Casting Technology Assignment

5/21/2018 Casting Technology Assignment

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CASTING TECHNOLOGY

ASSIGNMENTSAND PREPARATION AND SAND

MOUNDING TECHNIQUES

Done By: 114113073 to 114113083

Second year Production Engineering

2013-2017, NIT Trichy

8/28/2014


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Casting Technology Assignment 114113073 - 114113083

CONTENTS

SAND PREPARATION

A. Constituents of Moulding Sand

B. Sand Preparation and Conditioning

C. Properties of Moulding Sand

SAND MOULDING TECHNIQUES

A. Green-sand MouldingB. Dry-sand Moulding

C. Loam Moulding

D. Carbon dioxide Moulding

E. Resin Sand Moulding/Shell Moulding


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SAND PREPARATION

A. CONSTITUENTS OF MOULDING SAND:........................................................

1.Silica sand: It forms the bulk of the foundrysands and imparts refractoriness and other

properties to the moulding sand.

2.Binders:Binders give cohesiveness and strengthto the sand to enable it to retain the shape of the

mould after the withdrawal of the pattern from the

sand. Binders may be organic type (such as linseed

oil, molasses, dextrin, pitch or resins) and inorganic

type (such as clay or cement). Clay binders include

bentonite, limonite, fire clay etc. Clay binders areused for both green-sand and dry-sand moulding.

Increased clay content adds to strength, hardness

and toughness of the sand but reduces flowability and permeability of the sand.........

.

. ..............

3. Additives:These are used to improve the properties of the sand, either by improving

the existing properties of the sand or by imparting new properties to make the sandmixtures more useful. Common additives are: coal dust, when present(up to 10%) in the

moulding sand, reacts with oxygen present in sand pores at the time of casting and

produces a reducing atmosphere of CO2at the mould-metal interface, which gives smooth

castings. Cereal flour improves strength and collapsibility of the sand (to allow free

contraction of casting). Silica flour increases hot strength of mould and decreases metal

penetration into mould walls and thus gives smooth castings. Both dextrin and molasses

improve dry strength (after baking the mould) and decrease metal penetration into the

mould walls.

,


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Additives like wood flour, cereals and cellulose when added to moulding sand work as

cushion materials since they burn and forms gases during casting, thereby giving rise to

space to accommodate expansion of casting during its cooling. For example, wood flour and

saw dust reduce expansion defects in grey cast iron castings by promoting mould wall

movement and collapsibility. The grey cast iron expands (up to 2.5%) during solidification

and cooling, because of the period of graphitization that occurs during the final stages of its

solidification.

4. Water: Adding water to moulding sand is called tempering. Water imparts workability

to the sand for making the mould. Binders, such as clays, give green strength to the sand

due to the presence of water. A green-sand mould may contain water around 5% whereas

the sand for making a dry-sand mould has higher water content, up to 10%. Too high water

causes blow holes in the castings. Low water moisture sands are weak in green strength

and produce defects such as scabs or roughness on the surface of castings.

Properties of moulding sand are affected by size, type and distribution of sand grains,

amount of binding and additive materials and moisture contents. Amount of water added

to moulding sand is further governed by the type and amount of sand constituents and the

condition in which the mould sand is to be used, for example, moulding sand for dry-sandmould has initially (before baking) more water contents than that of green-sand mould.

Normally water content varies from 3% to 10%.

Effect of water (moisture) on

permeability is shown in (i) and on

the strength is shown in (ii) in the

figure aside. .


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B. SAND PREPARATION AND CONDITIONING: .

As mentioned earlier, none of the naturally available sands possesses the desired qualities

to the required degree for being considered as a good moulding sand. By the process ofblending or mixing varieties of sands together and by adding binding materials or additives

to this mixture, a new suitable sand mixture is produced for foundry purposes. .

.

New sands as well as used floor sand (which have been used several times in making

moulds) are properly prepared and mixed in a suitable ratio, for example, the ratio of used

floor sand (old) to new sand may be more for light castings but it decreases for medium and

heavy castings. . .

Conditioning of the sand is essential. Proper conditioning means uniform distribution of

clay bond and other additives over sand grains, even distribution of proper moisture and

sorting out of foreign matter like nails, and other metal pieces (which might have been used

for strengthening previous mould walls) by riddling and a thorough mixing and other

additives with hand shovels. Modern foundries have appropriate equipment forconditioning of the sand such as circular pan sand mixer using rotating stone wheels and

paddles. Testing of sands is also carried out for strength, permeability, moisture content,

etc. to predict its performance during use.

