Thermocouple metal sheath sealing technology is divided into traditional seam welding and modern laser seamless welding, which determines the air tightness, moisture resistance and anti-corrosion ability of the entire sensor. Most low-cost inferior thermocouple products adopt cheap seam welded sheaths, while high-quality dedicated hot runner sensors fully adopt laser integrated welding processes. The structural gap between the two welding modes leads to huge differences in service life and stability under mold high-temperature humid and corrosive environments.
Ordinary seam welded sheaths are formed by curling thin steel strips and welding the splicing gap with electric arc welding. The welding seam is thick and uneven, with tiny micro-cracks and pinholes inevitably left on the joint surface after molding. In hot runner working environments with water vapor and corrosive gas, moisture and volatile chemicals will continuously penetrate into the sheath interior through seam gaps, dampening magnesium oxide insulation powder and corroding internal thermoelectric alloy wires. Under repeated cold and hot cycles, the welding seam is easy to crack and expand, forming large leakage holes and directly scrapping the whole probe. Seam welded sheaths can only adapt to dry low-temperature intermittent trial molds, and are completely unsuitable for 24-hour continuous mass production lines.
Laser seamless welded sheath uses integral hollow alloy tube as raw material, and the measuring tip and wire outlet end are fully sealed by precision laser melting without any splicing gap. The welding area is smooth and compact without micro-pores, realizing complete air tightness isolation of the internal insulation structure. Water vapor, chlorine and sulfur corrosive gas cannot penetrate the sheath wall, fundamentally solving insulation damp and internal alloy corrosion faults. The laser welding layer has uniform thickness and strong tensile fatigue resistance, and will not crack after thousands of thermal expansion and contraction cycles of hot runners, maintaining long-term stable sealing performance.
Core performance advantages of laser welded thermocouples in hot runner application scenarios. First, ultra-long anti-damp service life: for water-cooled molds and high-humidity workshop production lines, the insulation resistance of laser welded probes can remain above 100MΩ for more than 8 months, while seam welded products fail due to damp insulation within 1–2 months. Second, stronger anti-corrosion ability: when processing halogen and sulfur-containing modified plastics, the sealed laser sheath blocks corrosive gas contact with internal alloy wires, slowing signal drift speed significantly. Third, stable high-temperature mechanical performance: the integral tube structure avoids welding seam fracture caused by bending and vibration, reducing intermittent open-circuit fault probability by over 70% for valve gate and stacked molds with frequent mechanical movement. Fourth, consistent precision retention: no internal moisture erosion means thermoelectric potential will not drift rapidly, and long-term temperature measurement error stays within the factory qualified tolerance range.
Cost comparison and matching suggestions. Laser welded thermocouples have slightly higher single purchase cost than seam welded products, but the replacement frequency is reduced by two-thirds in mass production, and the overall annual spare part expenditure of the workshop is lower. Seam welded cheap probes look cost-effective at first sight, but the frequent shutdown loss and mass scrap loss caused by early failure far exceed the saved procurement cost. For formal mass production molds, medical cleanroom molds and automotive precision molds, only laser seamless welded sheath thermocouples can be ordered; seam welded products are only allowed to be used for temporary small-batch trial mold debugging.
When receiving incoming thermocouple spare parts, simple appearance identification can distinguish the two welding processes: seam welded probes have obvious linear welding traces at the tube tip; laser welded tube ends are smooth and rounded without any linear splicing marks, which can be quickly screened before installation to avoid unqualified products entering the production line.
