The Sand Casting Process
Sand casting is an economical process for creating rough metal parts. Raw castings are then machined into finished products or components. Sand casting is the least expensive of all the casting processes, including die and investment casting.
Forming the Cavity
In the sand casting process, a pattern is made in the shape of the desired part. The pattern is typically made of wood, plastic, or metal. A single piece or solid pattern is used for simple designs. Patterns that are more complex are made in two parts, called split patterns. The upper part of a split pattern is called a cope, while the bottom section is called a drag. Where the cope and drag separate is known as the parting line. Both solid and split patterns can have cores inserted to complete the final part shape. When making a pattern, it is necessary to taper the edges so the pattern can be removed without breaking the mold.
The pattern is housed in a box called the flask, and then packed with sand. A binder helps harden the sand into a semi-permanent shape. Once the sand mold is cured, the pattern is removed. This leaves a hollow space in the sand in the shape of the desired part. The pattern is made larger than the cast to allow for shrinkage during cooling. Sand cores can then be inserted in the mold to create holes and improve the casting’s overall shape. Simple patterns are usually open on top, allowing molten metal to be poured into them. Two-piece molds are clamped together. Molten metal is poured into a pouring cup where it will then travel down a sprue and into the gating system. Vent holes are created to allow hot gases to escape during the pour. Ideally, the pouring temperature of the molten metal is a few hundred degrees higher than the melting point, assuring good fluidity. The temperature difference also prevents premature cooling and resulting voids and porosity. After the metal cools, the sand mold is removed and the metal part is ready for additional operations, such as cutoff and grinding.
Design Considerations
Properly locating the parting line reduces the number of cores and the amount of waste while increasing dimensional accuracy. Uniform casting thickness results in uniform cooling and solidification, as well as distortion-free castings. When uniform cross sections cannot be maintained, gradual changes in cross sections are required. This can be achieved by using a transition radius of 1/3 of the thicker section and blending in the radius with a 15-degree slope line. To minimize problems at intersecting points within the core, use staggered rather than continuous ribs and avoid large, unsupported areas. Maintaining minimum wall thickness will prevent voids and non-fill areas.
With a solid understanding of the process and its advantages, you can confidently select sand casting as the low-cost method for creating your metal components.
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