module-3 metal casting

MODULE : 3

MATHEW SAMUEL

LECTURER, SOE CUSAT

MODULE : 3METAL CASTING

1

REFER TEXTBOOKS

CASTING

A manufacturing process that pours a liquid material into a hollow mould until the material cools into a solidified shape.

A material in a liquid or semisolid form is poured or forced to flow into a die cavity and allowed to solidify, thus taking the solid shape of the cavity.

The process can be applied on metals and plastics.

Typical examples of the product produced by casting process Engine Block, Machine tool parts, etc.

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MATHEW SAMUEL, LECTURER SOE CUSAT

CASTING PROCESS

In pattern making, a physical model of casting, i.e. a pattern is used to make the mold.

Apatternis a replica of the object to be cast, used to prepare the cavity into which molten material will be poured during thecastingprocess.

The mold is made by packing some readily formed aggregated materials, like molding sand, around the pattern.

After the pattern is withdrawn, its imprint leaves the mold cavity , that is ultimately filled with metal to become the casting.

VIDEO OF CASTING PROCESS

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Steps involved in casting process

Pattern making.

Mould making.

Core making.

Melting of metal and pouring.

Cooling and solidification

Cleaning of castings and inspection

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PATTERN


PATTERN

Apatternis a replica of the object to be cast, used to prepare the cavity into which molten material will be poured during thecastingprocess.

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

Properly constructed patterns minimize overall cost of the casting.

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PROPERTIES OF PATTERN MATERIAL

It should be easily shaped, worked, machined and joined.

It should be resistant to wear and corrosion.

It should be resistant to chemical action.

It should be dimensionally stable and must remain unaffected by variations in temperature and humidity.

It should be easily available and economical.

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PATTERN MATERIALS

1.WOOD

Easily available

Low weight

Low cost

It absorbs moisture and hence dimensions will change

Lower life

Suitable for small quantity production and very large size castings.

Limitations: Inherently non uniform in structure, posses poor wear and abrasion resistance, cannot withstand rough handling, absorbs and gives off moisture.

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PATTERN MATERIALS

2.METAL

Used for mass production

For maintaining closer dimensional tolerances on casting.

More life when compared to wooden patterns

Few of the material used include CI, Al, Fe, Brass etc. Al is widely used.

Have a smooth surface

Can withstand rough handling

Resistant to wear, abrasion, corrosion and swelling.

Limitations : expensive, not easily repaired, heavier than wooden patterns

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PATTERN MATERIALS

3. PLASTIC

Low weight

Easier formability

Do not absorb moisture

Good corrosion resistance

Strong and dimensionally stable.

4. POLYSTYRENE

Used for prototype (single piece) castings

Also known as Disposable patterns.

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TYPES OF PATTERNS

SINGLE PIECE/SOLID PATTERN:

Simple shape castings are produced by this type of patterns.

It is the simplest form of pattern exactly like the desired casting.

It is inexpensive.

One piece Pattern is made from one piece and does not contain loose pieces or joints.

One piece Pattern is usually made of wood or metal depending up on the casting to be produced.

Stuffing box of a steam engine is an example of One piece pattern.

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SPLIT/PARTED PATTERN

Many patterns cannot be made in a single piece because of the difficulties associated with the moulding operations.

To eliminate this difficulty and for castings of intricate shape or unusual shape, split patterns are employed to form the mould.

These patterns are made in two parts. So that one part will produce the lower half of the mould and the other, the upper half.

The line of separation of two parts is called the Parting Line

Spindles, cylinders, steam valve bodies, water stop cocks and taps, bearings, small pulleys and wheels are few examples of castings that require the use of split patterns.

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LOOSE PIECE PATTERN

Certain patterns cannot be withdrawn once they are embedded in the molding sand.

Such patterns are usually made with one or loose pieces for facilitating their removal from the moulding box and are known as Loose Piece Patterns.

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MATCH PLATE PATTERN

Split pattern having the cope and drag portions mounted on opposite sides of a plate.

Several patterns can be mounted on one match plate if the size of the casting is small.

Piston rings of IC engines are produced by this method.

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GATED PATTERN

A gated pattern is simply one or more loose patterns having attached gates and runners.

These patterns are used for producing small castings in mass production system.

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COPE AND DRAG PATTERNS

Split pattern having the cope and drag portions each mounted on separate match plates

Used in production of large castings.

For higher rate of production, each half of the pattern is mounted on a separate molding machine, one operator working on the cope part of the mould and the other on the drag part of the mould

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SWEEP PATTERNS

Sweep pattern is preferred for producing large castings of circular sections and symmetrical shapes.

Sweep Pattern consists of a wooden board having a shape corresponding to the shape of the desired casting and arranged to rotate about a central axis.

When the board is rotated about the axis mould cavity of the desired shape is obtained in the mould.

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SEGMENTAL PATTERN

The segmented pattern is similar to Sweep Pattern , in the sense that both types employ a part of a complete pattern for getting the required shape of the pattern.

The segmented pattern is in the form a segment and is used for producing circular shapes.

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SKELETON PATTERN

For large casting having simple geometrical shapes(for example water pipes, turbine casings etc.), Skeleton Pattern are used.

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

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FOLLOW BOARD PATTERN

Follow Board is not a pattern but is a device used for various purposes.

It is used for supporting a pattern which is very thin and fragile which may break or collapse under pressure when the sand above is rammed.

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PATTERN ALLOWANCES

The amount of something that is permitted, especially within a set of regulations or for a specified purpose.

To compensate for any dimensional and structural changes which will happen during the casting process, allowances are usually made in the pattern

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TYPES OF PATTERN ALLOWANCES

SHRINKAGE ALLOWANCE

MACHINING ALLOWANCE

DRAFT OR TAPER ALLOWANCE

DISTORTION ALLOWANCE

RAPPING OR SHAKE ALLOWANCE

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SHRINKAGE ALLOWANCE

All most all cast metals shrink or contract volumetrically on cooling.

Provided to compensate for shrinkage of material

Pattern is made slightly bigger

Amount of allowance depends upon type of material, its composition, pouring temperature etc.

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

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SHRINKAGE ALLOWANCES(contd.)

The metal shrinkage depends upon:

The cast metal or alloy.

Pouring temp. of the metal/alloy.

Casted dimensions(size).

Casting design aspects.

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

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MACHINING ALLOWANCE

Provided to compensate for machining on casting.

Pattern is made slightly bigger in size.

Amount of allowance depends upon size and shape of casting, type of material, machining process to be used, degree of accuracy and surface finish required etc.

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MA depends on..

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Nature of metals: Ferrous metals need more allowance than Non Ferrous metals.

Size and shape of casting.

The type of machining operations to be employed for cleaning the casting.(grinding, sand paper, other machines)

Degree of finish.

Molding process employed.


DRAFT OR TAPER ALLOWANCE

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Provided to facilitate easy withdrawal of the pattern.

Typically it ranges from 1 degree to 3 degree for wooden patterns.

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.


TA depends on..

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Shape and size of pattern in the depth direction in contact with the mould cavity.

Moulding methods.

Mould materials.


DISTORTION ALLOWANCE

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Provided on patterns whose castings tend to distort on cooling


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MOLD


MOLD MATERIAL

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A mold material is one, out of which mold is made.

A mold material should be such that mold cavity retains its shape till the metal has solidified.

1. Permanent Molds made up of Steel or Grey Cast Iron

2. Temporary refractory molds made up of refractory sand and resins.

3. Molds may also be made up of wax, plaster of paris, carbon, ceramics etc.


MOLD(contd.)

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Refractory sands :They are

1. Silica sand

2. Zircon

3. Magnesite

4. Graphite etc

Refractory sand are the best molding sands.

They maintain their shape and characteristics even at high temperatures.

They have good permeability.

