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Investment inside casting pieces


#1

Hello,

I have encounter a problem to which I can’t figure out how to solve
it. I have a casting induction/centrifudge machine with a vacuum and
argon connection. On occations we cast pieces that once almost
finished we find that they have rather large cavities that contain
investment init. These cavities are not visible at first, they
appear as small minute holes, such as porousity. In an attempt to
close this porous with our laser machine we encounter that behind
that little hole lies a big space with investment.

I follow the investment quantities and burnout times to exact
precision.I have regulated the vacuum to only do -250 and the speed
of the centrifudge reduced to a bare minimun, but every now and then
we still have this issue? Has anyone encountered this problem, or
have any suggestions?


#2

Sounds like air bubbles trapped on the patterns when you pour your
investment. How are you debubbling your investment after pouring?

Craig
www.creativecutgems.com


#3

Hi Marco,

It sounds like you have air bubbles in your waxes. Sometimes they
are hard to see in heavy patterns, but look at them under a light
and they’ll show themselves. During the vacuum process, the air is
sucked out and creates a cavity.

Good luck,
Scott


#4

Sounds like you have a case of spalling. Has happened to me a few
times when I did pieces with large surface areas (6 to 10 ounce
cast containers). Have not had it with jewelry.

Try using distilled water for mixing the investment. I have been
using distilled water when I cast the larger pieces since I found
out about it and I have not had problem, but then it did not happen
every time with non-distilled water.

Check the description of spalling on page 5 of this article by Eddie
Bell.

www.gold.org/discover/sci_indu/GTech/2001_31/BELL.PDF


#5

Hi there,

As a person who has casted many years, my thoughts are several.
First, how long was your burn out. (did you follow thru as per the
specs. for your investment) What were the spru positions on the
incomplete? (sometimes the angles are off-enough that the metal
doesn’t complete the path) Did you wind up or activate your
caster-completely? (not enough force can cause the cast to not fill
out) If you are doing electro melt- was your machine calibrated
correctly? (perhaps the melt down was not up to complete temp.) How
long ago did you change your crucible? (that could be a breakdown of
the carbon crucible). The green haze could be too much flux…Just
some thoughts from a 30 year caster-still learning on a daily basis.

Ciao from sunny but chilly SF Bay Area!!!

Jo-Ann Maggiora Donivan


#6

Hello Marco;

I have encounter a problem to which I can't figure out how to
solve 

I’m not the expert on the subject of casting, here among our
Orchid posters, but I immediately thought of what might be the cause
of your problem. I have been casting for 30 some years, though, so I
figured I’d throw out my idea in case nobody else here considers it.

Since you have covered all the considerations such as proper
investment, burnout cycle, casting settings, and I’ll assume you
aren’t banging the flasks around after they’re burned out and before
their cast with… you are careful with them, right? Investment is a
little fragile when it’s burned out. There are two other
possibilities:

If the burnout schedule is ramping up too quickly in the beginning,
and the designs are such that there are delicate configurations of
investment left after the wax is gone, it could be the pressure of
the expanding wax is breaking the plaster free in those areas. It’s
obvious the plaster is coming from somewhere in the design (although
there is the possibility it’s coming from irregularities in the sprue
chamber). Study these castings when you find the these inclusions.
There should be a corresponding area in your design that has filled
with metal where it shouldn’t have. If the area that is breaking free
is too delicate, too long, or otherwise unable to support itself
through the process of the metal entering the cavity, you’ll have to
change that somehow. Might there be certain designs you have this
problem with more than others? Shoot some waxes and hold them up to a
strong light. It’s even possible there are bubbles within the wax
that the investment is managing to fill during investment. Good luck.

David L. Huffman


#7
On occations we cast pieces that once almost finished we find that
they have rather large cavities that contain investment init. 

I can think of two possible causes of your problem.

  1. There are bubbles in your waxes. When the investment/wax is
    vacuumed the bubbles pop allowing investment to penetrate the wax
    design. You might want to vacuum the wax before investing. If there
    are bubbles under the wax they will show up after vacuuming. You
    might also want to hold the design up against a light. The bubbles
    will show up as lighter sections. The bubbles can be filled with wax
    before investing.

