@Salty. I'll bring my metal casting experience to the table here, spending two decades as a dental technician in the army, casting chrome cobalt based partial denture frames, gold inlays and precious metal crowns etc. I spent five years teaching casting techniques at Dental School too. All that experience helped me for sure, when I took up casting my pure lead
.
Porosity, (bubbles) in a casting, can be caused by four key issues: uneven flow into the mold that incorporates air (turbulence), the mold not hot enough causing the metal to solidify too quickly, the metal not hot enough causing the metal to solidify too quickly, and, not allowing enough metal to fill the sprue where the cooling metal can draw (suck) from as the casting cools (it is a combination of the
poured molten metal and
the reservoir in the sprue that creates good fill out). All metals shrink when they cool, naturally drawing from the pool in the molten button, the last part of any pour to solidify. I like a healthy chubby button when I cast - tells me that I have enough metal to feed and fill the mold.
A hint too - if a few postmortem cuts of your poured balls show a consistent single void (air bubble) just below where you cut the sprue off, then change your technique to pour consistently leaving a good amount of metal in the button. I admire guys and girls who cast with great results using gang molds - two or more. I can only concentrate on a single cavity!!
@RobertIN did mention another notion too, and he's dead right - an inclusion of a foreign object or spur of already cooled metal can fall into the mix causing porosity; though, this would be a rarer event than the four key areas that I explained above.
Hope this helps. Best of luck with your endeavours! Casting is fun huh?
Cheers, Pete
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