In hot runner mold maintenance and accessory replacement, random matching of thermocouple models by appearance has become a common human error. Many operators only pay attention to probe length, lead diameter and appearance shape, ignoring the essential differences in alloy characteristics, temperature resistance grade and compensation parameters between different models. Mismatched thermocouples will bring systematic hidden dangers to temperature control stability, molding quality and equipment safety.
The first hazard is persistent temperature measurement deviation. K-type and J-type thermocouples have different thermoelectric potential curves. Using J-type instead of K-type in high-temperature molds will lead to serious drift of display temperature, the controller cannot accurately identify the actual runner temperature, resulting in long-term overheating or insufficient heating. Overheating causes melt decomposition and carbonization, while insufficient heating leads to poor melt fluidity and various filling defects, which cannot be solved by adjusting process parameters.
The second hazard is accelerated aging and frequent failure. Low-temperature resistant J-type thermocouples used in high-temperature working environment will be rapidly oxidized and attenuated, the probe fails in a short time, and frequent replacement is required, increasing mold downtime and maintenance cost. Non-corrosion-resistant ordinary thermocouples matched with corrosive plastic molds will be eroded by acidic volatile gas in a short time, resulting in sheath perforation and internal wire corrosion damage.
The third hazard is controller signal identification disorder. Hot runner temperature controllers are preset with fixed compensation parameters for K-type and J-type. Mismatched models make the cold junction compensation algorithm invalid, temperature display value jumps randomly, over-temperature and under-temperature alarms frequently, and the closed-loop constant temperature control function completely fails. In valve gate hot runner systems, temperature instability caused by model mismatch will change melt viscosity, interfere with valve needle opening and closing timing, and induce gate salivation, wire drawing and flash defects.
Standardized model management, unified use of original matching thermocouples and clear marking of mold wiring area can completely avoid model mismatch risks, ensure the long-term stable operation of hot runner temperature control system, and reduce unnecessary production loss and maintenance cost.
