MODULE -I

ME 220 MANUFACTURING TECHNOLOGY

Casting

• Casting is one of the oldest manufacturing process to shape

metals into useful products

• Casting involves pouring molten metal into a mould cavity,

upon solidification, the metal takes the shape of the cavity

• Casting first was used around 4000 B.C. to make ornaments,

arrowheads, and various other objects

• A Wide variety of products can be cast, and the process is

capable of producing intricate shapes in one piece, including

those with internal cavities, such as engine blocks

Outline of Metal Casting Process

Why Casting?

• Casting can produce complex shapes and can incorporate

internal cavities or hollow sections

• Very large parts can be produced in one piece

• Casting can utilize materials that are difficult or uneconomical

to process by other means

• The casting process can be economically competitive with

other manufacturing processes

Sand Casting

Sand casting consists of:

(a)Placing a pattern (having the shape of the desired casting) in

sand to make an imprint

(b)Incorporating a gating system

(c)Removing the pattern and filling the mould cavity with

molten metal

(d)Allowing the metal to cool until it solidifies

(e)Breaking away the sand mould

(f) Removing the casting

Steps in Sand Casting

Features of Mould in Sand Casting

Features of Mould in Sand Casting

• The flask supports the mould itself. Two-piece moulds consist of a cope ontop and a drag on the bottom; the seam between them is the parting line.

• When more than two pieces are used in a sand mould, the additional partsare called cheeks.

• Pouring basin or pouring cup: into which the molten metal is poured

• Sprue :through which the molten metal flows downward

• Runner: channel that carry the molten metal from the sprue to the mouldcavity.

• Gates are the inlets into the mould cavity.

• Riser : supply additional molten metal to the casting as it shrinksduring solidification

• Cores : They are placed in the mould to form hollow regions orotherwise define the interior surface of the casting

• Vents: which are placed in moulds to carry off gases produced whenthe molten metal comes into contact with the sand in the mould and thecore.

Sands

• Most sand-casting operations use silica sand (𝑆𝑖𝑂2) as themould material.

• Sand is inexpensive and is suitable as a mould materialbecause of its high-temperature characteristics and highmelting point

• There are two general types of sand: naturally bonded (banksand) and synthetic (lake sand).

• Sand having fine, round grains can be packed closely and,thus, forms a smooth mould surface and enhances mouldstrength

• Fine grains lower mould permeability (where fluids and gasespenetrate through pores)

• The mould also should have good collapsibility to allow thecasting to shrink while cooling and, thus, to avoid defects inthe casting

Types of Sand Moulds

There are three basic types of sand moulds:

1.Green-sand Mould :

• Mould is a mixture of sand, clay, and water.

• The term “green” refers to the fact sand in the mould ismoist or damp while the metal is being poured into it

• Green-sand moulding is the least expensive method ofmaking moulds

• Because of their higher strength, these moulds generallyare used for large castings

Types of Sand Moulds

2.Cold-box Mould :

• Various organic and inorganic binders are blended into the sand

to bond the grains chemically for greater strength

• Moulds are more dimensionally accurate than green-sand

moulds, but are more expensive.

3. No-bake mould :

• A synthetic liquid resin is mixed with the sand and the mixture

hardens at room temperature

• The bonding of the mould in this and in the cold-box process

takes place without heat, they are called cold-setting processes.

Patterns

• Pattern is a model of a casting, constructed in such a way that itcan be used for forming an impression(mould) in damp sand

• Patterns are used to mould the sand mixture into the shape of thecasting and may be made of wood, plastic, or metal

• The selection of a pattern material depends on the size andshape of the casting, the dimensional accuracy and the quantityof castings required, and the moulding process

• Patterns may be made of a combination of materials to reducewear in critical regions, and they usually are coated with aparting agent to facilitate the removal of the casting from themoulds.

