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Taking Your Temperature(s): A Degree of Difference

January 27, 2016

Melt Temperature.  Of all the hundreds of processing variables to control in injection molding, the temperature of the plastic, especially its melt temperature, is paramount, no doubt.  After all, temperature control is critical, and you should duplicate the plastic melt temperature as much as possible, from one run to another.
But molders forget about barrel temperature, which can be a more accurate measurement of what’s going on in your plastic injection molding machine.  “Can be” is the operative phrase here; many times barrel temperature can be misleading.

Consider a piece of a barrel with a typical thermocouple well and a typical thermocouple. Many times the contact area is not positioned properly.  Forget the spring mechanism, the real issue is you might find your thermocouple in the middle of the hole and not touching anything. The result? It is measuring only ambient air temperature.

As a result, many molders do not differentiate between the actual plastic temperature and the temperature shown on the temperature-control monitor. They are not the same and can differ by as much as 90 degrees Fahrenheit. This makes the true melt temperature impossible to measure during production. Worse yet, most molders interrupt the cycle and purge plastic into the air to take a measurement, a procedure that is almost guaranteed to introduce inaccuracy. What’s really needed is a method to measure melt temperature while the machine is running.

What we’re really talking about is improving barrel-temperature measurement and control. We’re not saying barrel-zone temperatures influence melt temperature — they do.  But most agree that about 20% of the energy to melt the plastic comes from the heater bands, which are vital to the process, especially at the beginning of it. So how can you ensure you are reading barrel temperature correctly?

The concept seems simple enough: Just use a thermocouple to measure the steel temperature and control the heater band with an ambient-temperature-reference PID (Proportional Integral Derivative) controller.  Nothing shocking here.  You can use J- or K-type thermocouples (TCs), as both have the same accuracy.

Make sure the wires aren’t damaged — no splices — and are routed so that molten plastic does not splash on them. In many cases, regrettably, the thermocouples and electrical wires hanging down from them, have gloppy plastic hanging from them. This increases downtime, which increases costs, not to speak of wire replacement, safety, and frustration. Fortunately, there is a simple solution:  just tie them up on the side not facing the operator.

More could be said about measuring temperature in the injection molding process, but that’s the subject of another blog post.


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