- November 26, 2012, Tom Bezigian
From film to sheet, cast film, extrusion coating, laminating, papermaking, etc., whenever there is a wound roll or any significant diameter, unless the thickness profile is VERY uniform, with very little point to point variation, die streaks are an issue.
Why does this happen? When a substance exits a die, if there is a defect in the die land area, or if there is a material buildup, such as degradation, the gap between the die is not uniform, or the flow is disrupted, and the result is inconsistent thickness across the web. As the web is wound onto a roll, after 10-15 cm (4"-6") of wrap, narrow gauge bands start to appear. These bands can be 1-4 mm wide, and will grow in height as roll diameter increases. It doesn't take much to experience quality problems, and these defects can either be seen or felt by hand. The rest of the discussion will assume that there are no physical defects in the die.
As these gauge bands grow, the web is deformed, and quality defects can often result. These can range from optical defects such as dull streaks in the polymer surface, to discontinuity in foil or metalized layers, to "baggy" areas which can wreak havoc with eye-marks on converting and packaging equipment. Obviously, the greater the defect or buildup, the worse the die streaks and gauge bands. Also, the large the diameter and tighter the roll is wound, the more evident the defects.
On one call to address this issue, I suggested the client take the extrusion laminator off-line to inspect and clean the die lips. After much chagrin, they agreed. While there was no physical damage to the die, there was, as expected, carbon buildup in many areas across the width of the die. This is of course indicative of thermal degradation. The extrusion process in general, and the extrusion coating and laminating process in particular, is prone to polymer degradation due to frequent stoppages, either planned or unplanned.
I asked the operator to clean the die to remove the carbon, and all he really did was remove the molten carbon. Ah ha!!! The extruder therefore really hadn't been cleaned properly in a LONG time. It is understandable why this is avoided, as it is hard work in a very hot environment to clean the die. I asked him to clean it again, without much more luck. I asked one more time, with me cleaning half and him cleaning the other half of the die so we could compare. I gave ample warning of what the results would be and that another shutdown would be required to get it right. Anyway... you know the results. There were no die streaks or gauge bands on the half of the die that I cleaned, and there was no change on the half that the operator cleaned. Lesson learned.
While it is not possible, even with the best equipment, operators and procedures in the world to completely eliminate this phenomenon, there are ways to minimize it and to "mask" the problem so as to eliminate defects caused by gauge bands. First, a change in formulation will help, such as the addition of A LOT of antioxidant... like 10,000 ppm and reduced melt temperature. Beware... adhesion will suffer, and process changes may be required, such as harder rubber nip rolls, higher nip pressure.
Second, a mechanical solution is the addition of a "randomizer" will distribute any variation to the point where they are not noticeable. This is most easily done by mounted a cam on a low-speed motor onto the back of the extruder carriage, causing the entire extruder carriage to move back and forth an inch or so (2-3 cm) every 20-30 seconds. The other end of the cam is attached to a bar that latches into a groove in the floor, thus fixing the carriage onto floor, allowing for the oscillating movement to occur. This randomized the variation much the way a rotating winder does on a blown film line.
Good luck with these suggestions. I hope everyone had an excellent Thanksgiving holiday.