UNIT 1 - CASTING

BY V.KAMALA

Assistant Professor, DOIE, Anna University,

Chennai.

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A foundry is:

A factory that pours molten metal into molds, producing cast metal objects.

Some typical cast

metal objects:

Turbine blades in jet engines

Engine blocks, axles

Aluminum pots

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SAND,

Mixed With Clay Binder & Water (So It Will Hold Its Shape) Plus Coal Dust To

Improve Surface Finish

PATTERN A copy of the shape you want to produce, made of wood, plastic or metal

CASTING

Container of molten metal (filled from furnace)

Top and bottom mold forms

(made of metal, open at top and bottom)

Rammer (tool to compact the sand;

often a pressing machine is used)

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MOLDING:

Sand placed into bottom mold

form & compacted

MOLDING:

Pattern placed into mold

A very basic summary of the sand casting process. . . First of all,

Mix the sand.

2

1 THEN

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MOLDING: Add the top mold form

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4 MOLDING: Fill top form with compacted sand.

A tube or pipe provides a path to pour the

metal in

Pattern is still inside!

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MOLDING: Take the top mold off and remove pattern & pipe or post

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6 MOLDING: Replace the top mold and fasten securely!

Pouring hole

In the middle of the sand is a cavity shaped

like the pattern!

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7 CASTING: Pour the metal (container is filled from furnace immediately before you are ready to pour)

8 Wait for the metal to cool (minutes to days, depending on the size of the casting)

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8 SHAKE OUT: Break apart the two halves of the mold & take out the part—usually requires vibrating or striking the mold to break apart the sand

CLEANING. Sand is cleaned off the part, the “tab” where metal flowed in must be removed.

9

A copy of the pattern has now been made

in metal

10 Mold forms are reused

11 Sand is broken up, screened to remove debris and clumps, and sent for remixing

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It is the process of making metal

parts, by pouring molten metal in

to the cavity of the required

shape and allowing it to solidify !

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CASTING TERMINOLOGY

FLASK The frame and support COPE The upper half of sand

mold DRAG The lower half of the sand

mold PATTERN Shape used to mold the

shape of the casting POURING BASIN Orifice where molten metal

is poured into the mold SPRUE Vertical channel through

which molten metal flows downward into mold

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CASTING TERMINOLOGY RUNNERS Channels that carry molten

metal from sprue to mold RISER Reservoirs to supply the

molten metal needed to make up shrinkage losses during solidification.

GATE Portion Of Runner Through

Which The Molten Metal Enters The Mold Cavity, Used To Trap Contaminants.

VENTS Openings used to carry off

gases given off by the metal and exhaust air.

CORES Insets used to produce

interior cavities.

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Patternmaking

Core making

Molding

Melting and pouring

Cleaning

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A pattern is made of wood or metal, is a replica of the final

product and is used for preparing mould cavity

Mould cavity 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.

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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.

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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.

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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

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It is a Replica Model of the final object with some modifications.

Shape and size of the pattern is made larger than the final casting to compensate the volume reduction occurred during the conversion of high to low temperature.

It is made up of wood, metal and plastics.

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Size and complexity of the shape

Number of components produced

Method of castings to be used

Dimensional Accuracy

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Made of single solid piece without joints most inexpensive of all types of patterns

Low quantity production

Made up of wood

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Made into two parts One part is used to produce the lower half of

the mould, Other part is used to produce the upper half of the mould

Two parts are assembled together in correct position by pins

Line separating the two parts is called parting line

Used for making symmetrical shaped castings • Eg. Cylinders , Bearings, Pulleys

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Pattern is made into two halves mounted on both sides of a plate.

Match plate is accurately placed between the cope and drag.

Many patterns can be mounted on the same plate.

Little hard work

– Eg. Piston rings of I.C Engine

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Patterns are made larger then the required castings

– Purpose of compensating the metal shrinkage,

– to provide extra metal which is to be removed during machining,

– for easy with drawl of pattern from the mould.

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Shrinkage allowance

Machining or finish allowance

Draft or taper allowance

Distortion or camber allowance

Rapping or shake allowance

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1. SHRINKAGE ALLOWANCE

All most all cast metals shrink or contract volumetrically on cooling. The metal shrinkage is of two types:

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.

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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

The casting shown is to be made in cast iron using a wooden pattern. Assuming only shrinkage allowance, calculate the dimension of the pattern. All Dimensions are in Inches

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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.

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

During machining some of the metal is removed from the casting, for this purpose the pattern is made larger than the required casting.

The amount of finish allowance depends on the material of the casting, its size ,volume of production, method of moulding etc.

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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

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The casting shown is to be made in cast iron using a wooden pattern. Assuming only machining allowance, calculate the dimension of the pattern. All Dimensions are in Inches.

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The casting will distort or wrap during cooling if it has irregular shape, flat long casting surface or V shape and also all the parts do not shrink uniformly.

Due to distortion ,the casting will not get the required shape.

To avoid this ,the shape of the pattern is slightly bent into the opposite direction.

