By

V. THULASIKANTHAssistant Professor

Mechanical Engineering Departmentvtkvsk@gmail.com

Introdcution to Casting

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Product Life Cycle

Manufacturing is the backbone of any industrialized nation.

Manufacturing and technical staff in industry must know the following;

Manufacturing processesMaterials being processed Tools and equipments for manufacturing different componentsProducts with optimal process plan using proper precautions Specified safety rules to avoid accidents

Introduction to Manufacturing Technology

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Production or manufacturing can be simply defined as value addition processes by which raw materials are converted into valued products.

Source for Fig 1.1 NPTL Manufacturing

Manufacturing Processes

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This refers to science and technology of manufacturing products effectively, efficiently, economically and environment-friendly through Application of any existing manufacturing process and system Proper selection of input materials, tools, machines and environmentsImprovement of the existing materials and processes Development of new materials, systems, processes and techniques

All such manufacturing processes, systems, techniques have to be

Technologically acceptable Technically feasible Economically viable Eco-friendly

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Broad classification of Manufacturing Processes (a) Shaping or forming Manufacturing a solid product of definite size and shape from a given material taken in three possible states: • in solid state – e.g., forging rolling, extrusion, drawing etc. • in liquid or semi-liquid state – e.g., casting, injection moulding etc. • in powder form – e.g., powder metallurgical process.

(b) Joining process Welding, brazing, soldering etc.

(c) Removal process Machining (Traditional or Non-traditional), Grinding etc.

(d) Regenerative manufacturing Production of solid products in layer by layer from raw materials in different form: • liquid – e.g., stereo lithography • powder – e.g., selective sintering • sheet – e.g., LOM (laminated object manufacturing) • wire – e.g., FDM. (Fused Deposition Modelling) 6

CastingIt is a earliest metal shaping technique known to human being since 3500BC.

Simple and complicated shapes can be made from any metal that can be melted.

Example of cast parts: frames, structural parts, machine components, engine blocks, valves, pipes, statues, ornamental artifacts.

Casting sizes range form few mm (teeth of a zipper) to 10 m (propellers of ocean liners).

Casting processes basically involve the introduction of a molten metal into refractory mold cavity, where upon solidification, the metal takes on the shape of the mold cavity.

Refractory mold pour liquid metal solidify, remove finish7

COPE

DRAG

1. Pour the molten metal into sand mold

2. Allow time for metal to solidify

3. Break up the mold to remove casting

4. Clean and inspect castingSeparate gating and riser system

5. Heat treatment of casting is sometimes required to improve metallurgical properties

Steps in Sand Casting

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Classification of casting processes

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AdvantagesIt is possible to cast any material be it ferrous or non-ferrous.

Tools required for casting moulds are very simple and inexpensive.

It is an ideal method for trail production or production of small lots.

Good directional properties.

Casting of any size and weight, even up to 200 tonnes can be made.

LimitationsDimensional accuracy and surface finish achieved is some what less.

Traditional casting methods are labour intensive.

Difficult to remove the defects in some materials due to moisture in sand.

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Applications

Cylinder blocks

Liners

Machine tool beds

Pistons

Piston rings

Mill rolls

Wheels

Housings

Water supply pipes

Bells etc.,air compressor frame

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Casting Terms1. Flask (Cope & Drag)

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2. PatternIt is a replica of the object to be made.

Mould cavity is made with the help of pattern

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3. Parting Line

4. Bottom Board

5. Facing Sand

7. Moulding Sand

8. Pouring Basin

It is a dividing line between cope, drag and also for split pattern.

It is made of wood, the pattern is first kept on the bottom board

Carbonaceous material is sprinkled on the inner surface of the moulding cavity to give better castings

Mixture of silica sand with 18 to 30 percent clay, having moisture content from 6 to 8%. It is used to make mould cavity.

It is a funnel shaped cavity at the top of the mould into which molten material is poured.

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Steps involved in making a sand mould1. Initially a suitable size of moulding box for suitable wall thickness is selectedfor a two piece pattern.

2. Next, place the drag portion of the pattern with the parting surface down on the bottom (ram-up) board as shown below;

3. The facing sand is then sprinkled carefully all around the pattern so that thepattern does not stick with moulding sand during withdrawn of the pattern.

