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Roll "Tin-Canning"
February 29, 2008
When you look at a roll of film it will be smooth and regular indicating that the film is wound with little or no distortion. However, this is not always the case. In the roll in the picture,
Rolls showing narrow buckles known as "tin-canning"
we see that there are a large number of closely spaced ridges in the roll of film. These are small closely spaced film buckles which are in the roll and which will remain in the fill as it is unwound. Often times these buckles are called "tin-canning," because it looks like the closely spaced ridges rolled into the can wall to stiffen it. These film buckles are very common in thin films and are an indicator of film which is being wound at too high a tension and/or which has a high film-to-film coefficient of friction. They are especially prevalent in thin films.
What is the cause of these ridges in the rolls, and why are thinner films more susceptible? Well, let's look at what they are and see if we can determine the source. Because the film is buckled, the width of the film must be longer than the width of the wound roll. How did that happen? It happened when we were winding the film. To transport and wind the film, we have to apply tension to pull it to the roll. Generally the tension is lower than the yield point, so we are operating in the elastic region of the tensile curve. When we pull a film (apply tension) in the elastic region, the film will get longer. But this takes place at constant volume, and to maintain the volume constant, the film must also get thinner and narrower. How much it thins and narrows depends on its mechanical properties through Poisson’s ratio. Generally, the major dimensional changes are to get longer and thinner.
Then, when the film enters the roll, the winding tension is released and because the film is elastic, the film will then shorten, and recover any other dimensional change, so it gets wider. But the film is held in place by the film wound under and over it. If the film cannot slide on itself, as it widens, compressive stresses will increase in the film. If the compressive stresses are high enough, the film will buckle forming the ridge in the roll of film. The size and spacing are very characteristic of the strength of the film. Thin films are more susceptible because they are more easily deformed and will buckle at a lower stress. Preventing the "tin-canning" requires films formulated for winding as well as tensions low enough to prevent the elongation and narrowing of the film prior to the formation of the roll itself.
Posted by Eldridge M. Mount on February 29, 2008 | Comments (0)