For those kinds of materials I have a suggestion or two. Here in L.
A. people are casting straight to gold from cad models of the polymer
variety. First they commit to a far stronger kind of investment.
Usually a dental crossover like Docs Casting Plaster. There is
likely a similar product you can get near you.
Then they really expand the burnout. Hours at 1500 or even 1600 F.
Then a long ramp down to casting temperatures. That’s what gets all
the model burned out.
My first observation is that I don’t understand your burnout cycle.
When I burnout I slowly ramp up to 650C and hold for 2-3 hours and
bring the kiln down to 450C and cast into the flasks straight out of
the kiln. This is for fairly large flasks. You can speed things up a
bit with smaller (e. g., 50mm) flasks.
It is not clear whether you were using plaster as a colloquial term
for investment or just plaster which is not used alone for casting.
In either case gypsum which is only component of plaster and a
substantial component of investment starts to break down at 730C and
can cause rough and porous castings.
In addition you seem to imply you cast in the flask at room
This might be my misinterpretation. As I have already suggested I
cast with a flask at 450C or perhaps higher.
As for the polymer from the 3D printer. I am not familiar with it
but I expect it should be capable of a clean burnout although the
fumes might be nasty.
You should be able to cast anything that can be invested and burned
with good results.
Assuming that you vacuumed and got all of the air bubbles out of
your investment plaster. the obvious problem I see is with your burn
out cycle and casting temp.
We always burn very low and slow at first to give the wax or plastic
time to melt out and evacuate the flask. If it’s taken up to high too
fast the wax just bubbles and boils and breaks down the investment
leaving a bad surface. We do the low burn for about two hours at
about 600 F depending on the size of the models.
Then we take it up to 1350 F for about two hours. After that we
reduce the temp for about one more hour and cast at 800-1000 F.
All in all it takes us most of the day to do a burnout. We used to
burn out at night while we sleep, but don’t do that anymore after our
kiln thermostat malfunctioned and burned up a few flasks. It actually
melted the steel flasks. We had visions of our house burning down.
Now we only do supervised burn outs.
The pictures are most puzzling. It does not look like the sort of
porosity I have encountered but looks more like imperfections in the
polymer pattern that were either present at the time of investment or
were produced during the investment process. I am assuming that the
patterns had a clean smooth finish prior to investing. One vague
possibility that occurs to me that the surface was damaged during the
vacuum debubblising perhaps because of small pockets of air trapped
beneath the polymer during the 3D printing process.
The other possibility is that the polymer you used is affected
either by water or exposure to high pH liquids such as investment
slurry. The former possibility suggests the need to avoid vacuum
debubbilising and instead carefully covering the pattern with
successive layers of investment with a fine brush before investing.
The second possibility suggests the use of a protective coating on
the pattern such as spray on art fixative. I use this in some of my
pieces incorporating natural materials. One consideration that
reinforces these possibilities is that you used an alternative
polymer and didn’t have the problem. All things considered It is
probably best to go back to the polymer you used previously.
All the best.
Tears of the Moon Artisan Jewellery