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  • - by Kishore 09331A0396

  • Introduction:

    Metal Casting is one of the oldest materials shaping methods known. Casting means pouring molten metal into a mold with a cavity of the shape to be made, and allowing it to solidify. When solidified, the desired metal object is taken out from the mold either by breaking the mold or taking the mold apart. The solidified object is called the casting. By this process, intricate parts can be given strength and rigidity frequently not obtainable by any other manufacturing process. The mold, into which the metal is poured, is made of some heat resisting material. Sand is most often used as it resists the high temperature of the molten metal. Permanent molds of metal can also be used to cast products.

  • Foundry: Foundry or casting is the process of producing metal/alloy component parts of desired shapes by pouring the molten metal/alloy into a prepared mould (of that shape) and then allowing the metal/alloy to cool and solidify. The solidified piece of metal/alloy is known as a CASTING.

  • Casting Terms: 1. Flask: A metal or wood frame, without fixed

    top or bottom, in which the mold is formed.

    Depending upon the position of the flask in

    the molding structure, it is referred to by

    various names such as drag - lower molding

    flask, cope - upper molding flask, cheek -

    intermediate molding flask used in three piece

    molding.

    2. Pattern: It is the replica of the final object to be

    made. The mold cavity is made with the help

    of pattern.

    3. Parting line: This is the dividing line between

    the two molding flasks that makes up the

    mold.

  • 4. Core: A separate part of the mold, made of

    sand and generally baked, which is used to

    create openings and various shaped

    cavities in the castings.

    5. Pouring basin: A small funnel shaped

    cavity at the top of the mold into which the

    molten metal is poured.

    6. Sprue: The passage through which the

    molten metal, from the pouring basin,

    reaches the mold cavity. In many cases it

    controls the flow of metal into the mold.

    7. Runner: The channel through which the

    molten metal is carried from the sprue to the

    gate.

  • 8. Gate: A channel through which the molten metal enters the mold cavity.

    9. Chaplets: Chaplets are used to support the cores inside the mold cavity to take care of its own weight and overcome the metallostatic force.

    10. Riser: A column of molten metal placed in the mold to feed the castings as it shrinks and solidifies. Also known as feed head.

    11. Vent: Small opening in the mold to facilitate escape of air and gases.

  • Basic Features: Pattern and Mould A pattern is made of wood or metal, is a replica of

    the final product and is used for preparing mould cavity.

    Mould material should posses refractory characteristics and with stand the pouring temperature.

    When the mold is used for single casting, it made of sand and known as expendable mold.

    When the mold is used repeatedly for number of castings and is made of metal or graphite are called permanent mould.

    For making holes or hollow cavities inside a casting, cores made of either sand or metal are used.

  • Melting and Pouring: Several types of furnaces are available for

    melting metals and their selection depends on the type of metal, the maximum temperature required and the rate and the mode of molten metal delivery.

    Before pouring provisions are made for the escape of dissolved gases. The gating system should be designed to minimize the turbulent flow and erosion of mould cavity.The other important factors are the pouring temperature and the pouring rate.

  • Solidification and Cooling: The properties of the casting

    significantly depends on the

    solidification time cooing rate.

    Shrinkage of casting, during cooling of solidified metal should not be

    restrained by the mould material,

    otherwise internal stresses may

    develop and form cracks in casting.

    Proper care should be taken at the design stage of casting so that

    shrinkage can occur without casting

    defects.

  • Removal, Cleaning, Finishing and

    Inspection:

    After the casting is removed from the mould it is thoroughly cleaned and the

    excess material usually along the parting

    line and the place where the molten metal

    was poured, is removed using a potable

    grinder.

    White light inspection, pressure test, magnetic particle inspection, radiographic

    test, ultrasonic inspection etc. are used

  • Schematic diagram of casting

    mould:

  • Steps involved in making a casting: 1. Make the pattern out of Wood , Metal or Plastic.

    2. Prepare the necessary sand mixtures for mould

    and core making.

