Thank-you for the responses regarding my investment casting fail.
So, I’ve learned the following:
Investment powder is likely to degrade if old or allowed to pick up
moisture. So, I’ll stop buying those small lots of investment that
come in baggies that originate from larger packages broken down by
the supplier.
The mix ratio of water to powder has to be exact, no more adding
just a little more water to make it easier to pour.
The flask must be perfectly still while “glossing off”; the
investment is beginning it’s initial set and any slight motion will
degrade the strength of the cavity.
Allow enough time for the flask to set after pour to give it some
strength before burn-out.
Ramp up the inital temperature in the kiln from zero to 300. I was
putting the flask in at 300-400.
Gradually ramp temperature from 300-1350. I may have gone over 1350
so that may have contributed to the cracks in investment. I also have
to check that temperature guage on the kiln for accuracy.
The mix ratio of water to powder has to be exact, no more adding
just a little more water to make it easier to pour.
Well, George, and all, I’ll temper this a bit. We use Satincast 20,
BTW, and always have. The mix of water to investment only needs to
be within certain limits, which, as someone else said, is like crepe
batter or thin pancake batter. There is advise out there to make a
thinner mix (like crepe batter) for finer pieces and a little
thicker for heavier work. We use measuring cups and aren’t
especially critical with it and get fine results for decades now.
What IS very, very important is that you mix it to death. Mix it
till it seems mixed and then mix it some more. It needs to be
absolutely homogenous.
There’s nothing wrong with ramping temperatures - professional
casters do it routinely. Our kiln is on a timer and it just clicks
on at 1AM, full temperature (from a cold start, of course). Again,
for decades with no problems. It’s not really critical - it is more
so if you’re casting 50 flasks. Certainly NOT ramping temperature is
not the reason for the cracks in this thread’s casting.
The recommendation is to let flasks set for at least two hours. If
you violate that you’ll have nothing but trouble. Ours set for 6-7
hours, generally.
I can’t say for certain the reason for the cracks, not having been
there and not watching the process. I can say that I’ve had
overheated flasks in burnout - from a dead or dying rheostat - and
had that very problem as a result, though.
What IS very, very important is that you mix it to death. Mix it
till it seems mixed and then mix it some more. It needs to be
absolutely homogenous.
Good point about the mixing, I was mixing the investment by hand
with a metal spatuala; so it's possible that the batch was not
activated properly.
Mixing by hand with a spatula or even a spoon or other implement is
fine, what you want to avoid is allowing the investment to sit still
in time period before the setting reaction really takes off. If you
do so the powder and water can start to separate by gravity
resulting in less than optimal results in your investment strength
and quality. So keep it in motion till you are ready to pour it into
the flasks which should be no more than a minute before gloss off
should start.
I recall at one point casting small handles for a pewter cup in a
mold made out of sawn and layered plywood. Worked fine.
Actually, you can cast bench blocks (like for stamping) out of lead,
tin, type metal and the like in a cardboard box. Wrap it with masking
tape to reinforce it, wipe it inside with Vaseline, put it on
something heat resistant and just pour it in. Before somebody
complains that it started on fire when they poured bronze into a box
that’s lead, tin (pewter), type metal, soft solder and the like,
all of which are in the 600F range or whatever. Done it many times,
works fine but it just gives you a block…
To elaborate a bit on Peter’s posting about spin casting, without
actually looking up numbers - a mold-making machine for spin casting
is $4-5K or something. The molds run $500-$700 or something and to
be efficient you need to make 100 models or whatever fits in a 2
foot circle. The molds are a giant pancake with a hole in the
middle, and the entire perimeter is generally packed with models.
It’s a big deal to do, not something people toy around with,
generally.
What IS very, very important is that you mix it to death. Mix it
till it seems mixed and then mix it some more. It needs to be
absolutely homogenous.
Good point about the mixing, I was mixing the investment by hand
with a metal spatuala; so it's possible that the batch was not
activated properly.
Casting is not my cup of pickle, so I did not paid much attention to
technical details of how investment should be mixed and etc. I do
have experience in working with plaster of paris however, which is
very similar to casting investment.
The best results, working with plaster of paris, are obtained with
minimal mixing. If proportion of plaster and water is right, casts
are very solid. Excessive mixing only introduces undesirable air
bubbles and could compromise it’s strength.
I guess my question is - “mix it to death” is a manufacture’s
recommended approach? And if it is, what is different about casting
investment requiring such radical treatment?
Excessive mixing only introduces undesirable air bubbles and could
compromise it's strength.
which is why we vacuum the bubbles out of investment. That, and
because bubbles (which can be on a wax surface even if not already in
the investment mix simply due to air being “trapped” by the surface.
I guess my question is - "mix it to death" is a manufacture's
recommended approach? And if it is, what is different about
casting investment requiring such radical treatment?
Manufacturers don’t call it that, but they do recommend initial
manual mixing to roughly incorporate the water into the powder
(reduces the amount of dry dust kicked up into the air too, by just
starting with a power mixer). Then they want a power mixer for a
couple minutes or so. In practice, you find that this looks like
they’re mixing well beyond the point where it looks fully uniform.
The reason this is needed for investment, and not for plaster of
paris is that with investment, if there is any slight lumpiness or
lack of uniformity in the mix, this will give variances in how the
investment resists thermal stress in burnout, and can increase
problems with molds cracking or breaking down. This need for a
really homogeneous mix is another reason to vacuum the mix, since
with sufficient vacuum, you’ve got the mix below the vapor pressure
of water, so it boils. The mix doesn’t just look like it’s boiling
because air bubbles are rising to the surface, but done right, it’s
actually boiling at room temp. What that does is to also add another
level of mixing, where any pockets of increased amounts of water
tend to boil more, causing a more even distribution of the powder and
water. It really does make a difference in casting quality, with
better surface details, and significantly lower incidence of mold
breakdown or cracking.
"mix it to death" is a manufacture's recommended approach? And if
it is, what is different about casting investment requiring such
radical treatment?
Entirely different animals - plaster and investment. If investment
isn’t mixed thoroughly, the water will be “free” and it will leave
water marks/trails (which Peter made reference to, also) on your cast
pieces. They look like ants walked across your waxes, sort of. It’s
not really even recommended by the manufacturers, that I know of
(haven’t read that stuff in35 years…) it’s from the trenches.