What Welding Techniques Are Used for Thermocouple Junctions in Hot Runners?

The junction-where the two dissimilar thermocouple wires are joined-is the heart of the thermocouple. In hot runner applications, this junction must be robust, free of contamination, and metallurgically sound to ensure long‑term stability. The most common welding technique is capacitive discharge welding, which uses a high‑voltage capacitor discharge to melt the wire ends together in a fraction of a millisecond. This method produces a clean, spherical bead with minimal oxidation and no introduction of foreign metals. The energy must be precisely controlled-too low and the weld is weak; too high and the alloy evaporates, changing the thermoelectric composition. The next technique is argon arc welding (TIG), which provides good control but introduces more heat, potentially annealing the wires near the junction and affecting sensitivity. TIG is often used for larger‑diameter sheathed thermocouples. A newer method is laser welding, which offers exceptional precision and minimal heat‑affected zone, resulting in very consistent junctions. For mineral‑insulated thermocouples, the junction is welded to the inside of the sheath at the tip, creating a grounded junction. The weld must fill the tip completely to ensure good thermal contact and mechanical integrity. For ungrounded (isolated) junctions, the wires are welded together inside the sheath but are insulated from the sheath by a thin layer of MgO. The quality of the weld directly affects drift-a poorly welded junction may develop microcracks over thermal cycles, causing intermittent open circuits or signal noise. Another critical aspect is the prevention of contamination: any finger oils, dirt, or fluxes on the wire ends before welding can introduce impurities that alter the thermoelectric voltage. Thus, welding is typically done in a clean, controlled environment. Some manufacturers use an automated welding station that controls the energy and atmosphere (argon gas purge) to ensure repeatability. After welding, the junction is often heat‑treated (annealed) to relieve residual stresses and stabilise the crystalline structure. The finished thermocouple is then subjected to a high‑temperature burn‑in test, which further stabilises the junction. For field repairs, technicians sometimes use a small welding kit to re‑weld broken leads, but this is not recommended for precision hot runner sensors because the energy and alloy match cannot be guaranteed. Instead, always replace the entire probe. Understanding these welding techniques helps users appreciate why factory‑built thermocouples from reputable brands outperform makeshift repairs-the junction quality is the foundation of thermocouple accuracy and longevity.