Here's my problem. My nickel whit castings look fine. Upen
clean up however, I often find rather large, subsurface pits,
one or two in number, at the point I've sprued to, ususally the
main, heaviest feeder sprue. These pits are round, contain no
flux, investment or "stuff" and they are really pissing me off!
We see the same thing, sometimes, with platinum castings when
the sprues/casting temp aren’t quite right. What you’ve got is a
shrinkage hole, caused by the intersection of the sprue and the
casting creating a small area that is heavier in cross section
than the sprue itself. So by the time it solidifies, the sprue
has already done so, and all shrinkage centers on that area
within the casting instead of being up in the sprue or button.
You need to sprue so that does not happen. flatten the sprue
slightly, so it flares but gets a bit thinner just where it meets
the model. don’t overdo it, as you can create undue levels of
turbulence in the metal flow, but you do want to make a little
bit of a nozzle there. The idea is to have it’s cross secional
area equal to the model there, but it’s thickness shouldn’t
exceed the model, and can be slightly less. The pinching off of
the sprue that you do to effect this causes the metal to speed up
as it flows through that junction, which makes it less likely to
solidify as quickly. The sprue itself is then free to be a bit
thicker, ensuring that it will freeze last, and it’s mass, right
next to the pinch, also helps keep that from freezing up till
the casting itself has done so. And at the same time, the
thinner but wider sprue attachment area creates less of an
increase in the bulk of the area being attached to than if the
sprue attachement point is real heavy, especially if there are
wide wax fillets between the model and the sprue.
Another technique that often works is to put a small reservoir
on the sprue just behind where it attaches to the model. Again,
same purpose, to keep the sprue liquid longer than the model, so
it can feed shrinkage in the model.
Another thing you can do to increase the sequential
solidification of the model from it’s farther reaches progressing
towards the sprue is to use a slightly higher flask temp when you
pull it from the oven, but let it sit a minute or two before
casting. That creates a cooler outside to the flask, but the
central core, with the main sprue, will be hotter still.
A final trick, especially useful with problematic platinum
castings, is to use a sacrificial “chiller” on the model if there
are areas that are especially thick. The cooling rate of a mass
of metal is proportional to it’s surface area. In heat sinks,
this is increased with fins attached to the metal. You can do
the same thing with a model that has an abruptly thicker area
that will tend to solidfy too slowly. Add a fin. These are
usually done by putting a couple thin cuts, like air release
cuts, in the rubber mold, so it creates a small fin the same as a
mold that’s injected with too much wax pressure or not enough
clamping pressure, except that this is an intentional fin. Looks
often like a little shark fin, a tad wider where it attaches to
the model, and thin at the top. Uses little additional metal, and
after casting is snipped off and added back to the metal for the
next cast. It’s job is to increase the total surface area of
that region of the model, hastening it’s cooling.
Hope this helps.
P.S.: Happy New Year!