Long-term high-temperature use, vibration corrosion and repeated disassembly will cause precision attenuation of hot runner thermocouples, and regular on-site calibration is the core means to ensure long-term stable temperature detection accuracy. Many injection molding factories only replace sensors after faults occur, lacking standardized regular calibration procedures, leading to long-term hidden temperature deviation and mass defective products. Complete on-site calibration of thermocouples can be completed with a portable constant temperature calibrator and multimeter, divided into static bench calibration and mold on-machine dynamic calibration two processes.
Static bench calibration is suitable for new incoming thermocouple inspection and spare parts precision screening before installation. Place the thermocouple measuring tip and standard calibrated reference thermocouple together into the constant temperature tank of the portable calibrator, set three gradient temperature points covering the actual production temperature interval of the mold: low temperature 100℃, medium temperature 250℃, high temperature 380℃. After the temperature of each point stabilizes for 15 minutes, record the temperature reading difference between the detected thermocouple and the standard reference sensor. For Class 1 automotive and medical thermocouples, the allowable error at each temperature point shall not exceed ±0.5℃; Class 2 packaging ordinary sensors allow error within ±1.0℃. Any probe exceeding the error range is marked as unqualified and returned or scrapped, avoiding installation of low-precision sensors on precision molds.
Mold on-machine dynamic calibration is aimed at thermocouples already installed on hot runners, eliminating error interference caused by installation gaps and manifold radiant heat. Heat the hot runner system to the normal production set temperature, keep constant temperature for 2 hours to reach thermal balance state. Insert the probe of portable standard contact thermometer into the reserved measuring hole of the manifold or closely attach it to the nozzle outer wall at the thermocouple installation position, record the actual metal temperature, and compare it with the temperature value displayed by the hot runner controller. If the deviation exceeds the allowable standard, two processing methods can be adopted: fine-tune the cold junction compensation parameter offset value of the controller channel to correct the reading, or disassemble the thermocouple to check whether there is carbon deposit on the contact surface leading to heat conduction delay.
Annual full batch calibration cycle specification is formulated according to production load. Molds operating 24 hours continuous mass production need full thermocouple calibration every 6 months; molds with single-shift intermittent production can implement one calibration per year. After calibration, record the error data of each sensor in the mold equipment file, mark the probes with obvious precision attenuation, and arrange batch replacement in the next shutdown maintenance window.
Common calibration misoperations that affect accuracy should be avoided. Do not conduct calibration when the hot runner is not fully thermally balanced, transient unstable temperature will lead to false error judgment; do not calibrate only at a single temperature point, local small-range precision cannot represent full working interval performance; do not use uncalibrated portable thermometers as reference standards, the reference equipment itself needs annual official metrology calibration to guarantee data credibility.
On-site calibration has low cost and simple operation, which can timely screen aging and drifted thermocouples before mass defects appear, maintaining stable temperature control accuracy of hot runner systems and reducing raw material waste and mold shutdown loss caused by inaccurate temperature measurement.
