Metal Casting Process 101

What is Metal Casting?

Metal casting is the process of making objects by pouring molten metal into a mold that has a shaped space. The metal then cools and hardens to a solidified part. Historically it is used to make complex and/or large parts, casting is often a less expensive way to manufacture a piece compared with machining the part out of a piece of solid metal.

There are many metal casting methods to choose from. What type of casting is most efficient depends on the metals used, the size of the run, and the complexity of the casting. Metal casting can be divided into two groups: processes with expendable molds and processes with permanent molds.

Sand, plastic, clay or wax are used for the casting process. These molds are discarded after use.

Sand Casting


Sand casting utilizes expendable sand molds to form complex metal parts that can be made of nearly any alloy. Because the sand mold must be destroyed in order to remove the part, called the casting, sand casting typically has a low production rate. 

Investment Casting


Also named lost-wax casting, it is an alternative to sand casting that can work with most grades of metal, even high-melting point ferrous alloys, and yet avoids some of these challenges of patternmaking in sand casting.

Lost Foam Casting


Lost foam casting is a type of evaporative pattern casting.It uses pattern materials that evaporate during the pour, which means there is no need to remove the pattern material from the mold before casting.

The molds are generally metal, but the method of performing the casting differs greatly from many expendable methods.

Gravity Casting


Gravity die casting is among the oldest known processes for fabricating metals and metal alloys. It involves the pouring of molten metal from a crucible into a mold under only the force of gravity, without the use of pressurized gases, vacuums, or centrifugal force.

Low Pressure Casting


In low pressure die casting, the die is filled with metal from a pressurised furnace, with pressures typically around 0.7 bar. In this process, the holding furnace is located below the cast and the liquid metal is forced upwards through a riser tube and into the cavity. 

Die Casting


The die casting process forces molten metal under high pressure into mold cavities (which are machined into dies). The die casting method is especially suited for applications where many small to medium-sized parts are needed with good detail, a fine surface quality and dimensional consistency.

Centrifugal Casting


The centrifugal casting process steps begin with molten metal being poured into a preheated, water cooled, spinning mold. The mold is rotated around its central axis at high speed. Centrifugal force pulls the liquid metal along the mold’s surface in an even layer.

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How Does Metal Casting Work?

Advantages of metal casting

As with any other manufacturing process, a basic understanding of the process, its underlying science, its pros and cons are essential for manufacturing low-cost quality engineer products.

  • Metal casting can create any complex structure economically.
  • 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 processes can be cast.
  • Compared to other manufacturing processes, casting is cheaper for medium to large quantities.
  • Castable materials are wide range.
  • Near net shape often without or very minor post-processing.

Below is a comparison of relative surface finishes that can be expected from various casting process:

Casting Process RMS Range
Die 20 – 120
Investment 60 – 200
Shell Mold 120 – 300
Centrifugal – Standard tooling 400 – 500
Centrifugal – Permanent Mold 20 – 300
Static – Permanent Mold 200 – 420
Normal Non-Ferrous Sand 300 – 560
Normal Ferrous Green Sand 560 – 900

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    Material choices & Applications

    Although almost all the metals can be used, the most common ones are iron, steel, aluminium and copper-based alloys such as bronze.

    Choosing the most appropriate alloy for a cast component can be a daunting process given the range of alloys available and their widely varying capabilities and limitations. The process of elimination usually starts with consideration of various performance indices for the product such as mass, energy density, and power-to-weight ratio.

    The values for these performance indices will depend on three factors: the function of the product, the geometry and dimensions of the product, and the environment under which the product needs to work. These factors determine the possible set of metal alloys that could be used in casting the component. It may also be that the identification of the most appropriate alloy will exclude some of the available casting processes, even at this early stage. If it were decided, for example, that one of the cast Stainless Steels or Cast Irons were the most appropriate alloy, then this would preclude the use of Die Casting.

    In addition, the designer will have to work within other parameters regarding the appropriate material to select. These could include material cost, manufacturing cost, end-product weight, the size of the product, and the temperature range that the selected material can hold.