Starvest High Heat Casting Investments
(Phosphate Bonded)
Why
is the investment setting up so fast or mixing thick?
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The best results are obtained when the average
temperature of both the liquid (after dilution with water
if necessary) and powder is about 72 degrees F (22 C). Since
this is the starting point in the investing process it is
necessary to take precautions at this juncture with regard
to the temperatures of the liquid and powder to assure satisfactory
results.
We highly recommend in getting a digital
thermometer which can read in at least 1 decimal digit to
assure the accuracy. Take the temperature of the powder after
opening the pouch and subtract it from the number 144 for
Fahrenheit or 44 for temperature in Celsius. The remaining
number should be the temperature of the liquid.
Remember that at Emdin all batches are tested
at 22 degrees C (72 F) and must have at least 5 to 7 minutes
working time. Therefore if the material appears to be becoming
thick too quickly leaving you to struggle with the investing
procedure, there is no question the lab is too warm. If there
is no means of determining the temperature properly, then
simply refrigerate the liquid and cool the mixing bowl in
refrigerator for 10 minutes before investing.
When the liquid is refrigerated we recommend
that the material be mixed as long as possible without causing
difficulty in investing (about 90 to 120 seconds). This will
cause the mixed investment to set up uniformly and snappily.
Why
am I having a rough sandpaper like surface?
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This is almost invariably due to not mixing
material long enough. Simply increase the mixing time by another
15 to 30 seconds and the problem should go away.
Why
are my castings tight?
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The fit of a casting depends on many factors.
Overall alloy shrinkage is the most contributing factor. Other
less obvious factors are temperature of liquid and powder,
liquid to powder ratio, liquid dilution with distilled water,
thickness of wax up, temperature of the wax, type of the wax
(dipping wax vs. buildup wax) and the softness of the wax,
as well as the wax pot and the age of the wax in the wax pot.
Generally speaking as the silver and copper
content of the alloy increases, its shrinkage will decrease
as well and therefore the recommended dilution for these gold
crown type III alloys is 50% water and 50% clear liquid. Clearly
other factors might influence the fit of the casting and consequently
this dilution would need to be adjusted by changing proportions
of liquid and water.
Conversely the more concentrated liquid
you use, the more expansion would be realized to make the
casting bigger. As the gold content of the alloys increases
the more concentrated liquid needs to be used by reducing
the dilution.
Palladium containing alloys require less
concentrated liquid than high gold alloys with no Palladium.
Those labs which have now switched to gold alloys because
of high prices of Palladium alloys would immediately notice
that their castings are tighter at liquid dilution they used
for high Palladium alloys.
Finally, the non-precious alloys pose the
most challenge. The quality of these alloys is all over the
map and requires several approaches. One approach is the use
of our Green high expansion liquid. However, if the green
liquid at 100% concentration along with less mechanical spatulation
as described in the instruction fails to yield satisfactory
fit, then it would be necessary to use a softer wax.
Laboratories have reported that use of Yeti
Preci Dip dipping wax allowed castings to fit better. Simultaneous
tests with another wax proven the non-precious castings to
not fit at all. This same wax worked fine with high
noble and type III gold alloys just fine.
What
should I do when using Green High Expansion Liquid?
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Generally when using full strength clear
liquid fails to produce a casting with good fit, it is time
to use green liquid. The starting point should be the liquid
powder ratio for that alloy with clear liquid. For example,
if clear liquid dilution for a 60g pouch is 9.5 ml liquid
and 5.0 ml distilled water, then the same ratio should be
the starting point with green liquid when the use of 100%
clear liquid failed to yield satisfactory results. Further
increase in concentration of the liquid should be in 2 ml
liquid increments.
What
should I do when top of the mold (ring less) blows off in
the fast burnout method?
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First the molds should be placed in the
oven within 5 to 10 minutes after they are separated from
the mold ring.
Generally, even though the molds can be
placed in the ring in a preheated oven from 500 C (900 F)
to 750 C (1400 F) keep in mind that as the liquid concentration
increases the temperature of the preheated oven should be
decreased to protect the mold from excessive thermal shock.
