What is the purpose of the pre-drum spreader roll?
Answer
The pre-drum spreader is to put some lateral tension into the web before the web sees the deposition zone where the rise in temperature will make the film want to expand. The initial expansion relaxes this tension and then as the expansion continues the web does not move on the drum and so a lateral compressive builds up. If the lateral compressive force is too much the web will buckle off the drum and you see this as the formation of a tramline.
This pre-spreader roll usually has a soft polymer surface and it relies on the softness of the polymer to deflect which results in the lateral spreading of the web to take place. Over time these rolls age and become harder and so lose some of their performance. The roll can have a rubber, elastomer or polymer material as the covering, depending on the material will depend on the aging process. Some age more quickly with heat others can be simply be dried out slowly by the vacuum and others can be hardened by an interaction with solvents that may be used to clean the rolls as part of routine maintenance.
Another complimentary method of reducing the wrinkling is to inject a gas between the web and the deposition drum. Part of the problem is that the web cannot easily move on the deposition drum hence as the web comes in contact with the cooled deposition drum it wants to shrink as it cools to reach the same temperature as the drum. The tension is applied to the web to hold it hard against the deposition drum to help maximise the heat removed from the film during the deposition process. The combination of the coefficient of friction and the tension applied to the web not only holds the web tight against the drum but also stops the web moving relative to the drum even though it is contracting and then expanding and finally contracting again with the changes of temperature. Injecting gas behind the web lubricates the back surface enabling the web to move relative to the drum and so reducing the chance of the web buckling. The trapped gas also increases the conduction part of the heat transfer coefficient and so reduces the film temperature too. Hence there are two mechanisms that help stop buckling.
A further point on buckling. Some work done by Mike McCann showed that any dirt on the drum (even down to a micron or more) is sufficient to lift the web off the drum and cause a local overheating and adding to the buckling forces. Thus wrinkles will happen more frequently on dirty machines than on immaculately clean machines.
The positioning of the wrinkles can also be affected by the residual stress in the web as too can any profile variations.
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