I am having a real issue with porosity. I am casting in
bronze. Here is my process. I use a vacuum casting machine to
de-bubble my flasks and for the casting as well. I use a electro
melt furnace to melt my metal.
2 hours at 300
2 hours at 700
3 hours at 1350
1 hour at casting temp( I have varied this from
It seems like that final burnout temperature is a tad high. (This is
Farenheit, right?) I've never needed to go above 1200F. When I've
inadvertantly let the temperature get to 1350 or so, I've noticed
some investment breakdown. You might also try a cooler flask
temperature. If the mold is relatively cool, the metal will chill
against the surface instead of staying liquid.
melting temp of the metal is 950 degrees, I am casting it at just
under 1000 C. I am using 100% new metal.
What kind of bronze are you using? Most people around here use
silicon bronze (hercaloy or everdur) because it melts cleanly without
flux and welds well too.
I usually give it about 15 minutes and I quench the flask in
That might be a little quick- try giving them 1/2 an hour.
Several of the parts I cast look like the surface has a type of
crystalisation. That is when I look at the surface under a magifying
glass it appears like many lines that have spaces between them
overlapping each other. Kind of like fiberglass. I hope I am clear
what I am trying to explain. And the porosity is strongest in the
center of the object. I have a link to a couple photos below so you
can see what I am trying to describe:
These pics are worst case scenarios. But I still get it to some
degree in all my casts. But not alway's this bad.
Sorry for the poor pics but I can only get so close to the object
with my digital camera before it losses focus.
Most porosity is due to inadequate gating: when there isn't enough
liquid metal to feed the piece as it shrinks, then the piece will
draw metal from its own semisolid areas toward the surface, resulting
in a shriveled surface with sub-surface porosity. From the look of
the pieces you show, this seems to be at least part of the problem.
Is that long skinny gate part of the piece? If it's necessary, point
that toward the top of the mold, and use a shorter and thicker gate
from the button to the thickest part of the piece. And just after
casting, put a piece of soft firebrick on top of the mold to keep the
heat in the button, which encourages that reservoir of melted metal
to stay liquid longer.