    3. Prepare the Mould and necessary Cores.

    4. Melt the metal/alloy to be cast.

    5. Pour the molten metal/alloy into mould and

    remove the casting from the mould after the

    metal solidifies.

    6. Clean and finish the casting.

    7. Test and inspect the casting.

    8. Remove the defects, if any.

    9. Relieve the casting stresses by Heat Treatment.

    10. Again inspect the casting.

    11. The casting is ready for shipping.

  • Advantages of metal casting:

    Casting is one of the most versatile manufacturing process.

    Casting provides the greatest freedom of design in terms of shape, size and the product quantity.

    Casting imparts uniform directional properties and better vibration capacity to the cast parts.

    Casting produces machinable parts.

    Shapes difficult and uneconomic to obtain otherwise may be achieved through casting process.

    ( Contd..)

  • A product may be cast as one piece, there

    by eliminating the need of metal joining

    processes.

    Very heavy and bulky parts which are

    otherwise difficult to get fabricated may be

    cast.

    Metals (like cast iron) dificult to be shaped

    by other manufacturing processes may be

    cast.

    Casting can be designed for equal

    distribution of loads and for minimum stress

    concentration in order to achieve more

    strength and increased service life.

    Casting process can be mechanised and

    usefully employed for mass production of

    components.

  • Limitations of casting: Dimensional accuracy and surface finish of

    the castings made by sand casting processes are a limitation to this technique.

    Many new casting processes have been developed which can take into consideration the aspects of dimensional accuracy and surface finish.

    Some of these processes are die casting process, investment casting process, vacuum-sealed molding process, and shell molding process.

    The metal casting process is a labor intensive process

  • Applications of Casting:

    Transportation vehicles

    Turbine vanes

    Power generators

    Railway crossings

    Agricultural parts

    Aircraft jet engine parts

    Sanitary fittings

    Communication, Construction and Atomic Energy applications, etc..

  • Different Sections in Foundry:

    1) Pattern making

    2) Sand mixing & preparation

    3) Mould and core making

    4) Mould assembly & handling

    5) Melting

    6) Pouring

    7) Shaking out

    8) Fettling & finishing

    9) Heat treatment

    10) Inspection & testing

  • Raw Materials for

    Foundry:

    1) Metals and alloys.

    2) Fuels (for melting

    metals).

    3) Fluxes.

  • Metals and alloys commonly

    used in Foundries: 1. Ferrous

    2. Non-Ferrous

    FERROUS: a. Cast irons

    b. Steels

    NON-FERROUS: a. Copper alloys

    b. Aluminium alloys

    c. Magnesium alloys

    d. Zinc alloys

    e. Nickel alloys

  • Fuels for Melting Metals:

    1. Coal

    2. Coke

    3. Gas

    4. Oil

    5. electricity

  • Fluxes:

    A flux is a low melting point material.

    When heated up, it melts and combines

    with ash, viscous slags, sand, metallic

    oxides etc., and makes a fluid and easy

    flowing slag.

    This slag being lighter comes on to the

    surface of the molten metal from where it

    can be easily skimmed, tapped or removed

    before pouring the metal into the mould.

    Slag protects molten metal from furnace

    atmosphere.

  • Refractories:

    o Refractories are heat resistant materials.

    o They can withstand high temperatures

    without being fused.

    o Crucibles and furnace sides and bottoms

    containing molten metal are made up of

    refractories.

    o These are used as ladles for pouring

    metal into the mould.

    o Refractories constitute furnace walls and

    roof and thus minimize heat losses.

  • Core and core prints:

    Castings are often required to have holes, recesses,

    etc. of various sizes and shapes.

    These impressions can be obtained by using cores.

    So where coring is required, provision should be made

    to support the core inside the mold cavity.

    Core prints are used to serve this purpose.

    The core print is an added projection on the pattern and

    it forms a seat in the mold on which the sand core rests

    during pouring of the mold.

    The core print must be of adequate size and shape so

    that it can support the weight of the core during the

    casting operation.

