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April 22, 1999

6 Min Read
The Troubleshooter, Part 30:A lesson in shear and diameters

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

A couple of weeks ago, I received a care package containing parts that looked like Army flashlights. I'm sure they were not real Army flashlights, but they did look like Army flashlights.

The molder's complaint was that he couldn't consistently fill the parts out, and he was getting a lot of rejects. He would get flash on the threaded end or short shots, but nothing consistent. I looked at the gate and could see the part was being filled from a stubby runner straight into the side wall of the part. What I couldn't see was what was feeding the stubby runner. It looked like maybe it was being fed by a heated sprue bushing, and maybe it was a single cavity mold, but I didn't want to assume anything, so I called the molder and asked him what it looked like. He told me it was a three-plate, three-cavity mold. Who would have believed it? I asked him to send me a runner. Next day, the runner came in, and I could see what the problem was.

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Figure 1. The part was flashing in some areas but shorting in others. The solution was to drill out the orifice of the sprue bushing to more closely match the diameter of the runner. Also, the land angle of the heated sprue bushing orifice from the drop to the stubby runner was changed from 90° to 45°.

Mold Problems
The three-plate runner was being fed by a heated sprue bushing, and the orifice in the heated sprue bushing was only half the diameter of the cold sprue where it fed into the runner. I told him he would need to increase the heated sprue bushing orifice to at least .275 inch (Figure 1) and change the land of the hot tip gate from a 90° angle to a 45° angle. I also suggested he drill the sprue bushing and the nozzle out to match the flow tube diameter of the heated sprue bushing. I told him I didn't care what the flow tube diameter was as long as he was sure it was the same dimension all the way through. I was hoping the flow tube diameter would measure at least .5 inch for the ABS material he was using, but I suspected a .375-inch flow tube would also work in this situation.

The gate lands were all too long where they fed into the parts from the stubby runner. This caused a shear point and a restriction to the material as it tried to fill and pack the parts themselves. I suggested he reduce the land at the gate to .030 inch from .090 inch. I also said he should reduce the land of the heated sprue bushing orifice into the stubby runner from the current .115 inch to only .005 inch.

I could see a grainy surface on the switch protector ribs on the part, which I guessed was caused by a lack of venting. When you trap and then compress air in these hard-to-vent areas, you will see dullness on the surface, then a grainy condition caused by the air being heated even more. I suspected the runner wasn't vented, and all the runner air was being shoved into the hard-to-vent areas on the parts.

I called the molder back, passed my recommendations on, and waited for a return call.

Making the Changes
It was two weeks before I heard from him, but when I did hear, it was all good. The toolmakers drilled the sprue bushing and the nozzle out to the same .5-inch diameter. They opened up the feed point to the runner from .150 inch to .275 inch. They left the gate diameter of the stubby runner at .100 inch but changed the length of the gate land from .090 inch to .030 inch.

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Figure 2. The surface of the switch guard appeared grainy, a sign of poor venting. Vents .003 inch deep were cut into the ends of the runners and polished to help make them self-cleaning.

They vented the runner like I told them, which was to make the vents .003 inch deep, then go out .060 inch from the parting line into a .040-inch-deep channel to atmosphere (Figure 2). Then they polished the vent lips to an A1 finish in the direction of air flow to make them self-cleaning.

The molder said he couldn't get much more venting on the part itself, but he did get the toolmaker to do some perimeter venting on both ends of the part that seemed to help. I think he was lucky to get the toolmaker to do this much venting. Toolmakers usually tell me the mold is already vented when I know it isn't. Something I've learned about toolmakers is that talking to them is tricky. You can't tell them much, but you can make suggestions, and if they make sense, they might do it or they might not. It really depends on how their day is going. But after you connect on the first mold, they'll usually be more cooperative.

Promising Results
He wound up with parts that were running without sink on one shot and flash the next. He was able to reduce the melt temperature of the material and could fill with less injection pressure. He could have sped up the cycle without producing warped parts, but decided to leave it at the current 45 seconds because the end caps seemed to fit better on the parts running at the 45-second cycle.

What I found so interesting was his last comment before he got off the telephone. He said he couldn't figure out why the mold didn't come to him already incorporating all the changes we had to make before he could run production parts. I just told him that this is his edge in the world of molding. If all the toolmakers knew these tricks of the trade, his competitors would mold parts as well as he is able to do.

This is just another good example of a mold being almost ready to run production but not quite. The runners and drops were sized correctly, and the gate diameter and stubby runners were OK.

It was just the nozzle orifice, heated sprue bushing orifice size, and feed point to the runner that were undersized. These restrictions, plus the lack of runner venting, caused all the problems. He was losing pressure through the runner system and fighting shear points all at the same time. He could slow injection speed to reduce shear, but then he couldn't pack out the sink. Well, if he had a new machine, he could, but he couldn't with the one he was using.

The molder told me that, after all was said and done, his parts had the glossy look he wanted, and he hadn't more than a handful of rejects since starting the mold up after the rework. I congratulated him on his willingness to accept new ideas and to make the changes necessary to help the mold run like it was supposed to when he first got it. I can't wait for the chance to work with him again.

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