Internal alloy wire breakage inside MI thermocouple sheaths is a frequent sudden production fault; disassembly inspection wastes plenty of maintenance time, while experienced technicians can quickly judge wire breakage types and breakage positions through controller display signals, equipment abnormal performance and simple multimeter testing without removing the mold. Thermocouple wire breakage is divided into complete open circuit, partial semi-break virtual connection and multi-segment broken wire three categories, with distinct external manifestations respectively.
Complete open circuit shows the most obvious controller alarm: the screen continuously displays "OPEN" fault code, corresponding heating zone stops heating completely, and no temperature reading changes regardless of actual manifold temperature. This fault originates from fully fractured internal positive or negative thermoelectric wires, mostly caused by severe sheath extrusion, long-term tensile fatigue or overheating oxidation melting of low-grade alloy wires. Multimeter continuity testing shows no conduction buzz between two terminal lugs, confirming thorough wire breakage requiring full sensor replacement.
Partial semi-break virtual connection is a hidden hard-to-judge fault, leading to intermittent temperature signal loss. Typical phenomena: temperature readings suddenly drop by 30–80℃ randomly during production, recover normally after mold vibration or slight wire pulling, and no stable open-circuit alarm appears. The internal alloy wire only retains a tiny contact section after fatigue cracking; vibration and thermal expansion separate the fracture temporarily, cutting thermoelectric signal transmission. During static shutdown testing, multimeter continuity may show normal conduction, misleading technicians to rule out thermocouple faults. Effective identification method: gently bend the MI sheath section by section while observing multimeter microvolt readings; sudden voltage disappearance at a certain bending position marks the semi-break point inside that sheath segment.
Multi-segment broken wires mostly occur on ultra-long thermocouple wires over 3 meters used for large automotive and new energy molds, with multiple fatigue fractures formed after long-distance reciprocating pulling. External characteristics include extremely unstable temperature readings jumping up and down without rules, frequent random low-temperature alarms that disappear after wire rearrangement. Multimeter testing shows intermittent conduction with irregular microvolt fluctuations when heating the sensing tip, and replacing the thermocouple extension wire alone cannot eliminate the fault, proving breakage exists inside the MI sheath instead of external connecting cables.
Breakage position judgment relies on segmented bending and thermal excitation testing. If abnormal signals appear when bending the wire outlet close to the manifold, the fracture locates at the sheath root suffering repeated tensile stress from thermal expansion and contraction; if signal failure occurs only when bending the middle wire section, the wire is broken at the mold wiring groove friction position; unstable readings concentrated at terminal lugs indicate crimping virtual disconnection instead of internal sheath wire fracture.
Preventive measures target different breakage root causes: reserve sufficient wire slack to avoid tensile fatigue, lay protective sleeves in wiring grooves to prevent friction sheath damage, and replace J-type iron alloy patch sensors with sealed MI K-type models for high-temperature working conditions to avoid oxidation wire breakage. Mastering rapid wire breakage judgment methods shortens troubleshooting time from 1–2 hours to within 15 minutes, drastically reducing production downtime losses caused by thermocouple internal fracture faults.
