Ask Dr. Nick: Can I fix gaps in a parting line?

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Dr. Nick Henwood

A badly fitting parting line is a regular pain in the neck, for a number of reasons.  The most notable annoyance is that, as the mold rotates in the initial stages of heating, powder spills out from any gaps that exist.  This wastage of powder can cause an under-weight part and, even if the spillage is small, the powder burns, makes a mess in your oven and creates a nasty smell.  Better to avoid the problem, if you can!

Recently I was given an old steel test mold from another lab; it was a hexagonal cylinder used to make 5 inch square plaques for the ARM Low Temperature Impact Test. (The procedure for this important test is available on the ARM website.)  The first time I put the mold on my machine, I noticed that I had a small powder spill from the parting line area.

gap edited

By good fortune, the next day I participated in one of ARM’s Troubleshooting Calls; we run these every month, as a free-to-member service.  One of the regular moderators is Sandy Scaccia of Norstar, who is one of our industry’s top mold experts.

During the call, I asked Sandy for some advice about what I could do to reduce, or hopefully eliminate, the parting line gaps.  He told me of a procedure he had used for aluminum molds: heat up the affected area and use an exterior clamp to squeeze the parting line shut while it is still hot.  He expressed some doubt that this would be as effective for a steel mold, but I thought it was worth a try.

The results of this are shown in the Table below.  For each of the six corners of the hexagonal mold, I measured the initial gap with callipers, did the treatment and then re-measured.  Actually I repeated the treatment one more time, followed by more measurement.

Screen Shot 2019-02-13 at 2.19.16 PM

Although the procedure wasn’t totally successful, the results are instructive.

Overall, I did achieve an average 24% gap reduction.  Actually, the spillage was a bit less, but it still persisted.  This is not entirely surprising, since the biggest particles in a standard 35 mesh powder are only approx. 25 thou in diameter.

If the initial gaps had been smaller, maybe this would have worked well enough to eliminate the spillage.

There also seems to be a law of diminishing returns at work; in some cases, the gap grew bigger as a result of the second treatment.

Details of the procedure are as follows:

  1. I used a large propane torch, set to a fierce blue flame.
  2. I heated the affected area until the metal glowed red.  For steel, this happens at approx. 900°F. On my mold (approx. 1/8 inch plate), it took about 3 minutes to heat to a red glow.
  3. I then quickly closed up the gap, using a G-clamp.  I was as quick as I could be with this, but I measured about a 200°F temperature drop, just in the few seconds it took to turn up the clamp screw.
  4. I decided to try to do some rudimentary stress relieving; I put the (empty) clamped-up mold in my rotomolding oven (set at approx. 450°F), heated it for 30 min, switched off the oven burner and let the oven and mold cool down overnight.

I think this procedure would have worked significantly better for an aluminum mold and could have fixed the problem.  Unfortunately, the steel mold appeared to be resistant to my treatment and I was only partially successful.

In order to completely fix the problem, I’ll put extra bolts on the corners, to make sure that the gaps pinch up.  However, it was worth a try and I’ve shared my experiences, to help other who face the same predicament.

Sometimes we can learn as much from our failures as our successes!

Before the ARM legal counsel freaks out, I should stress that, of course, I followed all necessary safety precautions, including use of appropriate PPE.

If nothing else, my experience reinforces the HUGE benefit of being able to freely discuss problems with other ARM members, especially acknowledged experts like Sandy. Remember that the Troubleshooting Call is a totally FREE service to members, so I would thoroughly recommend that you give it a try.  ARM sends out regular notification of these events, so keep a look out for them.

Happy rotomolding!

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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