Requirements of a Good Pattern

• Light in weight

• Simple in design and ease of manufacture

• Smooth and wear resistant surface

• Retain its dimensions and rigidity during the definite service life

• High strength and long life

• Ability to withstand rough handling

• Cheap and readily repairable

Types of Pattern

1. Single piece or solid pattern

2. Split pattern

3. Loose piece pattern

4. Match plate pattern

5. Gated pattern

6. Sweep pattern

7. Cope and drag pattern

8. Follow board pattern

Single Piece or Solid Pattern

Split Pattern

Loose Piece Pattern

Match Plate Pattern

Gated Pattern

Sweep pattern

Follow Board Pattern

Pattern Materials

Materials commonly used are:

1. Wood

2. Metal

3. Plastic

4. Wax

5. Quick setting compounds

Wood

• The wood used for pattern material should be properly dried and seasoned

• It should be straight grained

• It should be free from knots

• It should be free from insects and excessive sap wood

Types of wood commonly used for pattern making

• White pine

• Mahogany

• Mapple

• Teak

Metals

• Where durability and strength are required, patterns are made from metals

• A metal pattern can be either cast from master wood pattern or be machined by the methods of machining

• Metal patterns are used in machine moulding

Metals commonly used for pattern making

• Aluminium

• Brass

• Whit metal

• Cast Iron

Plastic

• Plastic patterns are highly resistant to corrosion

• Strong and dimensionally stable

• Surface of the pattern is smooth

Plastics employed for pattern making

• Phenol formaldehyde

• Polyester resin

• Ploy acrylates

• Polyethylene

• Poly vinyl chloride

Quick Setting Compounds

• Gypsum patterns are capable of producing castings with

intricate details and to very close tolerances

• Gypsum can be easily formed , has plasticity and can be easily

repaired

Pattern Allowances

• While making patterns certain dimensional allowances must be

given in the pattern so that casting obtained is of required

specification.

• The allowances usually provided in a pattern are

1. Shrinkage allowance

2. Draft or taper allowance

3. Machining allowance

4. Rapping or shaking allowance

5. Distortion allowance

Pattern Allowances (Cont…)

1. Shrinkage allowance: An allowance added to the pattern to compensate for the metal shrinkage that takes place while the metal solidifies

2. Draft or taper allowance: It is the taper provided on the verticalsurface of a pattern to facilities its removal from mould withoutexcessive rapping or breakage

3. Machining allowance :Machining or finishing allowance is the extramaterial provided on certain details of casting may be machined toexact dimensions

4. Rapping or Shaking allowance: This allowance is provided for theenlargement of the mould cavity because of excessive rapping

5. Distortion allowance : This allowance is provided on the pattern tocompensate for possible distortion of casting because of the unequalcooling rates at different sections of casting and uneven internalstresses

Cores

Cores

• For castings with internal cavities or passages, such as thosefound in an automotive engine block or a valve body, cores areutilized.

• Cores are placed in the mould cavity to form the interiorsurfaces of the casting and are removed from the finished partduring shakeout and further processing

• Cores must possess strength, permeability, the ability towithstand heat, and collapsibility

• Cores are anchored by core prints

• To keep the core from shifting, metal supports (chaplets) maybe used to anchor the core in place

Sand-moulding Machines

When large number of castings is to be produced, hand moulding

consumes more time, labour and also accuracy and uniformity in

moulding varies.

To overcome this difficulty, machines are used for moulding.

Based on the methods of ramming, moulding machines are classified

as follows:

1.Jolt moulding machine

2.Squeeze moulding machine

3.Jolt-squeeze moulding machine

4.Sand slinger

Jolt Moulding Machines

Jolt Moulding Machines

• A jolt machine consists of a flat table mounted on a piston-cylinderarrangement and can be raised or lowered by means of compressedair.

• In operation, the mould box with the pattern and sand is placed onthe table. The table is raised to a short distance and then droppeddown under the influence of gravity against a solid bed plate. Theaction of raising and dropping (lowering) is called 'Jolting'.

• Jolting causes the sand particles to get packed tightly above andaround the pattern. The number of 'jolts' may vary depending on thesize and hardness of the mould required. Usually, less than 20 joltsare sufficient for a good moulding.

