Mold purging is a necessary process to clean carbonized residue inside hot runner channels, but excessive purging frequency, prolonged high-temperature holding time and excessive injection pressure during purging will produce serious abrasion, oxidation and contamination damage to thermocouple probes, shortening the service life of armored sensors by more than half. Many production operators pursue thorough cleaning effect and execute over-purging operations every single shift, ignoring the irreversible damage to nozzle spring thermocouples and embedded manifold probes caused by long-term repeated purging cycles.
Excessive purging frequency brings continuous abrasive scouring damage to nozzle gate thermocouples. Even non-abrasive pure resin purging materials carry suspended tiny carbon particles stripped from runner walls during high-speed flow. Each purging cycle impacts the exposed sensing head of spring probes, forming uniform micro-scratches on the sheath surface. If purging is performed every 8-hour shift, the polished sheath protective layer is worn off within one month of continuous production, exposing the internal alloy base metal to plastic corrosive vapor and accelerating oxidation drift. For high-filler original production materials, residual glass fiber powder mixed in purging melt will deepen scratch abrasion on the probe surface, quickly wearing thin the sheath wall and inducing short-circuit hidden dangers.
Prolonged high-temperature holding during over-purging aggravates thermocouple thermal oxidation scaling. Operators often maintain purging temperature 40–60℃ higher than normal production parameters for 30–60 minutes to fully melt thick carbon deposits. Long-term ultra-high temperature above 400℃ forms thick loose black oxide scaling layers on the thermocouple sheath surface, which act as permanent thermal insulation barriers and cause fixed negative temperature offset that cannot be eliminated by calibration. J-type iron constantan probes suffer the most serious damage under over-temperature purging; the internal sensing junction oxidizes rapidly, generating large zero-point drift after only three consecutive over-purging operations.
Excessively high injection pressure and flow speed during purging deform spring thermocouple structures. High-pressure fast-flowing purging melt produces strong instantaneous impact force on the flat sensing head, denting the contact surface and compressing the internal spring to the limit repeatedly. After dozens of over-pressure purging cycles, the spring loses elastic rebound ability, cannot maintain tight contact between the sensing junction and nozzle core after heating expansion, forming persistent air gap temperature fluctuation faults. Offset bent probes for valve gate nozzles are also prone to bending deformation under high-pressure melt impact, leading to collision with reciprocating valve needles in subsequent formal production.
Over-purging also accumulates composite hard contamination layers on probe surfaces. Mixed carbon residue and cured purging resin adhere tightly to the sensing head after cooling, forming a hard film that cannot be wiped off by ordinary alcohol cleaning. Each over-purging cycle thickens the contamination layer, continuously increasing thermal resistance and forcing the controller to run heaters at full power, creating a vicious cycle of overheating and accelerated carbonization.
Standardized limited purging specifications reduce thermocouple wear fundamentally: set purging frequency to once every 24 hours for general molds and once every 48 hours for low-carbon low-temperature PP/PE molds; control purging temperature to no more than 20℃ above production setpoint with maximum holding time limited to 15 minutes; appropriately reduce injection pressure and speed during purging to weaken melt impact on nozzle probes. After each purging cycle, disassemble exposed spring probes and wipe off surface carbon residue with polishing pads before resuming production. Controlling purging frequency and parameters within standard ranges avoids unnecessary thermocouple abrasion and oxidation damage, extending the stable service cycle of hot runner temperature sensors significantly.
