One of the oldest and yet most fundamental industrial processes known to man is metal casting. Started in antiquity to create jewelry and weaponry, casting is a process where liquid metal is poured inside of certain molds to quickly produce complex shapes. In modern times, standard procedures have been developed to create highly intricate parts, specifically with a process known as investment casting. Investment casting uses wax, slurry, and molds to produce low-tolerance, high-resolution parts without the hassle of more traditional methods. The process of investment casting, how it works, its advantages, and its applications will be shown in this article so that designers can potentially implement this technique into their own projects.
What is investment casting, and how does it work?
Investment casting is a type of casting process by which highly complex parts can be quickly made (to learn more about casting, read our article on the types of casting processes). There are four main steps to the investment casting processes, shown in Figure 1, and explained below.
The first step involves creating the wax pattern that will eventually be the final shape of the part(s). Wax is utilized because it is easily melted and reused, but this also means that a wax pattern can only be used once each time a part is made. This limitation requires the manufacturer to have some sort of master mold, which can be reused to create the wax patterns. These can be expensive because they must be tailored to each part, and are tricky to perfect if especially low tolerances are needed. However, if many of these molds are made, they can be connected via a wax bar (known as a “runner”) which allows for one pour to cast many parts. Finally, a ceramic pouring cup (known as a “sprue”) is added to the top of the wax pattern, so that manufacturers will have a funnel to pour the molten metal into the final mold.
The second step in the investment casting process is when this finalized wax pattern, complete with runners and sprues, is dipped into a ceramic refractory slurry. This liquid usually contains extremely fine silica, water, and other binders. When dipped, the part will be covered in a thin layer of slurry, which is effectively a mold of the wax pattern. This dipping step is done many times over to achieve a certain coat thickness (often 5-10mm), and then the part is dried. After drying, the part is turned upside down and heated to both remove excess moisture and melt away the wax on the inside. Now, manufacturers are left with a hollow ceramic mold of their desired part.
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