C. PROPERTIES OF MOULDING SAND: .

Good moulding sand possesses the following characteristics, which are determined by the

various constituents present in a particular moulding sand mixture.

1. Refractoriness: This refers to the sand's ability to withstand the temperature of the liquid

metal being cast without breaking down. For example some sands only need to withstand

650 C (1,202 F) if casting aluminium alloys, whereas steel needs sand that will withstand1,500 C (2,730 F). Sand with too low a refractoriness will melt and fuse to the casting.


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2. Chemical inertness: The sand must not react with the metal being cast. This is especially

important with highly reactive metals, such as magnesiumand titanium are used.

3. Permeability: This refers to the sand's ability to exhaust gases. This is important because

during the pouring process many gases are produced, such ashydrogen, nitrogen, carbondioxide, and steam, which must leave the mould otherwise casting defects, such as blow

holes and gas holes,occur in the casting. Note that for each cubic centimetre (cc) of water

added to the mould 16,000 cc of steam is produced.

4. Surface finish: The size and shape of the sand particles defines the best surface finish

achievable, with finer particles producing a better finish. However, as the particles become

finer (and surface finish improves) the permeability becomes worse.

5. Cohesiveness(or bond): This is the ability of the sand to retain a given shape after the

pattern is removed.

6. Flowability: The ability for the sand to flow into intricate details and tight corners

without special processes or equipment.

7. Collapsibility: This is the ability of the sand to be easily stripped off the casting after it

has solidified. Sands with poor collapsibility will adhere strongly to the casting. While

casting, metals that contract a lot during cooling or with long freezing temperature ranges

are used, sand with poor collapsibility will cause cracking and hot tearsin the casting.

Special additives can be used to improve collapsibility.

8. Availability/cost: The availability and cost of the sand is very important because for

every ton of metal poured, three to six tons of sand is required. Although sand can be

screened and reused, the particles eventually become too fine and require periodic

replacement with fresh sand.
http://en.wikipedia.org/wiki/Magnesiumhttp://en.wikipedia.org/wiki/Hydrogen_gashttp://en.wikipedia.org/wiki/Hydrogen_gashttp://en.wikipedia.org/wiki/Nitrogen_gashttp://en.wikipedia.org/wiki/Carbon_dioxidehttp://en.wikipedia.org/wiki/Steamhttp://en.wikipedia.org/wiki/Casting_defectshttp://en.wikipedia.org/wiki/Gas_holehttp://en.wikipedia.org/wiki/Hot_tearhttp://en.wikipedia.org/wiki/Hot_tearhttp://en.wikipedia.org/wiki/Hot_tearhttp://en.wikipedia.org/wiki/Gas_holehttp://en.wikipedia.org/wiki/Casting_defectshttp://en.wikipedia.org/wiki/Steamhttp://en.wikipedia.org/wiki/Carbon_dioxidehttp://en.wikipedia.org/wiki/Nitrogen_gashttp://en.wikipedia.org/wiki/Hydrogen_gashttp://en.wikipedia.org/wiki/Magnesium


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SAND MOULDING TECHNIQUES

Basic Process

There are six steps in this process:

1. Place a patternin sand to create a mould.

2. Incorporate the pattern and sand in a gating system.

3.Remove the pattern.

4.Fill the mould cavity with molten metal.

5.Allow the metal to cool.

6.Break away the sand mould and remove the casting.
http://en.wikipedia.org/wiki/Pattern_(casting)http://en.wikipedia.org/wiki/File:Haandform-e.pnghttp://en.wikipedia.org/wiki/Pattern_(casting)


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Green Sand Moulding

These expendable moulds are made of wet sands that are used to make the mould's shape.

The name comes from the fact that wet sands are used in the moulding process. Green

sand is not green in colour, but "green" in the sense that it is used in a wet state (akin to

green wood). Unlike the name suggests, "Green sand" is not a type of sand on its own, but

is rather a mixture of:

Silica sand (SiO2), or Chromite sand (FeCr2O),

or zircon sand (ZrSiO4), 75 to 85%, or olivine,

or Staurolite, or graphite.

Bentonite(clay), 5 to 11%

Water, 2 to 4%

Inert sludge3 to 5%

Anthracite(0 to 1%)

There are many recipes for the proportion of clay, but they all strike different

balances between mouldability, surface finish, and ability of the hot molten metal to de-

gas. Due to presence of moisture, the mould lacks permeability and strength and this may

result in defects such as blowholes and pinholes in casting. Despite this drawback this is

most popular and used in almost 90% of the sand castings because of greater adaptability

and economical operation. The defects are kept under control by keeping moisture low.

