Closed Mold vs. Open Mold Cleaning

Many mechanical and chemical purges can be molded into parts when within their recommended service temperature range. Many molders prefer to perform closed mold purging so that they can minimize downtime. However, commercial purging compounds can be equally effective when using the open mold cleaning method.

If possible, closed mold hot runner purging is preferrable, because it minimizes downtime. Increase hot runner temperatures around 68°F – 86°F (20-30°C) prior to cleaning, to help dislodge carbon and materials deposits. Make sure that the temperatures do not exceed safe processing temperatures for the previous resin or purging compound.  It is imperative that the screw and barrel be completely clean prior to purging the hot runner. First, with the nozzle seated against the manifold and the mold safety door closed, remove all pack and hold velocities to avoid getting parts stuck in the cavities. Next, add about 50-90% barrel capacity of purging compound and begin molding short shots – about 50% of shot size is typically enough.Ejects the parts immediately to avoid sticking. If using a chemical purge, stop now for the recommended soak time before proceding to the next step.  Repeat the short shot process until parts are free of contamination. Finally, flush out the purging compound with the next production resin and begin production.

If the cavity geometry is too intricate or complex for molding the purging compound, the open mold method can be used to purge hot runners. Increase hot runner temperatures around 68°F – 86°F (20-30°C)  prior to cleaning, to help dislodge carbon and materials deposits. Make sure that the temperatures do not exceed safe processing temperatures for the previous resin or purging compound. Again, it is imperative that the screw and barrel be clean prior to purging the hot runner. Make sure there is a safe method to collect the purge and remove it from the machine after each shot, and always follow your plant-specific safety policies. With the nozzle seated against the mold, and the guards in place, begin extruding the purging compound through the hot runner system. Slowly increase the screw speed from slow to maximum safe speed. If using a chemical purge, stop now for the recommended soak time before proceding to the next step. Once the purge is flowing through all gates, begin executing injection shots (air shots) through the hot runner. When the purge from each gate is observed to be contaminant-free, flush the system with the next production resin and return to production once extrudate is free of the purging compound.