    Depending upon the requirement a core can be placed

    horizontal, vertical and can be hanged inside the mold

    cavity.

  • Pattern having core prints.

  • Pattern Making:

    A Pattern is a model or the replica of

    the object to be cast.

    Except for the various allowances a

    pattern exactly resembles the casting

    to be made.

    A pattern is required even if one object

    has to be cast.

  • Functions of Patterns:

    A Pattern prepares a mould cavity for the purpose of making a casting.

    A Pattern may contain projections known as core prints if the casting requires a core and need to be made hollow.

    Patterns properly made and having finished and smooth surfaces reduce casting defects.

    Properly constructed patterns minimize overall cost of the casting.

  • Selection of Pattern Materials:

    The following factors assist in selecting

    proper pattern material:

    No. of castings to be produced.

    Dimensional accuracy & surface finish.

    Shape, complexity and size of casting.

    Casting design parameters.

    Type of molding materials.

    The chance of repeat orders.

    Nature of molding process.

  • The pattern material should be: 1. Easily worked, shaped and joined.

    2. Light in weight.

    3. Strong, hard and durable.

    4. Resistant to wear and abrasion .

    5. Resistant to corrosion, and to chemical

    reactions.

    6. Dimensionally stable and unaffected by

    variations in temperature and humidity.

    7. Available at low cost.

  • Materials for making patterns:

    a. Wood

    b. Metal

    c. Plastic

    d. Plaster

    e. Wax.

  • 1. Wood Patterns: Advantages: Inexpensive

    Easily available in large quantities

    Easy to fabricate

    Light in weight

    Thay can be repaired easily

    Easy to obtain good surface finish

    Limitations: Susceptible to shrinkage and swelling

    Possess poor wear resistance

    Abraded easily by sand action

    Obsorb moisture, consequently get wraped

    Cannot withstand rough handling

    These are used where the no. of castings to be produced is small and pattern size is large.

  • 2. Metal Patterns: Advantages:

    Do not absorb moisture

    More stronger

    Possess much longer life

    Do not wrap, retain their shape

    Greater resistance to abrasion

    Accurate and smooth surface finish

    Good machinability

    Limitations: Expensive

    Not easily repaired

    Ferrous patterns get rusted

    Heavy weight

    These are employed where large no. of castings have to be produced from same patterns.

  • 3. Plastic Patterns: Advantages:

    Durable

    Provides a smooth surface

    Moisture resistant

    Does not involve any appreciable change in size or shape

    Light weight

    Good strength

    Wear and corrosion resistance

    Easy to make

    Abrasion resistance

    Good resistance to chemical attack

    Limitations:

    Plastic patterns are Fragile

    These are may not work well when subject to conditions of severe shock as in machine molding (jolting).

  • 4. Plaster Patterns:

    Advantages:

    It can be easily worked by using wood working tools.

    Intricate shapes can be cast without any difficulty.

    It has high compressive strength.

    Plaster may be made out of Plaster of paris or

    Gypsum cement. Plaster mixture is poured into a mould

    made by a sweep pattern or a wooden master pattern, in order to obtain a Plaster pattern.

  • 5. Wax patterns:

    Advantages: Provide very good surface finish.

    Impatr high accuracy to castings.

    After being molded, the wax pattern is not taken out of the mould like other patterns; rather the mould is inverted and heated; the molten wax comes out and/or is evaporated. Thus there is no chance of the mould cavity getting damaged while removing the pattern.

    Wax patterns find applications in Investment casting process.

  • Types of Patterns:

    1. Single piece pattern.

    2. Split piece pattern.

    3. Loose piece pattern.

    4. Match plate pattern.

    5. Sweep pattern.

    6. Gated pattern.

    7. Skeleton pattern

    8. Follow board pattern.

    9. Cope and Drag pattern.

  • (a)Split pattern

    (b) Follow-board

    (c) Match Plate

    (d) Loose-piece

    (e) Sweep

    (f) Skeleton

    pattern

  • 1. Single piece (solid) pattern:

    Made from one piece and does not contain loose pieces or joints.

