Breaking in a new mold

  • Last Post 01 December 2020
Boschloper posted this 28 November 2020

How do you break in a new mold?  Phil Sharpe says to cast until you get good bullets.

My wife gave me a new Lyman 2 cavity 429421 for my birthday 3 years ago. My standard procedure is to cast in batches of 110 bullets and inspect with a 2.5 X Optivisor. I typically yield 104 to 106 good bullets from a batch. The first 16 batches that I cast with this mold I had a terrible time with incomplete fill. I started casting batches of 120 and hoping to get 100 that were marginal but usable. 

I cast a batch 3 nights ago and knew by the 5th bullet that it was going to be a good night. My yield was 106 really good bullets from 110 cast. I feel confident saying that I finally have this mold broken in. 

Let me put it to all of you:  How do you break in a new mold?

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RicinYakima posted this 28 November 2020

I wash with Dawn dish soap and toothbrush. Dry well and put on the wood stove over night. Repeat. Then start casting and avoid "smoking" the mould unless it is a Lee.

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Shopdog posted this 28 November 2020

Since making a real nice mould oven,that was worth every bit of effort.

Just stick a new mould in it,set the heat on medium,under what they would be for casting. Warm them up good,and then hit them with brake clean. The heat brings out any oils. We do the same thing for weldments that are headed for paint.

I usually do a heat and clean,3 days in a row. I'm in the shop anyway so it isn't a big deal at all. But even after that,it still takes a few casting sessions for the mould to really start performing.

Good luck with your project.

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John Alexander posted this 28 November 2020

I think that "breaking in" is probably a misnomer for most molds and "cleaning" is probably closer to what is going on.  With apologies to Mr. Sharpe I have never had much luck cleaning a new mold by casting.  I do about what Ric does but with boiling or near boiling water.


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Ross Smith posted this 29 November 2020

I second John. Wash with dawn rinse repeat, dry , use break cleaner get mold hot and start to cast. Repeat with brake cleaner if necessary. I reccommend that you not use your wife's electric toothbrush.

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Eutectic posted this 01 December 2020

I use solvents to remove the oil. If possible I take the mold apart, especially the sprue plate. Then I soak it in a clean container submerged in clean mineral spirits for an hour or overnight. I remove most of the mineral spirits with a clean towel and flush the cavities twice with acetone. The acetone dries quickly and I sometimes coat the cavities with Mold Prep, a graphite suspension. I always coat the cavities of aluminum molds, but smoking with a butane lighter seems to work just as well. If pre-heated, the first cast is frequently perfect.

I only do this once as I store the mold in an ammo can with desiccant. Actually not true, I have had to repeat it when I put too much lubricant on the sprue plate bolt and it migrated into the cavities.

Do this outside and do not smoke, the acetone is very flammable.

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