How Does Thermocouple Cold Junction Compensation Stabilize Hot Runner Temperature Accuracy

Most moulders concentrate on the quality of the thermocouple probe, but neglect the important effect that cold junction compensation has on the accuracy of the hot runner temperature measurement. The basic functioning principle of thermocouples is based on the thermoelectric potential generated by the temperature difference between the hot end and the cold end. The hot end is close to the high-temperature runner and the cold end is set at the terminal of temperature controller, which is easily affected by the workshop ambient temperature, seasonal alternation, air conditioning airflow and equipment heat radiation. If the cold junction correction is not accurate, even the high-precision imported thermocouples will have apparent drift, which will break the thermal balance of the manifold and the nozzle.

Cold junction compensation is an algorithm that is integrated in the hot runner temperature controller and a hardware module. It detects the ambient temperature of the cold end in real time, automatically calculates the possible error caused by the fluctuation of the cold end temperature, and corrects the temperature display and output power. The cold end potential fluctuates simultaneously with the increase or decrease of the ambient temperature of the workshop. The compensation module can compensate the interference in real time, so that the thermoelectric potential difference only represents the actual temperature of the hot runner, and false high or false low temperature feedback is avoided.

For hot runner systems with multiple cavities or zones, the thermocouples have a lengthy wiring distance and the cold ends are distributed. The ambient temperature at the terminal of each channel is varied, causing uneven drift of each channel without adjustment. Each temperature control channel may be compensated separately with high precision for each . It ensures the temperature constancy of each zone and each cavity, reducing filling imbalance and product weight deviation. In the medical and automotive moulds with high tolerance requirements, the precision of cold junction compensation is strongly related to the qualification rate and batch stability of the product.

Improper use habits will also reduce compensation effect, such as direct air blowing to the terminal row, overheating of surrounding power equipment and mixed arrangement of signal lines and power lines, which will create cold end temperature disorder and compensation failure. The cold end should be placed in a closed constant-temperature wiring box, avoiding the influence of air flow and heat radiation, and the original thermocouple should be matched to guarantee that the thermoelectric characteristic curve is consistent with the controller compensation database. Long-term drift-free temperature regulation can only be achieved by perfect coordination of high-quality thermocouples and high-precision cold-junction compensation.