The Troubleshooter, Part 22: Parting Line Runners and Gates

Every Monday morning I get at least a few shipments of parts that have gone wrong. A customer sent some parts recently and asked me to take a look at the voids he was getting in the thick section of the part. The part appeared to be a handle for something light in weight, such as a handle for a kitchen utensil or maybe for something like a rat tail file. The shape of the handle is pretty much a 1/2-inch square with rounded corners and sides. It is about 4 inches long with a .150-inch diameter hole in one end of the part. The depth of the hole was about 2 inches. The material looked like glass-filled nylon.

The accompanying note said the problem was in the thick section of the part on the end that wasn't cored out. The customer included a part that had been cut open lengthwise to show the cored out hole and the void in the thick section. The void was just one continuous void that ran from quite close to where the cored out hole ended all the way through the thick section to the gate.

The parting line runs through the center of the part and the gate is on the parting line at the end of the part. The gate is round, which is a little unusual for an edge gate. This combination suggested to me that we were looking at a mold being run on a vertical press.

At this point, I had to stop and wait for more information. I wanted a runner to look at, so I called the customer who called his molder who sent me a runner. By looking at the handle I could see the problem was the flow path freezing off too soon--before the part fills, packs, and then, with extra hold time and pressure, fills out the voids as they form in the thick section.

Based on the wall thickness the gate was big enough and the lack of gate blush or jetting also confirmed the gate size was OK. I figured the void problem was due entirely to pressure losses in the runner or maybe premature freeze-off of the runner due to the diameter being smaller than needed for this type of material.

When the package arrived, I pulled the runner out. I couldn't believe how close I had come to what the problem was. I must have ESP or something. The runner is just a single straight piece about 2 inches long and only about .150 inch in diameter. Now I could see we were dealing with a mold that has parting line injection, probably a Boy or an Arburg type machine. You would have to see this runner to believe it. Simply amazing!

The end of the runner where the nozzle seats is only .160 inch in diameter. The diameter of the runner increased to .200 inch at the other end where there is a cone shaped section that feeds down to the round gate. Now I know why the gate is round. The diameter of the cone where it attaches to the runner is .250 inch and it tapers down to .150 inch at the gate. The gate diameter appears to be .130 inch.

So the runner is smaller in diameter than the cone itself. Not only that but the material flows through the runner from the .160-inch diameter into the .200-inch diameter, then into the gate cone, which is just backwards of what we like to see. We always want material to flow into decreasing dimensions to maintain pressure on the melt front, not into an increasing dimension where we will lose pressure.

With bigger runner systems, we always use a larger diameter main runner to feed smaller diameter subrunners to keep from losing pressure. The second part of the problem is that the diameter of the runner and the nozzle orifice need to be large enough to keep the flow path open long enough to fill any voids that form in the thick section of the part.

Usually the diameter of the runner that feeds the gate should be about 11/2 times the thickest wall dimension in the part so we can pack out the voids and eliminate sinks. This time we won't follow the rules. All we need is a diameter for this short runner that won't allow it to freeze off and choke off the flow of material before the voids are packed out.

We have a single cavity to fill, the runner is only 2 inches long, and it should take only 1 to 1.5 seconds to fill and pack this part. But I want to sit on the holding pressure for an extra 3 or 4 seconds to fill the airless void as it forms.

The question is more like what diameter runner will it take to stay open that long? In our favor is the fact that we will want to run a hot mold to bury the glass in the material to give us a nice "resin rich" surface on the part. I look in my memory book and see that a .250-inch diameter will stay open for at least the 5 seconds I need. We can start with the .250-inch sizing and if it doesn't stay open long enough we can open it up later. The difference with my version of steel safe and that of most toolmakers is that I am starting out with a .250-inch dimension instead of .150 inch like some might do.

Anytime I size a runner for amorphous materials, such as impact polystyrenes, ABS, ASA, acrylics, or polycarbonates, I always start at least this big with runner diameters. Any filled material also gets this same treatment. The only materials that I size smaller are the unfilled crystallines, such as polyethylene, polypropylenes, nylons, acetals, and PBT polyesters.

I called the molder and suggested he open the runner up to .250 inch, leave the cone and gate as is, and drill his nozzle orifice out to .225 inch.

I got a call back a couple of days later. The molder said he was finally making good parts. He was also able to lower his barrel heats, lower his injection pressure, raise his hold pressure, and even speed the cycle up a little bit. It was just another case of restricted flow paths causing major problems. This was a fairly simple mold but the problems are the same as with the more complex ones.