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楊宏智（台大機械系教授） 2

Chapter Outline 1. Introduction 2. Design Considerations in Casting 3. 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 part 2. Mold features that are needed to produce the desired casting

Design Considerations in Casting: General Design Considerations for Castings Design of Cast Parts  Corners, angles, and section thickness  Flat areas  Shrinkage  Draft  Dimensional tolerances  Lettering and markings  Finishing operations

Design Considerations in Casting: General Design Considerations for Castings 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

Design Considerations in Casting: General Design Considerations for Castings 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 Design Considerations in Casting: General Design Considerations for Castings

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 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

 Expendable-mold process cool much slower than die casting  Important design considerations are: 1. Mold Layout - must be placed logically and compactly with gates 2. Riser Design - consideration in size and placement of risers 3. Machining Allowance - require for additional finishing operations Design Considerations in Casting: General Design Considerations for Castings

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