. Increase stress of high thickness, larger diameter lanes, creating more bagginess viewed when unwinding after storage. Using a rubber covered nip helps reduce this greatly.
  5. Transient Nips–Nips are great until they aren’t there. In many turret winder designs, the winding nip is disengaged during the turret indexing portion of automatic roll changes. Some winders have secondary winding nips during indexing, but most of these are marginally successful at replacing the primary nips roller functions.

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