Metal Casting Process

Metal Casting

• Metal casting is one of the oldest materials shaping methods known.
• It involves pouring molten metal into a mold with a cavity of the shape to be made, and allowing it to solidify.
• The solidified object is called the casting.

Advantages of Metal Casting

• Molten material can flow into very small sections, making intricate shapes possible.
• Practically any material that is ferrous or non-ferrous can be cast.
• Metal can be placed exactly where it is required, achieving large savings in weight.
• Simple and inexpensive tools are required for casting molds.
• Certain parts made from metals and alloys can only be processed through casting.

Limitations of Metal Casting

• Dimensional accuracy and surface finish of castings made by sand casting processes are limited.
• The process is labor-intensive.

Components Used for Making a Mould Cavity

• Molding box: A suitable size of molding box is selected for creating a suitable wall thickness.
• Pattern: A replica of the final object to be made, used to create the mold cavity.
• Parting line: The dividing line between the two molding flasks that make up the mold.
• Molding sand: A mixture of silica sand, clay, and moisture in appropriate proportions.
• Facing sand: A small amount of carbonaceous material sprinkled on the inner surface of the mold cavity to improve surface finish.
• Core: A separate part of the mold, made of sand and baked, used to create openings and cavities in the castings.
• Pouring basin: A small funnel-shaped cavity at the top of the mold into which the molten metal is poured.
• Sprue: The passage through which the molten metal reaches the mold cavity.
• 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: Used to support the cores inside the mold cavity to overcome metallostatic force.
• Riser: A column of molten metal placed in the mold to feed the castings as it shrinks and solidifies.
• Vent: Small openings in the mold to facilitate the escape of air and gases.

Pattern

• A pattern is a model or replica of the object to be cast.
• It is embedded in molding sand and suitable ramming of molding sand is made around the pattern.
• The pattern is then withdrawn to generate a cavity in the molding sand.
• Objectives of a pattern include:
  • Preparing a mold cavity for the purpose of making a casting.
  • Possessing core prints that produce seats in the form of extra recess for core placement.
  • Establishing parting lines and parting surfaces in the mold.
  • Forming runners, gates, and risers.
  • Minimizing overall cost of the casting.
  • Helping to establish locating pins on the mold and casting.

Common Pattern Materials

• Wood: Cheap, easily available, and repairable, but susceptible to moisture, warping, and wearing out quickly.
• Metal: Stronger, more durable, and resistant to moisture, but more expensive and heavier than wood.
• Plastic: Lighter, stronger, and more resistant to moisture and wear, but more fragile and less resistant to sudden loading.
• Plaster: Suitable for producing highly intricate castings, but brittle and prone to breaking.
• Wax: Excellent for investment casting, with properties including low ash content, resistance to primary coat material, and high tensile strength and hardness.

Types of Patterns

• One-piece or solid pattern: A simple, single-piece pattern.
• Two-piece or split pattern: A pattern split into two parts for easy withdrawal.
• Cope and drag pattern: A pattern made in two halves, which are mounted on different plates.
• Three-piece or multi-piece pattern: A pattern made in three or more pieces for complex shapes.
• Loose piece pattern: A pattern made of separate pieces that are assembled to form the mold cavity.
• Match plate pattern: A pattern made in two halves, which are joined at the parting line.
• Follow board pattern: A pattern used for small castings.
• Gated pattern: A pattern with a gate attached to the pouring basin.
• Sweep pattern: A pattern used for making large castings.
• Skeleton pattern: A pattern used for making hollow castings.
• Segmental or part pattern: A pattern used for making large castings in segments.### Pattern Types
• Loose-piece pattern: a single piece made to have loose pieces to allow withdrawal from the mold after the molding process is completed (Fig. 6)
• Match plate pattern: a type of pattern that has two parts, one for one side and another for another side of the pattern, used for casting of metal, usually aluminum (Fig. 7)
• Follow board type pattern: a special pattern used to allow the mold to support structurally weak portions, made of wooden material (Fig. 9)
• Gated pattern: used for mass production of casings, multi-cavity moulds are formed by joining a number of patterns and gates, and providing a common runner for the molten metal (Fig. 10)
• Sweep pattern: used for forming large circular molds of symmetric kind, a template of wood or metal is attached to a spindle and revolves to form the desired shape (Fig. 11)
• Skeleton pattern: used when only a small number of large and heavy castings are to be made, a ribbed construction of wood that forms an outline of the pattern (Fig. 12)
• Segmental pattern: used to prepare the mold of larger circular casting, similar to sweep pattern, but the sweep pattern gives a continuous revolve motion to generate the part (Fig. 13)

Pattern Allowances

• Pattern may carry additional allowances to compensate for:
  • Metal shrinkage
  • Machining
  • Draft
  • Distortion
  • Rapping or shake
  • Mould wall movement
• These allowances are given to ensure that the final casting has the desired dimensions and shape
• Shrinkage allowance: a positive allowance to compensate for the contraction of metal during cooling (Fig. 14)
• Machining allowance: a positive allowance to compensate for the amount of material lost during machining or finishing (Fig. 15)
• Draft or taper allowance: a positive allowance to ensure easy withdrawal of the pattern from the mold (Fig. 16)
• Rapping or shake allowance: a negative allowance to account for the movement of the mold cavity during pattern withdrawal
• Distortion allowance: an allowance to compensate for the distortion of the casting during cooling due to thermal stresses
• Mould wall movement allowance: an allowance to account for the enlargement of the mold cavity due to heat and static pressure

