Metal Casting: Definition, Types, and Process

What is Metal Casting?

Metal casting is defined as the process in which molten metal is poured into a mould that contains a hollow cavity of a desired geometrical shape and allowed to cool down to form a solidified part. The term ‘casting’ is also used to describe the part made by the casting process which dates back 6000 years.

Historically it is used to make complex and/or large parts, which would have been difficult or expensive to manufacture using other manufacturing processes.

Metal casting is a modern process with ancient roots. In the metal casting process, metal shapes are formed by pouring molten metal into a mold cavity, where it is cooled and later extracted from the mold.

Metal casting is arguably the earliest and most influential industrial process in history. It’s used to make many of the metal objects used in our daily lives: automotive parts, train wheels, lamp posts, school bus pedals, and much more.

Plus, metal casting foundries rely on metal recycling as a cost-efficient source of raw material, significantly reducing wasted scrap metal that might end up in landfills.

The Metal Casting Process

1. Patternmaking

A pattern is a replica of the exterior of the casting. Patterns are typically made of wood, metal, plastic, or plaster. Patternmaking is incredibly important for industrial part-making, where precise calculations are needed to make pieces fit and work together.

2. Core making

If casting is hollow, an additional piece of sand or metal (called a core) shapes the internal form to make it hollow. Cores are typically strong yet collapsible so they can be easily removed from the finished casting.

3. Molding

To visualize the metal casting process so far, imagine yourself walking on the beach toward the ocean. Look at a footprint you leave behind in the wet sand. Your foot would be the core, and the impression left in the sand is a mold of your foot.

Molding is a multistep process that will form a cast around the pattern using molding sand. In casting, a mold is contained in a frame called a flask. Greensand, or molding sand, is packed into the flask around the pattern.

This is known as metal sand casting. Once the sand is packed tight, the pattern can be removed and the cast will remain. Alternatively, a two-piece, non-destructible metal mold can be created so that the mold can be used repeatedly to cast identical parts for industrial applications.

4. Melting and Pouring Molten Metal

After the metal is melted, it is poured into the cavity of the mold and left to solidify. Once solidified, the shakeout process begins: the molds undergo vibration to remove sand from the casting.

In industrial applications, equipment like our Two-Mass Shakeouts keeps production output high because of its efficient and smooth performance. Removed sand is typically collected, cooled, and reclaimed to be used once more in future castings.

5. Cleaning

In this final step, the cast metal object is removed from the mold and then fettled. During the fettling, the object is cleaned of any molding material, and rough edges are removed.

Types of Metal Casting

Metal casting can be divided into two groups by the basic nature of the mould design. i.e. expendable mould and permanent mould castings. It can be further subdivided into groups depending on their pattern material.

  • Reusable molds:
    • Permanent molds
    • Semi-permanent molds
    • Slush casting
    • Centrifugal casting
    • Pressure casting
    • Die casting
  • Expendable molds:
    • Sand casting
    • Shell molding
    • Investment casting (lost-wax)
    • Full mold or foam

Expendable mould casting

Expendable mould casting, as the name suggests uses a temporary non-reusable mould to produce the final casting as the mould will be broken to get the casting out. These moulds are typically made of materials such as sand, ceramics & plaster.

These are generally bonded using binders called bonding agent to improve its properties.  Complex intricate geometries can be cast using expendable mould casting.

Permanent mould casting

Sometimes called non-expendable mould casting, permanent mould casting uses permanent moulds that are reused after each production cycle. Although permanent mould casting produces repeatable parts due to re-use of the same mould, it can only produce simple castings as the mould needs to be opened to remove the castings.

Composite mould casting

As the name suggests these uses both expendable and re-useable casting moulds to produce castings. These normally include materials such as sand, wood, graphite and metal.

Application of Metal casting

A sector-wise casting consumption is given below which highlights the importance of casting in any industrial setup.

  • Transport: Automobile, aerospace, railways and shipping
  • Heavy Equipment: Construction, farming and mining
  • Machine Tools: Machining, casting, plastics molding, forging, extrusion and forming
  • Plant Machinery: Chemical, petroleum, paper, sugar, textile, steel and thermal plants
  • Defense: Vehicles, artillery, munitions, storage and supporting equipment
  • Electrical Equipment Machines: Motors, generators, pumps and compressors
  • Hardware: Plumbing industry pipes, joints, valves and fittings
  • Household: Appliances, kitchen and gardening equipment, furniture and fittings
  • Art Objects: Sculptures, idols, furniture, lamp stands and decorative items

Advantages of metal casting

  • Metal casting can produce complex shapes
  • Features like internal cavities or hollow sections can be easily achieved
  • Large components can be produced in one-piece cast
  • Materials that are difficult or expensive to manufacture using other manufacturing process can be cast
  • Compared to other manufacturing processes, casting is cheaper for medium to large quantities
  • Almost all the metals can be cast
  • Near net shape often without or very minor post-processing

Disadvantages of metal casting

  • Relatively coarse surface finish and hence wider tolerance has to be allowed and not suitable for mating interfaces
  • Metal casting such as shell moulding has a limit in terms of size and the pattern
  • Patterns are time-consuming and expensive to make although additive manufacturing processes such as binder jetting are being used lately to make a mould
  • Die casting can be very expensive for smaller to medium quantities due to high die cost
  • Part size and material choices depend on the casting process chosen. For instance, only non-ferrous metal can be used for permanent mould castings