How Mold Coating Treatments Affect Thermocouple Surface Heat Transfer Efficiency

Many hot runner manufacturers apply surface coating treatments, such as nitriding, DLC diamond coating, PTFE anti-stick coating on manifolds and nozzles to prevent mold steel corrosion, plastic adhesion and carbon deposition. The different coating thickness and material qualities modify the surface thermal conductivity of the measuring plane, which interferes the heat transfer between the metal substrate and the thermocouple sensing head, causing temperature measurement lag and fixed offset. This paper discusses the impact of conventional hot runner coatings and the matching adjustment techniques for thermocouple monitoring.

Nitriding treatment is the most popular hot runner surface strengthening coating with low thickness, the coating thickness is just 0.03~0.08mm, and the dense metal nitride layer has thermal conductivity near to mold steel. The influence on the heat transfer of thermocouple is small and the offset of temperature measurement is controlled within ±0.3℃, which practically does not affect the usual production control. The only hidden difficulty is the tiny uneven micro-pits generated on the nitrided surface, and carbon dust is easier to deposit in the pits, and long-term buildup will form thermal barriers to cause drift. Optimization matching: . For thermocouples installed on nitrided measuring surfaces, the thickness of thermal conductive paste should be increased accordingly to fill micro-pits and the carbon cleaning cycle be shortened by one week.

DLC diamond-like carbon anti-stick coating with a thickness of 0.05-0.15mm , Low thermal conductivity carbon-based coating, which creates an obvious thermal resistance layer between the hot runner steel substrate and thermocouple contact surface. The recorded temperature of the thermocouple is 1～1.8℃ lower than the temperature of the real flow channel metal at the same temperature, and the temperature response lag increases by 1～2 seconds. The offset becomes more pronounced as the DLC coating gets thicker. For hot runners with full DLC coating on the manifold measuring planes, two adjustment measures can be taken. One is to increase the coating amount of high thermal conductivity metal powder thermal conductive paste to break through the low-conductivity coating heat barrier. The other is to 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 at the measuring point only. Local coating damage will result in inconsistent surface wear and rapid carbon deposition at the boundary.

PTFE polymer anti-stick coating . It is a soft organic coating with very low thermal conductivity, thickness 0.1-0.3mm and most substantial interference to thermocouple temperature monitoring . Even a thin layer of PTFE will form a strong heat-insulating layer, causing a temperature reading lag of over 3 seconds and a fixed negative drift of 2~4℃. PTFE will decompose and carbonize gradually above 280℃, forming a composite insulation layer of organic residue and carbon dust, and the drift offset will grow day by day. Solution: In the mold design to avoid arranging the thermocouple measuring points on PTFE coated areas. 1. In case the measuring point is covered by PTFE coating, remove the PTFE 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. Avoid PTFE coated surfaces for long term high temperature hot runners above 300°C.

The hard chromium plating coating has a thickness of 0.08-0.2mm and a medium thermal conductivity. It has a smooth and dense surface that is not easy to develop carbon deposits. The offset of the temperature measurement is roughly 0.6℃, which can be minimized by the thin layer thermal conductive paste during the installation of the thermocouple. The smooth chromium plated surface minimizes the speed of carbon adhesion thereby lengthening the cleaning cycle of thermocouple sensing heads.

Recommendations of coating layout improvement in mold design phase. All thermocouple measurement sites should be centralized as far as possible on bare uncoated metal planes. If the complete mold is coated, choose thin-layer nitriding or strong chromium plating, avoid large-area DLC and PTFE coating at temperature measuring places. When customizing thermocouples for coated hot runners, advise the supplier of the coating type and thickness beforehand, and choose high-sensitivity ungrounded exposed bead sensors with ultra-thin contact structure to minimize heat transfer resistance loss. During normal calibration, record individually the fixed offset from the coating and make distinct correction parameters for each coated hot runner zone, to stabilize long-term accuracy of temperature measurement.