HOT RUNNERS: To purge or not to purge

Hot runner systems have hit their stride as the desire to reduce scrap and improve efficiency and productivity motivates mold buyers to shell out the added dollars for the technology.

A worthwhile investment, for sure, but there’s a downside. “A lot of plastics processors have rigorous preventative maintenance (PM) schedules for their molds but neglect the hot runner system,” said Martin Baumann, business manager, hot runners, with Husky Injection Molding Systems. 

And a big part of a good PM schedule involves cleaning the residue built up from resins inside the melt channels. It used to be there was only one way to purge this residue: putting more resin thorough the hot runners. And dinosaurs once ruled the earth too. Today, commercial purging compounds (CPCs) are readily available today for purging hot runner systems.

But still, not everyone is buying in. “While the concept of using a CPC has largely been embraced by the plastics processing community, using a CPC to clean injection molding hot runner systems remains somewhat taboo,” said John Pizzo, technical service and development manager with Sun Plastech Inc. “Usually, this reluctance either comes from never having used a CPC to clean hot runner tooling, or using it incorrectly and getting a bad result.”

Whatever the cause, giving CPCs a complete miss is probably a mistake. “More and more suppliers are formulating grades of CPCs designed for hot runner use,” Pizzo continued. “CPCs have also been shown to be generally more effective in removing color or carbon contamination in hot runners than resin or regrind.”

WHAT TO DO IT WITH

It sounds like the definition of a no-brainer, but probably bears saying anyway: The key to using CPCs on hot runners is to make sure the compound has, in fact, been designed for hot runners. “A lot of standard purging compounds have larger particles that don’t melt, clogging hot runner gates,” said Bill Rousseau, engineering and technical services manager with Synventive Molding Solutions. “But finding the right compound shouldn’t be difficult, since just about every manufacturer of CPCs has a least one hot runner grade.”

Case in point: The Dyna-Purge division of Shuman Plastics Inc. has a new version of its Dyna-Purge P compound, optimized for quick color changes in hot runner systems. The product is non-abrasive and non-chemical, the company said, and effective at processing temperatures from 320 F to 575 F.

HOW TO DO IT

If you’ve never purged hot runners with a CPC before, it’s often recommended to purge the screw and barrel first to ensure that, if there’s any contamination in the barrel, it doesn’t get dragged into the hot runners. The caveat? “If you’re certain the screw and barrel are relatively clean, it’s okay to start cleaning the hot runners without purging them, to save time and material,” John Pizzo said. “When in doubt, however, always clean the screw and barrel first.”

For the second step, hot runner temperatures, particularly gate temperatures, should be raised before purging to help loosen any deposits – the extra heat also helps improve the flow of the compound. “But take care when processing heat-sensitive resins, as raising temperatures may cause further degradation,” Pizzo cautioned.

And don’t forget to verify if there are any hot runner gate-clearance requirements for the CPC being used. “If the CPC has any type of filler, then it will likely require larger gate clearances,” Pizzo explained. “A glass-filled CPC is not recommended for purging most hot runner systems due to gate-clearance restrictions or potential damage to nozzle tips. Also, any effective CPC will remove carbonized resin or colorants, which could potentially block gates if their clearances are too small.”

The next step brings another level of complexity to the process: depending on a variety of factors, CPC suppliers recommend different purging procedures. “Some CPCs can be used with either a closed-mold or open-mold method, depending on the resin being processed, the mold design, and the cleaning difficulty,” Pizzo said. “Open-mold purging is usually sufficient for molds with fewer cavities, while higher-cavitation molds benefit more from closed-mold purging.” 

For each method, there’s no shortage of for achieving the best result. For closed-mold purging, for example, inject the CPC using short shots whenever possible – approximately 50 per cent shot size – to decrease the possibility of parts sticking in cavities. And if a short shot can’t be molded, then it’s important that the shrinkage rate of the CPC be similar to that of the processing resin to avoid parts getting stuck in the cavities. “Also, use a CPC grade that’s compatible with the processing resin being molded to ensure less residue left behind by the CPC,” Pizzo said. “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 CPC parts immediately while still warm.”

For open-mold purgings meanwhile, it helps to extrude the CPC through the hot runner system starting at a medium velocity; then increase screw speed to the maximum safe level to achieve maximum cleaning. “Once the CPC is flowing adequately through all the gates, inject air shots with the mold open,” Pizzo said.   

WHEN NOT TO DO IT

So now that you know how to purge hot runners with CPCs, when shouldn’t you do it? “Using resin remains the most economic way of purging hot runners,” said Martin Baumann. “We recommend using a chemical purge when moving from very dark to light colors, or when using reds and blues. If you’re moving from a light to a darker color, and are satisfied with your color change times, there’s no need to use a CPC.”

There’s also the issue of CPCs adding an extra step to the purging process. “In this regard, the question becomes, will the CPC help your color change more than the extra time spent purging?” said Bill Rousseau. “The answer is up to the processor. With a properly-designed hot runner system, it might not always be worthwhile, because the system will be designed to change colors after five to 10 shots. But if it’s a more difficult application, with color changes after 40 shots, then probably it is worth that extra 10 minutes for a CPC purge.”       

In the end, the choice is yours. Done properly, you’ve nothing to lose by using CPCs on the right occasions – other than the minutes you’re shaving off your color change times, of course.