The coal typically referred to in foundries as sea-coal, which is present at a ratio of

less than 5%, partially combusts in the presence of the molten metal leading to off gassing

of organic vapours. Green Sand for non-ferrous metals do not use coal additives since the

CO created is not effective to prevent oxidation.

The choice of sand has a lot to do with the temperature that the metal is poured. At

the temperatures that copper and iron are poured, the clay gets inactivated by the heat in

that the montmorillonite is converted to illite, which is non-expanding clay.

Silica is the least desirable of the sands since metamorphic grains of silica sand have

a tendency to explode to form sub-micron sized particles when thermally shocked during

pouring of the moulds. These particles enter the air of the work area and can lead to

silicosis in the workers. Iron foundries spend a considerable effort on aggressive dust

collection to capture this fine silica.

Often additives such as wood flour are added to create a space for the grains to

expand without deforming the mould like Olivine, Chromite, etc. They are used becausethey do not have a phase conversion that causes rapid expansion of the grains, as well as

offering greater density, which cools the metal faster and produces finer grain structures in

the metal.
http://en.wikipedia.org/wiki/Staurolitehttp://en.wikipedia.org/wiki/Graphitehttp://en.wikipedia.org/wiki/Bentonitehttp://en.wikipedia.org/wiki/Clayhttp://en.wikipedia.org/wiki/Sludgehttp://en.wikipedia.org/wiki/Anthracitehttp://en.wikipedia.org/wiki/Olivinehttp://en.wikipedia.org/wiki/Chromitehttp://en.wikipedia.org/w/index.php?title=Phase_conversion&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Phase_conversion&action=edit&redlink=1http://en.wikipedia.org/wiki/Chromitehttp://en.wikipedia.org/wiki/Olivinehttp://en.wikipedia.org/wiki/Illitehttp://en.wikipedia.org/wiki/Montmorillonitehttp://en.wikipedia.org/wiki/Foundryhttp://en.wikipedia.org/wiki/Anthracitehttp://en.wikipedia.org/wiki/Sludgehttp://en.wikipedia.org/wiki/Clayhttp://en.wikipedia.org/wiki/Bentonitehttp://en.wikipedia.org/wiki/Graphitehttp://en.wikipedia.org/wiki/Staurolitehttp://en.wikipedia.org/wiki/Olivinehttp://en.wikipedia.org/wiki/Zirconhttp://en.wikipedia.org/wiki/Chromitehttp://en.wikipedia.org/wiki/Silica


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Dry Sand Moulding

This method is used when the mould requires greater strength to withstand theweight of a large volume of metal or if a hard surface is required to avoid surface erosion.

The mould is prepared with specially processed sand and is then dried in an oven. This is a

mixture of:

Moulding sand

Burnt facing sand

Clay

Cinders( Boiler ash)

Moisture

The layer of facing sand surrounding the mould cavity is made up of fine moulding

sand, river sand of fine grained variety, a bond such as pitch or flour and water. The water

content in the dry sand mix is kept as high as 6-8% so that green properties are satisfactory.

After the mould is ready, it is dried in an oven maintained at 200-300 degree Celsius,

until all moisture is eliminated. Alternatively hot air may be circulated.

The castings from dry sand are flawless. They are shrink proof and more rigid and

strong. However the higher costs involved limit the application of this method to large

castings where the maximum depth of pouring is more than 1200mm and where a faultless

and well-finished casting is essential. Dry sand moulds possess higher resistance to cooling

contraction and so are unavailable for light and intricate castings of metals such as steel,

chills may be used to overcome this problem.


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CO2Moulding:

CO2Casting is a kind of sand casting process. In this process the

sand moulding mixture is hardened by blowing gas over the mould.

This process is favoured by hobby metal casters because a lot of cost

cutting can be done. In addition, one can be sure of gettingdimensionally accurate castings with fine surface finish. But, this

process is not economical than green sand casting process. The image

aside shows the finished product.

Process:The mould for CO2Casting is made of a mixture of sand and liquid silicate binder

which is hardened by passing CO2 gas over the mould. The equipment of the moulding

process includes CO2 cylinder, regulator, hoses and hand held applicator gun or nozzle.

Carbon dioxide moulding delivers great accuracy in production.

