The document discusses general design considerations for metal castings. It notes that successful casting requires controlling variables related to the metal, casting method, mold materials, mold design, and process parameters. It describes steps for robust casting design, including designing the part for easy casting, selecting suitable materials and process, locating parting lines and gates, and including features like risers. It also discusses avoiding defects by modifying part geometries, allowing for shrinkage, adding drafts and finishes, and considering post-casting machining needs.
2. Metal-Casting:
Design, Materials, and Economics
In the preceding two chapters, it was noted that successful casting
practice requires the proper control of a large number of variables.
These variables pertain to the particular characteristics of the metals
and alloys cast, method of casting, mold and die materials, mold
design, and various process parameters. The flow of the molten metal
In the mold cavities, the gating systems, the rate of cooling, and the
gases evolved influence the quality of a casting.
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.
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3. All casting operations share some characteristics, such as phase
change and thermal shrinkage during the casting cycle. Consequently,
a number of design considerations apply equally to, for example, sand
casting and die casting. However, each process will have its own
particular design considerations: Sand casting will require the
consideration of mold erosion and associated sand inclusions in the
casting, whereas die casting will not have this concern (although it has
its own problems, such as heat checking of dies which reduces die
life).
General Design Considerations for Castings
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
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4. Robust design of castings usually involves the following steps:
General Design Considerations for Castings
I. Design the part so that the shape is cast easily.
2. Select a casting process and a material suitable for the part, size,
required production volume, mechanical properties, and so on.
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.
6. Locate mold features, such as Sprue, screens, and risers, as
appropriate.
7. Make sure proper controls and good practices are in place.
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5. Design of Cast Parts.
Suggested design modifications to avoid defects in castings
Corners, angles, and section thickness
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7. Examples of designs showing the importance of maintaining uniform cross
sections in castings to avoid hot spots and shrinkage cavities.
Design of Cast Parts.
Because the cooling rate in regions with larger circles is lower, these regions are
called hot spots. They can develop shrinkage cavities and porosity
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9. Flat areas. Large flat areas (plane 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
One of the common techniques for avoiding either of these problems is to
break up flat surfaces with staggered ribs and serrations.
Shrinkage. To avoid cracking of the casting
during cooling, there should be allowances
for shrinkage during solidification.
In castings with intersecting ribs, the
tensile stresses can be reduced by staggering
the ribs or by changing the intersection
geometry. Pattern dimensions also should
allow for shrinkage of the metal during
solidification and cooling. Allowances for
shrinkage, known as patternmaker's
shrinkage allowances, usually range from
about 10 to 20 mm/m.
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11. Draft
A small draft (taper) typically is provided in sand-mold patterns to enable
removal of the pattern without damaging the mold
Drafts generally range from 5 to 15 mm/m. Depending on the quality of the
pattern, draft angles usually range from 0.5° to 2°.
Dimensional tolerances. Dimensional tolerances depend on the
particular casting process, size of the casting, and type of pattern used.
Tolerances should be as wide tolerances are usually in the range of
±0.8 mm for small castings and increase with the size of the castings.
Tolerances for large castings, for instance, may be as much as ± 6 mm.
Finishing operations.
In designing a casting, it is important to consider the subsequent machining and
finishing operations that may be required.
For example, if a hole is to be drilled in a casting, it is better to locate the hole on
a flat surface than on a curved surface in order to prevent the drill from
wandering. An even better design would incorporate a small dimple as a starting
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12. Selecting the Casting Process.
Design of Cast Parts.
Locating the parting Line.
Locating and Designing Gates
Runner Design.
The runner is a horizontal distribution channel that accepts
molten metal from the Sprue and delivers it to the gates
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
avoided, but the mold is filled compared to the solidification time required.
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15. Examples of undesirable (poor) and desirable (good) casting
designs. Source: Courtesy of American Die Casting Institute.
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17. Mechanical properties for various groups of cast alloys. Note that
even within the same group, the properties vary over a wide range,
particularly for cast steels..
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18. Typical Applications for Castings and Casting Characteristics
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21. An incorrect and a correct design for casting are
shown. Review the changes made and comment on
their advantages.
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22. Three sets of designs for die casting are shown. Note the changes made
to design 1 and comment on the reasons for then or them.
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