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
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.