Why Does Temperature Protection Fail in Hot Runner Systems?

Temperature protection function failure in hot runner systems is essentially the result of coordinated failure of four subsystems: sensing, control, actuation, and environment. The cause is not an isolated event, but rather an inevitable manifestation of long-term stress accumulation and design flaws. Sensor Layer Failure: Signal Distortion Thermocouple material oxidation causes thermoelectric potential drift. K-type thermocouple negative electrodes undergo selective oxidation during long-term operation above 200°C, with grain boundary corrosion leading to an irreversible decrease in the Seebeck coefficient, drifting approximately 0.3–0.5mV every 1000 hours. Insulation layer carbonization creates conductive paths. High-temperature melt leakage or localized overheating causes carbonization of magnesium oxide insulation, creating a micro-short circuit that bypasses the signal, resulting in temperature readings 30–80°C lower than actual. Electromagnetic coupling interference from inverters and servo drives couples to millivolt-level thermocouple signals through unshielded cables, with superimposed noise peaks reaching 50–200mV, far exceeding temperature controller resolution. Execution Layer Failure: Relay Sticking Relay sticking is the trigger for energy runaway. Each switching action generates a micro-arc; high temperature causes contact material (silver alloy) to evaporate and deposit on the opposite electrode, forming a metal bridge structure. Even when a power-off command is issued, current continues to conduct. The cold-state resistance of heating coils is only 1/10 of hot-state resistance, so instantaneous current during startup can reach 8–12 times rated value. Mechanical relay contacts are subjected to impacts far exceeding rated load, accelerating welding. Oil mist and moisture in injection molding workshops condense inside relay housings, forming electrolyte films that trigger micro-current corrosion, reducing contact resistance and promoting sticking. High ambient humidity combined with frequent start-stop and no surge suppression circuit increases relay sticking probability by over 300%. Control Layer Failure: Software and Parameter Drift PID parameters not retuned cause temperature overshoot reaching ±15℃. Aging heating coils increase thermal inertia; original parameters cannot match new thermal characteristics. Memory overflow and watchdog failure from poor heat dissipation of main control chips cause program stack overflow and RTOS deadlock, freezing control commands. Communication protocol conflicts between Modbus RTU and TCP on the same bus cause alarm signal loss. Significantly, 78% of false alarms stem from not executing Auto-tune, not sensor malfunctions-parameters are dynamic, not set and permanent. Power Layer Failure: Voltage Protection Absence Lack of voltage protection is a fatal vulnerability. Voltage spikes and surges damage controller power supplies and relay drivers, causing unpredictable behavior and complete system failure. Prevention Strategy Preventing temperature protection failure requires systematic attention to all four layers: use armored double-shielded thermocouples with 5–8 year lifespan; implement surge suppression circuits; perform Auto-tune after any mold or heater change; and maintain proper grounding and shielding.