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Production Technology: Casting Technology

• Casting is a manufacturing process where liquid material is poured into a mold with a hollow cavity of the desired shape. The liquid material solidifies, forming a casting that is removed from the mold.
• Topics covered in this presentation include: casting process definition, classifications and types of casting processes, casting advantages and disadvantages, sand casting process, and casting defects.

Casting Process Definition

• Casting is a manufacturing method involving pouring liquid material into a mold containing a hollow cavity.
• The liquid solidifies, creating a casting.
• The casting is then removed from the mold.

Casting Products

• Examples of casting products shown include various gears, pump components, and engine parts.

Classifications and Types of Casting Processes

• Casting processes are categorized based on the mold's nature.
• Expendable mold castings use molds that are destroyed after the casting is removed (e.g., sand casting).
• Mold materials: sand, wax, plaster
• Binders are also included in expandable mold materials.
• Permanent mold castings use molds that are reusable (e.g., die casting).
• Mold materials: metal or, less commonly, a ceramic refractory material

Types of Casting Processes

• Die casting: A metal casting process that forces molten metal under high pressure into a mold cavity. This process uses hardened tool steel dies and is used for mass production of highly accurate, well-finished parts.
• Investment casting: An older manufacturing process pouring molten metal into an expendable ceramic mold. A wax pattern forms the mold, and ceramic slurry hardens the mold. This process can also be referred to as "lost-wax" casting.
• Centrifugal casting: Molten metal is poured into rotating permanent molds. Centrifugal force throws the metal outward, solidifying against the mold walls.
• Sand casting: A metal casting process using sand as the mold material. Molten metal is poured into a pre-formed sand cavity (the mold). The metal solidifies, and the sand mold is broken apart to retrieve the casting.

Sand Casting Terminology

• Flask: A metal or wooden frame used to form the mold halves.
• Cope: The upper half of the mold flask.
• Drag: The bottom half of the mold flask.
• Mold: The flask, sand, and gating system together.
• Gating system: A set of channels (pouring cup, sprue, runners, gate) delivering molten metal to the mold cavity and channels for air and gas removal.
• Riser: A column of molten metal placed in the mold to compensate for shrinkage
• Core: A separate component of the mold, typically made of sand, to create specific shapes/cavities within the casting.
• Mold cavity: The hollow area in the mold where the metal solidifies.
• Pattern: A replica of the final object created to form the mold cavity.

Steps of Sand Casting

• Patternmaking: Creating a replica of the desired casting using materials like wood, metal, or wax.
• Core making: Creating a separate part of the mold (core) that defines the internal cavities of the casting.
• Molding: Preparing the sand mold around the pattern (and core).
• Melting and pouring: Melting the metal before pouring it into the mold.
• Cleaning and fettling: Removing sand, excess metal, and other unwanted parts from the casting.
• Inspection and tests: Inspecting and testing the casting to ensure quality.

Pattern Allowances

• Shrinkage allowance: The design increase added to the pattern to compensate for the contraction of the metal during cooling.
• Draft (or taper) allowance: The angle added to the pattern's vertical faces to facilitate its removal from the sand mold without tearing the mold surfaces.
• Machining Allowance: Extra metal on the casting surface needed for machining the casting to the necessary dimensions.
• Distortion (or camber) allowance: A compensatory allowance to the pattern to compensate for warpage/distortion of the casting during cooling.

Casting Defects

• Blow holes: Cavities in the casting caused by gases.
• Shifts: External casting defects due to core misplacement or mold mismatching.
• Inclusion: Contamination of the casting by external materials (e.g., slag).
• Misrun: The molten metal not completely filling the mold cavity, potentially due to high viscosity or missing/small risers.
• Shrinkage cavity: A void or depression in the casting due to contraction during cooling.
• Hot tears: Internal or external cracks caused by contraction stresses when the metal is cooling and relatively weak.
• Swell: Enlargement of the mold cavity due to insufficient ramming of the sand or rapid pouring of molten metal.