Casting

Process Part 1 (SSC JE Mechanical/ GATE/ONGC/SAIL

BHEL/HPCL/IOCL)

Mech Zone

Refractory mold pour liquid metal solidify, remove finish

Casting - Process of Producing Metallic Parts by Pouring

Molten Metal in to a Mould having a Cavity of the Part to

be Cast and then Allowing the Molten Metal to Solidify.

Moulding – Process of Making a Mould Using a Pattern.

Pattern – A Model or Replica of the Part to be Cast.

Advantages • Complex forms, low cost

• Certain shapes cannot be machined

• One piece parts vs. multiple piece parts

• Design changes are easily incorporated

• High volume, low skilled labor

• Large, heavy parts can be made easily

Disadvantages • Problems with internal porosity

• Dimensional variations due to shrinkage

• Trapped impurities, solids and gasses

• High-tolerance, smooth surfaces not possible

• More costly than stamping or extruding in some cases

• Sand Mould Casting – Molten Metal is Poured into the

Cavity of the Mould without Applying External

Pressure (Gravity Feed).

• Permanent Mould Casting - Gravity Die Casting

• Die Casting – Molten Metal is Forced under very high

Pressure in to the Die (Metallic Mould).

– Hot Chamber Die Casting

– Cold Chamber Die Casting

Forms of Casting and Terminology

Flask : The box containing the mold

Cope : The top half of any part of a 2-part mold

Drag : The bottom half of any part of a 2-part mold

Core: A shape inserted into the mold to form internal cavities

Core Print : A region used to support the core

Mold Cavity: The hollow mold area in which metal solidifies into the part

Riser: An extra cavity to store additional metal to prevent shrinkage

Gating System: Channels used to deliver metal into the mold cavity

Pouring Cup: The part of the gating system that receives poured metal

Sprue: Vertical channel

Runners: Horizontal channels

Parting Line / Parting Surface: Interface that separates the cope and drag of a 2-part mold

Draft: Taper on a pattern or casting that allows removal from the mold

Core Box: Mold or die used to produce cores

Casting: The process and product of solidifying metal in a mold

Metal Solidification

Metal Solidification

Dendrites

Tree-like structures that form during the solidification of alloys

Slow cooling rates produce dendrites with larger branch spacing; faster cooling rates produce finer spacing; very fast cooling rates produce no dendrites or grains

Liquid shrinkage

solid shrinkage

solidification shrinkage

Shrinkage or Contraction

Tp

Tf

Ta

Fluid Flow

Metal is poured through a pouring cup

Risers hold and supply metal to prevent shrinking during solidification

Gates are designed to prevent contaminants from reaching the mold cavity

Fluidity of Molten Metal

Fluidity

The capability of a molten metal to fill mold cavities

Viscosity

Higher viscosity decreases fluidity

Surface tension

Decreases fluidity; often caused by oxide film

Inclusions

Insoluble particles can increase viscosity, reducing fluidity

Solidification pattern

Fluidity is inversely proportional to the freezing temperature range

Fluidity of Molten Metal

• The metal that solidifies first is at the wall of the mold; this solid layer thickens as time passes

• Shrinkage during cooling can change the part dimensions and sometimes cause cracking; it is caused by the metal’s thermal expansion properties and the phase change between liquid and solid.

Heat Transfer

Classification of Casting Processes

Process Advantages Disadvantages Examples

Sand many metals, sizes, shapes,

cheap

poor finish &

tolerance

engine blocks,

cylinder heads

Shell mold better accuracy, finish, higher

production rate

limited part size connecting rods,

gear housings

Expendable

pattern

Wide range of metals, sizes,

shapes

patterns have low

strength

cylinder heads,

brake components

Plaster mold complex shapes, good surface

finish

non-ferrous metals,

low production rate

prototypes of

mechanical parts

Ceramic mold complex shapes, high accuracy,

good finish

small sizes impellers, injection

mold tooling

Investment complex shapes, excellent finish small parts, expensive jewellery

Permanent

mold

good finish, low porosity, high

production rate

Costly mold, simpler

shapes only

gears, gear

housings

Die Excellent dimensional accuracy,

high production rate

costly dies, small

parts,

non-ferrous metals

gears, camera

bodies, car wheels

Centrifugal Large cylindrical parts, good

quality

Expensive, few shapes pipes, boilers,

flywheels

Mech Zone 16

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Mech Zone… (SSC JE Mechanical/ GATE/ONGC/SAIL

BHEL/HPCL/IOCL)