Gating System

• Gating system: all elements connected with the flow of molten metal from the ladle to the mold cavity
• Elements of the gating system:
  • Pouring basin
  • Sprue
  • Sprue base
  • Well
  • Runner
  • Ingate
  • Riser
• Types of gating systems:
  • Horizontal gating system
  • Vertical gating system
  • Top gating system
  • Bottom gating system
  • Middle gating system
• Design requirements for a good gating system:
  • Completely fill the mold in the shortest time possible
  • Smooth metal flow without turbulence
  • Prevent unwanted material from entering the mold cavity
  • Control metal entry to prevent aspiration of atmospheric air
  • Maintain a proper thermal gradient
  • Prevent gating or mold erosion
  • Ensure enough molten metal reaches the mold cavity
  • Economical and easy to implement and remove after casting solidification

Solidification of Metals

• Pure metals: solidify at a constant temperature, with a clearly defined melting point (Fig. 17)
• Alloys: solidify over a range of temperatures, with a mushy zone (Fig. 19)
• Solidification of alloys: dendrites form in the mushy zone, with a liquid metal present between the dendrite arms
• Freezing range: the difference between the liquidus and solidus temperatures of an alloy (Fig. 17)
• Importance of freezing range: affects the microstructure and properties of the casting
• Nucleation agents: can be used to influence the microstructure and properties of the casting

Risers

• Riser: a source of extra metal that flows from the riser to the mold cavity to compensate for shrinkage during solidification

• Functions of risers:

  • Provide extra metal to compensate for volumetric shrinkage
  • Allow mold gases to escape
  • Provide extra metal pressure on the solidifying mold to reproduce mold details more exactly

• Design requirements of risers:

  • Riser size: must be last to freeze, with a larger volume-to-surface-area ratio than the casting
  • Riser placement: must be placed at the portion of the casting that is last to freeze
  • Riser shape: cylindrical risers are recommended, with a hemispherical bottom to increase the volume-to-surface-area ratio

• Riser design: must be designed to freeze after the main casting, with a minimum volume of metal in the riser### Core and Core Box

• A core is a compact mass of core sand that produces hollowness in a casting by not allowing molten metal to occupy a specific space.

• Cores are classified according to shape and position in the mold, with types including horizontal, vertical, balanced, drop, and hanging cores.

• The functions of cores include:

  • Producing hollowness in castings
  • Improving mold surface
  • Providing external undercut features in casting
  • Strengthening the mold
  • Forming gating systems
  • Achieving deep recesses in casting

Special Casting Processes

Centrifugal Casting

• Working principle: It works on the principle of centrifugal force, where a rotating mold forces molten metal to the outer wall, separating slag and other inclusions from the metal.
• Types:
  • True Centrifugal Casting: used for making symmetrical, round, hollow sections without cores.
  • Semi Centrifugal Casting: used for casting large, axi-symmetrical objects with a core inserted at the center.
  • Centrifuging: used for casting shapes that are not axi-symmetrical, with multiple mold cavities connected to a central sprue with radial gates.
• Applications:
  • Aircraft industry
  • Steam turbine bearing shells
  • Roller for steel rolling mills
  • Automobile industry
  • Electronic industries
• Advantages:
  • Dense metal with high mechanical properties
  • Unidirectional solidification
  • No cores required for hollow shapes
  • Gating system and runner eliminated
  • Lower pouring temperature
  • Lower casting defects
• Disadvantages:
  • Limited design capabilities
  • Only suitable for symmetrical shapes
  • High equipment cost
  • Not suitable for all metals
  • Higher maintenance required
  • Skilled operators required
  • Difficult to determine solidification time and temperature distribution

Investment Casting Process

• Also known as the lost wax process
• Begins with the production of wax replicas or patterns of the desired shape
• Patterns are prepared by injecting wax or polystyrene into metal dies
• The mold is prepared by surrounding the pattern with refractory slurry
• The mold is then heated to melt the pattern, leaving a clean cavity behind
• Basic steps:
  • Production of heat-disposable wax patterns
  • Assembly of patterns onto a gating system
  • Investing or covering the pattern assembly with refractory slurry
  • Melting the pattern assembly
  • Firing the mold to remove the last traces of the pattern material
  • Pouring
  • Knockout, cutoff, and finishing
• Advantages:
  • Formation of hollow interiors in cylinders without cores
  • Less material required for gate
  • Fine-grained structure at the outer surface of the casting
• Disadvantages:
  • Segregation of alloy components during pouring
  • Contamination of internal surface of castings with non-metallic inclusions
  • Inaccurate internal diameter

Ceramic Shell Investment Casting Process

• Difference from investment casting: the wax pattern is immersed in a refractory aggregate before dewaxing
• A ceramic shell is built around a tree assembly by repeatedly dipping a pattern into a slurry
• The shell is heated to burn out any residual wax and develop a high-temperature bond
• Advantages:
  • Excellent surface finish
  • Tight dimensional tolerances
  • Machining can be reduced or eliminated

Metal Casting

• Metal casting is one of the oldest materials shaping methods known.
• It involves pouring molten metal into a mold with a cavity of the shape to be made, and allowing it to solidify.
• The solidified object is called the casting.

