What Risks Does Reverse Thermocouple Polarity Bring to Hot Runner Production

Reversal of thermocouple polarity is a subtle but high risk hidden fault in hot runner system wiring that is commonly overlooked in mould commissioning and mass production. Different from evident open circuit or short circuit failures, the polarity reversal will not directly activate an alert, but continually distort the temperature feedback, mislead the PID temperature control and cause a series of plastic moulding flaws.

The inverted temperature potential signals at the thermocouple outputs are produced by the reversed positive and negative wires. The temperature controller misestimates the current temperature condition and automatically reduces the heating power when the melt is still low, or retains the heating continually when the nozzle and the manifold reach the set temperature. Such aberrant control logic easily generates two extreme situations: 1) Insufficient temperature: cold material obstruction, short shot and flow mark. 2) Overheating: material carbonisation, yellowing, wire pulling and salivation at the gate.

For multi-cavity balanced moulds, a reversed polarity in one zone will disturb the entire thermal balance. The anomalous temperature of individual cavities would result in variable melt viscosity and filling speed, leading to obvious weight variation, dimensional out-of-tolerance and uneven shrinkage of batch products. In severe circumstances, the long-term overheating will accelerate the ageing of the thermocouple wires and heater coils, therefore lowering the service life of the hot runner accessories and raising the cost of maintenance.

Polarity reversal also influences subsequent process optimisation and mould data inheritance. Process engineers can neither get true and correct temperature curve data, hence cannot set up stable and repeatable moulding parameters. And when the mould is transferred to other production lines, the same aberrant faults will continuously occur. Therefore, it is vital to thoroughly examine the colour coding and terminal labelling of thermocouples before power-on, inspect the temperature increase curve one by one during low-temperature preheating and eliminate polarity concealed risks in advance.