They can be molded into intricate shapes.


MOLDING SAND

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Ingredients of Molding Sand are :

1. Sand

2. Binders

3. Water

4. Additives

Sources of Molding Sand:

1. River Beds

2. Sea

3. Lakes

4. Desert


TYPES OF MOLDING SAND

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1. Natural Sands

2. Synthetic Sands

3. Loam Sands

NATURAL SAND

Natural sand is directly used for molding and contains 5-20% of clay as binding material.

It can maintain moisture content for a long time.

They are inexpensive.

It can be easily repaired. Used for casting cast iron and non-ferrous metals.


SYNTHETIC SAND

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Synthetic sand consists of silica sand with or without clay, binder or moisture.

It is a formulated sand i.e. sand formed by adding different ingredients.

These sands have better casting properties like permeability and refractoriness and are suitable for casting ferrous and non-ferrous materials.

These properties can be controlled by mixing different ingredients.

Synthetic sands are used for making heavy castings.


LOAM SAND

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Loam sand contains many ingredients, like fine sand particles, finely ground refractories, clay, graphite and fiber reinforcements.

In many cases, the clay content may be of the order of 50% or more.

When mixed with water, the materials mix to a consistency resembling mortar and become hard after drying.

Big molds for casting are made of brick framework lined with loam sand and dried.


PROPERTIES OF MOLDING SAND

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1. Flowability : It is the ability of the molding sand to get compacted to uniform density.

2. Green Strength : It is the strength of the sand in moist condition. A molding sand with adequate Green Strength will retain its shape, will not distort.

3. Dry Strength : It is the strength of the sand in dry condition.

4. Permeability : It is the ability of a molding sand to allow the passage of mold gases through them. Mold gases may be produced by the reaction of molten metal with the moisture or binders.

5. Refractoriness : It is the ability of the sand to withstand high temperature.


PROPERTIES OF MOLDING SAND(contd.)

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6. Adhesiveness : It is the ability of the sand to stick on to the mold walls.

7. Collapsibility : It is the ability of the sand to collapse after the casting solidifies.

8. Fineness : It is the ability of the sand to produce smooth surfaced castings.

9. Coefficient of Expansion : A good molding sand should have less coefficient of expansion.

10. Durability : It is the ability of the sand to be used again and again.


PROPERTIES OF CORE

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It must be strong to retain the shape while handling,

It must resist erosion by molten metal,

It must be permeable to gases,

It must have high refractoriness, and

It must have good surface finish to replicate it on to the casting.

CORE: Acoreis a device used incastingandmoldingprocesses to produce internalcavities. The core is normally a disposableitem that is destroyed to get it out of the piece.


GATING SYSTEM

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It refers to the passageway through which molten metal passes to enter mould cavity.


SPRUE

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Asprueis the passage through which liquid material is introduced into a mold.

Duringcastingormolding, the material in the sprue will solidify and need to be removed from the finished part.


POURING CUPS

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Funnel shaped cup which forms the top portion of the sprue.

It is used to direct the flow of molten metal to the sprue.

RUNNER

Connects sprue with gates.

GATES

Gate is a channel which connects runner with mold cavity.

Gates feeds molten metal to the mold cavity.


RISER

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It provide many advantages as follows,

In the initial stages of pouring it allows the air, steam and gases to go out of the mould.

On seeing the rising molten metal through it, it is ensured that the mould cavity has been completely filled up.

It act as a reservoir to feed the molten metal to the casting to compensate the shrinkage during solidification


Functions of a gating system

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Fill the mold cavity before solidifying.

Introduce the molten metal with less speed and less turbulence so that mold erosion and metal oxidation is prevented.

Regulate the rate at which the molten metal entering the mold cavity.

Should consume less metal. ie., less metal should be solidified in the gating system.


TYPES OF CORES

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1. Horizontal Core

2. Vertical Core

3. Hanging Or Cover Core

4. Balanced Core

5. Drop or Stop Off Core

6. Kiss Core


HORIZONTAL CORE

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Horizontal Core is placed horizontally in the mold.

