Investment Casting Materials: Metals & More

Investment casting, also known as precision casting or lost-wax casting, is a manufacturing process that uses a wide range of materials. The process involves creating a wax pattern, which is then coated with a refractory material, such as ceramic or plaster, to form a mould. This mould is then heated, melting the wax, which is drained out, leaving a cavity in the shape of the desired product. Molten metal is then poured into the cavity and left to cool and solidify.

Investment casting can be used with various materials, including most common metals and alloys. This includes stainless steel, brass, aluminium, carbon steel, and glass.

Stainless steel alloys

The use of stainless steel alloys in investment casting offers several advantages due to the material's properties. Stainless steel is a ferrous alloy that contains chromium, which provides a protective layer against staining and corrosion. This makes it ideal for applications requiring corrosion resistance, such as in the military, medical, and industrial sectors. Additionally, stainless steel exhibits heat resistance at high temperatures, making it suitable for applications in aerospace, automotive, and food and dairy industries.

There are various types of stainless steel alloys available for investment casting, including the 300 series and 400 series. The 300 series, or austenitic stainless steels, are non-magnetic chromium-nickel alloys known for their excellent corrosion resistance. On the other hand, the 400 series, or martensitic stainless steels, are chromium-magnesium alloys that provide high strength and machinability. They are magnetic and can be hardened through quenching and tempering, making them suitable for machine parts and valves.

Applications of Stainless Steel Alloys

Benefits of Stainless Steel Alloys in Investment Casting

One of the biggest advantages of using stainless steel alloys in investment casting is their versatility. The alloys can be heat-treated to further enhance their mechanical and corrosion-resistant properties, making them suitable for a wide range of applications. Additionally, stainless steel alloys offer excellent dimensional accuracy and repeatability, ensuring consistent and precise results. The alloys also provide smooth and fine surface finishes, reducing the need for secondary machining processes.

Challenges with Stainless Steel Alloys

While stainless steel alloys offer numerous benefits, there are some challenges to consider. One potential issue is the poor surface finish that can sometimes result from the interaction between the chromium and nickel content in the stainless steel and the refractory materials used in the investment casting process. However, this can be mitigated by using certain coatings and selecting appropriate refractory materials.

Brass

The process begins with the creation of a master die, or pattern, from wax. This can be done by hand, by machine, or through injection moulding. The wax pattern is then attached to a rod of wax called a sprue, and multiple wax patterns can be attached to the same sprue. The assembly is then dipped into an investment, or slurry, and left to dry. This process may be repeated several times to achieve the desired thickness. Alternatively, the assembly can be mounted in a flask and the slurry poured over it.

Once the investment has dried, the assembly is placed in an oven and heated to vaporise or melt the wax. The mould is then heated multiple times to remove any residual wax or moisture. Molten brass is then drawn into the mould through positive air pressure, a vacuum, or other centrifugal casting methods. The brass is then left to cool and harden before the mould is removed, revealing the brass casting.

However, a significant drawback of brass investment casting is its high cost compared to other manufacturing methods. Despite this, it remains a popular choice for industries such as aerospace, military, medical, automotive, and jewellery, where precision, complexity, and fine details are crucial.

Carbon steel

There are several types of carbon steel, classified according to their carbon content:

• Low-carbon steels: With less than 0.20% carbon, these steels offer a range of tensile properties via heat treatment and can be hardened to increase surface hardness and abrasion resistance. They also have good core ductility.
• Medium-carbon steels: With a carbon content of 0.20% to 0.50%, these steels are durable, pliable, and strong. They offer a range of tensile strengths, good ductility, and shock resistance. They are also readily machinable in the softened condition.
• High-carbon steels: With a carbon content of more than 0.50%, these steels are strong and have good shape memory. This makes them useful for spring manufacturers and heavy-duty applications.

When using carbon steel for investment casting, careful alloy selection is required. The 1000 series, for example, is the softest of the common carbon steels and is often used for parts that can undergo minor deformation, like cranks or handles. On the other hand, the 4000 series is used for parts that must not deform, such as firearm and automotive components. The 6000 series, known for its high shock resistance, is used for moving parts like gears and engine components.

The two main casting processes for carbon steel are Water Glass Casting and Silica Sol Casting. Water Glass Casting is the more common and affordable option, producing castings with a tolerance of CT7-CT8. Silica Sol Casting, on the other hand, offers better surface finishes and tighter tolerances (CT5-6), but at a higher production cost.

Aluminium

The precision investment casting process for aluminium involves using a wax pattern to create a ceramic mould. The molten aluminium alloy is then poured into the mould, resulting in an incredibly precise and detailed aluminium alloy casting.

Milwaukee Precision Casting, Inc. is an example of a company that uses aluminium investment casting to produce critical parts such as pump impellers and housings, covers, plates, high-velocity blowers, and more.

Glass

The investment casting of glass involves filling a flask with powdered glass, with a large hopper (also filled with glass powder) over the sprue. The whole setup is then placed in a kiln, where the glass melts and unifies within the mould. This ensures that the air space between powder particles is pushed out as the glass melts.

Water glass casting is a type of investment casting process that uses water glass as the binder for the shell. It is one of the most common investment casting processes in China, but it is seldom used in other countries. Water glass casting is suitable for steel castings, especially carbon steel and alloy steel castings. The process allows for the production of steel castings ranging from 0.5kg to 60kg.

Water glass investment casting uses water glass quartz sand as the moulding material, which can withstand temperatures up to 1800°C. This material is more expensive than wet clay sand, which is commonly used in sand casting.

The silica sol investment casting process uses silica sol zircon sand as the moulding material, which can withstand temperatures up to 2000°C. This material is very expensive.

The investment casting of glass allows for the production of parts with complex shapes, internal cavities, and thin walls, which are challenging to create using other casting methods.

Frequently asked questions