I took pretty much all the advice here, and I must say my castings
came out with a great deal of improvement.
I took a longer burnout cycle, and while the investment did not
flake, unfortunately it cracked on all three molds that were in there.
However, the cracks did not make their way down to the models, so
luckily the castings were not affected.
I’ll dial down the burnout cycle to something inbetween next time.
Thanks to all who have helped me… things are slowly improving :-).
When I learned to cast a number of years ago the person who taught me
thought it very important that all moisture be gone from the plaster
before casting. He took a piece of metal (about a 4" square sheet)
with a mirror finish and held it up to the door of the kiln, opened it
a crack and then checked the metal for any signs of steam on the
mirror surface and would not cast until all was clear. Sometimes this
meant holding the flasks quite some time at the casting temperature
while the moisture baked out. What do you experienced casters say
about this? This man was self taught so I don’t know where he picked
up this Annette
It would seem very important that no steam is still being released
from the flask, as that would mean that at least some of the
investment was still only at 212 degrees (the boiling point of water),
way to low for casting. This seems like a wildly inaccurate method of
judging temperature.
Jack Reisland
�Hola de nuevo! Moisture can only be a problem at the beginning of the
burning cycle and just in the case that you have not waited two hours
after investing, when the mold is still too wet, or if the kiln is
heating too fast (the recommendation is to pre-heat up to 300�F and
keep the mold at that temperture for 1 to 2 hours before rising again
the temperature); but, how can you possibly have moisture after
reaching and keeping 1,350�F for at least 1 to 2 hours? After a
complete burning cycle (from 6 to 12 hours, depending on different
factors)you definitely get rid of all moisture and, of course, you
should have eliminated all carbon residues. This might be the problem
that your teacher detected: fumes, not moisture. In this case, a good
burning cycle and the presence of oxigen inside the kiln (good
ventilation)should do. I hope this helps, Annette.
Tu amigo
Arq. David C. Duhne
Director Tecnico y Comercial
Diamantex S.A. de C.V.
Dear Annette, Both Dan Grandi and Robert Hood have made good points
about the importance of good investing proceedure. I am one that
wants to know as much as possible about the material I use to get best
possible results for my customers. I have tried many brands of
investment. Before I get a new batch (18, 100 lb. barrels) I want to
know how fresh it is. When it arrives I do a test batch to acertain
the gloss-off time. I too learned, when I began 25 years ago, to mix
the powder into the water until it looked like heavy cream. Yes, it
did work, and I was thrilled to see actual castings that I had done!
Back then, I worked as a caster for a company that had a nice line of
jewelry and I was responsible for the melt and spin. When things went
wrong I took the blame, not knowing exactly what happened. Tired of
that I began to call anybody I could think of to learn more than the
jewelry instructor had taught me. That’s when I began to thoroughly
investigate every aspect of the process. Guess what I discovered?!
It’s not how you kill the chicken and sprinkle the blood! And still I
actually had people tell me not to cast on a rainy day! What a great
day when I realized that spruing was one of the most crutial aspects
of the process and that I had control of cracking, spalling, bubbles
and water trails when I did the investing. Every time I teach casting
I stress that it is not necessary to hold a degree in metalurgy. Every
metal has a different “personality” and you can get to know them if
you care to. Investment is designed by folks that want it to do
certain things. It can’t do those things unless you follow their
rules. Your mention of the fellow with the polished metal to check for
moisture sounds like someone that casts by folklore. Yes, I have no
doubt that if moisture is present that it would condense on a cool
metal surface. And I have seen many wonderful jewelry creations done
in this manner. However to me it is more reassuring to know that the
water has left many hours before because of an understanding of the
process. Steam from the water will crack investment plaster when it is
heated too fast. Steam is a powerful force. In a previous post I
mentioned “free” and “chemically bound” water present in the invested
flask. A well planned burnout cycle for your kiln and the size of your
casting run is the capstone to your knowledge of the investment you
are using. Anything worth doing, is worth doing right. John,
J.A.Henkel Co.,Inc., Moldmaking Casting Finishing
Hi Annette and all, I had been following this and was only going to
add a little but since installing the new version of eudora, and it
really messed up my email programs, I have not been able to look up
the previous messages. I’m sure most of you have said most of the
things I am going to say here but I thought I should mention a few and
add my two cents!
