Back to Ganoksin | FAQ | Contact

Bubbles in casting


#1

I am having issues with air bubbles on the final items after
casting. I have tried debubblizer, but it only made the problem
worse. What is the most effective way to help cut down on bubbles and
flashing? This is broken arm casting, not vacuum casting.

Linda Graham


#2

Make sure you are vibrating the bubbles out of you investment
thoroughly! Vibrate or even better vacuum them out with a vacuum pump
and bell jar. Hope this helps, Steve


#3

Hi Linda

By far the most effective way of removing bubbles is using a vacuum
chamber firstly to debubbblise the investment after mixing and than
debubbblise the flask after investment. The issue of flashing may be
different and related to stale investment or incorrect proportions
of water and investment (it is best to accurately weigh your
investment powder and measure the volume of mixing water to an exact
40:100 ratio For my usual flask I use 750 grams of investment to
300ml of water. I mix with rainwater using a measuring cylinder for
the water and digital scales accurate to 1 gram for the investment.
Add the investment to water when mixing.

All the best
Jenny


#4

The problem is not in the casting but in the investment.

Did you vacuum the investment? The only way to absolutely prevent
bubbles is to vacuum the liquid investment prior to setting.

If you have no vacuum table, gentle rapping the flask several times
will cause the bubbles to release from the pattern. It will not
eliminate bubbles under the pattern. A gentle vibration will also
help but might injure the pattern/sprues.

Ben


#5

Linda,

Let me begin by saying the bubbles on the casting are there because
of improper amount of vacuuming. You also might have the mix too thin
leaving the flashing. During the vacuum process of the mix bowl first,
make sure you bang on the vacuum table, which should be mounted on
springs, with your fists pretty hard. After the pour into the flask,
down the side of it slowly, do the same process. This will get the
trapped air to the surface of the investment. You should have much
better luck then in your casting adventure.

Russ
The Jewelry CAD Institute


#6
I am having issues with air bubbles on the final items after
casting. I have tried debubblizer, but it only made the problem
worse. What is the most effective way to help cut down on bubbles
and flashing? This is broken arm casting, not vacuum casting. 

By air bubbles, I assume you mean the added nodules of metal that
occur when an air bubble stuck to the wax. If, instead, you mean pits
and bubbles IN the metal, that’s another issue altogether.

But with air bubbles, the most effective means, rather than
debubblizer (actually not all that useful with most commercial
casting investment, since these already have a wetting agent in
them), is simply to have a good vacuum pump for investing. Vaccuming
the investment cause any bubbles, even tiny ones, to expand greatly,
so then with a bit of agitation or vibration, they rise to the
surface of the flask, away from your model. Vaccum the investment
once in the mixing bowl, then again in the flask, and if you’re doing
it right, you should pretty much eliminate your problems. Your vac
pump should be good enough so the investment appears to boil (it
actually IS boiling, since lowering the pressure also lowers the
boiling point of the water). The boiling provides some agitation, as
well as also helping to ensure the investment if fully mixed.

Also pay attention to your water to powder ratio. If you use too
little water, your investment may be too thick, which can make
eliminating all the bubbles more difficult. Use a good scale (I like
gram scales, but any will work) to weigh both water and powder for
the proper ratios.

If you have trouble getting the investment to vaccuum properly,
perhaps due to not strong enough a pump, you can try using a bit more
water, which thins down the investment. It also slows setting time.
So then you use water which is a little warmer, like lukewarm or
more. This will then speed up the setting time again. And the warmer
water will make the investment boil at a slightly less good vacuum.
Doing this takes a little trial and error to get the ratio, and water
temp, balanced for a good setting time.

Be sure you’re got the timing down so that the investment is still
fluid as you finish vacuuming the flask and topping it off. It should
remain fluid for at least two or three minutes before starting to
gloss off. If it’s setting too fast, and is thickening up too much
while you’re still vacuuming the flask, then the bubbles caused by
boiling will just stay there, making things worse rather than better.

Now, if you don’t have a vac pump at all, you can help things by
painting on a thin coat of investment before actually pouring the
flask. work fast, as you don’t want the investment setting before the
flask is full. You can also fill the flask, and pour it back out
again, then use a brush to dislodge any bubbles you can see, then
refill the flask. This sort of method can work, especially with
flasks using only one or two waxes, but it’s not as easily
controlled. You’ll want a cowboy hat and spurs and a well practiced
"yeeehaww" yell to do this right… (grin. just kidding)

Fins suggest a couple other problems. If your investment sits too
long after pouring and before it glosses off, you can get some
separation of water and powder causing little raised "water mark"
lines on the castings. Increase mix time, pay attention to
water/powder ratios, and make sure your timing is right so the flask
doesn’t sit more than a few minutes before gloss off.

If the fins are wholely added metal, rather than little lines, then
that sounds like actual cracks in the investment. Causes can be using
too much water for an investment mix that’s too weak to take the
stress of a centrifuge, or a burnout thats too fast or too soon after
investing (or much later, after investment has dried out) Pattern
parts too close together can also give you places in the mold where
the investment is just too thin a wall, and breaks down then hot
metal hits it.

For the most part, the problems you describe are related to your
investing method, not the casting method. The only exception is that
the greater forces of centrifugal casting can sometimes require a
thicker (less water) investment mix in order to have the needed
strength to resist cracking. But if you’re properly weighing the
powder and water, and using proper temperature water, you’re not
likely to get a mix that’s too weak for any but the heaviest or most
complex models.

And if you use a decent commercial casting investment, skip the
debubblizers. Sometimes they can make surface quality worse, not
better. Usually they aren’t needed.

Peter Rowe


#7

Hi Linda,

When I first started out I had problems with bubbles as well. What
finally fixed it was to pay very close attention to the mixing of the
investment, the vacuuming of the investment in the mixing bowl and
the vacuuming of the investment in the flask. They are all three very
specific steps and make a vast difference in the outcome.

I use Kerr’s Satin Cast 20. I have a paper that I can share if you
would like. I believe it is posted on the Ganoksin Orchid list bench
tips or something like that or I can send it to you directly.

Ken Moore
www.kenworx.com


#8

Hi Linda

Some one told me tilt a little the flask after pouring the
investment when still liquid and tap it two or three times with a
hammer or stirrer handle to help to release the bubbles inside the
models. I have a vaccum casting machine so I don’t need to do that.
so I never try it.

Gustavo


#9

Linda.

I use my centrifuge for casting, but also use my vacuum to remove
any bubbles before burnout. My guess is that you are not getting all
the bubbles out of your investment. Are you vacuuming the investment
the proper length of time before burnout?

If debubblelizer does not work, you might dip your wax model in some
investment then place it in the flask and gently pour the remaining
investment over it. Be sure to vacuum it to remove any air bubbles.

Alma