Metal Casting Process: Types, Steps, Defects & Examples

Contents

• What Is Metal Casting?
• The Foundry Casting Process Step by Step
• How Metal Is Melted in a Foundry
• Advantages and Disadvantages of Casting
• Types of Casting Processes
• Parts of a Sand Casting Mold
• Sand Casting
• Shell Mold Casting
• Investment Casting
• Die Casting
• Casting Process Comparison Table
• Metals Used in Casting
• Automotive Castings
• Common Casting Defects
• Casting Design Guidelines
• Casting vs Forging vs Machining
• History of Casting
• Frequently Asked Questions

What Is Metal Casting?

Casting is the process of making parts in a foundry by pouring molten material into a mold. The metal cools and solidifies, taking the shape of the mold cavity. Casting is one of the oldest manufacturing processes and is still used for everything from engine blocks and pump housings to sculptures and jewelry. To see how casting fits among other shaping methods, read our overview of metalworking.

The Foundry Casting Process Step by Step

Although details vary by process, most foundry casting follows the same sequence:

1. Pattern making: a pattern, a replica of the part made slightly oversize to allow for shrinkage, is made from wood, metal, plastic or wax.
2. Mold making: the pattern is used to form a mold cavity, often with cores that create internal features.
3. Melting: metal is heated in a furnace above its melting point.
4. Pouring: molten metal flows through the gating system into the mold cavity.
5. Solidification: the metal cools and hardens. See solidification of metals and alloys.
6. Shakeout: the casting is removed from the mold. Expendable molds are broken apart.
7. Cleaning and finishing: gates, runners and risers are cut off, and the surface is cleaned and ground.
8. Heat treatment: some castings are heat treated to improve properties.
9. Inspection: parts are checked for dimensions and defects.

How Metal Is Melted in a Foundry

Metal is melted in electric or gas-fired furnaces and heated above its melting point so it flows easily into the mold. The heat energy required is the sum of:

• the energy to raise the metal to its melting point,
• the heat of fusion needed to change it from solid to liquid, and
• the heat needed to raise the molten metal to the desired pouring temperature.

High-quality molten metal is the first step toward high-quality castings.

Advantages and Disadvantages of Casting

Advantages of the Casting Process

1. Cost-effective and fast for complex parts.
2. Can create parts with complex internal and external geometries.
3. Produces near-net-shape components, so less machining is needed.
4. Scrap and parts are easy to recycle.
5. Works for a very wide range of part sizes.

Disadvantages of Casting

1. Mechanical properties are usually lower than those of wrought metals. The load-bearing capacity of a cast part is usually lower than that of a forged part.
2. Dimensional tolerances are generally looser than in wrought or machined processes.
3. Surface finish can be rough, especially in sand casting.
4. Safety hazards such as molten metal, heat and fumes.
5. Environmental concerns such as emissions, sand and slag disposal, and energy use.

Types of Casting Processes

Casting processes are grouped by the type of mold. The three most widely taught processes are sand casting (expendable mold, permanent pattern), investment casting (expendable mold, expendable pattern) and die casting (permanent mold). Sand casting is the most widely used of the three.

Expendable Mold Casting

Expendable mold processes use a mold that must be destroyed to remove the casting. Examples include sand, shell, plaster, ceramic, investment (lost wax) and lost foam casting. As part complexity increases, expendable molds become more likely. They are common for small runs of specialized castings such as pump housings, and also for mass production of intricate parts such as engine blocks and cylinder heads.

Permanent Mold Casting

Permanent mold casting uses a mold that is reused many times, so it suits consistent, higher production volumes. The mold is usually made of tool steel or another material able to withstand casting temperatures. Common types are gravity, slush, low-pressure and vacuum permanent mold casting, along with high-pressure die casting.

Parts of a Sand Casting Mold

A typical sand casting mold has two halves: the cope (top) and the drag (bottom). Molten metal is poured into the pouring cup or basin and flows through the gating system to fill the cavity. Risers and vents help make sure the cavity fills completely and that shrinkage is fed, which gives a higher-quality part. Key mold terminology:

• Cavity: the space that forms the part.
• Gating system: the channels that deliver metal, made up of the sprue (downsprue), well, runner and gates.
• Riser: a reservoir of molten metal that feeds the casting as it shrinks. Risers can be open or blind.
• Parting line: where the two mold halves meet.
• Flask, cope and drag: the box and mold halves that hold the sand.

Parts of a Sand Cast Part

As poured, a sand casting includes the casting itself plus the top riser, side riser, gate, runner, well, sprue and pouring cup. All risers and the gating system are waste metal that is cut off and re-melted to make more castings. To reduce waste, designers try to keep the volume of metal in the riser as small as possible.

Sand Casting: Process, Advantages and Disadvantages

Sand casting is the most widely used casting process. Sand serves as the mold material. Metal is melted, poured into a mold with the desired cavity shape, and left to cool and solidify.

