Selecting J-type or K-type thermocouples according to the processing temperature and chemical characteristics of plastic raw materials is the premise to ensure long-term stable operation of hot runner temperature measurement systems, and mismatched types will lead to rapid sensor failure and unstable molding quality. Type J iron-constantan thermocouples are suitable for general engineering plastics with processing temperature below 320℃, including ABS, PP, PE, PS, ordinary PC, TPU and household appliance composite materials. Its advantages lie in fast temperature response speed and low cost, matching open gate hot runners for packaging and consumer goods molds. However, the iron wire of J-type probes is extremely easy to oxidize above 330℃; when processing high-temperature modified materials such as PA66+GF30, continuous operation for more than one month will produce serious signal drift, so it is forbidden to use in high-temperature molding environments.
Type K chromel-alumel thermocouples are the universal choice for high-temperature engineering plastics, with a continuous working temperature up to 420℃ and strong anti-oxidation capacity, applicable to PPS, PEEK, LCP, PA66-GF50, high-temperature PC and all new energy vehicle lightweight composite materials. For plastic materials that release corrosive gas during melting, such as PVC, flame-retardant PA and recycled fluoroplastics, ordinary K-type stainless steel armored probes still have corrosion risks, and it is necessary to upgrade to Inconel or Hastelloy alloy sheathed K-type thermocouples to resist acid and halogen gas erosion. Medical-grade PEEK implant parts must adopt fully sealed high-purity K-type thermocouples to avoid metal ion pollution of high-temperature melt.
Special modified plastics have exclusive thermocouple matching schemes. Low-temperature soft rubber materials such as TPE and silica gel have a molding temperature range of 180–240℃, and low-cost unarmored J-type thermocouples can meet the demand, which effectively reduces mold manufacturing cost without high-temperature oxidation risk. Transparent optical PMMA and COP materials require extremely stable temperature control to eliminate flow marks and internal stress, so dual-point sealed K-type armored thermocouples must be equipped, and surface-mounted ring J-type probes are not allowed to avoid large temperature deviation. Biodegradable PLA and PBAT materials have narrow thermal stability windows, and slight overheating will cause material decomposition, so fast-response J-type spring thermocouples are installed at the nozzle tip to feed back gate temperature changes in real time.
In addition to temperature and corrosion factors, the fluidity of glass fiber filled plastics also affects thermocouple structure selection. Glass fiber has strong abrasiveness; when processing high-fiber content materials, the thermocouple sheath needs to increase wall thickness to prevent the melt from scratching the protective layer during long-term injection. Thin unarmored thermocouples will be worn through within 1–2 months, causing wire short circuit faults. Many mold factories previously adopted unified J-type thermocouples for all molds to save procurement costs, but the later maintenance and scrap losses far exceed the saved expenses. Standardized material matching rules can be formulated during mold design: low-temperature general plastics match J-type, high-temperature and corrosive plastics match K-type alloy armored probes, and transparent medical precision materials adopt customized fully sealed dual-point K-type thermocouples, balancing service life, temperature accuracy and comprehensive use cost.
