Signal drift of hot runner thermocouples is a common hidden fault in long-term continuous production, which will cause temperature control deviation and product defective rate rise, and its inducing factors cover material aging, working environment, installation process and mold structure. First of all, long-term high temperature working environment is the primary cause of thermocouple drift. Hot runner nozzles and manifolds work in the high temperature range of 200℃ to 400℃ for a long time. The thermoelectric wire material undergoes thermal aging and structural changes, resulting in deviation of thermoelectric potential output, gradual separation of displayed temperature from actual temperature, and slow drift of accuracy.
Mechanical vibration and frequent mold opening and closing impact will also accelerate drift and performance attenuation. In the high-cycle production process, the thermocouple probe installed in the manifold hole is subject to continuous vibration, resulting in loose contact between the temperature measuring junction and the mold steel, forming a tiny gap, increasing thermal resistance, delayed temperature sensing response and inaccurate feedback. At the same time, frequent bending and pulling of the wiring harness lead to internal fatigue of the thermoelectric wire, affecting signal stability.
Installation process irregularities are important human-induced factors causing drift. Insufficient cleaning of the thermocouple mounting hole, residual carbon deposits and oxide scale on the hole wall will isolate the probe from the mold matrix, resulting in slow heat conduction and distorted temperature measurement data. Improper insertion depth and insufficient compression and fixing will also lead to poor thermal contact state, and drift will appear after a period of production. In addition, chemical corrosion of plastic raw materials cannot be ignored. Flame retardant, modified and recycled plastics will produce corrosive gas at high temperature, corroding the thermocouple alloy sheath, damaging the internal thermoelectric wire structure, and further inducing signal drift and even local damage.
