Today wasn’t the best day, so I decided to cast some boolits after my kids went to bed. I wanted things to go well, but I went against the odds by going with an RCBS mold. Up to this point, I have just not been very satisfied with the sharpness of my boolit bases from an RCBS mold. I don’t like to get my mold too hot, and I’m too cheap to use too much tin. It seems that when I get sharp bases, my mold is too hot and lead sticks in unhelpful places. If I let things cool down so that I have sharp driving bands without lead sticking anywhere, I get slightly rounded bases. Where is that sweet point in the middle?
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I might have figured out how to get to that sweet point - by keeping the mold temperature of the sprue plate the same as the bottom of the mold. The sprue plate is naturally cooler than the mold blocks. It swings open & shut every cycle, getting some forced convection cooling. When held upright, natural convection cooling is the strongest at the sprue plate. So the first thing I tried is flipping the mold over while waiting for the sprue to cool. That helps, but it is not always enough. Sometimes I hold the mold upside-down for a few seconds, and then set it down (upside-down) on my cotton drop towel or on the cover plate for my RCBS bottom pour furnace. Setting it down like that really helps to cool the sprue without effecting the mold or sprue plate so much. That’s basically it - hold the mold upside while letting the sprue cool, and set it down on the sprue if that takes too long.
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I had the best result I have ever had with an RCBS mold today. I was using the 9mm-147-FN mold. Not the best choice for .357 Sig, but it works when you have a compressed charge of Enforcer. My alloy was 96-3-1, casting at 700F.