Inappropriate choice of thermocouple design, a real problem for Slovenian plastics converters

THERMOCOUPLE TIP – IT DOESN’T MATTER WHETHER THE PLUG-IN THERMOCOUPLE HAS A FLAT OR TAPERED TIP!

FIRST REVIEW of thermocouple design:

1. The surface on which the thermocouple was pressing was flat and the user was using a tapered design of plug-in thermocouples (118°). This resulted in a lack of proper loading at the measuring point and a lack of proper heat extraction from the measurement device (the plastic injection nozzle) to the measurement device (the thermocouple, which in this case serves as the temperature sensor).

Plug-in thermocouple with tapered tip for 118° bores.	Instead of a bore hole, the tool design was prepared for surface measurement.

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2. The smallest nozzle on the plastic injection moulding tool was over-impacted due to the small mass of the gauge. Therefore, the effect of the heater on the thermocouple was greater than the effect of the heater on the tool (in terms of the mass of the gauge and the mass of the gauge). As a result, the temperature was lower in the larger plots and higher in the smaller plot (last nozzle).

The smallest nozzle on the plastic injection moulding tool, with a plug-in thermocouple.

ELPRO LEPENIK – AN EXAMPLE OF GOOD PRACTICE

WHAT HAVE WE DONE?

1. The plug-in thermocouples have been changed to a flat-tip model to achieve a perfect fit at the measuring point. The temperature stabilised at ±2 °C, which is a very good result considering the surface measurement without additional insulation.
2. A clamping thermocouple was installed on the smallest nozzle, increasing the surface area of the thermocouple and resulting in an accurate control process.
3. Additionally, the actual surface temperature was measured using the comparison method and a correction was entered into the SHINKO controller to calibrate the display and the actual heating zone temperature – NOTE: the correction entered using the comparison method can only be carried out by a professionally qualified person, with the appropriate training and measurement equipment.

demonstration of the installed clamp-on thermocouple and comparative measurements with the special surface thermocouple TT-POV-TCK4.2x100R400x1000SS	display of the surface thermocouple on the TFN 520 thermometer and comparison with the display of the measuring system comparison: 176,8 °C measuring system 175 °C

ADVANTAGES OF THE SOLUTION

The advantages of working with the ELPRO Lepenik team, in this case:

• we have a 30-year relationship with the user, we know their processes and requirements (long-standing cooperation in different forms of organisation requires professionalism, understanding and excellent communication from both sides – which can result in selfless help and quick resolution of complex cases)
• the rapid response of our professionally trained staff – the execution was treated as an emergency, the damage to the products was extremely high, the line was at a standstill; the delivery of the products to the car manufacturer was jeopardised
• quick identification of the problem (based on years of experience – if you know what you are looking at you can quickly find out what is wrong; a relatively simple matter is often unsolvable for the lay user)
• field measurements of the extruder temperature were made with the appropriate knowledge and equipment and the user’s assumptions and testimony were confirmed
• with our own production of temperature sensors, new thermocouples (in this case calibrated to confirm the final measurements) were ordered for urgent production
• installation and simultaneous adjustment of the control (SHINKO controllers have been offered to the Slovenian market since 1996)
• field correction of temperature – on site calibration (to confirm the solution)
• we trained the user, who shared the good practice example with the toolmaker (The design flaw is with the toolmaker (there was no hole and the thermocouple could not be installed correctly). The user was not sufficiently trained on the correct choice of thermocouple, so they used what they had in stock. They have stock for other tools, such as those that actually use tapered plug-in thermocouples.)

CONCLUSION

The set and actual values on the temperature controller displays were “uniform” throughout the process, with no more hot spots and no more hot spots. The quality of the product is perfect, no more rejections. The inappropriate choice of thermocouple design has been resolved in this way.

Authors Rok Samec and Aleksandra Lepenik
March 2023

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Tel: +386 2 62 96 720.

They are always available for opinions and explanations:

• Measurement expert Zoran Lepenik
• Aleksandra Lepenik, expert in temperature measurements