Extrusion die drool: What you can—and can't—do about it

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January 14, 2016

"Drool" is a disturbing word, conjuring up images of rabid dogs and senile people. However, it does not upset everyone, as there is a beer named Moose Drool, which enough people drink to keep the company (Big Sky in Montana) in business. Anyway, I'm talking here about what are more politely called die deposits—the bits of molten plastic that stick to the face of the die and, occasionally, drop off onto the product.

This can cause serious problems, or no problem at all, if the product is black and surface blemishes don't matter in its application. For the rest, the result may be undesirable streaks or just bits of dark (oxidized) plastic that are visible on the product surface. If the product is subject to stresses, the bits can act as stress concentrators and lead to earlier or lower-stress failure. A classic case was a failure of a gas distribution pipe in Bowie, MD, where the installers bent the pipe over a rock in the trench, which sufficiently stressed the pipe at an "inclusion" for it to crack and leak enough gas into a home to fuel a fatal explosion.

Such inclusions at the inner surface of a pipe or tube (irreverently called "boogers") are especially nasty because they can't be seen, but just drop off the die onto the inner surface.

Did we make them or did they come in the resin or compound? It takes some time for the drool to accumulate, so if the problem was evident from the very beginning, it may have been in the feed. If it takes some hours to develop, it is not the material's fault . . . unless the compound is inadequately stabilized and, thus, more likely to degrade with heat. So, for many materials, such as PVC and some polyolefins, we may want a stability requirement to keep the die drool down.

What is it? The "magic" word here is Q-tip. Collect some of the drool on a Q-tip (that's a Unilever trademark, but any cotton swab will do) and have it analyzed. It is likely to be an oxidized low-molecular-weight polymer, but may also include some low-boiling additives. Running a cooler melt, especially at the die lips, will help, but it isn't always consistent with high-speed production. And if the additive content can be changed to a less-volatile one, that may help, too. If a concentrate is used, don't forget the carrier resin, which might be the culprit and may be changeable.
Letting it form but cleaning it off is a common strategy. This is usually done by hand, with the aforementioned Q-tips, but I've also seen a motorized device crossing and re-crossing the line where the extrudate emerges.

Do die materials and die angles matter? Fluorocarbon coatings may work but don't last long. Fluoroplastic die lips are too soft. Chrome plating on steel dies may help, and I suspect someone has studied such effects, but I haven't seen it yet. I like the idea of porous fluoroplastic-impregnated metal structures, but don't have enough evidence to promote it. Likewise with radius, or lack of same, at the die lips. Die buyers who plan to use easy-drooling materials should ask proposed manufacturers about angles and surfaces.

What does work for some polyolefins is a fluoroplastic-based processing aid, which lubricates the die lands and discourages the curl-up-and-out phenomenon that leads to die deposits. Another useful remedy is a thin stream of air directed at the lip-resin contact line.

 

Allan Griff is a veteran extrusion engineer, starting out in tech service for a major resin supplier, and working on his own now for many years, as a consultant, expert witness in law cases, and especially as an educator via webinars and seminars, both public and in-house. One-day extrusion seminars are scheduled this month in the Chicago area near O'Hare on Jan. 26 and in Atlanta near the airport on Jan. 28. Go to this article in PlasticsToday for further information about these in-person sessions. Griff wrote the first practical extrusion book back in the 1960s as well as the Plastics Extrusion Operating Manual, updated almost every year, and available in Spanish and French as well as English. Find out more on his website, www.griffex.com, or email him at algriff@griffex.com.

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