Originally Posted by subybaja
but that kind of porosity/roughness in a jewelery casting would indicate sprues (gates?) that are too small. The cooling, contracting liquid metal needs a reservoir to pull from, or it will pull from the piece.
The sprue is the part through which the metal travels from the pouring cup to the runner. The gates are where the metal travels from the runner to the casting.
The point of risers is to feed liquid metal into the casting to make up for this solidification shrinkage, based on Niyama criterion.
The gates were not too small, but the risers were (using a rule of thumb that the risers should have 120% the modulus (volume/area) of the local modulus of the casting).
Note, that was my first casting. The second one turned out much better. Once I am done with this take-home final (my last thing to do for this semester), I will update this thread more thoroughly.
It looked as though you leave the foam cores in the molds, and just let the iron burn them out? Weird.
How does it look like that?
Additionally, it's not that weird to actually leave the foam in. I chose not to because I wanted hot, good metal in there as fast as possible. In lecture, we discussed the metal front as it spends energy burning the foam out, and we actually saw an x-ray video of a casting doing just that.