Thermocouple probe length is one of the most basic installation parameters, and length mismatch is a common hidden problem in on-site replacement and non-standard matching. Using too long or too short probes instead of the original standard length will not only damage the internal structure of the thermocouple itself, but also disrupt the thermal field balance of the hot runner, induce temperature measurement deviation, and bring long-term hidden dangers to molding quality, heater service life and mold maintenance.
When the probe length is too short, the measuring head cannot reach the bottom of the mounting hole, forming a fixed air gap between the probe and the hole bottom. Air is a poor heat conductor, which seriously delays heat transfer speed. The thermocouple can only sense the ambient temperature of the upper part of the mounting hole, unable to capture the real temperature of the runner wall. The displayed temperature is permanently lower than the actual temperature, the controller keeps increasing heating power, resulting in long-term overheating of the nozzle and manifold, accelerating material carbonization, gate salivation and wire drawing, and also making the heating coil work under overload for a long time, which is easy to age and burn out.
When the probe length is too long, it will be squeezed and blocked when inserted into the fixed-depth mounting hole. The measuring head is forced to deform and bend, the internal thermoelectric alloy wire is squeezed and displaced, and the compacted magnesium oxide insulation layer is crushed and cracked, forming hidden micro short circuit and signal drift faults that are difficult to repair. The overlong probe is too close to the heating ring, directly sensing the radiant high temperature of the heater rather than the runner temperature, resulting in false high temperature feedback, the controller reduces heating power blindly, leading to insufficient melt temperature, poor fluidity and incomplete cavity filling. In addition, the redundant probe length will interfere with the internal runner structure and valve needle movement of the valve gate mold, affecting the normal action of the hot runner system.
Mismatched probe length also makes the bayonet spring compression stroke abnormal, losing the automatic gap compensation function, unable to adapt to thermal expansion and contraction of mold steel, resulting in loose contact after a period of production, temperature drift further aggravated. Therefore, in the process of thermocouple replacement and matching, the probe length must be strictly consistent with the original mold reserved hole depth, and non-standard length probes are prohibited from being used as substitutes, so as to avoid hidden operational risks caused by length mismatch.
