This article will cover what metal casting is, how it works, and walks you through the most common metal casting processes and the benefits manufacturers can attain by combining modern digital tools like 3D printing with traditional casting workflows.
Metal casting is an age-old metalworking process in which molten metal cools and solidifies in a mold to form metal parts. Despite its ancient roots, metal casting is still one of the most popular processes for companies looking to produce metal parts.
Get design guidelines for creating 3D printed patterns, walk through the step-by-step direct investment casting process, and explore guidelines for indirect investment casting and sand casting.
Once the metal cools down and solidifies, the parts get removed from the mold. Depending on the mold type, this can be done by vibrations in a shakeout process, washing away the investment material, or by ejector pins. Then, excess material, such as vents, gates, and feeders, are removed from the parts. Finally, the parts get filed, grated, machined, or sandblasted to smooth the surface and reach the final shape requirements.
During this step, the metal gets heated in a furnace until it melts. Depending on the application, manufacturers can use a variety of different metals, with the most commonly cast metals being iron, aluminum, aluminum alloy, steel, copper, and zinc, as well as precious metals like gold and silver. Once the metal melts, the manufacturer pours it into the mold cavity and allows it to cool and solidify.
The next step is creating a casting mold, which can be either reusable (non-expendable) or non-reusable (expendable). Non-reusable molds are usually made out of sand, plaster, wax, or by 3D printing, and just as the name suggests, they get destroyed in the casting process. Reusable molds are made out of metal and other durable materials and can be reused for multiple casting cycles.
When the casting piece is hollow, the manufacturer also creates a core of sand or metal to shape the internal form. This core gets removed upon completion of the casting.
A pattern is not an exact replica of the desired part. It has additional elements that make the casting process possible, including gates that allow molten metal to flow at a steady rate and vents for gas to escape. Additionally, patterns are also larger than the parts they represent to account for the shrinkage that occurs during cooling.
In order to begin the metal casting process, a manufacturer first must develop a representation of the desired pattern. This pattern is essential in designing the mold used for the cast. It is traditionally made from wood, foam, plastic, or wax and ensures that the mold accurately produces the finished metal part. Today, 3D printing is also a common method to produce patterns, which allows designers to create accurate patterns directly from digital CAD software tools .
Since the advent of metal casting, the methods have evolved and varied. Its core techniques, however, have remained constant. Here is a general step-by-step process for metal casting:
Though all metal casting techniques share the same core process, there are various methods better suited for different applications. Some of the most common methods include die casting, investment casting, and sand casting.
Die casting uses a steel mold and high pressure. (Source: buhlergroup.com)
Die casting is a metal casting process in which a manufacturer pushes molten metal into a steel mold cavity at a high pressure to quickly produce metal parts. In die casting, the manufacturer fixes two halves of a die or reusable mold together and uses a nozzle to inject pressurized molten metal into the die. When the metal cools, the die opens, and ejector pins push out the cast.
The two most common die casting processes are hot-chamber and cold-chamber casting. While the specifics of these processes vary, there are several shared characteristics of the die casting process as a whole.
Hot-chamber die casting is the most common of the two main die casting processes. Hot-chamber die casting machines have a built-in furnace to heat the metal within the machine. Once the metal reaches a molten state, the machine lowers a cylindrical chamber into the molten metal. The gooseneck shape of the metal injection system allows the chamber to quickly fill itself, and then push the material into the mold with air pressure or a piston.
Immersing the injection mechanism to fill it allows for rapid and streamlined mold injection in this casting process. Because the chamber is subject to direct heat from the molten metal, however, hot-chamber die casting systems are at risk for corrosion, making them a less viable option for metals with high melting points. Instead, it is better suited for materials with low melting points and high fluidity, like lead, magnesium, zinc, and copper.
By contrast, the cold-chamber die casting process works more slowly to avoid corrosion. With this method, a foundry worker ladles molten metal into the injection system. A piston then pushes the metal into the mold.