  2. Portions of investment are breaking off from somewhere in the
    mold. The breakage can be caused by too fast of a temperature
    increase at the start of the burn out cycle or by the molten metal
    breaking of a thin section of investment as the metal is poured.
    This broken off piece of investment will float in the mold until the
    metal is poured. The weight of the molten metal will float the piece
    of investment up against the surface of the mold. The broken off
    piece of investment will become trapped in the metal. I can send a
    sketch of this problem if you contact me directly. You might want to
    shake the mold with the sprue end down as you remove it from the
    burnout oven before placing it on the vacuum table. This might case
    the broken off piece of investment to drop out of the mold. Try to
    sprue the model so that the molten metal as it is poured does not hit
    a thin section of investment.


#8

Right you are, David. I experienced some similar problems once and
discovered the occasional air bubble in the wax. At the time, we
were recycling wax scrap…a perfect way to get bubbles. We ceased
that foolish practice immediately and the problem disappeared. I am
amazed at the number of benchies who insist on re-cycling wax, as if
it were expensive in the first place.

Wayne


#9
Sounds like air bubbles trapped on the patterns when you pour your
investment. How are you debubbling your investment after pouring? 

I’ve never seen air bubbles result in investment inclusions in a
casting. The flaws described are classic investment pockets caused,
usually, by a breakdown of investment. This can be many things:

-A thin or fragile finger or area of investment that breaks off due
to the flow of metal as it enters the mold chamber or if the flask is
droped or heated too fast.

Look for “filled in” areas of detail in the cast piece.

_A bit of the mold chamber wall has peeled back or crumbled off.
This can happen when a bit of investment was weak or poorly mixed
when investing or if the burned out flask has been dropped or shocked
or if too rapid burn out weakens the plaster. Also if the invested
flask was burned out at too high a final temp. which would begin
disintegrate the plaster. (There would be other clues in this case,
such as sulfur contamination and porosities.)

Look, in this case, for positives where there weren’t any. Sort of
like a rough tumor… Some hardened investment has broken off from
the mold chamber wall and fallen into the piece. The “tumor” is the
new area of the mold chamber (left vacant by the crumbling
investment) which now becomes an extension of the mold chamber (and
model) and fills with metal.

Another possibilty is that the bit of investment was introduced some
time during the melt. Perhaps the “nose” of the crucible rubbed off
some plaster from the button area of the flask. I’ve seen that
happen…

When this first happened to me it was the second scenario. I saw the
positive area on the casting and easily filed it off and
recontoured, smugly assuming that I was out of the woods. What I
didn’t realize was that the investment had to go somewhere. It
appeared as a tiny, jagged pit that only got bigger as I filed. It
was, truly, the tip of the iceberg.

Always look for clues. It’s really pretty basic. The hard part is
figuring out what caused it.

Hope this helps, Andy


#10

I have seen air bubbles in the wax cause this as well, if the air
bubbles are close enough to the surface of the wax that a relatively
thin wall of wax is left between the bubble wall and the outside wall
of the model. During investing the air in the bubble expands and
ruptures the wax model wall, allowing investment to creep in.

It depends on how efficient your vacuum is and how close the bubble
is to the exterior of the casting.


#11

David,

Another area I’ve seen plaster breaking from is an incompletely or
fluidly attatched sprue-- either at the model end or the button end.
If there is a little indentation or unmelted area at the juncture a
finger of investment can work its way in and then snap off.

A


#12
but every now and then we still have this issue? Has anyone
encountered this problem, or have any suggestions? 

There is much good advise on this. Probably the single biggest reason
for your problem is tiny holes. Spalling, too, is a good culprit, if
you don’t mix the invenstment thoroughly.

Little holes, especially deep ones, as in the ones under stone
setting, become tiny little pillars of investment after burnout. When
the metal comes tearing through, it just knocks them down, and the
investment gets mixed with the metal. If you look at the wax, and see
any tiny little hole or space, realize that it’s going to turn into a
tiny sliver of investment later on.

http://www.donivanandmaggiora.com


#13

Wayne,

Thanks. I hadn’t thought about bubbles coming for recycled wax. Dang.
I have done some heavy pieces where bubbles have been a problem. Time
to change a habit of being miserly about some materials. Wax is
certainly cheap enough compared to time spent filling holes or
recasting. I had always blamed the bubbles on not getting the vents
right in my molds. I had not thought about the wax possibly
contributing to the problem.

Bill Churlik
www.earthspeakarts.com


#14

Sounds like air bubbles trapped on the patterns when you pour your
investment. How are you debubbling your investment after pouring?

I've never seen air bubbles result in investment inclusions in a
casting. 

Just reread my first post… 'Twas early and I had envisioned
external air bubbles trapped in the investment and cast as "beads"
on the casting. Sorry…