PROCESS CONTROL OF INVESTMENT CASTING SLURRIES
Slurry testing in Investment casting plays an important role to maintain the Slurry which
results in lower rejection due to reduction in Slurry related defects.
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Investment Casting Slurries are made by mixing Colloidal Silica/Binders, Zircon/Silica
powder and other additives.
Control of Slurries is necessary for producing sound shells.
Sound shells are required to produce quality castings.
The common tests performed are.
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Slurry Viscosity
Binder solids
Refractory solids
Total solids
Slurry Density
Binder pH
Bacteria test
Slurry Temperature
Gel test
Frequency of Testing
Test
Frequency of testing
Slurry Viscosity
Twice -Each Shift
Binder Solids
Twice a week,
Binder pH
Twice a week
Slurry pH
Twice a week
Slurry Density
Twice a week
Refractory Solids
Twice a week.
Slurry Temperature
Twice Each shift
Gel Test
Once a month (If pH>9.2)
Introduction:
Here an effort is made to explain the procedure used for the Testing of Slurries as a means
of process control of investment Casting Slurries.
TEST Procedures
1. Slurry Viscosity (Flow Cup method)
One of the important parameters that affects the quality of the slurry and the final casting is
the viscosity. Viscosity is a measure of how thick or thin a fluid is, and how easily it flows. A
high viscosity slurry may not coat the wax pattern evenly, resulting in shell defects such as
cracks, surface finish, edge coverage, shell strength etc. A low viscosity slurry may not form
a strong enough shell, leading to deformation or collapse during dewaxing or casting.
Slurry composition alone cannot guarantee a defect free shell if the slurry is not
prepared properly. The main objective of slurry preparation is to produce a stable slurry
that must meet a given set of slurry parameters. To consider slurry as stable, the slurry
need to be thoroughly mixed and the viscosity value must be stable.
Therefore, it is essential to measure and control the VISCOSITY of the slurry daily during the
investment casting dipping process. One of the common methods for measuring the
viscosity of the slurry is the FLOW CUP method. This method involves filling a standard cup
with a known volume of slurry and measuring the time it takes for the slurry to flow out
through a small hole at the bottom of the cup. The flow time is proportional to the viscosity of
the slurry.
The following steps describe how to perform the flow cup method for measuring the
viscosity of the investment casting slurry.
1. Clean and dry a standard flow cup such as Zahn 4 or similar. Make sure there are no
obstructions or residues in the hole.
2. Dip the flow cup in the slurry tank, fill it with slurry up to the brim. Hold it there for a
minute or two to attain thermal equilibrium,
3. Lift the cup vertically above the slurry surface (about 15 cm) and start a stopwatch
as soon as the slurry begins to flow out from the hole.
4. Stop the stopwatch when the first definite break in the stream of slurry occurs at the
bottom of the cup.
5. Record the flow time in seconds as viscosity time (efflux time). and calculate the
viscosity of the slurry in “cST” using the formula:
V= K (t-c).
Where
V= Kinematic Viscosity (cST)
T= Viscosity time measured as above.
K , c = are the constants for a specific Flow Cup . As per ASTM D 4212, for Zahn#4
cup the values for K=14.8 , c=5
But for day today control one can use Viscosity time (efflux time) for managing the
slurries.
6. Repeat steps 2 to 5 at least three times and take an average of the flow times to
obtain a more accurate result.
7. Compare the measured viscosity with the specified range for your slurry formulation
and adjust accordingly by adding more water or binder if needed.
o If viscosity is higher than the specified range, then thin the slurry with 50/50
mixture of of binder and water during production dipping.
o If slurry is left idle overnight use only water to adjust the viscosity to
compensate water loss by evaporation.
8. Advisable to measure the Viscosity at the beginning and end of shift. However
Frequency depends on amount of dipping relative to the size of tank and
environment factors such as humidity, airflow and temperature.
The flow cup method is a simple and inexpensive way to measure and control the viscosity
of investment casting slurry. However, it has some limitations such as being sensitive to
temperature variations, not accounting for shear effects, and requiring manual operation.
Therefore, it is advisable to use other methods such as rotational viscometers or rheometers
for more precise and reliable measurements.
REFERENCE: Ceramics Testing Guidebook, Investment Casting Institute.
To be continued: