1. CASTING DESIGN, MATERIALS, AND ECONOMICS
CHAPTER 4
CASTING DESIGN, MATERIALS, AND ECONOMICS
DR . Ahmad Hassan

2. 1-Introduction:
The successful casting practice requires careful control of a large number of variables. These variables pertain to the particular characteristics of the metals and alloys cast,
1- Method of casting
2- Mold and die materials
3- Mold design
4- Various process parameters
5- The flow of the molten metal in the mold cavity
6- Gating systems
7- The rate of cooling
8- The gases evolved during casting
2-Design considerations:
As in all engineering practice and manufacturing operations, certain guidelines and design principles pertaining to casting have been developed over many years. Although these principles were established primarily through practical experience, analytical methods and computer-aided design and manufacturing techniques are now coming into wider use, improving productivity and the quality of castings. Moreover, careful design can result in significant cast savings.  

3. Fig. 1 Suggested design modifications to avoid defects in castings
2.1 Designing for expandable-mold casting:
The following guidelines generally apply to all types of castings. The most significant design considerations are identified and addressed.
2.1.1 Corners, angles, and section thickness:
1- Sharp corners, angles, and fillets should be avoided, Fig.1, as they may cause tearing and cracking during solidification of the metal. Note that sharp corners are avoided to reduce stress concentrations.
Fig. 1 Suggested design modifications to avoid defects in castings

4. 2- Fillet radii should be selected to reduce stress concentrations and to ensure proper liquid-metal flow during the pouring process. Fillet radii usually range from 3 mm to 25 mm. On the other hand, if the fillet radii are too large, the volume of the material in those regions is also large and, consequently, the rate of cooling is less.
3- Section changes in the castings should smoothly blend into each other.
The location of the largest circle that can be inscribed in the particular region is critical so far as shrinkage cavities are concerned, as shown in (Figs.2a&b). Because the cooling rate in regions with the large circle is less, they are called hot spots. These regions could develop shrinkage cavities and porosity, (Figs.2c&d). cavities at hot spots can be eliminated with small cores, (Fig.2e).
Fig.2 Examples of designs showing the importance of maintaining uniform cross-sections in castings to avoid hot spots and shrinkage cavities.

5. Other examples of design principles that can be used to avoid shrinkage cavities are shown in Fig.3.
Fig.3 Examples of design modifications to avoid shrinkage cavities in castings

6. 1- Although they increase the cost of production, metal paddings in the mold can eliminate or minimize hot spots. These paddings act as external chills. Such as that shown for casting of a hollow cylindrical part with internal ribs in Fig.4.
Fig.4 The use of metal padding (chills) to increase the rate of cooling in thick regions in a casting to avoid shrinkage cavities.

7. 2.1.2 Flat areas:
Large flat areas (plain surfaces) should be avoided. They may warp because of temperature gradients during cooling or develop poor surface finish because of uneven flow of metal during pouring. Flat surfaces can be broken up with ribs and serrations.
2.1.3 Shrinkage:
Allowances for shrinkage during solidification should be provided for, so as to avoid cracking of the casting. Fig.5a depicts a wheel with spokes. If the spokes are curved, the tensile stress in them resulting from contraction during solidification-and hence the tendency for cracking-is reduced. Another example is shown in Fig.5b, in which the original design has been altered slightly. Pattern dimensions should also provide for shrinkage of the metal during solidification and cooling. Allowance for shrinkage, also known as patternmaker’s shrinkage allowance, usually range from about 10 mm/m to 20 mm/m. Table1 shows the normal shrinkage allowance for some metals cast in sand molds.

8. Table 1 Normal shrinkage allowance for some metals cast in sand molds
Percent
Metal
Gray cast iron
2.1
White cast iron
Malleable cast iron
1.3
Aluminum alloys
Yellow brass
Magnesium alloys
Phosphor bronze
Aluminum bronze
2.6
High-manganese steel

9. Fig.5 Two examples of poor and good casting design practice to avoid tears caused by contraction during cooling.

10. 2.1.4 Parting line:
Recall that the parting line is the line, or plane, separating the upper (cope) and the lower (drag) halves of the molds, as shown in Fig.6. In general, it is desirable for the parting line to be along a flat plane, rather than contoured. Whenever possible, the parting line should be at the corners or edges of castings, rather than on flat surfaces in the middle of the casting. In this way, the flash at the parting line (material squeezing out between the two halves of the mold) will not be as visible. The location of the parting line is important because it influences:
1- Mold design
2- Ease of molding
3- Number and shape of cores
4- Method of support
5- The gating system
Three examples of casting design modifications are shown in Fig.7.

11. Fig.7 Examples of casting design modifications
Fig.6 Redesign of a casting by making the parting line straight to avoid defects.
Fig.7 Examples of casting design modifications

12. 2.1.5 Draft:
As we saw in the last chapter, a small draft (taper) is provided in sand-mold patterns to enable removal of the pattern without damaging the mold. Typical drafts range from 5 mm/m to 15 mm/m. Depending on the quality of the pattern, draft angles usually range from 0.5º to 2.0º. The angles on inside surfaces are typically twice this range. They have to be higher than those for outer surfaces because the casting shrinks inward towards the core.
2.1.6 Tolerances:
Tolerances-the permissible variation in the dimensions of a part- depend on:
The particular casting process
The size of the casting
The type of the pattern used
Tolerances should be as wide as possible, within the limits of good part performance; otherwise the cost of the casting increases. In commercial practice, tolerances usually are in the range of  0.8 mm for small castings and increase with the size of castings, say to  6 mm for large castings.

13. 2.1.7 Machining allowance:
Because most expandable-mold castings require some additional finishing operations, such as machining, allowance should be made in casting design for these operations. Machining allowances, which are included in pattern dimensions, depend on the type of casting and increase with the size and section thickness of castings. Allowances usually range from 2 mm to 5 mm for small castings, to more than 25 mm for large castings.
2.1.8 Residual stress:
The different cooling rates within the body of a casting cause residual stresses. Stress reliving may thus be necessary to avoid distortions in critical applications.

14. 2.2 Casting Alloys:
2.2.1 Nonferrous casting alloys:
Aluminum-base alloys:
Alloys with an aluminum base have a wide range of mechanical properties.
Advantages:
1- The alloys have various hardening mechanisms and heat treatments that can be used with them.
2- Their fluidity depends on oxides and alloying elements in the metal
3- These alloys have high electrical conductivity
4- They have generally good atmospheric corrosion resistance. They are nontoxic and light weight
5- They have good machinability.
Disadvantages:
1- Their resistance to some acids and all alkalis is poor and care must be taken to prevent galvanic corrosion
2- They have generally low resistance to wear and abrasion, except for alloys with silicon
Applications:
1- Aluminum-base alloys have many applications, including architectural and decorative use
2- Engine blocks of some automobiles are made of aluminum-alloy castings

15. 2.2.1.2Magnesium-base alloys: Advantages:
1- The lowest density of all commercial casting alloys are those made from the magnesium-base group
2- They have good corrosion resistance
3- They have moderate strength, depending on the particular heat treatment used 
Copper-base alloys:
Although somewhat expensive, copper-base alloys have many advantages.
1- Good electrical and thermal conductivity
2- Good corrosion resistance
3- The alloys are nontoxic
4- The alloys have wear resistance suitable for bearing materials
5- The mechanical properties and fluidity are influenced by the alloying elements
Zinc-base alloys:
1- The alloys have a low-melting point
2- Zinc-base alloys have good fluidity
3- The alloys have sufficient strength for structural applications
4- These alloys are commonly used in die-casting.

16. Properties and typical applications of cast nonferrous alloys

17. 2.2.1.5High-temperature alloys:
High-temperature alloys have a wide range properties and typically require temperatures of up to 1650 ºC for casting titanium and super alloys-and higher for refractory alloys. Special techniques are used in casting these alloys into parts for jet-and rocket-engine components. Some of these alloys are more suitable and economical for casting than for shaping by other manufacturing methods, such as forging. The following table shows the properties and typical applications of cast nonferrous alloys.
2.2.2 Ferrous casting alloys:
Cast irons:
Cast iron represents the largest amount of all metals cast. They generally possess several desirable properties, such as wear resistance, hardness and good machinability. The term cast iron refers to a family of alloys. They are classified as gray cast iron (gray iron), ductile (nodular or spherical) iron, white cast iron, and compacted graphite iron.
a- Gray cast iron:
Gray cast irons are specified by two-digit ASTM designation. Class 20, for example, specifies that the material must have a minimum tensile strength of 20 ksi (140 MPa). The mechanical properties for several classes of gray cast iron are shown in the next table.

18. Mechanical properties of gray cast irons
Hardness
(HB)
Elastic modulus (Gpa)
Compressive strength (MPa)
Ultimate tensile strength (MPa)
ASTM
Class
156
66 to 97
572
152 20
174
79 to 102
669
179 25
210
90 to 113
752
214 30
212
100 to 119
855
252 35
235
110 to 138
965
293 40
262
130 to 157
1130
362 50
302
141 to 162
1293
431 60

19. Advantages:
Castings of gray cast iron have relatively few shrinkage cavities and little porosity.
Uses:
1- Typical uses of gray cast iron are for engine blocks
2- Machine bases
3- Electric-motor housings
4- Pipes
5- Wear surfaces for machines
b- Ductile (nodular) iron:
Ductile irons are specified by a set of two-digit numbers. Thus, for example, class or grade indicates that the material has a minimum tensile strength of 80 ksi (550 MPa), a minimum yield strength of 55 ksi (380 MPa), and 6 percent elongation in 50 mm.
Typically used for machine parts, pipes, and crankshafts.

20. Properties and typical applications of cast irons
Elongation in 50 mm (%)
Yield strength (MPa)
Ultimate tensile strength (MPa)
Type
Cast iron
Pipe, sanitary ware
0.4
140
170
Ferritic
Gray
Engine block, machine tools
240
275
Pearlitic
Wearing surfaces
550
Martensitic
Pipe, general services 18
415
Ferretic
Ductile (nodular)
Crankshafts, highly stressed parts 6
380
H.S machine parts, wear resistant parts 2
620
825
Tempered martensite
Hardware, pipe fittings, general engineering service
365
Malleable
Railroad equipment, couplings 10
310
450
Railroad equipment, gears, connecting rods
700
Wear-resistant parts, mill rolls
White

21. e- Compacted graphite iron:
c- White cast iron:
Because of its extreme hardness and wear resistance, white cast iron is used mainly for liners for machinery to process abrasive materials, rolls for rolling mills, and railroad-car shoes.
d- Malleable iron:
Malleable irons are specified by a five-digit designation. Thus 35018, for example, indicates that the yield strength of the material is 35 ksi (240 MPa), and its elongation is 18 percent in 50 mm. The principal uses of malleable iron is for railroad equipment and various types of hardware.
e- Compacted graphite iron:
Compacted graphite iron has the properties that all between those of gray and ductile irons. Whereas gray iron has good damping and thermal conductivity but low ductility and ductile iron has poor damping and thermal conductivity but high tensile strength, compacted graphite iron has damping and thermal properties to gray cast iron and strength and stiffness comparable to those of ductile iron.
Advantages:
1- Because of its strength, parts made of compacted graphite iron can be lighter
2- It is easy to cast
3- Its machineability is better than ductile iron

22. 2.2.2Cast steels:
Because of the high temperatures required to melt cast steels, up to 1650 ºC, their casting requires considerable knowledge and experience. The high temperatures involved present difficulties in the selection of mold materials-particularly in view of the high reactivity of steels with oxygen-in melting and pouring the metal. Steel castings possess properties that are more uniform than those made by mechanical working processes. Cast steels can be welded; however, welding alters the cast microstructure in the heat-affected zone, influencing the strength, ductility, and toughness of the cast metal. Subsequent heat treatment must be performed to restore the mechanical properties of the casting. Cast weldments have gained importance where complex configurations, or the size of the casting, may prevent casting the part economically in one place.
2.2.3Cast stainless steels:
Casting of stainless steels involves considerations similar to those for steels in general. Stainless steels generally have:
1- A long freezing range and high melting temperatures.
2- They develop various structures, depending on their composition and the process parameters
3- Cast stainless steels are available in various compositions
4- It can be heat-treated and welded
5- These cast products have high heat and corrosion resistance
6- Nickel-base casting alloys are used for severely corrosive environments nd very high temperature service.

23. 2.3Economics of casting:
When looking at various casting processes, you will note that:
1- Some casting processes require more labor than the others
2- Some processes require expensive dies and machinery
3- And some take a great deal of time to complete.
These important characteristics are outlined in the following table. Each of thee individual factors listed affects to varying degrees the overall cost of a casting operation. As we can see from the following table, relatively little cost is involved in molds for sand casting. On the other hand, die-casting dies require expensive materials and a great deal of machining and preparation.
The cost of a product involves:
1- The cost of the materials
2- The labor cost
3- The tooling cost
4- The equipment cost
5- Preparation for casting a product include making molds and dies that are require raw materials, time, and effort, which can be translated into cost
6- In addition to molds and dies, facilities are required for melting and poring the molten metal into the molds or dies. These facilities include furnaces and related machinery; their cost depend on the level of automation desired
7- Finally, costs are involved in heat treating, cleaning, and inspecting the castings.

24. General cost characteristics of casting processes Process Cost
Production rate Pc/hr)
Die
Equipment
Labor
Sand L
L-M
Less than 20
Shell-mold
M-H
Less than 50
Plaster M
Less than 10
Investment H
Less than 1000
Permanent mold
Less than 60
Less than 200
Centrifugal

25. Heat-treating is an important part of the production of many alloy groups, especially ferrous castings, and is necessary to produce improved mechanical properties. However, heat-treating also introduces another set of production problems, such as scale formation and warpage, and can be a significant part of the production costs.
The amount of labor required for these operations can vary considerably, depending on the particular process and level of automation. Investment casting, for example, requires a great deal of labor because the large number of steps involved in this operation. On the other hand, operations such as highly automated die-casting can maintain high production rates with little labor required.
It can be noted from the above table that, however, that the cost of equipment per casting (unit cost) will decrease as the number of parts increases. Thus sustained high production rates can justify the high cost of dies and machinery. Thus not all manufacturing decisions are based purely on economic considerations, but also on the quality of the produced casting from the different casting processes. Then, if the part can be produced by more than one or two processes, the final decision rests on both economic and technical considerations.

26. THE END