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授課教師：楊宏智教授
1
楊宏智（台大機械系教授）
2
Chapter Outline
Introduction
2. Design Considerations in Casting
3. Economics of Casting
1.
Introduction
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Successful casting practice requires proper control of a
large number of variables
Flow of the molten metal in the mold cavities, the
gating systems, the rate of cooling, and the gases
evolved would influence the quality of a casting
Design Considerations in Casting
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All casting operations share the characteristics of phase change
and thermal shrinkage during the casting cycle
But each process will have its own design considerations
Sand casting will require mold erosion and sand inclusions in the
casting
Die casting will not have this concern
Defects frequently are random and difficult to reproduce
Design Considerations in Casting:
General Design Considerations for Castings
2 types of design issues in casting:
1. Geometric features and tolerances incorporated into
the part
2. Mold features that are needed to produce the desired
casting
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Design Considerations in Casting:
General Design Considerations for Castings
Design of Cast Parts
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Corners, angles, and section thickness
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Flat areas
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Shrinkage
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Draft
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Dimensional tolerances
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Lettering and markings
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Finishing operations
Design Considerations in Casting:
General Design Considerations for Castings
Locating the Parting Line
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Parting line is the line or plane separating the upper (cope) and
lower (drag) halves of molds
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Parting line should be along a flat plane rather than be contoured
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Location is important as it influences mold design, ease of
molding, number and shape of cores required, method of support
and the gating system
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Parting line should be low for less dense metals and located at
mid-height for denser metals
Design Considerations in Casting:
General Design Considerations for Castings
Locating and Designing Gates
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Multiple gates are preferable
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Gates should feed into thick sections of castings
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A fillet should be used
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Gate closest to the sprue should be placed sufficiently far away
from the sprue
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Minimum gate length should be 3 to 5 times the gate diameter
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Curved gates should be avoided
Design Considerations in Casting:
General Design Considerations for Castings
Runner Design
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Runner is a horizontal distribution channel that accepts molten
metal from the sprue and delivers it to the gates
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It is used to trap dross and keep it from entering the gates and
mold cavity
Designing Other Mold Features
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Goal in designing a sprue is to achieve the required metal flow
rates while preventing excessive dross formation
Design Considerations in Casting:
General Design Considerations for Castings
Designing Other Mold Features
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Goal in designing a sprue is to achieve the required metal flow
rates while preventing excessive dross formation
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Turbulent flow rates should be avoided
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A pouring basin is used to ensure uninterrupted metal flow into the
sprue
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Filters are used to trap large contaminants and more laminar flow
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Chills used to speed up solidification of the metal
Design Considerations in Casting:
General Design Considerations for Castings
Establishing Good Practices
 High-quality molten metal is essential for producing
superior castings
 Pouring of metal should not be interrupted
 Stress relieving necessary to avoid distortions of
castings in critical applications
Design Considerations in Casting:
General Design Considerations for Castings
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1.
2.
3.
Expendable-mold process cool much slower than die
casting
Important design considerations are:
Mold Layout
- must be placed logically and compactly with gates
Riser Design
- consideration in size and placement of risers
Machining Allowance
- require for additional finishing operations
Design Considerations in Casting:
Design for Permanent-mold Casting
Designs may be modified to eliminate the draft for
better dimensional accuracy
 Die-cast parts are net shaped to remove gates and
minor trimming to remove flashing and defects
 They do not require a machining allowance
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Design Considerations in Casting:
Computer Modeling of Casting Processes
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Casting involves complex interactions among material and
process variables
A quantitative study of these interactions is essential to the proper
design and production of high-quality castings
Studies consist of heat flow, temperature gradients, nucleation
and growth of crystals, formation of dendritic and equiaxed
structures, impingement of grains and movement of the liquid–
solid interface during solidification
Commercial software programs are Magmasoft, ProCast, Solidia
and AFSsolid
Economics of Casting
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Cost of each cast part (unit cost) depends on several
factors, including materials, equipment, and labor
Each of individual factors affects the overall cost of a
casting operation
Economics of Casting
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Costs are involved in melting and pouring the molten metal into
molds and in heat treating, cleaning, and inspecting the castings
Labor and skills required is also a consideration
Equipment cost per casting will decrease as the number of parts
cast increases
High production rate can justify the high cost of dies and
machinery