Advantages of Metal Casting

• Molten material can flow into very small sections, making intricate shapes possible.
• Practically any material that is ferrous or non-ferrous can be cast.
• Metal can be placed exactly where it is required, achieving large savings in weight.
• Simple and inexpensive tools are required for casting molds.
• Certain parts made from metals and alloys can only be processed through casting.

Limitations of Metal Casting

• Dimensional accuracy and surface finish of castings made by sand casting processes are limited.
• The process is labor-intensive.

Components Used for Making a Mould Cavity

• Molding box: A suitable size of molding box is selected for creating a suitable wall thickness.
• Pattern: A replica of the final object to be made, used to create the mold cavity.
• Parting line: The dividing line between the two molding flasks that make up the mold.
• Molding sand: A mixture of silica sand, clay, and moisture in appropriate proportions.
• Facing sand: A small amount of carbonaceous material sprinkled on the inner surface of the mold cavity to improve surface finish.
• Core: A separate part of the mold, made of sand and baked, used to create openings and cavities in the castings.
• Pouring basin: A small funnel-shaped cavity at the top of the mold into which the molten metal is poured.
• Sprue: The passage through which the molten metal reaches the mold cavity.
• 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: Used to support the cores inside the mold cavity to overcome metallostatic force.
• Riser: A column of molten metal placed in the mold to feed the castings as it shrinks and solidifies.
• Vent: Small openings in the mold to facilitate the escape of air and gases.

Pattern

• A pattern is a model or replica of the object to be cast.
• It is embedded in molding sand and suitable ramming of molding sand is made around the pattern.
• The pattern is then withdrawn to generate a cavity in the molding sand.
• Objectives of a pattern include:
  • Preparing a mold cavity for the purpose of making a casting.
  • Possessing core prints that produce seats in the form of extra recess for core placement.
  • Establishing parting lines and parting surfaces in the mold.
  • Forming runners, gates, and risers.
  • Minimizing overall cost of the casting.
  • Helping to establish locating pins on the mold and casting.

Common Pattern Materials

• Wood: Cheap, easily available, and repairable, but susceptible to moisture, warping, and wearing out quickly.
• Metal: Stronger, more durable, and resistant to moisture, but more expensive and heavier than wood.
• Plastic: Lighter, stronger, and more resistant to moisture and wear, but more fragile and less resistant to sudden loading.
• Plaster: Suitable for producing highly intricate castings, but brittle and prone to breaking.
• Wax: Excellent for investment casting, with properties including low ash content, resistance to primary coat material, and high tensile strength and hardness.

Types of Patterns

• One-piece or solid pattern: A simple, single-piece pattern.
• Two-piece or split pattern: A pattern split into two parts for easy withdrawal.
• Cope and drag pattern: A pattern made in two halves, which are mounted on different plates.
• Three-piece or multi-piece pattern: A pattern made in three or more pieces for complex shapes.
• Loose piece pattern: A pattern made of separate pieces that are assembled to form the mold cavity.
• Match plate pattern: A pattern made in two halves, which are joined at the parting line.
• Follow board pattern: A pattern used for small castings.
• Gated pattern: A pattern with a gate attached to the pouring basin.
• Sweep pattern: A pattern used for making large castings.
• Skeleton pattern: A pattern used for making hollow castings.
• Segmental or part pattern: A pattern used for making large castings in segments.### Pattern Types
• Loose-piece pattern: a single piece made to have loose pieces to allow withdrawal from the mold after the molding process is completed (Fig. 6)
• Match plate pattern: a type of pattern that has two parts, one for one side and another for another side of the pattern, used for casting of metal, usually aluminum (Fig. 7)
• Follow board type pattern: a special pattern used to allow the mold to support structurally weak portions, made of wooden material (Fig. 9)
• Gated pattern: used for mass production of casings, multi-cavity moulds are formed by joining a number of patterns and gates, and providing a common runner for the molten metal (Fig. 10)
• Sweep pattern: used for forming large circular molds of symmetric kind, a template of wood or metal is attached to a spindle and revolves to form the desired shape (Fig. 11)
• Skeleton pattern: used when only a small number of large and heavy castings are to be made, a ribbed construction of wood that forms an outline of the pattern (Fig. 12)
• Segmental pattern: used to prepare the mold of larger circular casting, similar to sweep pattern, but the sweep pattern gives a continuous revolve motion to generate the part (Fig. 13)

Pattern Allowances

• Pattern may carry additional allowances to compensate for:
  • Metal shrinkage
  • Machining
  • Draft
  • Distortion
  • Rapping or shake
  • Mould wall movement
• These allowances are given to ensure that the final casting has the desired dimensions and shape
• Shrinkage allowance: a positive allowance to compensate for the contraction of metal during cooling (Fig. 14)
• Machining allowance: a positive allowance to compensate for the amount of material lost during machining or finishing (Fig. 15)
• Draft or taper allowance: a positive allowance to ensure easy withdrawal of the pattern from the mold (Fig. 16)
• Rapping or shake allowance: a negative allowance to account for the movement of the mold cavity during pattern withdrawal
• Distortion allowance: an allowance to compensate for the distortion of the casting during cooling due to thermal stresses
• Mould wall movement allowance: an allowance to account for the enlargement of the mold cavity due to heat and static pressure

