Sand Casting

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Contents
introduction ..................................................................................................................................... 2 
Types of Casting Sand ...................................................................................................................... 3 
Capabilities of Sand Casting ............................................................................................................ 5 
Materials .......................................................................................................................................... 6 
Casting Defects: ............................................................................................................................ 6 
Defects related with sand molds : .............................................................................................. 6 
Properties of Casting  Sand ............................................................................................................. 7 
Casting Sand composition ............................................................................................................. 11 
TESTING SAND PROPERTIES ................................................................................................................. 11 
Reference ...................................................................................................................................... 17 
 
 
  

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introduction
 
Sand casting, also known as sand molded casting, is a metal casting process
characterized by using sand as the mold material. The term "sand casting" can
also refer to an object produced via the sand casting process. Sand castings are
produced in specialized factories called foundries. Over 70% of all metal castings
are produced via a sand casting process.
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Sand casting is relatively cheap and sufficiently refractory even for steel foundry
use. In addition to the sand, a suitable bonding agent (usually clay) is mixed or
occurs with the sand. The mixture is moistened, typically with water, but
sometimes with other substances, to develop strength and plasticity of the clay
and to make the aggregate suitable for molding. The sand is typically contained
in a system of frames or mold boxes known as a flask. The mold cavities and
gate system are created by compacting the sand around models, or patterns, or
carved directly into the sand.
Sand casting is used to make large parts (typically Iron, but also Bronze, Brass,
Aluminum). Molten metal is poured into a mold cavity formed out of sand (natural
or synthetic). The processes of sand casting are discussed in this section,
include patterns, spruces and runners, design considerations, and casting
allowance.
Sand casting, the most widely used casting process, utilizes expendable sand
molds to form complex metal parts that can be made of nearly any alloy.
Because the sand mold must be destroyed in order to remove the part, called the
casting, sand casting typically has a low production rate. The sand casting
process involves the use of a furnace, metal, pattern, and sand mold. The metal
is melted in the furnace and then ladled and poured into the cavity of the sand
mold, which is formed by the pattern. The sand mold separates along a parting
line and the solidified casting can be removed.
 
  

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Types of Casting Sand
 
According to the different binders, foundry sand used for moulding can be divided
into green sand, water glass sand and resin sand.

Green Sand (Clay Sand)
Clay sand is the mixture of natural silica sand, clay, additives and water. The clay
used to make wet clay sand is betonies clay. The compressive strength is
generally 0.05 to 0.1 Mpa. The water content is 3.5 to 5%, and air permeability is
more than 80. It is commonly used in machine molding and hand molding.

The sand made can be used to pour liquid metal without drying, which has the
advantages of low production cost and short production cycle. Wet clay sand
used in sand casting production accounts for about 60%. As the wet sand has a
high moisture content, low strength and air permeability, the castings can easily
have the porosity, coarse, sticky sand and sand expansion defects. In hand
molding, the dimensional accuracy is low, so it is generally used only for the
production of small and medium sized iron castings and non-ferrous alloy
castings; while in mechanical modeling, the castings have high dimensional
accuracy, so it is widely used for high-volume production of castings.

The clay used to make dry sand clay is ordinary clay, and its moisture content is
high. The sand made should be dried in the temperature of about 250 ~ 400 (
and then be combined cast, and it is generally used for steel castings. Because
of the high energy consumption, long production cycle, low dimensional
accuracy, dry sand is gradually phased out.

Water Glass Sand (Sodium Silicate Sand)

Sodium silicate sand is a kind of foundry sand made by sodium silicate (aqueous
solution of sodium silicate) as the binder. The sodium silicate added accounts for
6% to 8% of the sand quality.

Water glass hardening is necessary before pouring to improve strength. The
hardening methods are: chemical hardening with CO2 gas and drying the heating
surface. You can also put the hardening agent in the sand, and then it will harden
by itself. By eliminating or greatly reducing the drying process, the emergence of
sodium silicate sand simplified greatly the sand molding process.

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However, because its collapsibility is poor and falling sand, sand cleaning and
the sand recycling are very difficult, in iron casting, the sand sticks seriously, so it
is not suitable for iron casting, but mainly used in steel casting production.

Resin Sand (Phenol, Furan)

Resin sand is a kind of foundry sand with synthetic resin (phenolic resin and
furan resin, etc.) as the binder. The resin added is about 3% to 6% of the sand
quality. Resin sand can harden fast when heated 1 to 2 minutes, and its dry
strength is high, so the castings made are accurate in size, the surface is
smooth, and the collapsibility is excellent.

