We shall discuss a beginners’ guide to metal casting and an overview of metal casting in a foundry.
Our lives are filled with cast metal products. Many metal objects we take for granted—train wheels, trailer hitches, lampposts, large industrial equipment, and even statues—are cast in foundries.
The various applications of cast metal demonstrate its versatility: the metal can be cast into durable and complex metal parts with minimal machining or welding, reducing the need for expensive labor. In addition, metal foundries have become important users of recycled metal scrap, turning discarded metal items into useful products.
What is metal casting?
Casting is the process of creating solid metal shapes (casting) by filling the empty space in the mold with liquid metal and then cooling the object. The basic steps in the manufacture of castings are molding, forming, melting, pouring, cooling, grinding and cleaning, heat treatment, and inspection.
Pattern,
In casting terminology, a “model” is a copy of the object to be cast, which is used to shape the mold cavity. They can be made of different materials including wood, metal, or plastic.
Plaque-making is the process of producing these patterns. Since the pattern determines the shape of the casting, the casting cannot be better than the pattern that made it. To produce quality castings, models must be carefully designed, built, and finished.
The main functions of the model are:
Shape the mold cavity
Compliance with the characteristics of metal casting
Provide exact size
It provides a method of delivering liquid metal into the mold.
When solidified, the metal tends to shrink and/or deform (to varying degrees depending on the type of metal casting). The pattern is designed with internal shrinkage and deformation limits to compensate. It was also necessary to create a cone called a draft on the vertical wall, which was necessary to extract the pattern without disturbing the walls of the mold.
Hollow castings can be made using a core, which is an extra piece of sand or metal that forms the holes and channels inside the casting. Each core sits in the mold before the molten metal is poured. To keep each core in place, the template has grooves called core prints that hold the cores in place.
Formation
Wooden pattern next to a metal replica produced by the casting process
Wood grain is produced by casting (left) and resulting in solid metal casting.
Forming is the process of preparing the mold to receive the molten metal. There are two different types of mold processes: reusable and non-reusable.
As the name suggests, reusable templates can be reused. The casting process does not break the mold as the metal solidifies and cools. Reusable molds are usually made of metal.
In contrast, non-reusable molds are temporary objects that are destroyed during the solidification and cooling of the metal. The most widely used method of non-reusable molds is sand casting, a process in which special sand (“green” sand) is placed around the model and placed in a holder (flask). The mold is then removed, the core is placed in place, and a gate system is created to guide the molten metal into the mold.
Each of these general mold method categories has several subgroups that are specifically optimized for different cast metals and different levels of pattern complexity. Such methods include hollow casting, die casting, shell casting, and investment casting.
Melting and pouring
There are two types of metals from which castings are produced: ferrous (metals that contain iron) and non-ferrous metals (metals that do not contain iron). Iron alloys include steel, malleable iron, and gray iron. The most common non-ferrous alloys in casting are aluminum and copper, but alloys based on magnesium, nickel, and titanium are sometimes used for special applications.
The metal to be melted and cast—usually a mixture of recycled scrap and alloy metal—is loaded into a “charge” furnace. Once inside the furnace, the metal is exposed to extremely high temperatures until it reaches its melting point (typically over 2,500°F [1,370°C]). To reach such high temperatures.
The two main types of furnaces used in foundries are electric arc and induction.
Electric arc furnaces operate as a batch melting process, producing batches of molten metal known as “hot”. This metal is melted by supplying electricity into the furnace through graphite electrodes. Additional chemical energy is provided by oxyfuel burners and oxygen lances.
Oxygen is injected during the melting process to remove impurities and other dissolved gases. As the metal melts, slag is formed and floats to the top of the molten metal; Slag often contains undesirable impurities and is removed before tapping (the process of removing the metal from the furnace).
Induction furnaces transfer power by induction – a high voltage power source from the primary coil induces a low voltage and high current in the steel or secondary coil. Induction furnaces are capable of melting and alloying many metals with minimal melt loss but are not as capable of refining metal as electric arc furnaces.
Due to their advantages and disadvantages, electric arc furnaces are mostly used for melting ferrous metals, while induction furnaces are dominant in non-ferrous metal applications.
Crucibles, robotic arms, and gravity induction castings are used to transfer molten metal from one location to another. Skilled metalworkers will also use ladles to pour molten metal.
Molten metal is poured into the mold through a system of gates and elevators; The metal cools and solidifies, forever taking the shape of the interior (void) of the mold it occupies. The mold is then ejected from the mold or shaken out of the sand mold.
The gates and risers that feed the molten metal into the mold are also filled with molten metal in this process. The metal in the mold solidifies with the metal in the gate and riser system to form a single unit. After molding/breaking, the metal from the gate and mount system is still attached to the original casting. Remove excess metal during cleaning (usually with a chisel hammer or band saw). A combination of drum, blower, and pressure washer is used to remove any remaining sand or sediment.
Heat Treatment and Inspection: Service Life and Recycling
Some castings are used in industrial applications: they may need to maintain their precise shape at subfreezing temperatures, resist corrosion in humid environments, or support large weights. Heat treatment is used to change the physical properties of metals to desired specifications.
Heat treatment involves the use of heating and cooling, often at extreme temperatures, to reduce stress in the casting and/or change the physical properties of the metal. To achieve the desired physical properties, the temperature must be precisely controlled.
Castings are tested for physical properties and structural integrity before a full production run is considered. Test methods that require damage to the casting under test are called destructive tests, while those that do not damage the casting are classified as non-destructive tests.
The test method used depends on the specification requirements. For some purely aesthetic products, only a simple visual inspection of dimensional accuracy, cracks, and surface finish is required. On the other hand, if a casting is to be implemented in an industrial capacity, it is possible to fully test all its physical properties (adhesion, tensile strength, elongation, impact properties, hardness, etc.).
After passing casting inspection, they are shipped, sold, and put into service. Cast metal, especially heat-treated metal, is durable. Depending on the metal alloy and the application, their useful life can vary from months to decades. When the useful life of the casting is over, the cycle begins again. It is collected from scrap yards, recycled, and remade into a new object.
Interested in learning more about custom metal casting manufacturing? A deeper understanding of the metal casting process and how to communicate your needs to a production foundry will help ensure the success of your casting project.
Conclusion
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