    Inexpensive.

    Used for large size simple castings.

    Pattern is accommodated either in the cope or in the drag.

    Examples:

    1. Bodies of regular shapes.

    2. stuffling box of steam engine.

  • Fig: Single piece pattern

  • 2. Split piece pattern:

    Patterns of intricate shaped castings cannot be made in one piece because of the inherent difficulties associated with the molding operations (e.g. withdrawing pattern from mould).

    The upper and the lower parts of the split piece patterns are accommodated in the cope and drag portions of the mold respectively.

    Parting line of the pattern forms the parting line of the mould.

    Dowel pins are used for keeping the alignment between the two parts of the pattern.

    Examples:

    1. Hallow cylinder

    2. Taps and water

    stop cocks etc.,

  • Fig: split piece pattern

  • 3.Loose piece pattern:

    Certain patterns cannot be withdrawn once they are embedded in the molding sand. Such patterns are usually made with one or more loose pieces for facilitating from the molding box and are known as loose piece patterns.

    Loose parts or pieces remain attached with the main body of the pattern, with the help of dowel pins.

    The main body of the pattern is drawn first from the molding box and thereafter as soon as the loose parts are removed, the result is the mold cavity.

  • 4. Match plate pattern:

    It consists of a match plate, on either side of which each half of split patterns is fastened.

    A no. of different sized and shaped patterns may be mounted on one match plate.

    The match plate with the help of locator holes can be clamped with the drag.

    After the cope and drag have been rammed with the molding sand, the match plate pattern is removed from in between the cope and drag.

    Match plate patterns are normally used in machine molding.

  • Fig: Match plate pattern

  • 5. Sweep pattern:

    A sweep pattern is just a form made on a wooden board which sweeps the shape of the casting into the sand all around the circumference. The sweep pattern rotates about the post.

    Once the mold is ready, Sweep pattern and the post can be removed.

    Sweep pattern avoids the necessity of making a full, large circular and costly three-dimensional pattern.

    Making a sweep pattern saves a lot of time and labour as compared to making a full pattern.

    A sweep pattern is preferred for producing large casting of circular sections and symmetrical shapes.

  • 6. Gated pattern:

    The sections connecting different patterns serve as runner and gates. This facilitates filling of the mould with molten metal in a better manner and at the same time eliminates the time and labour otherwise consumed in cutting runners and gates.

    A gated pattern can manufacture many casting at one time and thus it is used in mass production systems.

    Gated patterns are employed for producing small castings.

  • castings

    Gating system

  • 7. Skeleton pattern:

    A skeleton pattern is the skeleton of a desired shape which may be S-bend pipe or a chute or something else. The skeleton frame is mounted on a metal base

    The skeleton is made from wooden strips, and is thus a wooden work.

    The skeleton pattern is filled with sand and is rammed.

    A strickle (board) assists in giving the desired shape to the sand and removes extra sand.

    Skeleton patterns are employed for producing a few large castings.

    A skeleton pattern is very economical, because it involves less material costs.

  • 8. Follow board pattern: A follow board is a wooden board and is used

    for supporting a pattern which is very thin and fragile and which may give way and collapse under pressure when the sand above the pattern is being rammed.

    With the follow board support under the weak pattern, the drag is rammed, and then the fallow board is with drawn, The rammed drag is inverted, cope is mounted on it and rammed. During this operation pattern remains over the inverted drag and get support from the rammed sand of the drag under it.

    Follow boards are also used for casting master patterns for many applications.

  • 9. Cope and Drag patterns:

    A cope and drag pattern is another form of split pattern.

    Each half of the pattern is fixed to a separate metal/wood plate.

    Each half of the pattern(along the plate) is molded separately in a separate molding box by an independent molder or moulders.

    The two moulds of each half of the pattern are finally assembled and the mould is ready for pouring.

