Casting Design

Metal-Casting Processes and Equipment

Manufacturing Engineering and Technology

10.Fundamentals of Metal Casting11.Metal-Casting Processes12.Metal Casting: Design, Materials and

Economics

2005 Pearson Education South Asia Pte Ltd

Economics

12. Metal Casting: Design, Materials and Economics

Chapter Objectives

• Design considerations for expendable and

permanent mold casting.

• General guidelines for successful casting.

• Characteristics and applications of nonferrous

and ferrous casting alloys.


and ferrous casting alloys.

• Economic considerations in metal casting.


Chapter Outline

1. Introduction

2. Design Considerations in Casting

3. Casting Alloys

4. Economics of Casting



12.1 Introduction

• This chapter describes general design

considerations and guidelines for metal casting and

presents suggestions for avoiding defects.

• It also describes the characteristics of the alloys that

are commonly cast, together with their typical


applications.


12.2 Design Considerations in Casting

• All casting operations share some characteristics,

such as phase change and thermal shrinkage during

the casting cycle.

• Troubleshooting the causes of defects very often is

complicated, and the considerations presented in


this chapter are by no means an exhaustive list.

• Also, defects often are random and difficult to

reproduce, further complicating the implementation

of corrective measures.


12.2.1 General design considerations for casting

• There are two types of design issues in casting: (a) geometric

features, tolerances, etc., that should be incorporated into the

part and (b) mold features that are needed to produce the

desired casting.

• Robust design of castings usually involves the following

steps:


steps:

1. Design the part so that the shape is cast easily. A number

of important design considerations are given in this chapter to

assist in such design efforts.

2. Select a casting process and a material suitable for the

part, size, mechanical properties, and so on. Often, the

design of the part will not be independent of the first step

given, and the part, material and process have to be specified

simultaneously.



3. Locate the parting line of the mold in the part.

4. Locate and design the gates to allow uniform

feeding of the mold cavity with molten metal.

5. Select an appropriate runner geometry for the

system.


system.

6. Locate mold features such as sprue, screens, and

risers, as appropriate.

7. Make sure proper controls and good practices are

in place



Design of cast parts

• Fig 12.1 shows the suggested design modifications

to avoid defects in castings.





• Corners, angles, and section thickness. Sharp

corners, angles, and fillets should be avoided as

much as possible, because they act as stress raisers

and may cause cracking and tearing of the metal (as


well as of the dies) during solidification. Fig 12.2

shows examples of designs showing the importance

of maintaining uniform cross-sections in castings to

avoid hot spots and shrinkage cavities.







• Flat areas. Large flat areas (plain surfaces) should

be avoided, since they may warp during cooling

because of temperature gradients, or they develop

poor surface finish because of an uneven flow of


metal during pouring.

• Shrinkage. To avoid cracking of the casting during

cooling, there should be allowances for shrinkage

during solidification. Table 12.1 shows the normal

shrinkage allowance for metal cast in sand molds.







• Draft. A small draft (taper) typically is provided in

sand-mold patterns to enable removal of the pattern

without damaging the mold.

• Dimensional tolerances. Dimensional tolerances


• Dimensional tolerances. Dimensional tolerances

depend on the particular casting process, size of the

casting, and type of pattern used. Tolerances should

be as wide as possible, within the limits of good part

performance; otherwise, the cost of the casting

increases.




• Lettering and markings. It is common practice to

include some form of part identification (such as

lettering or corporate logos) in castings.

• Finishing operations. In designing a casting, it is


• Finishing operations. In designing a casting, it is

important to consider the subsequent machining and

finishing operations that often have to take place.



Selecting the cast process

• Casting process selection cannot be separated from

a discussions of economics.




Locating the parting line

• A part should be oriented in a mold so that the large

portion of the casting is relatively low and the height

of the casting is minimized.

• Part orientation also determines the distribution of


• Part orientation also determines the distribution of

porosity.

• A properly oriented part then can have the parting

line specified.

• The parting line is the line or plane separating the

upper (cope) and lower (drag) halves of molds.



Locating the parting line

• In general, the parting line should be along a flat

plane rather than be contoured.

• The parting line should be placed as low as possible

(relative to the casting) for less dense metals (such


(relative to the casting) for less dense metals (such

as aluminum alloys) and located at around mid-

height for denser metals (such as steels).



Locating and designing gates

• The gates are the connections between the runners

and the part to be cast.

