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#41
Author: David R. Roisum, Ph.D.,
Email: drroisum@aol.com
Subject: Tin canning is a very common problem with thin films.
Tin canning is a very common problem with thin films. This defect is defined by narrow annular ridges (not on a diagonal or helix) that are somewhat evenly spaced on a pitch of a part of an inch and cover the center part of a roll. Ridges that have other characters may be a different defect such as a gage band or a corrugation. The tin canning gets worse with: decreasing caliper, decreasing MD modulus and increased time after winding. The trouble is also very grade dependent, profile dependent and may be sensitive to winding conditions. The defect is easily recorded by 'tracing' the surface with flat chalk or crayon
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 approximately even 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. Thus, we know that the result of the root causes must be 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 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 hours to days, 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 some cases so that 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 have a dilemma. Another known is that the roll will creep or settle during storage. More accurately, it is 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.
No matter what the cause, 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.
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