Metal Casting Explained: Processes, Materials, and When to Use Them
Why Metal Casting Still Matters
The metal casting process has been used throughout history and is now widely regarded as a cost-effective method of producing complex metal components at a high volume. Examples include engine blocks, pump housings, and industrial brackets; all of which can be produced by metal casting in ways that would be practically impossible if the material began as one solid piece.
Whether it’s 3D Printing, CNC Machining, Casting or Sheet Metal, all manufacturing methods have a place in today’s production landscape; however, a lot of people overlook casting (in many cases thinking of it as “old tech”) when, in actuality, it is an incredibly economical option for professionals looking at producing parts (and/or entire products) at medium-to-high volume levels.
This article breaks down how metal casting works, what types exist, what materials are used, and how to decide if it's right for your application.
What Is Metal Casting?
Metal casting refers to pouring hot metal into a form to cool off, creating the shape of the final piece. The mold is used to create the shape and fill the cavity with metal. Although simple in concept, this method is very complex and requires a great deal of expertise to complete successfully. The design of the final piece can vary from highly complex and precise detailed parts to very simple, strong parts that can be produced from only a few pieces up to hundreds of thousands of pieces in one year.
Casting can use various types of metals; however, aluminum, iron, steel, copper, brass, zinc, and magnesium are the most common. The type of metal selected is based on the characteristics necessary for the piece, such as weight limit, desired strength, and cost.
Common Metal Casting Processes There are various forms of metal casting available for use, but each has distinct tooling costs, surface finish quality, dimensional accuracy and production volume requirements.
Examples of Die Casting Die casting is the process by which molten metal is injected into a steel die (mould) at very high pressure, thus producing a component with close tolerances and an excellent surface finish, providing an accurate dimension and consistency over production runs.
Non-ferrous metals such as Aluminum, Zinc, and Magnesium are the best materials for using this process. Upfront tooling costs are significant but eventually each individual part becomes much less expensive with increased volume. Examples of common uses include automotive parts, housing for consumer electronic products, & plumbing fittings.
Sand Casting - Sand casting has been around for many years and is one of the least expensive methods of producing casting. The actual mold is made from compacted sand; this combined with the low tooling cost make this process very flexible to use. This process can be used cups with virtually any type of metal including Iron and Steel.
Dimensional accuracy and surface finish in sand casting are both inferior to die casting. Thus, sand castings are ideal for producing low to mid volumes of large, heavy parts, such as machine bases for heavy equipment, agricultural machinery components, and custom parts made for industrial purposes.
Precision Casting (Investment Casting)
The process of precision casting can also be referred to as 'investment casting' or 'lost wax' casting and is best suited where there is a requirement for high accuracy, fine detail and smooth surface finishes. The procedure is to create a wax pattern, coat the wax with a material that will set to form a ceramic shell, melt the wax out through heating and replace it with molten metal. Although it costs more than sand casting, it provides a much greater level of accuracy and therefore has many applications in aerospace, medical devices, or complex parts that would require excessive time or resources if machined.
Forging (Related Process)
Although forged metal isn't technically the same process as casting, because forging shapes (hence) metal via compressive force and not by pouring molten metal, the two methods often get compared in similar discussions. The reason for this is that forged parts generally have a better grain structure than cast parts; therefore, the strength of forged components can exceed that of cast components if the parts are subjected to repeated loads (fatigue) and require high strength as their main design criteria.
Comparing Metal Casting with Other Manufacturing Methods When selecting among casting, sheet metal fabrication, CNC machining, or additive manufacturing (3D Printing), the determining factors are typically geometry, volume, and performance requirements.
Sheet Metal Fabrication Sheet metal fabrication focuses on taking flat metal sheet stock and creating three-dimensional (3D) shapes through processes such as cutting, bending, stamping, and welding together flat pieces of metal. Several common sheet metal fabrication processes involve laser cutting, CNC bending, and stamping.
Typically, sheet metal fabrication is used to create parts such as enclosures, brackets, panels, and structural frames — typically for parts that are relatively flat or have a consistent wall thickness. Also, it is often faster and less expensive to produce parts using this process than by using casting for thin-walled parts; however, it is not capable of producing the very complex internal geometries that casting is able to produce with ease.