Sand Casting Advantages

• Nearly all cast alloys can be poured into sand molds.
• Part size and weight can range widely.
• Complex shapes are possible.
• Tooling cost is much lower than for other casting processes.
• Short lead times.

Sand Casting Disadvantages

• Rough surface finish from the sand texture.
• Lower dimensional accuracy, partly from solidification shrinkage.
• Some defects, such as porosity, are hard to avoid entirely, which can lower strength.
• Secondary machining is usually needed for better finish or tighter tolerances.

Shell Mold Casting: Process, Advantages and Disadvantages

Shell molding is an expendable mold process that uses resin-coated sand to form a thin, hard shell mold. Both ferrous and non-ferrous metals can be cast. Compared with sand casting, it offers lower labor cost, higher productivity and better dimensional accuracy.

• Advantages: higher accuracy than sand casting (which can reduce machining), good surface quality, thin walls, good permeability that reduces gas defects, and lower skill requirements.
• Disadvantages: usually economical only for higher volumes, pattern cost is high, and part size is limited.

Investment Casting (Lost Wax): Process, Advantages and Disadvantages

Investment casting, also called lost wax casting, is one of the oldest casting techniques. A wax pattern is coated in refractory ceramic, the wax is melted out, and molten metal is poured into the resulting cavity. The ceramic shell is then broken away. Because it makes highly accurate, complex shapes, it is a precision casting process. Ferrous and non-ferrous metals can both be cast.

• Advantages: complex and intricate shapes, excellent surface finish, tight tolerances, thin walls, low porosity, no draft required, cores often unnecessary, and low material waste.
• Disadvantages: expensive because of the many steps and labor, a new pattern is needed for every casting, and part size is limited.

Die Casting: Hot Chamber and Cold Chamber

Die casting forces molten metal into a reusable steel die under pressure. There are two main variations. In hot chamber die casting, also called gooseneck casting, the injection system sits in the molten metal, so it is used for low-melting-point alloys such as zinc and magnesium. In cold chamber die casting, molten metal is ladled into the shot chamber for each cycle, which suits higher-melting alloys such as aluminum. The metal is held under pressure while it solidifies.

• Advantages: good dimensional accuracy and surface finish, fast cycle times, economical for mass production, and a fine-grained structure with good mechanical properties.
• Disadvantages: die and equipment costs are high, so it is uneconomical for small runs, skilled operators are needed, and hot chamber machines only work with low-melting-point alloys.

Other Casting Processes

Other processes include lost foam, plaster and ceramic mold, centrifugal, squeeze, continuous and vacuum casting. Each trades cost, accuracy and part size differently.

Casting Process Comparison Table

Process	Mold type	Surface finish	Dimensional accuracy	Tooling cost	Best for
Sand casting	Expendable	Rough	Lower	Low	Large parts, low volume, most alloys
Shell molding	Expendable	Good	Medium to good	Medium	Medium to high volume, smaller parts
Investment casting	Expendable	Excellent	High	Medium to high	Small, intricate, precision parts
Permanent mold (gravity)	Permanent	Good	Medium to good	Medium to high	Consistent volumes, non-ferrous alloys
Die casting	Permanent	Very good	High	High	High-volume zinc, aluminum and magnesium parts

Metals Used in Casting

Metal	Typical processes	Typical applications
Cast iron	Sand casting	Engine blocks, brake rotors and drums, pipe fittings
Steel	Sand, investment	Heavy machinery, valves, structural components
Aluminum alloys	Sand, die, permanent mold, investment	Wheels, cylinder heads, transmission housings
Zinc alloys	Hot chamber die casting	Small housings, hardware, connectors
Magnesium alloys	Die casting	Lightweight housings and brackets
Copper alloys (bronze, brass)	Sand, investment, centrifugal	Bearings, fittings, sculptures

Automotive Castings: Which Car Parts Are Cast?

Internal combustion engine (ICE) vehicles contain more cast parts than electric vehicles (EVs). Many are in the powertrain: pistons, engine blocks, valve covers, transmission housings and rear differential housings. Chassis parts include wheels (rims) and brake rotors, discs and drums. The alternator housing is a cast part in the electrical system. Some manufacturers are also casting large sections of the body structure to reduce the number of parts in the body-in-white. See also vehicle lightweighting.

Casting Processes Used in the Automotive Industry

• Sand casting
• Die casting
• Lost wax (investment) casting
• Lost foam casting
• Cosworth process (a precision sand casting process for aluminum)
• Squeeze casting

Common Casting Defects

Defects are a major cost in casting, so foundries design the gating, risers and process parameters to prevent them.