Gating System

• Gating system: all elements connected with the flow of molten metal from the ladle to the mold cavity
• Elements of the gating system:
  • Pouring basin
  • Sprue
  • Sprue base
  • Well
  • Runner
  • Ingate
  • Riser
• Types of gating systems:
  • Horizontal gating system
  • Vertical gating system
  • Top gating system
  • Bottom gating system
  • Middle gating system
• Design requirements for a good gating system:
  • Completely fill the mold in the shortest time possible
  • Smooth metal flow without turbulence
  • Prevent unwanted material from entering the mold cavity
  • Control metal entry to prevent aspiration of atmospheric air
  • Maintain a proper thermal gradient
  • Prevent gating or mold erosion
  • Ensure enough molten metal reaches the mold cavity
  • Economical and easy to implement and remove after casting solidification

Solidification of Metals

• Pure metals: solidify at a constant temperature, with a clearly defined melting point (Fig. 17)
• Alloys: solidify over a range of temperatures, with a mushy zone (Fig. 19)
• Solidification of alloys: dendrites form in the mushy zone, with a liquid metal present between the dendrite arms
• Freezing range: the difference between the liquidus and solidus temperatures of an alloy (Fig. 17)
• Importance of freezing range: affects the microstructure and properties of the casting
• Nucleation agents: can be used to influence the microstructure and properties of the casting

Risers

• Riser: a source of extra metal that flows from the riser to the mold cavity to compensate for shrinkage during solidification

• Functions of risers:

  • Provide extra metal to compensate for volumetric shrinkage
  • Allow mold gases to escape
  • Provide extra metal pressure on the solidifying mold to reproduce mold details more exactly

• Design requirements of risers:

  • Riser size: must be last to freeze, with a larger volume-to-surface-area ratio than the casting
  • Riser placement: must be placed at the portion of the casting that is last to freeze
  • Riser shape: cylindrical risers are recommended, with a hemispherical bottom to increase the volume-to-surface-area ratio

• Riser design: must be designed to freeze after the main casting, with a minimum volume of metal in the riser### Core and Core Box

• A core is a compact mass of core sand that produces hollowness in a casting by not allowing molten metal to occupy a specific space.

• Cores are classified according to shape and position in the mold, with types including horizontal, vertical, balanced, drop, and hanging cores.

• The functions of cores include:

  • Producing hollowness in castings
  • Improving mold surface
  • Providing external undercut features in casting
  • Strengthening the mold
  • Forming gating systems
  • Achieving deep recesses in casting

Special Casting Processes

Centrifugal Casting

• Working principle: It works on the principle of centrifugal force, where a rotating mold forces molten metal to the outer wall, separating slag and other inclusions from the metal.
• Types:
  • True Centrifugal Casting: used for making symmetrical, round, hollow sections without cores.
  • Semi Centrifugal Casting: used for casting large, axi-symmetrical objects with a core inserted at the center.
  • Centrifuging: used for casting shapes that are not axi-symmetrical, with multiple mold cavities connected to a central sprue with radial gates.
• Applications:
  • Aircraft industry
  • Steam turbine bearing shells
  • Roller for steel rolling mills
  • Automobile industry
  • Electronic industries
• Advantages:
  • Dense metal with high mechanical properties
  • Unidirectional solidification
  • No cores required for hollow shapes
  • Gating system and runner eliminated
  • Lower pouring temperature
  • Lower casting defects
• Disadvantages:
  • Limited design capabilities
  • Only suitable for symmetrical shapes
  • High equipment cost
  • Not suitable for all metals
  • Higher maintenance required
  • Skilled operators required
  • Difficult to determine solidification time and temperature distribution

Investment Casting Process

• Also known as the lost wax process
• Begins with the production of wax replicas or patterns of the desired shape
• Patterns are prepared by injecting wax or polystyrene into metal dies
• The mold is prepared by surrounding the pattern with refractory slurry
• The mold is then heated to melt the pattern, leaving a clean cavity behind
• Basic steps:
  • Production of heat-disposable wax patterns
  • Assembly of patterns onto a gating system
  • Investing or covering the pattern assembly with refractory slurry
  • Melting the pattern assembly
  • Firing the mold to remove the last traces of the pattern material
  • Pouring
  • Knockout, cutoff, and finishing
• Advantages:
  • Formation of hollow interiors in cylinders without cores
  • Less material required for gate
  • Fine-grained structure at the outer surface of the casting
• Disadvantages:
  • Segregation of alloy components during pouring
  • Contamination of internal surface of castings with non-metallic inclusions
  • Inaccurate internal diameter

Ceramic Shell Investment Casting Process

• Difference from investment casting: the wax pattern is immersed in a refractory aggregate before dewaxing
• A ceramic shell is built around a tree assembly by repeatedly dipping a pattern into a slurry
• The shell is heated to burn out any residual wax and develop a high-temperature bond
• Advantages:
  • Excellent surface finish
  • Tight dimensional tolerances
  • Machining can be reduced or eliminated

Metal Casting

• Metal casting is one of the oldest materials shaping methods known.
• It involves pouring molten metal into a mold with a cavity of the shape to be made, and allowing it to solidify.
• The solidified object is called the casting.

