THIS PPT SAYS ABOUT THE VARIUS GRAIN SIZE AND THE PROPERTIES OF MOULDING SAND AND CALCULATION OF GRAIN FINENESS NUMBER

1. DETERMINATION OF AVERAGE GRAIN SIZE AND
DISTRIBUTION
Sand is the principal molding material in the foundry shop where
it is used for all types of castings, irrespective of whether the cast metal
is ferrous or non-ferrous, iron or steel. This is because it possesses the
properties vital for foundry purposes.
Molding sand is specified in terms of the size and shape of the
silica grains it contains, the clay content and the moisture content.
Among these characteristics in this report we will discuss about shape
and size of sand grains.
SIZE AND SHAPE OF GRAINS:
Sand grains are of paramount importance in molding sand
because they impart refractoriness ,chemical resistivity ,and
permeability to the sand .They are specified according to their average
size and shape. the finer the grains ,the more intimate will be the
contact and lower the permeability. However, fine grains tend to fortify
the mold and less than its tendency to get distorted. The shapes of the
grain may vary from round to angular (as shown in fig.) the grains are
classified according to their shape. The shape can be determined by
observing the grains under a magnifying glass or microscope.

2. 1) Rounded grains :
These grains have the least contact with one another in a
rammed structure, thereby making the sand highly permeable to gases.
Sand having rounded grain, however, lacks strength and does not pack
up to the maximum extent. The binder requirements are minimum.
2) Sub-angular grains:
These grains have comparatively lower permeability and
greater strength then the rounded ones.
3) Angular grains:
These grains have defined edges and surfaces are nearly flat.
They produce higher strength and lower permeability in the mold then
sub angular grains. The binder consumption is likely to be high.
4) Compounded grains:
In some cases, the grains are cemented together such that
they fail to separate when screened. They may consists of rounded, sub
angular or angular grains or a combination of three. Such grains are

3. called compounded grains and are least desirable due to their tendency
to break down at higher temperature. In practice, sand grains contain
mixed grain shapes, depending on origin. A sub angular to rounded
grain mixture would be the best combination.
The grain size of sand is expressed by a number called grain
fineness number. Grain fineness number is a system developed by AFS
for rapidly expressing the average grain size of a 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.
To determine this number for a given sand sample, it is
customary to use a standard sieve set which contains several sieves one
above the other, having a varying but known number of meshes. The
coarsest sieve is placed at the top and finest at the bottom. After
separating the clay and the moisture from the sand under test, the
sample is placed in the top sieve and the whole set is shaken in a sieve
shaking machine for a definite length of time the amount of sand
remaining in each sieve is then collected , weighed, and expressed as a
percentage of the original sample weight. The comparative sieve
designations of IS, BS, ASTM, sieves are given in table.
The percentage collected in each sieve is multiplied by its
own multiplying number a constant, one for each sieve and all the
products are added to the arrive at the total product. Thus
Grain fineness number =total product / total sum of percentages
Collected in each sieve

4. Flow chart of NFD sand controller procedures:
SUPPLIER
RECEIVED TO
STORE(PSG)
INSPECTION
SAND SHAKER TESTING
BULK RECEIVER
ACCEPTED (USE FOR
SAND PLANT)
REJECTED
IF NOT
SATISFIED
REQUIRED AFS
VALUE
SATISFICATION
55-65 =MOLDING SAND
45-55= CORE SAND

5. Chemical composition of moulding sand:
Silica is the main constituent but along with silica small amounts of
iron oxide, alumina, limestone, magnesia, manganese oxide, soda and
potash are present as impurities.
The chemical compositions of a moulding sand give an idea of the
impurities like lime, magnesia, alkalis, etc., present and thereby of the
refractoriness of the sand.
Grain size of moulding sand:
The grain size influences the properties of sand as follows;
Compactability : Finer the sand, the lower is the compactability and vice
versa. This results from the fact that the specific surface increases and
the grain size decreases. As a result, the number of point of contacts
per unit volume increases and this in turn raises the resistance to
compacting.
Green strength: The green strength has a certain tendency, admittedly
not very pronounced, towards a maximum with a grain size which
corresponds approximately to the medium grain size. As the grains
become finer, the film of bentonite becomes thinner, although the
percentage of bentonite remains the same. Due to thinning of
bentonite film, the green strength is reduced. With very coarse grains
and, therefore, the number of point of contacts per unit of volume
decreases so sharply that the green strength is again reduced.

