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It was late summer and I was running out of things to work on when the quick delivery company drove up in front of my office. It is a good thing I saw them coming because I had to quickly move Jake out from in front of the door so the driver could get through to deliver my package.

Bob Hatch

September 30, 2011

6 Min Read
The Troubleshooter: Runner connection causes part to run short of material

It was late summer and I was running out of things to work on when the quick delivery company drove up in front of my office. It is a good thing I saw them coming because I had to quickly move Jake out from in front of the door so the driver could get through to deliver my package.


The Troubleshoot: Bob Hatch

It turned out to be a big box with a lot of packing material on the inside to protect the gray ABS runner system and parts. I pulled the parts out of the box and gave them a good look-see.

I looked at the parts first and while they appeared to be well molded, I cut one of the parts in half to see what the inside of the part looked like. Turns out it is a good thing I did. The center of the part had a small diameter void running most of the length of the part.

Next I looked at the runner and it was pretty easy to visually spot a potential trouble spot. The connection between the main and sub runners was designed in a funny way. The main runner was kind of set on top of the sub runners instead of meeting in the center.

This runner connection was restricting the flow of material from the main runner into the sub runner by over 50%, according to my calculations. The sprue had been cut away from where it was connected to the main runner, but I could easily tell that the sprue was undersized from what it should be to inject plastic into the runner system in an effort to fill and pack these parts.

It was time to get tools out so I could do some measuring. My dial calipers were still in my seminar bag from my last road trip to put on a seminar for some real nice folks over in Michigan. I dug them out and started taking measurements.

The sprue measured .300-inch where it would have attached to the main runner. Using a standard taper of .0175" per inch and figuring the sprue length at about 4 inches long, I ended up with a sprue "O" diameter of .230", which in turn would mean out full taper nozzle orifice size would be 10% smaller or about .205" in diameter.

The full round main runner measured .312" or 5/16ths of an inch. The depth of the modified trapezoidal sub runner was also .312" but the width was only .250" by .190" and the worst part of this review was where the sub runner was only sitting half way down into the .312" full round main runner.

Next I moved to the gates and saw that all 4 gates were identical although they did appear to be cut into the steel manually. Not that doing something done manually is all bad, it is just that manual operations often leave undercuts on the length and diameter of the gate that often causes this kind of sub gate design to break off in the tunnel area; then the molding machine needs to be shut down while the tool makers do their thing. Needless to say this leads to a lot of finger pointing, but eventually the mold can be put in cycle and parts can be molded successfully.

Beyond the obvious problems I measured the gate end of the tunnel gate and that turned out to be .080". Where the tunnel attached to the sub runner I measured .150"and these dimensions were a little small for an ABS part with part walls that measured around .300" nominal.

But then I saw a gas pin location on the part and realized I was dealing with parts that were gas assist technology to core out the thick walls of the part with gas down the middle of the part and that is why I could see a gas channel down the center of the part that measured .100" by .150".

I knew it was time to re-group and figure out where I should go from this point. I went back to the main and sub runner connection that would be difficult to work with if this were normal injection molding, but since it is gas assist I will give the tool maker credit for attempting to eliminate the flow of gas back in through the runner channels.

At this point there are .080" diameter tunnel gates feeding into the reduced wall thickness of .150 typical around the gas channels. Now this is a problem because ABS should have gate diameters equal to 75% of the wall thickness, which would make the gate diameter requirement .113" instead of the .080" they had designed it to be.

Now it could be that the toolmaker knew the .080" gate diameter was smaller than it should be, but he could have figured he could always open up the gate it he needed to.

Since somebody sent me these parts to inspect, I am guessing the .080" diameter gates will need to be opened up. But what else needs to be changed to get this mold running again?

All's well that ends well

I know we found the sprue was too small to effectively fill and pack the part walls, and now that I know gas assist is involved I will recommend that the molder change the sprue bushing out for one that will make the sprue large enough in diameter to fill and pack the parts.

Then we will let the gas go ahead and run a channel through the thick sections to allow these parts to run at a faster cycle than would be possible without the gas assist, and avoid the typical sinks and voids that are usually associated with thicker wall parts.

The correct dimension for a sprue bushing feeding this runner system and using ABS as the material would be a spue "O" diameter of .344" and a diameter of .414", where the sprue attaches to the main runner. The full taper nozzle orifice would then be approximately .373".

From what I know about this mold, I think the molder will benefit from these changes I have suggested and still be able to use his gas assist to gain the benefits that this process offers. I went ahead and placed my call to the molder to see how close I came to reality and I must have caught him between customer visits, because he actually picked up the phone and talked my head off.

He said they had missed the sprue size and said he would get together with the toolmakers and see what they wanted to do; if they had any questions they would get back to me. Well, I could not hope for anything better. I will just wait and see if they call. It has already been 4 days since we had our phone conversation so I am guessing everything is going well.

My high priority project for the rest of the afternoon will be to take my trusty dog Jake out for a run and swim at the local park and lake area. He will like that because it has been a week or so since we have done that.

About the author: Consultant Bob Hatch of Bob Hatch & Assoc.  has more than 45 years of experience finding solutions to processing challenges. Reach him at [email protected].

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