Defect	What it is	Common causes
Gas porosity	Gas bubbles, pinholes or blowholes trapped in the metal	Moisture or gas in the mold or melt, turbulence, poor venting
Shrinkage porosity	Voids from metal shrinking as it solidifies	Thick sections cooling last, inadequate risers
Misrun	Cavity not completely filled	Low metal or die temperature, slow pouring
Cold shut	Seam where two metal streams meet without fusing	Low temperature, interrupted or turbulent flow
Hot tear	Crack formed while the metal is still weak	Restrained shrinkage, abrupt section changes
Mold shift	Misaligned casting halves	Misalignment of cope and drag or die halves

Types of Porosity in Castings

Molten metal can react with moisture and trap gas vapor, and the metal may solidify before the gas escapes, leaving a hole. High pour rates cause turbulence that can trap gas, while pouring too slowly can let the metal freeze before the mold fills. For non-eutectic alloys, dendrites form during solidification and can cause microporosity. Porosity falls into three groups:

• Macroshrinkage: large shrinkage cavities.
• Gas porosity: air or gas trapped as the metal solidifies.
• Microshrinkage (microporosity): small voids left when metal shrinks away from thick sections toward thin ones.

Solidification Shrinkage

Most metals shrink as they solidify, which is called solidification shrinkage. Molds are made slightly larger than the finished part to compensate, with the allowance depending on the alloy.

Why Chills Are Used in Casting

Chills are metal inserts placed in the mold to eliminate shrinkage porosity. In a casting with varying thickness, the thickest area cools last and often develops porosity. Placing a chill at the thickest section speeds up cooling there so the section solidifies in step with the rest of the part.

Casting Design Guidelines

• Keep wall thickness as uniform as possible to avoid hot spots and shrinkage porosity.
• Add generous fillets and radii instead of sharp corners, which concentrate stress and cause hot tears.
• Include draft angles so the pattern or part releases from the mold (investment casting is a notable exception).
• Plan the parting line to keep it simple and reduce the need for cores.
• Allow for shrinkage and machining stock where tight tolerances or smooth surfaces are required.
• Design for feeding: make thick sections accessible to risers so they can be fed as they solidify.

For the assembly side of the design, see design for assembly (DFA) and design for manufacture (DFM).

Casting vs Forging vs Machining

Aspect	Casting	Forging	Machining
How the shape is made	Pouring liquid metal into a mold	Compressive force deforms solid metal	Material is cut away
Geometric complexity	Very high, including internal cavities	Moderate	High, limited by tool access
Mechanical strength	Usually lower than wrought	Typically highest	Same as stock material
Tooling cost	Low (sand) to high (die casting)	High dies	Low to medium
Material waste	Low, with scrap re-melted	Low to medium	Highest, as chips

Learn more about forging, machining operations, rolling, extrusion and powder metallurgy.

History of Casting

Casting is the oldest manufacturing process. The first metal castings can be traced to roughly 4000 BC, with copper castings in Mesopotamia. Bronze casting was developed by the ancient Sumerians in western Asia around 3500 BC, and by about 1200 BC it appeared in Greece, Africa and other parts of Asia.

The Chinese produced cast iron as early as the 6th century BC, but iron casting was not widely adopted in the western world until roughly 1600 to 1800 AD.

Frequently Asked Questions About Metal Casting

What is metal casting?

Metal casting is a manufacturing process in which molten metal is poured into a mold cavity, allowed to cool and solidify, and then removed as a finished or near-finished part called a casting.

What are the steps of the foundry casting process?

The main steps are pattern making, mold making, melting the metal, pouring, solidification, shakeout, cleaning and finishing, heat treatment where needed, and inspection.

What are the three types of casting?

The three most commonly taught processes are sand casting (expendable mold, permanent pattern), investment casting (expendable mold and pattern) and die casting (permanent mold). Casting processes are generally grouped into expendable-mold and permanent-mold methods.

What are castings?

A casting is a part made by pouring molten material, usually metal, into a mold and letting it solidify. Examples include engine blocks, pump housings, wheels and brake rotors.

What metals can be cast?

Common cast metals include cast iron, steel, aluminum, zinc, magnesium and copper alloys such as bronze and brass. The choice of casting process depends on the metal’s melting point and the part requirements.

What are the most common casting defects?

Common defects include gas porosity, shrinkage porosity, misruns, cold shuts, hot tears and mold shift. They are mainly caused by trapped gas, poor feeding during solidification, low metal temperature and turbulence.

What is the difference between casting and a foundry?

Casting is the process of forming metal parts by pouring molten metal into a mold. A foundry is the factory or facility where casting is carried out.

What is the difference between hot chamber and cold chamber die casting?

In hot chamber die casting the injection system is submerged in the molten metal, so it is used for low-melting alloys such as zinc and magnesium. In cold chamber die casting the molten metal is ladled into the shot chamber for each cycle, which suits higher-melting alloys such as aluminum.

Conclusion

Metal casting turns molten metal into complex, near-net-shape parts. Choosing the right process, from low-cost sand casting to high-volume die casting and precision investment casting, comes down to part size, complexity, tolerance, volume and alloy. Explore more in the Manufacturing Processes category.