This process limits the corrosion that is more common in hot-chamber die casting. It is an ideal option for metals with high melting points, like aluminum and aluminum alloy.
The die casting process is rapid and produces highly detailed parts. It is ideal for the production of high volumes of complex parts and can also produce strong parts with smooth surface finishes. Die casting’s capacity to produce a high volume of parts makes it a crucial process in the automotive and aerospace industries.
As die casting tooling and equipment are expensive, this process is not cost-effective for smaller production runs. In addition, the malleability of metals used in the process can impact the complexity of the product.
Cast parts from SLA patterns printed in Clear Resin on a Formlabs 3D printer.
Investment casting, also known as lost-wax casting, is a process that uses wax, slurry, and molds to produce complex parts. It is one of the oldest metal casting techniques but is still valued for its ability to create precise metal parts with intricate shapes.
This process is still widely used for producing jewelry, dentistry, and art. Its industrial form, investment casting, is a common way to create precision metal parts in engineering and manufacturing.
Investment casting patterns are typically made out of wax or 3D printed polymers. The patterns are assembled into a tree-like structure and dipped into a slurry of silica, or put into a flask and surrounded by the liquid investment plaster. After the investment material dries, the flask is placed upside down into a kiln, which melts the pattern, leaving a negative cavity in the shape of the original model. Metal is melted and then poured, using gravity or vacuum pressure to pull the metal into the cavity. The casted parts are filed, ground, machined, or sandblasted to achieve final geometry and surface finish.
Sprue trees with cast rings.
Investment casting is a versatile process. It allows manufacturers to produce accurate and repeatable parts out of nearly any metal available for casting and complicated shapes that would be difficult or impossible with other casting methods. Casted parts also have excellent surface qualities and low tolerances, with minimal surface finishing or machining required.
These features make investment casting ideal for complex parts in automotive, aerospace, and industrial applications, medical tools, dental implants, as well as fine jewelry and art.
Investment casting is a complex and labor-intensive process. It requires specialized equipment, costly refractories and binders, as well many manual operations to make a mold. It can be difficult to cast parts that require cores and the process is better suited to small parts.
One half of a sand casting mold.
Sand casting is a metal casting method that was first in use 3,000 years ago but remains the most widely used casting method to this day. This process allows manufacturers to cast metal without relying on machining.
In the sand casting process, the manufacturer first creates a foundry pattern, or replica of the casting, most commonly from wood or plastic. The pattern is oversized to allow for shrinkage. Parts with features on one side only require an open-faced mold. For parts with multiple detailed surfaces, the manufacturer separates the foundry pattern into two mold boxes to form a closed cavity mold. The top half is called a cope and the bottom a drag.
Once the manufacturer creates the pattern, they tightly pack sand around the pattern. Then, they add sprues and gates to ensure that the molten metal flows smoothly through the mold cavity. The manufacturer removes the pattern then clamps the two halves of the sand mold together. Once the metal melts to a molten state, it is poured into the mold and left to cool. From here, the sand mold is removed using vibrations or high-pressure water. Finally, the manufacturer refines the part by removing sprues and gates, and polishing the cast metal part.
Sand casting is an adaptable process that functions outside the limitations of machinery. Because of this, it can create complex parts of virtually any size. Sand is inexpensive and plentiful, which lowers the setup cost and makes modifications possible. It is the only practical or economical way to produce very large castings. The lead time of sand casting is also short, making it a viable process for short production runs.
Sand casting’s versatility makes it a manufacturing option across a wide array of industries. It can produce medical equipment, automobile parts, electronic equipment, gas tanks, and engine blocks, and more.
Sand casting creates highly porous, textured metals. The shrinkage and rough surface finish also lower the dimensional accuracy of parts. This results in a low-strength final product that requires time-consuming post-processing to achieve a higher quality finish.
In order to choose the right industrial metal casting process, several factors must be considered. We’ve created this comparison table to help you compare die casting, investment casting, and sand casting in terms of types of metals, production volume, costs, production time, part complexity, and for which industries they are generally used.
