Casting Fundamentals and Basics Concepts.ppt

7/27/2019 Casting Fundamentals and Basics Concepts.ppt

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2MODULE

COURSE OUTCOMES

Fundamentals of Casting process

Solidification of metals

Fluid flow of molten metal

Various casting process

Casting defects & quality


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:Casting / Foundry

Casting processes basically involve the introduction of a molten

metal into a mold cavity, where upon solidification, the metal

takes on the shape of the mold cavity.

Applications :

Cylinder blocks, liners, machine tool beds, pistons, piston rings,

mill rolls, wheels, housings, water supply pipes, bells


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Examples of Cast Parts

Crank handle formed by casting; some areas

were machined and assembled after casting


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C-clamps formed by casting (left) and machining

(right)


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Complex part formed by casting


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Aluminum piston for an internal

combustion engine:

(a) as-cast (b) after machining.


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: A molding flask is one which holds the sand mould intactFlask

: Lower molding flaskDrag

: Upper molding flaskCope

Pattern : It is a replica of final object to be made with somemodifications. The mold cavity is made with the help of pattern material

: It is the freshly prepared refractory material used forMolding sandmaking the mold cavity.

: It is used for making hollow cavities in the castings.Core

Core Print : A region used to support the core

: A small funnel shaped cavity at the top of the mold intoPouring basinmolten metal is poured.


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Mold

cavity

chaplet

Mold

cavity

chaplet


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Parting Line / Parting Surface : Interface that separates thecope and drag

: The channel through which the molten metal is carriedRunnerfrom the sprue to the gate.

Gate: A channel through which the molten metal enters the moldcavity

: Chaplets are used to support the cores inside the moldChapletscavity .

: It is a reservoir of the molten metal provided in the casting .Riser

: Small opening in the mold to facilitate escape of air and gases.Vent

: The passage through which the molten material from theSpruepouring basin reaches the mold cavity.


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Pattern Material

Wood, metals & alloys, plastic, plaster of Paris, plastic

and rubbers, wax, and resins.

Material select io n depends on size and shape of cast ing .

To be suitable for use, the pattern material should be:

1. Easily worked, shaped and joined2. Light in weight

3. Strong, hard and durable

4. Resistant to wear and abrasion

5. Resistant to corrosion, and to chemical reactions6. Dimensionally stable and unaffected by variations

in temperature and humidity

7. Available at low cost


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Sand Casting


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Pattern Allowances

Pattern allowance is a vital feature as it affects thedimensional characteristics of the casting

The selection of correct allowances greatly helps to

reduce machining costs and avoid rejections.

1. Shrinkage or contraction allowance

2. Draft or taper allowance

3. Machining or finish allowance

4. Distortion or camber allowance5. Rapping allowance or shake allowances


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Shrinkage or Contraction Allowance

All most all cast metals shrink or contract volumetr ical lyon cooling.

The metal shrinkage is of two types:

(1) 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 thecasting, are provided in the mold.

(2) Solid Shrinkage: It refers to the reduction in volume during the

cooling of the cast metal to room temperature.

To account for this, shrinkage allowance is provided on the patterns.

The rate of contraction with temperature is dependent on the material.

For example steel contracts to a higher degree compared to

aluminum.


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Shrinkage

Metal Percent Contraction (-)Expansion(+)

Aluminum

Zinc

Gold

Copper

Brass

Carbon Steel

LeadGray Cast Iron

-7.1%

-6.5%

-5.5%

-4.9%

-4.5%

-2.5-4%

-3.2%+2.5%


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Draft or Taper Allowance

Taper is provided on all vertical surfaces of the pattern so that it

can be removed from the sand without tearing away the sides of the

sand mold.

Draft allowance varies with the complexity of the sand job.

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

The amount of draft depends upon the length of the vertical side of

the pattern to be extracted; the intricacy of the pattern; the method of

molding; and pattern material.


