How Are Temperature Sensors Calibrated in a Hot Runner System?

Calibrating temperature sensors in a hot runner system is essential to ensure that the displayed temperature accurately reflects the true temperature of the melt channel. The calibration process can be performed at three levels: factory calibration, in-situ calibration, and reference verification. Factory calibration is performed by the thermocouple manufacturer using a temperature-controlled bath or dry-block calibrator that is traceable to national standards. Each thermocouple is tested at several points (e.g., 100°C, 200°C, 300°C, 400°C) and the deviation is recorded on a calibration certificate. This is the baseline-it confirms that the sensor meets its tolerance class when it leaves the factory. However, installation, aging, and the environment can shift this baseline, so periodic in-situ calibration is recommended. In-situ calibration involves comparing the thermocouple's reading with a high-accuracy reference sensor placed as close as possible to the measuring junction. This can be done by inserting a reference probe into a separate hole adjacent to the working thermocouple, or by using a surface pyrometer at the same location (though surface measurement is less accurate). The system is heated to a stable operating temperature, and the deviation between the controller reading and the reference reading is noted. If the deviation is constant (e.g., the controller reads 3°C high), the controller's offset parameter can be adjusted to correct it. For more rigorous calibration, the hot runner is turned off, cooled, and the thermocouple is removed and tested in a portable dry-block calibrator on-site. This is more accurate but requires downtime. The third level is reference verification, where a certified temperature signal simulator is connected to the controller's input. The simulator generates a precise millivolt signal equivalent to the desired temperature (e.g., 250°C for Type K), and the controller's displayed value is checked. This verifies the controller's input circuit, but not the thermocouple itself. A common mistake is to rely solely on controller verification without checking the sensor. Best practice is to perform in-situ calibration quarterly and a full sensor calibration annually. All calibration records should be stored, particularly in medical and automotive applications where traceability is required. Calibration can be performed by the plant's maintenance team if they have the proper equipment and training, or by an external accredited laboratory. Using a calibration service that provides a certificate with uncertainty data gives the highest confidence. In summary, calibration is not a one-time event but an ongoing process that ensures the hot runner's temperature control remains within the tight tolerances demanded by modern molding.