Advantages of Metal Casting

• Molten material can flow into very small sections, making intricate shapes possible.
• Practically any material that is ferrous or non-ferrous can be cast.
• Metal can be placed exactly where it is required, achieving large savings in weight.
• Simple and inexpensive tools are required for casting molds.
• Certain parts made from metals and alloys can only be processed through casting.

Limitations of Metal Casting

• Dimensional accuracy and surface finish of castings made by sand casting processes are limited.
• The process is labor-intensive.

Components Used for Making a Mould Cavity

• Molding box: A suitable size of molding box is selected for creating a suitable wall thickness.
• Pattern: A replica of the final object to be made, used to create the mold cavity.
• Parting line: The dividing line between the two molding flasks that make up the mold.
• Molding sand: A mixture of silica sand, clay, and moisture in appropriate proportions.
• Facing sand: A small amount of carbonaceous material sprinkled on the inner surface of the mold cavity to improve surface finish.
• Core: A separate part of the mold, made of sand and baked, used to create openings and cavities in the castings.
• Pouring basin: A small funnel-shaped cavity at the top of the mold into which the molten metal is poured.
• Sprue: The passage through which the molten metal reaches the mold cavity.
• 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: Used to support the cores inside the mold cavity to overcome metallostatic force.
• Riser: A column of molten metal placed in the mold to feed the castings as it shrinks and solidifies.
• Vent: Small openings in the mold to facilitate the escape of air and gases.

Pattern

• A pattern is a model or replica of the object to be cast.
• It is embedded in molding sand and suitable ramming of molding sand is made around the pattern.
• The pattern is then withdrawn to generate a cavity in the molding sand.
• Objectives of a pattern include:
  • Preparing a mold cavity for the purpose of making a casting.
  • Possessing core prints that produce seats in the form of extra recess for core placement.
  • Establishing parting lines and parting surfaces in the mold.
  • Forming runners, gates, and risers.
  • Minimizing overall cost of the casting.
  • Helping to establish locating pins on the mold and casting.

Common Pattern Materials

• Wood: Cheap, easily available, and repairable, but susceptible to moisture, warping, and wearing out quickly.
• Metal: Stronger, more durable, and resistant to moisture, but more expensive and heavier than wood.
• Plastic: Lighter, stronger, and more resistant to moisture and wear, but more fragile and less resistant to sudden loading.
• Plaster: Suitable for producing highly intricate castings, but brittle and prone to breaking.
• Wax: Excellent for investment casting, with properties including low ash content, resistance to primary coat material, and high tensile strength and hardness.

Types of Patterns

• One-piece or solid pattern: A simple, single-piece pattern.
• Two-piece or split pattern: A pattern split into two parts for easy withdrawal.
• Cope and drag pattern: A pattern made in two halves, which are mounted on different plates.
• Three-piece or multi-piece pattern: A pattern made in three or more pieces for complex shapes.
• Loose piece pattern: A pattern made of separate pieces that are assembled to form the mold cavity.
• Match plate pattern: A pattern made in two halves, which are joined at the parting line.
• Follow board pattern: A pattern used for small castings.
• Gated pattern: A pattern with a gate attached to the pouring basin.
• Sweep pattern: A pattern used for making large castings.
• Skeleton pattern: A pattern used for making hollow castings.
• Segmental or part pattern: A pattern used for making large castings in segments.### Pattern Types
• Loose-piece pattern: a single piece made to have loose pieces to allow withdrawal from the mold after the molding process is completed (Fig. 6)
• Match plate pattern: a type of pattern that has two parts, one for one side and another for another side of the pattern, used for casting of metal, usually aluminum (Fig. 7)
• Follow board type pattern: a special pattern used to allow the mold to support structurally weak portions, made of wooden material (Fig. 9)
• Gated pattern: used for mass production of casings, multi-cavity moulds are formed by joining a number of patterns and gates, and providing a common runner for the molten metal (Fig. 10)
• Sweep pattern: used for forming large circular molds of symmetric kind, a template of wood or metal is attached to a spindle and revolves to form the desired shape (Fig. 11)
• Skeleton pattern: used when only a small number of large and heavy castings are to be made, a ribbed construction of wood that forms an outline of the pattern (Fig. 12)
• Segmental pattern: used to prepare the mold of larger circular casting, similar to sweep pattern, but the sweep pattern gives a continuous revolve motion to generate the part (Fig. 13)

Pattern Allowances

• Pattern may carry additional allowances to compensate for:
  • Metal shrinkage
  • Machining
  • Draft
  • Distortion
  • Rapping or shake
  • Mould wall movement
• These allowances are given to ensure that the final casting has the desired dimensions and shape
• Shrinkage allowance: a positive allowance to compensate for the contraction of metal during cooling (Fig. 14)
• Machining allowance: a positive allowance to compensate for the amount of material lost during machining or finishing (Fig. 15)
• Draft or taper allowance: a positive allowance to ensure easy withdrawal of the pattern from the mold (Fig. 16)
• Rapping or shake allowance: a negative allowance to account for the movement of the mold cavity during pattern withdrawal
• Distortion allowance: an allowance to compensate for the distortion of the casting during cooling due to thermal stresses
• Mould wall movement allowance: an allowance to account for the enlargement of the mold cavity due to heat and static pressure

