Mold purging is essential to clean carbonized plastic residue inside hot runner channels, but improper purging material selection, temperature setting and operation steps will scratch, corrode or contaminate thermocouple probes, accelerating signal drift and shortening service life drastically. Many operators only focus on cleaning effect of the runner and ignore protection of temperature sensors during purging, generating hidden thermocouple faults that surface after several production shifts.
The most serious damage source is abrasive high-filler purging compounds. Some factories use glass fiber-filled cleaning materials to remove thick carbon deposits; high-speed flowing abrasive purging melt scours the exposed thermocouple sensing head at nozzle gates, creating deep linear scratches on the sheath surface within one purging cycle. The scratched outer wall loses anti-corrosion and heat transfer performance, forming local thin-wall areas that are easily worn through during subsequent formal production. Ultra-miniature flat button thermocouples for micro molds suffer irreversible surface abrasion after one abrasive purging operation, requiring direct replacement. Non-abrasive pure resin purging materials without mineral or glass filler are the only permitted cleaning medium for molds equipped with precision thermocouples.
Excessively high purging temperature constitutes the second major damage factor. Operators often raise manifold and nozzle temperature by 30–50℃ above normal production parameters to accelerate carbon melting during purging. Long-term ultra-high temperature over 400℃ intensifies thermal oxidation scaling on thermocouple sheath surfaces, forming thick black insulating oxide layers that cause permanent temperature offset. For J-type iron constantan probes, over-temperature purging triggers rapid junction oxidation, generating severe zero drift that cannot be calibrated afterward. Standard purging specifications limit heating temperature to no more than 20℃ above regular production setpoints, with continuous high-temperature holding time controlled within 15 minutes.
Improper purging operation steps cause mechanical contamination and collision damage. Many technicians increase injection pressure and speed to strengthen cleaning effect; high-pressure fast-flowing purging melt carries solid carbon particles to impact spring-loaded nozzle thermocouple heads, denting the flat contact surface and weakening internal spring elasticity. After purging, residual purging material mixed with carbon residue solidifies on probe sensing junctions, forming hard composite contamination layers that block heat transfer. Direct air gun blowing without probe disassembly drives fine carbon dust into manifold deep measuring holes, accumulating around embedded thermocouple sheaths and creating persistent thermal resistance barriers.
Corrosive chemical cleaning agents bring hidden sheath corrosion risks. Strong acid mold carbon cleaning solvents sprayed inside manifolds penetrate tiny gaps of thermocouple sheaths during soaking, etching pinholes on stainless steel surfaces and oxidizing internal alloy junctions. If chemical solvent is not fully purged and dried before mold reheating, volatile acid vapor accelerates long-term probe corrosion during production cycles. When chemical cleaning is unavoidable, all thermocouple probes must be disassembled and sealed in dry storage boxes in advance.
Standard protective purging procedures eliminate thermocouple damage fundamentally: adopt non-abrasive purging resin, strictly control purging temperature and holding time, reduce injection pressure and speed appropriately, disassemble exposed nozzle spring probes before chemical cleaning, and fully blow clean manifold measuring holes with dry nitrogen after each purging cycle. Regulating mold purging operations according to protective standards avoids unnecessary thermocouple abrasion, oxidation and contamination, extending sensor service life by more than half and reducing post-purging temperature instability defects.
