The Troubleshooter: Part 97: When will they learn?

By admin
Published: December 31st, 2007
Messing with the sprue and runner size after they?re optimized is a mistake that can cause defective parts.


This article continues our series of troubleshooting reports from one of the leading on-the-spot problem solvers in the molding industry. Consultant Bob Hatch of Bob Hatch & Assoc. has more than 45 years of experience finding solutions to processing challenges.

The second time around for the same part shows it’s not smart to disregard the rules.

I received a part and runner the other day that seemed to be pretty well molded. I read the note that accompanied them and found it was from someone I met a long time ago. The note indicated the molder wanted me to do a final review of this part and send him a bill for the time spent on it. Then I remembered the first time I worked with him and what we were trying to accomplish.

When we worked on this part some time ago, it was cracking around its molded-in brass inserts. It also had cracking and crazing in many of the 90° angles designed into the part. We had gate blush issues and many areas of shear splay originating at the gate and radiating outward to the part’s surface. I guessed this molder just wanted to finish troubleshooting his part.

I looked it over and realized he had made most of the changes I recommended, such as radiusing all of the sharp corners to eliminate cracking and crazing. So what could be wrong now? I kept looking and, sure enough, I could see blush at the gate as well as a little bit of shear splay—same problem as before but not nearly as much. So what happened? Why was something we corrected a few years ago back again?

Blush at the gate always tells me the gate is too small or the injection speed is too fast, or maybe the material is not being run hot enough. Also, I could see from the parting line that the vents were not doing as good a job of getting rid of the air in the cavity as they were when we worked on this part previously. I decided to get back to basics and review everything as if I had never worked on this part.





Don’t undersize the sprue

I checked the sprue and runner and could see the sprue O-diameter was smaller than the main runner diameter. (I could also tell that the runner was no longer vented—more on that later). I’m just guessing, but I bet someone in the toolroom decided the sprue needed to be replaced with a smaller bushing—something I’ve often noticed over the years. The reason for this I’m usually told is that the toolmaker was trying to help out the molders by giving them a smaller-diameter sprue bushing so they could use a smaller-orifice nozzle, which eliminates the tendency toward stringing or drooling.

What the toolmakers don’t realize is that when the material flow path is undersized, the molding technicians usually raise the barrel and nozzle melt temperatures to fill and pack the parts. This, in turn, causes the cycle to slow down to keep the parts from warping, which increases the material’s residence time in the barrel and therefore causes the material to degrade.

To correct this sizing error, I recommended a new sprue bushing with a 0.312-inch O-diameter to replace the 0.210-inch bushing. I also suggested going back to a full-taper nozzle with a 0.290-inch orifice diameter of instead of 0.160 inch. (We changed this previously, but somehow the nozzle was changed to a general-purpose nozzle—maybe it was the only spare nozzle they had at the time.) Then, I recommended increasing the main runner diameter from 0.250 inch to 0.312 inch to better match up with the new, bigger sprue size.

The short subrunners needed to be increased from 0.240 inch to 0.312 inch—same as the new main runner. Since the material flow was not split to feed two subgates at each end of the main runner, the subrunner diameters didn’t need to be smaller than the main runner.

Now for the gates: At 0.090 inch, feeding a wall of 0.120 inch, the subgates appeared to be sized correctly, but the gate blush revealed that the gates were really too small for this application. First, the subgates needed to be rectangular, not round. The depth would stay at 0.090 inch but the width needed to increase to 0.180 inch. It would still be a regular subgate, but the change in shape would allow more flow. Remember, the gate width controls the volume of material that goes through the gate.

Venting

I couldn’t see any evidence of venting on the runner system, and that was causing a few defective areas on the parting line of both parts in this two-cavity, two-plate mold. The runner needed to be vented at the sprue puller and at each end of the main runner.

Sprue puller vents are treated the same as parting line vents—only 0.0015 inch deep. I recommended adding one vent per parting line inch and double venting the corners of each cavity.

To review venting, part vents are generally 0.200 inch wide. The land length of the vent lip is 0.040 inch, dropped into a 0.040-inch-deep channel to atmosphere. Finish by draw polishing the vent lips to an A1 or mirror finish to make them self-cleaning.

Main runner vents, on the other hand, should be 0.003 inch deep, as wide as the runner being vented, with a 0.060-inch land, dropped into a 0.040-inch-deep channel to atmosphere. The same draw polishing applies as with part vents.

That was all I could see that needed changing. I called the molder and his toolroom supervisor and passed on my recommendations, answered their questions, and told them I would like to hear from them as to whether or not we corrected all their molding problems.

It did not take long for them to call back and the report was good. They could not believe they had optimized this mold previously and all they had to do was pretty much repeat the process they went through the last time.

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