Die CastingInvestment CastingSand CastingCompatible metalsAluminum, copper, lead, magnesium, zinc Most metalsMost metalsProduction volumeHigh volumeLow to high volumeOne-off to medium volumeUnit costsLowModerate to highModerateTooling costsHighModerateLowCycle timeRapidLongModerateIndustriesAutomotive, aerospace, consumer products, furniture, power toolsAutomotive, aerospace, jewelry, medicine, dentistry, artAutomotive, aerospace, industrial equipment, electronics, consumer productsTake a walk to the nearest park or ball field and look down; that’s not just a bunch of rocks and dirt down there. Every metal known to humankind comes from the ores and minerals found underground. To a manufacturing person, all that stuff under our feet is what rocks our world. Pardon the pun.
Let’s take another walk, this one through the Periodic Table of the Elements. Yes, most of us learned about the elements in high school chemistry class, but it’s probably been a while, so a refresher might be in order.
We’ll zoom through most of it. Hydrogen, oxygen, and argon—gases are exciting to welders and neon sign makers, but unless we’re short of breath after a brisk walk, most of us take them for granted. Without silicon and germanium, computer chip manufacturers would need new jobs. Plutonium is important to bomb makers, as are lead and krypton to those in lighting. To everyone else, the elements are a pretty boring subject.
In between all those rare earths and noble gases, however, sit metals. Aluminum, titanium, iron, and nickel—these are the building blocks of modern society. Without the raw materials trapped in the earth’s crust, and the technology to extract and process those minerals into various alloys, humans would still be living in grass huts and chasing their food with wooden clubs. Protolabs uses a range of metals for its manufacturing services. These can be classified as either hard or soft, with metals like steel and stainless steel on one side of the fence, and brass, copper, magnesium, and aluminum on the other. The last on this list—aluminum—is the most abundant metal in the earth’s crust, and the third most common element after oxygen and silicon. Despite making up 8 percent of the earth’s crust by weight, aluminum is rarely found in its pure metallic form, however, since most of it is locked up in bauxite and other ores.
Elemental aluminum is soft and highly malleable, making it a poor candidate for mechanical purposes. Instead, aluminum is usually blended with a mix of other elements, including silicon, copper, magnesium, and zinc, then heat-treated to make the strong, lightweight alloys used today in airframes, automobiles, and various consumer products.
Protolabs’ machining service makes parts from two types of aluminum: 6061-T651 and 7075-T651. The T-suffix signifies how the material was processed, in this case mechanically stretched by 1 to 3 percent after heat treatment to eliminate residual stress, thus making it more stable when machined. 6061 aluminum is alloyed with magnesium and silicon, and in its wrought form offers yield strength of 40,000 psi or more. It is very corrosion resistant and weldable given the proper equipment, making it an ideal choice for low-fatigue applications such as structural components in machinery, hydraulic valve bodies, marine, and automotive parts, and most any application requiring robust, lightweight material.
The other horse in Protolabs’ aluminum stable is 7075 aluminum. Harder and stronger than 6061, it offers yield strength nearly twice that of its less robust cousin, but at nearly three times the cost. Its primary alloying elements are zinc, magnesium, and copper. The American military uses 7075 in many of its firearms, connecting rods made of forged 7075 aluminum are used in top fuel dragsters, and the wing spars in Boeing aircraft are made of 7075. It’s tough stuff. In fact, the only place where 6061 wins out is in corrosion resistance, and in parts that need a little more “give” than those made of 7075. Both materials offer easy machining, although 7075 is a bit abrasive.
Another popular lightweight material is magnesium, which is the fourth most abundant element in the earth’s crust. Two-thirds the weight of 6061 and nearly as strong, it is the lightest of all structural metals. Camera and cell phone bodies, frames for power tools, laptop computers chassis—magnesium is a preferred material wherever good strength and low weight is important. In an effort to improve fuel efficiency, automobile manufacturers make extensive use of magnesium in transmission cases, seat frames, and intake manifolds.
