There are several steps in the sand molding process. From mixing the sand particles to finishing the final product, great care and attention to detail are included in every step.
1. Create The Sand Mixture
The sand used in the green sand casting process is a mixture of sand particles, clay, wood flour, and water. The sand particles are typically silica sand, and the clay is usually bentonite clay. Combining these ingredients makes a compactable mixture that can be used for molding. While the sand is black, this mixture is known as green sand because it contains moisture. The ingredients are processed through a muller that mixes the sand, clay, and wood flour and provides forces that incorporate kneading and shearing for uniform consistency.
The physical characteristics of the sand mixture are some of the most important controls that must be maintained in the sand casting process. The grain size, moisture content, and compactibility are just a few of the variables. Here at Boose Aluminum, our quality lab performs numerous daily tests to maintain the properties of our sand at optimal conditions.
2. Create The Mold
One of the highlights of sand casting is the creation of the sand mold. This intricate procedure requires carefully packing the sand mixture around a mold pattern to achieve the desired shape. This pattern is then removed, leaving a mold cavity that retains the shape of the pattern.
More specifically, it takes three items to produce a mold: a pattern, a flask, and molding sand. In a foundry, a pattern replicates the object to be cast, mounted on a board. Patterns used in sand casting may be made of wood, metal, plastics, or other materials. A flask is a "box" with only sides, no top or bottom, and forms a frame around the pattern. Each flask is split horizontally and has an upper frame, known as the cope, and a lower frame, or drag.
The process starts with the cope side of the flask on a flat surface. A pattern is placed on the cope. The second part of the flask, or drag, is then placed on top of the pattern. The pattern is "sandwiched" between the cope and drag. The drag is packed with the molding sand, and the excess sand is struck off.
Then the whole "sandwich" is rotated, and the top half, or cope, is filled with sand, packed, and excess sand removed. Then, the worker cuts a sprue or column through the sand in the cope half to serve as the entryway for the metal to enter the gating system. The mold's top half is detached, and the pattern is removed. The top mold half is placed on top of the bottom half, leaving an open cavity between the two mold halves for the liquid metal to flow through.
3. Pour the Molten Metal
Once the sand mold is prepared, the next step is pouring molten metal. This liquid metal is carefully poured into the mold cavity through the sprue, which feeds the gating system. The gating system delivers the molten metal to the void in the mold created by the pattern. As the molten metal cools, it solidifies into the desired shape, taking on the intricacies of the mold pattern.
The pouring stage is critical in achieving a metal casting with precise dimensional accuracy and a good surface finish. As the metal cools, the molding sand around it helps to absorb the heat, facilitating an even cooling process.
Just like the sand preparation, the pouring of the metal has several variables that need constant monitoring, including the metal's temperature and the speed at which metal is introduced into the mold.
4. Remove and Finish the Product
After the poured metal has cooled and solidified, the sand mold is broken apart to reveal the casting. This cast metal piece goes through a series of cleaning and finishing processes to remove any adhering sand and achieve the required surface finish. Various techniques can be employed to refine the surface finish, such as grinding, sanding, or shot blasting.
