Manufacturing Technology II (ME-202)

Overview of Casting

Processes

Dr. Chaitanya Sharma

PhD. IIT Roorkee

Title of slide

Lesson Objectives In this chapter we shall discuss the following:

Learning Activities 1. Look up

Keywords 2. View Slides; 3. Read Notes, 4. Listen to

lecture

Keywords:

Casting

• Most widely used casting process, accounting for a significant majority of total tonnage cast

• Nearly all alloys can be sand casted, including metals with high melting temperatures, such as steel, nickel, and titanium

• Castings range in size from small to very large

• Production quantities from one to millions

Fig: A large sand casting weighing over 680 kg

for an air compressor frame

Casting Flow Diagram

Steps in Sand Casting

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 casting 5. Heat treatment

Pattern Making

• In pattern making, a physical model of casting, i.e. a pattern

is used to make the mold.

• 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• After the pattern is withdrawn, its imprint leaves the mold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 casting.

• In case, the castings is required to be hollow, such as in the

case of pipe fittings, additional patterns, known as cores,

are used to develop these cavities.

Making the Sand Mold

• The cavity in the sand mold is formed by packing sand around a pattern, then separating the mold into two halves and removing the pattern.

• The mold must also contain gating and riser system • If casting is to have internal surfaces, a core must be

included in mold.

•A new sand mold must be made for each part produced

Melting and Pouring

• Melting is a process of preparing the molten material for casting.

• It is generally done in a specifically designated part of foundry,

• The molten metal is transported to the pouring area wherein the molds are filled.

Two Categories of Casting Processes

1. Expendable mold processes – sand mold is sacrificed to remove part – Advantage: more complex shapes possible

– Disadvantage: production rates often limited by time to make mold rather than casting itself

2. Permanent mold processes - mold is made of metal and can be used to make many castings – Advantage: higher production rates

– Disadvantage: geometries limited by need to open mold

The Pattern

• A full-sized model of the part, slightly enlarged to account for shrinkage and machining allowances in the casting.

• Pattern materials: – Wood - common material because it is easy to work, but it warps

– Metal - more expensive to make, but lasts much longer

– Plastic - compromise between wood and metal

Top center is the clay original, then the two part

plaster mold used for casting the lead at above, and

wax cast from mold, sprued for better brass casting,

not yet cast.

homepages.waymark.net/mikefirth/tapper6881b.jpg

Types of Patterns

Types of patterns used in sand casting are shown below:

– Solid pattern

– Split pattern

– Match-plate pattern

– Cope and drag pattern

Pattern Allowances

• Pattern is having different size as compared to casting because it carries certain allowances due to metallurgical and mechanical reasons.

• The various allowances are as follows:

1. Shrinkage or contraction allowance.v

2. Machining or finish allowance.

3. Draft or taper allowance.

4. Distortion or camber allowance.

5. Shake or rapping allowance (-).

6. Mould wall movement allowance (-).

Moulds

• Mould or Mould cavity contains molten metal and is essentially a negative of the final product.

• Mould is obtained by pattern in moulding material (sand).

• Mould material should posses refractory characteristics and withstand the pouring temperature.

Types of Moulds

Basically moulds are two types:

1. Expendable moulds-

– are made of sand and is used for single casting which break upon solidification.

2. Permanent moulds-

– are made of metal or graphite (costly) and used repeatedly for large number of castings which do not break upon solidification.

Fig. Expendable moulds

Types of Sand Mold

• Green-sand molds : mixture of sand, clay, and water;

– “Green" means mold contains moisture at time of pouring

• Dry-sand mold - organic binders rather than clay

– And mold is baked to improve strength

• Skin-dried mold - mold cavity surface of a green-sand mold is dried to a depth of 10 to 25 mm.

Open mould

closed mould

Moulding

• Moulding is the process of making sound mould of sand by means of pattern.

Types of moulding:

1. Hand moulding-

are used for odd castings generally less than 50 no. and ramming is done by hands which takes more time.

2. Machine moulding-

are used for simple castings to be produced in large numbers. Ramming is done by machine so require less time.

Desirable Mold Properties

• Strength - to maintain shape and resist erosion

• Permeability - to allow hot air and gases to pass

through voids in sand

• Thermal stability - to resist cracking on contact with

molten metal

• Collapsibility - ability to give way and allow casting to

shrink without cracking the casting

• Reusability - can sand from broken mold be reused to

make other molds?

