Some rewinders can apply torque in either a locked- or slip-core mode. In locked-core mode, every daughter roll across the width must turn at the same RPM because the expandable shaft connects them all together. Locked-core works fine if the gage profile is quite level or the material is compressible. The problem is that gage variations are a way of life for certain incompressible grades such as film.

As seen in the accompanying figures, the individual rolls tend to build to reflect the incoming gage variations. Diameter is not, as commonly believed, proportional to local gage. (The actual gage variation could be an order of magnitude greater than the resulting diameter variation because the winding process tends to be self-leveling.) In any case, thick lanes wind bigger and thin lanes wind smaller. As we look at the side view, (bottom left) we see that the bigger roll will turn at a faster surface speed, thus higher draw, and will necessarily pull a higher tension than the neighboring small roll.

However, the real problem may be on the small roll. It will turn at a slower surface speed and won't be able to take up as much material. The small roll then winds looser than the big roll. How loose? Quite possibly loose enough to make a gathering puddle on the floor, if you could run that way.

What's the operator to do? The first thing to do is to crank the only knob available; winding tightness. If you pull really hard, you might be able to muscle your way through. If this doesn't do it for whatever reason, you will next look at the root cause—namely, gage variation. Unfortunately, the pleas of the winder operator are always ignored, even if they assemble a bullet-proof case such as:

• The smallest diameter rolls wind loose,
• The small diameter rolls are softer by a hardness measurement,
• The small diameter rolls weigh less,
• The small diameter rolls can be temporarily fixed by throwing scrap into the roll (to make up for the shortage of material from manufacturing).

There are many reasons that manufacturing will ignore the protests of the winder operator:

• They do not understand the physics,
• They do not want to acknowledge the fact that they are the root of the problem,
• They hide behind the flimsy shields of "The scanner/test lab gage measurements do not show troubled rolls." "We are in spec." "It's the best we can do."

Thus, getting absolutely no satisfaction and probably no sympathy either from extrusion, coating or whoever is responsible, the winder operator is back where he started. Now, he has only one card left to play to reduce waste levels: differential winding. With differential winding, also called slip-core winding, each roll can turn at its own RPM, is independently torqued and thus independently tensioned.

920/725-7671 DRroisum@aol.com, www.roisum.com