Metal Casting

elaine meszaros
30 July 18

What is metal casting process?

Metal casting is defined as the process in which molten metal is poured into a mold that contains a hollow cavity of a desired geometrical shape and allowed to cool down to form a solidified part. 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.

Primarily casting produces ingots and shapes. An Ingot is a casting produced into a simple shape and intended for further processing such as metal extrusion, forging etc. Shape casting is for near or net shape castings to produce complex geometries which are closer to the final part.

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

Elements of the gating system

Mold is made out of two halves. Contained inside a box called flask, the upper half is called the cope and the drag is the bottom half. As shown in the image above the flask is also divided into two halves. The line that separates the two halves is called the parting line.

The gating system is the channel or the path by which the molten metal flows into the cavity. As shown above, the gating system consists of a pouring cup and a down sprue through which the metal enters the runner which leads into the main cavity. Pouring cup minimizes the splash and turbulence when the metal flows through the sprue which is tapered to aid the flow.

Most of the casting suffers from shrinkage during cooling and to minimize the shrinkage issue, a riser is used. A riser is a simple reservoir in the mold that feeds molten material to the shrinking sections to compensate as it solidifies. There are four different types of risers, viz. top riser, side riserblind riser and open riser.

Types of Metal casting

Metal casting can be divided into two groups by the basic nature of the mold design. ie Expendable mold and permanent mold casting. It can also further subdivided into groups depending on its pattern material

  • Expendable Mold
    • Permanent pattern
      • Sand casting
      • Plaster molding
      • Shell mold
      • Ceramic mold
  • Permanent mold
    • Gravity casting
    • Low pressure/vacuum
    • Die casting

Following factors needs to be considered before choosing a suitable metal casting for a given engineering product design.

  • Part shape and size
  • Required quantity
  • Tolerance requirement
  • Material

Expendable mold casting

Expendable mold casting as the name suggests uses a temporary non-reusable mold to produce final casting as the mold will be broken to get the casting out. These molds are typically made of materials such as sandceramics & plaster. These are generally bonded using binders called bonding agent to improve its properties.  Complex intricate geometries can be cast using expendable mold casting.

Permanent mold casting

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

Composite Mold casting

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

Advantages of metal casting process

  • 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
  • Material 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

Because of the above reasons metal casting is one of the important net shape manufacturing technologies. Others include net shape forging, stamping of sheet metal, additive manufacturing and metal injection molding.

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

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