The Most Popular Casting Methods
Casting is a manufacturing process that has been around for approximately 7,000 years! Over this time frame, the knowledge around this manufacturing process has made casting an extremely popular method within manufacturing industries. Many years of development has allowed casting processes to improve their products by increasing size, better mechanical properties and lower costs. Here we will discuss the most popular casting methods which you are likely to encounter during your engineering career.
Want to understand the fundamentals of casting before proceeding? Read a post about casting here.
Metal Casting Method Categories
Casting methods fall into one of two categories: expendable mold processes and reusable molds.
The Most Popular Casting Methods
Expendable Mold Methods
Investment Casting
Investment casting is based on the oldest casting method, which is also known as lost-wax casting. Investment casting is a casting process that pours molten metal into a wax pattern to give the desired product.
The direct method is when molten metal is poured directly into the original wax mold. The removal of the mold occurs following the solidification of the molten metal. There is a variety of methods to remove the mold, including hammering and media blasting.
The general process of indirect investment casting for this method of casting starts with creating a master pattern. From the master pattern, a master mold is created. Next, wax patterns are formed from the master mold and assembled (if a single part requires multiple wax patterns).
To get the pattern into an acceptable temporary mold various additives and materials are added to the wax to create a ceramic. The mold is made into a ceramic because a ceramic will not melt when the molten metal enters but wax will. Following the creation of the ceramic, the mold is put in a furnace to melt the wax.
Like direct investment casting, the molten metal is poured into the ceramic mold, with the mold removed via the processes stated above.
Like the majority of casting methods, post-casting repairs are necessary to remove any impurities. The most popular post-casting repair method is grinding and welding.
Investment casting proves to be one of the most popular casting methods in manufacturing industries due to the excellent surface finish and high dimensional accuracy that is achievable with the method.
Sand Casting
As the name suggests, sand casting is a popular manufacturing process that uses sand as the mold material. It is the most popular casting process, with over 50% of metal castings produced using the sand casting method.
Sand molds are cheap and require additives and other materials such as clay to bond the sand together and make it appropriate for casting.
The mold is created by placing a pattern in the top and bottom half (cope and drag) with the sand packed around both patterns. Once a mould is present around the pattern, a gatting system is added to allow a passage for the molten metal. The patterns are then removed from the sand resulting in a negative in the sand. The two halves are assembled, and molten metal is now ready to be poured into the sand mold.
Following solidification, the sand mold is removed, unveiling the metal casting.
Reusable Mold Casting
Die Casting
Die casting is one of the most popular casting methods that utilises pressures to force the molten metal into the mold. The pressure used for the process are high and generally between 150 to 1200 bar. As a result of the high pressure, the molten metal enters the mold rapidly – as high as 540 km/h.
Non-ferrous alloys are the typical type of metal used to cast using the die casting process, for example, zinc, aluminium, magnesium, copper and lead. Zinc is the easiest to cast, magnesium is the easiest to machine and aluminium, copper, and lead have excellent dimensional stability.
Before 2019, the sizes of the products were limited by their weight when using this method. However, the development of the Giga press has allowed die casting of products of over 100 kg. The Giga press is the largest aluminium die casting machine in the world, producing a humungous clamping force of 55,000 to 61,000 kilonewtons. However, this comes at a cost with the machine weighing 410–430 tonnes and takes up a vast area.
The Giga press is well known is producing the chassis of the Tesla Model Y. Using the Giga press gives Tesla a large advantage as it allows them to manufacture the chassis at a quicker rate than their rivals.
The distinct advantages of die casting are the excellent dimensional accuracy and smooth cast finishes – 2 um surface roughness. However, the process is limited to metals with high fluidity, otherwise, a high scrappage rate will occur, and there have been previous issues with casting weights.
Centrifugal Casting
Centrifugal casting is a popular casting method that utilises a centrifugal force, which is created by rotating the mold. The axis of rotation can be either the vertical or horizontal axis.
A horizontal casting machine manufactures products that have a greater length than diameter – long thin tubes. However, a vertical casting machine produces products with a greater diameter than length – rings and bearings.
A centrifugal casting makes products from a range of materials. Usual materials consist of cement, concrete, glass and metals such as iron and steel.
Speeds between 300 – 3000 rpm are reached during centrifugal casting – generating up to a 75 G force. The high rotational speeds allow the casting material to spread across the internal faces of the mold. The material which contacts the mold wall first is the first portion to solidify like many casting processes.
An advantage of centrifugal casting is that it produces different wall thicknesses from the same mold. Additionally, the centrifugal forces give the casting material good mechanical properties because of its effect on grain structure. The main disadvantage of the process is that it is limited to rotationally symmetric material.
Although not commonly documented, centrifugal casting is one of the most popular casting methods.
Summary
Casting is a process that appears simple however a lot of intricacies are in the various methods. Each stage of the process is carefully monitored to stop the formation of impurities in the casting material.
The size of the component is generally larger than its final form for most casting methods because most methods cannot provide the high dimensional accuracy required. It is common for cast components to be machined to obtain high dimensional accuracy where necessary.
When performed correctly, casting provides the industry with a consistent and economical approach to manufacturing components. Its importance is highlighted by its popularity with the majority of manufacturers in the sector.
The Graduate Engineer Social Media
