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Porosity at direct castyng acrylic plastic

#1

Hi, I casted directly UV curable acrylic plastic made by 3D printer
to 750 gold but as a result they have a lot of porosity like a
cheese and also got many cracking.

I have many experience about centrifugal casting and vacuum casting
by casting wax but I have never seen such so many porosity at
result…

I would like to ask how I can prevent this terrible porosity on
acrylic plastic next time.

What I did, UV curable acrylic plastic ring size US16 0.06inch/1.5mm
thickness arm ring. I took mold in plaster.

Stainless ring size is 2.9921inch/7.6cm diameter, 3.937inch/10cm
height 0.0787inch/2mm thickness
electric kiln setting in turns,

  1. 550C/1022F Hold 1hour
  2. 675C/1247F Hold 30mins
  3. 775C/1427F Hold 10mins
  4. 680C/1256F Hold 90mins
  5. I took the plaster mold out from kiln and stayed at room 5 mins to
    cooled it down in room temperature 28C/82.4F.

I used oxygen torch with propane gas for melting 750 gold and my
casting machine is Romanoff Deluxe Roma-Vac Tabletop System 110V
http://www.ganoksin.com/gnkurl/ep80jm

UV curable acrylic plastic heat Softening temperature is Celsius 80
degrees.

Was this problem cause of gas porosity by UV curable acrylic plastic
of did I too much heat up plaster mold? I put a pinch of borax into
gold when I melt it to prevent gas inclusion.

I m thinking I should take silicon mold from acrylic plastic and
convert it to injection wax next time…

Mike

#2

Hi

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.

I hope that helps

Daniel Ballard
Precious Metals West

#3

Hi Mike,

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
temperature.

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.

All the best
Jen

#4

Mike- Wish we had some photos of your casting.

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.

Have fun and make lots of jewelry.

Jo Haemer
timothywgreen.com

#5

Hi, thank you very much for your advise Yves, Daniel, Jen.

as Daniel and Jen described, I’m using Casting plaster can proof
against higher temperature than 1330F.

I will try again to burnout slowly ramp up and hold longer 2-3hours
(depends on flask size).

and will burnout 1500F and long ramp down to casting temperature
930F.

I will report you what will happens on my next challenge.

Thank you for the great advice!
Mike

#6

Jo,

Thank you for your advise.

Here I uploaded porosity pictures on my gold ring.

Please let me know if you have ever seen same kind pf porosity and
can guess any cause for that.

Last time when I casted different plastic is called Fine
Polyamide(melting point is 172 - 180 $B!k (BC), I got successful
result without any porosity in same burnout procedure.

So I think it’s weird. UV acrylic’s melting point is obvious lower
than former plastic. I thought it should melt clearer than former
one…

As you described $B!! (BI’ve heard if it $B!G (Bs taken up to high
to fast wax/plastic just bubbled will be cause of break the internal
surface of investment.

So I will try to burnout and take up to high very slowly from about
600F 2hours and set to ramp up slowly.

Although I can’t let my kiln burn whole night because my studio is
not my living place.

Your burnout procedure sounds really artistic though.

Thank you.
Mike

#7

Hi Mike,

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.
Jenifer Gow
Tears of the Moon Artisan Jewellery

#8

Jenifier,

Thank you for your great advice.

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.

Reminds me-when I made a hole and carved on UV Acrylic Polymer
internal one were softer than surface somehow.

Those printed UV Acrylic Polymer are translucent and I didn’t
recognize any trapped tiny bubble through magnifier.

I think covering UV Acrylic Polymer by spray coating is worth to
try.

May I ask you what sort of spray do you use usually for your natural
material? Is regular lacquer fixative?

The material I used previously and got successful conclusion was not
UV Acrylic was rough surface plastic is called Fine Polyamide PA
2200.

Its surface is harsh as if sand sculpture.

So it’s not appropriate for small scale of jewelry casting.

Kind Regards,
Mike