I have been casting jewellery for more than ten years now but I
continue having a problem with investment inclusions in my castings.
The investment generally breaks away inside the patterns rather than
from the tree or sprues. I have better luck casting thin items than
thick ones. I use a furnace with an electronic cycle controller and have
an electric melting furnace. Any suggestions?
My guess is that you are either mixing the investment too thin or you
are too rough in handling them after the burnout. Make sure that
your sprue-formers are fairly clean so that they leave a smooth sprue
cavity too. Could be you are washing bits of investment down into
the cavity that came from the sprue chamber area. I sometimes smooth
my sprue button area with an old spoon after I pull off the bottom of
the flask, and you can rinse it with a little water too, but not too
long. Make sure you let the plaster set up long enough too. Don’t
go pulling that rubber bottom off after 20 minutes. I’ll risk it
after 45 minutes, but I like to let it set up for an hour.
Dear David,
Good advice on investment breakdown, but I would like to add another
pitfall that seems to be linked to investment breakdown. I have found
that it is essential that, when using de-bubbleizer, you make certain
that your wax is completely dry after dipping. I always set up a
small fan trained on the wax model after dipping. I let it go for
about a half an hour or until there is absolutely no wetness.
De-bubbleizer does seems to interact with investment…believe me, I
learned the hard way ! Ron at Mills Gem, Los Osos, CA.
There are several things that can cause investment inclusion
problems.
The first is measuring and mixing. follow the manufacturers water to
weight ratios exactly, weigh both the investment and water with an
accurate scale not a dial type but a balance or digital scale (BTW
water weighs 1 gram per cc which makes weighing it easy). Many people
use large diameter measuring vessels to measure the water and you can
be off by 10% without really noticing it and the spring scales that
are often used are just not accurate enough.
Many municipal water systems have chemicals like chlorine and
fluoride that are added to the water that affect the investment
setup. The water can vary in mineral content which can slow down or
speed up the investment setting time and affects strength of the
investment. Use distilled or deionized water it will make your setup
times repeatable and prevent contamination that can affect the
investments strength.
Hotter water will cause the investment to set up faster and it will
be stronger as the chemical reaction is more complete but it shortens
the setup time and unless you are fast it can setup before you are
ready for it so start off with 80 F ( 26 C ) water. I know some
people that use 110 F (43 C ) water and it makes very hard
investment but it is very tricky to time it right. One other thing
hot water will do is it makes the investment very thin and the
bubbles come out much easier so if you are at high altitude and
always have bubble problems increase your water temp it will
compensate for the low vacuum that can be pulled at higher
elevations.
Let it set up for at least one hour before touching the flask the
chemical reaction that you start when you add the water to the
investment is not finished when the investment glosses off, it is
ongoing for at least the next hour . Vibration and motion during the
first few minutes after gloss off can cause cracking in the
investment so don’t touch it. The investment will achieve most of
its green(unfired) strength during the first hour
The second area is burnout processes.
Put the flask in a preheated kiln and slowly ramp the temperature up
to 1200 F ( 649 C ) If you go any higher than this you run the risk
of breaking down the investment and reducing its strength and
increasing the amount of gas porosity due to the release of sulfur
dioxide from the thermal breakdown of the calcium sulfate( gypsum )
binder in the investment. The key is slow change in temperature if
you try to ramp the kiln up too fast early on the water that is
trapped in the investment can crack and degrade the surface of the
investment by forcing it out too rapidly.
The third is mechanical shock. treat the investment gently as it is
rather fragile especially when it is first setting up and after
burnout…
The fourth area is in the wax. You must be careful to not create
thin areas of investment that can flake off when struck by the molten
metal when it enters the mold make sure all your sprue attachment
points are filleted and smooth. Look at your model think of it in
the negative form, are there any thin protrusions of investment in
line with where the metal is going to flow in? If so you may want to
reposition the sprue.
I hope some of this is helpful.
Jim
jbin@well.com
James Binnion Metal Arts
4701 San Leandro St #18
Oakland, CA 94601
510-533-5108
Excellent point on debublizer, Ron; I read on the label of a bottle of
the stuff that you shouldn’t use it for vacuum investing. I don’t
usually use it with a vacuum machine, but before I had one it needed
it. I read the label that it needs to dry before investing, although
for a long time I never did, and I know it gave me problems, I just
didn’t know they were avoidable. I’d like your take on the use of
debulizer. Is it helpful if you are vacuum investing?
