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Blog
Making vs. winding film
June 8, 2008
It seems a bit unusual but in many instances it is easier to make film than it is to wind the film. In an earlier posting I spoke about the formation of a defect called “tin canning” which was due to the over tensioning of the film as it is wound. This week I am out making a very thin film for a capacitor application. We have studied the orientation parameters and identified the basic parameters of preheat temperature and stretch ratio’s and have a good handle on that. However, we are working on a line which is not really designed for the tension levels of the film.
So I expect to have a fairly easy time of the stretching and the real work will be how to control the film tension in the web transport section and at the winder. When I was working to design metallized films I did a survey of film returns to find out why our films were returned from Customers. What I discovered was eye opening. Almost none of the film came back for properties not being right. Almost all of the film came back because the rolls were poor quality, wound to tight, edge damage, crushed cores etc.
Based on this and the need to metalize films in a vacuum, without the benefit of air to help with the roll formation, I developed a concept which I term formulating for winding. The key idea is to formulate the film to control the film to film spacing as the roll forms without the benefit of an air layer. This is done with antiblock particles to space the film. Ideally the film spacing is larger than the air layer which would be developed if winding and transporting the film in air. I will discuss this some more in the future.
The problem with the film to be made this week is its strength relative to the tension ranges possible on the line. This is however true on many lines particularly on a line designed for thick films being used to make thin films. For instance, assume you are making I.0 mil (25 micron) film with a yield stress of 1,000 psi at 6% elongation. What is the applied force we must not exceed to keep the film elongation below 2%? First we must know the film width to determine the film area so we can calculate the load at the yield point. For a 100 inch wide film this would be 100 pounds force or 1 pound per inch (PLI) at the yield point (6% elongation). At 2$ elongation this would reduce to 33 pounds force or 0.33 PLI. If I now move to make the film ½ mil, (12.5 micron) then to maintain the film elongation at 2% or less, I will have to reduce the applied tension to 16.5 pounds or 0.16 PLI. The active question is can the winding section be operated stability at this reduced tension? If not then no matter that you can make the film, you will ruin it at the winder
Posted by Eldridge M. Mount on June 8, 2008 | Comments (0)