Tin canning is a very common problem on wound rolls of thin films, but is rarely if ever seen on other materials. This defect is defined by narrow annular ridges (not on a diagonal or helix) that are somewhat evenly spaced. The pitch between the ridges may vary from as little as a centimeter for thin films to several centimeters on thicker films. Ridges that have other characters may be a different defect, such as a corrugation or gage band. For example, a corrugation occurs in a narrow annular region, but the buckles within the defect are oriented at an angle and have the appearance of a twisted rope. Also, the pitch spacing is not regular on either corrugations or gage bands.

The tin canning gets worse with: decreasing caliper, decreasing MD modulus and increased time after winding. The trouble is also grade dependent, profile dependent and is sensitive to winding conditions. The defect is easily recorded by 'tracing' the surface with flat chalk or crayon, much like we traced leaves in kindergarten.

Unfortunately, as common as this problem is there has been no published study on the defect. One hint that science does gives us, however, is that the nearly uniform CD spacing is a reflection of the preferred buckling pitch. This is similar to a yardstick that wants to buckle as a half-sine when compressed on its axis. Because of this, we know that the root cause must somehow involve CD compression. Also, the experience of the industry is inconsistent, probably because there are several ways to generate the defect. Many will blame poor film as the cause, but are not specific about what aspect of the film created the defect and how. Many will claim that changing winding tightness via web tension and, more particularly, layon roll pressure can reduce the severity. It is generally believed that winding looser is beneficial, but there appear to be many exceptions.

The fact that the defect sometimes gets worse with time in storage in the roll also hints as to possible mechanics. We know for a fact that the air inevitably entrained into the wound roll will weep out the edges of the roll over the course of minutes to hours, exactly the same sort of time period that we see the ridges develop and the roll harden. Thus, it is likely that air is involved in these cases. Excluding air during winding by aggressive layon roll pressure may be helpful.

However, to exclude air is to wind tighter and we know that many wound roll defects get worse with tightness. Thus, we seem to have a dilemma. Another known is that rolls may creep or settle during storage. More accurately, the layers slide on each other ever so minutely rather than the material yielding. Thus, we find that the top of a core-supported roll is more buckled than the bottom due to the compression of beam bending. In these cases, wide rolls are at a much greater risk.

No matter what the cause, however, the first thing we try to do with most winding defects is to see if we can move the problem with the tightness knob. Run two trials. The first will be so tight that the web is almost necking and the layon is at a pressure that is at the threshold of wrinkling on the roll. The second will be so loose that the web is baggy and floppy entering the windup, and the layon is at the lowest pressure that can be maintained consistently without the roller bouncing off. Compare the rolls only on the basis of the tin-can defect described above. Move winder settings on the basis of what you see with the exaggerated trials.

David Roisum, Ph.D.

Consulting Technical Editor

920/725-7671

DRroisum@aol.com

www.roisum.com