David L. Huffman
Most investments available to the jeweler now have surface tension
reducing agents ( debublizer) in them now. So any additional
debublizer is really not needed and may actually cause problems with
the cast surface.
jbin@well.com
James Binnion Metal Arts
4701 San Leandro St #18
Oakland, CA 94601
510-533-5108
Hello Hoods, I have read some of the posts on your problem. There are
many good suggestions there. I will add a few things. Investment these
days contains a surfactant (soap) which eases the surface tension so
the bubbles can be released. A wax dip is no longer necessary, the
added ingredients in the investment do the job. Hotter water in the
mix does not “thin” the investment but actually activates the
surfactants to release the bubbles. Do a dummy flask and time the
gloss off and plan to stop work 1 1/2 - 2 minutes before that time. I
believe James touched on burnout. Investment when hardened carries
both "free’ water and “chemically bound water”. The free water leaves
in the early stages of burnout, under 500f. Chemically bound water
releases at approximately the same time the christobalite (rock
powder) is begining to expand at 700f. If you push past this temp zone
the combined forces of this expansion with the chemically bound water
turning to steam will result with the symtoms you mentioned. It is
called spauling. The solution is to creep up to 700f and hold there
for at least an hour and then proceed with the rest of your burnout.
John, J.A. Henkel Co.,Inc. Moldmaking Casting Finishing
David, Theoretically debubbleizer shouldn’t be necessary when vacuum
casting. If you invest using a vacuum, there shouldn’t be any bubbles
left in the investment nor on the wax pattern. I do all of my casting
using either horizontal or vertical centrifugal casting machines and
I cling to the old method of using a vibratory debubbleizer.
Furthermore, I always use a hand held mini massager on the flask to
bring the bubbles to the top of the flask ( ring ) after investing. I
definitely don’t belong to the school of die hard formula freaks who
carefully weigh the investment , measure the volume of water, take
the water temperature and consult a spiritualist. I just throw a glob
of investment into whatever is handy and stick it under the
waterspout and whip up a batch of what might just as well be pancake
batter. If my investment is old and tired, I goose it up with warmer
water. I definitely allow plenty of time for the investment to set
up…I usually let it take over night under a cover that is
relatively air tight. As for burn out, I prefer to start with a cold
oven and set the RATE of temperature increase to a low level and cook
the flask to about five hundred degrees farenheit at which point I
turn the flask over , with the sprue opening pointing up, and cook
the hell out of it using the highest rate of temperature increase.
With my oven, I can bring the temp up to 1350 in about an hour and a
half. After cooking at 1350 for a half hour , I open the oven and
check the color of the investment…if it is bone white, I turn off
the oven and follow the temp down to about 900 degrees at which point
I cast. One of the details that I have overlooked is the fact that I
do almost all of my casting with an antique vertical casting machine
which uses very small flasks and, therefore, does not require
prolonged burnout times. The amount of burnout time is directly
proportional to the mass of the investment and the weight of wax
patterns and should be adjusted accordingly. Casting is certainly a
complex function, but it need not be relegated to voodoo ritual.
Trial and error will take the mystery out of it. Have faith,
perservere and yee shall overcome…! Ron at Mills Gem, Los Osos,
CA.
I'd like your take on the use of debulizer. Is it helpful if you are vacuum investing?
All the major brands of investement now include wetting agents in
their formula. Kerr actually recommends AGAINST the use of additional
debubbelizers. With the included wetting agents already in the
investments, debubbelizers have no real purpose other than to continue
to make money for the manufacturers, decades after the investment
makers have caught up. For investing without vacuum, the
debubbleizers may have a slight beneficial effect, but it’s marginal,
and you can get into almost as much trouble with them, as you have
without them. I prefer without. For vacuum casting, the stuff is
clearly a detriment. Investment surfaces where it’s been used are more
prone to breakdown.
Several other products whice ARE useful can also cause problems with
investment. That nice citrus smelling orange colored liquid solvent
widely sold for polishing wax models is a bad one in this regard,
especially if you’re doing platinum casting. Though the bottles all
say the stuff does not contaminate investment, test runs I’ve seen
show otherwise. With phosphate curing platinum investments, for
example, the surfaces of the platinum are MUCH rougher where the wax
polish was used, unless the waxes are actually washed to remove
residues. The contaminate with lower temp investments is much less,
but it’s still there, especially if you don’t let the stuff completely
dry before investing.
Peter Rowe