What is metal casting process?
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. 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
Mould (or 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 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 mould 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.
Types of Metal casting
Metal casting can be divided into two groups by the basic nature of the mould design. ie Expendable mould and permanent mould casting. It can also further subdivided into groups depending on its pattern material
- Expendable Mould
- Permanent mould
- 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
Expendable mould casting
Expendable mould casting as the name suggests uses a temporary non-reusable mould to produce 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, this 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.
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 moulding.
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.