Gating System

• Gating system: all elements connected with the flow of molten metal from the ladle to the mold cavity
• Elements of the gating system:
  • Pouring basin
  • Sprue
  • Sprue base
  • Well
  • Runner
  • Ingate
  • Riser
• Types of gating systems:
  • Horizontal gating system
  • Vertical gating system
  • Top gating system
  • Bottom gating system
  • Middle gating system
• Design requirements for a good gating system:
  • Completely fill the mold in the shortest time possible
  • Smooth metal flow without turbulence
  • Prevent unwanted material from entering the mold cavity
  • Control metal entry to prevent aspiration of atmospheric air
  • Maintain a proper thermal gradient
  • Prevent gating or mold erosion
  • Ensure enough molten metal reaches the mold cavity
  • Economical and easy to implement and remove after casting solidification

Solidification of Metals

• Pure metals: solidify at a constant temperature, with a clearly defined melting point (Fig. 17)
• Alloys: solidify over a range of temperatures, with a mushy zone (Fig. 19)
• Solidification of alloys: dendrites form in the mushy zone, with a liquid metal present between the dendrite arms
• Freezing range: the difference between the liquidus and solidus temperatures of an alloy (Fig. 17)
• Importance of freezing range: affects the microstructure and properties of the casting
• Nucleation agents: can be used to influence the microstructure and properties of the casting

Risers

• Riser: a source of extra metal that flows from the riser to the mold cavity to compensate for shrinkage during solidification

• Functions of risers:

  • Provide extra metal to compensate for volumetric shrinkage
  • Allow mold gases to escape
  • Provide extra metal pressure on the solidifying mold to reproduce mold details more exactly

• Design requirements of risers:

  • Riser size: must be last to freeze, with a larger volume-to-surface-area ratio than the casting
  • Riser placement: must be placed at the portion of the casting that is last to freeze
  • Riser shape: cylindrical risers are recommended, with a hemispherical bottom to increase the volume-to-surface-area ratio

• Riser design: must be designed to freeze after the main casting, with a minimum volume of metal in the riser### Core and Core Box

• A core is a compact mass of core sand that produces hollowness in a casting by not allowing molten metal to occupy a specific space.

• Cores are classified according to shape and position in the mold, with types including horizontal, vertical, balanced, drop, and hanging cores.

• The functions of cores include:

  • Producing hollowness in castings
  • Improving mold surface
  • Providing external undercut features in casting
  • Strengthening the mold
  • Forming gating systems
  • Achieving deep recesses in casting

Special Casting Processes

Centrifugal Casting

• Working principle: It works on the principle of centrifugal force, where a rotating mold forces molten metal to the outer wall, separating slag and other inclusions from the metal.
• Types:
  • True Centrifugal Casting: used for making symmetrical, round, hollow sections without cores.
  • Semi Centrifugal Casting: used for casting large, axi-symmetrical objects with a core inserted at the center.
  • Centrifuging: used for casting shapes that are not axi-symmetrical, with multiple mold cavities connected to a central sprue with radial gates.
• Applications:
  • Aircraft industry
  • Steam turbine bearing shells
  • Roller for steel rolling mills
  • Automobile industry
  • Electronic industries
• Advantages:
  • Dense metal with high mechanical properties
  • Unidirectional solidification
  • No cores required for hollow shapes
  • Gating system and runner eliminated
  • Lower pouring temperature
  • Lower casting defects
• Disadvantages:
  • Limited design capabilities
  • Only suitable for symmetrical shapes
  • High equipment cost
  • Not suitable for all metals
  • Higher maintenance required
  • Skilled operators required
  • Difficult to determine solidification time and temperature distribution

Investment Casting Process

• Also known as the lost wax process
• Begins with the production of wax replicas or patterns of the desired shape
• Patterns are prepared by injecting wax or polystyrene into metal dies
• The mold is prepared by surrounding the pattern with refractory slurry
• The mold is then heated to melt the pattern, leaving a clean cavity behind
• Basic steps:
  • Production of heat-disposable wax patterns
  • Assembly of patterns onto a gating system
  • Investing or covering the pattern assembly with refractory slurry
  • Melting the pattern assembly
  • Firing the mold to remove the last traces of the pattern material
  • Pouring
  • Knockout, cutoff, and finishing
• Advantages:
  • Formation of hollow interiors in cylinders without cores
  • Less material required for gate
  • Fine-grained structure at the outer surface of the casting
• Disadvantages:
  • Segregation of alloy components during pouring
  • Contamination of internal surface of castings with non-metallic inclusions
  • Inaccurate internal diameter

Ceramic Shell Investment Casting Process

• Difference from investment casting: the wax pattern is immersed in a refractory aggregate before dewaxing
• A ceramic shell is built around a tree assembly by repeatedly dipping a pattern into a slurry
• The shell is heated to burn out any residual wax and develop a high-temperature bond
• Advantages:
  • Excellent surface finish
  • Tight dimensional tolerances
  • Machining can be reduced or eliminated

Metal Casting

• Metal casting is one of the oldest materials shaping methods known.
• It involves pouring molten metal into a mold with a cavity of the shape to be made, and allowing it to solidify.
• The solidified object is called the casting.

