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A Plastic Injection Molding Process Problem – Troubleshooting Methods (cont.)
(continued from previous page) With that information, it appeared that the material residence time in the barrel of hot runner issues would be the most likely cause of the issues. We ran multiple moisture checks on the resin, which all came back good with a moisture content of 4% (.04) or less, and most readings were 2% or less (.02). At this point we started looking harder at the residence time of the barrel with a fairly low part weight (150 grams) and a fairly large shot size 70 ozs. To test this theory, we let the machine sit down for 10 minutes to test out the ability of the material to with stand the heat over a long period. About 8 minutes into this test, we noticed smoke coming from the mold out of two different valve gates. We checked all of our hot runner cables, connections, and controller readings and everything appeared to be yet, it continued to smoke. This must be it. It would account for both issues and we can see it happening. The tool was pulled and the hot runner system was checked. We found that two of the thermocouples were bad and were giving us false readings that were almost 200 degrees less than the actual temperature. After correcting this problem, we put the tool back into the machine to test our work. What we found was less than encouraging though, as it did correct the #2 issue, the contamination & black spec problem, but the splay problem remained.
We decided to test the residence time theory again, as this still could be the potential cause of the splay problem on our part. We let the material sit for 10 minutes and shot parts, and saw nothing significant. This was repeated at 20 & 30 minute intervals of sitting in the barrel cooking, but once again it had no significant effect on the splay problem even at 30 minutes of residence time. We also ran numerous trials with varying valve gate positions and fill speed to no avail or significant effect on our intermittent splay issue. Once the problem showed up, it would be persistent, and if you purged the barrel, the problem would go away for as much as 2 hours at a time, just as the Technicians had noted earlier. What was going on?
We went back over our data and troubleshooting records and found nothing significant that we could see that we missed. We still felt that wet material would be the likely suspect because of the consistency of the problem when it showed up, but didn’t at first explain why it would go away after purging for up to two hours. While looking at the machine, we noticed that the hopper on the machine held about 25 lbs of resin when full. The feed system would draw about 20 lbs. of resin at a time and sit idle until the next time the material level dropped to a point where it called for more once again. Now knowing that our part weight with runners weighed in at 150 grams or .33 lbs., could it be that our material was getting wet before it was used? We calculated the residence time of the material sitting in the hopper out to be 1.5 hours. We calculate this by the hopper full weight of 25lbs. x 454 = 11,350 grams/150 grams = 76 shots in the hopper when full. Cycle time was 72 seconds or 50 shots per hour, thus 1.5 hours to empty. To check our theory, we waited till the hopper was almost empty, and took a sample to run though the moisture analyzer. It came out at 5.5% (.055). There it was.
Had this been during the winter, this would not likely have been a noticeable issue, but our summers are very humid and that is enough time for our resin to absorb enough moisture to cause our problem. That is why when the Technicians purged it out, the problem went away for almost 2 hours. The solution was to add a small auxiliary dryer and hopper at the machine and the problem all but disappeared. I hope that this shows you one reason why you need to look at all the possibilities and keep digging till you find the root cause of your problem. I hope that it also shows some of the interaction of your injection molding processes with each other and how one thing can affect others.
Written by: WM8C, August 28th, 2006. Not for use without written permission
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