Alloy wire aging is an irreversible natural degradation process of hot runner thermocouples under long-term high-temperature load, leading to temperature drift, slow signal response and unstable readings. Many maintenance staff replace sensors only after open-circuit or short-circuit alarms, ignoring early aging signs that already cause mass defective products. Summarizing multi-dimensional judgment methods including appearance observation, multimeter testing, temperature calibration and operational curve analysis enables early identification of aging thermocouples and proactive replacement before quality failures occur.
Visual appearance inspection captures preliminary aging characteristics without disassembly. After long-term high-temperature baking, the outer sheath of aging thermocouples shows uniform dark oxidation discoloration; severe aging forms thick rust spots and pitting pits on the metal surface. The outgoing fiberglass protective sleeve turns brittle, appears cracked and sheds powder when gently folded. The terminal plug pins generate thick green oxide rust, which cannot be completely wiped off with clean cloth. For exposed sensing heads, aging alloy wires show thin brittle oxide layers at the welding bead; once touched slightly, the oxide layer peels off, exposing fragile internal metal wire. If any above appearance defects exist, the alloy wire inside has already undergone partial crystal transformation, and the thermocouple cannot recover original precision even after cleaning and calibration.
Multimeter static resistance testing reflects internal wire aging degree. Use a high-precision digital multimeter to measure the total loop resistance of thermocouple positive and negative wires, and compare with the standard resistance value of new same-model sensors of equal length. Aging alloy wires produce tiny metal oxide particles inside the sheath, increasing overall loop resistance significantly. If the measured resistance exceeds the standard value by over 15%, the alloy wire has obvious aging and will generate severe signal attenuation during operation. Meanwhile, switch the multimeter to insulation resistance mode; aging thermocouples with degraded magnesium oxide insulation will show insulation resistance lower than 500MΩ, bringing hidden short-circuit risks. This test is simple and efficient, suitable for batch inspection of spare thermocouples in inventory.
High-temperature calibration test is the most authoritative aging judgment standard. Conduct three-point temperature calibration at 200℃, 350℃ and 400℃ with a constant temperature dry block furnace, recording the deviation between tested thermocouple and standard reference sensor. New qualified K-type thermocouples maintain deviation within ±0.8℃; after serious aging, the drift error expands to ±2℃ or higher, and the deviation value increases obviously after 72-hour high-temperature aging simulation. N-type high-temperature thermocouples with aging alloy wires lose anti-drift performance, showing unstable error fluctuation at 420℃ ultra-high temperature point. Calibration data with excessive deviation confirms irreversible alloy wire crystal transformation, and such thermocouples must be scrapped directly instead of continuing to use after manual offset correction.
Dynamic temperature curve analysis judges real-time aging during production. Observe the temperature change curve on the hot runner controller touchscreen during heating and heat preservation stages. New intact thermocouples show smooth, stable curve without irregular fluctuations; aging sensors present slow temperature rise lag during heating, and small-range random jumps during constant temperature holding. When adjusting the heating power setpoint, the curve of aging thermocouples delays for more than 5 seconds before responding, which is a typical feature of oxidized alloy wires weakening thermoelectric signal transmission. If multiple batches of plastic parts have consistent scorching or incomplete filling defects accompanied by slow temperature response curve, the thermocouple alloy wire aging is the core root cause.
Classified disposal schemes for aging thermocouples avoid production risks. Slightly aging sensors with drift deviation within 1℃ can be temporarily used for low-precision packaging and toy molds after controller offset calibration, with shortened inspection cycle to every two weeks. Thermocouples with deviation over 1.5℃, increased loop resistance or brittle outer sleeves are prohibited from being used in automotive and medical precision molds and must be replaced immediately. Regular aging detection every month for continuous mass production lines can eliminate hidden quality hazards caused by degraded alloy wires fundamentally.