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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 88 to 32

3.00

1.50

1.00

0.750.50

3.00

2.50

1.50

1.001.00

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

Draft Allowances of Various Metals


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Taper on patterns for ease of removal from the sand mold


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Machining or Finish Allowance

Machining or finish allowances are added in the pattern dimensionto have good surface finish or dimensionally accurate

The amount of machining allowance to be provided is affected by

the method of molding and casting used viz. hand molding or machine

molding, sand casting or metal mold casting.

The amount of machining allowance is also affected by the size andshape of the casting; the casting orientation; the metal; and the degree

of accuracy and finish required.


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Metal Dimension (inch) Allowance (inch)

Cast ironUp to 1212 to 20

20 to 40

0.120.20

0.25

Cast steel

Up to 6

6 to 2020 to 40

0.12

0.250.30

Non ferrous

Up to 8

8 to 12

12 to 40

0.09

0.12

0.16

Machining Allowances of Various Metals


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Distortion or Camber Allowance

Castings get distorted, during solidification, due to their typical shape.

For example, if the casting has the form of the letter U, V, T, or L etc. itwill tend to contract at the closed end causing the vertical legs to look

slightly inclined.

This can be prevented by making the legs of the U, V, T, or L shaped

pattern converge slightly (inward) so that the casting after distortionwill have its sides vertical

The distortion in casting may occur due to internal stresses.

These internal stresses are caused on account of unequal cooling of

different section of the casting and hindered contraction.

To prevent the distortion in castings include:

i. Modification of casting designii. Providing sufficient machining allowance to cover the distortion

affect

iii. Providing suitable allowance on the pattern, called camber or

distortion allowance (inverse reflection)


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Distortions in Casting


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Rapping Allowance

Before the withdrawal from the sand mold, the pattern is rapped all

around the vertical faces to enlarge the mold cavity slightly, which

facilitate its removal.

Since it enlarges the final casting made, it is desirable that the

original pattern dimension should be reduced to account for thisincrease.

There is no sure way of quantifying this allowance, since it is highly

dependent on the foundry personnel practice involved.

It is a negative allowance and is to be applied only to those

dimensions that are parallel to the parting plane.


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Fluid flow

2 principles of fluid flow are relevant togating design: Bernoullis theorem and

the law of mass continuity.


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Fluid flow

Bernoullis theorem

Based on

- principle of conservation of energy

- frictional losses in a fluid system

Conservation of energy requires that,

Constant2

2

g

v

g

ph

f

g

v

g

ph

g

v

g

ph

2

2

2

22

2

2

11

h= elevation

p= pressure at elevation

v= velocity of the liquid

= density of the fluid


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Fluid flow

Mass continuity

States that for an incompressible liquid

the rate of flow is constant.

Subscripts 1 and 2 pertain to two different

locations in the system.

2211vAvAQ

Q= volumetric rate of flow

A = cross-sectional area of the liquid stream



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Fluid flowSprue profile

Relationship between height and cross-

sectional area at any point in the sprue is

given by

Velocity of the molten metal leaving the

gate is

When liquid level reached heightx, gate

velocity is

1

2

2

1

h

h

A

A

ghcv2

xhgcv 2


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Fluid flow

Flow characteristics Reynolds number, Re, is used to characterize

aspect of fluid flow.

It represents the ratio of the inertia to the viscous

forces in fluid flow and is defined as

vDRe


D= diameter of the channel

= density

n = viscosity of the liquid.


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Flow Characterist ic s 0 < Re < 2000 => laminar flow

2000 < Re < 20 000 =>mixture of laminar and turbulent

flow , generally regarded as harmless in gatingsystems.

Re > 20 000 => severe turbulence

In gating systems, Re typically ranges from 2000 to20,000

Techniques for minimizing turbulence

Dross or slag can be eliminated by vacuum casting

Use of filters eliminates turbulent flow in the runnersystem


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Turbulence can be reduced by the design of a gating system

that promotes a more laminar flow of the liquid metal.

Sharp corners and abrupt changes in sections within the

casting can be a leading cause of turbulence. Their affect

can be mitigated by the employment of radii.


