What is Metal Casting

March 3, 2022

Metal casting is a 7,000-year-old process used in both manufacturing and fine art. During metal casting, molten metal is poured from a crucible which is a container for melting, into a mould to create a metal cast object. A mould is a hollowed-out block that is filled with metal that hardens inside the mould, adopting its shape. The metal object or the casting is removed and finished after the metal and mould are cooled.

Traditional metal casting techniques include lost-wax casting, plaster mould casting, die casting, and sand casting among others.

Metal casting processes have been widely used for creating sculptures, jewellery, transportation, weapons and tools. The first known cast object is a copper frog that dates to 3200 BC, found in present-day Iraq. Around 1000 BC, India was among the first ancient civilizations to cast silver and copper coins for currency. Later, around 500 BC, the Zhou Dynasty introduced iron casting. Metal casting technology flourished in the twentieth century, eventually producing processes that most contemporary methods are built on.

The basic metal casting process involves creating a pattern and a mould, then pouring molten metal into the mould. The solid metal casting is then extracted and finished. This process is customisable for different types of metals or alloys, shapes and sizes. The following steps are involved in the making of a casting.

Step 1: Create the pattern

A pattern that will decide the shape of the mould needs to be created first before the mould is made. The pattern can be a 3-dimensional model of the final cast and may be shaped in wax, sand, plaster, ceramic or even wood. The pattern should account for any anticipated shrinkage when the metal cools. 

Step 2: Make the mould

After the pattern is created, the mould is made. The mould can be a reusable metal mould or a single-use mould that may be made from sand, plaster, or ceramic shell. The methods for making moulds can be optimised for different casting metals and various levels of pattern complexity. If you’re using a wax or plastic pattern, you can bake the mold within a kiln to burn out the pattern.

Step 3: Choose the metal or alloy

All metal castings are produced from either ferrous or non-ferrous metals and alloys. Metals that are composed primarily of iron are known as ferrous metals. Non-ferrous metals are those metals that do not have iron or iron components. Alloys are a mixture of elements that provide the best mechanical properties for the casting application. Ferrous alloys include steel, malleable iron and grey iron. Non-ferrous alloys that are most used are aluminium, bronze and copper. Precious metals used include silver, copper, gold and platinum.

Step 4: Melt the metal

Melting processes vary between metals/alloys because each metal/ alloy will have a different melting temperature. Essentially, melting consists of placing the solid metal in a crucible for melting and heating it over an open flame or inside of a furnace.

Step 5: Pour the metal into the mould

Pour the molten metal into the mould cavity. If it is a small casting, it can be simply poured from the crucible where the metal was heated, directly into the mould. A larger casting may require a small team to support heating the metal inside of a furnace and transferring the metal into a larger crucible or ladle before pouring the metal into the mould.

It is important to follow all the recommended safety guidelines when pouring the molten metal. Ensure that you wear protective clothing, insulated gloves and safety goggles. The workspace should be well-ventilated to avoid any risks from dangerous fumes and the walkway between the furnace and the mould should be kept clear. Allow the mould to solidify before moving onto the next step.

Step 6: Remove the casting from the mould

When the metal has cooled and solidified, it can be removed from the mould. If it has been cast into a single-use mould, the mould can be broken away from the casting. For reusable moulds, ejector pins may be used to extract the casting. An ejector pin’s main purpose is to apply a force to remove a component from the mould.

Step 7: Finish the casting

File and polish the solid casting. This may involve cleaning the casting like scrubbing away the excess mould material in water, breaking off the casting gates with clippers for small objects or an angle grinder for large pieces. A gate is simply an opening between the runner and part cavity that ushers the molten metal into the cavity and feeds the casting as it solidifies. A runner is a small channel that directs molten metal to the part.

Advantages of metal casting:

·       Metal casting can produce complex shapes and parts of varying sizes. Large parts can be produced as a one-piece cast

·       Features like internal cavities or hollow sections can be easily achieved

·       Metals and alloys that are difficult or expensive to manufacture using other manufacturing processes can be cast

·       It is a process highly adaptable to the requirements of mass production; many castings can be produced quickly as in the case of cast engine blocks and transmission cases

Disadvantages of metal casting:

·       Relatively coarse surface finish and prone to casting defects

·       Metal casting such as shell moulding has limits in terms of size and the pattern

·       Patterns are time-consuming and expensive to make

·       Part size and material choices depend on the casting process chosen. For instance, only non-ferrous metals can be used for permanent mould castings

·       Uneconomical for mass production

Some of these limitations can be addressed through an additive technology such as 3D printing.

What is 3D printing?

3D printing is an additive technology used to manufacture parts. It is ‘additive’ in that it doesn’t require a block of material or a mould to manufacture physical objects, it simply stacks and fuses layers of material. It is typically fast, simple and allows affordable in-house manufacturing with low fixed setup costs and can create more complex geometries than metal casting with an ever-expanding list of materials.

SPEE3D, an Australian company has developed the world’s first metal 3D printer using supersonic 3D deposition technology to deliver manufacturing-grade printing at production speeds. The company’s cutting-edge technology allows application across several industries including marine, defence and aerospace.

SPEE3D’s metal 3D printing process differs from traditional metal 3D printing in a few key ways. Instead of using heat, the process involves a rocket engine accelerating compressed air with an added metal powder. The technology builds parts that are ready for use, eliminating the need to wait for melting and re-solidification. It is the world’s first metal 3D printing production cell which has a metal 3D printer integrated with a heat treatment oven and a CNC 3 axis milling machine with support provided by SPEE3D’s experts in metal additive manufacturing.