Reasons and Improvement Measures for Low Temperature Feedback of Thermocouples

A low temperature feedback fault is a common aberrant temperature measurement event in manufacturing, when the temperature value displayed by the controller is consistently lower than the actual operating temperature within the hot runner. This flaw will result in ongoing overcompensation heating of the system, invisible thermal breakdown of plastic melt, and several quality-related hidden risks to injection molding items.

Excessive carbon buildup on the probe surface is the most frequent motivation. Plastic breakdown remnants stick to the sensor head and create thick, hard carbon layers during prolonged high-temperature work. Because of the carbon deposit's low thermal conductivity, heat transfer between the thermocouple probe and the hot runner metal matrix is isolated. Persistent low temperature feedback results from the sensing element's inability to record actual high temperature data. When making recycled materials and easily carbonized modified plastics, this behavior is particularly dangerous.

Another significant factor is an unreasonable installation state. When the insertion depth is too shallow, the probe only detects the low-temperature position on the hot runner's surface and is unable to reach the core high-temperature area. Poor contact gaps caused by oxide scales and sundries in the installation hole will slow down the rate of heat conduction and result in a low detected temperature. This temperature deviation issue will also be made worse by loose thread fixation and a small displacement of the probe during vibration.

Low temperature readings can also be caused by heat dissipation interference from the external environment. The temperature sensor is either directly facing the cooling air flow in the workshop or too near to the cooling water circuit. The feedback temperature is lower than the actual melt temperature inside the flow channel due to rapid heat dissipation, which lowers the surrounding temperature. Fixed systematic low temperature deviation will result from long-term operation.

Such errors may also be caused by internal performance aging. Long-term use causes aging thermocouples' thermoelectric conversion efficiency to drop, their thermal drift value to rise, and their output signal to consistently be low, all of which cannot be fixed with straightforward parameter adjustments.

Targeted improvement efforts aimed at the aforementioned flaws must be implemented promptly. To completely remove surface carbon deposits and restore the probe's smooth heat conduction, disassemble the thermocouples on a regular basis. To guaranty close contact, re-standardize the installation depth, clean the hole of contaminants, and tighten the thread. To prevent direct heat dissipation interference, change the direction of the workshop air outflow and the mold water flow. Enter appropriate compensation values and promptly calibrate temperature deviation data. Replace old thermocouples directly if their performance has significantly declined. To prevent hidden risks of material scorching and product defects caused by low temperature feedback, do a constant temperature test after troubleshooting to make sure the displayed temperature matches the real temperature.