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To take the pattern out of mould cavity it is slightly rapped or shaked to detach it from the mould cavity.

To avoid this the pattern is made slightly smaller.

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Design of casting

Number of castings to be produced

Degree of accuracy and surface finish required

Shape, complexity and size of the castings

Castings or moulding methods adopted

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Advantages: Inexpensive Easily available in large quantities Easy to fabricate Light in weight They 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 Absorb 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.

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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.

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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

Advantages:

It can be easily worked by using wood working tools.

Intricate shapes can be cast without any difficulty.

It has high compressive strength.

Advantages:

Provide very good surface finish.

Impart 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.

It maintains shape at very high temperature

It makes a mould porous

It is inexpensive

It mainly consist of Refractory sand

Binder

Additives

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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.

GREEN SAND

DRY SAND

LEAN SAND

CO2 sand

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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.

1. The most popular clay types are: 2. Kaolinite or fire clay (Al2O3 2 SiO2 2 H2O) and

Bentonite (Al2O3 4 SiO2 nH2O) 3. Of the two the Bentonite can absorb more water

which increases its bonding power. 4. To bring the property of cohesiveness

To improve the properties like strength, refractoriness and permeability.

To give a good surface finish. • Sea Coal

• Silica flour

• sawdust

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REFRACTORINESS

POROSITY or PERMEABILITY

STRENGTH or COHESIVINESS

PLASTICITY or FLOWABILITY

FINENESS

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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 refractoriness.

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.

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.

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.

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.

MIXING OF SAND – Mixing of sand , Binder and moisture

TEMPERING OF SAND – Spraying of water

CONDITIONING OF SAND - controlling the moisture

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1. Moisture content test

2. Clay content test

3. Grain finenss test

4. Permeability test

5. Refractoriness test

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1. A Measured Quantity prepared sand is placed in the pan and is heated by an infrared heater bulb for 2 to 3 minutes.

2. The moisture in the moulding sand is thus evaporated.

3. Moulding sand is taken out of the pan and reweighed.

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The percentage of moisture can be calculated from the difference in the weights, of the original moist and the consequently dried sand samples.

Percentage of moisture content =

– ((W1-W2)/(W1) )%

– Where, W1-Weight of the sand before drying,

– W2-Weight of the sand after drying

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Clay influences strength, permeability and other moulding properties. It is responsible for bonding sand particles together.

Procedures are:

1. Small quantity of prepared moulding sand was dried

2. Separate 50 gms of dry moulding sand and transfer wash bottle.

3. Add 475cc of distilled water + 25cc of a NaOH.

4. Agitate this mixture about 10 minutes with the help of sand stirrer.

5.Fill the wash bottle with water up to the marker.

6. After the sand etc., has settled for about 10 minutes, Siphon out the water from the wash bottle.

7. Dry the settled down sand.

8. The clay content can be determined from the difference in weights of the initial and final sand samples.

• Percentage of clay content = (W1‐W2)/(W1) * 100

• Where, W1‐Weight of the sand before drying,

• W2‐Weight of the sand after drying

The grain size, distribution, grain fitness are determined with the help of the fitness testing of moulding sands. The apparatus consists of a number of standard sieves mounted one above the other, on a power driven shaker

Grain fineness test:

• The shaker vibrates the sieves and the sand placed on the top sieve gets screened and collects on different sieves depending upon the various sizes of grains present in the moulding sand.

• The top sieve is coarsest and the bottom‐most sieve is the finest of all the sieves. In between sieve are placed in order of fineness from top to bottom.

AFS grain Fineness number

= ∑ Wi Fi/ ∑ Fi

Wi =Multiplication factor

Fi = Retained sample

The quantity of air that will pass through a standard

specimen of the sand at a particular pressure condition is called the permeability of the sand

An inverted bell jar, which floats in a water.

Specimen tube, for the purpose of hold the equipment

A manometer (measure the air pressure)

1. The air (2000cc volume) held in the bell jar is forced to pass through the sand specimen.

2. At this time air entering the specimen equal to the air escaped through the specimen

3. Take the pressure reading in the manometer.

4. Note the time required for 2000cc of air to pass the sand

5. Calculate the permeability number

6. Permeability number (N) = ((V x H) / (A x P x T))

Where,

V-Volume of air (cc)

H-Height of the specimen (mm)

A-Area of the specimen (mm2)

P-Air pressure (gm / cm2)

T-Time taken by the air to pass through the sand (seconds)

The refractoriness is used to measure the ability of to sand to with stand the higher temperature.

