The Troubleshooter: When is the regrind percentage just too much?

A chunky runner gets a review.

I was visiting the Phoenix area to do some in-house seminars and spend time with my son, who’s attending a golf school in Chandler. The last thing I expected to see pulling up outside was a big brown truck with a package, but sure enough, the UPS driver found me. Must be some new technology at work.



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. You can reach him at [email protected] signed my name and opened the box. I found parts, a runner, and processing information, but no personal note. The return address on the outside of the box wasn’t much help, but I know that sooner or later I always hear from the molder.

The process sheet indicated they were getting sinks and the occasional void in the thicker sections of the parts. This meant that we needed to check the runners and gates for pressure loss and flow restrictions.

I started my review the way I always do, at the subrunner that feeds the gate, or the cold drop that feeds the gate, on the interior section of the part, but in this case I didn’t see much wrong with the drops, other than the fact that they were huge.

I moved on to the subrunner that connects the drops to the main runner. It measured 0.250 inch deep by 0.290 inch wide—pretty good so far.

Next was the main runner measuring 0.250 inch deep by 0.290 inch wide—same as the subrunner. Well now, we all know this just is not the way it should be. The main runner depth has to be deeper than the subrunner to avoid pressure losses in the runner system. How much deeper is a mystery to many toolmakers and mold designers. I tell them to make the main runner 0.025 inch deeper than the subrunner, or in this case, 0.275 inch to pressurize the subrunners properly. We would set the width at 0.300 inch.

(Incidentally, if you’re on the metric system, I suggest using millimeter differences. Therefore, if the subrunner diameter is 6 mm, for example, the main runner should measure 7 mm.)

Then I checked the feedpoint to the runner. It had an orifice diameter of 0.250 inch feeding into a cone with a diameter of 0.312 inch. Once we deepened the main runner to 0.275 inch, this feedpoint would need to be made larger—more like 0.312 inch feeding into a 0.344-inch-diameter cone.

This would correct the runner system problems, and so I moved on to the gate sizing to see how it measured up. With wall sections as thick as 0.300 inch and 0.360 inch, I was sure the gates would need to be larger than the present 0.090-inch diameter.





Thick-wall parts are susceptible to sink if the gates are not properly sized.

Correct gate dimensions were critical; otherwise the molder would get sink in the thick sections of the part farthest away from the gate. Typically, I would recommend gate sizing of 75% of the thick wall sections, but that would be too big for this application. So I recommended a gate diameter of 0.125 inch for these gates and we’d see what the results were. Besides, we could always make the gates a little bigger if we still got a few parts with sinks or airless voids in the thicker sections.

The biggest problem with this runner was that it produced so much scrap with each shot. I estimated that at least 30% of the shot weight was ending up in regrind, and that’s just ridiculous in this day of high-priced materials.

My next suggestion for this molder was to sit down with his customer to review this part and figure out what the yearly production volume would be for the next three, five, and seven years and the life expectancy of the mold. If the numbers looked good, I would suggest building a hot runner mold to replace it; but if the volume was declining and the expected life of the mold was uncertain, then I would recommend continuing with the present mold.

I suspect the key decision point here was what the part quality issues had been, particularly in the area of visual clarity. If they were adding back in a large amount of regrind to the material drying system, this left them open to a tremendous amount of material tinting. However, if they were not putting the regrind back in, they could pay for the new hot runner mold with just the material savings.

I passed along my suggestions to the molder, and it was just a couple of days later when I heard back. He had assembled his senior staff to be on a conference call with me to let me know that the changes I suggested had their parts looking better than ever, and the cycle time was a full 5 seconds faster than before.Was I surprised? No, but then not much surprises me anymore.

Troubleshooter’s 
notebook
Parthttps://www.plasticstoday.com/material: Acrylic faucet handle.
Tool: Eight-cavity cold runner
Symptomshttps://www.plasticstoday.com/problem: Sinks and voids in the thick part sections.
Solution: Enlarge gate diameters and the feedpoint to the cold runner.