Finer the grain

low is the permeability

Addition of water

increases permeability upto a limit

Fig (a) Effect of grain size permeability

Fig (b) Water content on permeability

Material used for making green sand moulds consists following: 1. Sand (70-85%): to provide refractoriness

2. Clay (10-20%): to act as binder, along with water, impart tensile and shear strength to the molding sand

3. Water (3-6%): to activate the clay and provide plasticity

4. Organic additives (1-6%): to enhance desired sand properties

• Moulding sand composition must be carefully controlled to assure

Satisfactory and consistent results.

• Good molding sand always represents a compromise between

conflicting factors such as: – Size of sand particles, Amount of bonding agent (such as clay),

Moisture content, Organic matter

Composition Of Moulding Sand

Constituent Of Moulding Sand CLAY

Clay is generally used as binding agent in the molding sand to provide the strength, because of its low cost and wider utility.

The most popular types of clay used are: 1. Kaolinite or fire clay (melting point: range of 1750 to 1787°c )

2. Bentonite (melting point: range of 1250 to 1300 0c), two types

3. Sodium bentonite or western bentonite

4. Calcium bentonite or southern bentonite

Bentonite can absorb more water which increases its bonding

power.

Sodium bentonites produce better swelling properties (volume

increases some 10 to 20 times), high dry strength which lowers the

risk of erosion, better tolerance of variations in water content, low

green strength and high resistance to burnout which reduces clay

consumption.

• Water activates clay so that it develops the necessary plasticity and strength.

• Amount of water used should be properly controlled.

• Water in molding sand is often referred as “tempering” water.

• Water in excess -------- free water

• A part of the water absorbed by clay helps in bonding while the remainder up to a limit helps in improving the plasticity.

• Excessive water decreases the strength, permeability and formability.

• Normal percentages of water used are from 2 to 8%.

Constituent of Moulding Sand Water

Fig. Effect of Water content on (a) sand properties (b) green strength

• Additives are added to sand to enhance the specific properties.

• Since molding material is often reclaimed and recycled, the temperature of the mold during pouring and solidification is also important.

• If organic materials are added to provide collapsibility, a portion will bum during the pour. Some of the mold material may have to be discarded and replaced with new one.

Constituent 0f Moulding Sand Additives

Variables Affecting Molding Sand Properties

Following are the main variable:

1. Sand grain shape and size

2. Clay and water

3. Method of preparing sand mold

• Coarse grains: good permeability and better refractoriness.

• Finer grains: lower permeability but better surface finish.

• Purity of sand grains improves the refractoriness.

• Uniform-size grains: good permeability, while a wide distribution of sizes enhances surface finish.

• Round grains: good permeability and require less amount of clay.

• Angular grains: better green strength.

Sand Grain Shape And Size

Clay and Water

• An optimum amount of water is to be used for a given clay content to obtain maximum green compression strength.

• During the sand preparation clay is uniformly coated around the sand grains.

• Water then reacts with the clay and forms a linkage of silica-water-clay-water-silica throughout moulding sand.

• Any additional amount of water increases the plasticity and dry strength but reduces the green compression strength.

Degree of ramming increases the bulk density.

Increased ramming increases the strength.

Permeability of green sand decreases with degree of

ramming.

Machine moulding provide better and uniform density.

Sling moulding is better than jolt moulding

Method of Preparing Sand Mold

Effect of Moisture, Grain Size And Shape On Mould Quality

CORES

• Full-scale model of interior surfaces of part. • It is inserted into the mold cavity prior to pouring • The molten metal flows and solidifies between the mold cavity. • Core forms the casting's external and internal surfaces. • Cores may require supports to hold it in position in the mold

cavity during pouring.

Fig: (a) Core held in place in the mold cavity by chaplets, (b) possible chaplet design, (c)

casting with internal cavity.

Core Parts

• A core consists of two portions:

– The body of the core and

– one or more extensions called prints

• The body of the core is surrounded by molten metal

during casting process.

• Body of core has all the features which are required in

final internal surface (e.g. hole) of the castings.

• The prints are necessary to support the core in the

mould.

• They also conduct the heat (and gases produced by a sand

core) to the mould.

Core, Core Print & Core Box

CORE: sand body that is

inserted into the mold to

produce the internal features

of a casting, e.g. holes.

