casting investment

I've been casting (as an amateur) for at least 20 years. I may have
'missed' 6 or 7 castings in the last 5 years - not bad for a non-
professional. However, I don't know exactly what goes on during the
last part of the burn out cycle. I use steam to dewax the molds, then
use a 300 degree F. in kiln to finish the dewax. When the temp is
raised to about 800, all the 'organic' material is eliminated. So far
so good. Then the temp is raised to 1250 d. F for about 2 hours before
being lowered to casting temp of 900 to 1,000 degrees. Ok, what
happens at 1250? I know that the investment I'm using will start to
break up at about 1300 degrees, releasing sulphur into the cavity. But
why does the cycle need the 1250 degrees?

My standard flask is 2.5 inches diameter by 3 inches tall, with no
more than 5 ounces of SS. I have done bigger flasks, but I try not to.

Charlie

On Mon, 25 Sep 2006 18:28:30 -0700, in rec.crafts.jewelry Charlie Leo
wrote:

I've been casting (as an amateur) for at least 20 years. I may have
'missed' 6 or 7 castings in the last 5 years - not bad for a non-
professional. However, I don't know exactly what goes on during the
last part of the burn out cycle. I use steam to dewax the molds, then
use a 300 degree F. in kiln to finish the dewax. When the temp is
raised to about 800, all the 'organic' material is eliminated.

There's the possible flaw. at 800, by far the majority of the organic material
will indeed be gone, at least as far as organid compounds. But a small amount
of the wax, which didn't flow out during steam dewaxing or later, but instead,
soaked into the investment, has by now turned to carbon. You can verify this by
stopping the burnout at this point with a scrap test flask, letting it cool,
and breaking it apart with a hammer to examine the investment walls in the mold
cavity. Usually, you'll find they're not pristene white, but instead, gray.
Now, to a certain extent, this can act as a deoxidizer for metal if it's still
in the flask, but it can also be a major cause of gas porosity, as well as
no-fills, since when it reacts with oxygen, it forms carbon dioxide or carbon
monoxide. The influx of molten metal can cause some of it to do exactly that,
and combined with the turbulence of the casting operation and all, some will be
incorporated into the metal. Not only can it cause gas porosity, but it can
also, with some metals, cause the formation of metallic carbide compounds, which
show up as hard annoying speck like inclusions in the surface. Some types of
white golds are especially prone to this. The other thing that this carbon does
is to take up space in the pores in the investment. It simply makes it less gas
permiable by clogging it. So that makes it more difficult for the air in the
mold cavity to get out of the way of the incoming metal.

That's the reason for raising the temp to 1250 or so. it ensures that all the
carbon in the mold, not just that visible at outer surfaces, but that close to
the inner mold cavities, has been burned away.

With that said, it's not always essential to raise the temp that high, at least
not with standard gold/silver investments. Since you're steam dewaxing, most of
the wax, and therefor the carbon source, is gone, minimizing the problem. And
if you raise it just a little higher, and then hold for a more extended time,
you can get the same quality of casting. Casters who cast pieces with the
diamonds or other stones already set in the wax, often will need to do this,
since the stones (especially diamonds) won't always withstand the higher
temperatures without damage. So the flask is simply raised to about 900 or 950,
no higher, and held there long enough for a complete burnout, and then cast at
that same temperature. The hold times at this temp, though, can be quite a bit
longer to get a properly clean burnout.

So far
so good. Then the temp is raised to 1250 d. F for about 2 hours before
being lowered to casting temp of 900 to 1,000 degrees. Ok, what
happens at 1250? I know that the investment I'm using will start to
break up at about 1300 degrees, releasing sulphur into the cavity. But
why does the cycle need the 1250 degrees?


Only because at that temp, full burnout is fastest and most consistantly
assured. That temperature level does not cause any intrinsic change in the
investment structure or anything like that. Now, higher temperature
investments, such as some dental investments or platinum investments, may need
their full high temperature range (often higher than 1250) in order to properly
sinter together, to be strong enough to withstand the casting operation. But
that's not the case, so far as I know, with the standard sulphate bond
investments like Satin Cast 20, or the like.

peter

On Tue, 26 Sep 2006 01:47:56 GMT, "Peter W.. Rowe,"
wrote:


So the flask is simply raised to about 900 or 950,
no higher, and held there long enough for a complete burnout, and then cast at
that same temperature. The hold times at this temp, though, can be quite a bit
longer to get a properly clean burnout.


Peter - Not only did you give me a good answer, but you may have
solved a major problem as well. I do demo casting at our annual
Houston Gem & Mineral Society show. We have a programable kiln timer
to run the burn-out cycle during the night so we can cast the next
morning. Unfortunately we have had a lot of electrical problems within
the building that we are using. Even though we have built an
uninteruptble power source for the controller timer, the cycle still
gets 'knocked' out.

If I'm reading you correctly, I can do a steam elimination of the wax
and then set a manual controler to 950 degrees and let it run
overnight (from 6 pm until 10 am next morning. It wouldn't matter if
the electricity was off for a couple of hours during the night - as
soon as the power came back on, the controler would bring the temp
back to the 950. Show opens at 10 am - we start casting at about
10:30.

Thanks much

Charlie