machines: motors, generators, pumps, compressors
•Household: appliances, kitchen and gardening equipment, furniture, and fitting
•Art objects: sculptures, idols, furniture, lamp stands, and decorative items.
The advantages of casting have, in the past, been offset by significant disadvantages compared with wrought products. Castings are considered to be less ductile than the equivalent wrought product, and they have a less consistent performance in fatigue; they also have inferior integrity. The difference in ductility may be more apparent than real, however. A forged or rolled component may have a higher ductility than a casting in the direction of forging or rolling but a significantly lower transverse ductility. This is a distinct advantage if the longitudinal direction has to resist the principal stress, but it is not necessarily a sign of inferiority of the casting process.
Metal casting processes may be classified in several different ways:
•According to the mold type: (1) expendable mold (destroyed after each casting) and (2) permanent mold (reused many times);
•According to the type of pattern used for making a sand mold: (1) expendable pattern (melted for each mold), the pattern material being wax; and (2) permanent pattern (reused for many molds), the pattern material being wood or metal.
•According to the type of core used for producing a hole in casting: (1) expendable core (used in both sand and metal molds), the core material being sand; and (2) a permanent core (used with a permanent mold only), the core material being metal.
•According to the method by which the mold is filled: (1) gravity (sand casting, gravity die casting); (2) pressure (low and high pressure die casting); and (3) vacuum (vacuum investment casting).
Sand casting is a metal-forming process in which a molten metal compound is poured into a sand mold to produce a workpiece’s desired shape.
Sand casting has historically been the most popular casting method, producing by far the greatest tonnage of castings used in any country. Today, however, with the widespread conversion of automotive components from ferrous metals to aluminum, sand casting’s position as the dominant molding method is threatened. It is usually the least expensive way of making a component; its inherent cost advantage over other methods continues to make it an attractive molding method.
Basically, sand casting consists of six production steps:
Pattern. Preparing and placing a pattern having the shape of the desired workpiece.
Molding. Making a mold and incorporating a gating system using a molding machine.
Pouring. Pouring the molten compound metal into the mold.
Cooling. Cooling and solidifying the metal in the mold to form a desired shape.
Sand removal. Removing sand and scales from the surface of a separated workpiece, and removal of risers and gates.
Inspection. Performing in-process and preshipment inspection in accordance with standards.
Silica sand (SO2) is the molding aggregate most widely used by the foundry industry. Silica’s high fusion point, 1760°C (3200°F) and low rate of thermal expansion produce stable cores and molds compatible with all pouring temperatures and alloy systems. Its chemical purity also helps prevent it from interacting with catalysts or with the curing rate of chemical binders. There are two basic types of silica sand that are commercially available as a mold material. The first type is a round-grain silica sand containing roughly 99% or higher silica with minimal amounts of trace materials. The second type is lake sand. This sand contains approximately 94% silica, with the balance containing iron oxide, lime, magnesia, and alumina. Since impurities are removed, round-grain sands possess higher refractoriness than the lake sands. However, because of this much higher refractoriness, round-grain sands may have a higher propensity for veining and metal penetration defects. Lake sand has lower refractoriness but also has a lower tendency for casting defects. For proper functioning, molding sand must be able to withstand the high temperatures of molten metals, hold the shape of the mold when moist (usually with the aid of a bonding agent such as clay), be permeable enough to release gases, have sufficient strength to support the weight of the metal, and be of a fine enough texture to result in a smooth casting.
A pattern is used to make a cavity in the sand mold into which molten metal is poured. The pattern is a full-size model of the part, enlarged to account for shrinking and machining allowances in the final casting. The selection of the material used to make the pattern depends on the size and shape of the casting, the dimensional accutance and the quantity of castings required, and the molding process. Generally, material used to make patterns include wood, plastics, and metals. Patterns may be made of a combination of materials to reduce wear in critical regions, and they usually are coated with a patting agent – a liquid used over a patterns that leaves a slick film–to facilitate the removal of the casting from the molds.
There are four types of patterns: solid patterns, split patterns, match-plate patterns, and cope-and-drag patterns.
a) Solid Patterns
Figure 2.1 shows a solid pattern, also called loose pattern, made of one piece, used for simple shape and low-quantity production; its geometry is the same as the casting, adjusted in dimensions for shrinking and machining. Generally, it is made from wood and is inexpensive. However, determining the location of the parting line between two halves of the mold and positioning the gate system can be a problem.
Fig. 2.1 Solid pattern.
b) Split Patterns
A more complex pattern (Fig. 2.2) for round or irregular-shaped workpieces made in two or more parts is called split pattern. The two pattern halves usually predetermine the parting line of the mold. Split patterns are used for complex shape of the workpieces and moderate production quantities.
Fig. 2.2 Split pattern.
c) Match Plate Patterns
Match plate patterns are split patterns assembled on opposite sides of wooden or metal plates known as match plates (Fig. 2.3). Holes in the plate allow the top (core) and bottom (drag) sections of the mold to be aligned accurately. For long runs and top quality, match plate patterns, or cope-and-drag plate, are used. The major advantage to this is that a single machine can make both cope and drag molds from one pattern.
