Hazards Caused by Excessive Carbon Deposition on Hot Runner Thermocouple Probes

In the long-term operation of hot runner systems, plastic melt will inevitably produce decomposition residues under high temperature, and gradually form hard carbon deposits attached to the surface of thermocouple probes. Excessive accumulated carbon deposits are invisible hidden dangers in temperature control work, which will bring a series of adverse effects on temperature measurement accuracy, temperature control stability, product molding quality and the service life of sensing elements. First of all, thick carbon deposits will form a dense heat insulation layer on the surface of the measuring head, which seriously hinders the normal heat conduction efficiency. The heat inside the hot runner flow channel cannot be quickly transmitted to the internal sensing core wire through the probe, resulting in serious temperature measurement lag. The temperature value fed back by the thermocouple is always lower than the actual internal temperature of the flow channel. The temperature controller will continuously increase heating power for temperature compensation under wrong data guidance, resulting in invisible overheating of local melt inside the hot runner, which is very easy to cause thermal decomposition, discoloration and material scorching of plastic raw materials, and directly produce defective products such as black spots and yellow lines on the surface of finished products. Secondly, carbon deposition accumulation will interfere with the normal constant temperature adjustment logic of the system. Due to inconsistent carbon deposition thickness on different thermocouple probes in each temperature zone, the temperature lag degree of each measuring point is different, resulting in unbalanced temperature distribution of the whole hot runner flow channel. The melt viscosity of each branch flow is uneven, and the filling speed and pressure cannot be unified in the injection process, which easily leads to unstable product size, large weight difference between cavities, warping deformation and other quality problems, and seriously affects the consistency of batch production products. Thirdly, long-term coverage of carbon dirt will accelerate the aging and damage speed of thermocouples. The corrosive substances in carbon deposits will slowly erode the outer anti-oxidation protective layer of the probe, gradually corrode the internal metal matrix, cause thinning and deformation of the measuring head, and reduce the structural firmness. At the same time, the long-term high-temperature closed environment formed by carbon deposition will increase the internal thermal load of the thermocouple, accelerate the drift of thermoelectric potential, make the precision decline faster, and greatly shorten the stable service cycle of the sensing element. In addition, excessive carbon deposits will also increase the difficulty of later equipment maintenance. It is difficult to remove the hardened carbon dirt thoroughly, and long-term accumulation will also cause blockage of local small flow channels, affecting the smooth flow of melt. Therefore, enterprises must formulate regular probe cleaning plans, thoroughly remove surface carbon deposits in combination with machine halt maintenance, optimize production parameters to reduce material residence time, fundamentally slow down the formation speed of carbon deposits, ensure that thermocouples maintain efficient and accurate temperature sensing state, and eliminate various quality and equipment hidden dangers caused by carbon deposition.