CORE PRINT: region added to

the pattern, core, or mold which

is used to locate and support

the core within mold

CORE BOX: the mold or die

used to produce casting cores

Essential Characteristics of Core (Sand)

A good core must possess followings:

High permeability to allow an easy escape to gases formed.

High refractoriness to withstand high temperature of

molten metal

Smooth surface.

High collapsibility i.e. it should be able to disintegrate

quickly after the solidification of the metal is complete.

Sufficient strength to support itself.

Functions (Purposes) of Cores

Cores are required for following :

• The cores are used to form the internal cavities.

• Cores are used to form a part of a green sand mould.

• Cores are used to strengthen the moulds.

• Cores are used as a part of the gating system.

Post Solidification Operations

In general following operation are performed on castings:

1. Trimming: Removal of sprues, runners, risers, parting-line flash, fins, chaplets, and any other excess metal.

2. Removing core: Cores are fall out own, as the binder deteriorates or are removed by shaking casting or dissolved chemically.

3. Surface cleaning: Casting surface are cleaned by tumbling, wire brushing, buffing, and chemical pickling etc. to enhance surface appearance and detect defects.

4. Inspection: To detect defects & assure quality of castings

5. Repair : to fix general and casting related defects.

6. Heat treatment : Castings are often heat treated to enhance properties and relieving stresses.

• Discontinuities in castings that exhibit a size, shape, orientation, or location that makes them detrimental to the useful service life of the casting

• Some casting defects are remedied by minor repair or refurbishing techniques, such as welding

• Other casting defects are cause for rejection of the casting.

Casting Defects

• Fins are excessive amounts of metal created by solidification into the parting line of the mold.

Fins are removed by grinding or sandblasting.

• Swells are excessive amounts of metal in the vicinity of gates or beneath the sprue.

• Scabs are surface slivers caused by splashing and rapid solidification of the metal when it is first poured and strikes the mold wall.

Casting Defects: Metallic Projections

– Blowholes and pinholes are holes formed by gas entrapped during solidification

– Shrinkage cavities are caused by lack of proper feeding or non-progressive solidification and have a rougher shape.

Porosity is pockets of gas inside the metal caused by micro-shrinkage, e.g. dendritic shrinkage during solidification.

Casting Defects: Cavities

• Cracks in casting and are caused by hot tearing, hot cracking, and lack of fusion (cold shut)

– A hot tear is a fracture formed during solidification because of hindered contraction

– A hot crack is a crack formed during cooling after solidification because of internal stresses developed in the casting

– Lack of fusion is a discontinuity caused when two streams of liquid in the solidifying casting meet but fail to unite

Rounded edges indicate poor contact between various metal streams during filling of the mold

Casting Defects: Discontinuities

• Casting surface irregularities that are caused by incipient freezing from too low a casting temperature

• Wrinkles, depressions and adhering sand particles

Casting Defects: Defective Surfaces

• Particles of foreign material in the metal matrix

• The particles are usually nonmetallic compounds but may be any substance that is not soluble in the matrix

– Slag, dross, and flux inclusions arise from melting slags, products of metal treatment, or fluxes

They are often deep within the casting

– Mold or core inclusions come from sand or mold dressings and are usually found close to the surface

Casting Defects: Inclusions

Gating System

• Gating systems refer to all those elements which are connected with the flow of molten metal from the ladle to the mould cavity.

• Following are the elements of gating systems:

1. Pouring Basin

2. Sprue

3. Sprue Base Well

4. Runner

5. Runner Extension

6. Gate or Ingate

7. Riser

Objective of The Gating System

Four main points, which enables a proper gating system, are:

1. Clean molten metal.

2. Smooth filling of the casting cavity.

3. Uniform filling of the casting cavity.

4. Complete filling of the casting cavity.

• The mold cavity must be filled with a clean metal so that it prevents the entry of slag and inclusions into the mold cavity, which in turn minimizes the surface instability.

• If the mold has smooth filling then it helps to reduce the bulk turbulence. If it has a uniform filling it means that the casting fill is in a controlled manner.

• Complete filling of the cavity makes the metal thin with minimum resistance at the end sections.

