Why parts change

When molding small precision parts, like gears, sometimes you find even though the cavities are made exactly the same, some cavities produce parts that are slightly out of tolerance and either perform poorly or are rejected.  OR, over time, for no apparent reason some cavities produce off spec parts.

Why is this?  The cavities aren't filling evenly.

Here's an interesting test:  Get a weigh scale that can measure at least to .1 gram if not .01 grams.    If the parts are small get at least ten sequential shots, sort the parts by cavity, weigh the parts and get the average cavity weight.  What you'll usually see is the defective parts weigh significantly different than the good parts.  Do a second experiment:  Begin backing off the shot size and look at the parts.  Eventually you'll see a shot where the plastic is up to the gates on some cavities, and other cavities are already partially filled.  This is the second proof on an unbalanced system.  Some cavities are being over-packed to fill and pack out the remaining cavities.  This is the reason for the differing part weights and why parts that were functional are now marginally functional or simply rejects.

How did the mold get this way?  Somebody diddled with the gate sizes.

When the gates are blocked people unblock them with drills, rat tailed files or a variety of other creative tools.  The end result is that they make the gate bigger.  Bigger gates allow easier fill for that cavity.

Making a gate bigger is easy.  Making them smaller and rebalancing the tool is expensive and time consuming.  Cavity filling is an exercise in flow.  You don't need to reweld and recut the gates.  Your local mold component supplier sells a component called a “Runner Shutoff Insert”.  This is a round slug pressed into a hole drilled through the runner.  It has the runner's geometry in it but it can be rotated with a special tool to block off a cavity.  Put these inserts in the early filling cavities and slightly restrict the flow to the cavity.  Do your weight study getting an overall average part weight, and then adjust your ‘restrictors’ until each cavity is within 5% of your overall average.  You'd be surprised how nice things turn out.

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