The safest way to prevent blowouts is to
put the molds in the oven at 500 C (900 F) and it should almost
always be safe.
How
fast can I do the slow burnout?
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It depends. When starting burnout in a cold
oven after a 30 minutes bench set, you can go straight to
casting temperature at 15 to 20 degrees F per minute. The
longer you let it sit on the bench the lower should be the
heating rate. For overnight burnout cycles where the molds
sit in the oven for several hours, it is best to follow the
2-stage heating cycle described in the instructions.
Also, the heating rate depends on the size
of the mold. Molds larger than 200g should be heated at lower
recommended burnout cycle. A fast burnout should be avoided.
What
should I do when molds show cracks during bench set?
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Always use distilled water and monitor to
make sure that temperatures of the liquid, water, and powder
is about 72 degrees F (23C). When all three temperature are
in low 60's F (16 C) then mold becomes weak and cannot
withstand the forces of setting expansion This results in
micro cracks on the surface, sometimes leading to large cracks.
Generally micro cracks will not cause any harm during burnout.
All batches are tested for this behavior
up to mold sizes of 200g and rarely do we see even micro cracks.
For molds of 250g or more we recommend that
metal rings be used to prevent cracks in the mold.
Why
are there bubbles on the casting?
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There are two kinds of bubbles - round and
irregular. The round bubbles are either air or gases coming
from the mixed investment. To prevent air bubbles do the following:
after hand-spatulation of the powder into liquid, place the
cover on and apply vacuum for 10 second and then spatulate
mechanically for another 45 to 90 seconds depending upon the
temperatures of the liquid and powder. A longer mixing is
desirable because it would also minimize the gas bubbles due
to longer hold under vacuum and more rapid reaction between
the binder ingredients.
The second kind of bubbles are irregular
like pointed pyramids. This is frequently the result of a
very rapid burnout during slow burnout method or doing a fast
burnout in an oven preheated to a very high temperature. Another
consequence of such irregular bubbles will be inclusions of
investment particles in the casting.
Although the molds can be placed in an oven
preheated to as high as 1500 degrees F, it is far more preferable
to place the molds at 900 degrees F (500 C) oven and let the
temperature rise as rapidly as possible to the desired temperatures.
What
is the reason for finning on the casting?
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There are several reasons for finning, the
commonest of all is a very rapid rise during slow burnout.
It is rather interesting that no finning is observed during
a fast burnout in a preheated oven but it frequently shows
up during slow burnout when the heating rate is very high
- say 20 degrees F per minute or higher.
As a rule follow the manufacturers recommendations
for slow burnout.
The finning also shows up when the mold
has been sitting on the bench or in the oven for later or
overnight burnout. For overnight burnout it is highly desirable
to let the temperature rise at a low rate of 5 degrees F per
minute up to 500 to 600 degrees F and then let the temperature
rise at 10 to 15 degrees F per minute up to 1000 degrees F
and let it rise at still higher rate. This is 3 stage burnout.
If the oven does not allow a 2 or 3 stage
burnout then use the lowest heating rate suggested above.
If the finning occurs on the margins only,
then it is either due to the wax preparation too close to
the top or the finning occurred during the casting process.
Increase the distance from the top of the mold to the top
of the wax crown to about 3/8 inch rather than 1/4 inch.
If the finning occurs on the sprues but
not on the crowns, then using softer wax sprues will help.
In any case a slower heating rate will help
minimize all kinds of finning.
How
many turns should I give on the broken arm casting machine?
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Generally speaking the number of turns will
depend upon the density of the alloy and the physical condition
of the spring in the casting machine. Some brands of Casting
machines require one to two additional turns.
As for the density of the alloy, lighter
the alloy more the turns. It is best to consult the manufacturer
of the alloys and the casting machines because this matter
is unrelated to the casting investments. However, giving too
many turns can cause finning as well as blow through the mold
thereby losing much of the alloy through the back of the mold.
Also, It is very important that the casting
arm is completely balanced. This is true for both manual as
well as induction casting machines.
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