Processing Tips to Prevent Gauge Bands - Part 4

Published: June 02, 2014, By Tom Bezigian

In the previous 3 articles in this mini-series, we discussed processing tips and operating procedures to help reduce or eliminate gauge bands.  Rather than shim the die whenever gauge bands appear, or oscillate the winder or extruder to dampen the effect of gauge bands, the best strategy is to not get them in the first place.  This article will review resin additives to achieve this same goal.

DISCLAIMER:  The following recommendations are general guidelines that are applicable to most extrusion coating lines using modern equipment, processing low density polyethylene (LDPE).  Your equipment or materials may require special conditions not covered here.  Be sure to consult with your resin supplier, machine supplier or technical experts to ensure that these conditions are applicable to your equipment and materials.  Improper start-up procedures have been known to damage equipment and/or cause explosions. 

This last section on reducing die lines has to do with formulation changes to reduce polymer degradation.  I will only give general advice here.  You will have to rely on internal R&D experts or your additive suppliers to determine the exact anti-oxidants to use.  That being said, this topic is quite simple, and will be approached from a manufacturing or process engineering perspective so that product quality is improved.

An effective way to stop oxidation is with the addition of an anti-oxidant to the formulation.  This can be done on a routine basis with all products, or just during shutdown, which, when combined with start-up, is when most of the polymer degradation occurs.

So, one strategy would be to shut down with an anti-oxidant concentrate in the barrel.  That is, the very last thing introduced into the hopper and purged through the extruder would be a pure anti-oxidant concentrate.  Add enough concentrate to be 100% ensure that it has gone through the barrel, pipes, adapters, coex block and full width of the die before stopping the extruder.   Doing this in conjunction with the shutdown procedure described in part 3 of this series will stop degradation of the polymer, thus eliminating gauge bands and facilitating a fast and trouble free-start-up.  Again, I have seen "start-up" times go from 24 hours down to 2 hours using the tips I have described in this series.

If you feel that you are seeing too many die streaks or other visual deformities during operation that are unrelated to start-up and shutdown issues, then add some anti-oxidant to your resin and observe what happens.  In my experience, I have found that a loading of 10,000 ppm of a suitable anti-oxidant(s) is sufficient to inhibit polymer degradation under normal conditions.

At this level of anti-oxidant loading, you will experience other issues, such as poor adhesion of the molten poly to your substrate.  So, you either optimize the anti-oxidant loading to balance adhesion with polymer degradation.  This is an ideal situation for a DOE to determine the optimal loading level with other process conditions, such as nip roll pressure, rubber roll hardness, etc.  Don't be surprised if you require nip roll pressures from 10-100 times greater than normal.  This is not for the faint of heart, but the rewards are world-class quality.

Please feel free to contact me on any of the points made in this series in regards to die lines, gauge bands, operating procedures, etc.