Why Sand Casting is Still an Effective Method Today
Sand casting is a method that has been used for thousands of years and remains one of the most versatile and widely used methods for casting metal today. Sand casting is the process of pouring molten metal into a form made from condensed sand. The sand mold holds the metal’s shape as the metal cools. Once the metal is cooled the manufacturer separates the mold to reveal the shaped metal below, destroying the sand casting in the process. This basic process has not changed considerably over time, but there have been advances throughout the years in the technology used for it.
The use of sand casting is widespread throughout the foundry industry. A majority of metal castings are manufactured using a sand casting process because it is a cost effective and extremely reliable method for complicated parts and components requiring internal cavities. Sand casting allows for the design flexibility needed to create highly complex shapes in almost any size or weight from nearly every type of engineering alloy.
Perhaps the largest drawback to sand casting is that the finished product relies heavily on the quality of the sand casting itself. Molds provide the best outcome possible when they can withstand the high temperatures of liquid metal they hold, and when they release the metal product cleanly. Defects in the sand-casting process can detract from the quality or functionality of the finished product.
Creating Quality Sand Casting Molds
When making a sand cast, manufacturers begin by creating a reusable pattern that features the same details as the finished part they wish to create. They build in a small allowance for thermal contraction which is the amount the metal part will shrink as it cools and hardens. The pattern also includes metal pathways that feeds the desired cast design and manages thermal contraction, metal flow, and gas venting. Depending on needed volume and tolerance, patterns may be made from metal, wood, synthetics, expendable polystyrene (EPS) or other materials.
Mold designers then pour sand around the pattern. The sand often contains clay, bonding materials or other components that help it stand up to the metal pouring process. In some cases, designers may machine the desired shaped cavity directly from a block of sand. Machining the sand casting is common during the product development stage, when designers want to implement or manage changes quickly, and avoid storing patterns.
The sand-cast mold generally has two main components: the top “cope” half and the bottom “drag” half. The mold may also contain cores that redirect and solidify the metal to create internal features and geometry within the final product. Once the sand has set, workers separate the halves and remove the pattern. A refractory wash is added that serves as a parting compound, helping the part release from the mold properly. The refractory wash also gives the finished product a better surface finish and protects the mold from the hot metal. It typically contains water or solvent based liquid, graphite solids, or magnesite talc.
After the refractory wash, the sand-casting mold goes through sand core dryers to seal the refractory wash onto the surface of the mold. A conveyor belt is used to transport the washed sand cast molds and cores through the oven. The core dryer, also known as a core drying oven or mold drying oven, is the final step in preparing sand casting molds for use. Contact Wisconsin Ovens for more information about sand core dryers or mold drying ovens.