Thermocouple mounting screw torque directly determines contact tightness between the sensing junction and hot runner metal, while excessive torque crushes internal probe structures and insufficient torque forms insulating air gaps; standardized torque adjustment specifications are the core installation operation standard to balance stable heat transfer and long probe service life, differentiated for spring nozzle thermocouples, ring washer manifold thermocouples and embedded deep-hole probes.
Spring-loaded nozzle tip thermocouples have the strictest torque control requirements, with standard recommended torque range of 0.8–1.2 N·m. The built-in compression spring inside the probe needs reserved elastic stroke to offset thermal expansion displacement of the nozzle during heating. If technicians tighten screws with torque exceeding 1.5 N·m, the flat sensing head is pressed hard against the nozzle core metal, compressing the internal spring to the limit and crushing the thin dissimilar metal sensing junction inside the armored sheath. Tiny cracks appear on the alloy junction after over-tightening, leading to gradual signal drift and eventual TC OPEN alarms after repeated thermal cycles. Conversely, torque below 0.6 N·m cannot maintain stable spring compression force; after the nozzle expands thermally at high temperature, loose contact gaps form between the sensing head and nozzle wall, generating temperature reading deviation of 10–25℃. During installation, torque-limited screwdrivers must be used instead of ordinary hand screwdrivers relying on subjective force feeling to avoid human error.
Surface ring washer manifold thermocouples adopt a medium torque standard of 1.5–2.0 N·m. The metal washer needs uniform pressure to fit closely with the manifold measuring boss surface and eliminate air gaps between the flat washer and manifold metal. Torque lower than 1.2 N·m causes uneven washer contact, with local gaps accumulating metal shavings and release agent residue that form thermal barriers. Excessive torque above 2.5 N·m deforms the thin metal ring washer, bending the edge upward and creating permanent uneven contact surfaces that cannot recover after mold cooling, requiring replacement of the washer and probe together. Before tightening, the manifold measuring boss surface must be polished clean without burrs or plastic flash to ensure uniform pressure distribution on the washer.
Embedded deep-hole manifold thermocouples fixed by side locking screws use low torque of 0.6–1.0 N·m. The armored sheath is inserted into the measuring hole with reserved 0.3mm axial clearance to accommodate manifold thermal expansion. Over-tightening the side locking screw with torque over 1.2 N·m scratches the polished sheath outer wall, wearing down the internal magnesium oxide insulation layer and inducing short-circuit hidden dangers. Insufficient locking torque allows the probe to shift slightly inside the deep hole during thermal expansion, moving the sensing junction away from the central melt flow channel and distorting real melt temperature feedback.
Standard torque operation steps unify on-site installation standards. First, manually screw the mounting screw until the washer or probe head touches the metal surface without pressure. Second, use a calibrated torque screwdriver to tighten to the specified torque value in one steady rotation, avoiding repeated forward and backward twisting that loosens contact pressure. Third, after completing mold heating to production temperature and holding for 30 minutes, execute a secondary torque inspection after the manifold fully expands thermally to compensate for slight pressure loss caused by metal expansion.
Recording torque values in mold maintenance logs after each thermocouple replacement facilitates fault tracing when temperature drift occurs later. Strictly implementing classified torque adjustment standards for different thermocouple structures eliminates two major installation faults: over-tightening crushed junctions and loose contact air gaps, stabilizing long-term temperature measurement accuracy and extending the service life of hot runner thermocouple probes by more than 80%.
