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Expansion of the investment during burnout


#1

Hello…By way of introduction…a tip.

When casting, shrinkage is, for the most part, expansion of the
investment during burnout. To reduce the expansion immerse the mold
in 105F water immediately after pouring the investment. Leave it in
the water until it sets. I use a 5 gallon aquarium with a heater so
the temp stays constant. I learned this trick 35 years ago when I
set up a dental lab and was casting crowns. They have to fit
perfectly the first time. I’ve not found an investment that this
doesn’t work with but try it on a non critical piece first.

Chris
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45 43’03.24"N 123 53’45.81"W


#2

Continue from:
https://orchid.ganoksin.com/t/expansion-of-the-investment-during-burnout

Dear Chris,

When casting, shrinkage is, for the most part, expansion of the
investment during burnout. To reduce the expansion immerse the mold
in 105F water immediately after pouring the investment. Leave it in
the water until it sets. I use a 5 gallon aquarium with a heater so
the temp stays constant. I learned this trick 35 years ago when I
set up a dental lab and was casting crowns. They have to fit
perfectly the first time. I've not found an investment that this
doesn't work with but try it on a non critical piece first.

There are not too many of us who were casting 35 years ago who are
still at it, congratulations. On the subject of expansion of
investment during burnout I thought you would be interested in an
update on the current technology of investment. I am not really sure
what happens to a jewelry investment mold when you put it in warm
water but as far as I know it wouldn’t have any affect on thermal
expansion of the jewelry investment sold today. There may have been
a good reason to soak the dental investment 35 years ago but
whatever it was I don’t think it applies to jewelry investment on
the market today. I will try to explain why without getting too
technical.

Investment manufacturers can control the thermal expansion of the
mold by adjusting the composition of the investment. We have three
basic ingredients to deal with; the calcium sulfate binder (plaster
of Paris), and silica refractory in the form of the minerals
cristobalite and quartz. In short the proportions of these three
ingredients control the thermal expansion of the investment.

Figure 1, Scanning Electron Microscope image of the broken
surface of a fired investment mold. From: 2005 Santa Fe
Symposium Paper by Dr G.M. Ingo, Univ. of Rome 

Figure 1 shows the structure of the investment after it is burned
out. It is a haystack of calcium sulfate (CaSO4) crystals and grains
of sand. It is a very open structure because all the space that was
occupied by water when the investment set is now empty. This open
structure provides the permeability through which the gas in the
mold cavity will escape when the metal fills it. You can see that
all this is only glued together at the little points where parts of
it happen to touch.

Figure 2. The thermogram evidences the chemical-physical reactions
experienced by the investment during the burnout process. At 104C the
first water evaporates; at 246C the CaSO4 becomes anhydrous; at 466C
an unimportant alfa-CaSO4 to beta-CaSO4 phase transition occurs. At
607C the alfa-quartz to beta-quartz phase change occurs. From: 2005
Santa Fe Symposium paper by Dr. G.M. Ingo

In Figure 2 the upper graph line shows investment weight change from
water loss and thermal decomposition (starting at 990C) as it is
heated. The lower line shows the endothermic and exothermic behavior
of the investment as water evaporates and material phase changes
take place. In Dr. Ingo’s graph you can see that the investment
undergoes a two-stage dehydration process. He marks the first
dehydration stage at 104C where most of the water is removed. The
second stage is marked at 246C where crystallization water
evaporation takes place and the CaSO4 shrinks as it becomes
anhydrous. Looking at Figure 1, you can imagine the CaSO4 crystals
shrinking and breaking away from each other and the grains of sand
if something didn’t counteract their lost volume. Cristobalite comes
to the rescue. It so happens that cristobalite goes through a phase
transformation with a noticeable volume expansion at about the same
temperature and rate that calcium sulfate shrinks.

Figure 3. Temperature Differential Analysis (TDA) graph showing
the expansion and contraction of investment formulation sample
#2 as it is heated and cooled. 
Figure 4. TDA graph showing the expansion and contraction of
investment formulation sample #5 as it is heated and cooled. 

Here is how it works. As the mold temperature rises, the binder
shrinks and the cristobalite expands. You can see in Figures 3 and 4
that happens between 200 and 270C in both cases. However, you can
also see that at the 270C mark one sample expanded 0.580% and the
other expanded only 0.477%. On both Figure 3 and 4 you can see that
at 580C the quartz starts to expand but the final expansion is
always relative to the percentage of binder, cristobalite and quartz
(this was a test to see if I have put you to sleep yet. The 607C
temperature on Dr. Ingo’s graph is the peak). So you can see that it
is how much of each of the major ingredients there are in the
formulation that controls how much thermal expansion the investment
mold will have and your only control is the casting temperature of
the flask.

Additional notes: If you experience cracked investment try extending
the time up to 240C and dwelling at 240 for an hour or so. Looking
at the cooling curve on Figures 3 and 4 you can understand why the
investment gets very weak if a burned out flask is cooled below
270C. Remember the binder shrank and the cristobalite expanded to
keep everything tight. Then going down from 270C the cristobalite
transforms back to alpha form and shrinks but the binder doesn’t
expand and the bonds are broken. I am always interested in other
views and opinions and welcome comments from any and all.


#3
Here is how it works. As the mold temperature rises, the binder
shrinks and the cristobalite expands. You can see ..... 

This is very interesting. Not too technical here is what I take away
from it.

– Slow ramp up to initially burn off the water and let contraction
and expansion of materials take place

– Never burnout, cool all the way to room, reheat and then pour. It
is subject to cracks

– I’m glad I don’t have to mix my own investment

Regards RLW