- April 29, 2010, Tom Bezigian
I’m sorry about the long gap between posts. I have been working as an expert witness on a very interesting and demanding case which has taken all of my time for the last two months. Now… back to die lines.
The best way to get rid of die lines is to never have them in the first place. Other than a physical defect in the die lips themselves, die lines are quite often caused by carbon buildup in the die land area, i.e., the area just before the plastic exits the die. The pressure drop across this area is in the range of 250-750 psi (1.72-5.17 MPa), so it is not possible for the carbon or other contamination to enter the die from the outside, so the carbon comes from either the resin itself or the extrusion manufacturing process. In my experience, it is usually the latter of these two possibilities.
While die design varies between processes, the extrusion process is universal. If the temperature the polymer is exposed to is too hot, the rate of polymer degradation will accelerate. If the polymer is exposed to high temperatures for an excessive period of time, degradation will occur. The degradation process is goes something like this: polymer - too hot for too long - reduced viscosity - gels - soft carbon - medium brown carbon - dark carbon & gas. A good analogy is when the fluid from the apple pie dish spills over onto the bottom of the oven, it slowly goes from its original medium brown color to various stages of darker and darker brown until it finally turns into dark black carbon. I happen to like baking apple pies, so I have learned this through experience. The same process happens to the polymer in the extruder and die.
It may seem obvious that the temperatures have to be set correctly, but on one assignment at a flexible packaging converter who was experiencing die lines, their extrusion coating die was set at 675?F (357?C), which is excessive. As soon as we opened the die to inspect for carbon buildup, the polymer caught on fire. In addition to being set too hot, we later found that the temperature controllers were not calibrated, and the actual temperature was 30?F (17?C) hotter than the readout indicated. They got the point, and changed their process conditions and maintenance procedures to my recommendations, and the problem went away.
So, as far as preventing die lines, the most important thing to do is to prevent carbon build up in the die. To do this, it is important that all your temperature set to temperatures that will not degrade the plastic and that the temperature controllers are calibrated and working properly. Next, it is critical that the polymer flow (output) does not go to zero or anywhere near zero. Let me give a detailed example that I have direct experience with to illustrate my point.
Let’s assume the following: A converter has a 5-layer flexible packaging barrier blown film coextrusion line designed to produce a structure with all layers being 15 microns (0.6 mils) thick for a total thickness of 75 microns (3.0 mils). These are ultimately used as flexible barrier packaging bags for use in meat and cheese packaging. Over the past 20 years, the screw and barrel were not maintained properly, and the radial flight clearance of the 45 mm screws increases from 125 microns (5 mils) to 625 microns (25 mils), and output drops from the original design of 500 lbs/hr (227 kg/hr) to 300 lbs/hr (136kg/hr). Let’s also say that in an attempt to reduce costs, the thickness of the expensive EVOH barrier layer is arbitrarily reduced from 15 microns to 6 microns. What has effectively happened is that the output of that layer has been reduced from the original design criteria by 76%. This means that instead of the EVOH having a bulk residence time of approximately 8 minutes in the extruder and die, it now has a residence time of about 30 minutes. This is not good.
Let’s also say for example that the outer layer of the structure is nylon, which requires a higher temperature than the rest of the structure, and that the die temperature is set at 470?F (243?C) to accommodate the nylon. This is approximately 30?C higher than it should be, meaning that the EVOH is being subjected to about 8 times more heat input than it was designed for. When you couple the extra heat with the extra time the EVOH is subjected to in the extruder and die, we have a situation where the EVOH is receiving a minimum of 30 times more energy input than the original design specification.
Let’s also say that this converter is in the Southeastern part of the United States, which is subject to frequent thunderstorms and lightning strikes. A lightning strike can take down an entire plant in an instant, quite often leaving the polymer in the extruder to overheat and bake for 20-30 minutes. This in and of itself is devastating. When this happens, it is imperative to immediately purge the extruders with the proper purge compounds to remove the degraded polymer as soon as possible so it does not degrade to carbon and thus build up in the die. Any hard carbon in the extruder should be caught by the screen packs in the breaker plate.
It should also be obvious that routine disassembly of the die and extruders is necessary to maintain consistent quality for demanding applications such as barrier food packaging films. Having a spare die is very important for two reasons. First, it is the heart of the blown film system, and without it, you cannot make film. Second, when you do disassemble the extruder for cleaning and maintenance, having a clean spare die will greatly reduce the downtime associated with cleaning the die. That is, the spare die can be cleaned offline.
I consulted with the manufacturer, Gloucester Engineering in this case, and they confirmed my conclusions. I actually got my Plastics Engineering degree at UMASS-Lowell with Bill, the chief engineer at Gloucester, so it was fun to reconnect with him. Had this film manufacturer followed Bill’s advice, they would not have experienced the problems they did experience with their bags, and ultimately would not have gone out of business.
This is obviously an extreme case of the effect of die lines on product quality, but it shows the importance of having a knowledge of the degradation process, and applying that knowledge to the extrusion process to maintain product quality. Lastly, if you don’t have in-house expertise, it is important to hire outside help to get you through these important issues.