    Cope and drag patterns are used for producing big castings which as a whole cannot be conveniently handled by one moulder alone.

  • Fig: Cope and drag pattern

  • Pattern Allowances:

    A pattern is larger in size as

    compared to the final casting, because it

    carries certain allowances due to

    metallurgical and mechanical reasons for

    example, shrinkage allowance is the result

    of metallurgical phenomenon where as

    machining, draft, distortion, shake and

    other allowances are provided on the

    patterns because of mechanical reasons.

  • Types of Pattern Allowances:

    The various pattern allowances are: 1. shrinkage or contraction

    allowance.

    2. Machining or finish allowance.

    3. Draft of tapper allowances.

    4. Distortion or chamber allowance.

    5. Shake or rapping allowance.

  • 1.Shrinkage Allowance: All most all cast metals shrink or contract

    volumetrically on cooling.

    The metal shrinkage is of two types: 1. Liquid Shrinkage:

    it refers to the reduction in volume when the metal changes from liquid state to solid state at the solidus temperature. To account for this shrinkage; riser, which feed the liquid metal to the casting, are provided in the mold.

    2. Solid Shrinkage:

    it refers to the reduction in volume caused when metal loses temperature in solid state. To account for this, shrinkage allowance is provided on the patterns.

  • Almost all cast metals shrink or contract volumetrically after solidification and therefore the pattern to obtain a particular sized casting is made oversize by an amount equal to that of shrinkage or contraction.

    Different metals shrink at different rates because shrinkage is the property of the cast metal/alloy.

    The metal shrinkage depends upon:

    1. The cast metal or alloy.

    2. Pouring temp. of the metal/alloy.

    3. Casted dimensions(size).

    4. Casting design aspects.

    5. Molding conditions(i.e., mould materials and molding methods employed)

  • Rate of Contraction of Various Metals :

    Material Dimension Shrinkage allowance

    (inch/ft)

    Grey Cast Iron Up to 2 feet

    2 feet to 4 feet

    over 4 feet

    0.125

    0.105

    0.083

    Cast Steel Up to 2 feet

    2 feet to 6 feet

    over 6 feet

    0.251

    0.191

    0.155

    Aluminum Up to 4 feet

    4 feet to 6 feet

    over 6 feet

    0.155

    0.143

    0.125

    Magnesium Up to 4 feet

    Over 4 feet

    0.173

    0.155

  • 2. Machining Allowance: A Casting is given an allowance for machining, because: i. Castings get oxidized in the mold and during heat

    treatment; scales etc., thus formed need to be removed.

    ii. It is the intended to remove surface roughness and other imperfections from the castings.

    iii. It is required to achieve exact casting dimensions.

    iv. Surface finish is required on the casting.

    How much extra metal or how much machining allowance should be provided, depends on the factors listed below:

    i. Nature of metals.

    ii. Size and shape of casting.

    iii. The type of machining operations to be employed for cleaning the casting.

    iv. Casting conditions.

    v. Molding process employed

  • Machining Allowances of Various Metals:

    Metal Dimension (inch) Allowance (inch)

    Cast iron

    Up to 12

    12 to 20

    20 to 40

    0.12

    0.20

    0.25

    Cast steel

    Up to 6

    6 to 20

    20 to 40

    0.12

    0.25

    0.30

    Non ferrous

    Up to 8

    8 to 12

    12 to 40

    0.09

    0.12

    0.16

  • 3. Draft or Taper Allowance:

    It is given to all surfaces perpendicular to parting line.

    Draft allowance is given so that the pattern can be easily removed from the molding material tightly packed around it with out damaging the mould cavity.

    The amount of taper depends upon: i. Shape and size of pattern in the depth

    direction in contact with the mould cavity.

    ii. Moulding methods.

    iii. Mould materials.

    iv. Draft allowance is imparted on internal as well as external surfaces; of course it is more on internal surfaces.