Magnesium is most commonly alloyed with aluminum and zinc. It has excellent dampening characteristics, is very machinable, and readily molded or die-cast.
In addition to magnesium’s susceptibility to corrosion, another drawback is its reduced strength at high temperatures, although Volkswagen used magnesium successfully in the crankcase of its air-cooled Beetle engine for more than 50 years. Price-wise, it’s more expensive than aluminum, but this is largely mitigated by the relative ease with which magnesium components are manufactured. Note that Protolabs no longer manufacturers magnesium parts.
Rounding out the soft metal lineup are brass and copper, the kissing cousins of the metal family. Of the two, brass is by far the most versatile. With the exception of environments high in ammonia and some acids, it is extremely weather and corrosion resistant. If you’ve ever replaced a car radiator, soldered a kitchen faucet, or played the French horn, you’ve handled parts made of brass.
Protolabs offers parts machined from C260 cartridge brass, long a favorite for ammunition casings. It contains 70 percent copper and 30 percent zinc, and is considered the most general purpose of all brass alloys. There are literally dozens of brass grades though, all with subtle differences and distinct uses. Cutting back on the copper percentage by a few points produces a brass suitable for rivets and screws. Cut back a bit more, add a little iron, and you’ve created Muntz metal, good for architectural trim and lining the bottoms of boats. Increase the copper content, toss in some manganese and a pinch of nickel, and you have the makings of Sacagawea one dollar coins. Brass is the ultimate switch hitter.
To a machinist, brass is as easy as it comes: coolant is optional, tool life exceptional, and feedrates quite high. Don’t let its easygoing nature fool you, however—brass is sturdy stuff, offering tensile strength rivaling that of mild steel. Ironically, copper is a far different story. Even though it’s the primary ingredient in brass, unalloyed copper’s machinability is roughly five times worse, and even the most patient of machinists avoid it due to copper’s tough, stringy nature. Chips are virtually impossible to break and, due to its high thermal conductivity, the material heats up very quickly during cutting.
Copper is only second to silver in electrical conductivity, a factor that makes it one of the most important metals in use today. Copper (and aluminum) wiring basically make electricity possible. Without it, lights would remain unlit, cars wouldn’t run and it would be impossible to blend a frozen margarita.
Copper is easy to braze but difficult to weld. Its extreme ductility makes it both strong and flexible, a rare occurrence among metals. Yet copper does far more than conducting the power needed to heat our grills. It’s used in semiconductor manufacturing as an element of high-temperature superconducting, in glass-to-metal seals such as those needed for vacuum tubes, and has even been approved by the United States EPA for use in hospitals and public places as an antimicrobial surface.
Because elemental copper exists in nature, people first started pounding it into coins and cutlery millennia ago. Today, it’s an ingredient in more than 570 different metallic alloys, of which cartridge brass is one. Tellurium copper, nickel copper, bronze, gunmetal, aluminum, and steel alloys—the list goes on. Copper can also be used for electrodes in electrical discharge machining (EDM), a technology often seen in injection molding and metal stamping. In the modern world, copper is indeed king.
The world needs hard metals as well. Steel is used in everything from cars to cruise ships, cables to crescent wrenches. Regardless of alloy type, steel is mostly composed of iron. Iron smelting and limited steel manufacturing has been in use for thousands of years, but it wasn’t until the Bessemer steel process, invented in the mid-1800s, that mass production of high-quality steel was made possible. Thus began the industrial revolution.
As with the soft metals, a small quantity of alloying elements can have a dramatic effect on steel’s properties—the addition of less than 1 percent carbon and manganese, along with a little metallurgical legerdemain, is what makes brittle iron into tough 1018 steel. And 4140 alloy steel, suitable for aircraft use, is made by combining an equally small amount of chromium along with a dusting of molybdenum.
Carbon steels such as these can be hardened to one extent or another, and are easily welded. There’s just one problem: They rust, making plating or painting a requirement for most any application involving carbon steel.
