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Metal Casting

Metal Casting

What is Metal Casting?

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. 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.

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.

Types of Metal Casting

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

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

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

  • Part shape and size
  • Required quantity
  • Required tolerance
  • Material

Expendable mold casting

Expendable mold casting, as the name suggests uses a temporary non-reusable mold to produce the final casting as the mold will be broken to get the casting out. These molds 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 mold casting.

Plaster mould aluminium casting
Figure 2 aluminium alloy a356 copier frame plaster mold aluminium casting(credit_link)

Permanent mold casting

Sometimes called non-expendable mold casting, permanent mold casting 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.

Permanent mould casting
Figure 3. Permanent mold casting (credit – Alibaba)

Composite mold casting

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

Composite mould casting
Figure 4. Composite mold casting (credit – highlywell-china)

How does metal casting work?

Metal casting steps

  1. Patternmaking – A replica of the part to be cast is made using a suitable material such as wood, metal plastic or plaster.
  2. Mold making – Mold making is a multi-step process in which patterns and cores are used to create a mold. The type and how the molds are made would vary depending on the type of metal casting. For example, sand casting uses sand inside a flask to create molds and die casting uses hardened tool steel molds.
  3. Metal melting & pouring – Liquid is then melted and poured into the mold cavity either by gravity or by high pressure. Then the cast is allowed to solidify before the cast parts are removed from the mold. Again, the cast part removal will vary depending on the type of metal casting.
  4. Post processing – 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.

Elements of the gating system

One of the most common metal casting process is Sand casting and its mold is made of two halves. Contained inside a box called flask, the upper half is called the cope and the bottom half is called the drag. As shown in the image (Figure 5) below the flask is also divided into two halves. The line that separates the two halves is called the parting line.

Metal casting gating system

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 that leads into the main cavity. Pouring cup minimises 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 minimise 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 riser, blind riser and open riser.

Material suitability

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

Zinc, aluminium, magnesium and brass are widely used in die casting whereas aluminium alloy, brass alloy, cast iron and cast steel are very popular sand-casting materials.

Typical application

Nearly every engineering product we use from washing machines to pillar drills, cars to bicycles are manufactured using metal parts which are most likely to be made using one of the metal casting processes. This age-old manufacturing process has improved its precision and tolerances over time.

Typically, castings are used to make car engine blocks, crankshafts, power tool housings such as, pillar drills, plumbing parts, turbine blades, metal statues, some gears and gearbox housings.

Advantages and disadvantages of metal casting

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

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

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.

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 mold
  • 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 mold castings

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