Many hot runner molds are kept in low-temperature heat standby for days or weeks during order intervals, and thermocouples under long-term constant low heat will produce hidden zero drift, resulting in inaccurate temperature readings after resuming formal production. Zero drift refers to the fixed offset error existing even when the hot runner is close to room temperature, which is easily mistaken for controller parameter deviation and cannot be eliminated by simple on-site offset correction. This article analyzes the formation mechanism of standby zero drift and provides complete elimination and preventive solutions.
Long-term low-temperature heat preservation causes slow alloy wire microstructure change, which is the core cause of zero drift. When the thermocouple is maintained at 180–220℃ standby temperature for a long time without molding cycle temperature fluctuation, the internal nickel alloy wire produces uniform slow recrystallization without thermal shock relief. Tiny oxide precipitates accumulate on the grain boundary, changing the zero-point thermoelectric potential difference at room temperature. When the mold is reheated to production temperature, the original zero-point offset is superimposed on the working temperature measurement value, forming a fixed positive or negative drift of 1–2℃. K-type thermocouples are most prone to this standby drift, while N-type rare earth alloy wires have 70% lower zero drift tendency under the same standby conditions.
Secondary inducement: sealed wire outlet internal moisture accumulation during long standby. The mold standby temperature is lower than normal production temperature, and the workshop humid cold air continuously penetrates the tiny gaps of the thermocouple wire outlet sealing layer. Moisture adheres to the alloy wire surface inside the sheath, forming thin conductive oxide films at room temperature, which shift the zero-point millivolt output value. After formal heating, the moisture evaporates but the oxide film remains, leading to persistent zero drift that cannot be eliminated by drying alone. Generic thermocouples without double-layer silicone sealing have far more serious zero drift after long standby than high-standard sealed sensors.
Third factor: carbon deposit layer static thermal aging on the sensing head. Under long low-temperature standby, the thin carbon film attached to the thermocouple surface undergoes slow thermal aging and densification, forming a stable heat insulation layer. At room temperature, the carbon layer does not affect zero-point detection, but once heated to production temperature, the thermal resistance difference generates fixed measurement offset, which is essentially zero drift derived from heat transfer barriers. Molds processing flame-retardant and recycled plastics accumulate aging carbon layers faster, and zero drift after standby is more obvious.
Standard zero drift inspection steps before resuming production. After long standby, do not directly heat up to molding temperature. First cut off the heating power and cool the hot runner completely to ambient temperature (20–25℃). Use a calibrated heat gun constant temperature block to test each thermocouple at 25℃ room temperature standard point, record the displayed value of the controller. If the reading deviates more than ±0.8℃ from the standard room temperature value, zero drift is confirmed. Then perform a two-stage heating test: heat to 200℃ and hold for 30 minutes, then heat to production temperature, continuously track temperature deviation changes to distinguish alloy wire drift from carbon layer drift.
Classified zero drift elimination solutions.
1. Alloy wire inherent zero drift: Complete three-cycle high and low temperature alternating aging treatment. Heat the hot runner to 380℃ and hold for 1 hour, then cool naturally to room temperature, repeat three times. The temperature alternating shock eliminates part of the alloy recrystallization stress and reduces zero drift offset. If the offset still exceeds ±1℃ after aging treatment, the thermocouple has irreversible internal damage and must be replaced.
2. Carbon layer induced zero drift: Fully disassemble the thermocouple, remove all carbon deposits on the sensing head and sheath with neutral cleaning agent and copper brush, dry completely and re-coat thin thermal conductive paste before reinstallation. This type of drift can be completely eliminated after cleaning.
3. Moisture-induced zero drift: Bake the entire thermocouple at 120℃ for 2 hours to fully evaporate internal moisture, reapply high-temperature sealing glue at the wire outlet to block subsequent moisture penetration.
Preventive operation specifications to avoid standby zero drift.
First, optimize mold standby temperature strategy. Avoid long-term fixed low-temperature heat preservation; if standby exceeds 48 hours, turn off all hot runner heating power completely instead of maintaining low heat. For short standby within 48 hours, reduce the standby temperature to below 150℃ to slow alloy recrystallization speed.
Second, pre-clean thermocouples before long mold standby. Remove surface carbon deposits to prevent static aging of carbon layers during heat preservation.
Third, select high-sealed N-type thermocouples for molds requiring frequent long standby, which inherently suppress zero drift caused by alloy and moisture factors.
Fourth, carry out zero-point calibration at room temperature every time production resumes after standby, and adjust the controller cold junction offset in advance to avoid mass defective products caused by drift.