• The disadvantage of this type is that, the density and hardness of therammed sand at the top of the mould box is less when compared toits bottom portions.

Squeeze Moulding Machine

Squeeze Moulding Machine

• In squeeze machine, the mould box with pattern and sand in it is

placed on a fixed table as shown in figure

• A flat plate or a rubber diaphragm is brought in contact with the upper

surface of the loose sand and pressure is applied by a pneumatically

operated piston.

• The squeezing action of the plate causes the sand particles to get

packed tightly above and around the pattern.

• Squeezing is continued until the mould attains the desired density.

• In some machines, the squeeze plate may be stationary with the mould

box moving upward.

• The disadvantage of squeeze machine is that, the density and hardness

of the rammed sand at the bottom of the mould box is less when

compared to its top portions.

Jolt Squeeze Moulding Machine

Jolt Squeeze Moulding Machine

• Jolt squeeze machine combines the operating principles of 'jolt' and 'squeeze'machines resulting in uniform ramming of the sand in all portions of the moulds

• The machine makes use of a match plate pattern placed between the cope and thedrag box.

• The whole assembly is placed on the table with the drag box on it.

• The table is actuated by two pistons in air cylinders, one inside the other. Onepiston called 'Jolt piston' raises and drops the table repeatedly for a predeterminednumber of times, while the other piston called 'squeeze piston' pushes the tableupward to squeeze the sand in the flask against the squeeze plate. In operation, sandis filled in the drag box and jolted repeatedly by operating the jolt piston.

• After jolting, the complete mould assembly is rolled over by hand.

• The cope is now filled with sand and by operating the squeeze piston, the mouldassembly is raised against the squeeze plate. By the end of this operation, the sandin the mould box is uniformly packed.

• The match plate is now vibrated and removed. The mould is finished and madeready for pouring.

Sand Slinger

Sand Slinger

• A sand slinger is an automatic machine equipped with a unit that

throws sand rapidly and with great force into the mould box. Figure

shows a sand slinger. Sand slinger consists of a rigid base, sand bin,

bucket elevator, belt conveyor, ramming head (sand impeller) and a

swinging arm.

• In operation, the pre-mixed sand mixture from the sand bin is picked

by the bucket elevator and is dropped on to the belt conveyor.

• The conveyor carries the sand to the ramming head, inside which there

is a rotating impeller having cup shaped blades rotating at high speeds

(around 1800 rpm).

Shell Mould Casting

Shell Mould Casting

• Shell-mould casting can produce many types of castings with closedimensional tolerance and good surface finish a low cost

a)A mounted pattern made of a ferrous metal or aluminium is

heated to 175 ̊ C -370 ̊ C

b) Coated with a parting agent such as silicone

c) Clamped to a box or chamber

• The box contain fine sand, mixed with 2.5% to 4% thermosetting resinbinder, such as phenol-formaldehyde that coats the sand particles

• The box is either rotated upside down or the sand is blown over thepattern , allowing it to coat the pattern

• The assembly is then placed in an oven for a short period of time tocomplete the curing of the resin

• The shell hardens around the pattern and is removed from the patternusing built-in –ejector pin

Ceramic Mould Casting

Ceramic Mould Casting

• Also called cope and drag investment casting

• Uses refractory mould materials suitable for high temperatureapplications

• The slurry is a mixture of fine grained Zircon (ZrSi𝑂4 ),aluminium oxide and fused silica, which are mixed withbonding agent and poured over the pattern, which has beenplaced in a flask

• The pattern may be made of wood or metal

• After setting , moulds are removed, dried, burned off to removevolatile matter and baked

Investment Casting

Investment Casting

• Also called lost wax process

• The pattern made of wax or plastic

• The pattern is made by injecting molten wax or plastic into a

metal die in the shape of the pattern

• The pattern is then dipped into a slurry of refractory material

such as fine silica and binder(such as water, ethyl silicate and

acids)

• After tis initial coating has dried, the pattern is coated

repeatedly to increase its thickness

• The term investment derives from the fact that the pattern is

invested with the refractory material

Investment Casting (Cont…)

