Extrusion basics: A primer on key process variables

In past columns, I've written about measurement of the key process variables in extrusion: Melt temperature, melt pressure and motor load . . . the vital signs. However, I shouldn't assume that all of you out there in Ex-Ex-Land remember it all, so here's a short review.

The inspiration for this subject was the need to measure my own body temperature to check for fever, as I have had an annoying cough for days. My HMO says to call if I am over 100.4° F (oral) for three days and arrange a phone consultation or personal doctor's visit. Last night's 102.4 concerned me, until I realized I just had a cup of hot tea. Sure enough, a few spoonfuls of cold yogurt later, it was down to 100.2°. Still sick, but not ER-sick. Lesson learned, and applied to extrusion: Know where you are measuring, what to expect and what may throw off the results.

horsepower
When it comes to motor load, if you are up to your limit in amps, check the speed range of transmission. Maybe motor adjustments or a set of retrofit pulleys can get your horses out of the stable. Image courtesy Sgbeer; file:Pferdestaerke.svg. creativecommons.org/licenses
/by-sa/3.0), via Wikimedia Commons.

There are no miracles in science, and changes in vital signs have their reasons, often interactive. A colder die temperature means higher back pressure, which means more heat development in the screw. Without experience and/or a computer, there’s no way to predict the temperature of what comes out. If melt temperature is measured way back at the screens or, worse yet, before the screens, it may be quite different at the exit of a large die; additional measurement near die lips or even of the extrudate itself (IR gun) may be in order. 

A significant change in melt temperature may mean a different resin (even if the melt index is the same), or a change in additive nature/amount/carrier, or a change in barrel/die heats, or a stuck-on or burnt-out heater or maybe just a 10% boost in line speed. As I’ve been saying to my seminar attendees for a long time: “If you know what good is, you’ll know what fishy is.”

Melt temperature data are deceptively consistent if measured at the same place and insertion depth, but the real temperature may be very different elsewhere. In some classic experiments done 20 to 30 years ago, variation across a circular flow path was shown to be as high as 55° F, and if it was only 10° F that was considered very good! Nevertheless, measurement at variable depth is rare; at two locations, it is even rarer. 

Why bother to measure melt temperature? If it’s too high, you risk degradation and black specks in product (this also depends on the die design and thermal stability of the resin). Hotter melt also makes cooling more difficult, may affect sizing in downstream equipment and can cause a premature reaction with certain additives and colors.

Melt pressure typically is measured at the screw tip and before the screens, as a guard against blowing off the head. While safety should, indeed, come first, we have other reasons for wanting to know melt pressure there. This value represents the resistance of the path downstream of the gauge, usually including screens, adapters and the whole head and die. If there is a gear pump, there are other pressure gauges at the pump. The one at the screw tip relates to melt temperature, as the higher the resistance the screw has to work against, the more power is needed and the more energy in the form of heat is generated in the screw.

Melt pressure depends on die design, to be sure, as that’s the pathway through which the melt has to flow, but it also depends on formulation, as process aids and lubricants can reduce the resistance to flow; fillers and crosslinkers can increase melt pressure; and resin selection sets the starting point. Also, tighter screens remove more contamination and raise back pressure and thus improve mixing, but they also increase melt temperature and require more frequent changing. 

Today’s instruments can easily produce continuous records of melt pressure—the problems are getting the right people to look at them and setting alarms that will sound an alert if values get over or under aim limits.

Motor load was the least complicated of the vitals when DC motors were everywhere. Now AC reigns, and there are different kinds of AC, which means for every drive someone has to set a limit (amps or percentage of load—be sure which one you’re using!) and get it visible next to the display. Better yet is two limits: A yellow warning—“load getting too high, say something to someone”—and a red limit—“motor in danger, shut down as soon as possible.”

A final bit of maybe-good news: If it looks like you are up to your limit in amps and can’t do anything with formulation or conditions to help, all is not lost. Check the speed range of transmission; you may not be up to your limit in HP or kW, and maybe motor adjustments or a set of retrofit pulleys can get your horses out of the stable. Incidentally, I think that cars should all have a picture of a horse on the inside of the hood to remind us how much better things are today.

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. He 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 e-mail him at algriff@griffex.com.

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