Sounds like air bubbles trapped on the patterns when you pour your
investment. How are you debubbling your investment after pouring?
I've never seen air bubbles result in investment inclusions in a
casting. The flaws described are classic investment pockets caused,
usually, by a breakdown of investment. This can be many things:
-A thin or fragile finger or area of investment that breaks off due
to the flow of metal as it enters the mold chamber or if the flask is
droped or heated too fast.
Look for "filled in" areas of detail in the cast piece.
_A bit of the mold chamber wall has peeled back or crumbled off.
This can happen when a bit of investment was weak or poorly mixed
when investing or if the burned out flask has been dropped or shocked
or if too rapid burn out weakens the plaster. Also if the invested
flask was burned out at too high a final temp. which would begin
disintegrate the plaster. (There would be other clues in this case,
such as sulfur contamination and porosities.)
Look, in this case, for positives where there weren't any. Sort of
like a rough tumor... Some hardened investment has broken off from
the mold chamber wall and fallen into the piece. The "tumor" is the
new area of the mold chamber (left vacant by the crumbling
investment) which now becomes an extension of the mold chamber (and
model) and fills with metal.
Another possibilty is that the bit of investment was introduced some
time during the melt. Perhaps the "nose" of the crucible rubbed off
some plaster from the button area of the flask. I've seen that
When this first happened to me it was the second scenario. I saw the
positive area on the casting and easily filed it off and
recontoured, smugly assuming that I was out of the woods. What I
didn't realize was that the investment had to go somewhere. It
appeared as a tiny, jagged pit that only got bigger as I filed. It
was, truly, the tip of the iceberg.
Always look for clues. It's really pretty basic. The hard part is
figuring out what caused it.
Hope this helps, Andy