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Fluidity of molten metal

Fluidity of Molten Metal : The capability of molten metal to fill mold

cavities is called f lu id i ty .

The following influence fluidity

Characteristics of molten metal Viscosity (How runnyis it when hot)

Surface tension (Development offilm ) Inclusions

Solidification pattern of the alloy

Casting parameters

Mold design (Risers, runners, gates, etc.) Mold material and its surface characteristics

Degree of superheat

Rate of pouring

Heat transfer


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Heat Transfer

Important consideration in casting

Heat flow in the system

Complex

Depends of flow characteristics

Solidification Time A function of the volume of a casting and its surface

area

Solidification time = C volume 2

surface area

Effects on solidification time Mold Geometry

Skin thickness


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Heat transfer

Temperature distribution in the mold-

liquid metal interface is shown below.


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Sol id i f icat ion o f Metals

Involves liquid metal turning back in to solid metal

The process is different for Pure metals and alloys

Can be divided into two steps:

Formation of stable nuclei

Growth of crystals

Pure Metals

Have a clearly defined melting point

Temperature remains constant during freezing

Solidifies from the walls of the mold toward the center

of the part


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Grain Structu re for Pure Metals

Two types of grains are formed for a pure metal

Fine equiaxed grains Columnar

Rapid cooling at the walls produces fine equiaxed grains

Columnar grains grow opposite of the heat transferthroughout the mold following the chill zone

Equiaxed Grains

If crystals can grow approximately equally in all directionsequiaxed grains will grow.

Large amounts of under cooling is needed near the wall ofthe mold.

Ill t ti f C t St t


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Illustration of Cast Structures


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Al loys

Solidification in alloys begins when the temperature drops below

the liquidus TL and is complete when it reaches the solidus, TS.

All


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Alloys

Within the TL and TS Temperature range, the alloy is like a slushy withcolumnardendrites


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Effects o f Coo l ing Rates

Slow cool rates results in cou rse grain struc tures(102 K/s)

Faster cooling rates produce f iner grain s tructures(104 K/s)

For even faster cooling rates, the structures are amorphous

(106

108

K/s)

Grain size influences strength of a material

Smaller grains have higher ductility and strength

Smaller grains help prevent ho t tearingand/or cracks in thecasting


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Casting Process Classifications

Expendable Mold / Reusable Pattern.1

Expendable Mold / Expendable Pattern.2

Permanent Mold / No Pattern.3

Sand CastingShell moldingCeramic-Mold Casting

Investment Casting

Evaporative-Foam Casting

Permanent Mold Casting

Die Casting

Centrifugal Casting

( Wax , Plastic ,Polystyrene

Foam )

( Wood ,Plastic ,metal )

Sh ll ldi


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Shell molding


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Sh ll ldi


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a mounted pattern, made of a ferrous metal or

aluminum, is heated to 175-370 0 C, coated with aparting agent such as silicone, and clamped to a box

or chamber containing a fine sand coated with a 2.5 -

4.0% thermosetting resin binder

the sand mixture is blown over the heated pattern,coating it evenly

the assembly is placed in an oven to complete the

curing of the resin

the shell is formed by removing the pattern two half shells are made and are clamped together in

preparation for pouring

Shell molding

S


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Shell molding

Advantages

Better surface finish

Better dimensional tolerances.

Reduced Machining.

Less foundry space required.

Semi skilled operators can handle the process.

The process can be mechanized.

Sh ll ldi


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Disadvantages

The raw materials are relatively expensive.

The process generates noxious fumes which must be

removed.

The size and weight range of castings is limited. (Size

limits of 30 g to 12 kg )

Shell molding


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Shell Molding

Applications

-Crankshaft fabrication

-Steel casting parts, fittings

-Molded tubing fabrication

-Hydraulic control housing fabrication

-Automotive castings (cylinder head andribbed cylinder fabrication).


