- March 18, 2010, Tom Bezigian
We’ve all seen them. We really don’t like to see them. We’ve invented ingenious ways to deal with them, like oscillating and rotating dies and extruders, but we just can’t seem to get rid of them permanently, and like a bad penny, they always show up when least desired and expected.
Some extrusion coating converters blame this phenomenon on the substrate. This is a carry over from “the old days” when wide gauge bands were seen on paper rolls. However, from experience, we know we don’t see them in rolls of incoming oriented PET film. As can be seen in the attached picture, they are present in rolls of converted PET film. This leads us to the logical conclusion that gauge bands are caused in the converting operation… either due to damaged or dirty die.
In a perfect world, there would be no die lines. But, as we all know, nothing is perfect in manufacturing. When we stop momentarily to change orders or when the web breaks or heaven forbid when the proverbial “lightning strike” shuts down the plant, the molten polymer has the opportunity to remain stationary on a very hot metal surface, in the presence of oxygen, resulting in polymer degradation. Small amounts of carbon are then deposited on the die lands, and these deposits, if not uniform, cause irregularities in the thickness of the polymer film. This translates into die lines, or gauge bands as they are commonly called.
One solution is to move the die relative to the winder. In blown film applications it is very common to rotate the inner mandrel of the die. With flat dies used in cast film and extrusion coating, oscillating the casting section/laminator or extruder carriage itself achieves the same purpose. This is the “if you can’t beat ‘em, join ‘em” strategy whereby the die streaks are randomized so they are not concentrated on one location on the finished roll.
Blown film die manufacturers are very familiar with rotating dies… nothing new here. It is a different story when it comes to oscillating a casting section of a cast film or extrusion coating line. With these, a cam is slowly rotated against the fixed object (the laminator), which is on casters. The trick is fixing the extrusion coating station so that it can roll in and out as needed, but become fixed so that it can oscillate to remove the die streaks. Here a simple toggle is dropped into place to fix the extruder carriage once it is online so the cam can oscillate against it. It takes some engineering, but it can be done.
Another solution to this problem is to optimize formulas to improve polymer stability, use proper operating and preventative maintenance procedures to minimize downtime and carbon buildup, and to use proper die cleaning techniques so that the die lines are eliminated during scheduled maintenance. Easier said than done. We’ll talk about this next time.