6. Permeability: The sands with grains equal but coarse in size have
greater void space and have therefore, greater permeability than the
finer sands. This is more pronounced if the sand grains are equal in size.
Grain size distribution of moulding sand:
The grain size distribution influences the properties of sand as
follows:
Compactibility: Sand with wide range of particle size has higher
compactibility than sand with narrow distribution. The broadening of
the size distribution or in both directions simultaneously and a sand of
higher density will result. Broadening to the coarse side has a great
effect on density than broadening the distribution to the fine side.
Green strength: A wide range distribution favours the green strength
while narrow grain distributions reduce it.
Permeability: The grain size distribution has a pronounced effect on
permeability. A sand contained many fines and a wide range of particle
size will have low permeability as compared to that containing grains of
average fineness but of the same size.
The grain fineness number is a concept that can be used
for comparing fineness of different sands. The distribution of different
grain sizes present in a sand is a more significant test. For good
compaction of sand, the amount retained on 3 or 4 consecutive sieves
should be in the range of 75-80%. In addition, the sieves distribution
(percentage of sand retained on various sieves) should not show a
double peak when the relationship between sieve size and percentage

7. of sand retained are plotted. The distribution should show a normal
curve with a single peak. Table suggested values of grain fineness
numbers of new sands suitable for casting various metals and alloys.
COMPARISON OF SIEVE SIZES
Sieves used for sand grading are 200mm diameter and are
now usually metric sizes, designed by their aperture size in
micrometers. The table lists sieve sizes in the British standard metric
series together with other sieve types.
CALCULATION OF AVERAGE GRAIN SIZE
The adoption of the ISO metric sieve means that the
old AFS grain fineness number can no longer be calculated. Instead, the
average grain size, expressed as micrometers is now used. This is
determined as follows:
1) Weigh a 100 g sample of dry sand.
2) Place the sample into the top sieve of a nest of ISO
sieves on a vibrator. Vibrate for 15 minutes.
3) Remove the sieves and, beginning with the top
sieve, weigh the quantity of sand remaining on each sieve.
4) Calculate the percentage of the sample weight
retained on each sieve, and arrange in a column.
5) Multiply the percentage retained by the appropriate
multiplier and add the products.

8. 6) Divide by the total of the percentages retained to
give the average grain size.
Grain size distribution of moulding sand
The grain size distribution influences the properties of sand as
follows:
Compactibility: Sand with wide range of particle size has higher
compactibility than sand with narrow distribution. The broadening of
the size distribution or in both directions simultaneously and a sand of
higher density will result. Broadening to the coarse side has a great
effect on density than broadening the distribution to the fine side.
Green strength: A wide range distribution favours the green strength
while narrow grain distributions reduce it.
Permeability: The grain size distribution has a pronounced effect on
permeability. A sand contained many fines and a wide range of particle
size will have low permeability as compared to that containing grains of
average fineness but of the same size.
CALCULATION OF AFS GRAIN FINENESS NUMBER
Using either the old BS sieves or AFS sieves, follow 1-4
above.
5) Arrange the results.
6) Multiply each percentage weight by the preceding
sieve mesh number.

9. 7) Divide by the total of the percentage to give the AFS
grain fineness number.
Example:
BS sieve number
Or
Mesh no.
Percent of sand
retained on sieve
weight(%)
Multiplied by
previous sieve
no.
Product
12 0 6 0
20 0.2 12 2.4
30 1.32 20 24.4
40 3.48 30 104.4
45 4.16 40 106.4
50 3.74 45 168.3
70 26.98 50 134.9
100 44.06 70 3084.2
140 13.28 100 1328
200 1.3 140 182
270 0.76 200 154
PAN 0.24 270 64.8
TOTAL 99.52 6627.9
AFS grain fineness number=6627.9/100
=66.59 or 66 AFS
% of sand retained in 45-100 mesh =78.94%
Foundry sands usually fall into the range 150-400µm,
with 220-250µm being the most commonly used. Direct conversion

10. between average grain size and AFS grain fineness number is not
possible, but an approximate relation is shown below:
AFS
grain
fineness
number
35 40 45 50 55 60 65 70 80 90
Average
grain
size(µm)
390 340 300 280 240 220 210 195 170 150
While average grain size and AFS grain fineness number are useful
parameters, choice of sand should be based on particle size
distribution.