CONSTITUENTS OF MOLDING
SAND

The main constituents of molding sand involve silica sand, binder, moisture content and additives.

Silica sand

Silica sand in form of granular quarts is the
main constituent of molding sand having enough
refractoriness which can impart strength,
stability and permeability to molding and core sand. But along with silica small amounts
of iron oxide, alumina, lime stone, magnesia, soda and potash are present as impurities. The chemical composition of
silica sand gives an idea of the impurities
like lime, magnesia, alkalis etc. present. The presence of excessive amounts
of  iron oxide, alkali oxides and lime
can lower the fusion point to a considerable extent which   is undesirable. The silica sand can be specified according to the size (small, medium
and large silica sand grain) and the shape
(angular, sub-angular and rounded).

Effect of grain shape and size of silica sand

The shape and size of sand
grains has a significant effect on the different properties of molding and core
sands. The shape of the sand grains in the mold or core sand determines the possibility of its application in various types of foundry
practice. The shape of foundry
sand grains varies from round to angular.
Some sands consist almost entirely of grains of one shape, whereas
others have a mixture of various shapes. According to shape, foundry sands
are classified as rounded, sub-angular, angular
and compound. Use of angular
grains (obtained during
crushing of rocks hard sand stones) is avoided as these grains have a large
surface area. Molding sands composed of angular grains will need higher amount
of binder and moisture content for
the greater specific surface area of sand grain. However, a higher percentage of binder is required to bring in
the desired strength in the molding sand and core sand. For good molding
purposes, a smooth surfaced sand grains are preferred. The smooth surfaced grain has a higher sinter
point, and the smooth surface secures a mixture of greater permeability and
plasticity while requiring a higher percentage of blind material. Rounded shape
silica sand grain sands are best suited for making permeable molding sand.
These grains contribute to higher bond strength in comparison to angular grain.
However, rounded silica sand grains
sands have higher thermal expandability than angular silica grain sands. Silica sand with rounded silica sand
grains gives much better compactability under the same conditions than the
sands with angular silica grains. This is connected with the fact that the silica sand with rounded grains having
the greatest degree of close packing of particles while sand with angular
grains the worst. The green strength increases as the grains become more
rounded. On the other hand, the grade of compactability of silica sands with
rounded sand grains is higher, and other, the contact surfaces between the
individual grains are greater on rounded grains than on angular grains. As
already mentioned above, the compactability increases with rounded grains. The
permeability or porosity property of molding sand and core sand therefore, should increase with rounded grains and
decrease with angular grains. Thus the round silica sand grain size greatly influences the properties of molding sand.

The
characteristics of sub-angular sand grains lie in between the characteristics
of sand grains of angular and rounded kind. Compound grains are cemented
together such that they fail to get separated when screened through a sieve. They may consist of round, sub-angular, or angular sub-angular sand
grains. Compound grains require higher amounts of binder and moisture content also. These grains are
least desirable in sand mixtures because they have   a tendency to disintegrate at high temperatures. Moreover the compound
grains are cemented together and they fail to separate
when screened.

Grain sizes and their distribution in molding sand influence greatly the
properties of the sand. The size and
shape of the silica sand grains have a large bearing upon its strength and other
general characteristics. The sand
with wide range of particle size
has higher compactability than
sand with narrow distribution. The broadening of the size distribution may be
done either to the fine or the coarse side of the distribution or in both
directions simultaneously, and a sand
of higher density will result. Broadening to the coarse side has a greater effect    on
density than broadening the distribution to the fine sand. Wide size
distributions favor green strength, while narrow grain distributions reduce it.
The grain size distribution has a significant effect on permeability. Silica sand containing finer and a wide range of
particle sizes will have low permeability as compared to those containing
grains of average fineness but of the same size i.e. narrow distribution. The
compactability is expressed by the green density obtained by three ram strokes.
Finer the sand, the lower is the compactability and vice versa. This results from the fact that the specific
surface increases as the grain size decreases.
As a result, the number of points of contact per unit of volume
increases and this in turn raises the resistance to compacting. 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 silica sand grains become finer, the
film of bentonite becomes thinner, although
the percentage of bentonite remains the same.
Due to reducing the thickness of binder film, the green strength is
reduced. With very coarse grains, however, the
number of grains and, therefore, the number of points of contact per unit of
volume decreases so sharply that the green strength is again reduced. The sands
with grains equal but coarser in size
have greater void space and have, therefore greater permeability than the finer silica sands.
This is more pronounced if sand grains
are equal in size. 

     Binder

In general, the binders can be either
inorganic or organic substance. The inorganic group includes clay sodium silicate
and port land cement etc. In foundry
shop, the clay acts as binder which
may be Kaolonite, Ball Clay, Fire
Clay, Limonite, Fuller’s earth and Bentonite.
Binders included in the organic group are dextrin, molasses, cereal
binders, linseed oil and resins like phenol formaldehyde, urea formaldehyde etc. Organic binders
are mostly used for core making.
Among all the above binders, the
bentonite variety of clay is the most common. However,
this clay alone can not develop bonds among sand grins without the
presence of moisture in molding sand and core
sand.

     Moisture

The amount of moisture content in the molding
sand varies generally between 2 to 8 percent. This amount is added to the
mixture of clay and silica sand for developing bonds. This is the amount of water required to fill the
pores between the particles of clay without separating them. This amount of water is held rigidly
by the clay and is mainly responsible for developing the
strength in the sand. The effect of clay and water decreases permeability with
increasing clay and moisture content. The green compressive strength first
increases with the increase in clay content,
but after a certain value,
it starts decreasing.

For increasing the molding sand
characteristics some other additional materials besides basic constituents are
added which are known as additives.

    
Additives

Additives are the materials generally added to
the molding and core sand mixture to develop some special property in the sand. Some common used
additives for enhancing the properties of molding and core sands.