A few months ago I was having trouble getting sharp edges at the base of the RCBS 45-230-CM, which was resolved by filling each cavity without connecting the sprue puddles for each cavity. I’m using an RCBS bottom pour pot, and the valve would open and close for each cavity. After the two cavities were filled, I would pour a swath of lead over both sprues to keep things hot.
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Using the method of pouring each cavity separately, I found a surprisingly high rate of boolits with excellent fill out except for voids and sinking on the bases. The edges of the bases are sharp, but the flat midsection has voids & sinking. I thought that perhaps my COWW +1%Sn alloy was a bit dirty, but today I observed that perhaps it was the pouring technique.
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I got low on lead and added SOWW ingots to the tune of 4:3 COWW:SOWW without adding more tin. Then I cast up some NOE 454-237-RN boolits starting with the “pour each cavity independently” method described above, and I saw the sinking & voids again. Then I went to my old method of keeping the valve open while pouring all cavities (i.e. connecting the sprues) and the voids were eliminated. It was the same alloy, same mold, same casting session. Maybe a chunk of oxides were stuck in the valve and broke free at the same time I changed the pouring method, or maybe connecting the sprues gives less porosity.
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Pouring each cavity helped with the RCBS mold, because the bottom driving band was thin. I may have to increase the tin for that boolit and pour both cavities at once.