Metal Casting Basics

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• The pattern is then withdrawn to generate a cavity in the molding sand.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

Learn about the importance of metal casting in various industries and its role in shaping our economy. Discover the different types of metal cast products used in everyday life.