Introduction
For one to achieve success in any given casting practices, there must be a proper control in the variables required during the process. The variables may involve certain characteristics of the alloys, and metals used, the casting method, design of the mold, material of the die used, and the parameters used during processing. The casting quality is influenced by the cooling rate, gating systems, and the evolved gasses during the entire process. Precautions must be, therefore, taken to avoid defects during metal casting design. This paper also outlines the economic factors that are relevant and basic in casting operations.
Metal casting
Design considerations in casting
It is known for a fact that most of the casting processes share some principles. This means that most of the design consideration characteristics are applied to almost all processes. Examples include: die casting and the sand casting although each has specific design considerations. In particular, sand casting requires mold erosion consideration in association with sand inclusions in the sand casting. The difference with die casting is that the latter does not check on his consideration. It has its own considerations like checking the heat of the dies which may reduce the life of the die material selection for the part to be produced is vital since it determines the quality and the characteristics of the final part. Any defects due to material selection are expensive to correct, and troubleshooting is very complicated. There are two main issues that are considered for casting; these are (i) geometric features, tolerances and so on that should be included in the cast part and (ii) the mold features needed to produce the required casting. The most important considerations in designing parts include section thickness, corners and angles of the part, flat areas, ribs, shrinkage, draft, dimension tolerances, lettering and marking, and finishing operations. It is worth noting that the casting processes cannot be selected without considering the economic influences.
Casting alloys
The properties and the applications of cast metals and their alloys are extensively discussed in this section. Apart from casting alloys characteristics, their weldability and machinability are important considerations. This is because the alloys are assembled with other parts to produce a complete part. Gray iron is the mostly used casting alloy followed by ductile iron, aluminum, zinc, lead, copper, malleable iron, and magnesium.
Non-ferrous Casting alloys
Aluminum alloys - There are many hardening mechanisms and heat treatment methods. These give aluminum a wide range of mechanical and electrical properties which is useful in many engineering applications. Aluminum alloys combined with magnesium alloys produce parts known as light metal castings. These castings have a high electrical conductivity and have good corrosion resistance. However, they have a poor resistance against acids and bases, and when they react, galvanic corrosion occurs.
Magnesium alloys - These alloys have the least know density compared to other alloys. They have good corrosion resistance and have moderate strength. This strength is determined by the heat treatment method used. They find applications in automotive industries where they are used to make wheels, engine blocks, and housings.
Copper alloys - These alloys have very good electrical and thermal conductivity. They are good in corrosion resistance and have good wear properties. These properties make them more suitable for use in bearing materials. Examples of copper alloys include aluminum bronzes, brasses, tin bronzes and phosphor bronzes.
Other non-ferrous alloys include zinc-based alloys which are known to have a low melting point and very good in corrosion resistance. They have good fluidity and moderate strength for structural applications. Tin alloys have low strength but are good corrosion resistors and are mainly used for bearing surfaces. Lead alloys are like tin alloys although they are highly toxic.
Ferrous alloys
Cast irons - Cast irons form the largest quantity of all metal cast. They can be cast with ease to intricate shapes. Cast irons have the following desirable characteristics; wear resistance, good machinability, and high hardness. They are classified as gray cast iron, ductile iron, white cast iron and malleable cast iron. Each of the classification has its own properties.
Cast steels - Cast steels require special consideration due to the high temperatures required to melt them. These high temperatures present difficulties making selections of the mold materials. These steels have a high reactivity factor with oxygen during the melting and the pouring of the molten metal. The last type of ferrous alloys is the cast stainless steel. This type of steel involves many considerations that are similar to other steels. They are associated with high melting temperatures and freezing ranges. Several structures can be developed from the cast stainless steeling depending on their composition and the processing parameters.
Economics of casting
The unit cost of each part depends on several factors. These factors include materials, the equipment required and the labor involved. Some of the various casting processes demand more labor than others, some involve more expensive dies and other machinery than others, and some require more production times than others. Each of these factors affects the total cost involved in making the required molds for casting. Costs are incurred when the molten metal is being poured into the molds, in heating, cleaning and while the inspection is being done on the molds. Expensive labor and skills required for these operations vary considerably mainly depending on the specific operation being carried out. The technology involved in the foundry shop also determines the skills and labor of the personnel. A good example is investment casting which requires much labor due to the number of steps required in the operation. In some instances, advanced technology like the use of robots is possible. This automation increases the rate of production at reduced labor.
Conclusion
There are general guidelines that have been established to guide the production of the casting that are ready for use. Measures are taken to make sure that these parts are free from defects; they have the right tolerances, good surface finish, desirable service requirements and specified standards. There are wide varieties of ferrous and non-ferrous casting alloys in the market today. These metals have a wide range of properties, casting characteristics and various applications. Several factors such as machinability, weldability and surface characteristics are considered to be important. This is because most casting parts are assembled with other mechanical parts. The economics of castings is also an important consideration. The costs are mainly affected by the cost of the materials, dies, molds, equipment required and the labor. Each of these varies with the particular casting method and operations.
Reference
Kalpakjian, S, & Steven Schmid (2014).Manufacturing Engineering & Technology.Pearson
