Hot runner runner carbonization is a common stubborn fault in injection molding production, which will cause product black spots, material impurities, runner blockage and frequent mold disassembly and cleaning. Most operators attribute carbonization to poor plastic raw materials or unreasonable process parameters, but ignore that thermocouple failure and temperature measurement deviation are the core inducing factors of runner carbonization. Unstable temperature sensing will lead to long-term overheating of local runners, making the melt decompose and carbonize continuously.
When the thermocouple probe ages, oxidizes or contacts poorly, the feedback temperature is significantly lower than the actual temperature of the runner. The temperature controller mistakenly judges that the temperature is insufficient and continuously increases heating power, resulting in actual local temperature far exceeding the decomposition temperature of plastic materials. The melt stays in the dead angle of the runner for a long time under ultra-high temperature, gradually decomposes, cokes and forms carbonized deposits. These carbonized impurities will flow out with the melt, polluting the product surface and affecting appearance quality.
Thermocouple open circuit fault will cause temperature runaway of the hot runner. The heating coil keeps working at full power, the manifold and nozzle temperature rises sharply in a short time, and a large amount of melt carbonizes rapidly, which not only blocks the runner channel, but also adheres to the inner wall of the runner and is difficult to clean thoroughly. Even after cleaning, residual carbonized substances will accelerate new melt deterioration and form repeated carbonization problems.
In multi-zone manifold hot runners, local thermocouple accuracy deviation leads to unbalanced temperature distribution. Partial branch runners are overheated and carbonized, while other runners are in normal temperature state, resulting in inconsistent cavity quality and batch unqualified products. Regular inspection and calibration of thermocouple accuracy, timely replacement of aging and drifted sensors, can keep the runner temperature within the optimal process range, avoid local overheating from the source, reduce melt residence decomposition and carbonization probability, extend the cleaning cycle of hot runner molds, and improve continuous production efficiency.
