Though they have been refined through the ages, significant differences exist between the two methods used to make metal parts.
Originally published in the April 2016 issue of Machine Design by Brad Done, Reliance Foundry Co.
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.
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.
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:
- Form a gelatin mold around a solid, 3D model of the final product.
- Create a metal mold of the final product, and then fill it with hot wax.
- Carve and machine a replica of the product out of wax.
Due to the complexity and preparation needed, investment casting is often much more expensive than sand casting. But sand casting cannot always make the small, intricate parts possible with investment casting. And while investment castings can make parts weighing only fractions of an ounce, they are limited in terms of the size and weight of parts made with sand casting.
For comparison, investment castings can weigh a fraction of an ounce, such as for dental braces, or more than 1,000 lb. for complex aircraft engine parts. Smaller components can be cast at hundreds per tree, while heavier castings often are made using an individual tree.
Another major difference between investment and sand casting is the surface finish of the final product. With sand casting, the mold has to be split apart to release the final product. As a result, finished parts have a seam left by the parting lines in the mold. And the relatively rough sand leaves an equally rough surface on cast parts. In many instances, the parting line and roughness are smoothed over in secondary machining processes, but this requires additional time and labor.
One advantage of sand casting is that it is relatively simple to change the mold (by modifying the pattern or core boxes, often made of wood) to accommodate design changes. With investment casting, technicians may need to modify or replace the solid metal pattern or make new molds for turning out wax versions, which can be a more complicated process.
(Originally printed in the April 2016 issue of MACHINE DESIGN.)
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