It is supported in core seats at both the ends.

It may have any shape, circular or some other section depending up on the shape of the cavity required in the casting.

These are generally placed at the parting line


VERTICAL CORE

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A Vertical Core is placed vertically in the mold.

A Vertical Core is supported in core seats at both the ends.

It may have any shape, circular or some other section depending up on the shape of the cavity required in the casting.

A big portion on the vertical core remains in the drag.

Vertical Cores are generally placed at the parting line.


HANGING OR COVER CORE

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A hanging core is supported from above & it hangs vertically in the mold cavity.

A hanging core has no support from bottom.


BALANCED CORE

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A Balanced Core is supported in one end only.

A Balanced Core requires a long core seat so that core does not sag or fall into the mold.


DROP OR STOP OFF CORE

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A Drop Core is used when a hole or cavity is required in a casting not in the parting surface but above or below the parting surface.

Tapering is given to the core for easy location.


RAM UP CORE

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Ram up core is placed in the sand along with the pattern before ramming the sand.

It cannot be placed after the mold has been rammed.

Ram up core is placed in the mold so as to obtain cavities which are inaccessible after the mold is prepared.


KISS CORE

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A kiss core is not supported by core seats.

A kiss core is placed between cope and drag due to the pressure exerted by cope on the drag.

A number of kiss core can be simultaneously positioned in order to obtain a number of holes in casting.


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TYPES OF MOLDING PROCESS


GREEN SAND MOLDING

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Green sand is a sand which contains water.

In a Green Sand Mold the molten metal is poured when is in the green state i.e., in the un dried state.

Intricate shapes can be produced in Green Sand Mold.

These are suitable for producing small and medium sized castings.

It is specially employed for producing non-ferrous castings.


ADVANTAGES

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No mold baking or drying is required.

There is less mold distortion than in dry sand molding.

Time and cost associated with mold baking or drying is eliminated.

Green sand molds having smaller depths permit the escape of mold gases without any difficulty.

Green sand molding provides good dimensional accuracy.


DISADVANTAGES

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Lower strengths.

They are less permeable.

There are more chances of defects (like blow holes etc.) occurring in castings made by green sand molding.

Green sand molding does not impart good surface finish on castings.


DRY SAND MOLDING

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Dry molding sand differs from the green molding sand in the sense that it contains binders (like clay, bentonite, molasses etc.) which harden when the mold is heated or dried.

Dry sand molds are actually made with molding sand in green condition and then the entire mold is dried in ovens.


ADVANTAGES

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Possess high strength.

More permeable as compared to green sand molds.

Castings produced from dry sand molds possess clean and smooth surfaces.

Less defects.

Better overall dimensional accuracy to the molds and castings as compared to green sand molding.

Preferred for large sized castings.


DISADVANTAGES

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Dry sand molding involves more labour and consumes more time in completing the mold.

Mold baking is an extra work as compared to that required in green sand molding.

Dry sand molding is more expensive as compared to green sand molding.


SKIN DRIED MOLDING

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The mold is made with the molding sand in green condition.

Then the skin of the mold cavity is dried up with help of gas torches or radiant heating lamps.

Skin Dried mold is dried up to a depth varying from 6mm to 25 mm.

A Skin Dried mold possess strength and other characteristics in between green and dried sand molds.


AIR DRIED MOLDING

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The mold is made with the molding sand in green condition and then it is kept open to the atmospheric air for a certain period of time.

During that period some of the moisture from the mold surface gets evaporated and consequently the mold skin dries thereby increasing the strength and hardness of the mold surface.


CEMENT BONDED SAND MOLDING

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Cement bonded sand mold material consists of 85.5% clean silica sand, 10%potland cement and 4.5% water.

Mold Sand does not require much ramming.

Cement-Sand Mold develop strength and hardness owing to the setting action of the Portland cement.