Factor Affecting The Performance Of Gating System

• To achieve sound casting and other objectives following factors should be controlled properly:

1. The type of ladle and ladle equipment.

2. The size, type, and location of sprue and runner.

3. The size, number & location of gates entering mold cavity.

4. The rate of pouring.

5. The position of the mold during casting.

6. The temperature and fluidity of the metal.

Elements of Gating System

1. Pouring basin : collects the molten metal, which is poured, from the ladle.

2. Sprue : leads the molten metal from the pouring basin to the sprue well.

3. Sprue Well : It changes the direction of flow of the molten metal to right angle and passes it to the runner.

4. Runner : takes the molten metal from sprue to the casting.

5. Ingate: moves molten metal from runner to the mold cavity.

6. Slag trap : It filters the slag when the molten metal moves from the runner and ingate.

ELEMENTS OF GATING SYSTEM Pouring basin

• A reservoir for the molten metal poured from the ladle.

• This is otherwise called as bush or cup.

• It is circular or rectangular in shape.

• It collects the molten metal, which is poured, from the ladle.

• It prevent the mould erosion.

• Prevent slag and other impurities from entering the mould cavity.

ELEMENTS OF GATING SYSTEM

• Sprue : It is circular in cross section. It leads the molten metal from the pouring basin to the sprue well.

• Sprue Well : It changes the direction of flow of the molten metal to right angle and passes it to the

runner.

ELEMENTS OF GATING SYSTEM

• Slag trap : It filters the slag when the molten metal moves from the runner and ingate.

• It is also placed in the runner.

ELEMENTS OF GATING SYSTEM

• Runner : The runner takes the molten metal from sprue to the Ingates of casting.

• This is the final stage where the molten metal moves from the runner to the mold cavity.

Types of Runners

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TYPES OF GATES

1. Horizontal Gating System : This is used most widely. This type is normally applied in ferrous metal's sand casting and gravity die-casting of non-ferrous metals. They are used for flat casting, which are filled under gravity.

2. Vertical Gating System : This is applied in tall castings were high-pressure sand mold, shell mold and die-casting processes are done.

3. Top Gating System : this is applied in places where the hot metal is poured form the top of the casting. It helps directional solidification of the casting from top to bottom. It suits only flat castings to limit the damage of the metal during the initial filling.

4. Bottom Gating System : it is used in tall castings where the molten metal enters the casting through the bottom.

5. Middle Gating System : It has the characteristics of both the top and bottom

GATES OR INGATES

• Top gate

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Fluid Flow & Solidification Time

Sprue design

A1

A2

h2

h1

Mass continuity

Q A1v1 A2v2

Bernoulli’s theorem

hp

gv2

2g constant

Reynolds number

Re vD

Chvorinov’s Rule

Solidification time =CVolume

Surface Area

n

Casting Design

1

2

111 F

g

vPh

2

2

222 F

g

vPh

v2A2

v1A1

)( ossc TTAhq

Solidification of Casting

•During solidification metal experience shrinkage which results in void formation. •This can be avoided by feeding hot spot during solidification. •Riser are used to feed casting during solidification.

Solidification Time For Casting

Methods Of Riser Design

• Following are the methods for riser design:

1. Caines Method

2. Modulus Method

3. NRL Method

CAINES METHOD

• Caines equation

Where X = Freezing ratio Y = Riser volume / Casting volume A, b and c = Constant

Freezing ratio

Constant For Caines Method

Example

Modulus Method

• Modulus is the inverse of the cooling characteristic ( surface area/ Volume) and is defined as

Modulus = Volume / Surface area

• In steel casting riser with height to diameter ration of 1 is generally used.

• Volume of cylindrical riser =

• Surface area =

• For sound casting modulus of riser shoud be greater than the modulus of casting by a factor of 1.2. Therefore

Mr = 1.2 Mc

• On simplification D =6 Mc

• Considering contraction of metal

MO

DU

LI

OF

SIM

PL

E S

HA

PE

S

NRL METHOD

NRL method is essentially a simplification of Caine’s method,

defines a shape factor to replace freezing ratio

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CHILLS

• Chills are pieces of material placed in the mold to speed up heat transfer in thicker areas of the part to prevent shrinkage porosity

• Internal chills are left within the cast part; external chills are removed

Chills

FIGURE 5.35 Various types of (a) internal and (b) external chills (dark areas at corners), used in castings to eliminate porosity caused by shrinkage. Chills are placed in regions where there is a larger volume of metal, as shown in (c).