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

Investment casting and sand casting are two methods for creating metal parts. They involve pouring molten steel into three-dimensional molds. While these methods have been around for thousands of years, they have been improved over time to reduce the metal required and minimize any machining or finishing. There are significant differences between the two methods of making metal parts. Let's first look at the basic principles behind each.

Basics of Sand-Casting

During sand casting, molten metal is poured into the molds made from sand.

Sand casting involves pouring molten metal (usually iron, steel or bronze, brass, aluminum and magnesium) into a two-piece mold. Molds are made by compressing sand, which is most often mixed with clay to form a bonding agent. The molds then get moistened with water and molded around a model or pattern of the final product. The pattern is removed and the mold is broken apart. The mold is reassembled and the void filled with molten steel. After the metal has cooled sufficiently, the mold can be opened and the sand removed. The part is then taken out.

Multiple parts of the same part can be cast simultaneously, or different parts can share one mold. While molds can be destroyed during the process, new molds are simple to make. Sand can often be reclaimed and used multiple times.

Sand casting molds usually have two halves, which are securely fastened together when they are being used.

Basics of Investment-Casting

Investment casting is also known as lost-wax cast. It uses molten metal to make parts, such as stainless-steel alloys and brass, aluminum, carbon steel, and carbon. First, you will need to create a wax model of the final product. You can do this in one of these three ways:

You can cast multiple parts at once by using several wax patterns. The wax runners and sprues are carefully connected to make a tree. After the "tree" has dried, it is dipped in a ceramic glaze that hardens as it dries. After several layers of ceramic have dried, the "tree" and its patterns can be heated to melt the wax. The wax from the sprues, gates and leaves also flows out of trees and forms pipes and tubes for molten steel to follow.

The now empty mold is filled with molten metal and allowed to cool. To remove any part, the ceramic mold (the investment) is destroyed.

Differences

Investment casting can be more costly than sand casting due to the amount of preparation and complexity involved. Aluminum Sand Casting is not always able to produce the intricate, small parts that investment casting can. Investment castings can produce parts as small as a fraction of an ounce but they have a limit on the size and weight of parts made from sand casting.

Investment castings, on the other hand, can weigh less than an ounce (e.g. for dental braces) or more that 1,000 lb. Complex parts of aircraft engines. Casting smaller components at hundreds per tree is possible, while castings of larger parts are often made with one tree.

Investment castings are limited in weight due to the equipment used at the casting facility. The majority of U.S. facilities can cast parts up 20 lb. Many domestic facilities are becoming more capable of casting larger parts. Components in the 20-120 range are becoming more common.

The final product's surface finish is another major difference between investment casting and sandcasting. Sand casting requires that the mold be dismantled in order to release the final product. The mold's parting lines will leave a seam in the finished parts. Cast parts also have a rough finish due to the sand. Although the roughness and parting lines can be smoothed out in secondary machining, this takes additional time and labor.

Investment Permanent Mold Casting uses hardened ceramic molds. The finished parts have smooth surfaces, low tolerances and thin walls. Cast parts are often left unprocessed after removing the sprues and gates.

The molds are made using liquid slurry. This allows engineers to create complex designs and intricate features. Parts cast from sand must be tapered (with draft angles), or shaped to emerge easily from the compacted, sand with minimal friction. Sand-cast parts that have internal cavities or voids can be difficult to cast. To form the interior of the part, cores must be properly shaped and inserted into the mold. It is possible to need multiple cores, which can make it time-consuming and difficult to secure each one within the mold.

Sand casting has the advantage that you can easily modify the mold to suit your design needs. Investment casting can require technicians to modify or replace solid metal patterns, or create new molds for wax versions. This can be more difficult.