4. The drag is then filled with loose prepared moulding sand and ramming of the moulding sand is done uniformly in the moulding box around the pattern. Fill the moulding sand once again and then perform ramming. Repeat the process three four times,

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5. The excess amount of sand is then removed using strike off bar to bring moulding sand at the same level of the moulding flask height to completes the drag.

6. The drag is then rolled over and the parting sand is sprinkled over on the top of the drag

7. Now the cope pattern is placed on the drag pattern and alignment is done using dowel pins.

8. Then cope (flask) is placed over the rammed drag and the parting sand is sprinkled all around the cope pattern.

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13. Rap and remove both the cope and drag patterns and repair the mould suitably if needed and dressing is applied

14. The gate is then cut connecting the lower base of sprue basin with runner and then the mould cavity.

15. Apply mould coating with a swab and bake the mould in case of a dry sand mould.

16. Set the cores in the mould, if needed and close the mould by inverting cope over drag.

17. The cope is then clamped with drag and the mould is ready for pouring

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PatternsPattern is the replica of the casting and it is embedded in moulding sand. The pattern is then withdrawn for generating cavity (known as mould) in moulding sand, so it is a mould forming tool.

Objectives of a pattern

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

2.Pattern possesses core prints which produces seats in form of extra recess for

core placement in the mould.

3.It establishes the parting line and parting surfaces in the mould.

4.Runner, gates and riser may form a part of the pattern.

5.Properly constructed patterns minimize overall cost of the casting.6.Properly made pattern having finished and smooth surface reduce casting

defects.

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Types of Patterns1. Single-piece or solid pattern

Solid pattern is made of single piece without joints, partings lines or loose pieces. It is the simplest form of the pattern.

2. Two-piece or split pattern

• When solid pattern is difficult for withdrawal from the mold cavity, thensolid pattern is splited in two parts.

• Split pattern is made in two pieces which are joined at the parting line by means of dowel pins.

• The splitting at the parting line is done to facilitate the withdrawal of the pattern.

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Types of Patterns3. Gated pattern

•In the mass production of casings, multi cavity moulds are used.

•Such moulds are formed byjoining a number of patterns and gates and providing a common runner for the molten metal.

•These patterns are made of metals, and metallic pieces to form gates and runners are attached to the pattern.

4. Cope and drag pattern

•Cope and drag part of the mould are prepared separately.

•This is done when the complete mould is too heavy to be handled by one operator.

•The pattern is made up of two halves, which are mounted on different plates.22

Types of Patterns

6. Loose-piece PatternIt is used when pattern is difficult for withdrawl from the mould.

Loose pieces are provided on the pattern and they are the part of pattern.

First main pattern is removed finally the loose piece is withdrawal separately

5. Match plate pattern

Cope and drag patterns along with Gating , risering are mounted on a single matching plate on either side.

After removing match plate mould cavity along with gating will be formed.

Several patterns can be fixed to a single match plate if they are small in size.

Used for small castings with higher dimensional accuracy and & large production.

This pattern is used in machine molding.

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8. Sweep Pattern

Used for forming large circular moulds of symmetric kind by revolving a sweep attached to a spindle.

Sweep is a template of wood or metal and is attached to the spindle at one edge and the other edge has a contour depending upon the desired shape of the mould.

The pivot end is attached to a stake of metal in the center of the mould.

Types of Patterns7. Follow board pattern

Used for castings which are structurallyWeak.

Bottom board is modified as follow board to follow the contour of weak pattern and give support during ramming

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Types of Patterns9. Skeleton patternUsed for large castings having simple geometric shape.

These are simple wooden frames that outline the shape of the part to be cast.

Used in Pit or floor moulding process.

For round shape pattern is made into two halves.

Used for casting water pipe turbine castings etc.

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Pattern Materials

Wood, Metal, Plastic, Plaster, Wax, Etc…

Wood is most commonly used because of easily available, machinability and low Weight.

Metal patterns are used for large scale casting productions, close tolerances, Smooth, Surface finish. Aluminium and white metals are commonly used.

Plastics are used because of their low weight, easier formability, smooth surface.

Choice of pattern material depends on Size, No of Castings, Dimensional accuracy, Expected life.

