The reliability of thermocouple wires is mainly affected by the purity of raw materials, manufacturing process, compositional uniformity, physical structure consistency, and long-term high-temperature stability. These factors collectively determine the accuracy and service life of temperature measurement in hot runner systems.
1. Raw Material Purity: High-purity (≥99.99%) base metals such as nickel-chromium, platinum, and rhodium are fundamental to ensuring stable thermoelectric performance.
Impurity elements (such as Fe, Co, and Si) can cause local Seebeck coefficient anomalies, leading to thermoelectric potential drift.
High-quality brands (such as Omega and TECO) use vacuum induction melting to effectively control impurity content.
2. Manufacturing Process Level
Process Steps and Their Impact on High-End Control Standards
Wire Drawing Process: Die wear or uneven tension leads to diameter fluctuations. Diameter tolerance ≤ ±0.005mm (high-end brands)
Annealing Treatment: Eliminates internal stress, forming fine equiaxed crystal structure. Heating at 1100℃ for 2 hours prevents grain coarsening.
Multi-Pass Processing: Improves microstructure density and compositional uniformity. TECO and Kanthal use 5 or more wire drawing and annealing cycles.
Key Point: Process stability directly affects batch consistency; avoid sudden temperature control changes after on-site changes.
3. Composition and Microstructure Uniformity
Compositional segregation (e.g., uneven Cr and Rh distribution) leads to localized thermoelectric property differences, causing measurement errors >1℃.
Poor metallographic structure (columnar crystals, abnormally large grains) reduces creep resistance and accelerates aging.
Detection Methods: XRF/ICP-MS analysis of composition, SEM+EDS observation of microsegregation.
4. Physical Dimensional Consistency: Diameter fluctuations > ±0.01mm will affect the stability of electrical signal transmission, especially with significant cumulative errors over long distances.
Surface defects (scratches, oxide scale) can easily become crack initiation points, leading to premature failure at high temperatures.
Laser diameter gauge scanning along the entire length is a necessary acceptance test.
5.Long-Term High-Temperature Stability: If the thermoelectric potential drift > 2μV (approximately 1.3℃) during continuous operation at rated temperature, it indicates insufficient material reliability.
Type B (Platinum-Rhodium 30-Platinum-Rhodium 6) exhibits minimal drift at 1600℃, serving as a benchmark for long-term stability.
Type K is prone to "greening" (selective oxidation) during long-term use at temperatures >1100℃, significantly reducing reliability.
Measured Data: Omega Type B filament exhibits a drift < 1μV in a 100-hour aging test, leading the industry.
