What Causes Hot Runner Material Degradation Defects

Product yellowing, discoloration, black lines, bubbles, brittle texture, and a quick reduction in mechanical qualities are all signs of material thermal deterioration, a common high-grade defect in hot runner injection molding production. Batch product scrapping will result from large-area deterioration. Temperature control state, material residence time, thermocouple accuracy, and flow channel environment are the primary factors that affect this defect; effective prevention may be achieved through clear cause analysis.

Plastic deterioration is mostly caused by an excessively high local temperature. The actual internal flow channel temperature is much greater than the shown temperature when hot runner thermocouples have a significant negative variation in temperature measurement, which causes the raw materials to remain in a very hot environment for an extended period of time. The plastic's molecular chain will break and disintegrate, releasing carbonized impurities and volatile gas that flow into the mold cavity with the melt to create a variety of performance and cosmetic flaws. Thermal deterioration is most likely to occur in materials that are very sensitive, such PC, PET, and clear optical plastics, when they are somewhat overheated.

Another core-inducing issue is an excessively lengthy material residence time inside the hot runner flow channel. The melt stays in the high-temperature flow channel for a considerable amount of time without discharging when the production line stops periodically, shifts are switched, or molds are debugged. Long-term constant-temperature standing will also cause raw materials to gradually age and decompose, even if the total set temperature is within the authorized range. Static materials can easily build up in the dead corners and small spaces inside the manifold and nozzle flow channels, which serve as the primary sites for carbonization and deterioration.

Raw materials will deteriorate more quickly in an unclean flow channel environment. Qualified new materials will quickly discolor and decay due to the catalytic degradation reaction caused by residual old carbon deposits and deteriorated impurities left in the flow channel after prior manufacture. Furthermore, combining recycled materials with materials that have been reused for too long would lower the thermal stability of raw materials, increasing their vulnerability to high-temperature breakdown in hot running systems.

Degradation risks will also be increased by unreasonable heating zone matching and temperature control system failure. Thermocouple failure causes individual heating zones to become uncontrollable, creating localized overheating zones that turn into concentrated locations for material breakdown. Unreasonable PID parameter settings and an excessively quick start-up heating speed will also result in an instantaneous temperature overshoot, which will have a short-term influence on ultrahigh temperatures and harm raw material performance.

Make sure all hot runner thermocouples are precise and efficient in order to prevent material degrading flaws. Then, calibrate and replace malfunctioning sensors on a regular basis to maintain temperature control within the ideal safe range. Establish fixed flow channel cleaning cycles, routinely empty stationary materials during shutdown, and strictly regulate material residence time. In order to significantly lower the incidence rate of thermal degradation problems, optimize the hot runner internal flow channel design to minimize retention dead corners and match targeted temperature parameters in accordance with various raw material thermal stability.