Temperature drift of thermocouples and heater aging failure produce similar abnormal molding phenomena including product warpage, color difference and unstable filling, which are easily confused by on-site technicians during troubleshooting. Misjudgment will lead to unnecessary replacement of intact heaters or ignoring gradually drifting probes, resulting in repeated mold disassembly and maintenance waste. Mastering multi-dimensional discrimination methods from temperature data, equipment test and physical inspection can quickly distinguish hidden thermocouple drift faults and heater system failures.
Judgment from temperature controller operation data is the fastest screening method. Hidden thermocouple drift features stable temperature readings without alarm codes, but there is a fixed constant difference between displayed temperature and actual melt temperature, and the deviation value gradually expands with production time. When adjusting the set temperature up and down by 5°C, the displayed value changes synchronously normally, only the benchmark reading has fixed offset. In contrast, heater aging failure shows slow temperature rise after startup, the controller outputs full power heating for a long time but cannot reach the set temperature, or temperature drops rapidly after cutting off heating power, with obvious slow heating response. Short-circuited heaters will trigger overheating alarm, while broken heater wires directly display heating zone temperature unable to rise, completely different from silent drift without alarm of thermocouples.
Cross-replacement test verification is the most accurate discrimination means under mold production conditions. Swap the suspected abnormal thermocouple with a brand-new fully calibrated probe of the same specification on the same heating zone. If after replacement, the temperature deviation disappears and product defects are eliminated, the original fault source is thermocouple drift. If abnormal temperature state remains unchanged after replacing a new probe, the problem lies in the heater, manifold heat conduction structure or temperature controller output module. For multi-zone manifold molds with multiple identical heating zones, exchange thermocouples between normal and abnormal cavities to observe whether the temperature deviation transfers to the new cavity following the probe, directly confirming sensor drift failure.
Mold cooling state static inspection can distinguish heat conduction failure and sensing drift. Shut down heating power, cool the mold to room temperature, then power on and set all zones to the same low temperature of 150°C to hold constant temperature for one hour. If the abnormal zone reading still maintains fixed deviation compared with other cavities after uniform low-temperature heating, it is confirmed as thermocouple drift, because low-temperature static heating eliminates uneven heat loss interference of the heater and manifold. If each zone temperature reading is consistent under low temperature holding, and deviation only appears after rising to production high temperature, the root cause is uneven heat conduction of the heater or manifold thermal deformation, not sensor drift.
Physical disassembly inspection after mold cooling serves as final confirmation. Pull out the suspected thermocouple and check the sensing tip for carbon deposits, loose springs and oxidized alloy wires; heavy carbon isolation layers, fatigue-lost springs and discolored welding points are typical drift-inducing damage. Remove the heater assembly to inspect heating wire breakage, local thinning and oxide scaling on the heating sleeve surface; uneven heating wire wear and thick oxide layers lead to insufficient heating efficiency, belonging to heater system failure without relation to thermocouple performance.
Misjudgment hazards are obvious: replacing intact heaters for drifting thermocouples increases spare parts cost and mold disassembly downtime; ignoring long-term thermocouple drift and continuously adjusting heater power parameters will aggravate plastic thermal degradation and batch defective products. Establish standardized troubleshooting flow: first perform probe cross-replacement test to exclude thermocouple drift before disassembling heaters for inspection, optimizing workshop fault handling efficiency.