CNC Machining
CNC machining removes material from a solid block using computer-controlled cutting tools. It's the go-to for tight tolerances, high surface finish, and low-volume production.
With metal casting, CNC machining is usually used as a secondary operation to clean up surfaces, drill holes, or achieve dimensions that cannot be achieved with just the casting operation. In other words, they work in conjunction with one another. Additive manufacturing is also called 3D printing and makes parts layer-by-layer as opposed to all at once.
Additive manufacturing is ideal for creating prototype parts and parts that need to be made in relatively small quantities. It can also create complex internal geometries that would be difficult or impossible to create with either cast or machined processes.
Metal three dimensional printing technology (3D printing) - either titanium, aluminum or stainless steel has advanced quite a bit. However, when you are producing a lot of parts there is still a high cost per part when compared to castings. As a result, many manufacturers use 3D printing to validate their designs before they build tooling for casting parts.
Selecting the Appropriate Process for Your Component Consider the following as a good way to approach the decision process:
Volume has the greatest effect. High-volume manufacturing (thousands of parts per year) will typically be best suited to diecast or sandcast processes since you will recover your tooling investment quickly.
Geometry drives the choice. Internal channels, undercuts, and complex 3D shapes point toward casting. Simple flat or prismatic shapes may be better served by sheet metal or machining.
Surface finish and tolerances make up the required base to get a part produced. If a part requires consistent, close tolerances on all your parts' features, you'll need to use CNC machining (which could be both your primary machining process and as one of your final steps) to achieve this. Casting may get your shape close enough, however, machining will give you the precision you require.
Variable Material Limits Various Options: Both sand casting and investment casting can cast almost any kind of metal. However, die casting can only produce nonferrous alloys, such as aluminum or zinc, and cannot produce either cast iron or cast steel; thus, if steel or cast iron is needed, the normal option will be to do so with sand casting.
Commonly Used Metals in Metal Casting The most popular material for metal casting is aluminum alloy as it is lightweight, corrosion resistant, and provides easy machining after the casting process. These properties make aluminum alloy an ideal choice for a wide variety of industries ranging from automotive, aerospace to consumer products.
Ductile iron and gray iron are commonly used for components that require vibration damping, resistance to wear, and a high compressive strength. Products that contain these materials would include engines, hydraulic bodies and tooling for a machine tool. Steel castings have the ability to withstand extreme temperatures and carry very heavy loads, which makes them ideally suited for a wide range of applications, including mining, construction, and manufacturing equipment.
Marine fittings, valves and various electrical components require a good degree of conductivity and/or corrosion resistance. These types of components will be manufactured primarily using metal alloys that contain copper (brass and bronze). Zinc alloys are also commonly used for producing die cast small detailed parts such as locks, electrical connectors and decorative hardware.
Surface Treatment After Casting After the metal alloy has been cast, the castings do not go directly to assembly. Most castings will need to undergo some sort of combination of the following processes:
Shot blasting or sand casting; to clean the surface - Heat treatment; to relieve any stresses or improve mechanical properties
Using CNC Machines to Mill Critical Surfaces - Coating/Plating - Anodizing (for aluminum), Powder Coating, Electroplating, Painting, etc. Which post processing method to use is dependent on ambient environment, load requirements, and look standards for the final part.
Finally,
Metal casting has been well established and is seen as a dependable process due to its key role in the manufacture of products. This is attributed to its ability to produce large quantities of complex geometric shapes using many different types of metals and to work seamlessly with various processes such as CNC machining, sheet metal fabrication and additive manufacturing as part of the overall production process. Metal casting also serves as part of a closed-loop manufacturing system where recycled castings can have a second life as new metal castings.
The key is matching the process to your part's actual requirements — volume, geometry, material, and tolerance. When that match is right, casting is hard to beat on cost and scalability.
If you are involved in an OEM (original equipment manufacturer) project and require custom metal parts, one of the best ways to determine the most efficient and affordable way to manufacture your finished product is by evaluating a supplier’s complete array of manufacturing capabilities, such as; castings, stamped sheet metals, CNC machined parts, and 3D printed components through an integrated manufacturing system.
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