  • Table 2 : Draft Allowances of

    Various Metals:

    Pattern material Height of the given surface

    (inch)

    Draft angle

    (External

    surface)

    Draft angle

    (Internal

    surface)

    Wood

    1

    1 to 2

    2 to 4

    4 to 8

    8 to 32

    3.00

    1.50

    1.00

    0.75

    0.50

    3.00

    2.50

    1.50

    1.00

    1.00

    Metal and plastic

    1

    1 to 2

    2 to 4

    4 to 8

    8 to 32

    1.50

    1.00

    0.75

    0.50

    0.50

    3.00

    2.00

    1.00

    1.00

    0.75

  • Fig: taper in design

  • 4. Distortion or cambered

    allowance: A casting will distort or wrap if :

    i. It is of irregular shape,

    ii. All it parts do not shrink uniformly i.e., some parts shrinks while others are restricted from during so,

    iii. It is u or v-shape,

    iv. The arms possess unequal thickness,

    v. It has long, rangy arms as those of propeller strut for the ship,

    vi. It is a long flat casting,

    vii. One portion of the casting cools at a faster rate as compared to the other.

  • 5. Shake allowance:

    A patter is shaken or rapped by striking the same with a wooden piece from side to side. This is done so that the pattern a little is loosened in the mold cavity and can be easily removed.

    In turn, therefore, rapping enlarges the mould cavity which results in a bigger sized casting.

    Hence, a ve allowance is provided on the pattern i.e., the pattern dimensions are kept smaller in order to compensate the enlargement of mould cavity due to rapping.

    The magnitude of shake allowance can be reduced by increasing the tapper.

  • Pattern Layout:

    Steps involved: Get the working drawing of the part for

    which the pattern is to be made. Make two views of the part drawing on a

    sheet, using a shrink rule. A shrink rule is modified form of an ordinary scale which has already taken care of shrinkage allowance for a particular metal to be cast.

    Add machining allowances as per the requirements.

    Depending upon the method of molding, provide the draft allowance.

  • Pattern Construction:

    Study the pattern layout carefully and establish,

    a. Location of parting surface.

    b. No. of parts in which the pattern will be made.

    Using the various hand tools and pattern making machines fabricate the different parts of the pattern.

    Inspect the pattern as regards the alignment of different portions of the pattern and its dimensional accuracy.

    Fill wax in all the fillets in order to remove sharp corners.

    Give a shellac coatings(3 coats) to pattern.

    impart suitable colors to the pattern for identification purposes and for other informations.

  • Pattern Colours:

    Patterns are imparted certain colors and shades in order to:

    i. Identify quickly the main body of pattern

    and different parts of the pattern.

    ii. Indicate the type of the metal to be cast.

    iii. Identify core prints, loose pieces, etc.,

    iv. Visualise the surfaces to be machined,

    etc.

  • Moulding Materials Major part of Moulding material in sand casting are

    1. 70-85% silica sand (SiO2)

    2. 10-12% bonding material e.g., clay cereal etc.

    3. 3-6% water

    Requirements of molding sand are:

    (a) Refractoriness

    (b) Cohesiveness

    (c) Permeability

    (d) Collapsibility

    The performance of mould depends on following factors:

    (a) Permeability

    (b) Green strength

    (c) Dry strength

  • Molding Material and Properties:

    A large variety of molding materials is

    used in foundries for manufacturing

    molds and cores. They include

    molding sand, system sand or backing

    sand, facing sand, parting sand, and

    core sand. The choice of molding

    materials is based on their processing

    properties. The properties that are

    generally required in molding

    materials are:

  • 1. Refractoriness:

    It is the ability of the molding material to

    with stand high temperatures

    (experienced during pouring) with out

    1. Fusion,

    2. Cracking, buckling or scabbing,

    3. Experiencing any major physical

    change.

    Silica sand have high

    refractriness.

  • 2. Permeability: During pouring and subsequent solidification of a

    casting, a large amount of gases and steam is generated.

    These gases are those that have been absorbed by the metal during melting, air absorbed from the atmosphere and the steam generated by the molding and core sand.

