An overview of metal casting in a foundry
Our lives are filled with cast metal products. Many of the metal objects we take for granted – train wheels, trailer hitches, lamp posts, large scale industrial equipment, and even sculpture – are cast in a foundry. The sheer number of different applications for cast metal demonstrates its versatility: metal can be cast into durable, complex metal components with minimal machining or welding, thereby decreasing the need for expensive labor. More importantly, metal casting foundries have become a significant user of recycled scrap metal, taking obsolete metal objects and transforming them into useful products.
What is Metal Casting?
Casting is a process that creates solid metal shapes (castings) by filling voids in molds with liquid metal, and then letting the object cool. The basic steps involved in making castings are patternmaking, molding, melting and pouring, cooling, shakeout and cleaning, heat treating, and inspection.
In casting terms, a “pattern” is a replica of the object to be cast which is used to shape the mold cavity. They can be made from a wide range of materials, including wood, metal, or plastics. Patternmaking is the process for producing these patterns. Because the pattern determines casting form, a casting can be no better than the pattern from which it is made. To produce a quality casting, it is essential that the pattern is carefully designed, constructed, and finished.
The main functions of a pattern are to:
- shape the mold cavity
- accommodate the characteristics of the metal cast
- provide accurate dimensions
- provide a means of delivering liquid metal into the mold (the gating system)
As it solidifies, metal tends to shrink and/or distort (to varying degrees depending on the metal cast). The pattern is designed with built-in shrinkage and distortion allowances to compensate. It must also be built with a taper in the vertical walls, called a draft, which is necessary to extract the pattern without disturbing the mold walls.
Hollow castings can be created with the use of a core – an additional piece of sand or metal that shapes the internal holes and passages of a casting. Each core is positioned in the mold before the molten metal is poured. In order to keep each core in place, the pattern has recesses called core prints where the core can be anchored in place.
Molding is the process of preparing a mold to receive molten metal. There are two distinct types of mold processes: Reusable and non-reusable.
As the name suggests, reusable molds can be used repeatedly. The casting process does not break down the mold during the metal solidification and cooling process. Reusable molds are usually made from metal.
In contrast, non-reusable molds are temporary objects that are destroyed during the metal solidification and cooling process. The most widely used non-reusable mold method is sand casting, a process in which specially treated sand (“green” sand) is rammed around the pattern and placed in a support (flask). The pattern is then removed, cores are set in place, and the gating system is established to guide molten metal into the mold.
Each of these general mold method categories has many specialized sub-types optimized for different casting metals and various levels of pattern complexity. Such methods include slush casting, pressure casting, shell molding, and investment casting.
Melting & Pouring
There are two categories of metals that castings are produced from: ferrous (metals that contain iron) and non-ferrous (metals that do not contain iron). Ferrous alloys include steel, malleable iron, and gray iron. The non-ferrous alloys most commonly used in casting are aluminum and copper, however magnesium, nickel, and titanium based alloys are sometimes used for specialized applications.
The metals to be melted and cast – usually a mix of recycled scrap and alloying metals – are loaded to “charge” the furnace. Once inside the furnace, the metal is subjected to extremely high temperatures until the melting point (often in excess of 2500 degrees Fahrenheit [1370°C]) is reached. Specialized furnaces are necessary to reach such elevated temperatures.
The two dominant types of melting furnace employed by foundries are electric arc and induction.
The electric arc furnace operates as a batch melting process, producing batches of molten metal known “heats”. The metal is melted by supplying electrical energy to the furnace interior via graphite electrodes. Additional chemical energy is supplied by oxy-fuel burners and oxygen lances. Oxygen is injected to remove impurities and other dissolved gasses during the melting process. As the metal melts, slag forms and floats to the top of the molten metal; the slag, which often contains undesirable impurities, is removed prior to tap out (the process of removing metal from the furnace).
An induction furnace transfers electrical energy by induction – a high voltage electrical source from a primary coil induces a low voltage, high current in the steel charge, or secondary coil. Induction furnaces are capable of melting and alloying a wide variety of metals with minimal melt loss, however when it comes to metal refinement they are less capable than electric arc furnaces.
Due to their respective strengths and weaknesses, electric arc furnaces are more widely used for melting ferrous metal, while induction furnaces are more dominant in non-ferrous applications.
Solidification, Ejection, and Cleaning
Crucibles, robotic arms, and gravity induced pouring machines are used to move molten metal from one location to another. Skilled metal workers will also pour molten metal using ladles. Molten metal is poured into the mold through a system of gates and risers; the metal cools and solidifies, permanently adopts the shape of the mold interior (void), it occupies. The casting is then ejected from the mold, or removed from a sand mold by shakeout.
The gates and risers that deliver molten metal to the mold interior also fill with molten metal in the process. Metal in the mold solidifies along with metal in the gate and riser system, forming a single piece. Immediately after ejection/ shakeout, metal from the gate and rider system is still attached to the main casting body. That excess metal is removed in the cleaning process (a chipping hammer or band saw are commonly used). A mix of tumbling barrels, air-blast units, and pressure washers are used to clean off any remaining sand or scale.
The resulting casting should be identical in shape and proportion to the original pattern, although it may be slightly smaller due to metal shrinkage.
Heat treatment and Inspection
Some castings are used in demanding industrial applications: they may need to maintain their exact shape in freezing temperatures, resist corrosion in a wet environment, or bear up under immense weight. Heat treatment is used to alter the physical properties of metal to the required specifications.
Heat treating involves the use of heating and chilling, often to extreme temperatures, to reduce stress in a cast part, and/or modify the physical properties of the metal. The temperature must be controlled with precision to achieve the desired physical properties.
Before a production run of castings can be considered complete, its physical properties and structural integrity are tested. Testing methods that require the destruction of the casting being tested are known as destructive tests, while those that do not damage the casting are categorized as non-destructive.
The testing methods used depend on how demanding the specifications are. For some purely aesthetic products, only a brief visual inspection for dimensional accuracy, cracks, and surface finish is required. On the other hand, if the casting will be required to perform in an industrial capacity, it may have all of its physical properties (ductility, tensile strength, elongation, impact properties, hardness, ect.) tested exhaustively.
Service Life and Recycling
After a casting has passed inspection, it is shipped, sold, and put into service. Cast metal products, especially those that have undergone heat treatment, are durable. Depending on the metal alloy and application, their service life might range from a few months to several decades. Once the service life of a casting ends, the cycle begins again. It is collected by a scrap yard, recycled, and recast into a new object.
Interested in learning more about custom metal casting production? A deeper understanding of the metal casting process and how to communicate your needs to a production foundry will help ensure your casting project is successful.