How Humidity and Temperature Cycling Accelerate Thermocouple Internal Insulation Failure

Alternating temperature and humidity cycling is a composite environmental stress that leads to significant damage to magnesium oxide insulation inside armored thermocouples. This is a factor of aging that is easily neglected in comparison with single high temperature or single humidity damage. The alternating cycle of cold-hot and dry-wet continuously leads to the condensation of water vapor in the sheath, which gradually decomposes the compact insulation powder, reduces the insulation resistance, and induces the intermittent short-circuit fault. This article describes the process of damage cycle and focused preventative measures for hot runner workshops with significant temperature and humidity swings.

Stage 1: High temperature heating stage, the internal air thermal expansion discharges part of the air from the wire outlet gap, while plastic volatile acid gas enters into the sheath interior. High temperature acid vapors condense on the surface of magnesium oxide insulation particles in order to generate small conductive salt impurities. At this level the insulating resistance is marginally reduced with no apparent indicators of malfunction.

Stage 2: The internal air shrinks in the cooling down of mold shutdown, and the external humid air in workshop is sucked back into the sheath through the wire outlet sealing gap. Water vapor reacts with acid salt impurities bound to the insulation powder and forms conductive electrolyte solution. The insulation resistance between the positive and the negative alloy wires is lowered remarkably. The controller shows erratic low-temperature short-circuit alarms from time to time, which would disappear after reheating and evaporating moisture, making the maintenance workers mistake as transitory signal interference.

Stage 3: Accumulation of conductive electrolyte residues via repeated Cold-hot cycling. After dozens of heating-cooling cycles, the electrolyte residue hardens between insulation particles in order to establish permanent conductive channels, and the insulation resistance drops down to less than 300MΩ for a long period. At present, the thermocouple will cause a continuous short-circuit fault during heating, resulting in the heating zone outputting full power, causing hot runner overheating and plastic carbonization. If corrosion causes micro-perforations of the sheath, the rate of penetration of moisture and acid gasses will increase more than 3 times and the insulation will fail in 1-2 months.

1) Important workshop environments that increase the damage of temperature-humidity cycling: • Coastal injection molding workshops with high air humidity throughout the year; • Workshops with mold water-cooling pipes arranged near hot runner molds, producing a lot of water vapor; • Workshops in seasonal rainy seasons with poor ventilation; • Molds that frequently start and stop production every day, leading to multiple cold-hot cycles each shift. Low-cost generic thermocouples with single-layer thin silicone sealing at the wire outlet are particularly susceptible to this composite damage, whereas high-standard double-layer sealed thermocouples may block more than 80% of vapor infiltration.

Multi-level protection solutions for gradual insulation breakdown.

1. Thermocouple grade selection priority. For production lines with high humidity and frequent starts and stops, consider products with double-layer high-temperature silicone sealing at the wire outlet and Inconel anti-corrosion sheath. Avoid cheap single-seal 304 stainless steel sheath sensors.

2. Control of workshop environment: installation of dehumidifiers to ensure relative humidity of 40%-65% RH; heat insulation baffles to isolate mold cooling water pipelines to reduce water vapor volatilization around the mold area; strengthening of workshop ventilation to timely discharge acid plastic volatile gas

3. Optimization of operation process Reduce frequent mold startup and shutdown. If the shutdown period is more than 30 minutes, reduce the hot runner heat preservation temperature to slow down the internal thermal expansion and contraction cycle of the thermocouple sheath. Slow evaporation of internal residual moisture during segmented low temperature stages heat up after extended term shutdown and cooling before entrance to formal production temperature.

4. New regular inspection items: Monthly maintenance of thermocouple, insulation resistance test using a megohmmeter, if the resistance value is less than 500MΩ, clean and dry the thermocouple to remove internal moisture. If the resistance value cannot be recovered after drying, replace the thermocouple immediately.

5. Protection of spare parts storage: It is recommend to keep the standby thermocouples in sealed moisture proof boxes with desiccant to prevent absorption of moisture before installation.