[Document title]

1. Mold and molding practices

The major chunk of the global casting production of castings comes from sand molds.
Although, the sand metal ratio varies depending on the type and size of casting and the
molding method employed, it is considered that for production of 1 ton of casting, about 4 – 5
tons of molding sand is required. Consequently, sand remains a major consumable in casting
industry. Sand casting involves various modifications of the process i.e. green sand molding,
dry sand molding, CO2 molding, shell molding etc.
Sand:
According to the American Foundrymen Society (AFS), sand is one of the most important
factors as it is a primary cause for about 30 percent defects and is considered to play a
secondary role for the formation of around 42 percent defects in castings.
The basic requirement of a good foundry sand is that it should be readily moldable and produce
a defect free casting. Some of the properties on which the sand quality for foundry application
is evaluated include,
a. Green strength f. Refractoriness
b. Dry strength g. Flowability
c. Hot strength h. Collapsibility and
d. Permeability i. Reusability etc.
e. Thermal stability
a. Green strength- the green sand after water has been mixed into. It must have adequate
strength and plasticity for performing and handling of the mould.
b. Dry strength - a casting is poor, said adjacent to the hot metal quickly loses its water at
steam. The dry sand must have strength to resist erosion and also metal static pressure
of the molten metal or else the mould will enlarge.
c. Hot strength- After the moisture has been evaporated, the sand may be required to
possess strength at some elevated temperature above hundred degrees Celsius. The
static pressure of the liquid metal bearing against the mould wall may cause mould
enlargement, or if the metal is still flowing, erosion, cracks or breakage may occur unless
the sand possesses adequate hot strength.
d. Permeability - Heat from the casting causes, a green sand, mould to evolve a great deal
of steam and other gases.the mould must be permeable that is, porous, to permit the
gases to pass off, or the casting will contain holes.
e. Thermal stability- heat from the casting causes rapid expansion of the sand surface at
the mould metal interface. The mould surface may then crack buckle or flake off ( scab)
unless the moulding sand is relatively stable dimensionally under rapid heating.
f. Refractoriness - Higher pouring temperatures, such as those for ferrous alloys at around
1300°C to 1760°C, require greater refractoriness of the sand. Loo ping temperature
metals for example, aluminium poured at around 700°C do not require high degree of
refractoriness from the sand.
 g. Flowability – The sand should respond to moulding process.
h. Collapsibility- heated sand, which becomes hard and rock like his difficult to remove
from the casting and may cause contracting metal to tear or crack
The properties of the green molding sand and its behavior in the mold is a function of some
important factors, namely
Sand ingredients:
A synthetic sand is basically an aggregate of silica particles, clay, water and special additives. As
the silica particles which form the bulk aggregate come from the base sand, the attributes as
average fineness number and particle size distribution of the base sand, have a direct influence
of the green sand during molding.
The fineness of the sand and particle size distribution is assessed by sieve analysis. The three
major fractions of the sieve analysis that need to be considered are, the bulk, the coarse and
the fine fractions. The bulk fraction constitutes more than 80 percent of the molding aggregate.
The coarse and the fine fractions each constitute less than 10 percent by weight.
Shape and distribution of sand grains:
Generally the molding sand used is a mixtures of new sand and reclaimed sand. The reclaimed
contains reclaimed molding sands as well as also core sands. The grain shape decides the sand
surface area and the grain size distribution decides the permeability of the mold. Higher is the
surface area higher is the requirement of amount of bonding material (normally clay and
water) for proper bonding. Thus, any change in surface area, perhaps due to a change in sand
shape or the percentage of core sand being reclaimed, calls for a corresponding change in the
amount of bond required.
Rounded grains have a low surface-area-to-volume ratio and are therefore used for making
cores because require the least amount of binder. However, when they are recycled into the
molding sand system, their rounded shape becomes disadvantageous, especially where
automatic molding is used. These automatic molding systems use high percentage of clay and
water to assist rapid, automatic molding.
Angular sands have the greatest surface area (except for sands that fracture easily and produce
a large percentage of small grains and fines) and therefore require more mulling, bond, and
moisture. The angularity of a sand increases with use because the sand is broken down by
thermal and mechanical shock
Uniformity of aggregate:
A green sand mold must have adequate strength to tolerate the erosion caused by the metal
hitting and flowing over the sand surface. If the individual sand grains are not held firmly in
place during metal flow, the result will be loose sand grains that will wash - into the casting
cavity and cause a defective casting Sand grains are held in place by a combination of two
mechanisms a wedging action in which the sand grains are mechanically locked to adjacent
grains, and the clay-water bond established between the grains. The combined action of these
two mechanisms forms the basis of the sand strength developed in the mold cavity.
The total coarse fraction should be restricted in amount, to about less than 4 per cent. The limit
is necessary since a higher percentage of coarse particles promotes a poor casting surface
finish. Additionally, coarse particles get easily dislodged from the mold cavity surface and form
dirt in the casting.
Similarly, the fine fraction must also be controlled, and usually is not permitted to exceed 5
percent. This limit is necessary because an excess of fine particles causes what is called as
balling to occur during mulling. In presence of water, the fines and clay agglomerate form balls
during mulling. Consequently, the clay cannot get thoroughly distributed throughout the mass.
Although the percentage of fines must be restricted to a maximum 5, certain minimum
percentage of fines is necessary. In the absence of the fines, clay balling can occur. During
mulling the clay and water form balls if there are no fines present. Thus, agglomerates of
uniformly dispersed ingredients are not formed from mulling. The balling, occurring due to
either of the reasons, leads to lower green strength. For effective mulling i.e. uniform
distribution of the ingredients requires proper balance between coarse, bulk, and fine sand
particles in the base sand.
Particle size:
Permeability of the sand is critical to the production of quality castings. Molten metal, on
entering a green sand mold, leads to generation of steam and gases, because of the thermal
decomposition of the binder and other additives. Low permeability does not allow the
generated gases to escape, resulting in increased mold pressure, thereby inhibiting the flow of
molten metal, or in extreme cases may even cause the metal to be blown out of the mold. On
the other hand, penetration of molten metal into the sand is prevented due to the generated
gases. This helps in minimizing the burn on sand grains and the following issues associated with
cleaning and machining of the castings. Thus it is essential to maintain proper balance between
mold permeability and gas generation.
Permeability is the result of the size and the amount of voids between the compacted sand
grains. These attributes of the voids, in turn, are dependent on size, shape and particle size
distribution of the sand grains.
The finer sand grain sizes decrease the permeability. A sand with high percentage of fines and a
varied range of particle sizes will have low permeability. The coarser sands with greater void
space have greater permeability as compared to the finer sands.
Typically targeted range for permeability is 90-130*. (Source: Green Sand Solutions for Iron Foundries –
A practitioner’s handbook by Deepak Chowdhary, published by MPM Private Limited)

