CS0101S2B03_10

授課教師：楊宏智教授

1

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楊宏智（台大機械系教授）2

Chapter Outline1. Introduction2. Design Considerations in Casting3. Economics of Casting

Introduction 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 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 part2. Mold features that are needed to produce the desired

casting


Design of Cast Parts Corners, angles, and section thickness Flat areas Shrinkage Draft Dimensional tolerances Lettering and markings Finishing operations


Locating the Parting Line Parting line is the line or plane separating the upper (cope) and

lower (drag) halves of molds Parting line should be along a flat plane rather than be contoured Location is important as it influences mold design, ease of

molding, number and shape of cores required, method of support and the gating system

Parting line should be low for less dense metals and located at mid-height for denser metals


Locating and Designing Gates Multiple gates are preferable Gates should feed into thick sections of castings A fillet should be used Gate closest to the sprue should be placed sufficiently far away

from the sprue Minimum gate length should be 3 to 5 times the gate diameter Curved gates should be avoided

Runner Design Runner is a horizontal distribution channel that accepts molten

metal from the sprue and delivers it to the gates It is used to trap dross and keep it from entering the gates and

mold cavity

Designing Other Mold Features Goal in designing a sprue is to achieve the required metal flow

rates while preventing excessive dross formation


Designing Other Mold Features Goal in designing a sprue is to achieve the required metal flow

rates while preventing excessive dross formation Turbulent flow rates should be avoided A pouring basin is used to ensure uninterrupted metal flow into the

sprue Filters are used to trap large contaminants and more laminar flow Chills used to speed up solidification of the metal


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


Expendable-mold process cool much slower than die casting

Important design considerations are:1. Mold Layout

- must be placed logically and compactly with gates2. Riser Design

- consideration in size and placement of risers3. 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

Design Considerations in Casting:Computer Modeling of Casting Processes 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 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 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

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CS0101S2B03_10