Is It Hot Enough for You?

Measuring heat is one of the, or perhaps THE critical parameter for reliable plastics processing.

Everyone knows how to use their household thermostat, so it’s natural to expect you can simply forget it exists after setting the temperature.

If only plastics processing was that simple!

In reality, however, it’s about measuring a range of temperatures oscillating above and below the magic set points, then adding just enough current to the heaters. Plus, molders may have to heat — and monitor — everywhere from the sprue to hot runners, which could be further than a metre from the nozzle.

I can’t stand complex things, so let’s break it down to the simplest possible concepts.

We use temperature to measure the effect of adding heat to the melt. But keep in mind that temperature and heat are not the same thing.

The resin is heated by electric heaters as well as through the mechanical shearing and pressing action of the screw, and also during part of the injection cycle.

Don’t overlook the contribution of the screw’s mechanical action to the heating process; thermoset molders sometimes cool the barrel to prevent their resin from setting prematurely. However, thermoplastic molders, like us, add heat, then measure temperature to discover how much heat we’ve added and how much we still need.

METAL BONDS

Thermocouples measure the temperature of the various zones along the barrel and within hot runner systems.

Essentially they are simply tiny slugs made of two different metals that are bonded together with a lead wire attached to each metal. They work by a bizarre property of physics — the Seebeck Effect — whereby heating the junction of two dissimilar metals generates a tiny electric current.

Just to note, the current is generated at the junction, and not in the lead wires — an important distinction. Insulating or sleeving leads is a good idea to prevent shorts and abrasion but in heat terms, the only electrical property that changes in the leads is resistance, and even that effect is usually negligible for plastics processing.

POPULAR MISCONCEPTIONS

The other important thing to consider is that the thermocouple effect only works where there is a temperature gradient across a dissimilar metal junction.

If both sides of the junction are at the same temperature, nothing happens, so get ready to forget another widely held misconception about thermocouples: You can solder crimp, or splice leads with dissimilar materials without affecting the thermocouple’s output, provided the splice’s junction is at an even temperature.

But if constant temperature around the joint means no thermocouple effect, i.e., no current, how do we measure temperature when the thermocouple’s business end — the junction — is jammed up against a uniformly hot barrel or runner surface?

The answer lies within a little circuit contained within the sensor body, which electrically simulates a “cold” — actually only slightly less hot — side to keep the junction operating. It works through a sort of bridge circuit using resistors with carefully chosen properties, mainly the resistor’s ability change its resistance with changing temperature.

It’s basically self-compensating and uses surprisingly little power. Plus, the whole circuit is sealed into the cartridge and most techs don’t even know it’s there.

HANDLE WITH (SOME) CARE

And from a maintenance perspective, they’re easy to replace and are rugged enough that you don’t have to handle them like plutonium — although I have seen them fractured with rough handling.

Although standard types are available, I like to use the same brand of thermocouple throughout the entire zone I’m trying to control.

This strategy simply removes one variable out of troubleshooting and means redundant technical literature is available, along with one-call advice if you’re really in trouble. And believe me, trouble with temperature is easy to get into, as I’ll talk to you about in September’s issue.