Refractoriness of the silica sand:

Sand grains of high purity and coarser size are known to have excellent refractoriness. Washed
& dried silica sand of AFS number 30 to 45, has a fusion point of 16500C. Presence of impurities
such as limestone, feldspar or iron oxide, adversely affects this property.
Clay and water:
For the given type of clay and its content, there is an optimal water content that should be
added. Too much water causes excessive plasticity and dry strength. Too little water fails to
develop adequate strength and plasticity. Control of moisture in the molding sand so that the
best properties are developed is a necessary basis of sand control.

Variation of green strength with water content for western bentonite addition.
Ref: R.F. Grim and FL Cuthbert, Engineering Experiment Station Bulletin 357, University of Illinois, 1945

For the aggregate mixture to develop plasticity and adequate strength, clay activation is
necessary. For activation of the clay, a water content of 1.5 to 8 percent is necessary. The water
in the molding sand is called as tempering water. This water gets adsorbed by the clay particles
and is rigidly held on to the particle surface only to a limited extent. It is this adsorbed water
that is effective in developing the intended strength.
These rigidly coated particles get wedged against each other thereby developing the desired
strength. Excessive water, increases the plasticity & moldability of sand, but decreases its
strength.
If dissolved minerals are present, the tempering water may engage in ion exchange with the
clay.
Typical range for active clay 6 – 12%*
Typical range for moisture 2.8 – 4%*
In addition to the three basic ingredients, namely sand, clay and water other materials are
often added to the molding sand, to develop specific properties. They are called as additives.
Special additives –