Advantages of Metal Casting

• Molten material can flow into very small sections, making intricate shapes possible.
• Practically any material that is ferrous or non-ferrous can be cast.
• Metal can be placed exactly where it is required, achieving large savings in weight.
• Simple and inexpensive tools are required for casting molds.
• Certain parts made from metals and alloys can only be processed through casting.

Limitations of Metal Casting

• Dimensional accuracy and surface finish of castings made by sand casting processes are limited.
• The process is labor-intensive.

Components Used for Making a Mould Cavity

• Molding box: A suitable size of molding box is selected for creating a suitable wall thickness.
• Pattern: A replica of the final object to be made, used to create the mold cavity.
• Parting line: The dividing line between the two molding flasks that make up the mold.
• Molding sand: A mixture of silica sand, clay, and moisture in appropriate proportions.
• Facing sand: A small amount of carbonaceous material sprinkled on the inner surface of the mold cavity to improve surface finish.
• Core: A separate part of the mold, made of sand and baked, used to create openings and cavities in the castings.
• Pouring basin: A small funnel-shaped cavity at the top of the mold into which the molten metal is poured.
• Sprue: The passage through which the molten metal reaches the mold cavity.
• 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: Used to support the cores inside the mold cavity to overcome metallostatic force.
• Riser: A column of molten metal placed in the mold to feed the castings as it shrinks and solidifies.
• Vent: Small openings in the mold to facilitate the escape of air and gases.

Pattern

• A pattern is a model or replica of the object to be cast.
• It is embedded in molding sand and suitable ramming of molding sand is made around the pattern.
• The pattern is then withdrawn to generate a cavity in the molding sand.
• Objectives of a pattern include:
  • Preparing a mold cavity for the purpose of making a casting.
  • Possessing core prints that produce seats in the form of extra recess for core placement.
  • Establishing parting lines and parting surfaces in the mold.
  • Forming runners, gates, and risers.
  • Minimizing overall cost of the casting.
  • Helping to establish locating pins on the mold and casting.

Common Pattern Materials

• Wood: Cheap, easily available, and repairable, but susceptible to moisture, warping, and wearing out quickly.
• Metal: Stronger, more durable, and resistant to moisture, but more expensive and heavier than wood.
• Plastic: Lighter, stronger, and more resistant to moisture and wear, but more fragile and less resistant to sudden loading.
• Plaster: Suitable for producing highly intricate castings, but brittle and prone to breaking.
• Wax: Excellent for investment casting, with properties including low ash content, resistance to primary coat material, and high tensile strength and hardness.

Types of Patterns

• One-piece or solid pattern: A simple, single-piece pattern.
• Two-piece or split pattern: A pattern split into two parts for easy withdrawal.
• Cope and drag pattern: A pattern made in two halves, which are mounted on different plates.
• Three-piece or multi-piece pattern: A pattern made in three or more pieces for complex shapes.
• Loose piece pattern: A pattern made of separate pieces that are assembled to form the mold cavity.
• Match plate pattern: A pattern made in two halves, which are joined at the parting line.
• Follow board pattern: A pattern used for small castings.
• Gated pattern: A pattern with a gate attached to the pouring basin.
• Sweep pattern: A pattern used for making large castings.
• Skeleton pattern: A pattern used for making hollow castings.
• Segmental or part pattern: A pattern used for making large castings in segments.### Pattern Types
• Loose-piece pattern: a single piece made to have loose pieces to allow withdrawal from the mold after the molding process is completed (Fig. 6)
• Match plate pattern: a type of pattern that has two parts, one for one side and another for another side of the pattern, used for casting of metal, usually aluminum (Fig. 7)
• Follow board type pattern: a special pattern used to allow the mold to support structurally weak portions, made of wooden material (Fig. 9)
• Gated pattern: used for mass production of casings, multi-cavity moulds are formed by joining a number of patterns and gates, and providing a common runner for the molten metal (Fig. 10)
• Sweep pattern: used for forming large circular molds of symmetric kind, a template of wood or metal is attached to a spindle and revolves to form the desired shape (Fig. 11)
• Skeleton pattern: used when only a small number of large and heavy castings are to be made, a ribbed construction of wood that forms an outline of the pattern (Fig. 12)
• Segmental pattern: used to prepare the mold of larger circular casting, similar to sweep pattern, but the sweep pattern gives a continuous revolve motion to generate the part (Fig. 13)

Pattern Allowances

• Pattern may carry additional allowances to compensate for:
  • Metal shrinkage
  • Machining
  • Draft
  • Distortion
  • Rapping or shake
  • Mould wall movement
• These allowances are given to ensure that the final casting has the desired dimensions and shape
• Shrinkage allowance: a positive allowance to compensate for the contraction of metal during cooling (Fig. 14)
• Machining allowance: a positive allowance to compensate for the amount of material lost during machining or finishing (Fig. 15)
• Draft or taper allowance: a positive allowance to ensure easy withdrawal of the pattern from the mold (Fig. 16)
• Rapping or shake allowance: a negative allowance to account for the movement of the mold cavity during pattern withdrawal
• Distortion allowance: an allowance to compensate for the distortion of the casting during cooling due to thermal stresses
• Mould wall movement allowance: an allowance to account for the enlargement of the mold cavity due to heat and static pressure

