While plastic film made by the blown film process can be controlled closely, there are limitations imposed by the interdependent factors of the process and plastic resin selection.
To understand the physics of the blown film process, think soap bubble blown vertically. When the bubble starts out, the wall is thick and stable. When it gets larger, the wall is thinner and starts shaking. The relationship between the bubble size and the circular die, or aperture that the melt exits from, is a key factor called blow up ratio or BUR for short. Gauge profile is easiest to control at low blowup ratios. Problem: PE film made with a low blowup ratio can be weak and exhibit the dreaded straight line tear which results from too much orientation in the machine ( MD ) direction. While the textbook happy spot is a blowup ratio of 2:1 ( bubble diameter = twice die diameter ), this is not always possible in the real world. If scheduling realities dictate a higher blowup ratio, the operator must use tricks of the trade to achieve a flat, even plastic film.
The other component is resin selection. Resins with higher viscosities or lower melt index, or “MI” for short, lend themselves to an even thickness profile.
Of course, in the real world not every plastic film can be made from a very low preferably “fractional melt index” resin at a low blowup ratio. Ironically the least critical blown film application – shrink film – is made with low melt index, low blowup ratio. Anymore blown film plant managers are chefs who manage what winemakers call cuvees, or blends, of resins.
PE film with excessive variation is maddening when heat sealing and laminating. “Baggy film” is a perennial complaint which will bounce a roll of film a country mile without hesitation.
There is a fundamental difference between point-to-point thickness variation and what is known in the converting industry as Yield or area derived from a specific weight. Here are PE film conversion factors in both English and Metric units:
A good starting point for agreeing on thickness variation is the Flexible Packaging Association spec B 11. Sorry, we can’t link to it. You have to buy a copy.
Over the years the capital equipment manufacturers have endeavored to make blown film lines which guarantee + – tight tolerances with non-adjustable features which take the operator variable out of the equation. For now these luxury store bought turnkey lines are too rich for our blood.
We can’t help believe blown film is still an art and the operator should take responsibility for his workmanship. It is our tradition that every roll we make has the operator’s ( or should we say artists ?) name on it.