Any existing pattern can be used for the moulding purpose which can be placed in the

mould before the mould is hardened. This method helps in producing strong mould and

cores that can be used for high end applications. If the process is carefully executed then

casting can be as precise as produced by the shell casting method.

Carbon dioxide casting is favoured both by the commercial foundry men and hobbyist

for a number of reasons. In commercial operations, foundry men can assure customers ofaffordable castings which require less machining.

The moulding process which can be fully automated is generally used for casting

process that require speed, high production runs and flexibility. In home foundries this is

one of the simplest processes that improve the casting quality.

Applications:CO2casting process is ideal where speed and flexibility is the prime

requirement. Moulds and cores of a varied sizes and shapes can be moulded by thisprocess.

Advantages:This process has many advantages in comparison to other forms of castings

some of them are as follows:

Compared to other casting methods cores and moulds are strong

Reduces fuel cost since gas is used instead of to other costly heating generating

elements

Reduces large requirement for number of mould boxes and core dryers Provides great dimensional tolerance and accuracy in production

Moisture is completely eliminated from the moulding sand

This process can be fully automated.


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Loam Moulding:

Loam is one type of clay which is made with sand mixed with water to form a thin

plastic mixture from which moulds are made. Loam sand also contains ganisters or fire clay.

The loam must be sufficiently adhesive so that it can

cling to the vertical surface. It always requires specialprovision to secure adequate ventilation. The object is

opened out pores in the otherwise compact, closely

knit mass, by artificial means. There are various kinds

of organic matter such as chopped straw, and

particularly horse manure, is mixed up with the sand, a

typical loam sand mixture is given below:

Silica Sand Clay

Coke

Moisture

This applied as plaster to the rough structure of the mould usually made of brick work

and the exact shape is given by a rotating sweep around a central spindle. Cast iron plates

and bars are used to reinforce the brick work which retains the moulding material. Loam

moulds also are prepared by the use of skeleton pattern made of wood. The surfaces ofloams are blackened and are dried before being assembled.

Loam moulds are employed chiefly in the making of large casting for which it would

be expensive to use full pattern and ordinary flasks equipment. Objects such as large

cylinders, chemical pans, large gears, round bottoms, kettles and other machining parts are

produces in the loam moulding.

Highlights: Has about 50 per cent sand grains and 50 per cent clay.

Still practiced in some shops, particularly in making huge

manganese- bronze propellers.

No pattern is required, as sheet-steel sweeps are so

shaped that they generate proper casting contour as the sweep

arm is moved back and forth over a fixed spindle.

The chief advantages of this process are savings in pattern

cost and storage; pattern storage alone is an important andexpensive item in most foundries.

Is slow and laborious, and special moulders are required;

all work must be done by hand as the process is very much an art.


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Resin Sand Moulding/Shell Moulding:

The shell moulding process is a precision sand casting process capable of producing

castings with a superior surface finish and better dimensional accuracy than conventional

sand castings. These qualities of precision can be obtained in a wider range of alloys and

with greater flexibility in design than die-casting and at a lower cost than investment

casting.

The feature of this process is the use of fine-

grained, high purity sand that contributes the

attributes of a smooth surface and dimensional

accuracy to moulds cores and castings alike. In

conventional sand moulding the use of such fine

sand is precluded because it would dramatically

reduce mould permeability. However, thedistinguishing feature of the shell moulding

process is that the mould is literally a shell (10mm thick). It was the ability to produce such

a thin shell mould, which made the process a revolutionary development in metal founding.

The coincident development of plastics, like Bakelite, which were based on thermosetting

resins such as phenol formaldehyde, provided the basis for shell moulding.

In shell moulding the fine sand is coated with a thermosetting resin which provides

the relatively high strength required enabling a thin section, or shell, mould to be produced.

The requirement that the mould should accurately replicate the pattern detail and

dimensions if a precision casting is to be produced is also met by the shell moulding

process. This is achieved because the resin bond is developed whilst the mould is in contact

with a heated pattern plate. Furthermore, the mould is separated from the pattern without

the need to enlarge the cavity, as is the case in green sand moulding. These features apply

equally to the production of cores by the process.

Advantages: lower capital plant costs

good utilisation of space, low sand to metal ratio

lightweight moulds which are readily handled and have

good storage characteristics

skilled labour is not required

shells have excellent breakdown at the knockout stage

lower cleaning an fettling costs

Disadvantages: the raw materials are relatively expensive

the size and weight range of castings is limited

the process generates noxious fumes which must be effectively extracted

Casting Technology Assignment

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