Consistent shots
 

The key to precision molding is consistent shots sizes.  Well Duh!  But when was the last time you really looked to see how ‘precise’ your machine is?  Your screw is a volumetric pump.  At a given backpressure every turn should pump a specific amount of material.  But in many cases it doesn’t.  While it may take a bit of hunting, look up in your machinery manual the screen that will give you a history of screw position.  If the software only gives it to you shot by shot, record a few measurements.  If your check-ring is doing what it should do, measuring the ‘full back’ position after decompression should land the screw within + 5% of the same position every time.  Less than + 5% is good for precision molding, between +5-9 % is OK (but not particularly good) for basic commercial molding where part weight will vary and dimensions will slightly drift. Greater than 10% variation is only acceptable for wide tolerance general molding because the check ring is leaking back into screw during the injection and packing portion of the cycle.

There’s an old phrase in the molding business that still holds true today:  “There are only two kinds of check rings:  Those that work, and those that leak.”  The same goes for worn out screws and barrels.  Keep everything maintained and you’ll get lower scrap and higher profits.

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