Many mold factories repeatedly clean, calibrate and patch aging and damaged thermocouples to save spare parts cost, but repaired faulty sensors will repeatedly break down in a short time, triggering unplanned downtime and batch defective products. Clear scrapping judgment standards can help maintenance personnel distinguish repairable minor faults and irreversible aging damage, avoid ineffective maintenance labor waste and hidden production risks. This article summarizes multi-dimensional scrapping judgment indicators from appearance, electrical performance, calibration precision, internal structural damage and service life limit.
Appearance irreversible damage scrapping standards. The thermocouple must be directly scrapped when any of the following appearance defects occur: the armored sheath has obvious crack, deformation, pitting perforation or large-area oxidation rust spots that cannot be removed by polishing; the outer fiberglass/PTFE protective sleeve is severely embrittled, powdered and cracked, and the internal shielding braid and alloy wire are exposed; the sensing head welding bead has fallen off, the flat contact surface is permanently deformed and cannot fit the measuring plane; the terminal plug pin is severely corroded, broken or deformed, and cannot form stable contact after polishing and anti-rust treatment. Minor surface carbon deposits and slight plug pin oxidation belong to repairable faults, which can be reused after cleaning; perforated sheath, cracked wire sleeve and fallen sensing head are irreversible damage, and repair cannot restore the original protection and heat transfer performance.
Electrical performance index exceeding the limit scrapping standards. Use multimeter and megohmmeter to test two core electrical indicators: loop resistance and insulation resistance. If the total loop resistance of the thermocouple exceeds the standard value of the same model new product by more than 20%, it means the internal alloy wire is seriously oxidized and thinned, and must be scrapped; slight resistance increase within 10% can be calibrated and temporarily used for low-precision molds. When the insulation resistance measured under 500V DC is lower than 300MΩ at room temperature and cannot recover after full drying and cleaning, it indicates that the internal magnesium oxide insulation powder has pulverized and absorbed moisture, and hidden short-circuit risks exist, which cannot be repaired and must be scrapped. Repeated open-circuit and intermittent short-circuit faults after cleaning and drying prove that the internal wire has multiple fracture points, and patch repair will lead to repeated faults, so direct scrapping is required.
Calibration precision drift over-limit scrapping standards. After three-point high-temperature furnace calibration, if the temperature measurement deviation exceeds the grade allowable error for two consecutive calibrations, the thermocouple is judged to be irreversibly aging and scrapped. Specific limit values: K-type ordinary thermocouple deviation over ±1.5℃, N-type high-temperature thermocouple over ±1.2℃, E-type medical precision thermocouple over ±0.8℃. Even if the deviation can be offset by adjusting the controller channel parameter, the internal alloy wire recrystallization will make the drift error expand rapidly within one month, and frequent repeated calibration will waste maintenance time and cannot guarantee stable product quality, so it is not recommended to continue use. After 72-hour high-temperature aging simulation test, the deviation increase exceeds 0.5℃, which also meets the scrapping standard, indicating poor anti-drift performance and unsuitable for continuous mass production.
Internal structural fatigue damage scrapping standards. Flat-head and button thermocouples with completely failed elastic compression springs that cannot maintain tight contact after spring replacement are scrapped; the contact structure has produced permanent deformation and cannot eliminate air insulation gaps. Armored thermocouples that have undergone multiple bending repairs and still have internal wire fracture hidden dangers after fatigue test are scrapped; the bending section alloy wire has formed fatigue brittle layers, which will break again under slight vibration. Thermocouples used for corrosive plastic molding with invisible micro-perforations on the sheath detected by insulation test are scrapped; corrosive gas has polluted internal insulation powder, and cleaning cannot remove contaminated impurities.
Service life limit mandatory scrapping rules formulated by production load. Three-shift 24-hour continuous mass production thermocouples have a maximum service life limit of 6 months; two-shift production limit 12 months; single-shift intermittent production limit 18 months; discontinuous trial mold thermocouples shall not exceed 24 months even if no obvious faults appear. After reaching the service life limit, the internal alloy wire has accumulated enough aging crystal transformation, and hidden drift and short-circuit risks will break out at any time, so mandatory scrapping replacement is required regardless of appearance and test data. Medical and automotive high-precision mold thermocouples shorten the service life limit by half to ensure zero hidden quality hazards.
Classified disposal of scrapped thermocouples. Scrapped sensors cannot be mixed with ordinary mold spare parts for repeated use; collect them in special metal waste boxes, classify them by sheath alloy material for centralized metal recycling, and record the scrapping serial number, reason and date in the spare parts account to realize full life cycle traceability. Strictly prohibit refurbishing scrapped thermocouples for production use to avoid mass production quality accidents caused by hidden damage.
