Title
Sand Testing Preparation (GFN) (Foundry)
Objective
To determine the sand grain size and calculates the Grain Fineness Number (GFN) for moulding material of sand casting.
Introduction (Background and Theory)
Casting and Molding Processes
Molding processes can be divided into four main categories: sand casting processes;
permanent mold process, ceramic processes; and rapid prototyping. While determining the
best process for the product to suit needs, there are several factors to be considered:
• Surface quality
• Dimensional accuracy
• Type of pattern/core-box equipment
• Cost of making the mold
• Effect of selected casting process on design of the casting
Among these casting processes, sand casting method has been used extensively in industry;
therefore sand casting process will be the main issue of laboratory experiments during whole
semester.
Figure 1 : Typical Foundry Process Flow
Sand Casting Processes
A mold is produced by shaping a refractory material to form a cavity of a desired shape in
which the molten metal is poured. The mold cavity must retain its shape until the metal has
solidified and the product is removed. There are variety of different refractory materials as
casting sands including silica, olivine, chromite and zircon sands.
The mold made of one or combination of these sands must:
• Have sufficient strength to sustain the weight of the molten metal
• Be stripped away from casting cleanly and easily after it has sufficiently cooled
• Be inexpensive, because large amounts of sand are used in a casting facility.
Size and size distribution
The size and size distribution of sand grain is one of the most important factors for a healthy
casting process. The size of the sand grains affects the quality of the casting. The grain
fineness of molding sand is measured using a simple sieve analysis test. Grain Fineness
Number (GFN) is a measure of the average size of the grains in sand. Likewise, AFS Grain
Fineness Number (AFS-GFN), introduced by American Foundry Society, is a measure of
grain fineness of a sand system. AFS-GFN is used to verify the molding sand to be staying
within specification for the castings being produced to avoid potential casting problems. Too
fine grains may cause low permeability, results in gas defects; too coarse may create high
permeability which leads to metal penetration into mold, and affects surface roughness of the
casting. The ideal GFN depends on the type of metal poured, pouring temperatures, heavy or
light casting and required surface roughness.
The size distribution of sand grains is also related to the quality of the sand system. Porosity
is directly related to permeability which is the ability of the mold to permit gas escape
through the mold. If sand grains of the mold are having nearly same size, the porosity is
maximized. This may lead to poor surface quality and metal penetration into the mold.
Therefore one should decide on an optimum size distribution to avoid such defects.
Formula below used to calculate Grain Size Fineness;
GFN=Σ FΣC
×100 %
Apparatus
Num. Diagram Name
1.
Green Sand
2.
Sieve Shaker
3.
Digital Scale
Balance
Num. Diagram Name
4.
Sieve
5.
Steel Bowl
Industrial Apparatus
Num. Diagram Description
1. High Capacity Sieve Shakers A061-96
Activated by electromagnetic impulses using
triple vibrating action (vertical, lateral and
rotational) they are recommended to perform
sieving tests where high precision and
performance are important.
Simple and sturdy construction can hold up
to 10 sieves and they are suitable for wet
sieving test
The sieving time from 1 to 999 minutes
Power supply are 230V 50/60Hz 1ph
450/750W
2. Sieve Shaker Part No. 206650
The Sieve Shaker has a uniform mechanical
action comprising both a horizontal circular
motion and a vertical tapping motion that
allows particles to stratify and seek critical
openings, ensuring accurate, repeatable
results.
Test sieve size is 8 in. diameter (20.3cm). A
maximum of six 2 in. high (5.1cm) or
thirteen 1 in. high (2.5cm) sieves can be
used for testing at one time. Shaker features
vertically mounted 1/4hp motor and has a
built-in 99-minute digital timer/clock
accurate to 0.1 second.
Recommended for testing applications
requiring analysis of particles from 5 in.
(12.7cm) to 20 microns wide (635 mesh)
Experimental Procedures
a) The sieves was cleaned by turning the sieve face down and striking the rim evenly
on the table.
b) The sand sample is weighed to 100 grams.
c) The sieve is then stacked together by putting the biggest mesh size on top until the
smallest sieve on the bottom (35, 45, 60, 70, and 80,100,120,170,230) and next with
the sieve pan at the last one.
d) The stack of sieves is then put on top of the Sieve Shaker Octagon 2000 machine.
e) The silica sand was put into the sieve.
f) For a period of 10 minutes the sieve was shaken continuously.
g) After shaking for 10 minutes, the top sieve is taken apart and left over sand on the
sieve was wiped using a brass brush and carefully weighed. The weight then recorded
in column C.
h) The step in (g) was repeated until the last over sand in the sieve was weighed and
the value is recorded.