• Cereal binders, in the form of starch obtained from various sources, are commonly used
in foundries to increase the green strength or collapsibility or the dry strength. It’s often
added up to 2.0 percent.
• Ground pitch, a by product obtained during cokemaking, is used up to 3 percent.
• Sea coal (containing @ 36-40% volatile matter) is commonly added to molding sand,
especially during cast iron castings production. It helps improve surface finish. Added
usually up to 2-8% by weight
• Natural or synthetic graphite is used from 0.2 to 2.0 percent for good surface finish and
better moldability
The major defects arising out of sand related issues include

• Blows
• Shot iron
• Cuts and washes
• Drops
• Stickers
• Rough surface
• Penetration
• Fusion
• Swells
• Ramoff
Constituents of Molding Sand
The main constituents of molding sand involve silica sand, binder, moisture
content and additives.

1.1 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
(CaCO3), 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 sand grain size and the shape (angular, sub-angular
and rounded) of the sand.

1.2 Binder

Binders can be either inorganic or organic substance. Binders included in the inorganic
group are clay sodium silicate and port land cement etc. In foundry shop, the clay acts
as binder which may be Kaolinite, 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. Binders of
organic group are mostly used for core making. Among all the above binders, the
bentonite variety of clay is the most commonly used. However, this clay alone can’t
develop bonds among sand grins without the presence of moisture content in molding
sand and core sand.

1.3 Moisture

The amount of moisture content in the molding sand varies from 2 to 8%. 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.
1.4 Additives

Additives are the materials generally added to the molding and core sand mixture to
develop some special property in the sand. Some commonly used additives for
enhancing the properties of molding and core sands are coal dust, corn
flour, dextrin, sea coal, pitch, wood flour, silica flour.

1.4.1 Coal dust

Coal dust is added mainly for producing a reducing atmosphere during casting
process. This reducing atmosphere results in any oxygen in the poles becoming
chemically bound so that it cannot oxidize the metal. It is usually added in the molding
sands for making molds for production of grey iron and malleable cast iron castings.

1.4.2 Corn flour

Corn flour belongs to the starch family of carbohydrates and is used to increase the
collapsibility of the molding and core sand. It is completely volatilized by heat in the
sand mould, thereby leaving space between the sand grains. This allows free
movement of sand grains, which finally gives rise to mould wall movement and
decreases the mold expansion and hence defects in castings. Corn sand if added to
molding sand and core sand improves significantly strength of the mold and core.

1.4.3 Dextrin

Dextrin also belongs to starch family of carbohydrates that behaves also in a manner
similar to that of the corn flour. Dextrin increases dry strength of the molds.

1.4.4 Sea coal

Sea coal is the fine powdered bituminous coal which positions its place among the
pores of the silica sand grains in molding sand and core sand. When heated, sea coal
changes to coke which fills the pores and is unaffected by water. Because to this, the
sand grains become restricted and cannot move into a dense packing pattern. Thus,
sea coal reduces the mould wall movement and the permeability in mold and core
sand and hence makes the mold and core surface clean and smooth.

1.4.5 Pitch

Pitch is distilled form of soft coal. It can be added from 0.02 % to 2% in mold and core
sand. Pitch enhances hot strengths, surface finish on mold surfaces and behaves
exactly in a manner similar to that of sea coal.
1.4.6 Wood flour

Wood flour is a fibrous material mixed with a granular material like sand. Wood flour
is relatively long thin fibers prevent the sand grains from making contact with one
another. wood flour can be added in between 0.05 % to 2% in mold and core sand.
Wood flour volatilizes when heated, thus allowing the sand grains room to expand.
Wood flour will increase mould wall movement and decrease expansion defects. Wood
flour also increases collapsibility of both mold and core.

