In order to prevent mold steel corrosion, plastic adhesion and carbon deposition, many hot runner manufacturers adopt surface coating treatments such as nitriding, DLC diamond coating, PTFE anti-stick coating on manifolds and nozzles. Different coating thickness and material properties change the surface thermal conductivity of the measuring plane, interfere with heat transfer between the metal substrate and thermocouple sensing head, and cause temperature measurement lag and fixed offset. This article analyzes the influence of mainstream hot runner coatings and matching adjustment schemes for thermocouple measurement.
Nitriding treatment is the most common low-thickness hot runner surface strengthening coating, with coating thickness only 0.03–0.08mm, dense metal nitride layer with thermal conductivity close to mold steel. The impact on thermocouple heat transfer is minimal, and the temperature measurement offset is controlled within ±0.3℃, which basically does not affect normal production control. The only hidden trouble is the tiny uneven micro-pits formed on the nitrided surface; carbon dust is easier to deposit in the pits, and long-term accumulation will form thermal barriers to induce drift. Matching optimization: When installing thermocouples on nitrided measuring surfaces, increase the coating thickness of thermal conductive paste appropriately to fill micro-pits, and shorten the carbon cleaning cycle by one week.
DLC diamond-like carbon anti-stick coating has a thickness of 0.05–0.15mm, low thermal conductivity carbon-based coating, which forms an obvious thermal resistance layer between the hot runner steel substrate and thermocouple contact surface. Under the same temperature, the measured value of the thermocouple will be 1–1.8℃ lower than the actual flow channel metal temperature, and the temperature response lag increases by 1–2 seconds. The thicker the DLC coating, the more obvious the offset. For hot runners with full DLC coating on manifold measuring planes, two adjustment measures can be taken: first, increase the coating amount of high thermal conductivity metal powder thermal conductive paste to break through the low-conductivity coating heat barrier; second, preset a fixed positive offset of 1–2℃ on the corresponding controller channel to compensate for coating-induced under-reading. It is not recommended to machine off the DLC coating only at the measuring point, as local coating damage will cause inconsistent surface wear and accelerated carbon deposition at the boundary.
PTFE polymer anti-stick coating is a soft organic coating with extremely low thermal conductivity, thickness 0.1–0.3mm, and the most serious interference to thermocouple temperature measurement. Even a thin layer of PTFE will create a strong heat insulation barrier, resulting in temperature reading lag of more than 3 seconds and fixed negative drift of 2–4℃. PTFE will gradually decompose and carbonize above 280℃, forming composite insulation layers of organic residue and carbon dust, and the drift offset will expand day by day. Solution: Avoid arranging thermocouple measuring points on PTFE coated areas during mold design; if the measuring point is covered by PTFE coating, remove the coating at the contact position of the sensing head during mold processing, and reserve a bare metal measuring plane for direct contact of the thermocouple flat head or probe tip. Do not use PTFE coated surfaces for long-term high-temperature hot runners above 300℃.
Hard chromium plating coating has medium thermal conductivity, thickness 0.08–0.2mm, smooth and dense surface that is not easy to accumulate carbon deposits. The temperature measurement offset is about ±0.6℃, which can be eliminated by thin-layer thermal conductive paste during thermocouple installation. The smooth chromium-plated surface reduces carbon adhesion speed, extending the cleaning cycle of thermocouple sensing heads.
Coating layout optimization suggestions at mold design stage. Centralize all thermocouple measuring points on bare uncoated metal planes as much as possible; if the whole mold must be coated, select thin-layer nitriding or hard chromium plating, and avoid large-area DLC and PTFE coating at temperature measuring positions. When customizing thermocouples for coated hot runners, inform the supplier of coating type and thickness in advance, and select high-sensitivity ungrounded exposed bead sensors with ultra-thin contact structure to reduce heat transfer resistance loss. During regular calibration, record the fixed offset caused by the coating separately, and make independent correction parameters for each coated hot runner zone to stabilize long-term temperature measurement accuracy.
