Most injection molding workshop sites are not equipped with professional thermocouple calibration furnaces and precision resistance testers, and frontline technicians can only rely on temperature controller display data, visual inspection and simple multimeter tools to quickly judge various signal abnormalities of hot runner thermocouples. Mastering intuitive judgment methods without high-precision testing equipment can quickly locate faulty zones during mold trial run and mass production, reduce mold downtime caused by sensor failure, and avoid mass defective products caused by unrecognized temperature drift faults.
The first judgment method is to read the fault code and temperature display value of the temperature controller, which is the most direct screening means. When the controller screen shows the maximum temperature value and pops up TC open circuit alarm code, it can be directly judged that the internal alloy wire of the thermocouple has broken an open circuit. At this time, the heating zone will stop heating automatically, and the melt temperature will drop rapidly, resulting in short shot and cold material blockage. If the screen always displays zero temperature and triggers low-temperature alarm, it is judged as thermocouple short circuit: the positive and negative wires inside the sheath are in contact, and the heater will keep heating at full power without temperature feedback, which is easy to cause nozzle melt burning and carbonization. When the temperature value jumps up and down continuously without stable fixed value, the core problem is poor contact or electromagnetic interference. Technicians can temporarily separate the thermocouple signal wire from the heater power wire for test; if the temperature returns to stable, it can be confirmed that the signal wire is interfered by high-current power lines. If the display temperature is always 5~15°C lower or higher than the actual process set temperature without alarm, it belongs to hidden temperature drift fault, which cannot be eliminated by adjusting controller parameters and needs to be cleaned or replaced.
The second intuitive judgment means is visual inspection of thermocouple physical components under mold shutdown cooling state. After cooling the mold to room temperature, pull out the thermocouple plug outside the mold plate and check the terminal metal shrapnel: if there is white oxide powder on the surface, it will cause poor contact and intermittent signal failure, and the oxide layer needs to be polished with fine sandpaper. Check the outer sheath of the probe for scratches, perforations and melt burn traces; small holes on the sheath will lead to melt entering the interior and triggering short circuit after heating. Observe the bending part of the cable for cracking of the outer braided layer and exposed internal wires; exposed alloy wires are easy to contact the mold steel to form a short circuit. For spring bayonet probes, pull the cable gently to check whether the spring tip can stretch and rebound normally; if the spring loses elasticity and cannot pop out tightly against the hole wall, a gap will be formed to cause temperature drift. The integrated heater-thermocouple assembly can check the wire outlet of the heating sleeve for torsion deformation; long-term torsion will crack the internal thermoelectric wires and lead to intermittent open circuit.
The third simple auxiliary detection is resistance measurement with a portable multimeter, which is the most common basic tool in the workshop. Adjust the multimeter to the resistance gear, connect the two test pens to the positive and negative terminals of the thermocouple plug respectively. If the resistance value is infinite, the internal wire is completely broken, and the probe is scrapped directly. If the resistance value is extremely close to zero, the internal two wires are short-circuited. Normal intact thermocouples have a stable resistance value according to the wire length: the longer the cable, the higher the resistance, and the fluctuation range will not exceed 10% under normal conditions. If the resistance value fluctuates continuously when shaking the cable, it indicates that the internal wire has a hidden break point, which will form an open circuit after heating and thermal expansion, and such probes need to be replaced in advance. When distinguishing K-type and J-type miswiring faults, you can touch the sensing tip of the probe with your hand to heat it slightly; if the displayed temperature drops instead of rising, it proves that the positive and negative wires are reversed, and the wiring terminals only need to be swapped.
The fourth auxiliary discrimination method is process contrast verification during mold trial production. After replacing a new thermocouple for a faulty heating zone, if the product defects such as flow marks, warpage and color difference disappear, it can be confirmed that the original thermocouple had drift or contact failure. When multiple heating zones of the same manifold have inconsistent temperature readings under the same set parameters, the probes with obvious temperature deviation are judged to have inconsistent calibration accuracy and need unified calibration. These simple judgment methods do not rely on professional calibration equipment, and frontline operators can quickly troubleshoot thermocouple faults on site, greatly improving the efficiency of mold maintenance and production line fault handling.