1.4.7 Pulverized Silica or Silica flour

Silica flour is called as pulverized silica. Pulverized silica can be easily added up to 3%
which increases the hot strength and finish on the surfaces of the molds and cores. It
also reduces metal penetration in the walls of the molds and cores.

2. Different types of Molding Sand:

Molding sands can also be classified into various types according to their
use are backing sand, core sand, dry sand, facing sand, green sand, loam sand, parting
sand, system sand.

2.1 Backing sand or floor sand

Backing sand or floor sand is used to back up the facing sand and is used to fill the
whole volume of the molding flask. Backing sand is sometimes called black sand
because of old, repeatedly used molding sand is black in color due to addition of coal
dust and burning on coming in contact with the molten metal.

2.2 Core sand

Core sand is used for making cores and it is sometimes also known as oil sand. Core
sand is highly rich silica sand mixed with oil binders such as core oil which composed of
linseed oil, resin, light mineral oil and other bind materials. Pitch or flours and water
may also be used in large cores for the sake of economy.

2.3 Dry sand

Green sand that has been dried or baked in suitable oven after the making mold and
cores is called dry sand. It possesses more strength, rigidity and thermal stability. Dry
sand is mainly used for larger castings. Mold prepared in this sand are known as dry
sand molds.
2.4 Facing sand

Facing sand forms the face of the mould. It is next to the surface of the pattern and it
comes into contact with molten metal when the mould is poured. Initial coating around
the pattern and hence for mold surface is given by facing sand. Facing sand have high
strength refractoriness. Facing sand is made of silica sand and clay, without the use of
already used sand. Different forms of carbon are used in facing sand to prevent the
metal burning into the sand. A facing sand mixture for green sand of cast iron may
consist of 25% fresh and specially prepared and 5% sea coal. They are sometimes
mixed with 6-15 times as much fine molding sand to make facings. The layer of facing
sand in a mold usually ranges between 20-30 mm. From 10 to 15% of the whole
amount of molding sand is the facing sand.

2.5 Green sand

Green sand is also known as tempered or natural sand which is a just prepared
mixture of silica sand with 18 to 30% clay, having moisture content from 6 to 8%. The
clay and water furnish the bond for green sand. It is fine, soft, light, and porous. Green
sand is damp, when squeezed in the hand and it retains the shape and the impression
to give to it under pressure. Molds prepared by this sand are not requiring backing and
hence are known as green sand molds. Green sand is easily available and it possesses
low cost. Green sand is commonly employed for production of ferrous and non-ferrous
castings.

2.6 Loam sand

Loam sand is mixture of sand and clay with water to a thin plastic paste. Loam sand
possesses high clay as much as 30-50% and 18% of water. Patterns are not used for
loam molding and shape is given to mold by sweeps. Loam sand is particularly
employed for loam molding used for large grey iron castings.

2.7 Parting sand

Parting sand without binder and moisture is used to keep the green sand not to stick
to the pattern and also to allow the sand to the parting surface the cope and drag to
separate without clinging. Parting sand is clean clay-free silica sand which serves the
same purpose as parting dust.

2.8 System sand

In mechanized foundries where machine molding is employed. System sand is used to
fill the whole molding flask. In mechanical sand preparation and handling units, facing
sand is not used. The used sand is cleaned and re-activated by the addition of water
and special additives. This is known as system sand. Since the whole mold is made of
this system sand, the properties such as strength, permeability and refractoriness of
the molding sand must be higher than those of backing sand.

3. Properties of Molding sand

The basic properties required in molding sand and core
sand are adhesiveness, cohesiveness, collapsibility, flowability, dry strength, green
strength, permeability, refractoriness described as under.

3.1 Adhesiveness

Adhesiveness is a property of molding sand to get the stick or adhere to foreign

material such sticking of molding sand with the inner wall of molding box.

3.2 Cohesiveness

Cohesiveness is property of molding sand by virtue which the sand grain particles
interact and attract each other within the molding sand. Thus, the binding capability of
the molding sand gets enhanced to increase the green, dry and hot strength property
of molding and core sand.

