improve a basic shape that has been produced by casting or deformation processing, as well as to produce the basic shape itself.
PRODUCTION EQUIPMENT AND TOOLING
Equipment. The type of equipment and especially the tools used in manufacturing depends on the manufacturing process. Machine tools are among the most versatile of all production equipment. They are used not only for producing tools and dies, but also for producing components for other products and production equipment, such as presses and hammers for deformation processes, rolling mills for rolling sheet metal, and so on.
The name of the equipment usually follows from the name of the process. The productivity, reliability, and cost of equipment used for shaping processes are extremely important factors, since they determine the economics and practical application of a given process. For example, in both sheet and bulk forming the stroking rate of forming machines continues to get faster. Because of this machine dynamics and machine rigidity and strength are of increasing concern. As in other unit processes, the use of sensors for process monitoring and control is essential and continues to increase. Sensors are also being increasingly used to monitor the condition of the tooling during operations. Such monitoring systems cannot only improve part quality but can also enable longer tool life.
Tooling. The design and manufacture of tooling are essential factors determining the performance of shaping processes. The key to a successful shaping process lies in the tool design, which generally, to a very large extent, is based on the experience of the designer(s). Innovative multi-action tool designs have recently been developed that are capable of near-net shaping of increasingly complex parts, such as gears and universal joint components. These tooling approaches can be expanded. Many companies are already using computer-aided engineering and computer-aided manufacturing to design and fabricate process tooling. Advanced heat treatment and coating techniques can extend tool life. Based on a developing understanding of the mechanisms of erosive tool wear, studies are being conducted to measure and predict lubricant behevior and heat transfer at the tool material interface. This is an extremely important area, since tool life directly influences the economics of deformation processes.
In this part we discuss casting, molding, and other processes in which the starting material is a heated liquid. The three chapters deal with the fundamentals of metal casting, metal casting processes, and metal casting design and materials (Fig. P.1). Special attention is given to the heating of metals, an analysis of the processes from an engineering perspective, solidification and cooling of metals, defects in casting, sand casting, permanent mold casting, and other important factors in casting processes.
Fig. P.1 An outline of the casting and molding processes described in Part I.
1
FUNDAMENTALS OF METAL CASTING
1.5 Solidification and Cooling of Metals
Metal casting is a process whereby molten metal is poured into a mold the shape of the desired finished product. Casting is one of the oldest metal shaping processes; according to Biblical records, casting technology reaches back almost 5500 years BCE, or 7500 years ago. Gold was the first metal to be discovered and shaped according to prehistoric people’s fancy. The Chinese made iron castings around 1000 BCE, and steel was fabricated in India about 500 BCE.
Four main elements are required in the process of casting: a pattern, a mold, cores, and the work-piece. The pattern, the original template from which the mold is prepared, creates a corresponding cavity in the casting material. Cores are used to produce tunnels or holes in the finished mold, and the workpiece is the final output of the process.
The cast metal remains in the mold until it has solidified, and it is then ejected or revealed to show the fabricated part or casting.
The casting process is divided into two broad types of casting:
•expendable-mold casting processes, and
•nonexpendable-mold casting processes.
Expendable-mold casting is a general classification that includes sand, plastic, shell, and investment (lost wax technique) moldings. All of these involve the use of temporary and nonreusable molds, and they all need gravity to help force the molten fluid into the casting cavities. In this process, the mold in which the molten metal solidifies must be destroyed in order to remove the casting. It is used only once.
Nonexpendable mold casting differs from expendable processes in that the mold need not be destroyed after each production cycle. This technique includes at least four different methods: permanent, die, centrifugal, and continuous casting.
Steel cavities of molds are coated with some refractory wash layer like acetylene soot before processing to allow for easy removal of the workpiece and to promote longer tool life. The useful lifetime of permanent molds varies depending on maintenance: when the useful life is over, such molds require refinishing or replacement. Cast parts from a permanent mold generally show a 20% increase in tensile strength and a 30% increase in elongation as compared to the products of sand casting. Typically, permanent mold castings are used in forming iron, aluminum, magnesium, and copper-based alloys. The process is highly automated.
There are many different factors and variables that go into metal casting; each one can change the final product. That is why it is important that each one be considered in order to get a successful casting process.
Because different types of metal casting processes are available, it is one of the most used manufacturing processes. Casting technology is used to fabricate a large number of the metal components in designs we use every day. The reasons for this include the following advantages:
•Casting can produce very complex part geometries with internal and external shapes.
•It can be used to produce very small workpieces (a few hundred grams) to workpieces of very large size (over 100 tons).
•With some casting processes it is possible to manufacture final shapes that require no further manufacturing operations to achieve the required dimensions and tolerances of the parts.