    If these gases are not allowed to escape from the mold, they would be entrapped inside the casting and cause casting defects.

    To overcome this problem the molding material must be porous.

    Proper venting of the mold also helps in escaping the gases that are generated inside the mold cavity.

  • 3. Green Strength:

    The molding sand that contains moisture

    is termed as green sand.

    The green sand particles must have the

    ability to cling to each other to impart

    sufficient strength to the mold.

    The green sand must have enough

    strength so that the constructed mold

    retains its shape.

    Green strength helps in making and

    handling the moulds.

  • 4. Dry Strength: A mould may either intentionally be dried, or

    a green sand mould may lose its moisture and get dried while waiting for getting poured or when it comes in contact with molten metal being poured.

    The sand thus dried must have dry strength to

    1. Withstand erosive forces due to molten metal,

    2. Withstand pressure of molten metal,

    3. Retain its exact shape,and

    4. Withstand the metallostatic pressure of the liquid material.

  • 5. Hot Strength:

    As soon as the moisture is eliminated, the

    sand would reach at a high temperature

    when the metal in the mold is still in liquid

    state.

    The strength of the sand that is required to

    hold the shape of the cavity is called hot

    strength.

    In the absence of adequate hot strength, the

    mold may

    1. enlarge

    2. break, erode or

    3. get cracked.

  • 6. Collapsibility:

    Collapsibility determines the readiness with which the molding sand,

    1. Automatically gets collapsed after the casting solidifies, and

    2. Breaks down in knock out and cleaning operations.

    If the mould or core does not collapse, it may restrict free contraction of solidifying metal and cause the same to tear or crack.

  • 7. Flowability:

    It is the ability of the molding sand to

    get compacted to a uniform density.

    Flowability assists molding sand to

    flow and pack all-around the pattern

    and take up the required shape.

    Flowability increases as clay and

    water contents increase.

  • 8. Adhesiveness:

    It is the property of molding sand owing

    to which, it

    1. Sticks with the walls of molding

    boxes,

    2. Sticks with gaggers, and

    3. Thus makes it possible to mold

    cope and drag.

  • 9. Fineness:

    Finer sand mould resist metal

    penetration and produce smooth

    casting surfaces.

    Fineness and permeability are in

    conflict with each other and hence

    they must be balanced for optimum

    results.

  • Molding Sand Composition:

    The main ingredients of any molding

    sand are:

    Base sand,

    Binder, and

    Moisture

  • 1. Base Sand:

    Silica sand is most commonly used

    base sand.

    Other base sands that are also used

    for making mold are zircon sand,

    Chromite sand, and olivine sand.

    Silica sand is cheapest among all

    types of base sand and it is easily

    available.

  • 2. Binder:

    Binders are of many types such as:

    1. Clay binders,

    2. Organic binders and

    3. Inorganic binders

    Clay binders are most commonly used binding agents mixed with the molding sands to provide the strength.

    The most popular clay types are:

    Kaolinite or fire clay (Al2O3 2 SiO2 2 H2O) and Bentonite (Al2O3 4 SiO2 nH2O)

    Of the two the Bentonite can absorb more water which increases its bonding power.

  • 3. Moisture:

    Clay acquires its bonding action only in the presence of the required amount of moisture.

    When water is added to clay, it penetrates the mixture and forms a microfilm, which coats the surface of each flake of the clay.

    The amount of water used should be properly controlled.

    This is because a part of the water, which coats the surface of the clay flakes, helps in bonding, while the remainder helps in improving the plasticity.

