The tricky bits – Machine nozzles

Most molding machines are ‘general purpose’ machines unless you specified something unique.  General purpose means the manufacturer has designed his machine to be best suited for the largest component of the plastics market place – the automotive industry.

No one questions PVC and the Nylon family of materials process differently than ABS and PolyPro.  Everyone knows the ball radius of the sprue must mate with the ball radius of the machine's nozzle and the nozzle orifice can't be larger than the initial sprue orifice.  Everyone knows this.  But few people pay attention to it.

When I'm in a client's plant, it's common to hear “This mold runs wonderfully in machine #8 but when we put it in #5 we fight it all the way through the run.”  Usually the cause of this problem is simple:  Let's say Machine #8’s nozzle's orifice was 3/16”.  The orifice on the sprue bushing was 7/32.  We do the mold qualification in this machine, get the process conditions and the mold runs well.  But, by not paying attention, someone fails to notice even though the ball radius of both machine nozzles are the same the machine orifice on #5 is 1/8” diameter.  High school algebra tells us the area of a circle is a function of the square of the radius.  By using a smaller diameter opening in the nozzle in machine #5 we've reduced the opening the material can flow out of the machine by a factor of 4 compared to the other machine.

The setup techs (resourceful folks that they are) simply turn up the heats and the pressure and try molding the parts in machine #5 by brute force.  No wonder they fight this problem.

Machine nozzles cost about $25 in the USA.  Think about this:  What if you bought your mold with machine nozzle whose orifice mates with the mold's sprue bushing and put it in a sheet metal container attached to the mold?  You'd never have a problem with the right/wrong nozzle/sprue bushing combination again.  Or, better yet:  Most shops use a maximum of about 6 different machine nozzle configurations.  Stamp or deeply engrave letters on each nozzle.  Stamp or engrave on the mold the letter of the nozzle type to be used for this mold.   Match up the letters, problem solved.

For the complainers who say changing a nozzle tip takes too much time – with pipe fitters high temperature anti seize compound you only wrestle with a nozzle the first time.  After that, it comes off easily.  It depends of how much money you want to lose in scrap before you see the logic in this.

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