The mold is permitted to continue setting for a period of about 72 hours.

Accurate and smooth surfaced casting.


METAL MOLDS

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Metal molds are generally made of Grey Cast Iron or steel.

Metal Molds are preferred for casting non-ferrous metals and alloys (ie. Al,Mg,Zn and Pb base alloys).

Aluminum Alloy (I.C Engine) pistons are cast in metal molds.

Used in Permanent mold casting, Pressure die casting, Centrifugal casting


SHELL MOLDING

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Shell molds are produced with the help of heated iron or steel patterns.

A mixture of fine sand and resin is used to produce shells.

Shells are assembled to form the mold which is poured with molten metal.

Shell molds produce exceptionally good surface finish and dimensional accuracy.

Shell molding is suited to ferrous and non-ferrous alloy castings.


INVESTMENT MOLDING OR LOST WAX MOLDING

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Molten wax is poured in the die and obtain a wax pattern(a).

Then the wax pattern is pre-coated by repeatedly dipping it into a slurry of a fine refractory material.

Pre-coating is done to impart good surface finish to the casting.

Then the investment molding mixture is poured around the pre-coated wax pattern (b).

A typical investment mixture consists of 91.2% sand, 3.8% water, 6.5% primary calcium phosphate and 2.3%MgO-mesh.


IM(contd.)

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Allow the investment to harden.

Heat the mold in the furnace (at 200 to 300 F).

This further hardens the mold and melts the wax (c).

The investment mold is then heated to between 1000 and 1800 F and after heating, the molten metal can be poured into it (d).


CERAMIC MOLDING

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Ceramic Mold is a variation of Investment Mold.

If instead of Investment molding mixture, a slurry composed of refractory sand and ceramic binder is poured on the wax pattern, it results in ceramic thin-wall shell.

Wax is removed in the same manner as Investment Mold.


PLASTER MOLDS

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Make a metal pattern or of any other moisture resistant material.

Make a slurry of mixtures of gypsum or plaster of paris, and ingredients such as talc, asbestos fiber, silica flour etc., with water.

The slurry is poured over the pattern and is allowed to set.

Pattern is taken out and is heated in oven to about 600F for several hours.

It removes moisture from the plaster mold


GRAPHITE MOLDS

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They are used to make Titanium Alloys castings.

Graphite molds are prepared by squeezing the graphite mold material around the pattern at 4 to 8.5 kg/cm2 pressure.

The mold is dried and fired at 1800 to 2000F.

Graphite have been used for casting of railroad and car wheels.


SODIUM SILICATE-CO2 MOLDS OR CO2 MOLDING

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Mixture of silica sand and sodium silicate binder is rammed around the pattern in the mold box.

Carbon dioxide gas is forced into the mold at about 1.4 to 1.5 kg/cm2.

Na2 Si O3 +CO2 Na2 CO3 + Si O2

(Silica Gel)

Silica GEL hardens and forms a bond between the sand grains


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TYPES OF CASTING PROCESS


DIE CASTING

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Die casting is a permanent mold casting process in which the molten metal is injected into the mold cavity at an increased pressure.

The mold used in the die casting process is called a die.

A die casting machine consists of four basic elements:

1. Frame : Holds the die. The frame consists of a stationary part holding a stationary die and a movable part holding a movable die.

2. Source of molten metal : A Holding furnace for feeding metal.

3. Die-casting die

4. Metal injecting mechanism.


HOT CHAMBER DIE CASTING

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Cycle in hotchamber casting:

(1) with die closed and plunger withdrawn, molten metal flows into the chamber



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Cycle in hotchamber casting:

(2) plunger forces metal in chamber to flow into die, maintaining pressure during cooling and solidification



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Metal is melted in a container, and a piston injects liquid metal under high pressure into the die

High production rates - 500 parts per hour

Applications limited to low meltingpoint metals that do not chemically attack plunger and other mechanical components

Casting metals: zinc, tin, lead, and magnesium


COLD CHAMBER DIE CASTING

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Molten metal is poured into unheated chamber from external melting container, and a piston injects metal under high pressure into die cavity

High production but not usually as fast as hotchamber machines because of pouring step

Casting metals: aluminum, brass, and magnesium alloys

Advantages of hotchamber process favor its use on low meltingpoint alloys (zinc, tin, lead)


COLD CHAMBER DIE CASTING

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VIDEO


CENTRIFUGAL CASTING

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In Centrifugal Casting liquid metal is introduced into a rotating mold so that metal being poured is thrown to the outer surface of the mold cavity.

