Reels » Film Reel
Process of Rotating The Die While Processing Good Quality Film
By rotation of the extrusion die these gauge bands can be moved around the surface of the film as the bubble is being extruded. The bubble itself does not rotate. In this fashion they are evenly distributed across the face of the roll at an evenly distributed wound as you would reel in fish line on a fishing reel and build a cylindrical roll of plastic film of perfect symmetry. Without rotation these faults would build up in one place on the roll of film it would create a roll of film whose surface would look like something got caught in the roll and make an un-even impression or bulge on the roll.
Unfortunately rotation of the die can introduce problems of its own that the bulge called a gauge band now gradually moves across the face of the collapsing frames. When you have such actions the web moves back and fourth between the two wood or plastic frames and the lay flat begins to wonder back and fourth in the down stream equipment starting at the nip rolls. With out pulling trim off of the both sides of the film a web guide would be required.
Generally gauge bands are caused by drafts of air or a heat rise off of the front of the extruder. As for a consequence the roll of film may be tapered or have a convex or concave face as the different thicknesses of film build up upon themselves in the roll.
Again as the bubble or die diameter is increased so is the transverse speed of the gauge bands across the face of the A-Frames which will also change the rotational speed. This can cause bubble instability, intermittent wrinkling in the nips and web wondering downstream that can be corrected by reducing the rotational speed.
However one rotation should never be less than the time it takes to build a roll of film or the gauge bands will not have time to be uniformly distributed across the entire face of the roll of film.
dbanig@prflexbag.com
http://www.prflexbag.com
Source: www.articledashboard.com