  • A Typical Composition of

    Molding Sand:

    Molding Sand Constituent Weight Percent

    Silica sand 92

    Clay (Sodium Bentonite) 8

    Water 4

  • Effect of moisture, grain size and shape

    on mould quality

  • Classification of casting

    Processes: Casting processes can be classified into following FOUR categories:

    1. Conventional Molding Processes Green Sand Molding a. Dry Sand Molding

    b. Flask less Molding

    2. Chemical Sand Molding Processes a. Shell Molding

    b. Sodium Silicate Molding

    c. No-Bake Molding

    3. Permanent Mold Processes a. Gravity Die casting

    b. Low and High Pressure Die Casting

    4. Special Casting Processes a. Lost Wax

    b. Ceramics Shell Molding

    c. Evaporative Pattern Casting

    d. Vacuum Sealed Molding

    e. Centrifugal Casting

  • 1.Green Sand Molding: Green sand is the most diversified molding method used in

    metal casting operations. The process utilizes a mold made of compressed or compacted moist sand. The term "green" denotes the presence of moisture in the molding sand. The mold material consists of silica sand mixed with a suitable bonding agent (usually clay) and moisture.

    Advantages: Most metals can be cast by this method.

    Pattern costs and material costs are relatively low.

    No Limitation with respect to size of casting and type of metal or alloy used

    Disadvantages: Surface Finish of the castings obtained by this

    process is not good and machining is often required to achieve the finished product.

  • 2.Dry Sand Molding:

    When it is desired that the gas forming materials are lowered in the molds, air-dried molds are sometimes preferred to green sand molds. Two types of drying of molds are often required.

    Skin drying and

    Complete mold drying. In skin drying a firm mold face is produced. Shakeout of the

    mold is almost as good as that obtained with green sand molding. The most common method of drying the refractory mold coating uses hot air, gas or oil flame. Skin drying of the mold can be accomplished with the aid of torches, directed at the mold surface.

  • 3.Shell Molding Process:

    It is a process in which, the sand mixed with a thermosetting resin is allowed to come in contact with a heated pattern plate (200 oC), this causes a skin (Shell) of about 3.5 mm of sand/plastic mixture to adhere to the pattern..

    Then the shell is removed from the pattern. The cope and drag shells are kept in a flask with necessary backup material and the molten metal is poured into the mold.

    This process can produce complex parts with good surface finish 1.25 m to 3.75 m, and dimensional tolerance of 0.5 %.

    A good surface finish and good size tolerance reduce the need for machining. The process overall is quite cost effective due to reduced machining and cleanup costs.

    The materials that can be used with this process are cast irons, and aluminum and copper alloys.

  • 4.Sodium Silicate Molding

    Process In this process, the refractory material is coated with a sodium

    silicate-based binder. For molds, the sand mixture can be compacted manually, jolted or squeezed around the pattern in the flask.

    After compaction, CO 2 gas is passed through the core or mold. The CO 2 chemically reacts with the sodium silicate to cure, or harden, the binder. This cured binder then holds the refractory in place around the pattern. After curing, the pattern is withdrawn from the mold.

    The sodium silicate process is one of the most environmentally acceptable of the chemical processes available.

  • The major disadvantage of the process is that the binder is very hygroscopic and readily absorbs water, which causes a porosity in the castings.. Also, because the binder creates such a hard, rigid mold wall, shakeout and collapsibility characteristics can slow down production.

    Some of the advantages of the process are:

    A hard, rigid core and mold are typical of the process, which gives the casting good dimensional tolerances;

    good casting surface finishes are readily obtainable;

  • .

    .

  • Review:

  • Foundry activities

  • 2 types of moulding flask designs

  • Sand mold - opened

  • Sand mold - closed

  • Cooling curve for pure metal

  • Mixing moulding sand with binders & adhesives

  • Filling sand in moulding flasks

  • Melting furnace

  • Pouring molten liquid

  • Knock out

  • Heat treatment

  • Machining

  • final products of casting

  • Reference: 1. A text book of Production Technology Vol. I /

    O.P. Khanna / Dhanpat Rai Publications

    2. A text book of Production Technology (Manufacturing Processes) / P. C. Sharma / S. Chand & Company Ltd

    3. Manufacturing Technology (Second Edition) / P N Rao / Tata McGraw-Hill Publishing Company Ltd

    &

    http://nptel.iitm.ac.in/

    http://nptel.iitm.ac.in/

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