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MoldExpendable

Uses a polystyrene foam pattern which evaporates

with molten metal to form a cavity for the casting.

Polystyrene foam pattern includes sprue, risers,gating system and internal cores (if needed)

Polystyrene inexpensive and easily processedinto patterns


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Ad t f d d l t


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Advantages of expanded po lystyrene process:

1. Pattern need not be removed from the mold

2. Simplifies and speeds mold-making, because two

mold halves are not required as in a conventional

green-sand mold

Disadvantages:

1. A new pattern is needed for every casting

2. Economic justification of the process is highlydependent on cost of producing patterns

E ti P tt C ti f E i Bl k


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(a) Metal is poured into mold for lost-foam casting of a 60-hp. 3-

cylinder marine engine; (b) finished engine block.

(b)(a)

Evaporative Pattern Casting of an Engine Block


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I t t C ti


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Investment Casting

Investment Cast ing Character ist ics


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Investment Cast ing Character ist ics

Advantages:

Complex shapes possible

Thin wall sections possible

High production rates

High dimensional accuracy

Disadvantages:

Limited weight range

Expensive Machinery & Dies

Expensive Unit Cost, Labor Intensive

Mold is not reusable

Typical parts produced by investment cast ing.


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yp ca pa s p oduced by es e cas g

Products such as rocket components, and jet

engine turbine blades

Die casting


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Die casting typically makes use of non-ferrous alloys.

The four most common alloys that are die cast are

Aluminum alloys, Copper alloys, Magnesium alloys,Zinc alloys

The molten metal is injected into die cavity under highpressure

Pressure maintained during solidification

Die casting


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Hot chamber die

casting


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Cold chamber die casting

Advantages of die casting


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Advantages of die casting

Excellent dimensional accuracy

Smooth cast surfaces

Thinner walls can be cast

Inserts can be cast-in (such as threaded inserts,

heating elements, and high strength bearing

surfaces).

Reduces or eliminates secondary m/c ing operations.

Rapid production rates.

Disadvantages of die casting


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The main disadvantage - very high capital cost.

To make die casting an economic process a large

production volume is needed.

Die casting is limited to high fluidity metals (Zinc,Aluminum, Magnesium, Copper, Lead and Tin) (Not

applicable for high melting point metals and alloys

(eg. steels)

Casting weights must be between 30 grams and 10 kg

Limited die life

Disadvantages of die casting

Centrifugal Casting
http://en.wikipedia.org/wiki/Capital_costhttp://www.substech.com/dokuwiki/doku.php?id=steels_and_cast_irons&DokuWiki=b1069e39936ec6f4469f6b221001434bhttp://www.substech.com/dokuwiki/doku.php?id=steels_and_cast_irons&DokuWiki=b1069e39936ec6f4469f6b221001434bhttp://en.wikipedia.org/wiki/Capital_cost


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Centrifugal Casting

In this process, the mold is rotated rapidly about its

central axis as the metal is poured into it.

Because of the centrifugal force, a continuous pressure

will be acting on the metal as it solidifies.

The slag, oxides and other inclusions being lighter, getseparated from the metal and segregate towards the

center.

This process is normally used for the making of hollowpipes, tubes, hollow bushes, etc., which are ax- symmetric

with a concentric hole.

Centrifugal Casting


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The mold can be rotated about a vertical, horizontal or an

inclined axis or about its horizontal and vertical axes

simultaneously.

The length and outside diameter are fixed by the mold

cavity dimensions while the inside diameter is

determined by the amount of molten metal poured.

Since the metal is always pushed outward because of the

centrifugal force, no core needs to be used for making the

concentric hole.

Centrifugal Casting

Defects Cavities


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Defects

Metallic projections

Fins

Flash

Massive projections Swells

Rough surfaces

Cavities

Internal or external

Blow holes

Pin holes

Shrinkage cavities

Discontinuities

Cracks

Cold or hot tearing

Cold shunts

Casting Defects


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Casting Defects

Documents

Casting Fundamentals and Basics Concepts.ppt