Workshops frequently record thermocouple faults with vague descriptions such as "temperature abnormal" or "sensor broken", making it impossible to quantify root cause proportions, optimize procurement standards or adjust maintenance cycles. Establishing a unified standardized defect classification system enables digital fault statistics, supplier quality evaluation and targeted preventive maintenance plans. This article divides all common thermocouple failures into 8 major categories with clear identification criteria and corresponding root cause labels.
1. Thermal Drift Defect
Judgment standard: Three-point calibration deviation exceeds grade allowable error; no obvious appearance damage, electrical resistance and insulation resistance meet standard limits.
Root cause tags: Long-term high-temperature alloy recrystallization, carbon layer thermal barrier, long mold standby zero drift, frequent thermal shock cycling.
Typical scene: Gradual temperature offset increasing week by week without alarm codes, plastic scorching/cold material lines emerging slowly.
2. Open-Circuit Fault
Judgment standard: Multimeter measures infinite loop resistance; controller displays ultra-high temperature open-circuit alarm code.
Root cause tags: Wire tension fracture, armored bend fatigue crack, terminal crimp separation, sensing head welding bead shedding, sheath corrosion perforation oxidizing alloy wire.
Typical scene: Sudden zone heating shutdown during production, no temperature reading on touchscreen.
3. Short-Circuit Fault
Judgment standard: Multimeter measures near-zero positive/negative wire resistance; controller pops ultra-low temperature alarm, heating coil runs full power continuously.
Root cause tags: Moisture infiltration pulverizing insulation powder, sheath abrasion wire contact, plug pin conductive carbon dust accumulation, corrosive vapor internal contamination.
Typical scene: Hot runner local overheating, plastic carbonization, occasional sudden machine halt.
4. Slow Response Lag Defect
Judgment standard: Temperature rise/fall delay exceeds 4 seconds; infrared contact probe comparison shows consistent reading offset with large time lag.
Root cause tags: Thick carbon deposit insulation layer, loose contact air gap, low-conductivity mold coating barrier, deteriorated internal insulation powder voids.
Typical scene: PID temperature curve fluctuates wildly, thin-wall product short shots and gate stringing.
5. Intermittent Signal Fluctuation Fault
Judgment standard: Temperature jumps ±2~3℃ randomly without regular pattern, no permanent open/short circuit.
Root cause tags: Vibration loose plug pins, insufficient shielding electromagnetic interference, internal insulation partial moisture, ground loop clutter noise.
Typical scene: Frequent false temperature abnormal alarms, unstable melt viscosity between molding cycles.
6. Appearance Mechanical Damage Defect
Judgment standard: Visible sheath crack, deformation, pitting perforation, embrittled outer wire sleeve, deformed flat sensing head.
Root cause tags: Improper disassembly prying, mold transport tension pulling, sharp mold edge abrasion, chemical corrosion pitting.
Typical scene: Hidden internal faults develop rapidly within 1–2 weeks after mechanical damage occurs.
7. Polarity Reversal Wiring Defect
Judgment standard: Temperature rises extremely slowly, controller continuously outputs full heating power; heat gun millivolt test reads negative voltage value.
Root cause tags: Mixed ANSI/IEC wire color codes, careless maintenance plug replacement, mismatched extension wire splicing.
Typical scene: Severe hot runner overheating without obvious sensor hardware damage.
8. Aging Service Life Limit Defect
Judgment standard: Cumulative working hours reach production load service life cap; multiple calibrations show increasing drift, resistance slightly elevated.
Root cause tags: Natural alloy wire aging after long-term continuous production, irreversible insulation powder degradation.
Typical scene: Frequent calibration required to maintain process tolerance, high risk of sudden open/short circuit.
Workshop statistical application rules. Assign a unified two-digit defect code to each category for MES system data entry during maintenance record filing. At the end of each month, count the proportion of each defect type: if thermal drift accounts for over 40% of total faults, upgrade to N-type anti-drift thermocouples and shorten carbon cleaning cycles; if open-circuit faults dominate, optimize wiring tension protection and vibration isolation measures. Supplier quality evaluation uses defect frequency data: brands with high mechanical damage and drift rates are marked for procurement volume reduction or supplier replacement.