I’ll start with the investing and work towards the casting.
When investing it is important to use the exact ratios as stated by
the investment manufacturer. Too little water and too much will cause
many problems you don’t want. The investment while being mixed goes
through a mechanical and chemical change. So it needs to be wet enough
for these changes to take place but not too wet. The reason there are
a couple of suggested ratios for investment is so you can find the
proper one for your conditions. That is the first consideration the
second is the finish on your cast pieces. the more water you use the
rougher your surfaces. the less water the smoother the surface of your
castings. The more water the less strength the investment will have
because the water is 'pushing the investment particles away from each
other. You lose the mechanical bond. Too little water and now you lose
the chemical and mechanical bond.
Unless you build a room that is sealed and the environmental
conditions are completely controlled you will have variations in your
investment mixes from day to day and season to season. Humidity and
temperature changes all the time so it will affect your mixing. There
is a lot of trial and error that goes on in this until you have cast
enough to instinctively know what is needed and when. In our shop we
had gas heat and swamp coolers for cooling. We went from the heaters
removing almost all moisture in the air to having very high humidity
during the summer! This has many effects not only on the investing but
other steps in the whole process as well.
The majority of moisture should be removed at the lower temps. Again
if the times are not long enough the moisture will not be eliminated.
The lower temps are for melting the wax out and eliminating moisture.
As the oven temp is raised the purpose is to bring the flask slowly up
to total burnout temp. and to burn the wax and eliminate moisture.
Your top temp. is to completely burnout the wax (organics).
If the lower temps are rushed the wax will not melt out completely
and the moisture will not be eliminated. When the moisture is heated
it will expand and evaporate. If the moisture is still in the flask it
will expand and break up the investment. If the wax has not been
completely melted out it will tend to penetrate into the investment
creating a barrier for the moisture and preventing the moisture from
leaving the flask. There will be a small amount of wax that penetrates
into the investment no matter what you do. Steam dewaxing reduces this
the most.
One practice we used was to invest using exactly the recommended
ratios for the investment. Then set them to harden for about 30
minutes. We used a steam dewaxer to eliminate the wax. After steam
dewaxing the flasks were set out to dry for about an hour. Then placed
in the oven. the first temp. on the program was 150 degrees F. and it
was held for 1-1/2 hours. then the temp was brought up to 300 degrees
F. and we set it to take 30 minutes to bring it up to this temp. Then
it was held for another 1-1/2 hours. And so on. The first couple of
temps., times and ramps are critical as are the last couple. The
temps. times and ramps in the middle are important but not as much.
They are mainly for burning the wax and bringing the flask up to temp.
Again you don’t want to rush these as this could also cause problems.
We used flasks ranging in size from 2" diameter up to 6" diameter.
All in the same ovens and all with the same burnout program. The only
time we had casting problems was if someone mixed the investment wrong
or sprued a piece too close to the edge of the flask or at the
beginning of a season. The ovens were electric and we were switching
to gas kilns for a better burnout. The gas kiln will allow a faster
burnout because it can remove the wax and moisture so much faster and
it gets more oxygen into the cavity so combustion of the wax takes
place more completely and faster. However a gas kiln is not for
everyone.
You are right in that you need to get all the moisture out of the
flasks during burnout. But the moisture should be out by the time the
flasks are ready to cast. The rule of thumb is to hold the high temp.
So for the rule a four inch diameter flask needs a ‘soak’ time of 4
hours. A 2 inch diameter flask needs a 2 hour soak. The length of the
flask isn’t important.
