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Why
does it take so long for the investment to set?
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The binder component of the investment takes
advantage of a unique property of developing strength as the
invested material matures during the long setting period on
the bench. During setting the particles coated with the binder
settle gradually. As they settle the excess water of mixing
appears on top of the flask. After several hours the excess
water is removed by gentle suction and the mold is left resting
on the bench for several more hours to develop full green
strength before the flask can be placed in the oven.
Does room
temperature and humidity have any effect on burnout of the
mold?
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Absolutely. This is very critical because
inappropriate conditions might not cure the mold properly
so that additional time to cure the flask might be required
and even that might not be sufficient.
Generally speaking, the molds the room temperature should
be higher and higher as the relative humidity rises. At ideal
humidity of about 30 to 40%, room temperature around 23 degree
C would be sufficient. On the other hand, at humidities in
excess of 70% the room curing temperature ought to be above
30 degrees C (about 86 degrees F).
Another difficulty arises when the molds are left curing overnight
and the night temperatures drop while the humidity remains
high. This is surely not good for the best curing of the investment.
Why do the
castings have an "orange peel" like surface?
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This is one of the effects of molds not
curing properly indicating low room curing temperature, high
humidity or both. It has been noticed by some that in such
cases the curing needs to be done over several days. The best
procedure would be to place the molds in the casting room
where at least the room temperature would be higher even if
the humidity is higher as well.
What else
can be done to achieve faster curing of the investment?
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A heating cabinet can be constructed where
flasks (molds) can be placed after the water of top of the
flask has been removed. However, provision must be made to
remove the moisture laden air in the cabinet by exhaust continuously.
Feeding cooler air with less moisture and removing the moisture
laden from within the cabinet and maintaining cabinet at about
82 to 85 degrees would appear to be the best answer.
Why does the
material seem to mix so thick in the beginning, then gradually
thinning out over time?
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This is by design. We do not want too much
water in the flask after curing because it would make the
mold weak when it comes time to cast after burnout. This is
specially true if the castings are done on a centrifugal casting
machine.
Because of this unique requirement, different batches or material
even within the same batch might appear to behave slightly
differently during the mixing start. The best thing is to
keep about 1 to 2 % extra water and add it if necessary.
Is there
an optimal time to complete the mixing?
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This depends very much on the mixing equipment
and the amount of the investment being mixed. The investment
can be mixed in as large a quantity as 300 lb. due its unlimited
working time until invested ( poured ) into the flask. Small
amounts of 50 lb. or less can be mixed in about 30 minutes
in a good heavy duty bakery mixer. Again mixing time would
depend of energy out put of the mixing equipment.
There is a finite mixing time though. Eventually the slurry
viscosity would come to a constant and that can be measured
by utilizing a viscosity measuring cup (zahn cup).
What should
the burnout cycle be?
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This would depend on the size and the number
of flasks in the oven as well as on the curing conditions.
For best results the flasks should be heated at a very slow
rate for several hours to about 800 degrees F (425 C) and
then raise it to casting temperature, hold for long enough
and then cast.
Why do some
flasks blow up during casting process?
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This happens when flasks are not cured properly
or the casting machine is not balanced. Another reason would
be too many flasks in the oven without enough air (actually
oxygen) to burn out the wax and plastic completely. Inadequate
wax removal can be checked out by placing ceramic cones in
the oven. The cones come to melt at different temperatures.
If a cone to melt at oven's final temperature does not bend,
then the oven never reached the casting temperature. If the
cone color turns GREEN then there was not enough air to complete
the burnout and removal of wax and plastic.
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