Relying on 35 years of experience BE helps to provide methods to quickly identify the most common problems and ways to systematically tackle them.
BE has identified some common issues that blown film processors routinely need to oververcome. In this issue we troubleshoot haze lines and melt instability. We will provide some suggestions on how to identify the causes and correct these problems so the you can reduce downtime and eliminate scrap. In the next issue of Between the Lines we will discuss gels, interfacial instability and poor gauge.
1. Haze lines on the film in machine direction – mean poor film quality and lots of rolls destined for the scrap bins. The visible lines on the film are usually hazier than the rest of the film.
Contributing factors for haze lines include:
Processing temperatures
Set the correct processing temperatures from the extruder barrel zones all the way to the exit of the die. Be sure that you know the processing temperatures of each component of blends. A variable depth thermocouple installed at the extruder adapter prior to the entry to the die so that the melt temperature of the polymer can be recorded. The metal temperature of the die should be similar or slightly hotter than the recorded melt temperature. Too cold a metal temperature of the die can cause haze lines as well, since it forces a melt temperature gradient in the melt stream in the die.
Temperature gradient
It is important to have a uniform melt temperature gradient throughout the extrusion process, from the screws to the die. The easiest way to check for melt temperature gradient is to use a variable depth thermocouple. A melt stream with a large temperature gradient as measured in the adapter prior to the die introduces a material with very different viscosities. The temperature variance contributes to different flow behaviour in the spirals. Use a variable depth melt temperature probe to help optimize extruder melt temperature profiles for improved melt temperature control and mixing.
Screw and barrel conditions
Check into the screw design, as well as the screw and barrel conditions. Worn barrel and screws contribute to poor mixing of the different materials if a blend is used. This will also result in large temperature gradients. It is a good idea to establish a program that tracks screw, barrel wear via physical inspection and/or pumping rate, melt temperature measurements. BE recommends replacing them before they are worn beyond acceptable tolerances. This practice will significantly reduce gel issues and melt temperature related quality issues.
Die design
If the processing conditions have very little effect on eliminating or reducing the haze lines, and the haze lines correspond to the poor gauge uniformity coming out of the die, then it ‘s time consider a different die design.
For troubleshooting haze lines look at processing temperatures, temperature gradient, screw and barrel conditions and die design.
 BE Film Technoloogy Centre
2. Unstable melt is another troubling problem that results in lower output rates and poor quality film.
In designing film structures to achieve certain physical and/or barrier properties, consider the economics of the raw material costs AND the economics associated with productivity rates on a given film line. Balancing these physical properties, blow-up ratios and the available equipment configurations is critical in obtaining and maintaining product quality consistency and profitability.
When designing film structures for a multilayer blown film line it is particularly important to look at each specific material’s processing temperature requirements, the physical properties, barrier properties, as well as melt strength.
Consider extruder sizing by layer, required layer percentage or output, processing conditions to ensure that the polymers are not overheated from excessive extruder RPM or processing temperatures.
Product layflat and blow-up ratio requirements, as a function of die lip diameter, are critical. Too large a blow-up ratio with weak melt strength resins will result in lower production rates, which can have a negative effect on profitability. Large blow-up ratios can also contribute to poor film flatness and finished roll quality. This can have a negative effect on secondary processes such as printing and high speed converting.
For troubleshooting haze lines look at processing temperatures, temperature gradient, screw and barrel conditions and die design.
Avoid melt instability by designing your structure for specific resin characteristics, layer ratios, BURs and your machinery capabilities.
In the next issue of Between the Lines we will discuss gels, unmelts and interfacial instability.next issue
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