Individual thermocouple probes suffer partial melting and breakage of the measuring junction after long-term operation in high-temperature hot runner zones, completely cutting off the thermoelectric signal loop and triggering permanent open-circuit faults. Many maintenance personnel only replace the damaged probe without finding the root inducement, leading to repeated melting damage of new replacement thermocouples in the same temperature zone. This article summarizes four core inducements of measuring junction melting and targeted radical solutions.
Inducement one: Long-term severe temperature overshoot far exceeding the thermocouple rated temperature resistance. The fundamental reason for continuous overshoot is poor contact thermal resistance of the measuring base or spring ring; the sensor feeds false low-temperature signals to the controller, making the heater output full power for a long time. The local temperature of the measuring junction exceeds 500℃, surpassing the long-term temperature limit of K/J alloy wires, and the thermoelectric junction alloy melts gradually. Typical phenomenon: the corresponding hot runner zone has frequent material burning and salivation defects long before the thermocouple melts, and the actual metal temperature measured by a portable thermometer is 80–120℃ higher than the controller displayed value. Radical solution: disassemble and polish the measuring contact surface to remove carbon deposits, apply thermal conductive paste and retighten fasteners to eliminate thermal resistance, restore normal temperature closed-loop adjustment to avoid overshoot.
Inducement two: Local electric arc heating at the measuring junction caused by sheath short-circuit. When the thermocouple sheath has micro-cracks and internal insulation powder absorbs moisture, the positive alloy wire contacts the metal sheath to form a short-circuit loop. The tiny contact point generates high-temperature electric arc under weak thermoelectric current, instantly melting the measuring junction alloy. Precursor abnormal signs include frequent short-circuit alarms of the temperature zone, irregular jumping temperature readings and gradual decline of insulation resistance detected by megohmmeter. Solution: Replace the laser welded fully sealed thermocouple to block moisture and gas penetration, optimize wire layout to avoid sheath abrasion damage, regularly test insulation resistance to screen damp probes in advance.
Inducement three: Chemical high-temperature corrosion thinning the measuring junction alloy. Molds processing PVC, high-halogen flame retardant plastics and sulfur-containing recycled materials release corrosive gas that continuously erodes the measuring junction alloy at high temperature, thinning the alloy wire year by year. After long-term corrosion, the cross-sectional area of the thermoelectric junction becomes too small, and it melts and breaks under the same heating temperature that originally could work normally. Prevention scheme: Uniformly adopt Inconel sheath anti-corrosion coated thermocouples for corrosive material production lines, shorten the cleaning cycle of measuring points to remove corrosive residue deposits, and replace probes with obvious corrosion thinning every 4 months.
Inducement four: Improper welding process of inferior thermocouple measuring junctions. Low-cost inferior thermocouples adopt spot welding with small welding area instead of full circumferential welding, the connection strength between alloy wire and sheath wall is insufficient, and the heat conduction efficiency is poor. The local temperature of the tiny welding point accumulates sharply under normal hot runner heating, eventually melting the welding junction. New probes with this hidden danger usually fail within 1–2 months after installation. Solution: When purchasing spare parts, only select full laser welded measuring junction thermocouples, carry out spot check welding quality with a microscope during batch incoming inspection, reject spot-welded inferior products.
Standard post-failure handling process after measuring junction melting. After discovering melting damage, do not directly install a new thermocouple for trial production. First detect the actual maximum temperature of the hot runner zone to confirm whether there is long-term overshoot; test the insulation resistance of the hot runner heating coil to rule out leakage short-circuit; check whether the processing raw material releases strong corrosive volatile gas; eliminate all root inducements before installing a new sealed high-quality thermocouple, to avoid repeated melting damage of replacement probes and repeated mold shutdown losses.