• Some of the considerations in designing gating

systems are:


systems are:

• Multiple gates often are preferable and are

necessary for large parts. Multiple gates have the

benefits of allowing lower pouring temperature and

reducing the temperature gradients in the casting.

• Gates should feed into thick sections of castings.




• A fillet should be used where a gate meets a

casting; this feature produces less turbulence than

abrupt junctions.

• The gate closest to the sprue should be placed


• The gate closest to the sprue should be placed

sufficiently far away so that the gate can be easily

removed. This distance may be as small as a few

millimeters for small castings and up to 500 mm for

large parts.




• The minimum gate length should be three to five

times the gate diameter, depending on the metal

being cast. The cross-section should be large

enough to allow the filling of the mold cavity and


should be smaller than the runner cross-section.

• Curved gates should be avoided, but when

necessary, a straight section in the gate should be

located immediately adjacent to the casting.



Runner design

• The runner is a horizontal distribution channel that

accepts molten metal from the sprue and delivers it

to the gates.

• Runners are used to trap dross and keep it from


• Runners are used to trap dross and keep it from

entering the gates and mold cavity.



Designing other mold features

• The main goal in designing a sprue is to achieve the

required metal flow rates while preventing aspiration

or excessive dross formation.

• Flow rates are determined such that turbulence is


• Flow rates are determined such that turbulence is

avoided, but the mold is filled quickly compared to

the solidification time required.



Establishing good practices

• It has been observed widely that a given mold

design can produce acceptable parts as well as

defective ones and rarely will produce good or only

defective parts.


• To check for defective castings, quality control

procedures are necessary.




• Some of the common concerns are the following:

• Starting with a high-quality molten metal is

essential for producing superior castings. Pouring

temperature, metal chemistry, gas entrainment, and


temperature, metal chemistry, gas entrainment, and

handling procedures all can affect the quality of

metal being poured into a mold.

• The pouring of metal should not be interrupted,

since this can lead to dross entrainment and

turbulence. The meniscus of the molten metal in the

mold cavity should experience a continuous,

uninterrupted, and upward advance.




• The different cooling rates within the body of a

casting cause residual stresses. Stress relieving

thus may be necessary to avoid distortions of

castings in critical applications.



Example 12.1 Illustration for poor and good casting designs

Several examples of poor and good designs I permanent-mold and die

castin are shown in Fig. 12.3. The significant differences in design are

outlined here for

each example.

a. The lower portion of the design on the left has a thin wall with no

apparent functional role. This location of the part thus may fracture if

subjected to high forces or impact. The good design eliminates this


subjected to high forces or impact. The good design eliminates this

problem and also may simplify die and mold manufacturing.

b. Large flat surfaces always present difficulties in casting metals (as

well as nonmetallic materials), as they tend to warp and develop uneven

surfaces. A

common practice to avoid this situation is to break up the surface with

ribs and serrations on the reverse side of the casting. This approach

greatly reduces

distortion and, furthermore, does not adversely affect the appearance

and function of the flat surface.



c. This example of poor and good design is relevant not only to castings

but also to parts that are machined or ground. It is difficult to produce

sharp internal radii or corners which may be required for functional

purposes, such as inserts that are designed to reach the bottom of the

part cavity. Also, in the case of lubricated cavities, the lubricant can

accumulate at the bottom and, being incompressible, will prevent full

insertion of a part into the cavity. Placement of a small radius at the


insertion of a part into the cavity. Placement of a small radius at the

corners or periphery at the bottom of the part eliminates this problem.

d. The function of such a part could be, for instance, a knob to be

gripped and rotated—hence the outer features along its periphery. Note

in the design on the left that the inner periphery of the knob also has

features which are not functional but help save material. The casting die

for the good design is easier to manufacture.



e. Note that the poor design has sharp fillets at the base of the

longitudinal grooves, which means that the die has sharp (knife

edge) protrusions. Because of their sharpness, it is possible that

over extended use of the die these edges can chip off.

f. The poor design on the left has threads reaching the right face

of the casting. It then is possible that during casting some molten


of the casting. It then is possible that during casting some molten

metal can penetrate this region, thus forming a flash and

interfering with the function of the threaded insert, such as when a

nut is used. The good design uses an offset on the threaded rod,

eliminating this problem. This design consideration also is

applicable for the injection-molding of plastics.




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Casting Design