The reason for the soak time is the entire flask will not get to the
oven temp if this minimum time is not reached. So even though your
oven is at a temp it doesn’t mean your flask is. And even if the
outside of the flask is at temp the inside may not be if the oven has
not been held at the specified temp according to the rule.
From this you can see that there should not be any moisture left in
the flask if they are soaked for the proper times and temps.
So if you are using something and sticking it in the door to see if
there is moisture condensing on it the burnout has been way too fast
and you may have problems that can be easily eliminated by extending
the length of the burnout.
One thing I didn’t see anybody say in the postings and I may have
missed, was when taking the flask out of the oven be sure it is taken
out the way it is in there. For instance: if you have the flasks in
with the spur opening facing down pull the flask out with the spur
opening facing down then tip the flask very slightly and blow across
the surface where the spur opening is. Do not blow into the opening!
What you are doing is getting rid of any loose material on the
investment so it won’t fall into the opening when it is turned over.
Some people will brush the surface to get the loose material off but
it still will need to be blown off.
Also when turning the flask over watch the opening closely. If you
see anything fall into the opening turn it upside down again. If it
doesn’t come out your out of luck. There isn’t much you can do about
it.
One thing I want to mention here is the fact that investment is very
hazardous to your health. Please make sure you have very good
ventilation and when mixing investment and handling the flasks
whenever possible wear a respirator even if your have plenty of
ventilation. When getting a respirator make sure you get one that fits
properly If you don’t know the proper procedure for test fitting I can
help with that or as much as I hate to say it call OSHA. Also take a
breathing test to see if you can work with a respirator. Do not use
one unless you take a breathing test. Heavy smokers won’t pass a
breathing test! So if you can’t pass the breathing test don’t invest.
Hire someone who can wear a respirator do the investing. Your life and
health are worth the cost of these safety items.
By the way the investment you see is not what is harmful! Anything
you can see if you breath it in you can breath it back out. The
harmful stuff is what you cannot see. These particles are not
respirable (you can’t breath them back out). When you get these in
your lungs they stay there forever. They are very sharp and stay that
way. When they land on your lung tissue they will cut it and as you
breath they cut again and again. What ends up happening is that your
lungs don’t heal where these particles keep continuously cut them. So
when you have enough build up of these particles in your lungs the
lungs build up some scar tissue and are at the same time being cut up
repeatedly. This reduces your ability to get oxygen into your blood.
Eventually you will die from this. So let me say this again. Your life
and health are worth the cost of these safety items.
There is more I could write about burnouts, investing and casting But
if unless this has raised any questions I’ll stop here.
The hardest part of casting is experience. You can’t buy it unless
you hire someone with it and you can’t get your own without doing the
work. Which means a lot of trial and error sometimes and a whole lot
of research.
Good luck with your casting and remember if at any time you need
anything let me know. Were not here to just sell equipment and
repairs. I’ll answer any questions anyone might have even if it means
sending you to buy from someone else. Honesty is better than making an
unhappy customer.
Ken Kotoski
MPG Repair
www.mpgrepair.com
Hi All,
John touched on one item I forgot to in my last post. The sprueing is
very important to investment flaking. If you have a sharp angle on
your tree to your sprued part the metal can and most times will hit
that thin poin of investment and break it off. Be sure when you sprue
to have a smooth rounded junction from the tree to the sprue and from
the sprue to your part.
Ken Kotoski
MPG Repair
Ken you hit the nail on the head again! In the dental lab we have to
concern ourselvs with the rate of thermal expansion that occurrs with
the investment as it is heated to the final temp. Fit is for us the
ruling factor. It is imperative that we do read and follow
manufacturers instructions when it comes to mixing investment for
casting crowns, bridges, and inlays for your mouth. Following the
manufacturers instructions is the area I wanted to echo here as it
realy affects the final casting.
