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 technical programs manager for resin distributor Channel Prime Alliance. Before his present assignment, Bob managed a molding operation for 25 years. You can reach him at bob.hatch@channelpa.com.

I had an interesting problem come up last week. A package landed on my desk that contained a cap and the runner system that goes with it. The note said the material was PC/ABS and the problem was blush at the gate.

I looked at the part but couldn’t see the blush mentioned in the note until I used a bright light and a 5X magnifier. Then I could see a substantial amount of blush on the outside of the part curvature and right at the gate. The reason it was so hard to see is that the part was colored white—one of the best colors to use when you don’t want defects to show up. Whether or not this was why they chose it as the production color I don’t know, but it certainly did the job on this part.

Runner Review

As usual, I began with the basics by reviewing the runner system, starting with the extensions of the main runner that turned 90° to feed into the cavities. They were full round, which is correct for any amorphous material but is particularly well suited for anything with polycarbonate in it. The main runner diameter was .300 inch, but it only needed to be .250 inch. The .300-inch diameter extended from each side of the sprue, through the 90° turn, and out to each of the cavities, just as it should have been.

However, at .225 inch, the sprue O-diameter was some .120 inch too small. To feed this main runner of .300 inch, the sprue O-diameter should have been larger than the main runner diameter. In this case, since the main runner was already .300 inch, the sprue O-diameter would need to be .344 inch. If the sprue O-diameter were .344 inch, then the diameter of the sprue where it attached to the main runner would need to be .445 inch. Either the sprue bushing needed to be modified or a new one would have to be purchased.

I now had just two items left to review on this runner system. The first was whether or not the runner was vented, which all runners need to be. Although I couldn’t tell from these parts what type of venting the mold carried, there’s always time for a quick review on venting: Runner vents should be .003 inch deep and as wide as the runner channel. Vent land should be .060 inch from the runner parting line. Drop into a .040-inch-deep channel to atmosphere. Finish each vent by draw polishing the vent lip to an A1 or mirror finish to make each of them self-cleaning.

The second item in this final check was to find out what kind of nozzle was being used on the molding machine. Again, this wasn’t obvious from the parts and runners, but it should have been a full taper design for amorphous materials with an orifice of .325 inch to feed into the new sprue O-diameter of .344 inch.

The Ballpoint Test

Next I looked at the part. There wasn’t much to review. I checked the nominal wall thickness (.180 inch), ribs (.065 inch), and whether or not the proper radiuses had been put in the appropriate places. I also checked thread dimensions, but the threads were just rings around the inside of the cap outside wall; it looked like an interference fit was used instead of a thread pattern. I took out a medium point ballpoint pen and drew a line between the rib and the nominal wall to see if it made a single or a double line. If it had been single, the radius would have been an adequate 25% of the nominal wall.

A double line would indicate the angle was too sharp and there wasn’t enough radius to keep the projections—ribs in this case—from cracking or breaking. The cracking or breaking could show up right away or later in the life of this part, but either way, it posed a potential notch sensitivity problem. Proper radiusing is the only way to correct it in the design stage.

Did I see single or double lines? Well, I saw a single line where the ribs attached and a double line where the inside rings were designed into the inside curvature of the outside wall. This meant that the ribs were OK, but the rings were not. This was not unusual, but somewhat surprising since it’s usually the ribs that don’t have enough radiusing to stop cracking and breakage.

Edge Gate Positioning

Finally, I came to the gate, blush and all, on the outside edge of the part. It was a standard edge gate, .135 inch deep and .200 inch wide, with a .090-inch or longer land length.

For a wall of .180 inch we needed a gate depth of .175 inch to feed a polycarbonate-type product into the cavity area. The part had a depth of .135 inch—not deep enough for this material.

The required width of the edge gate for a part of this size and volume is generally twice the gate depth. This meant that the gate should have been .350 inch wide since the proposed new gate depth was .175 inch. This was kind of hard to do since the main runner diameter was only .300 inch, but it wouldn’t hurt to have a slightly fanned shape to the gate. Anything up to a 15° fan usually works when help is needed to fill and pack out a part like this.

The land length of any edge gate should only be one half of the depth, but should never exceed .030 inch for any material. Since one half of the new .175-inch depth was .088 inch, we needed to use the default land length of .030 inch.

I almost missed one more thing. The edge gate was coming off the side of the full round runner. This is never a good idea as it makes the edge gate into a high-shear gate, just like feeding an edge gate with a trapezoidal runner. The cosmetic problems you can expect from using a high-shear gate are jetting, blush, flow lines, or flow rings in a hemisphere pattern around the gate, which was exactly the complaint with this part. Actually, blush was the complaint but, depending on the material, mold temperatures, and injection speed, any one of those defects could have appeared at the gate.

It was a simple fix: Modify or buy a new sprue bushing, resize the gate and move it to the center of the runner feeding into the cavities, be sure the runner is vented, and make sure the radiuses are adequate.

TROUBLESHOOTER’S NOTEBOOK Part: PC/ABS cap. Tool: Two plate, cold runner. Symptoms: Blush at the gate. Problem: Sprue O-diameter was too small, inside rings weren’t designed with sufficient radiusing, gate depth and width were too small, land length was too long, edge gate feeding off the runner caused shear. Solution: Replace or modify the sprue bushing, resize the gate and move it to the center of the runner, create proper radiuses.