The Troubleshooter, Part 32: Problems with an acetal part

Parts arrived this morning from one of my best students, and I couldn't imagine why he needed help. I told myself that I had taught him all I know, so what could he possibly need help with this time?

I opened the box and found several parts and a couple of runners. I looked the runner over and could see it needed some help; then I looked at the parts and could tell from the blush at the gate, sink on the part surface, and flow marks pretty much all over the part (Figure 1) that I needed to call him and find out what was going on.

I called but didn't reach him, so I sat down at my computer to write a review that I could fax to him. I started the fax message by telling him all of the problems I saw on the parts and runner system, and then I ended my message by telling him to call me when he could find the time to tell me what happened to this mold. A few minutes later the phone rang and it was Peter. The first thing he told me was that this was not a mold he had anything to do with; he just wanted me to give him my thoughts on how to straighten out this mold, so he could compare my ideas with his.

Identifying the Problems
I told him that, from the greasy feel of the parts, blush at the gate, and the flow marks on the part, I could tell the material was acetal and he confirmed that it was. I reminded him that acetal needs big gates, but the runners can be on the smaller side. I took my verniers out of their corner in my desk drawer and started to measure the runner diameters. Based on the dimensions I was getting, I told Peter that he was already too large on one of the runner diameters, so we would have to put up with big runners and size the rest of the runner system accordingly. Then we would make the gates bigger to get the acetal to run nice looking parts at a decent cycle.

The thick section of the part was about .125 inch as best as I could tell without sectioning the part, so I told Peter that the runner that feeds the gate on a part this size should be approximately 1 1/2 times that, or .190 inch. Then, as we moved backwards through the runner system, we would need to increase the diameter of the runner at each new sub level (see Figure 2). In this case, the existing runner was sized so that the subrunner feeding the gate went from .185 inch to .200 inch, into a .245-inch runner, and then to a .245-inch main runner. This last runner was fed by a sprue with a .245-inch O diameter and a nozzle orifice of .200 inch.

I told Peter that it would have been better if the runner that feeds the gate had been sized at .190 inch, maintaining that increase through the next two levels of subrunners-.215 inch and .240 inch. The main runner would increase to .265 inch, the sprue O diameter to .312 inch, and the nozzle orifice to .290-inch. I told him I could see blistering on the current sprue bushing that indicated to me that the nozzle is being run too hot, no doubt due to the undersize nozzle orifice.

The gates should be .110 inch, based on the wall thickness of the part, instead of the current .090-inch size. The venting looked pretty good, but I could see gloss at the parting line that told me he was heating trapped air at the parting line.

I went on to tell Peter that he needed to check if the runner was vented and if not, to put in runner vents that are .003-inch deep, as wide as the runner, and then go out .060 inch from the parting line and drop into a .040-inch-deep channel to atmosphere. Then, to finish off the runner vents, he would need to make them self-cleaning by draw polishing the vent land to a mirror finish.

Part vents for acetal need to be only .0005 inch deep and .150 inch wide. I suggested that, due to the size and shape of these parts, I would put in two vents per parting line inch for each of the cavities, which would give him 30 percent of the parting line vented. I would also double vent each of the corners and carefully vent each of the air trap areas, such as ribs and bosses. Part vents are finished by going out .040 inch from the parting line, then dropping into a .040-inch-deep channel to atmosphere and draw polishing to make them self-cleaning.

Peter listened patiently, and finally said, "You know I didn't have anything to do with designing or building this mold." I could tell he felt like I was beating up on him for all the mistakes. He went on to say that he had already figured out most of the things that I had just told him, and he just wanted confirmation on those changes. I told him to make the changes and let me know how the parts looked and whether the cycle time changed at all.

Mission Accomplished
Peter wrote me a note several weeks later and said the changes had been made. The gate blush, sink, and surface imperfections were gone. The cycle stayed pretty much the same, even though they had lowered the barrel melt temperatures into the lower 300F range. It was more convenient for them to leave the cycle where it was so it better matched the speed of the assembly line. He said the lower barrel heats helped eliminate the formaldehyde odor that is so often associated with molding acetal, and that was a big plus for their operation. He also said that the deposits they used to get in the mold from the acetal had basically disappeared, so the toolmakers were happy. I just love it when the toolmakers are happy.

So they had fixed another problem mold and it only took a few hours in the tool room to make the changes and an hour or so on the molding floor to get the processing conditions where they wanted them for heats, speeds and pressures. All in all, they were pleased with the return they received for the time they had invested in this mold. Actually, all they did was finish the mold the way it should have been when it came in from the moldmaker. The moldmaker could have made the same changes if he had only known what to do.

Unfortunately, there aren't any schools moldmakers can go to for the sizing information they need. Even the current apprenticeship programs don't address these concerns. Moldmakers can get a piece of the puzzle from one material supplier or another, or from an engineering service here and there, but even then they don't always get what they need the most. If they can find and use this information, they can help their customers-the molders-do a better job of molding parts and maybe even make a little money while doing it.