I recently read that if you cast pieces with thick sections you will
have more problems with turbulence porosity. My work tends to be
chunky stuff and have problems with subsurface bubbles, I wonder if
any of the new people here have this problem and what methods they
use to avoid it…Dave
Dave,
I am not a new people by a long shot. For thick sections drop your
flask temps until it scares you and then some more 
I would
really suspect metal shrinkage more than turbulence as the root to
your problem. In the past I was charged with casting stuff with a 1
cm cross section, much trial and error and 500 - 600 F flask temps
for casting worked in the end. Centrifugal machines, I’m not sure
about vac or steam.
jeffD
Demand Designs
www.gmavt.net/~jdemand
It depends.
If you have a piece that is uniformly chunky, a casting should
theoretically be fine. If you have a piece that is chunky here, thin
there, then you will come up against possible casting issues.
If you are sending your work out to a reliable casting house, then
it’s their problem. You shouldn’t fear though, good casting houses
know what they’re doing, and will cast jobs with uneven metal
distribution with other jobs to reduce or eliminate casting defects.
The problem with an uneven chunky casting is that the thinner
sections of the piece will cool faster than the thicker sections, and
this is where casting defects happen easily in the home shop.
There’s a couple of tricks to avoid this in the home shop. Make
multiple items and invest them in opposite directions. This evens
out the heat on the piece.
Another way is to incorporate thick vents and risers where the
pieces are thin, however you are left with thick vents and risers
that just as easily could have been jewellery.
Regards Charles A.
If you really have to cast thick and chunky, at least try to hollow
it out from behind. The reason for the problem is that thick metal
cools slower than thin metal. One way to cast really thick is to put
a very large sprue right on the thick sections. Make the sprue as
heavy as your thickest point and pour with plenty of extra metal and
have a very large button. This will help. Every time I’ve ever cast
really thick, heavy rings, it has been because the customer insists
on having all the gold on their finger. But no matter what you do,
you’'ll need to do a lot of surface finishing. There is not really
any way around it.
Kevin
Hi Kevin,
I agree with almost everything you’ve stated regarding spruing. A
heavier sprue is always better. But let me add a little trick. If you
surround the sprue with a little bead about two to three times the
diameter of the sprue and about one diameter of the sprue away from
the object you are casting, any pullback from the button cooling
faster than the object you are casting will be eliminated. This
eliminates a lot of porosity at the sprued point.
Where I might disagree with you is your point of using a larger
button. The button, being in air, will cool faster than the sprued
object in the investment and shrink faster, pulling against the sprue
and the object you are casting. The larger the button, the more
shrinkage, and the more likelihood of porosity at the sprued point.
The bead around the sprue helps eliminate this problem as it is a
reservoir of heat and any pullback will cause the bead to have the
porosity and not the object.
All the above is in regards to casting a single item. If you look at
images of gang casting of multiple items, you’ll see that the main
sprue is several diameters of the object sprues. As it cools last in
the flask, due to it’s larger diameter, any shrinkage/porosity will
occur in the main sprue, and not the objects being cast.
When I used to cast crowns and bridges, I learned to run a “chill
vent” from the heavy sections of a crown (or coping). This was a
"blind" vent, dead-ending out in the mass of investment surrounding
the mold chamber. The logic is that this vent will fill with metal
which will conduct heat away from the large mass of cooling metal,
thereby balancing the rates and speeds of cooling between thicker
and thinner areas, avoiding “hot tears” and shrink spot porosity.
Works the same with jewelry, of course.