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, when it comes to mass production, casting is often a cheaper way to manufacture parts than machining them from a solid piece of 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.

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.

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.

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 pressurized 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.

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, and 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 a 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 a 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 processes:

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

More Material Choices

Although almost all metals can be used, the most common ones are iron, steel, aluminum 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.




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