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Pattern Allowances

1. Shrinkage allowances Metal Shrinks on solidification and contracts further cooling at room temperature.

Liquid Shrinkage refers to reduction in volume when metal changes from liquid to

solid state. Risers are used to compensate this.

Solid Shrinkage refers to reduction in volume when metal loses temperature in

solid state. Shrinkage allowance is used to overcome this.

Rate of contraction with temperature is dependent on the material.

Shrink rulers are used for different castings.

Final dimensions of casting are different from pattern because of various reasons

Cast Iron, Malleable iron

10mm/m

Brass, Cu, Al 13mm/mSteel 21.0mm/mZinc, lead 25mm/m

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Pattern Allowances2. Draft allowances

Vertical faces of pattern are continual contact with the sand and may damage during withdrawal.

It is a positive allowance and is given on all the vertical surfaces of pattern so that its withdrawal becomes easier.

Inner details of the pattern require higher draft than outer surfaces.

More draft is needed for hand moulding

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Pattern Allowances3. Machining allowance To get better surface finish for casting

Depends on type of metal and finish

4. Shake allowance Pattern is rapped all around the faces to remove and it enlarges the final casting.

So original dimensions should be reduced to overcome this.

Purely depends on skill of labour

It is negative allowance and one way to overcome is increase the draft.

5. Distortion allowance

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Moulding TypesCommonly used traditional methods of molding are bench molding, floor molding, pit molding and machine molding.

1. Bench Molding

This type of molding is preferred for small jobs.

Whole molding operation is carried out on a bench of convenient height.

In this process, a minimum of two flasks, namely cope and drag molding flasks are necessary.

But in certain cases, the number of flasks may increase depending upon the number of parting surfaces required.

2. Floor MoldingThis type of molding is preferred for medium and large size jobs.

In this method, only drag portion of molding flask is used to make the mold

The floor itself is utilized as drag

It is usually performed with dry sand.30

Moulding Types3. Pit MoldingUsually large castings are made in pits instead of drag flasks because of their huge size.

In pit molding, the sand under the pattern is rammed by bedding-in process.

The walls and the bottom of the pit are usually reinforced with concrete and a layer of coke is laid on the bottom of the pit to enable easy escape of gas.

The coke bed is connected to atmosphere through vent pipes which provide an outlet to the gases.

One box is generally required to complete the mold, runner, sprue, pouring basin and gates are cut in it.

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Moulding Types4. Machine MoldingFor mass production of the casting

The main advantage of machine molding, besides the saving of labor and working time, is the accuracy and uniformity of the castings which can otherwise be only obtained with much time and labor.

Cost of machining on the casting can be reduced drastically because it is possible to maintain the tolerances within narrow limits.

Molding machines thus prepare the moulds at a faster rate and also eliminate the need of employing skilled molders.

The main operations performed by molding machines are ramming of the molding sand, roll over the mold, form gate, rapping the pattern and its withdrawal.

Loam Molding and Carbon-Dioxide Gas Molding32

Moulding Sand•It is used to prepare mold cavities

•Molding sands may be of two types namely natural or synthetic.

The common sources of molding sands available in India are as follows:

1 Batala sand ( Punjab)

2 Ganges sand (Uttar Pradesh)

3 Oyaria sand (Bihar)

4 Damodar and Barakar sands (Bengal- Bihar Border)

5 Londha sand (Bombay)

6 Gigatamannu sand (Andhra Pradesh) and

7 Avadi and Veeriyambakam sand (Madras)

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Constituents of molding sand Silica sandIt is the main constituent of moulding sand having enough refractoriness which can impart strength, stability and permeability to moulding and core sand.

BinderIt can be either inorganic or organic substance.

Inorganic group includes Kaolonite, Ball Clay, Fire Clay, Limonite, Fuller’s earth and Bentonite (mostly used).

Organic group includes dextrin, molasses, cereal binders, linseed oil and resins like phenol formaldehyde, urea formaldehyde etc.

MoistureMoisture content in the molding sand varies between 2 to 8 percent.

Amount of water is held rigidly by the clay and is mainly responsible for Developing the strength in the sand.

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AdditivesGenerally added to the moulding and core sand mixture to develop some special property in the sand.