• The one-piece mould is dried in air and heated up to a

temperature of 90̊ C to 175̊ C

• It is held in an inverted position for about 12 hours to melt out

wax

• The mould is then fired to 650̊ C to 1050̊ C for about 4 hours

depends on metal to be cast

• After the metal has been poured and has solidified, the mould is

then broken up and the casting is removed

• Number of patterns can be joined to make one mould, called a

tree

Vacuum Casting

Vacuum Casting

• A mixture of fine sand and urethane is moulded over metal dies

and cured with amine vapour

• The mould is then held with a robot arm and immersed partially

into molten metal held in an induction furnace

• The vacuum reduces the air pressure inside the mould to about

two-thirds of atmospheric pressure, thus drawing the molten

metal into the mould cavities through a gate in the bottom of the

mould.

• It begins to solidify within a very short time.

• After the mould is filled, it is withdrawn from the molten metal

Slush Casting

Slush Casting

• Solidified skin develops in a casting and becomes thicker withtime

• Hollow castings with thin walls can be made by permanent-mould casting using this principle: a process called slushcasting.

• This process is suitable for small production runs and generallyis used for making ornamental and decorative objects (such aslamp bases and stems) and toys from low-melting-point metalssuch as zinc, tin, and lead alloys.

• The molten metal is poured into the metal mould.

• After the desired thickness of solidified skin is obtained, themould is inverted (or slung) and the remaining liquid metal ispoured out.

Pressure Casting

Pressure Casting

Pressure Casting

• In pressure casting (also called pressure pouring or low

pressure casting), the molten metal is forced upward by gas

pressure into a graphite or metal mould

• The pressure is maintained until the metal has solidified

completely in the mould

• The molten metal also may be forced upward by a vacuum,

which also removes dissolved gases and produces a casting

with lower porosity

• Pressure casting generally is used for high-quality castings,

such as steel railroad-car wheels

Hot-Chamber Die Casting

Hot-Chamber Die Casting

• A piston a certain volume of metal into the die cavity through a

gooseneck and nozzle.

• Pressures range up to 35 MPa, with an average of about 15 MPa

• The metal is held under pressure until it solidifies in the die.

• To improve die life and to aid in rapid metal cooling (thereby

reducing cycle time) dies usually are cooled by circulating water

or oil through various passageways in the die block

• Low-melting-point alloys (such as zinc, magnesium, tin, and

lead) commonly are cast using this process

Cold-Chamber Die Casting

Cold-Chamber Die Casting

• Molten metal is poured into the injection cylinder (shot

chamber).

• The chamber is not heated-hence the term cold chamber.

• The metal is forced into the die cavity at pressures usually

ranging from 20 to 70 MPa, although they may be as high as

150 MPa

Centrifugal Casting

Centrifugal Casting

• The centrifugal-casting process utilizes inertial forces (causedby rotation) to distribute the molten metal into the mouldcavities

• In true centrifugal casting, hollow cylindrical parts (such aspipes, gun barrels, bushings, engine-cylinder liners, bearingrings with or without flanges, and street lampposts) areproduced

• Molten metal is poured into a rotating mould

• The axis of rotation is usually horizontal, but can be vertical forshort work pieces.

• Moulds are made of steel, iron, or graphite and may be coatedwith a refractory lining to increase mould life

Gating System

Gating System

• The molten metal is poured through a pouring basin or cup; it

then flows through the gating system (consisting of sprue,

runners, and gates) into the mould cavity.

• A good gating design ensures distribution of the metal in the

mould cavity at proper rate without excessive temperature loss,

turbulence and entrapping gases and slag

• The design of gating system depends on both the metal and

mould composition

Gate Ratio (Gating Ratio)

It is defined as the ratio of sprue area to total runner area to total

gate area i.e., Sprue area : Runner area : Gate area

Types of Gating System1. Pressurised (Choked) Gating System

In this system, the ingate serve as the choke

This system maintains a back pressure and causes the entire

gating system to be pressurised

In this system molten metal enters mould cavity uniformly

Typical gate ratio in this system 4: 3: 2

This system is adopted for metals like iron, steel, brass, etc.