Mike & Dale
Lone Star Technical Service
Hej to you all on Orchid I am a newbee. and my English is not good. but I cannot resist
I have greath pleasure in following this casting update. and the
use of a metal mirror near the door of the kiln make sence to me. not
the waterstory. I would gess that it was not the water he was looking
for. But the damp or wax-moisture. water vil not condence but the
wax might ???. that is, until there is a wax free inviroment in the
kiln. this might be a good method to se if the wax is gone. sorry for
my spelling. i flunkt in English in my schooldays 
Peter Fr�sig The
sitting Dane Peterfs@post5.tele.dk
Hi All, I actually somewhat disagree with the statement that sharp
edges on sprueing can cause flaking and other problems on thin areas
and I suggest looking at your water ratio and other areas for a
solution…
The reason I state this is that on many designs that are cast, there
are sharp , thin areas in the design itself and this problem never
occurs.As a matter of fact, there are all kinds of shapes involved in
a casting where metal has to flow … far more complex than a sprue .
What can happen in the sprue area is if there are tiny cracks or
small pits in the sprue area where dipping into a wax wash will allow
liquid to enter those cracks and not evaporate for sometime, then,
when the investment dries, there is now a weak spot in the
investment. This can happen anywhere on the wax pattern where liquid
may be retained.This will cause problems.
We have received over the years, many molds from many customers where
the sprue was not attached correctly ( soldered or cut into the mold)
and we have never had a problem with flaking if the sprue was in the
correct location.
This does not mean that it is a good idea to have sharp edged
sprues… The importance of sprueing is apparent and will cause
problems in a casting if the sprue is placed in a position that goes
against the flow of the metal As you are now asking the metal to flow
in a pattern that is not suited to proper filling of the piece.This
happens to be one of the areas where we have had to remove many
sprues on models supplied by customers and re-attach them in a better
location.We find that many of our customers who have had lousy
casting from their previous caster had these problems more because of
sprue location than any other reason and that the caster they had
used was the person responsible for telling them where to put the
sprue. This is also why we ask our customers to send us models with
no sprue attached as we feel we are better at making this judgement
call . Realize that that is all it is… a judgement call…If you are
wrong , then you have the consequences of a bad casting .
If your metal cannot flow through the piece correctly, you will most
likely have to increase your metal and flask temperature so that the
area that is not filling correctly will do so… and because of this,
you may get problems in your castings as the item is generaly over
heated in the areas that have good metal flow.
Also realize that in many casting businesses, mold makers and model
makers are not casters and casters are not the mold maker… most
mold makers and model makers are not experienced casters and will make
mistakes in sprueing .As a matter of fact, many casters can’t make
molds or models !!!
The best combination of talents is when the person setting up the job
is an experienced model maker, mold maker and caster… this person
can and more than likely will make the "best guess " at sprue
location for the whole process.There are few casting businesses that
are like this.
Daniel Grandi casting /finishing/model making/mold making for
people in the trade.
I have seen castings with ‘investment inclusions’ . I noticed a
tiny pinhole on the surface of the casting and start to excavate a bit
with a needle or tiny ball burr and opened up a small crater filled
with investment. I have seen this caused by two things. Improper
sprueing ( the sprue wasn’t totally connected leaving an undercut that
upon investing gets filled with a very thin fin of investment) and
from pierce out work (windows or azures) in the model that taper to a
very sharp point. During casting the molten metal breaks off these
very thin fins or sharp skinny points of investment and mixes them in
with the molten metal.
Kate Wolf
Over night drying is preferred. A ‘wet’ flask is going to crack . .
. a slow drying out of the flask for an 1 hour, depending on the size
of the flask should handle the problem of cracking while Increasing
the temp to 212 f slowly and let sit for 1/2 to hour.
Most of all … some of us were met to be vac casters others
’spinners’ . . . after 5 years of vac casting and all the problems
that went with it … im now a happy, pleased, less problemed
spinner! I return to my vac machine when things have gone just to
well for a while to bring me back to earth!! Jim