Casting cools and solidifies from outside towards the axis of rotation.


ADVANTAGES OF CENTRIFUGAL CASTINGS :

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Castings produced are sound.

Rejection is less.

Production rate is sufficiently high.

Thin and intricate shapes can be cast.

Directional solidification is obtained.

DISADVANTAGES

All shapes cannot be cast.

Initial investment is more.


TYPES OF CENTRIFUGAL CASTINGS

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True Centrifugal Casting.

Semi-Centrifugal Casting.

Centrifuge Casting.


TRUE CENTRIFUGAL CASTING

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True Centrifugal Casting is used for making castings having more or less outer symmetrical shape.

To cast a hollow cylinder for example a cylindrical mold is made to rotate about its own axis at a speed such that the metal being poured is thrown to the outer surface of the mold cavity.

The metal solidifies in the form of a hollow cylinder.

Since the metal is always pushed outward because of the centrifugal force, no cores needs to be used for making concentric hole.


SEMI-CENTRIFUGAL CASTING

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Like Centrifugal castings, Semi-centrifugal

castings also uses the rotation of the mold about its axis.

Unlike True Centrifugal Casting, a core is used to form a central cavity, for producing internal shapes that could not be formed by any other method.

Semi- Centrifugal Casting technique is used to produce castings which are symmetric about the axis of rotation.

Gear blanks, wheels are produced using Semi-Centrifugal castings.


CENTRIFUGE CASTING OR PRESSURE CASTING

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Parts which are not symmetric about any axis of rotation can be cast using a group of molds arranged in a circle to balance each other.

The set up is revolved around the center of the circle to induce pressure on the metal in the molds.

Variety of smaller shapes can be cast.

Molten metal is passed to the mold cavities through a central sprue.


SLUSH CASTING

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Similar to Rotational Molding

Slush poured into mold

Slush is made from a mixture of resin and plastisols

Mold rotated

Part forms on surface of mold

Part removed

Slush molding is an excellent method of producing open, hollow objects, including rain boots, shoes, toys, dolls and automotive products, such as protective skin coatings on arm rests, head rests and crash pads.

VIDEO


INVESTMENT CASTING

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LOST WAX METHOD

****ALREADY DISCUSSED****


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CASTING DEFECTS


CASTING DEFECTS

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Imperfections in the castings is called as Defects or Flaws in castings.

Classification of defects :


CLASSIFICATION OF CASTING DEFECTS

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1.DEFECTS CAUSED BY PATTERNS AND MOLDING BOX EQUIPMENTS.

Mismatch or Mold Shift : Castings does not match at the parting line.

There is a mismatch of the top and bottom parts of the casting.

Fins, Flash and Strains : Occur at the parting line and result in excess metal which has to be removed.

Crush : Displacement of sand while closing the mold, thereby deforming the mold surface.

Causes : Excess weight cope portion mold.


2. DEFECTS DUE TO IMPROPER MOLDING AND CORE MAKING MATERIALS.

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BLOW HOLES :

Blow holes are smooth round holes which are not visible from outside. Blow Holes are entrapped gas bubbles with smooth walls.

Causes : Excess moisture, less permeability etc.

DROP :

A Drop occurs when cope surface cracks and breaks, thus the pieces of sand fall into the molten metal.

Causes : Low Green Strength, low strength of mold.

SCAB :

It is the penetration of molten metal into the molding material.