Types of Gating System (Cont..)

2. Unpressurised Gating System

In this system sprue base serves as choke

The typical gating ratio in this system are 1:2:2, 1:2:4, 1:3:3,

1:4:4

Such a system is adopted for light oxidisable metals like

aluminium, magnesium, etc.

Gates

• Also called ingates, are the opening through which the molten

metal enters the mould cavity

• Depending on the application, various types of gates are used in

the casting design

1.Vertical Gate

2.Bottom Gate

3.Horizontal Gate

Gating Design-Vertical Gating

𝒗𝒈 = 𝟐𝒈𝒉𝒕

Where, 𝑣𝑔=Velocity of metal at gate

g = Acceleration due to gravity

ℎ𝑡=ℎ𝑐 + ℎ𝑠

ℎ𝑐= Pouring basin (cup) height

ℎ𝑠= Sprue height

𝒕𝒇 =𝑽

𝑨𝒈𝒗𝒈

Where, 𝑡𝑓 =Time taken to fill up the mould

𝐴𝑔=Cross sectional area of the

gate

V= Volume of the mould

Gating Design-Bottom Gating

𝒗𝒈 = 𝟐𝒈(𝒉𝒕 − 𝒉)

Where, 𝑣𝑔=Velocity of metal at gate

ℎ𝑡=ℎ𝑐 + ℎ𝑠

h= static head

ℎ𝑡-h= effective head

𝒕𝒇 =𝑨𝒎

𝑨𝒈

𝟏

𝟐𝒈𝟐( 𝒉𝒕 − (𝒉𝒕 − 𝒉) )

Where, 𝑡𝑓 =Time taken to fill up the mould

𝐴𝑔and 𝐴𝑚=Cross sectional area

of the gate and mould

Fluidity of Molten Metal

• The capability of molten metal to fill mould cavities iscalled fluidity

• It consists of two basic factors:

(1) characteristics of the molten metal and

(2) casting parameters

Characteristics of the Molten Metal

• Viscosity : As viscosity and its sensitivity to temperature (viscosity index)increase, fluidity decreases.

• Surface Tension: A high surface tension of the liquid metal reducesfluidity.

• Inclusions : Because they are insoluble, inclusions can have a significantadverse effect on fluidity

• Solidification Pattern of the Alloy : The manner in which solidification takes place can influence fluidity

Fluidity of Molten Metal (Cont…)

Casting Parameters

• Mould Design : The design and dimensions of the sprue, runners, andrisers all influence fluidity

• Mould Material and its Surface Characteristics : The higher thethermal conductivity of the mould and the rougher its surfaces, thelower the fluidity of the molten metal.

• Degree of Superheat : Superheat (defined as the increment oftemperature of an alloy above its melting point) improves fluidity bydelaying solidification

• Rate of Pouring : The slower the rate of pouring molten metal into themould, the lower the fluidity because of the higher rate of coolingwhen poured slowly

• Heat Transfer. This factor directly affects the viscosity of the liquidmetal

Solidification Time

• The solidification time is a function of the volume of a casting and its

surface area (Chvorinov’s rule):

𝑺𝒐𝒍𝒊𝒅𝒊𝒇𝒊𝒄𝒂𝒕𝒊𝒐𝒏 𝒕𝒊𝒎𝒆 = 𝑪𝑽𝒐𝒍𝒖𝒎𝒆

𝑺𝒖𝒓𝒇𝒂𝒄𝒆 𝑨𝒕𝒓𝒆𝒂

𝒏

• where C is a constant that reflects

(a) the mould material

(b) the metal properties (including latent heat), and

(c) the temperature

• The parameter n has a value between 1.5 and 2,

• But usually is taken as 2

Caine’s Method

• Method for determining riser size based on experimentallydetermined hyperbolic relationship between relative freezingtimes and volumes of casting and riser

• According to Caine if the casting solidifies infinitely rapid, theriser volume should be equal to solidification shrinkage and ifthe riser and casting solidify at the same rate, the riser should beinfinitely large

