What Special Considerations Apply to High‑Temperature Engineering Plastics?

Processing engineering plastics such as PEEK, PPS, LCP, and high‑temperature nylons pushes hot runner thermocouples to their limits. These materials require melt temperatures between 350°C and 420°C, which is near or beyond the maximum continuous rating of standard Type K thermocouples. At these extreme temperatures, oxidation accelerates, causing rapid drift in K‑type sensors unless they are made with special high‑purity alloys and thicker sheaths. For sustained operation above 400°C, Type N thermocouples (Nicrosil‑Nisil) are increasingly recommended because they offer superior oxidation resistance and thermal stability, although they are less common and cost more. The sheath material must be Inconel 600 or even Superalloy 625, as standard 304 stainless steel will scale and lose mechanical strength. Mineral insulation (MgO) must be very dense and dry, as any moisture will cause insulation resistance to plummet at high temperatures. Installation depth becomes even more critical because thermal expansion of the mold at these temperatures is significant. Spring‑loaded designs with high‑force springs are essential to maintain probe‑to‑wall contact throughout the heating cycle. Another special consideration is the risk of chemical attack. Many high‑temperature plastics contain fillers such as glass fibre, carbon fibre, or PTFE, which can be abrasive or release corrosive by‑products. The thermocouple's protective tube must be sufficiently thick to withstand abrasion, and the connector area should be sealed to prevent corrosive fumes from entering. Fast response is also crucial because these materials degrade quickly if overheated-just 5°C above the recommended range can cause cross‑linking, discoloration, or gas evolution. Therefore, the controller must be tuned with very aggressive PID parameters to minimise overshoot. Some processors employ redundant thermocouples-one for control and one for high‑limit safety-to protect the mold and prevent catastrophic failures. Because of the high replacement cost of these thermocouples (often double or triple the price of standard units), careful handling and preventive maintenance become economically justified. Regular calibration at process temperature is mandatory, as drift that might be acceptable at 250°C becomes unacceptable at 400°C. When specifying thermocouples for high‑temperature plastics, always consult the supplier with the exact material and maximum operating temperature, and request test data that validates long‑term stability at those conditions.