The Final InspectionThe Final Inspection

Editor's note: Consultant Bill Tobin of WJT Assoc. helpsmolders diagnose molding problems and offers these guidelineson tool inspection.

Thereare two things many people leave out of a mold purchase order:a request for a final inspection and part interchangeability.When I started in plastics we used duplicating models but stillhad to hand-finish cavities. They were within tolerance, but stillhand finished. We're now in the age of CNC and CAD/CAM, wherehigh precision is expected, if not always delivered. Still, whena mold is completed, we're usually in such a rush for parts thatwe skip the mold inspection and jump to the tryout. In so doingwe miss an important point: The mold is the means of producingthose parts. If it's not well made, what's the point?

What if the water circuits aren't labeled? What if the balldiameter of the nozzle is different from the one you'll be using?What if the parts come out on the ejector pins but it takes atow truck to yank them off?

These silly questions bring us to the crux of the problem:The mold should be inspected before it's used. This is especiallyimportant when the moldbuilder is far from the production source.A client of mine in New Mexico had his molds built in Italy. Ifproblems weren't caught there, by the time the mold reached NewMexico it was cheaper to pay for changes locally than to shipit back. So here's what we did.

The Inspection

Before it was shipped to the tryout house, we asked for themold and a toolmaker for an afternoon. We put the mold on a benchand completely disassembled it. We were looking for shims, a predictorof failure potential. We found none.

We then drew a sketch of the cavity layout and removed allcavities and components (bubblers, inserts, etc.). Then we mixedup the components. We subassembled the core and cavity insertsand put them into any hole we could find in the mold base. If,as we specified, everything was interchangeable, then it didn'tmatter what went where. Sure enough, everything was keyed to fitin the proper location for the core and cavity as well as thevents. The spare components also interchanged.

Next we took a file and did a hardness test. The base willusually show file marks, but hardened cavity material will not.Another test uses ferric chloride; if the mold is made of toolsteel, this solution leaves a black stain. It won't stain platedor stainless steel.

Reassembly

We were ready to reassemble the mold. We put all the componentsin their proper place until the mold was fully assembled. We thenair-pressure tested the waterlines. This is done with a seriesof one-way valves, two female waterline connectors, a pressuregauge, and a bleed valve. We isolated one line, charged it to60 psig of compressed air, and waited 10 minutes. If the pressureis still 60 psig, we bleed the air and test the next circuit.The reason for this is simple: Air's viscosity is much lower thanwater's. If the circuit is air tight, it's water tight too.

Next, we used a depth micrometer to make sure all cavitieswere the same height. If one is higher than the others, it willcrush the vents. If lower it will flash without proper clamp pressure.If you can (with pin gauges or other instruments), check gatediameters. Unless they're computer balanced, all gates shouldbe the same size.

In the next phase we checked screws, bolts, and water fittings.We'd specified English linear waterline fittings and holes drilledto NPT standards. The screws and bolts, however, were metric.We told the toolmaker to put four engraved plaques on all sidesof the mold to indicate that metrics were used.

Little Things

We then looked for the mold identification. Was it stampedin a recessed pocket? Were waterlines labeled? Was there a hotrunner wiring plaque that included voltages, wattage, and manufacturername?

Finally, we checked for options that make things run easier.Were the quick-change pins installed on the platen side and werethey properly sized and distanced for our machine? Did we havehandling holes for all plates? Was the top labeled? Was the safetystrap on the operator's side? Were both hoist rings permanentlyattached to the mold plates?

With a clean bill of health, now it's time for the tryout.If there are corrections to be made, do them before running themold. Writing good tooling standards or a good purchase orderis a must if you want a good tool.