i) Using the Grain Fineness Number (GFN) formula at the below we could calculate
the Grain Fineness Number (GFN)
GFN=Σ FΣC
×100 %
Results And Data Analysis
A B C D E FNO Sieve no Opening (µ) Sand
mass(g)Sample % AFS
multiplierAFS Product
(C X E)1 35 500 0.00 0.00 10 0.002 45 355 0.00 0.00 20 0.003 60 250 29.00 29.59 30 870.004 70 212 17.00 17.35 40 680.005 80 180 9.00 9.18 50 450.006 100 150 15.00 15.31 70 1050.007 120 125 12.00 12.24 100 1200.008 170 90 9.00 9.18 145 1305.009 230 63 4.00 4.08 200 800.00
10 Pan Pan 3.00 3.06 300 900.00Total accumulated sand mass (g) ∑C= 98.00 ∑F= 7255.00
Original Mass of Sample before Sieving (g)
100
GFN=Σ FΣC
×100 %=7255.0098.00
×100 %=74.03<100(course)
Discussion
In this experiment, we have to measure grain size and calculates grain fineness number
(GFN)., We have followed the procedures given on the manual lab and recorded the data in
the respective table and sample of calculations in order to measure and calculate the grain.
From the experiment, the value of GFN we obtained for the sand silica sand in the sieve
analysis is 74.02 which is the silica sand has type of course grain. So, it can be simply said
that it is a good quality of sand.
The total accumulated mass of the sand mass (g) is 98.00. This is to be expected from
the beginning due some sand have trap at each sieves and not 100% pass. From the original
mass we were weighed before the sieve testing, there was a little bit decrease in the mass
weight where the original sand mass on sieve shaking was 100g. This occur due to the some
errors like not wipe all the left over sand in each sieve. In addition, the digital balancing scale
also one of the factor that cause error where it is not perform in proper condition. The reading
value always change and not fixed. However the data still accepted since the error are not too
much. For this experiment, there are several precaution we must take to avoid and prevent
errors from occurring. The screen on the sieves should be clean carefully in order to avoid
any grain sands still stuck in the sieves. The stack of sieves on the Sieve Shaker must be
locked tidily to avoid them from moving away or fall over during shaking process. We also
must clean the area around digital scale balance to get accurate readings and avoid the
environmental effects. Student also can use a softer bristle brush to gently wipe the screen.
i) Explain briefly what is GFN?
The definition ‘Grain Fineness Number’ or also known as (GFN) is a system
developed by AFS for rapidly expressing the average grain size of given sand. It
approximates the number of meshes per inch of that sieve that would just pass the sample if
its grains of uniform size. It is approximately proportional to the surface area per unit of
weight of sand, exclusive of clay.
ii) Based on GFN value and the distribution obtained, suggest the suitability of the sand
for castings.
To get the perfect match and sand casting suitability, there are several factor and
qualities that can be used as a reference. Firstly, there is refractoriness, bond strength,
permeability, collapsibility, grain fineness and size and lastly grain shape and roundness. The
most suitable sand for casting should have good strength. It is important to avoid the mould
from broken during the process. It also has high refractoriness to withstand the high
temperature of the molten metal.
The sand also must have good permeability. The sand must be porous so that the
gases generated are allowed to escape. Size of the sand and its shape it’s depend on the
materials and casting process. The small size provided better surface finish but the large grain
size is more permeable. The sand should have good thermal conductivity, so that the heat
from casting is quickly transferred. From the experiment, we could say that our sand sample
is good to use for casting as it have a balance amount of porosity and permeability to be used
for small cast operation and it will also have a good surface quality.
Conclusion
From the experiment, we can found that the value of GFN of the silica sand specimen
by using the sieve analysis is 74.03. Hence, the sand sample used in the experiment has the
most suitable coarse number value not too low of value and not too high value. As too low
will had poor surface quality in casting and too fine will make it less permeability. However
some error did occur during the experiment as the total accumulates sand mass is differ from
the original mass which is 98.00 gram while the original mass is 100g. To minimize the error
we need to take extra precaution in order to get the accurate data in this experiment and avoid
errors. As a conclusion the objective of this experiment was achieved and successfully.
Recommendation
There is a couple of things and procedure than can be improved by modifying certain steps or
we may even remove it. Firstly, the cleaning process of the screen on the sieves should be
done carefully in order to remove all grain sands. It also imperative to avoid contact the
sieves screen with the fingers. In the cleaning process, we should also gently brush the sieves
to make sure the sieves are not damage. In the shaking process, the stack of sieves on the
Sieve Shaker must be locked tidily to avoid them from moving away during. Then, we have
to make sure the left over sand in each sieve is transferred to the container use in weighing
process. Lastly, the area around digital scale balance should also be cleaned to get accurate
readings and this would avoid the environmental effects.
References
Books
1. Serope Kalpakjian, Steven R. Schmid, Manufacturing Technology and Fundamental,
5th Edition, Prentice Hall, 2004.
2. M. Y. b. H. M. al-Rangi, Manufacturing Process, Malaysia, 2009
3. Rao, Manufacturing Technology Vol-I 3E, Volume 1, Tata McGraw-Hill Education,
2009
Internet
1. http://wiki.answers.com/Q/
What_is_the_important_about_grain_fineness_number_for_sand_casting.
[Accessed On 27 October 2015]
2. http://www.atilim.edu.tr/~kazim.tur/mate401/Dosyalar-LAB/MATE%20401-Lab-
Exp.01-AFS%20grain%20fineness%20number.pdf
[Accessed On 27 October 2015]
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