All About Sand Casting – What it is and How it Works
Metal casting is one of the most basic yet most useful manufacturing methods available to designers. These processes involve pouring molten metal inside of a preformed mold, which becomes a finalized part when cooled. The ability to shape metal without the need for machining has allowed for the mass production of complex parts that are both durable and inexpensive. As a result, there are many processes used to cast metal, and this article will highlight the most widely used casting method, sand casting. This process uses sand to create any number of complex mold shapes, and this article will show how this process works, how it fares against other methods, and where sand casting is used in industry today.
Sand casting is a casting process by which sand is used to create a mold, after which liquid metal is poured into this mold to create a part. To learn about the other forms of casting, visit our article on the types of casting processes. Sand is used in this method because it insulates well, it is relatively cheap, and it can be formed into any number of mold shapes. There are defined steps to this process (shown simplified in Figure 1), and this article will walk through each of these steps to illustrate exactly how this casting procedure is conducted.
The first step in the sand casting process involves fabricating the foundry pattern – the replica of the exterior of the casting – for the mold. These patterns are often made from materials such as wood or plastic and are oversized to allow the cast metal to shrink when cooling. They are used to create the sand mold for the final part, and can potentially be reused depending upon the pattern material. Often times, two pattern halves are separately created which provides cavities when put together (shown in Figure 1). Cores are internal mold inserts that can also be used if interior contours are needed, but are typically disposable after one casting. The type of pattern and its material is dictated not only by the desired part dimensions but also by the number of castings needed from each mold.
The second step is the process of making the sand mold(s) from these patterns. The sand mold is usually done in two halves, where one side of the mold is made with one pattern and another side is made using the other pattern (shown in Figure 1). While the molds may not always be in two halves, this arrangement provides the easiest method of both creating the mold and accessing the part, once cast. The top part of the mold is known as the “cope” and the bottom half is the “drag”, and both are made by packing sand into a container (a “flask”) around the patterns. The operator must firmly pack (or “ram”) the sand into each pattern to ensure there is no loose sand, and this can be done either by hand or by machine. After ramming, the patterns are removed and leave their exterior contours in the sand, where manufacturers can then create channels and connections (known as gates/runners) into the drag and a funnel in the cope (known as a “sprue”). These gates/runners and sprues are necessary for an accurate casting, as the runners and gates allow the metal to enter every part of the mold while the sprue allows for easy pouring into the mold.