MODULE 2 COURSE OUTCOMES COURSE OUTCOMES

Fundamentals of Casting process

Solidification of metals

Fluid flow of molten metal

Various casting process

Casting defects & quality

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

Examples of Cast Parts

Crank handle formed by casting; some areas were machined and assembled after casting

Examples of Cast Parts

C-clamps formed by casting (left) and machining (right)

Examples of Cast Parts

Complex part formed by casting

Aluminum piston for an internal combustion engine :

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

Flask : A molding flask is one which holds the sand mould intact

Drag : Lower molding flask

Cope : Upper molding flask

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

Molding sand : It is the freshly prepared refractory material used for making the mold cavity.

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

Core Print : A region used to support the core

Pouring basin : A small funnel shaped cavity at the top of the mold into molten metal is poured.

Mold cavity

chaplet

Mold cavity

chaplet

Parting Line / Parting Surface : Interface that separates the cope and drag

Runner: The channel through which the molten metal is carried from the sprue to the gate.

Gate: A channel through which the molten metal enters the mold cavity  

Chaplets: Chaplets are used to support the cores inside the mold cavity.

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

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

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

Pattern MaterialWood, metals & alloys, plastic, plaster of Paris, plastic and rubbers, wax, and resins.

Material selection depends on size and shape of casting.

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

1.     Easily worked, shaped and joined2.     Light in weight3.     Strong, hard and durable4.     Resistant to wear and abrasion 5.     Resistant to corrosion, and to chemical reactions6.     Dimensionally stable and unaffected by variations in temperature and humidity7.     Available at low cost

Sand Casting

Pattern Allowances

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

The selection of correct allowances greatly helps to reduce machining costs and avoid rejections.

1.     Shrinkage or contraction allowance2.     Draft or taper allowance3.     Machining or finish allowance4.     Distortion or camber allowance5.     Rapping allowance or shake allowances

Shrinkage or Contraction Allowance

All most all cast metals shrink or contract volumetrically on 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 the casting, 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.

ShrinkageMetalPercent Contraction (-)

Expansion(+)

AluminumZincGold

CopperBrass

Carbon SteelLead

Gray Cast Iron

-7.1%-6.5%-5.5%-4.9%-4.5%

-2.5-4%-3.2%+2.5%

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.

Pattern material

Height of the given surface

(inch)

Draft angle(External surface)

Draft angle(Internal surface)

Wood

11 to 22 to 44 to 88 to 32

3.001.501.000.750.50

3.002.501.501.001.00

Metal and plastic

11 to 22 to 44 to 88 to 32

1.501.000.750.500.50

3.002.001.001.000.75

Draft Allowances of Various Metals

Taper on patterns for ease of removal from the sand mold

Machining or Finish Allowance

Machining or finish allowances are added in the pattern dimension

to 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 and

shape of the casting; the casting orientation; the metal; and the degree

of accuracy and finish required.

MetalDimension (inch)Allowance (inch)

Cast ironUp to 1212 to 2020 to 40

0.120.200.25

Cast steelUp to 66 to 2020 to 40

0.120.250.30

Non ferrousUp to 88 to 1212 to 40

0.090.120.16

Machining Allowances of Various Metals

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. it will 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 distortion will have its sides verticalThe 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 affectiii.  Providing suitable allowance on the pattern, called camber or distortion allowance (inverse reflection)

Distortions in Casting

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 this

increase.

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.

Fluid flow

• 2 principles of fluid flow are relevant to

gating design: Bernoulli’s theorem and

the law of mass continuity.

Fluid flow

Bernoulli’s theorem

• Based on - principle of conservation of energy - frictional losses in a fluid system

• Conservation of energy requires that,

Constant 2

2

g

v

g

ph

fg

v

g

ph

g

v

g

ph

2

2

222

2

211

h = elevationp = pressure at elevationv = velocity of the liquidρ = density of the fluid

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.

2211 vAvAQ Q = volumetric rate of flowA = cross-sectional area of the liquid streamv = velocity of the liquid

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 height x, gate velocity is

1

2

2

1

h

h

A

A

ghcv 2

xhgcv 2

Fluid flowFlow 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

vD

Rev = velocity of the liquidD = diameter of the channelρ = density n = viscosity of the liquid.

Flow CharacteristicsFlow Characteristics 0 < Re < 2000 => laminar flow

2000 < Re < 20 000 =>mixture of laminar and turbulent flow , generally regarded as harmless in

gating systems.

Re > 20 000 => severe turbulence

In gating systems, Re typically ranges from 2000 to 20,000

Techniques for minimizing turbulence

• Dross or slag can be eliminated by vacuum casting

• Use of filters eliminates turbulent flow in the runner system

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. 

 

Fluidity of molten metal

Fluidity of Molten Metal : The capability of molten metal to fill mold cavities is called fluidity.

The following influence fluidity Characteristics of molten metal

– Viscosity (How runny is it when hot)– Surface tension (Development of film )– 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

Heat TransferHeat 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

Heat transfer

• Temperature distribution in the mold-liquid metal interface is shown below.

Solidification of MetalsSolidification of 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 nucleiGrowth 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

Grain Structure 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 transfer throughout the mold following the chill zone

Equiaxed Grains

• If crystals can grow approximately equally in all directions – equiaxed grains will grow.

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

Illustration of Cast StructuresIllustration of Cast Structures

AlloysAlloys• Solidification in alloys begins when the temperature drops below

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

AlloysAlloys• Within the TL and TS Temperature range, the alloy is like a slushy with columnar

dendrites

Effects of Cooling RatesEffects of Cooling Rates

• Slow cool rates results in course grain structures (102 K/s)

• Faster cooling rates produce finer grain structures (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 hot tearing and/or cracks in the casting

Casting Process Classifications

1 .Expendable Mold / Reusable Pattern

2 .Expendable Mold / Expendable Pattern

3 .Permanent Mold / No Pattern

Sand CastingShell moldingCeramic-Mold Casting

Investment CastingEvaporative-Foam Casting

Permanent Mold CastingDie CastingCentrifugal Casting

( Wax , Plastic ,Polystyrene Foam )

) Wood ,Plastic ,metal(

Shell molding

– a mounted pattern, made of a ferrous metal or aluminum, is heated to 175-370 0 C, coated with a parting 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

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.

 

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

Shell Molding

Applications

• -Crankshaft fabrication

• -Steel casting parts, fittings

• -Molded tubing fabrication

• -Hydraulic control housing fabrication

• -Automotive castings (cylinder head and ribbed cylinder fabrication).

Expendable Mold

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 processed into patterns

polystyrenepattern

patternsupport

sand

moltenmetal

polystyreneburns;gas escapespolystyrene

pattern

patternsupport

sand

moltenmetal

polystyreneburns;gas escapes

Advantages of expanded polystyrene 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 highly dependent on cost of producing patterns

)  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

Investment Casting

Investment Casting – Characteristics

• 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 casting .

Products such as rocket components, and jet engine turbine blades

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 high pressure

Pressure maintained during solidification

Die castingDie casting

Hot chamber die casting

Cold chamber die casting

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.

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

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, get separated from the metal and segregate towards the center.

This process is normally used for the making of hollow pipes, tubes, hollow bushes, etc., which are ax- symmetric with a concentric hole.

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 Centrifugal Casting

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 DefectsCasting Defects