What is sand casting, and how does it work?
Sand castings are 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.
The third main step in sand casting is clamping the drag and cope together, making a complete mold. If a core is needed for some internal contours, it would be placed into the mold before the clamping step, and any gating/runners are also checked for misalignments.
The fourth step begins when the desired final material (almost exclusively some metal) is melted in a furnace, and is then poured into the mold. It is carefully poured/ladled into the sprue of the mold, where the molten metal will conform to the cavity left by the patterns, and then left to cool completely. After the metal is no longer hot, manufacturers will remove the sand from the mold (via vibrations, waterjets, and other non-destructive means, known as “shakeout”) to reveal the rough final part.
The fifth and final step (not shown in Figure 1) is the cleaning step, where the rough part is refined to its final shape. This cleaning includes removing the gating system and runners, as well as any residual mold/core parts the remains in the final piece. The part is trimmed in areas of excess, and the surface of the casting can be sanded/polished to a desired finish. After major cleaning, each part is inspected for defects and is tested to ensure compliance with the manufacturer’s standards of quality, so that they will perform as intended in their respective applications.
Valves are mechanical or electro-mechanical devices that are used to control the movement of liquids, gases, powders, etc. through pipes or tubes, or from tanks or other containers. In most instances, valve relys on some form of mechanical barrier—a plate, a ball, a diaphragm, for example—that can be inserted and removed from the flow stream of the material passing by. Some valves are designed as on-off varieties, while others allow very fine control of the passage of media.
Material selection and valve parts play an important role in specifying valves to ensure the compatibility of the wetted parts of the valve with the fluid or powder passing through it. Sizing is determined by the pipe or tubing diameter, flow rate, and the width between flanges for pipeline valves being installed as replacements.