The 300-series stainless steels offered by Protolabs carry at least 20 percent chromium along with a fair amount of nickel, making them more difficult to machine. Still, these popular materials are commonly used for medical instruments, vacuum and pressure vessels, and for food and beverage equipment. 300-series stainless is quite tough, but cannot be hardened like carbon steel. If hardness is a requirement for your application, consider kicking it up a notch with 17-4 PH.
This versatile but very tough material contains nickel, chromium, and copper. Although considered part of the stainless steel family, its machinability in the annealed state approaches superalloy status. When heat treated, it easily achieves hardness of 45 Rc and tensile strength of 150,000 psi or higher, three times that of carbon steel. It’s most commonly used in the medical, aerospace, and nuclear industries, or anywhere a combination of high strength and good corrosion resistance is needed.
Since rust never sleeps, metallurgists developed stainless steel. By increasing the amount of chromium to at least 10.5 percent, corrosion resistance is greatly enhanced. Stainless steel is widely used in the chemical industry, textile processing, and for marine applications. Many stainless steels are temperature resistant as well, and are able to withstand temperatures upwards of 2,700 degrees F, hot enough to turn aluminum, brass and copper into molten puddles. 316 stainless, for example, is excellent for heat exchangers, and sees regular use in steam turbines and exhaust manifolds.
If you’re looking for some truly robust alloys, look no further than cobalt chrome and Inconel. Protolabs doesn’t machine these materials, but its 3D printing service is happy to sinter them for you through a direct metal laser sintering (DMLS) process. Each material has unique, high-performance properties.
Inconel contains 50 percent or more of nickel, giving it excellent strength at a range of temperatures. It’s used for extreme demands such as gas turbine blades, jet engine compressor discs, and even nuclear reactors and jet engine combustion chambers. The high nickel content makes Inconel one of the most difficult materials to a machine, requiring wear resistant coated carbide and a rigid machine tool. Sitting right next to nickel on the periodic table is cobalt, the main ingredient in cobalt chrome alloy. This material is known for superb wear resistance and human biocompatibility, making it ideal for dental implants, hip and knee replacements, and arterial stents.
Finally, there’s titanium. This lightweight element is alloyed with aluminum and vanadium, providing a strong, corrosion-resistant material. Like cobalt chrome, titanium is biocompatible and is used extensively for bone screws, pins, and plates. Its tensile strength is roughly twice that of mild steel but weighs just half as much. This makes titanium appealing to the aerospace industry and high-performance vehicle manufacturers.
Metallurgy—it’s a pretty cool subject, right? As we’ve seen, a dozen or so raw elements provide for hundreds of important, life-altering materials. None of these metals would be worth a wooden nickel without the means to shape them, however. Principal among these is machining, which evolved in lockstep with steel processing. Over the past 150 years, machine tools have grown from crude pulley and steam driven devices to the high-tech, ultra-precise computer numerical control (CNC) equipment of today.
Protolabs employs a veritable army of these machine tools, one that’s several hundred strong, standing ready to machine custom parts from most of the materials just discussed. Chief among these are machining centers, which work by rotating a cutting tool such as an end mill or drill to remove material. The workpiece is gripped in a vise or similar clamping device and moved in one or more axes against the cutter, thus creating complex geometries. Five-axis machining centers may use all axes simultaneously to generate the free-form shapes common in artificial knees and propellers, or indexed to machine multiple sides of the workpiece in one clamping.
CNC lathes use a chuck or collet to grip the workpiece and rotate it against a fixed cutting tool. Need to cut a set of candlestick holders or a fitting for a garden hose? Lathes make short work of these parts and more. Mill-turn machines, like Protolabs uses, take lathes one step further with the addition of rotating tools and secondary spindles, eliminating what were once secondary machining operations.
For large-volume production, machined parts are often transitioned to casting or molding processes. Metal injection molding, or MIM, is the process whereby metal powders such as nickel steel, 316 stainless, 17-4 PH or chrome-moly are mixed with a binder composed of wax and thermoplastic.