• Relative freezing time or freezing ratio (𝑅𝑓) is defined as

𝑅𝑓 =

𝐴𝑉

𝑐𝑎𝑠𝑡𝑖𝑛𝑔

𝐴𝑉

𝑟𝑖𝑠𝑒𝑟

Caine’s Method (Cont…)

• Volume ratio (𝑅𝑣) is given by

𝑅𝑣 =𝑉𝑟𝑖𝑠𝑒𝑟𝑉𝑐𝑎𝑠𝑡𝑖𝑛𝑔

Caine’s formula

𝑹𝒇 =α

𝑹𝒗−𝒃+ 𝒄

Where, α= Freezing characteristics constant for the metal

b= Contraction ratio from liquid to solid

c= Relative freezing rate of riser and casting

Defects in Castings

Defects in Castings (Cont…)

Defects in Castings (Cont…)

1. Metallic projections, consisting of fins, flash, or projections suchas swells and rough surfaces

2. Cavities, consisting of rounded or rough internal or exposedcavities including blowholes, pinholes, and shrinkage cavities

3. Discontinuities, such as cracks, cold or hot tearing, and cold shuts.

• If the solidifying metal is constrained from shrinking freely,cracking and tearing may occur

• Cold shut is an interface in a casting that lacks complete fusionbecause of the meeting of two streams of liquid metal fromdifferent gates.

4. Defective surface, such as surface folds, laps, scars, adhering sandlayers, and oxide scale.

Defects in Castings (Cont…)

5. Incomplete casting, such as misruns (due to premature

solidification), insufficient volume of the metal poured, and runout

(due to loss of metal)

6. Incorrect dimensions or shape, due to factors such as improper

shrinkage allowance, pattern-mounting error, irregular contraction,

deformed pattern, or warped casting

7. Inclusions, Which form during melting, solidification, and

moulding; these are generally non-metallic

Design Considerations for Castings

There are two types of design issues in casting:

(a) Geometric features, tolerances, etc., that should be incorporated

into the part

(b) Mould features that are needed to produce the desired casting

Steps involved in design of casting

1. Design the part so that the shape is cast easily

2. Select a casting process and a material suitable for the part, size, required production volume, mechanical properties, and so on.

3. Locate the parting line of the mould in the part.

4. Locate and design the gates to allow uniform feeding of the mould cavity with molten metal.

5. Select an appropriate runner geometry for the system.

6. Locate mould features, such as sprue, screens, and risers, as appropriate.

7. Make sure proper controls and good practices are in place.

Design Considerations for Castings

(Cont…)

• Corners, angles, and section thickness. Sharp corners, angles, and fillets should be avoided as much as possible, because they act as stress raisers and may cause cracking and tearing of the metal (as well as of the dies) during solidification

• Shrinkage. To avoid cracking of the casting during cooling, thereshould be allowances for shrinkage during solidification

• Draft. A small draft (taper) typically is provided in sand-mouldpatterns to enable removal of the pattern without damaging the mould

• Dimensional tolerances. Dimensional tolerances depend on theparticular casting process, size of the casting, and type of pattern used.

• Lettering and markings. It is common practice to include some formof part identification (such as lettering or corporate logos) in castings

• Finishing operations. In designing a casting, it is important toconsider the subsequent machining and finishing operations that maybe required.

Design Considerations for Castings

(Cont…)

• Selecting the Casting Process. Casting processes cannot be selectedseparately from economic considerations

• Locating the Parting Line. A part should be oriented in a mould so thatthe large portion of the casting is relatively low and the height of thecasting is minimized.

• Locating and Designing Gates. Gates are the connections between therunners and the part to be cast

• Multiple gates often are preferable and are necessary for large parts

• Gates should feed into thick sections of castings

• A fillet should be used where a gate meets a casting

• The gate closest to the sprue should be placed sufficiently far awayfrom the sprue so that the gate can be easily removed

• Runner Design. The runner is a horizontal distribution channel thataccepts molten metal from the sprue and delivers it to the gates.