Different plastic materials impose demands on thermocouples. Temperature range, corrosiveness, abrasiveness, and required accuracy all vary by resin. Selecting the wrong thermocouple leads to premature failure, inaccurate readings, and quality defects. This article provides a resin-based selection guide.
Polyolefins (PE, PP). These general-purpose resins melt at 180–260°C. They are non-corrosive and non-abrasive. Standard 316L sheathed Type K thermocouples work well. Response speed is more important than chemical resistance-use small-diameter grounded probes for thin-wall packaging.
Styrenics (PS, ABS, SAN). Melt temperatures range 200–260°C. These resins are stable, releasing minimal corrosive gases. Type K with 316L sheath is adequate. However, ABS can produce styrene vapor at high temperatures-ensure the sheath is sealed at the termination.
Polycarbonate (PC). PC melts at 280–320°C and requires high-temperature stability. PC itself is not corrosive but can degrade if overheated. Use Inconel 600 sheaths for better oxidation resistance. Type K is acceptable, but consider Type N for long-term accuracy.
Polyamides (PA6, PA66, PA12). Nylons melt at 220–290°C and are often processed with glass fiber (GF). GF is highly abrasive-choose Inconel 600 or hardened stainless steel with thicker walls (1.5–2.0 mm). Nylons also release ammonia-like compounds at high temperatures, which can corrode copper-based alloys-avoid Type T.
Polyesters (PET, PBT). PET melts at 260–290°C, PBT at 230–260°C. They are often filled with glass or minerals. Corrosion is moderate, but degradation products include acidic species. Inconel 600 is recommended. For food-contact applications, ensure sheath material complies with FDA regulations.
PVC (Plasticized and Rigid). PVC melts at 160–200°C but decomposes above 190°C, releasing HCl gas which is highly corrosive. Standard 316L may fail within weeks. Use Inconel 600 or titanium sheaths. Keep melt temperatures as low as possible. Avoid ungrounded junctions because HCl can penetrate the MgO insulation.
Fluoropolymers (PTFE, PFA, FEP, PVDF). These melt at 300–340°C and release HF and other halogen compounds. Corrosion is extreme-only Inconel 600, Inconel 625, or Hastelloy C-276 are suitable. Expect shorter life; use thicker sheaths and inspect frequently.
High-Temperature Engineering Plastics (PEEK, PEI, PPS, LCP). Melt temperatures range 320–400°C. Type K drifts rapidly above 350°C-Type N is much more stable. Sheath material: Inconel 600 or Haynes 230. Regularly calibrate-these high temperatures accelerate drift.
Selection Workflow. Step 1: Identify the resin and any fillers/additives. Step 2: Determine maximum melt temperature including startup overshoot. Step 3: Check for corrosive byproducts. Step 4: Choose sheath material per guidelines. Step 5: Select thermocouple type (K or N). Step 6: Choose diameter and junction based on response needs. Step 7: Verify connector and lead wire ratings.
