The Troubleshooter, Part 42:Coping with thick walls and gatesThe Troubleshooter, Part 42:Coping with thick walls and gates

This article continues our series of troubleshooting reportsfrom one of the
leading on-the-spot problem solvers in the molding industry. BobHatch is manager of technical service and customer support forPrime Alliance, the Des Moines-based resin distributor. Beforehis present assignment, Bob managed a molding operation for 25years.

Ireceiveda package this week with parts and runners inside, just the wayI like them. I always tell people that I can get more out of lookingat runners than I can just looking at parts. This time I foundmyself examining good-looking, thick-wall parts and an almostgreat cold runner. The material was polycarbonate and the onlymistake I could see was the size of the nozzle orifice feedingthe sprue bushing.

The sprue O diameter was .280 inch and the nozzle orifice wasonly .125 inch. Ideally the sprue O diameter for a cold spruefeeding this size runner system would be .312 inch, or maybe .375inch with a nozzle orifice of .290 inch or .350 inch. But whatwas interesting was that the parts being molded looked great inspite of this discrepancy in the nozzle/sprue bushing relationship.

I called the molder and he said he was pleased with the waythe parts looked. His main concern was how to get the parts offof the runner without having a big chunky-looking gate. He wantedto know if he could make the gates smaller to improve the cosmeticsat the gate. I suggested that he didn't want to make the gatessmaller because that would probably cause gate blush or jetting,and possibly flow lines on the parts.

He agreed he didn't want to trade cosmetic defects for gatesize but wondered what else he could do. I suggested heated bladedegaters for a start. Air-assisted degaters with a rheostat wiredinto the blades can raise the blade temperature to a range from150 to 275F for acrylic and PC respectively.

The blades could also be thin and sharpened to a beveled edgeto allow them to slide into the land area of the gate. This wouldmake the blades about .030 inch thick. He said he hadn't messedwith heated gate cutters in the past but would look into it. Itold him that I see heated gate cutters being used with polycarbonateand acrylic parts all the time.

I told him that if the gate cutters worked, he could builda shear fixture on an Arbor press and accomplish the same thingwithout all the manual motions that handheld gate cutters require.I asked him to call me back when he was done and tell me how everythingworked out.

When I got the call about two weeks later the molder told mehe checked into the heated gate cutters, found exactly what hewanted, and ordered them. He took the cutters out of the box,plugged them in and waited for them to heat up. When they reachedtemperature he squeezed the handles together and sliced the biggates off as easily as a hot knife cuts through butter. He alsosaid that it looked like he could slice off a finger just as simply;he thought a little extra training for the operators would bea good idea.

I asked him if he wanted to go ahead and optimize the runner,sprue, and nozzle orifice size since he was so close to havingit sized correctly. He asked what he could do with the mold fullyoptimized that he couldn't do now. I told him that once he increasedthe diameter of the sprue bushing and opened up the nozzle orificehe would be able to bring the barrel melt temperatures down 20to 30 deg F and speed the cycle up a little bit. That soundedgood to him, so he said to fax him the changes and he would seeif I was right about the cycle time.

You'll recall that initially he had a .340-inch main runnerfed by a cold sprue with a .280-inch O diameter, which was fedby a .125-inch orifice in a full-taper nozzle. (At least he wasusing a full-taper nozzle; I recommend it for amorphous materials.)

The connector diameter between the main runner and the subrunnerswas .330 inch; subrunners were .300 inch. The ends of the subrunnerswere only .230 inch in diameter and tapered down to the gate,which was .200 inch deep and .210 inch wide, with a .030-inchland. The taper was the result of a conversion from subgates toedge gates.

The gates, subrunners, connector piece, and the main runnerswere all OK for this application. The problem was strictly inthe sprue and nozzle. To work with the size of main runner theyhad, the sprue bushing had to be opened up so that the O diameterwas .375 inch. The nozzle orifice could then be opened up to atleast .350 inch; this change alone will reduce the shear effectof the material shooting through the nozzle and reduce any pressureloss in the runner.

Then, barrel heats could be reduced and the injection pressurewould likely drop to about 600 to 800 psi. The hold pressure wouldprobably drop a similar amount, but with these thick-wall partsthe hold pressure would still be higher than the injection pressure.I guessed that the process would end up at 900 psi for the injectionpressure and 1100 psi for the hold pressure.

After optimization, it was just a formality to drop the barrelheats into the 500F range and speed the cycle up as the heatscame down. It's not an exact science, but it works every time.

We started with a gate cosmetic defect problem and ended upwith a fully optimized mold, which speeded up the cycle and eliminatedmany rejects over the first year of production with this mold.

These suggestions not only worked with these polycarbonateparts, but they also would work on other amorphous materials,such as acrylic, SAN, and polystyrenes. Even polysulfone-typematerials will respond to this treatment. It's another fine exampleof what can be accomplished in the molding shop if the moldmakersdo their part to help.