debugging porosity in a Bismuth cast

All,

I'm trying out bismuth as a practice metal for casting and milling. Lead is a no-no since I also care for a small child. The bismuth machines very nicely on my Taig mill and it is quite forgiving on my endmills. I'm getting some excellent practice.

One small problem...

Porosity. I am getting voids about 1/32 of an inch in places, usually around the center of my ingot cast. The mold is vertical, and I have it at ambient temperature before pouring molten bismuth at about 600 to 700 degrees from my Kerr manual electro-melt. Some places I am reading tell me that I need to have the mold preheated to about the same temperature, then pour, then allow the filled mold to cool slowly, and that would cure most of my porosity problems.

Would that be true? About the only other source of heat I have outdoors which could keep my mold at Bismuth melting point is a charcoal fire...

Comments?

The Eternal Squire

you'll get this with an aluminium casting as well. the outside of the cast cools and pulls outward the centre mushy partially molten remnants. the method I was taught which helps correct this is to have a decent size riser that you pour into the mould with. and have a long wooden plunger about the diameter of the riser tube. after the pour plop the wooden plunger/rammer in to the riser and push down. wait until the top of the riser has almost solidified before doing this and ignore the charring of the wood. what you are doing is pressure feeding the riser metal into the mould to replace the lost volume as the metal shrinks on cooling. replacing the shrinkage in the centre of the mould should get rid of the void problem.

works for me.

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