Our legal counsel notes: Dr. Henwood speaks for himself and not for ARM as a whole. We encourage feedback but comments should be on technical matters raised. Dr. Henwood’s responses are solely his responsibility and not ARM’s.
As part of a recent consultancy assignment, I needed to make a long run of parts in one of my hexagonal test tools. At the end of the job, the two ends of the tool were coated with a lumpy deposit of overcooked polyethylene (PE). I was aware that this deposit was building up as I continued to mold parts, but I was under severe time pressure, needed to keep the job going and didn’t have time to clean the mess off every cycle. This type of thing shouldn’t happen, but it sometimes does, even in a well-regulated molding operation.
So, at the end of the job, I was left with a nasty mess to clean up. When you repeatedly cook and cool PE, it can have a tendency to crosslink and the result is an extremely tough polymer layer which is extremely hard to remove. This was certainly the case on this occasion and my attempts to remove it with a plastic scraper were totally ineffective. I didn’t want to take a metal scraper to it and I really didn’t want to send my tool away for sandblasting, so I decided to try a trick that an experienced molder friend had told me about.
I poured some new PE powder into the mold, so it completely covered the mess at each end of the tool. Then I baked it in my oven until the powder was completely melted, carefully took it out and let it cool down completely. My hope was that the new powder would fuse to the old powder mess and that the whole thing would become more easy to remove.
My first result was a disappointment. The amount of original material removed was small; at this rate it would take many repeats to get the surface anywhere near clean. However, I persisted and was surprised to find that the next time a huge amount of deposit was removed. Two more repeats and the tool surface was essentially clean.
The photo below shows what happened. The two specimens at the left hand of the picture (one above the other) are my first attempt (one for each end of the mold being treated). The next two specimens are the result of the second attempt, then the third, then the fourth (at the far right). You can see that, progressively, all the mess got lifted off.
Why did this happen? I can’t be sure, but I suspect that there was some sort of surface layer on the original deposit (perhaps it was residual mold release agent) that was removed on the first attempt. Once this was gone, new material fused on to old material a lot better.
My mold was relatively small, so I was able bake it in my oven without having to fix it to the arm of the molding machine. For a big mold, you might need to rotate the mold with material inside and treat the whole tool surface. This would be less convenient, but could still be a lot less drastic than mechanical abrasion.
Many of you may have heard of this trick, tried it once and been disappointed by the result, as I was. It just goes to show that it’s sometimes worth persevering, even when things initially don’t go as you think they should!
Dr Nick Henwood serves as the Technical Director for the Association of Rotational Molders. He has 25 years-plus experience in rotomolding, specializing in the fields of materials development and process control. He operates as a consultant, researcher and educator through his own company, Rotomotive Limited, based in UK.
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