Sand Casting

Sand casting, the oldest known casting process, has been practiced since the 1000s BC. While process controls, material options, tolerance capabilities, ability to produce precision parts, and wide size ranges certainly represent major developments, the fundamentals of the application have largely remained the same. In its simplest form, sand casting is the creation of a cavity in the desired part shape and the pouring of molten metal into it. Sand casting is the most versatile and probably the most widely used metal casting method for thousands of years. When choosing how to manufacture a product, the design requirements (including shaping and dimensional needs), part and tooling costs, required volume, and even production feasibility shape which metal processing processes (including which casting methods) are most suitable. Sand casting uses techniques that produce shaped parts for almost any design, including very large parts and those containing internal passages. For any specific product, there may be casting or metal processing methods better suited depending on required tolerances, design complexity, volume, tooling availability, or delivery time. However, it is possible to produce a casting using sand processing in the required configuration, and this decision is left to the design engineer. The process is called sand casting because the mold, which contains the cavity into which the metal is poured, is made of compressed or consolidated sand. The sand may also contain reinforcing materials to maintain its shape.

Sand Casting Process A. Producing the Casting Pattern - Desired Product

The process uses a reusable pattern with the same details as the desired finished part. It carries a tolerance for thermal shrinkage or contraction.

B. Casting Pattern: Gates and Risers: Metal Distribution System

The casting pattern produced in step "A" also includes metal paths that will feed the desired casting design with appropriate gates and risers. This directs inevitable thermal shrinkage to acceptable areas (somewhere other than the actual desired final product) and manages metal flow and required gas venting. Casting patterns are made from many different materials depending on required volume and tolerance, such as wood, metal, synthetic, expendable polystyrene (EPS), and others.

C. Mold Making

A refractory material stable at high temperature (sand in this example) is formed around the casting pattern. The material must be strong enough to support the weight of molten metal during casting and resistant to reaction with the metal, yet brittle enough to separate easily from the solidified metal after the casting cools. There are various sand materials that can be used to make the mold. Sand is typically reinforced with other materials such as clay or other chemical binders to withstand the casting process. Alternatively, the mold can also be formed by directly machining the desired shaped cavity into a sand block. This technique is commonly applied during product development because design changes can be managed quickly and readily, or used for infrequently produced parts to avoid storing and maintaining a physical pattern. The mold is generally produced in two pieces: an upper and lower half. When the sand is set (conventional/without machine), the halves are separated and the casting pattern is removed. A refractory coating is added to provide better surface finish and to protect the mold from turbulence of the poured metal. The halves are assembled together and left with a cavity in the shape of the casting pattern. The mold may also contain cores, a method used to produce the internal passages desired in the final product.

D. Pouring Metal into the Mold

Molten metal is poured directly into the static mold. It fills the cavity defining both the finished part and the risers. The risers feed the casting process with a continuous source of liquid metal. Finally, because they are designed for cooling and solidification, shrinkage and potential voids concentrate in the riser rather than the desired section. There are various variations of tilted pouring, a process that allows the metal to flow more smoothly into the casting and eliminates turbulence. Less turbulence can help prevent the formation of oxides and casting defects. Nearly every alloy can be produced using this process. For materials particularly reactive with oxygen, a procedure such as argon shielding can be used to keep air away from the molten metal.

E. Shakeout

The casting, containing both the desired part and the additional metal needed to create it, solidifies and cools. The sand is broken apart in a shaking process. Most of the sand used to form the mold is collected, reconditioned, and reused.

F. Finishing Operations

Gates, runners, and risers are cut from the casting, and when necessary, shot blasting, grinding, etc. are performed to dimensionally finish the casting. Generally, sand castings require at least some additional processing to reach final dimensions or tolerances. Parts may be heat treated to improve dimensional stability or properties. Nondestructive inspection may also be performed. This may include fluorescent penetrant, magnetic particle, radiographic, or other inspections. Final dimensional checks, alloy test results, and NDT are verified prior to shipment.

References • Metal Casting: A Sand Casting Manual for the Small Foundry, Volume 2. Steve Chastain, 2004. • The Complete Handbook of Sand Casting. C. W. Ammen. McGraw Hill Professional, Mar 22, 1979. • Metal Casting: Principles And Practice. Ramana Rao. New Age International, 2007.