Geometries such as length, diameter and core wall thickness must be specified when ordering cores. Length is the easiest. The core is typically sized to be the same as the width of the roll for most materials.

An exception is some films, nonwovens and textiles where the core is intended to stick out beyond the end of the roll. This stickout is used to either support the roll during shipment or to protect the delicate edge of the roll during handling. If length is intended to match roll width, it may not for one of three reasons.

First, the slitters could be set up improperly.

Second, the core cutter could have been set improperly.

Third, the cores could be 'green'. If cores are not dried to equilibrium with the converting environment, they may shrink as they dry. This shrinkage takes place over the course of a day or two.

Wall thickness is also easy. The wall should be just thick enough so that very few cores collapse due to internal winding pressure or external handling loads. In the former case, we would also consider winding less tightly. In the latter case, we would wind tighter or use core plugs to protect the core.

The optimum wall size is determined as the minima of the core cost and core-crush costs. If you crush too often, then the wall size should be increased. If you never crush, the walls are too thick. The law of diminishing returns indicates that "zero defects" may not be a wise target. Defect rates of 1-100 parts per 10,000 could maximize system efficiency depending on the situation.

The most difficult geometry is nominal core size. The ubiquitous 3" diameter core must give way to larger diameters as roll size increase. As we increase either roll diameter or roll length, the inexorable pull of gravity can do us in.

The list of troubles when winding or unwinding heavy rolls on small diameter cores is so large that it must be given careful respect. This list includes very serious difficulties such as blocking, bulk loss, core failure, telescoping, vibration, wrinkling and many more. In fact, if you or your customer has troubles near the core, it may well be due to the core being too small—rather than some limitation of the winding machine or tension settings.

The problem is that customers insist on running ever-larger rolls on the same old core size. If you try unilaterally to educate them, they may not listen and simply go to your competitors. If you offer to upgrade their chucks and shafts to larger sizes, they will always find someone else willing to supply large rolls on wimpy cores. Some will claim there is no justification for spending extra money on bigger cores nor losing the extra few percent of roll capacity. However, this is no more valid than trying to run a bicycle tire on an automobile because they are cheaper. While a 1" wide wheel might carry 100 lbs comfortably, it takes a 10" wide wheel to carry car loads.

One of the largest users of cores, the paper printing industry, fought increasing core diameters for decades as roll weights increased from a modest 2,000 lb. to a crushing 10,000 lb. Finally it became clear to most everyone that they had gone far too far. But it took mega-tons of waste, endless trials and tribulations and uncountable meetings before consensus was reached that core sizes must be increased.

Europe led the move to 4" and 6" cores and the U.S. is now following. Some pockets of resistance still remain. Scientists have even studied the results of inadequate cores, yet management still wanted a more comforting answer like winding tighter. After many skirmishes, the battle is just now beginning in converting.

David Roisum, Ph.D.

Consulting Technical Editor

920/725-7671

DRroisum@aol.com

www.roisum.com