How to Select Thermocouples for Hot Runner Systems Processing High-Temperature Engineering Plastics?

High-temperature engineering plastics such as PEEK, PPS, LCP, and PEI present extreme demands on hot runner thermocouples due to their elevated processing temperatures, typically ranging from 350°C to 420°C. The first selection criterion is the thermocouple type. For sustained operation above 350°C, Type K thermocouples are commonly used, but they experience accelerated oxidation at these temperatures, leading to drift. Type N thermocouples (Nicrosil-Nisil) offer superior oxidation resistance and stability at high temperatures, making them the preferred choice for high-temperature applications. They have a lower drift rate-typically 0.5°C per 1000 hours at 400°C, compared to 1.5°C for Type K. The second criterion is the sheath material. Standard stainless steel 304 will oxidize and scale at 400°C, reducing its mechanical integrity and potentially contaminating the resin. Inconel 600 or 625, with their high nickel-chromium content, form a protective oxide layer that resists scaling and maintains strength. For ultra-high temperatures above 400°C, consider superalloys like Haynes 214 or alloy 59. The third criterion is the probe diameter. Thinner probes (0.5-1.0 mm) offer faster response, which is essential for high-temperature plastics that degrade quickly if overheated. However, thinner probes are more susceptible to oxidation-a trade-off that must be balanced. A 1.0 mm Inconel probe is a good compromise. The fourth criterion is the insulation material. High-purity, densely packed magnesium oxide (MgO) is standard, but for temperatures above 400°C, ensure the MgO is specially processed to remove moisture and to have a high density (>95% theoretical density) to prevent electrical leakage. The fifth criterion is the cold end sealing. At high temperatures, the cold end must be sealed with a high-temperature epoxy or a glass-to-metal seal that can withstand 250°C at the connector area. The sixth criterion is the compensating cable. The cable insulation must be rated for the ambient temperature at the mold. Fiberglass-insulated cables with a stainless steel overbraid are recommended, as silicone insulation may degrade above 300°C. The seventh criterion is calibration. High-temperature thermocouples should be calibrated at the actual processing temperature, not just at lower points. Request a calibration certificate with points at 350°C and 400°C. The eighth criterion is the replacement interval. Given the accelerated drift, replace high-temperature thermocouples more frequently-typically every 3,000-5,000 operating hours, or annually. By carefully selecting thermocouples based on these criteria, processors of high-temperature engineering plastics can achieve stable temperature control, minimize degradation, and extend sensor life, ensuring the production of high-performance parts with consistent quality.