Hot junction oxidation is the primary root of silent temperature drift for thermocouples used in high-temperature hot runners processing PEEK, LCP, PPS and glass fiber reinforced engineering plastics. The welding point where positive and negative alloy wires connect is exposed to continuous high-temperature radiation and corrosive plastic volatile gas, gradually forming thick oxide layers on the alloy surface that change thermoelectric potential output, creating fixed measurement deviation without any controller alarm. If oxidation is not treated in time, the oxide layer will expand to form micro cracks, evolving into intermittent hidden open-circuit faults that disrupt mass production. A complete set of prevention, regular cleaning and repair solutions targets hot junction oxidation for long-term high-temperature molding molds.
Three core prevention designs to slow hot junction oxidation during thermocouple customization. First, vacuum-sealed hot junction welding process. Conventional open-air welding leaves tiny air bubbles inside the welding point; under high temperature, residual oxygen reacts with chromel-alumel alloy to generate oxide rapidly. Vacuum welding eliminates internal air, forming a dense uniform welding joint without internal oxidation channels, slowing oxide layer growth by over 60%. All high-temperature dedicated K-type thermocouples must adopt vacuum welding, while cheap entry-level probes use open-air welding with poor anti-oxidation performance. Second, high-temperature anti-oxidation passivation coating on the welding point surface. After vacuum welding, a thin inert protective film is plated on the hot junction surface, isolating corrosive sulfide and halogen gas decomposed from plastics from contacting the alloy matrix, cutting off chemical oxidation reactions. Third, fully sealed mineral-insulated probe structure. The magnesium oxide filling inside the sheath tightly wraps the hot junction, preventing external corrosive gas from penetrating the probe interior to erode the welding point; split un-sealed probes with gaps at the sheath front end suffer accelerated hot junction oxidation within one month of high-temperature production.
Standardized weekly cleaning operation for moderately oxidized hot junctions. Shut down and fully cool the mold, remove the thermocouple probe and wipe the sensing tip with alcohol-dampened dust-free cloth to clear carbon deposits attached to the hot junction surface. Carbon residues contain corrosive gas that accelerates oxidation, so thorough removal delays oxide accumulation. For probes with thin uniform oxide films without cracks, soak the tip in weak alkaline cleaning fluid for 3–5 minutes (only for 316L and Hastelloy sheath probes), rinse repeatedly with deionized water and bake dry at 80°C to eliminate residual liquid that causes secondary oxidation. This cleaning method restores partial measurement accuracy for slightly oxidized probes and extends service life by 1–2 months.
Replacement judgment standard for severely oxidized hot junctions. After cleaning, observe the welding point surface: dark gray, black brittle oxide layers covering more than half of the hot junction, visible micro cracks and loose welding structures mean irreversible severe oxidation. Even if the probe still conducts electricity normally, the thermoelectric potential has deviated seriously, and the temperature drift will expand rapidly within two weeks of production. Such probes cannot be reused and must be scrapped directly to avoid batch defective products caused by inaccurate temperature feedback. For 24-hour continuous high-temperature production molds, implement monthly visual inspection of hot junctions to screen severely oxidized probes in advance.
Daily production auxiliary anti-oxidation operation specifications. After daily production shutdown, maintain hot runner temperature at 200°C for 30 minutes to volatilize residual corrosive gas inside flow channels, preventing gas from wrapping the thermocouple hot junction during mold cooling and triggering overnight oxidation. Avoid frequent rapid heating and cooling thermal shock cycles, as alternating stress accelerates oxide layer cracking and internal alloy corrosion. When selecting thermocouples for high-temperature molds, completely abandon J-type iron wire probes; iron hot junctions oxidize completely within weeks under temperatures above 700°C, leading to permanent failure.
Combining vacuum welding sealed probe selection, regular weekly cleaning and standardized shutdown exhaust procedures can effectively slow hot junction oxidation speed, reducing the replacement frequency of high-temperature hot runner thermocouples and cutting long-term spare part procurement costs for engineering plastic molding factories.