Some commonly used additives;

Coal Dust

Corn flour

Dextrin

Sea coal

Pitch

Wood flour

Silica flour

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Properties of moulding sand

RefractorinessPermeabilityCohesivenessGreen strengthDry strengthFlowability or plasticityAdhesivenessCollapsibility

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Types of moulding sand1. Green sandSilica sand + Clay 18 to 30% + Moisture 6 to 8%

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.

Moulds prepared by this sand are not requiring backing and hence are known as green sand moulds.

This sand is easily available and it possesses low cost.

It is commonly employed for production of ferrous and non-ferrous castings.

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2. Dry sand Green sand that has been baked in suitable oven after the making mould

and coresIt possesses more strength, rigidity and thermal stability.It is mainly suitable for larger castings.

3. Facing sand It is sprinkled on the inner surface of the moulding cavity to give better

castingsIt is directly next to the surface of the pattern and it comes into contact

molten metal when the mould is poured.It is made of silica sand and clay, without the use of used sand.

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4. Backing sandIt is used to back up the facing sand and is used to fill the whole volume of

the moulding flask. Used moulding sand is mainly employed for this purpose. Black in colour due to addition of coal dust and burning on coming in

contact with the molten metal.5. Parting sandThis is clean clay-free silica sand which serves the same purpose as parting dust.

To keep the green sand not to stick to the pattern and also to allow the sand on the parting surface the cope and drag to separate without clinging.

6. Core sandCore sand is used for making cores and it is sometimes also known as oil sand

This 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.

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Cores

Cores are made of sand which are used to make cavities and hallow

projections.

Characteristics of CoreGreen strength – sufficient strength to hold up its shape till it is baked.

Dry strength – sufficient strength to resist bending forces due to hydrostatic pressure from the liquid (molten metal), when core is placed inside the mould

Refractoriness – core is surrounded on all sides by molten metal and should have high refractoriness.

Permeability – gases evolved may pass through the core to escape andshould posses sufficient permeability.

Collapsibility – should shrink as molten metal shrinks during solidification

Friability – should get dismantled easily once the casting is completely cooled.

Smoothness – surface of core should be smooth to have better surface finish.

Low gas emission – emission of gases from core should be as low as possible to avoid voids formed inside core 41

Core Sand

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Core sand must be stronger than moulding sand

Core sand = Sand grains + Binders + Additives

Sand grainsSand containing more than 5% clay is not used to make core

Excessive clay reduces the permeability and collapsibility of the core. Coarse silica used for making steels and finer one for cast iron an non- ferrous alloysBindersOrganic binders tend to burn away under the heat of molten metal and hence increases the collapsibility of the core.

Organic binder develop strength by polymerisation and cross-linking and hence cores are baked.

Some of the binders are linseed oil, dextrin, molasses, resins etc.

Core Prints

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Core prints are extra projections provided on the pattern that form a seat in the mould. Core prints support the core in the mould cavity.

Core shifts and chapletsChaplets are used to support the cores which tend to sag without adequate supports. Chaplets are made of the same material as that of the casting.

Types of Cores

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Horizontal cores – It is held horizontally along the parting line of the mould.Ends of core rests in the seats provided by core prints on the pattern.

Vertical cores –

Two ends of the mould sits on the cope and drag portion of the mould.Amount of taper on the top is more than the taper at the bottom of the core.

Balanced cores –

When openings are required at only one end, balanced cores are used.Core prints are available at one end of the pattern.Core prints need to be sufficiently longer to support the core in case of longer

holes.

Types of Cores

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Hanging cores –

They are used when the casting is made in drag.

Core is supported from above and hangs into the mould.

Fastening wires or rods are used and hole is made in the upper part of the core so that molten metal reaches the mould cavity.

Cover cores –

In cover core, core hangs from the cope portion and is supported by the drag.

Core acts as a cover and hence termed as cover core.

Wing cores –A wing core is used when hole or recess is to be obtained in casting.

Core print is given sufficient amount of taper so that core is placed readily in the mould.

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Gating system in mold

Pouring basinSprueSprue BaseRunnerGateRiser

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Functions of Gating system

To provide continuous, uniform feed of molten metal in to mould

cavity and to reduce the turbulence flow.

