How to Choose Between Filled and Unfilled Wax Processes for Investment Casting Parts

Investment casting is one of the oldest and most sophisticated casting processes, allowing intricate shapes to be produced, while still maintaining high dimensional accuracy and high surface quality. Investment casting can be used to cast almost any metal, satisfying application requirements in diverse end markets.

The metal forming process of investment casting uses a wax pattern that is created in the shape of the workpiece. This wax is then surrounded by a ceramic shell. After the ceramic shell dries, the wax is melted out (or dewaxed), leaving the ceramic mold. Because the wax mold is melted, this is also referred to as the lost wax process.

Once the wax is eliminated, molten metal is poured into the ceramic mold to form the desired casting component. When the metal has cooled, the ceramic mold is removed, leaving the casting component. Some investment castings require a secondary machining process after casting. This article looks at the types of wax that can be used in the wax pattern production part of the process.

What Types of Wax are Used in Investment Casting?

Typically, the process of making a shell starts with a liquid wax mixture that is injected into a die. After cooling, the wax is removed from the die, to serve as a pattern for the ceramic shells. In the oldest forms of investment casting, beeswax was used for this purpose. In modern times, however, there are several types of wax that can be used. The two that we will focus on here are filled wax and unfilled wax.

• Filled Wax: Filled wax simply means that some type of filler is added to the investment casting wax. These fillers can provide certain properties to the casting to ensure strength, dimensional stability, lower thermal expansion, and minimal shrinkage. The types of fillers found in filled pattern waxes vary based on the supplier, yet some common filled wax mixtures may include bisphenol-A (BPA), organic fillers, terephthalic acid, cross-linked polystyrene, and even water. Due to the addition of the filler, some wax may remain in the ceramic shell after the dewaxing process, because it takes longer for the wax to be burnt out. Filled pattern waxes can also be more difficult to reclaim and recycle, depending on the density of the filler material.
• Unfilled Wax: In contrast to filled wax, unfilled wax blends contain less filler material. This type of wax also offers consistent mechanical performance and thermal performance. Unfilled wax is typically used for small and medium wax patterns, or investment castings which include complex geometries and well-defined patterns. Unfilled wax has exceptional flow properties. It also completely dewaxes from the ceramic shell, making it easier to reclaim and recycle.

How to Choose Filled or Unfilled Wax for Your Investment Casting Parts

Waxes can exhibit different properties, so the choice of filled or unfilled wax can have an influence on pattern behavior and the ultimate quality of the investment castings produced. The type of wax used will be based on several factors including flow properties, whether the wax will be reclaimed and recycled, dimensional consistency, surface finish, and the requirements of the application.

• When to Choose Filled Wax for Investment Casting Parts: Both small and large wax patterns can be created with filled wax. The major advantage of filled wax is that it has low thermal expansion and minimal shrinkage, eliminating the need for a wax chill to be placed into the pattern cavity to prevent shrinkage as the wax solidifies. Filled wax is often used when more dimensional control of the investment casting or component is required, or when using a wide range of injection temperatures.
• When to Choose Unfilled Wax for Investment Casting Parts: The primary advantage of unfilled wax is that it provides a high surface quality. This results in a ceramic mold with fewer cracks and defects. The biggest concern with unfilled wax, however, is that it has a slower solidification, which could result in sinking of the pattern surface. There could also be shrinkage along cross-sections of the pattern, depending on the wax temperature. The use of wax chills may be necessary with unfilled wax to avoid material shrinkage, depending on the configuration of the investment casting.

Learn More About Filled or Unfilled Wax for Investment Casting Parts

Impro offers investment casting for a wide range of applications including automotive, medical, aerospace, and industrial equipment. We are a global, integrated manufacturer of high-precision, high-complexity, and mission-critical components for diversified markets. We use state-of-the-art process technologies and equipment throughout our manufacturing operations. If you are interested in finding more information about investment casting and our process capabilities, please contact our team of experts today.