Hot runner systems are the backbone of the injection molding industry, but they require maintenance to ensure optimal performance. One way to keep hot runners clean is by using purging compounds. Proper use of purging compounds in injection moulding can take color changes to new heights while saving time and reducing material waste.
Despite the benefits, some plastics processors remain reluctant to use purging compounds, either because they have never used them before or because of a bad experience in the past. However, purging compounds have been proven to be more effective than using resin or regrind in removing color or carbon contamination in hot runners.
The key to using purging compounds on hot runners is ensuring the compound is designed for this purpose. Many standard purging compounds have larger particles that don't melt, which can clog hot runner gates. However, finding the right compound shouldn't be difficult, as virtually every purging compound manufacturer offers at least one hot runner grade.
Before using a purging compound on hot runners, it's recommended to purge the screw and barrel first. This prevents any contamination in the barrel from getting dragged into the hot runners. If the screw and barrel are clean, it's okay to start cleaning the hot runners without taking this step to save time and material.
Once the hot runner temperatures are raised, the purging compound can be injected to help loosen any deposits and improve the flow of the compound. However, it's important to be cautious when processing heat-sensitive resins, as raising temperatures may cause further degradation. It's also important to verify any hot runner gate-clearance requirements for the purging compound being used. Some compounds may require larger gate clearances, while others may not be suitable for purging most hot runner systems due to gate-clearance restrictions or potential damage to nozzle tips.
Different purging procedures are recommended depending on a variety of factors, such as the resin being processed, the mold design, and the cleaning difficulty. Closed-mold purging is usually sufficient for molds with fewer cavities, while higher-cavitation molds benefit more from open-mold purging.
Each method has valuable tips for achieving the best result.
For closed-mold purging, it's recommended to inject the purging compound using short shots whenever possible to decrease the possibility of parts sticking in cavities. If a short shot can't be molded, then it's important that the shrinkage rate of the purging compound be similar to that of the processing resin to avoid parts getting stuck in the cavities. It's also recommended to use a purging compound grade that's compatible with the processing resin to ensure less residue left behind by the purging compound. Spray the cavities with mold release between cycles, turn off pack and hold velocities and pressures but don't adjust pack and hold time, and then eject purged parts immediately while still warm.
For open-mold purging, it's helpful to extrude the purging compound through the hot runner system starting at a medium velocity, then increase the screw speed to the maximum safe level to achieve maximum cleaning. Once the purging compound is flowing adequately through all the gates, inject air shots with the mold open.
Using a purging compound to clean hot runner systems can produce striking results. It can save time and reduce material waste, while improving efficiency and productivity.
Ultra Purge manufactured by Chemtrend and supplied by Novanative in India, able to purge hot runner system with a compatible polymer. With proper use and in the right circumstances, Ultra Purge purging compounds can help take hot runners to new heights by improving efficiency and life of hot runner system as it keeps the hot runner system in original state along with the screw and barrel of injection moulding machine.
