The care and feeding of Hot Runners

With the constant concern on less cycle time and less regrind, hot runners are becoming a very viable option.  While not cheap, the fact that a runner system isn’t needed and therefore has no need to be cooled is seductively tempting.  One thing we forget consistently is the precision of the hot runner’s nozzle tips:  these are precise pieces of metal working, machined to very close tolerances for exacting fills and quick freeze offs.  Unfortunately, being precise, they are also easily damaged.

While the molding machine’s magnet drawer will pick up anything ferromagnetic, it will gleefully pass through copper staples, wads of paper, coffee cups, French fries, and burritos that fell into the grinder, and even the occasional rag that manages to get gobbled up by the vacuum system and pulverized by the screw.  With nothing to stop them, these bits of non-plastic will merrily go through the machine and into the molded parts if they get that far.  Usually they can get past the machine’s check ring, however they can become neatly lodged in your mega-expensive hot runner system.  Then the Horrors of Horrors occurs:  Joey (your setup tech) attempts to remove the offending item by drilling out the clogged nozzle tip.  His odds of doing this without damaging your hot runner system are less than winning Lotto!  Congratulations!  You now have a hefty repair bill when you send the system back to the manufacturer.

This could have been avoided for a few hundred dollars by installing an in-line filter between the machine nozzle and the extruder.  These things look a little like a gun silencer mounted on the injection unit. Their purpose is to trap those pesky pieces of Flotsam and Jetsam stuck in the machine’s melt stream.  Because they filter, there is nothing to get into your hot runner system so you avoid the repair bill and Joey’s attempt at brain surgery.  One word of warning:  As with all filters, they do clog up.  These are easy to dismantle and clean.  However, if for no ‘apparent’ reason you feel you have to increase the fill and packing pressures this is the give away to clean the in-line filter.  DON’T force the situation.  Trying to muscle your way through a clogged filter will only damage everything.

For just a little money you can avoid a very expensive problem.  Think about it.

Contact me with questions put in the subject line QUESTIONS.
Bill Tobin, WJT Associates, E-mail: bill4012@hotmail.com