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The Troubleshooter, Part 38: Acetal flow lines

March 1, 2000

6 Min Read
The Troubleshooter, Part 38: Acetal flow lines

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 manager of technical service and customer support for Prime Alliance, the Des Moines-based resin distributor. Before his present assignment, Bob managed a molding operation for 25 years.

Some parts that arrived recently made me think not only was I going to call the customer with some suggestions for corrective action, but I was probably going to have to schedule a flight to go and stand by the machine to get the problem solved.

The material was an acetal copolymer, and the molder was getting some flow lines on a round, flat surface just inside the gate on the part (see Figure 1). Just what was causing the flow lines was something I didn’t really know for sure. It could be jetting at the gate; or, since this was a hot runner mold it could also be that the land of the gate had not been relieved at a 45-degree angle back up into the flow path of the material. Possibly the injection speed was too slow or even too fast for this gate configuration. It could be a dozen possibilities, so rather than guess I went to see for myself.

The reception committee consisted of molders, toolmakers, and design engineers, all anxious to see if I could solve the problem. We looked at all the bad parts that had been molded. I listened to all the corrective action attempts they had tried, such as higher barrel heats, lower barrel heats, faster injection speeds, slower injection speeds, drying the acetal, not drying the acetal. From what I was hearing they had tried everything I would normally try to correct the problem.

So what was left? I asked if we could look at the gate areas in the mold itself. We then all visited the toolroom where we found one of the most knowledgeable toolmakers I have had the pleasure of meeting. He was a student of my On the Road with Bob Hatch troubleshooting book, and he claimed to have followed my suggestions exactly. Not being one to step on a student’s toes, I asked him to show me the gate detail to see if he had the hot tip diameter and 45-degree land relief as it should be. If it was less than 45 degrees, it would cause premature freeze-off of the material as it flows from the hot runner drop into the gate—something that also happens to tunnel gates when the angle is too sharp on a two- or three-plate mold. He quickly picked up a block of steel that had a gate insert in it, but the angle was not visible.

However, there was a Micro-Vue system in the quality control lab that would make the gate area easy to see, so we paraded over to the QC lab for a look. What a difference! We zoomed in on the gate land area and it was easy to see that they did indeed have the land at a 45-degree angle and the taper went right down to the gate diameter. The land angle was sharp, as it should be, and I’m sure if we had measured the land dimension it would have been at the .002 inch we recommend. (I say we, but it is a recommendation that I picked up at Mold-Masters when I last attended one of their seminars.) The gate diameter was 90 percent of the thick-wall portion of the molded parts and the land was relieved properly back into the flow path of the material.

Both the mold and what had been done to troubleshoot the parts seemed first-rate and thorough. Now it was time to move to the molding floor to watch the mold run. The molding technician was as good as any I have run into and he showed me around the press and reviewed the control panel displays for me. We looked at all the heats, speeds, and pressures, but I still couldn’t find anything unusual after spending 20 minutes at the machine. The material was not being dried, but that is really optional with a low-moisture material like acetal, especially in the winter. Besides, the cosmetic defect on the flat round part looked more like flow lines than splay so I didn’t jump on the lack of drying as the cause of this problem.

I watched the feed indicator as the material was being injected into the mold and thought the process of injecting plastic and recovering the screw was OK. Then something caught my eye. The mold temperature was only 140F. Acetal is a funny material in how it reacts to certain injection speeds and mold temperatures. Fast injection gives you a glossy part, which is what we wanted in this application, and a 180F mold will give you a more rigid part, which we also wanted. By contrast, a slow injection speed will give you a matte finish and a cold mold, such as 50F, will give you a more flexible part. So what does the 140F mold temperature give us?

The molding technician thought the 140F mold water was OK because the data sheet provided by the material supplier suggested mold temperatures from 75 to 180F. So much for some of the information you receive from material suppliers. Most of it is right on target but here was one that was too broad. We needed more information on which to base our temperatures settings—such as whether or not we wanted a rigid or flexible part.

I suggested they run the mold temperature up to 180F and walk away for 20 minutes to see what would happen. When we returned to the machine, the flow lines were gone. Just to be sure, we ran the heats up to 200F and waited to see if it got even better; it didn’t, so we dropped them back down to 180F.

Since the hot runner system was optimized, the parts did not warp when we raised the mold temperature, nor did we have to slow down the cycle. Optimizing a hot runner system means drilling the molding machine nozzle out to the same diameter as the flow tube diameter in the hot runner, which is a 1/2-inch diameter in this mold.

For the record, in this mold the flow tube diameter is 1/2 inch, the crossover tube is 3/8 inch and the drops are 1/4 inch tapered down to 1/8 inch, then tapered again to the .090-inch gate diameters that feed into the part. Hot runners are pretty easy to run when the tool is optimized. Of course the nozzle stays with the mold after this sizing operation, but that is a small price to pay for a mold that runs good-quality parts at an efficient cycle.

This is about all we had to do. The venting was adequate, and with the barrel melt temperatures at an optimized 350F we were getting good parts every shot. These techniques will work with most materials and they are easy to incorporate. The proof that all was well was the molder’s phone call a week later. He said the customer was extremely happy and glad the problem was resolved. The parts looked better to him than anything they had run since the mold was brand-new.

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