Since it has quick-drying and self-hardening characteristics, the modeling
process is easier to become mechanization and automation. Resin sand is a
promising new type of modeling material, and at present, it is mainly used in the
manufacture of complex sand cores.

Since different sands have different advantages, in metal casting, we should
adopt the right one to meet different needs.

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Capabilities of Sand Casting
 

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Materials
 
Sand casting is able to make use of almost any alloy. An advantage of sand
casting is the ability to cast materials with high melting temperatures, including
steel, nickel, and titanium. The four most common materials that are used in
sand casting are shown below, along with their melting temperatures.
 
Casting Defects:
1) Misruns (due to rapid solidification in the runner)
2) Cold shuts (due to rapid solidification before complete filling of the mold)
3) Cold shots (due to splattered globules of metal during pouring)
4) Shrinkage cavity (due to lack of riser system)
5) Microporosity (due to localized solidification shrinkage)
6) Hot tearing (due to the die's prevention of contraction)

Defects related with sand molds :
1) Sand blow
2) Pinholes
3) Sand wash
4) Scabs
5) Penetration
6) Mold shift

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7) Core shift
8) Mold crack
Properties of Casting Sand
 
1. Strength:
· The sand should have adequate strength in its green, dry and hot states
· Green strength is the strength of sand in the wet state and is required for making
possible to prepare and handle the mould.
· If the metal is poured into a green mould the sand adjacent to the metal dries and in
the dry state it should have strength to resist erosion and the pressure of metal.
· The strength of the sand that has been dried or basked is called dry strength









· At the time of pouring the molten metal the mould must be able to withstand flow and
pressure of the metal at high temperature otherwise the mould may enlarge, crack, get
washed or break
· Strength of the mounding sand depends on:
1. Grain size and shape
2. Moisture content

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3. Density of sand after ramming
· The strength of the mould increases with a decrease of grain size and an increase of
clay content and density after ramming. The strength also goes down if moisture content
is higher than an optimum value.
2. Permeability:
· The moulding sand must be sufficiently porous to allow the dissolved gases, which are
evolved when the metal freezes or moisture present or generated within the moulds to
be removed freely when the moulds are poured. This property of sand is called porosity
or permeability.
3. Grain size and shape:
· The size and shape of the grains in the sand determine the application in various types
of foundry. These are three different sizes of sand grains.
1. Fine
2. Medium
3. Coarse
· Fine sand is used for small and intricate castings. Medium sand is used for benchmark
and light floor works. If the size of casting is larger coarse sand is used
· Sand having fine, rounded grains can be closely packed and forms a smooth surface.
Although fine-grained sand enhances mould strength.
4. Thermal stability:
· The sand adjacent to the metal is suddenly heated and undergoes expansion. If the
mould wall is not dimensionally stable under rapid heating, cracks, buckling and flacking
off sand may occur.

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5. Refractoriness:
· Refractoriness is the property of withstanding the high temperature condition moulding
sand with low refractoriness may burn on to the casting
· It is the ability of the moulding material to resist the temperature of the liquid metal to
be poured so that it does not get fused with the metal. The refractoriness of the Silica
sand is highest.
6. Flow ability:
· Flow ability or plasticity is the property of the sand to respond to the moulding process
so that when rammed it will flow all around the pattern and take the desired mould
shape. High flow ability of sand is desirable for the sand to get compacted to a uniform
density and to get good impression of the pattern in the mould.










· Flow ability is also very important in machine moulding
· Flow ability of sand increases as clay and water content are increased.
7. Sand texture:
· As mentioned earlier the texture of sand is defined by its grain size and grain size
distribution.
· The texture chosen for an application should allow the required porosity, provide
enough strength and produce the desired surface finish on the casting.

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8. Collapsibility:
· The moulding sand should collapse during the contraction of the solidified casting it
does not provide any resistance, which may result in cracks in the castings. Besides
these specific properties the moulding material should be cheap, reusable and should
have good thermal conductivity
9. Adhesiveness:
· It is the important property of the moulding sand and it is defined as the sand particles
must be capable of adhering to another body, then only the sand should be easily attach
itself with the sides of the moulding box and give easy of lifting and turning the box when
filled with the stand.












