All answers are courtesy of Allan Griff, extrusion expert and the host of our webinar series. Don't miss the rest of the series; the next webinar, scheduled for February 24, will have Allan discussing what happens between the end of the screw and the die: screening, mixing, pumping, and the rest. The registration link for that event is already active here.

Today's article covers two processors' questions pertaining to measuring melt temperature.

Question: What is the value of flush-mounted thermocouples for measuring melt temperature?
AG: Extruders don't normally have flush-mounted melt thermocouples in the adapter or die. The only ones I've seen are the rather common and rather useless combination gauges, where a thermocouple is inside the pressure gauge at the screw tip. There was an infrared gauge sold 20-30 years ago (Vanzetti), but it quietly disappeared.

The problem is usually PVC, where there is fear that material caught behind the probe will initiate degradation. That is technically possible, but most systems have other places where degradation is more likely -- bends, long spaces after the breaker plate, and the like. If the probe is the main concern, you are doing a lot of other things right. I am more comfortable with knowing the melt temperature, even for PVC, and suggest a variable-depth device with minimal penetration in its "rest" position, but the ability to be adjusted to move away from the wall and toward the center of the stream.

You may be surprised at the variation from center to wall. There were two classic SPE papers written on this topic in the 1980s; they discovered as much as 55ºF (30ºC) variation in some cases, and concluded that if there is as little as 10 F (6 C) variation, you're doing very well.

Indirect methods may be useful, too - an infra-red sensor on the extrudate as it leaves the die, or manipulation of an adapter heater control to find the temperature that it just turns on and quickly off again.

Question: We use an infrared sensor to measure melt temperature as the extrudate comes out of the die. What else can we use to not only monitor the melt temperature, but also collect data for analysis?
AG: Melt thermocouples work just like the metal-sensing thermocouples, but their tip is immersed in the melt (see my slides 24-27). On the most useful ones you can adjust the probe depth, but these also are the least rugged and require added attention in use. Pay attention to where the probe is placed - it should be somewhere in the adapter after the screens. There is a temptation to combine it with the pressure gauge and put it into the hole at the screw tip. Pressure reading will still be OK, but temperature won't be representative. It's better than nothing, as you can still compare data, but can't trust their absolute values.

Regarding data collection, check with the maker of the controllers as well as the extruder itself. There are a few specialized companies who can do this, too.

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