Unless you have extruded some heavy gage tubing or very large rolls of blown film tubing (or ‘J’ sheet) possibly using an edge guide, you may not have experienced “flanges” on the edge of a roll of film. It is not unusual to see rolls of blown film with folded edges on one or both ends of the roll where the edges are 5 percent larger in diameter than the remaining roll.
Some film customers or their buyers seem to prefer rolls of film that are completely flat and have smooth ends with an appearance of the grooves in a CD, as you may see on a roll of single-wound, trimmed sheeting. This appearance supposedly represents high quality, and the roll is expected to track perfectly when they unwind it into their process without any edge guiding. The common zigzag blown film roll edges caused by a rotating die or oscillating nips are scorned as a poor roll configuration even when the entire roll is flat with a uniform hardness and uniform circumference.
In my poor grade school, many many years ago, we were taught how to take one sheet of paper, fold it, crease it and then tear it in half cleanly, in order to save paper and cut down on usage. This is not easily done with a sheet of polyethylene film, but the effects of creasing are the same. There is no way to fold a solid sheet of anything 180 degrees without stretching the outside radius.
Thankfully, polyethylene it somewhat extensible and it will do its best to recover as near as possible to its original thickness after being stretched. It will resist being creased and stretched as its edge folds are layered upon each other, but since it cannot stretch to zero thickness, something has to give.
In the case of zigzag roll ends, the folds have room to remain relatively round and this spreads the stretching. This rounder larger film cross section moves from side to side as the web wanders back and forth. On the other hand, without zigzag roll ends, the roll edges are aligned and therefore the folded edges will become harder as they are layered and creased and they will eventually build up to a larger diameter than the remaining roll.
When these edges can no longer maintain a flat roll surface, they will also begin stretching the folds lengthwise as the roll’s end diameter exceeds the remaining roll diameter. This will then produce a flange on the roll end(s) in order to make room for the thicker folded edges. If the roll is 12 inches diameter with a ¼-inch high flange, then the folded edges have had to wrap a larger 12 ½ inch diameter which means that the folded edge also had to stretch in the machine direction more than 1 ½ inches longer during every roll wrap! Now the folded edge has been stretched in two directions at the same time, and the fold is thinner. Where might this film fail during its final use? Should an operator increase the film gage in order to overcome any film weakness from a creased, stretched fold in this film, or should he reduce this stretching so that he can run this film at a thinner gage while maintaining its integrity?
Technology to the Rescue?
A few years ago I was relieved and delighted to discover that a modern electronic edge guide had the ability to actually produce a zigzag edge on a roll of blown film. The distance of the sideways oscillation on this feature could be set, and also the interval between oscillations. This feature could be vital on some film roll products that have one or more folded edges. Obviously, the resultant roll ends will not have the appearance of a CD.
Since blown film always has at least two folded edges going through the primary nip, and possibly more nips downstream, there are other locations where folded edges can get creased beyond what is inevitable. These nips are another whole story for another day, but suffice to say that four factors are involved in creasing film folds to their weakening point.
- Primary nips are notoriously hot and this high heat is going to add to more severe, permanent creasing of film folds.
- Any set of nip rollers must be perfectly level with each other when closed in order to prevent cross-axis molestation, slowly grinding the two film surfaces in opposite directions.
- The durometer (softness) of the “rubber” roller must allow deformity enough for folds to pass through without undue creasing.
- Then, the air pressure on a primary nip closing cylinders needs to be the minimum pressure necessary to trap air in the bubble, assuming both rollers have flat/round surfaces.
I have never had to sell rolls of tubing or any folded film to a customer and so I cannot say how those who must sell this product can ever overcome the natural tendency for customers to insist on “perfect looking roll ends” for their unwinds. If the film is failing at the folds it may require extra thickness to reduce the problem. I suppose that if you are selling film by the pound, and yields are not a consideration then maybe this is a non-issue for the film extruder.
About the Author
John R. Myers has been an engineer in plastics extrusion for 50 years and an independent consultant to the plastics blown film industry worldwide since 1974. Myers has been instrumental in establishing 7 blown film operations, but mostly works with existing plants to expand, design or improve processes and technology. Email him at Johnrmyersco@aol.com.