10. Reusability:
· Since large quantities of sand are used in a foundry it is very important that the sand be
reusable otherwise apart from cost it will create disposal problems
11. Easy of preparation and control:
· Sand should lend itself to easy preparation and control by mechanical equipment
12. Conductivity:

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· Sand should have enough conductivity to permit removal of heat from the castings.
Casting Sand composition
 
The sand that is used to create the molds is typically silica sand (SiO 2) that is
mixed with a type of binder to help maintain the shape of the mold cavity. Using
sand as the mold material offers several benefits to the casting process. Sand is
very inexpensive and is resistant to high temperatures, allowing many metals to
be cast that have high melting temperatures. There are different preparations of
the sand for the mold, which characterize the following four unique types of sand
molds.
 Greensand mold - Greensand molds use a mixture of sand, water, and a clay
or binder. Typical composition of the mixture is 90% sand, 3% water, and 7%
clay or binder. Greensand molds are the least expensive and most widely
used.
 Skin-dried mold - A skin-dried mold begins like a greensand mold, but
additional bonding materials are added and the cavity surface is dried by a
torch or heating lamp to increase mold strength. Doing so also improves the
dimensional accuracy and surface finish, but will lower the collapsibility. Dry
skin molds are more expensive and require more time, thus lowering the
production rate.
 Dry sand mold - In a dry sand mold, sometimes called a cold box mold, the
sand is mixed only with an organic binder. The mold is strengthened by
baking it in an oven. The resulting mold has high dimensional accuracy, but is
expensive and results in a lower production rate.
 No-bake mold - The sand in a no-bake mold is mixed with a liquid resin and
hardens at room temperature.
TESTING SAND PROPERTIES
 
The moulding sand after it is prepared, should be properly tested to see that
the requisite properties are achieved. These are standard tests to be done as
per Indian Standards.
Sample Preparation
Tests are conducted on a sample of the standard sand. The moulding sand
should be prepared exactly as is done in the shop on the standard equipment
and then carefully enclosed in a container to safeguard its moisture content.

. The sample is then weighed. The weight difference in grams when
multiplied by two would give the percentage of moisture contained in the

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moulding sand. Alternatively a moisture teller can also be used for
measuring the moisture content. In this sand is dried by suspending the
sample on a fine metallic screen and allowing hot air to flow through the
sample. This method of drying completes the removal of moisture in a
matter of minutes compared to 2 hours as in the earlier method. Another
moisture teller utilizes calcium carbide to measure the moisture content. A
measured amount of calcium carbide ( a little more than actually required for
complete reaction) in a container alongwith a separate cap consisting of
measured quantity of moulding sand is kept in the moisture teller. Care has
to be taken before closing the apparatus that carbide and sand do not come
into contact coming out will be collected in the space above the sand raising
the pressure. A pressure gauge connected to the apparatus would give
directly the amount of acetylene generated which is proportional to the
moisture present. It is possible to calibrate the pressure gauge directly read
the amount of moisture.

Clay Content
The clay content of the moulding sand is determined by dissolving or
washing it off the sand. To determine the clay percentage a 50 g sample is
dried at 105 to 110
and the dried sample is taken in a onelitre glass flask and added with 475 ml
of distilled water and 25ml of a one percent solution of caustic soda(NaOH
25 g per liter). This sample is thoroughly stirred. After the stirring, for a
period of five minutes, the sample is diluted with fresh water up to a 150mm
graduation mark and the sample is left undisturbed for 10 minutes to settle.
The sand settles at the bottom and the clay particles washed from the sand
would be floating in the water. 125 mm of this water is siphoned off the
flask and it is again topped to the same level and allowed to settle for five
minutes.
To find out the sand grain size, a sand sample which is devoid of moisture
and clay such as the one obtained after the previous testing is to be used. The
dried clay free sand grains are placed on the top sieve of a sieve shaker
which contains a series of sieves one upon the other with gradually
decreasing mesh sizes. The sieves are shaken continuously for a period of 15
minutes. After this shaking operation, the sieves are taken apart and the sand
left over on each of the sieve is carefully weighed. The sand retained on each
sieve expressed as a percentage of the total mass can be plotted against sieve
number. But more important is the Grain Finesses Number (GFN) which is a
quantitative indication of the grain distribution. To calculate the GFN each
sieve has been given a weight age factor as given in the Table –IV

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Sieve shaker

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Reference


http://en.wikipedia.org/wiki/Sand_casting
http://www.efunda.com/processes/metal_processing/sand_casting_intro.cfm
http://www.custompartnet.com/wu/SandCasting
http://dtzone.com/resmat/m_ind_sand_casting_step_by_step.htm
http://www.ehow.com/list_7243760_types-casting-sand.html
http://www.iron-foundry.com/foundry-sand.html
http://www.mechanicalengineeringblog.com/2960-properties-of-moulding-sands/
http://www.scribd.com/doc/18096886/Moulding-Sand-Properties