Gating System

• Gating system: all elements connected with the flow of molten metal from the ladle to the mold cavity
• Elements of the gating system:
  • Pouring basin
  • Sprue
  • Sprue base
  • Well
  • Runner
  • Ingate
  • Riser
• Types of gating systems:
  • Horizontal gating system
  • Vertical gating system
  • Top gating system
  • Bottom gating system
  • Middle gating system
• Design requirements for a good gating system:
  • Completely fill the mold in the shortest time possible
  • Smooth metal flow without turbulence
  • Prevent unwanted material from entering the mold cavity
  • Control metal entry to prevent aspiration of atmospheric air
  • Maintain a proper thermal gradient
  • Prevent gating or mold erosion
  • Ensure enough molten metal reaches the mold cavity
  • Economical and easy to implement and remove after casting solidification

Solidification of Metals

• Pure metals: solidify at a constant temperature, with a clearly defined melting point (Fig. 17)
• Alloys: solidify over a range of temperatures, with a mushy zone (Fig. 19)
• Solidification of alloys: dendrites form in the mushy zone, with a liquid metal present between the dendrite arms
• Freezing range: the difference between the liquidus and solidus temperatures of an alloy (Fig. 17)
• Importance of freezing range: affects the microstructure and properties of the casting
• Nucleation agents: can be used to influence the microstructure and properties of the casting

Risers

• Riser: a source of extra metal that flows from the riser to the mold cavity to compensate for shrinkage during solidification

• Functions of risers:

  • Provide extra metal to compensate for volumetric shrinkage
  • Allow mold gases to escape
  • Provide extra metal pressure on the solidifying mold to reproduce mold details more exactly

• Design requirements of risers:

  • Riser size: must be last to freeze, with a larger volume-to-surface-area ratio than the casting
  • Riser placement: must be placed at the portion of the casting that is last to freeze
  • Riser shape: cylindrical risers are recommended, with a hemispherical bottom to increase the volume-to-surface-area ratio

• Riser design: must be designed to freeze after the main casting, with a minimum volume of metal in the riser### Core and Core Box

• A core is a compact mass of core sand that produces hollowness in a casting by not allowing molten metal to occupy a specific space.

• Cores are classified according to shape and position in the mold, with types including horizontal, vertical, balanced, drop, and hanging cores.

• The functions of cores include:

  • Producing hollowness in castings
  • Improving mold surface
  • Providing external undercut features in casting
  • Strengthening the mold
  • Forming gating systems
  • Achieving deep recesses in casting

Special Casting Processes

Centrifugal Casting

• Working principle: It works on the principle of centrifugal force, where a rotating mold forces molten metal to the outer wall, separating slag and other inclusions from the metal.
• Types:
  • True Centrifugal Casting: used for making symmetrical, round, hollow sections without cores.
  • Semi Centrifugal Casting: used for casting large, axi-symmetrical objects with a core inserted at the center.
  • Centrifuging: used for casting shapes that are not axi-symmetrical, with multiple mold cavities connected to a central sprue with radial gates.
• Applications:
  • Aircraft industry
  • Steam turbine bearing shells
  • Roller for steel rolling mills
  • Automobile industry
  • Electronic industries
• Advantages:
  • Dense metal with high mechanical properties
  • Unidirectional solidification
  • No cores required for hollow shapes
  • Gating system and runner eliminated
  • Lower pouring temperature
  • Lower casting defects
• Disadvantages:
  • Limited design capabilities
  • Only suitable for symmetrical shapes
  • High equipment cost
  • Not suitable for all metals
  • Higher maintenance required
  • Skilled operators required
  • Difficult to determine solidification time and temperature distribution

Investment Casting Process

• Also known as the lost wax process
• Begins with the production of wax replicas or patterns of the desired shape
• Patterns are prepared by injecting wax or polystyrene into metal dies
• The mold is prepared by surrounding the pattern with refractory slurry
• The mold is then heated to melt the pattern, leaving a clean cavity behind
• Basic steps:
  • Production of heat-disposable wax patterns
  • Assembly of patterns onto a gating system
  • Investing or covering the pattern assembly with refractory slurry
  • Melting the pattern assembly
  • Firing the mold to remove the last traces of the pattern material
  • Pouring
  • Knockout, cutoff, and finishing
• Advantages:
  • Formation of hollow interiors in cylinders without cores
  • Less material required for gate
  • Fine-grained structure at the outer surface of the casting
• Disadvantages:
  • Segregation of alloy components during pouring
  • Contamination of internal surface of castings with non-metallic inclusions
  • Inaccurate internal diameter

Ceramic Shell Investment Casting Process

• Difference from investment casting: the wax pattern is immersed in a refractory aggregate before dewaxing
• A ceramic shell is built around a tree assembly by repeatedly dipping a pattern into a slurry
• The shell is heated to burn out any residual wax and develop a high-temperature bond
• Advantages:
  • Excellent surface finish
  • Tight dimensional tolerances
  • Machining can be reduced or eliminated

This quiz covers the basics of metal casting, a crucial process in manufacturing. It explains the importance of cast metal products in various industries and their applications.