What’s the Difference Between Investment Casting and Sand Casting?

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Sand casting and investment casting are methods of creating metal parts by pouring molten metal into three-dimensional molds. Although methods are thousands of years old, both have been refined over the years to minimize the amount of metal used and reduce any extensive machining and finishing required. But significant differences exist between these two methods of making metal parts. First let’s take a look at the basics behind each.

Sand-Casting Basics

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Molten metal gets poured into the sand-based molds during sand casting.
In sand casting, molten metal, usually iron, steel, bronze, brass, aluminum, magnesium, and other non-ferrous alloys, is poured into a two-piece mold. The molds are created by compacting sand—most often mixed with clay as a bonding agent and moistened with water—around a pattern or model of the final product. The mold is split apart and the pattern removed. The two halves of the mold are put back together and the void is filled with molten metal. Once the metal cools sufficiently, the mold is opened, the sand removed, and the part taken out.

Several of the same parts can be cast at the same time, or a range of different parts can share a mold. Molds are destroyed in the process, but new ones are easy to make. The sand is often reclaimed and reused multiple times.

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Molds for sand casting usually have two distinct halves that are tightly fastened together when being used.
Investment-Casting Basics

Investment casting, also called lost-wax casting, makes parts from molten metal, usually stainless-steel alloys, brass, aluminum, and carbon steel. The first step in the process is to build a wax version of the final product. This can be done in one of three ways:

1. Form a gelatin mold around a solid, 3D model of the final product.
2. Create a metal mold of the final product, and then fill it with hot wax.
3. Carve and machine a replica of the product out of wax.

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A “tree” of individual molds is heated to remove wax patterns used to shape the molds. (Photo courtesy of Kirt Edblom)
To cast several parts at once, several wax patterns can be assembled into a tree with wax runners and sprues carefully connecting them. The “tree” is then repeatedly dipped in a ceramic slurry that hardens when it dries. After receiving several layers of ceramic that are allowed to harden, the tree and its patterns are heated to remove the wax. The wax in the sprues and gates also flows out of the tree and forms tubes and pipelines for molten metal to follow and completely fill the mold.

Molten metal is poured into the now-empty mold and left to solidify. The ceramic mold, the investment, is destroyed to remove the part(s).

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