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.

extrusion system with vital signs monitor

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

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