Checking and Controlling Nozzle Body Temperature
Temperature control is critical from feedthroat to nozzle tip. Yet despite advances in machinery, this continues to be an issue for molders. Here’s what to do about it.
Injection molding machines continue to evolve in the electric, hydraulic and hybrid categories. Certainly, real improvements have been made. New presses are more energy efficient, faster and loaded with sophisticated software and controllers. Good news on efficiency and speed; not so sure there is such good news on controllers.
The bad news is some basic problem areas still seem to be ignored, such as on-stream melt-temperature sensing and the topic of this column, temperature control of the nozzle body. On-stream temperature sensing is no easy chore and I do not have an answer yet, but temperature control of the nozzle body is something else. Temperature control is critical from feedthroat to nozzle tip. Most machines control the heating zones nicely, but not the nozzle body. This is a major pet peeve of mine.
The nozzle body is often at a vastly different temperature than what the screen reports on the controller. What I am seeing lately for temperature control in the nozzle body is simply unacceptable. In checking six machines over the last two months, the difference between setpoint and actual has been near 40º C (85º F) for all of them. To make matters worse, the screen temperature agrees with the setpoint. The nozzle body is on every machine and is a critical element of every machine, yet it is apparently neglected by the industry.
The nozzle body is often at a vastly different temperature than what the screen reports on the controller.
First let’s make sure we are on the same page as to what a nozzle body is. Figure 1 shows a nozzle body (with heater band), nozzle tip, and thermocouple, which is common to nearly all injection molding machines.
Why take time to make sure the nozzle body is controlling temperature? Here are my reasons:
2. Gloss gradients;
5. Color variation;
6. Texture variation;
7. Inconsistent properties;
9. Cold slug in the nozzle tip;
10. Drooling or stringing;
11. Burn marks/streaks;
12. Asymmetrical filling;
13. Shot-to-shot variations;
17. Black specks.
Bottom line: This significant problem costs molders thousands of dollars a month. Adjusting the process does not fix incorrect or broken equipment. Few pay attention to it and it is easy to check or test out.
First, to convince yourself you may have a problem, take a walk through the production floor. Check where the thermocouple is located on the nozzle body and/or tip on a few of your machines. I bet you will notice there is no consistency. Sometimes the thermocouple is on the heater band, others have it in the hex nut or in a drilled hole in the hex nut, or on a clamp or the nut of the heater band. With no consistency in placement. you have no chance of providing a somewhat accurate melt temperature. The next step is to get some data. How do you get an actual reading of the melt temperature in the nozzle body?
It is not that difficult or time consuming. It may not be fun, but good temperature control will make your life easier on the shop floor and improve your profit margin. To check the actual melt temperature in the nozzle body and compare it to the setpoint on the controller, all you need is a common pyrometer and appropriate thermocouple. So, find your shop’s trusty pyrometer, find an appropriate wire or probe thermocouple (preferably one that is long enough to go all the way into the end cap), type J or K.
Next, pick a couple of machines and check them. If you have any machines with the thermocouple on the outside of the hex nut or on the clamp or heater band, they should be your first targets. Use the proper safety equipment—heat-protective gloves and goggles—and get a sense of the plastic temperatures and pressure you will be dealing with. Once ready, push the probe slowly into the nozzle tip through the nozzle body as far as you can get it, ideally all the way to the end cap. Wait until the temperature on the readout of the pyrometer stabilizes (stops climbing). It will take longer than you expect and you will learn why these types of probes do not provide correct melt temperatures of purges.
After the temperature stabilizes, pull the probe out 50 mm (2 in.) and take another reading, but wait until it stops trending. Continue until you measure the temperature of only the nozzle tip. Check yours out and send me the results, setpoint vs. actual temperatures. I will follow up with another article summarizing the results. So what’s next?
If your nozzle-body temperature is within 10o C (20o F) of setpoint, that is acceptable. Controlling the nozzle-body temperature is not easy to do, so if it is within this range compensate by changing the set temperature. If it is off by more than the above range, replace the thermocouple with a spade type. Location is critical. Measure the length of the nozzle body and tip. Put a spade-type thermocouple one-third the total distance of the nozzle body and tip, measuring from the nozzle tip. Use some thermal paste underneath it. Hold it in place with one full round wrap of a high-quality glass tape, then add three to four short pieces of glass tape over the thermocouple to insulate it. Now carefully put the heater band back on and make sure the thermocouple is in the gap of the heater band.
If your nozzle-body temperature is within 10o C (20o F) of setpoint, that is acceptable.
Once back up to appropriate melt temperature, check the actual temperature in the nozzle body as mentioned above. It will rarely be exactly at setpoint but it should be higher or lower than setpoint by less than 10o C (20o F).
In short, you cannot trust your machine for proper nozzle-body temperature control. Here’s some data for two recent nozzle-body checks I conducted:
Machine 1: setpoint 210 C (410 F); actual 257 C (495 F).
Machine 2: setpoint 304 C (580 F); actual 355 C (670 F).
Do not just believe me—check a couple of your machines. Save yourself some time and aggravation by reconfiguring the nozzle-body thermocouple. It will provide better melt-temperature control and consistency. This in turn will provide “Murphy” some naptime, and keep him from visiting you.
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