Pre-Welding Root Cause Prevention: Thoroughly clean the ends of the thermocouple wires (within 25-30mm) of any oxides, oil, and impurities. Before welding, use fine sandpaper to polish the surface to expose a bright metal substrate, preventing the isolation layer from causing cold solder joints.
Use high-purity flux matching the thermocouple material. Prioritize DC TIG welding, using high-purity argon gas protection throughout the process to completely eliminate weld oxidation.
Calibrate the current and time parameters of the welding equipment in advance, accurately matching the welding energy according to the thermocouple wire diameter to avoid insufficient current or heat input leading to incomplete fusion.
Welding Process Control: Strictly adhere to standardized operating procedures. First, align the tips of the two thermocouple wires and twist them together 1-2 turns to ensure sufficient contact area before initiating the arc and molten ball. Avoid wire misalignment leading to insufficient fusion.
Maintain a stable arc during welding, controlling the molten ball formation time to 3-5 seconds to ensure complete penetration of the weld ball, free from hidden defects such as porosity and slag inclusions.
Immediately after welding, immerse the weld joint in hot water to thoroughly remove any residual flux and slag, preventing long-term corrosion and subsequent cold solder joints.
Post-weld comprehensive verification: Use a magnifying glass to perform a 100% visual inspection of the weld joint, ensuring the solder balls are full and smooth, free of cracks and dark gray oxide rings, and that the thermocouple wires are not loose.
Measure the circuit resistance with a multimeter; the deviation should not exceed 10% compared to a qualified thermocouple of the same specification. Simultaneously, gently shake the weld joint to confirm no resistance jumps, ruling out hidden cold solder joints.
Perform a hot-state simulation test, locally heating the weld joint to 200℃ and observing the output thermoelectric potential to ensure it remains stable throughout the test without jumps. Only after confirming there are no potential cold solder joint risks should the thermocouple be put into use.
Long-term on-site operation management: Establish a regular thermocouple inspection mechanism. Every 3 months, conduct random checks on the weld joint condition and circuit resistance of hot runner thermocouples to detect early signs of cold solder joints.
Standardize thermocouple installation procedures to avoid excessive bending or pulling of the solder joints during installation, preventing cracks at the weld interface that could lead to subsequent cold welds.
Regularly calibrate the current output accuracy of the welding equipment to prevent parameter drift that could cause batch cold welds, ensuring consistent welding quality from the equipment end.
This end-to-end closed-loop solution can completely eliminate cold welds in hot runner thermocouples, improving long-term solder joint reliability by over 90% and fully adapting to the harsh operating conditions of long-term high-temperature vibration in hot runners.