Causes : Too fine sand, uneven mold ramming etc.


DEFECTS DUE TO IMPROPER MOLDING AND CORE MAKING MATERIALS (contd.)

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PIN HOLES :

They are small holes revealed on the surface of a casting.

Causes : High Moisture content of the sand, faulty metals.

HOT TEARS :

They are internal or external cracks on castings occurring due to unwanted cooling stresses.

Causes : Very hard ramming, faulty metal, faulty design of casting, too much shrinkage of metal while solidifying.


3. DEFECTS CAUSED BY MOLDING, CORE MAKING, GATING ETC.

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Hot Tears, Shifts,Fins and Flash, Crush

Cold Laps (Shuts) and Misrun :

If the molten metal is too cold, entire mold cavity may not be filled during pouring before the metal starts solidifying and the result is Misrun. The defect may appear like a crack.

If molten metal enters through two or more ingates or otherwise if two streams of metal which are too cold, physically meet in the mold cavity which do not fuse together, they develop Cold Shut Defect.

Causes : Too cold molten metal, too thin casting sections, too small gates, etc.


DEFECTS DUE BY MOLDING, CORE MAKING, GATING ETC (contd.)

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SLAG HOLES : They are smooth depressions on the surface of castings. Slag Holes result when slag enters the mold cavity.

SHRINKAGE DEFECTS : Molten Metals shrink as they solidify. If this shrinkage is not compensated by risers, voids will occur on the surface or inside.


4. DEFECTS OCCURRING WHILE CLOSING AND POURING THE MOLDS.

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Shift or Mismatch, Misrun, Cold Laps or Cold Shifts

INCLUSIONS:

Any separate undesirable foreign material present in the metal of a casting is called Inclusion. An inclusion may be oxides, slag etc.


5. DEFECTS CAUSED BY THE MOLTEN METAL.

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MISRUNS, COLD SHUTS

SAND CUTS AND WASHES :

Molten metal as it flows over the mold and cores surfaces, erodes the same and results in defects called Cuts and Washes.

Causes: Weak Sand, Soft ramming etc.

FUSION :

Sand may fuse and stick to the casting resulting in rough surface to the casting.

Causes : Too high molten metal temperature.

SHOT METAL :

If molten metal is relatively low temperature during pouring into mold, a few small particles get separated from main stream, they solidify and form shots. These shots, if do not fuse with rest of molten metal, get embedded in the casting causing a defect called Shot Metal.


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6. DEFECTS OCCURRING WHILE FETTLING(CLEANING) :

Sand and scales not properly removed from castings, distorted castings not properly removed.

7. DEFECTS DUE TO FAULTY HEAT TREATMENT. : Uncontrolled heating, fast cooling rates may cause cracking of brittle castings.


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INSPECTION OF CASTING



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1. VISUAL INSPECTION

Most surface defects can be seen

2. PRESSURE TEST

The casting is filled with pressurized air after closing all the openings

E.g. gear boxes, pressure vessels, look for leaks by submerging in special liquids

Pressurized oil can also be used in some cases

3. RADIOGRAPHIC EXAMINATION

Usually x-rays or g rays

x-ray method is used for voids, non metallic inclusions, porosity, cracks.

Defects appear darker than surrounding



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4. ULTRASOUND EXAMINATION

Ultrasound across the casting

Sound transmitted across homogenous metals

However discontinuities reflect sound back.

5. DYE PENETRATION INSPECTION

To detect invisible surface defects in non magnetic castings

A dye of fluorescent material is sprayed or applied near the surface. The surface is then wiped and viewed in darkness

Cracks will be visible



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6. MAGNETIC PARTICLE INSPECTION

Induce magnetic field through section under inspection

Powdered Ferro-magnetic magnetic material is spread onto the surface

Voids or cracks result in abrupt changes in permeability of material leads to leakage in magnetic field

Particles concentrate on the disrupted field or on the crack.


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END OF THE MODULE


module-3 metal casting

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