3.3 Collapsibility

After the molten metal in the mould gets solidified, the sand mould must be collapsible
so that free contraction of the metal occurs and this would naturally avoid the tearing
or cracking of the contracting metal. In absence of collapsibility property the
contraction of the metal is hindered by the mold and thus results in tears and cracks in
the casting. This property is highly required in cores.

3.4 Dry strength

As soon as the molten metal is poured into the mould, the moisture in the sand layer
adjacent to the hot metal gets evaporated and this dry sand layer must have sufficient
strength to its shape in order to avoid erosion of mould wall during the flow of molten
metal. The dry strength also prevents the enlargement of mould cavity cause by the
metallostatic pressure of the liquid metal.
3.5 Flowability or plasticity

Flowability or plasticity is the ability of the sand to get compacted and behave like a
fluid. It will flow uniformly to all portions of pattern when rammed and distribute the
ramming pressure evenly all around in all directions. Generally sand particles resist
moving around corners or projections. In general, flowability increases with decrease in
green strength and vice versa. Flowability increases with decrease in grain size of sand.
The flowability also varies with moisture and clay content in sand.

3.6 Green strength

The green sand after water has been mixed into it, must have sufficient strength and
toughness to permit the making and handling of the mould. For this, the sand grains
must be adhesive, i.e. they must be capable of attaching themselves to another body
and. therefore, and sand grains having high adhesiveness will cling to the sides of the
molding box. Also, the sand grains must have the property known as cohesiveness i.e.
ability of the sand grains to stick to one another. By virtue of this property, the pattern
can be taken out from the mould without breaking the mould and also erosion of
mould wall surfaces does not occur during the flow of molten metal. The green
strength also depends upon the grain shape and size, amount and type of clay and the
moisture content.

3.7 Permeability

Permeability is also termed as porosity of the molding sand in order to allow the
escape of any air, gases or moisture present or generated in the mould when the
molten metal is poured into it. All these gaseous generated during pouring and
solidification process must escape otherwise the casting becomes defective.
Permeability is a function of grain size, grain shape, and moisture and clay contents in
the molding sand. The extent of ramming of the sand directly affects the permeability
of the mould. Permeability of mold can be further increased by venting using vent rods.

3.8 Refractoriness

Refractoriness is defined as the ability of molding sand to withstand high

temperatures without breaking down or fusing thus facilitating to get sound casting. It
is a highly important characteristic of molding sands. Refractoriness can only be
increased to a limited extent. Molding sand with poor refractoriness may burn on to
the casting surface and no smooth casting surface can be obtained. The degree of
refractoriness depends on the SiO2 i.e. quartz content, and the shape and grain size of
the particle. The higher the SiO2 content and the rougher the grain volumetric
composition the higher is the refractoriness of the molding sand and core sand.
Refractoriness is measured by the sinter point of the sand rather than its melting point.

3.9 Miscellaneous properties of molding sand

In addition to above requirements, the molding sand should not stick to the casting
and should not chemically react with the metal. Molding sand need be economically
cheap and easily available in nature. It need be reusable for economic reasons. Its
coefficients of thermal expansion need be sufficiently low.
 References
1. Modern Casting, American Foundry Society, Vol. 11, No. 1, January 2021
2. P.F. Wilson, L.D. Dell, and G.F. Anderson, Root Cause Analysis A Tool for Total Quality
Management, Quality Press, 1993, p 7
3. F.W. Breyfogle III, Implementing Six Sigma: Smarter Solutions Using Statistical Methods,
John Wiley & S 1999, p xxvii
4. Management Manual, Guidelines for the Naval Aviation Reliability-Centered
Maintenance Process, NAVAIR 00-25-403.
5. ASM Metals Handbook, Vol. 15, Casting, p 1962
6. Mahi Sahoo & Sudhari (Sam Sahoo), Principles Metal Casting, Mcgraw Hill Education
(India) Private Limited, 2014, p 719