Metal casting, or foundry, consists of pouring molten metal into a mold cavity and then allowing that mold to cool and the metal to solidify. The process is used to manufacture objects of all sizes, from mere grams to thousands of kilograms, for medical, automotive, aerospace, and other industries.
Sounds simple, but there are many different methods of casting, all with unique characteristics.
Metal Casting Process Characteristics
Several features unite different variations of the metal casting process:
- The production process is high volume and low cost.
- Complex shapes may be cast, eliminating or minimizing the use of other manufacturing processes like machining, forging, and welding.
- It offers the ability to cast large-scale objects like pump housings or valves.
- Uniform material properties like machinability, bearing, and strengths are obtained on the cast object.
- Cutting weight reduction is obtained by allowing products to be cast as thin-wall casting.
Beyond these shared traits, the metal casting process features two main categories: expendable and reusable.
- Reusable molds can be reused after the castings are ejected.
- Expendable molds are consumed during the casting process.
Most molds consist of a minimum of two pieces forming a cavity where the liquid metal is poured to produce the metal castings.
Different Methods Of Casting Commonly Used In Manufacturing
Sand Casting
Sand casting is reusable; in most cases, it uses permanent patterns to produce molds. Liquid metal is poured through orifices or into the mold to form the casting, and the casting has to cool down before being ejected.
The advantages of this method are that it can produce complex shapes, has a low cost, and is suitable for materials with poor elasticity. The disadvantages of this method are that surface finishes are usually rough and have an imprecise tolerance.
Shell Mold Gravity Casting
This method is expandable and uses chemically bonded sands that utilize polymeric or chemical glues to hold the sand grains together. It uses a permanent pattern in most cases to create the molds.
A heated pattern is placed over a sandbox with chemically bonded sands at the bottom in the shell mold process. The box is turned, so the bonded sand lays on the heated pattern forming the molds. The shell mold is then cured in a heated oven, allowing the properties to become uniform. Finally, the shell mold is removed, obtaining the final molds for casting.
The main advantages of this process are the casting surface finish quality, the ability to produce thin wall thickness, and complex castings. The main disadvantages of this process are the high production costs and casting prices, high pattern cost, and size and weight limitations of casting.
Low- & High-Pressure Die Casting
The low-pressure die casting method fills a mold and lets the casting solidify under pressure, and the metal is forced into the die from below low-pressure die casting.
The high-pressure die casting forces liquid or semisolid metal into a mold under high pressure and lets the casting solidify under pressure. Both methods are reusable.
The principal advantages of these processes are that the parts are relatively cheap, labor cost is reduced through the automatization process, and the final product has a high accuracy dimensioning. The main disadvantages of these processes are that the high melting-point metals are not suitable, long lead time, air trapped in the mold chamber affects the final product, and changes to parts and designs are usually time-consuming.
Investment Casting
The investment casting process commonly uses a wax replica or any polymer pattern to which a thick ceramic coating or refractory material is applied. The wax or refractory material is removed, the ceramic fired, and the molten metal poured into the cavity created by the pattern.
The main advantages of this process are that complex parts can be created using a single piece of investment casting, a smooth surface finish is achieved, dimensional precision is achieved, and wax material is reused. The main disadvantages of this process are the high cost due to specialized equipment, multi-step processes, and parts that are difficult to cast if they require small and deep holes.
Tilt Casting
The tilt casting process involves a connection between the casting crucible, the mold, and the coordinated tilting of both the crucible and the mold itself.
The main advantages of this process are automation, standardization, waste minimization, and scalability. Some disadvantages are the possibility of damaging moving parts and the difficulty of working with them in constraint spaces.
Centrifugal Casting
The centrifugal process consists of reusable molds. The main characteristic of this process is that a measured volume of liquid metal runs into a spinning mold. The melted metal is transported to the outer side of the mold due to the centrifugal forces created by the centrifugal forces.
The main advantages of this process are the high density achieved that results in increased mechanical strength product, inclusion, and lighter impurities, and no gates and risers are needed. The main disadvantages of this process are the inaccurate inner diameter of the product and the limitation of alloys that can be cast through centrifugal casting.
Lost Foam Casting
The lost foam casting process uses a foam replica coated with a refractory material which is then immersed in loose sand, creating a mold. The foam then evaporates when the melted metal enters the mold.
This process's main advantages are ease of use, the simplified procedure, and high dimensional accuracy. The main disadvantages of the lost foam casting process are that it is more expensive when compared with different methods of casting and creates a low-strength final product.
Vacuum Die Casting
This process uses a vacuum tank and pumps to extract the gasses in the mold cavity when the molten metal is poured into the hole.
The main advantages of this process are superior final casting mechanical properties and surface quality, reduction of cavity back pressure, and improvement of filling conditions. The disadvantages of this process are the difficulty of installation and sealing, leading to a higher cost and the need for specialized labor personnel.
Squeeze Casting
The squeeze casting process utilizes a two-part mold, a cope (top) and a drag (bottom). Molten metal is poured in the drag, and the cope squeezes the metal giving it a final shape.
The main advantages of this process are eliminating internal pores, shrinkage, and other defects, its ease of automatization, and its low surface roughness. The main disadvantages of the squeezing die casting process are the high setup cost, the long lead time, and the difficulty of modifying a design.
Choosing a Metal Casting Method
Comparing and evaluating the different methods of casting can be challenging. A cast can have multiple ways of manufacturing, and selecting the process will depend on the casting needed and its characteristics.
The best method is using a simulation casting software, where you can emulate and visualize the entire process, analyze the advantages and disadvantages of each approach, and gather information about the final product. Simulation casting software helps manufacturers reduce error, identify production problems, and analyze product characteristics.
Contact Transvalor USA to learn more.
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