Steps involved are:

1. Prepare a cylindrical specimen of sand

2. Heating the specimen at 1500 C for 2 hours

3. Observe the changes in dimension and appearance

4. If the sand is good, it retains specimen share and shows very little expansion. If the sand is poor, specimen will shrink and distort

MOULDING TOOLS

• SHOVEL

• RIDDLE

• RAMMER

• TROWEL

• SLICK

• STRIKE OFF BAR

• LIFTER

• VENT WIRE

• SPRUCE PIN

• RISER PIN

• GATE CUTTER

• DRAW SPIKE

• SWAP

• MALLET

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Ramming the moulding sand

Rapping the pattern fro easy removal

Removing the pattern from the sand

JOLTING MACHINE

SQUEEZING MACHINE

SAND SLINGER

JOLTING MACHINE • Pattern is placed in

the flask on the table

• The table with flask is raised about 80mm and suddenly dropped

• It is mainly used for horizontal surface

• Operations is noisy because of jolting

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SQUEEZING MACHINE • Moulding sand in the flask

is squeezed between the machine table and squeezer head

• The mould board is clamped on the table

• The flask is placed on the mould board

• The pattern is placed inside the disk

• The table is raised by table left mechanism against the squeezer head

• After the squeezing is over, the table comes down to the starting position

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SAND SLINGER • Pattern is placed on a

board • Flask is placed on it • The slinger has an

impeller rotates at different speeds

• It throws a stream of sand at great velocity into the flask

• Slinger can be moved to pack the sand uniformly around the pattern.

• Ramming will be uniform with good strength

• Used for large and medium size moulds

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A core is a body made of refractory material which is set in to the prepared mould before closing and pouring it, for forming through holes, projections and internal cavities.

CORE MAKING MATERIALS

Core Sand

Refractories like silica sand ,zircon, liven etc

• Binders

• Vegetable oil, core flour, resins water, fire clay, urea.

• Additives

• Wood flour, coal powder, seal coal, graphite etc.,

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Sand core is usually made of river sand mixed with a binder.

Sand is weighed and put into the Muller.

Dry binders are filled in the Muller.

Muller is started and allowed to work for a little time.

Weight quantity of water is added to dry mixture.

After some time , the binder is added.

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CORE BOXES • HALF CORE BOX

• DUMP OR SLAB CORE BOX

• SPLIT CORE BOX

• STRICKLE CORE BOX

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• HALF CORE BOX

• Used to make one half of the symmetrical core pieces

• After baking, two core pieces will be pasted to form the full core

• DUMP OR SLAB CORE BOX

• Used for making a full core

• Used for making slab or rectangular cores

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CORE BOXES • SPLIT CORE BOX

• Two similar half boxes

• Box is assembled in correct position by dowel pins before filling the sand

• Boxes are separated after ramming the sand

• STRICKLE CORE BOX

• Dump core box is filled up with core sand

• Strickle board is pressed and swiped to form the profile

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Ovens Are Used For Heating The Cores To Obtain The Hardness

BATCH TYPE OVENS

CONTINUOUS TYPE OVENS

DIELECTRIC BAKING OVENS

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BATH TYPE OVENS • Small and medium

cores are baked

• These are fired with coal or oil

• It has several drawers

• Each drawer is loaded with batch of cores

• Cores are heated batch by batch

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CONTINUOUS TYPE OVEN • Heating is done continuously

• Cores are loaded on a conveyor at one side of the oven and it moves slowly inside the oven

• Cores are heated ,after that unloaded from the other side

• Heating time is controlled by conveyor speed

• It is suited for mass production

• The temperature of the oven varies from 2500 c to 2700c

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DIELECTRIC BAKING OVEN

• Two parallel electrodes

• Cores are placed in between electrodes

• High frequency current is supplied to the electrodes to heat the electrodes uniformly

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According to the state of core

Green sand core

Dry sand core

According to the position of the core

Horizontal core

Vertical core

Balanced core

Hanging core

Drop core

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• GREEN SAND CORE It is formed by pattern itself

Green sand core is made out of the same sand from which the rest of the mould has been made

• DRY SAND CORE It is made separately and

positioned in the mould

• HORIZONTAL CORE Placed horizontally in the mould

It may have any shape depending on the design

It is supported in the core seats at the ends

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• VERTICAL CORE • It is positioned vertically in

the mould • Ends of the cores rest on

core seats in core and drag • HANGING CORE • These are supported above

and hang into the mould • No support from the

bottom

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• BALANCING CORE • Core is supported and

balanced from its end • It requires a long core seat

so that core does not sag or fall into the mould

• It is used when the blind holes along a horizontal axis are to be produced

• DROP CORE • This core is used when a

hole is not in line with the parting surface is to be produced at a lower surface

• Hole may be above or below the parting line of the mould

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Melting is an equally important parameter for obtaining a quality castings. A number of furnaces can be used for melting the metal, to be used, to make a metal casting. The choice of furnace depends on the type of metal to be melted. Some of the furnaces used in metal casting are as following:.

– Crucible furnaces

– Cupola

– Induction furnace

– Rotatory

– Reverberatory furnace

ADVANTAGES OF CUPOLA

Simple design and easier construction

Low initial cost as compared to other furnaces of same capacity

Simple to operate and maintain in good condition

Less floor space requirements as compared to those of other furnaces of capacity

Cupola can be continuously operated for many hours.

Electric Power is not required.

Offer very high melting rate ( 1 to 35 tons per hour)

High Power Consumption

High installation costs

Shell casting

Investment casting

Ceramic mould

Lost Wax process

Pressure die casting

Centrifugal casting

CO2process