Casting Process
Manufacturing Technology – Mechanical Engineering
By Home Academy
Introduction to Casting
Casting is one of the oldest and most important manufacturing processes used to produce solid metal components known as castings. In this process, molten metal is poured into a specially prepared mold cavity that represents the desired shape of the final product. After pouring, the metal is allowed to cool and solidify, taking the exact shape of the mold cavity.
Casting provides exceptional flexibility in manufacturing complex, intricate, and large components that are difficult or uneconomical to produce using other manufacturing methods. It is also well suited for high-volume production, where material usage can be efficiently controlled, reducing waste and overall cost. The casting process is also commonly referred to as founding.
Foundry
A foundry is an industrial facility where metal castings are produced. In addition to casting, foundries offer a wide range of related services including mold and core making, metal melting, pouring, degassing, heat treatment, surface cleaning, inspection, and finishing operations. Foundries may specialize in ferrous, non-ferrous, or steel castings depending on their production capabilities.
Major Casting Processes
Centrifugal Casting
Centrifugal casting is a process in which a reusable mold is rotated at high speed while molten metal is poured into it. Due to centrifugal force, the molten metal is forced outward against the mold walls, resulting in dense and high-quality castings.
This process allows the production of components with uniform wall thickness and eliminates the need for cores. Finished products are free from gates, risers, and parting lines. Centrifugal casting is widely used for manufacturing pipes, tubes, bushings, and cylindrical components that may later undergo machining.
Die Casting
Die casting is a process in which molten metal is injected into a hardened steel mold (die) under high pressure. The mold consists of two halves that are clamped together during injection. Once the metal solidifies, the dies are opened and the casting is ejected.
Die casting produces components with high dimensional accuracy, excellent surface finish, and intricate details, often requiring minimal machining. However, the high cost of die manufacturing makes this process suitable mainly for large-scale production. Ferrous metals are rarely used; aluminum, zinc, and magnesium alloys are common.
Investment Casting (Lost Wax Casting)
Investment casting is a precision casting process used for producing complex and high-accuracy components with smooth surfaces and no visible parting lines. Due to its high setup cost, it is ideal for high-volume production of intricate parts.
Steps in Investment Casting:
Wax patterns are produced using injection molds.
Wax patterns are coated with ceramic material to form disposable molds.
The ceramic molds are heated to melt and remove the wax.
Molten metal is poured into the ceramic molds.
After solidification, the ceramic shell is broken to obtain the casting.
Gates, risers, and excess metal are removed.
This process is widely used in aerospace, medical, and precision engineering industries.
Sand Casting
Sand casting is one of the most commonly used casting methods in foundries. It uses sand as the mold material, typically mixed with clay and water to provide strength and plasticity. The mold cavity is formed by packing sand around a pattern placed inside a flask.
After pouring and cooling, the sand mold is broken to retrieve the casting. Although surface finish is relatively rough and machining is often required, sand casting is extremely versatile and economical. It can be used for both small batch and mass production, and for a wide range of metals.
Metals Used in Casting
Alloy
An alloy is a mixture of two or more elements, at least one of which is a metal. Alloys are produced to enhance properties such as strength, hardness, corrosion resistance, melting point, and cost effectiveness.
Aluminum
Aluminum is a lightweight, non-ferrous metal with excellent corrosion resistance and castability. It is commonly alloyed with copper, silicon, magnesium, zinc, and manganese.
Carbon Steel
Carbon steel contains 0.12–2.0% carbon and limited alloying elements. Higher carbon content increases hardness but reduces ductility. Carbon steel does not include stainless steel.
Cast Iron
Cast iron contains approximately 2–4% carbon and 1–3% silicon. It is known for good castability and wear resistance. Common types include gray cast iron and white cast iron.
Ductile Iron
Ductile iron is an advanced form of cast iron produced by adding magnesium or cerium to form spherical graphite. It offers superior ductility and impact resistance and is also known as nodular or spheroidal graphite iron.
Ferrous and Non-Ferrous Alloys
Ferrous alloys contain iron as the main constituent, while non-ferrous alloys contain little or no iron.
Stainless Steel
Stainless steel contains 10.5–30% chromium, which forms a protective oxide layer providing corrosion resistance. Additional alloying elements may include nickel, molybdenum, and titanium.
Steel Alloy
Steel alloys are produced by adding elements such as manganese, chromium, nickel, and molybdenum to improve mechanical properties and resistance to wear and corrosion.
Steel Casting
Steel casting is a specialized casting process involving carbon and alloy steels. It is widely used for producing components that require high strength, toughness, and wear resistance, such as machine parts, valves, gears, and structural components.
Important Foundry and Casting Terms
Annealing is a heat treatment process involving heating followed by controlled cooling to improve ductility and reduce hardness.
Case hardening increases surface hardness while maintaining a tough core.
Cope and drag are the upper and lower halves of a mold respectively.
A core is an insert used to create internal cavities that cannot be formed by patterns alone.
Degassing removes unwanted gases from molten metal to prevent porosity.
Draft is a taper provided on patterns to facilitate easy removal.
Flask is the container holding the sand mold.
Gates and gating systems guide molten metal into the mold cavity.
Green sand casting uses moist sand with clay binders for high strength molds.
Machining removes excess material to achieve final dimensions.
Risers compensate for shrinkage during solidification.
Sprue is the channel through which molten metal enters the mold.
Shrinkage refers to contraction of metal during cooling.
Soundness indicates the absence of defects such as porosity or inclusions.
Tolerance defines the permissible dimensional variation in cast components.
Conclusion
The casting process is a cornerstone of manufacturing technology in mechanical engineering. With its ability to produce complex shapes, handle a wide variety of metals, and support both low and high production volumes, casting remains indispensable in modern industry. A sound understanding of casting processes, materials, and foundry terminology is essential for engineers, technicians, and competitive exam aspirants.
By Home Academy – Mechanical Engineering Manufacturing Technology