is economical, with very little wastage. The extra metal in each casting is remelted and reused.
•Casting metal is isotropic: It has the same physical and mechanical properties in all directions.
•Some types of metal casting are very suitable for mass production.
There are also some disadvantages for different types of castings. These include the following:
•poor finish, wide tolerance (sand casting);
•limited workpiece size (shell molds and ceramic molds);
•patterns have low strength (expendable pattern casting);
•expensive, limited shapes (centrifugal casting);
•porosity (all types);
•environmental problems (all types).
To accomplish a casting process the worker must heat metal to the desired temperature for pouring. Heat is the energy that flows spontaneously from a higher temperature object to a lower temperature object through random interactions between their atoms. The heat energy required for heating metal to a pouring temperature is the sum of:
•the heat needed to raise the temperature to the melting point;
•the heat of fusion needed to convert it from a solid to a liquid;
•the heat needed to raise the molten metal to the desired temperature for pouring.
This energy can be expressed as a sum of phase energy by the following formula:
| Q | = | Qs + Qf + Ql | (1.1) | |
| Q | = | ρV[cs(Tm − T0) + Lf + c1(Tp − Tm)] | (1.1a) |
where
| Q | = | total heat energy, J (Btu) |
| Q s | = | heat energy for solid metal, J (Btu) |
| Q f | = | heat energy for fusion, J (Btu) |
| Q l | = | heat energy for liquid metal, J (Btu) |
| ρ | = | density, kg/m3 |
| V | = | volume of metal being heated, m3 (in3) |
| c s | = | specific heat for solid metal, J/kg°C (Btu/lbm°F) |
| c l | = | specific heat for liquid metal, J/kg°C (Btu/lbm°F) |
| T 0 | = | starting temperature, °C (°F) |
| T m | = | melting temperature of the metal, °C (°F) |
| T p | = | pouring temperature, °C (°F) |
| L f | = | heat of fusion of the metal, J/kg (Btu/lbm). |
The heat of fusion is the amount of heat required to convert a unit mass of a solid at its melting point into a liquid without an increase in temperature.
Equation (1.1a) can be used for the approximate calculation of the total heat energy because values cs, and c1 vary with the temperature; in addition, significant heat losses to the environment during heating are not implied by this equation.
Figure 1.1 shows a phase-change diagram of the process of heating for metal casting.
Fig. 1.1 Phase change diagram of the process of heating metal to a molten temperature sufficient for casting.
After the metal is heated to pouring temperature Tp the metal is ready for pouring. As the introduction of the molten metal into the mold includes the fluid flow in casting, we will describe a basic gravity casting system as shown in Fig. 1.2.
Fig. 1.2 Cross-section of a typical two-part sand mold.
Figure 1.2 presents a cross-section of a typical two-part sand mold and incorporates many features of the casting process. These are:
Drag. The drag is the bottom half of any of these features.
Core. A core is a sand shape that is inserted into the mold to produce internal features of a casting, such as holes or passages for water cooling.
Flask. The flask is the box that contains the molding aggregate.
Cope. In a two-part mold, the cope is the top half of the pattern, flask, mold, or core.
Core print. A core print is the region added to the pattern, core, or mold; the core print is used to place and support the core within the mold.
Riser. Risers serve as reservoirs of molten metal to supply any molten metal necessary to prevent shrinking during solidification.
Gating system. The gating system is the network of channels used to deliver the molten metal from outside the mold into the mold cavity.
Pouring basin. The pouring basin or cup is the portion of the gating system that initially receives the molten metal from the pouring vessel and controls its delivery to the rest of the mold. From the pouring basin the metal travels down the sprue (the vertical portion of the gating system), then along horizontal channels (called runners), and finally through controlled entrances, or gates, into the mold cavity.
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