Thermocouple sensors in hot runner systems are exposed to a range of environmental conditions. Condensation from water-cooling systems and humidity in plant air can cause moisture to seep into the thermocouple junction, affecting accuracy and accelerating corrosion. Understanding this impact is essential for maintaining sensor integrity and ensuring reliable temperature measurement.
The Mechanism of Moisture Ingress. Thermocouple mineral insulation (magnesium oxide) is hygroscopic-it absorbs moisture from the air. When the probe cools below the dew point, condensation forms on the sheath. Over time, moisture migrates through microscopic gaps in the sheath or connector seals into the MgO insulation, reducing its electrical resistance and altering the thermoelectric signal.
How Moisture Affects Readings. Moisture in the MgO creates electrical leakage paths between the thermocouple wires and the sheath. This shunts part of the thermoelectric signal to ground, causing the controller to read lower than actual temperature-typically 2–10°C low depending on moisture content. This is a gradual effect, often mistaken for sensor drift.
Accelerated Corrosion. Moisture combined with heat accelerates corrosion of both the sheath and the internal alloy wires. Corrosion products further degrade insulation resistance and can cause complete open-circuit failure. In aggressive environments with chloride or acidic gases, moisture acts as a carrier that accelerates chemical attack.
Inspection Signs. Visual indicators of moisture damage include greenish or white powder deposits on the connector pins, rust-colored stains near the sheath termination, and a cloudy appearance of the connector insulation. The affected sensor will typically show low insulation resistance (below 10 MΩ) when tested with a megohmmeter.
Prevention Strategies. Use hermetically sealed thermocouples with moisture-proof terminations. Store spare thermocouples in sealed bags with desiccant. Avoid routing cables through areas prone to condensation. When the mold is idle for extended periods, keep the hot runner at a low holding temperature (50–80°C) to prevent condensation.
Recovery from Moisture Contamination. If a thermocouple has absorbed moisture but is not yet corroded, drying can restore performance. Bake the sensor at 100–120°C for 4–8 hours (with the connector end open to allow moisture to escape) before reinstallation. This process drives moisture out of the MgO insulation and can recover normal insulation resistance.