Proper directional solidification

To fill the mould cavity in a less time to avoid thermal gradient

To provide minimum excess metal

To prevent erosion of mould walls

To prevent the foreign materials to enter in mould cavity

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Elements of Gating system

1. Pouring basinIt is the conical hollow element or tapered hollow vertical portion of

the gating system

It makes easier for the ladle operator to direct the flow of molten

metal from crucible to pouring basin and sprue.

It helps in maintaining the required rate of liquid metal flow.

It reduces turbulence and vortexing at the sprue entrance.

It also helps in separating dross, slag and foreign element etc.

Skim core plays very important role in removing slag.

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Elements of Gating system

2. SprueIt is channel in cope side connected at bottom

of pouring basin which will carry molten metal to

the parting plane.In straight sprue due to vortex flow air bubbles may enter in to the

cavity this can be compensated by providing taper to it.It is tapered with its bigger end at to receive the molten metal the

smaller end is connected to the runner.It some times possesses skim bob at its lower end.

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Elements of Gating system 3. Sprue Base WellIt acts as a reservoir for metal at the bottom of sprue in order to

reduce moment of molten metal.

4. RunnerIt will be in trapezoidal cross section which will connect the sprue to

ingates of mould cavity.For ferrous metals runner will be in cope sideMetal flow rate from runner should be more than flow rate in ingates

then only slag will be trapped.

5. GateIt is a small passage or channel being cut by gate cutter which connect runner with the mould cavity.

It feeds the liquid metal to the casting at the rate consistent with the rate of solidification.

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6. RiserIt is a passage in molding sand made in the cope portion of the mold. Molten metal rises in it after filling the mould cavity completely. It compensates the shrinkage during solidification of the casting

thus avoiding the shrinkage defect in the casting.It also permits the escape of air and mould gases. It promotes directional solidification too and helps in bringing the

soundness in the casting.Types of Gates

Top Gate

Bottom Gate

Parting Gate

Step Gate

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Chills

In some casting, it is required to produce a hard surface at a particular place in the casting.

At that particular position, the special mould surface for fast extraction of heat is to be made.

The fast heat extracting metallic materials known as chills will be incorporated separately along with sand mould surface during molding.

The main function of chill is to provide a hard surface at a localized place in the casting by way of special and fast solidification.

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Special Casting Process

Shell CastingInvestment CastingDie castingCentrifugal Casting

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Shell CastingIt is a process in which, sand with thermosetting resin is poured on

heated metallic pattern to form a thin shells which act as a mould

cavity.

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1. In this process a pattern is placed on a metal plate and it is then coated with a mixture of fine sand and Phenol-resin (20:1).

2. In order to get thermosetting characteristics resign is mixed with hexa-methylene-tetramine (hexa) up to 14-16%.

3. Sand, hexa and additives which are all dry are mixed in mueller for 1min.

4. The pattern is heated first and silicon grease is then sprayed on the heated metal pattern ( grey cast Iron) for easy separation.

5. The pattern is heated to 205 to 230°C and covered with resin bounded sand. After 30 seconds, a hard layer of sand is formed over pattern.

6. Pattern and shell are heated and treated in an oven at 315°C for 60 secs.,

7. Layer of about 4 to 10 mm in thickness is stuck on the pattern and the loose material is then removed from the pattern.

8. Then mold is ready for pouring.

Process

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AdvantagesVery suitable for thin sections like petrol engine cylinder.Excellent surface finish.Good dimensional accuracy of order of 0.002 to 0.003 mm.Negligible machining and cleaning cost.Occupies less floor space.Skillness required is less.Molds can be stored until required.Mass production.

Disadvantages(i) Initial cost is high.

(ii) Specialized equipment is required.

(iii) Resin binder is an expensive material.

(iv) Limited for small size.

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ApplicationsSuitable for production of Al, Cu and ferrous metalsCylinder heads for air cooled IC enginesAutomobile transmission partsCast tooth bevel gearsBreak beamRollers for crawler tractorsChain seat bracketRefrigerator valve plate etc.

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Investment Casting (lost wax casting)

(b) Multiple patterns assembled to wax sprue

(a) Wax pattern (injection molding)

In this process mould is prepared around expandable pattern.

Pattern is made of wax and same shape die has to be prepared first

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(c) Shell built immerse into ceramic slurry immerse into fine sand (few layers)

(d) dry ceramic melt out the wax fire ceramic (burn wax). Hot vapor solvent trichloro-ethylene is used to remove remained wax.

(e) Mould is preheated before pouring the melt. Pour molten metal (gravity) cool, solidify [Hollow casting: pouring excess metal before solidification

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(f) Break ceramic shell (vibration or water blasting)

(g) Cut off parts (high-speed friction saw) finishing (polish)

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AdvantagesComplex shapes can be producedVery thin sections can be producedBecause of using fine grain sand products with good surface finish can be

produced Little or no machining is requiredControlled mechanical properties

Disadvantages(i) Limited to size and mass of casting

(ii) More expensive

ApplicationJewellery, surgical instruments, vanes and blades of a gas turbine Fire arms, Steel valve bodies and impellers for turbo chargers, etc.

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PERMANENT MOLD OR GRAVITY DIE CASTING

Used in;

Carburetor bodies

Oil pump bodies

Connecting rods

Pistons etc.

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Molten metal is poured into the mold under gravity only and no external pressure is applied to force the liquid metal into the mold cavity.

The metallic mold can be reused many times before it is discarded or rebuilt.

These molds are made of dense, fine grained, heat resistant cast iron, steel, bronze, anodized aluminum, graphite or other suitable refractoriness.

The mold is made in two halves in order to facilitate the removal of casting from the mold.

The mold walls of a permanent mold have thickness from 15 mm to 50 mm.

For faster cooling, fins or projections may be provided on the outside of the permanent mold which provides the desirable chilling effect.

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DIE CASTING or Pressure Die-CastingIt is a process of injecting the molten metal with high pressure in metallic die.

The pressure is generally created by compressed air or hydraulically means

The pressure varies from 70 to 5000 kg/cm2 and is maintained while the casting solidifies.

Any narrow sections, complex shapes can be easily produced.

1. It consists of stationary die or cover die which is fixed to the die casting machine.

2. Second one is ejector die is moved out for the extraction of casting.

3. First lubricant is sprayed on the die cavity manually or automatic system.

4. Two dies are closed using the clamp.

5. Required amount of metal is injected in to the die cavity and is allowed to solidified

HOT CHAMBER and COLD CHAMBER DIE CASTING MACHINES

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HOT CHAMBER

Die is closed, gooseneck cylinder is filled with molten metal

Plunger pushes molten metal through gooseneck into cavity

Metal is held under pressure until it solidifies

Die opens, cores retracted; plunger returns

Ejector pins push casting out of ejector die

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COLD CHAMBER:•Die closed, molten metal is ladled into

cylinder•Plunger pushes molten metal into die cavity•Metal is held under high pressure until it

solidifies•Die opens, plunger pushes solidified slug

from the cylinder•Cores retracted•Ejector pins push casting off ejector die

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COLD CHAMBER DIE

Casting

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Centrifugal CastingIn this casting process, molten metal is poured into a revolving mold and allowed to solidify molten metal by pressure of centrifugal force.

3 -Types1. True centrifugal casting

2. Semi-centrifugal casting

3. Centrifuging

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True Centrifugal Casting

The process setup contains an accurately machined metal mold or die surrounded by cooling water.

The machine is mounted on wheels and it can be move lengthwise on a straight on a slightly inclined track.

At one end of the track there is a ladle containing proper quantities of molten metal As pouring proceeds mould is rotated and moved slowly down the track so that the metal is laid progressively along the length of the mould wall.

After completion of pouring the machine will be at the lower end of its track with the mold that rotating continuously till the molten metal has solidified in form of a pipe.

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Advantages

Good mechanical properties can be achieved

In order to make holes cores are not required

No need for gates and runners.

LimitationsCastings which are axi-symmetric and having concentric holes are suitable.

Equipment is expensive.

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Semi - Centrifugal Casting

Used for articles which are more complicated than those possible in truecentrifugal casting, but are axi-symmetric in nature.

A particular shape of the casting is produced by mold and core and not by centrifugal force.

The centrifugal force aids proper feeding and helps in producing the castings free from porosity.

For larger production rates moulds are stacked one over the other.

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Centrifuging CastingTo obtain high metal pressures during solidification When the shapes are not axi-symmetrical the centrifuging is used.

Suitable for only small jobs.

Radial runners are used to combine all the jobs

Similar to Semi-centrifugal casting.

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