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MT-284 MANUFACTURING PROCESSES. INSTRUCTOR: SHAMRAIZ AHMAD MS-Design and Manufacturing Engineering Shamraiz_88@yahoo.com Topic: Casting Defects and Mold Numerical Problems . Today’s Lecture. Casting Defects Use of chills Design considerations Mold design numerical problems .
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MT-284MANUFACTURING PROCESSES INSTRUCTOR: SHAMRAIZ AHMAD MS-Design and Manufacturing Engineering Shamraiz_88@yahoo.com Topic:Casting Defects and Mold Numerical Problems
Today’s Lecture • Casting Defects • Use of chills • Design considerations • Mold design numerical problems
Defects in Casting • Depending on casting design and the practices employed, several defects can developed in casting • Metallic Casting: consisting of fins, flash, rough surfaces • Cavities: consisting of rounded or rough internal or exposed cavities, including blowholes, pinholes, and shrinkage cavities • Discontinuities: such as cracks, cold or hot tearing, and cold shuts. • Defective Surface: such as surface folds, laps, scars, adhering sand layers and oxide scale
Defects in Casting • Incomplete casting: such as misruns, insufficient volume of metal poured: causes:- molten metal being at too low a temperature or pouring the metal too slowly, and run outs • Incorrect Dimension or shape: owing to factors such as improper shrinkage allowance, pattern mounting error, uneven contraction, deformed pattern
Chills FIGURE 5.35 Various types of (a) internal and (b) external chills (dark areas at corners), used in castings to eliminate porosity caused by shrinkage. Chills are placed in regions where there is a larger volume of metal, as shown in (c).
General Design Considerations • There are two types of design issues in casting e.g. geometric features, tolerances etc. It usually involves the following steps: • Design the part so that the shape is easily cast • Select a casting process and a material suitable for the part, size, dimensional accuracy, surface texture and mechanical properties • Locate the parting line of the mold or die adequately. • Locate and design the gating system to allow uniform feeding of the mold cavity with molten metal including risers, and sprue
Solidification Time • Total solidification time TTS = time required for casting to solidify after pouring • TTS depends on size and shape of casting by relationship known as Chvorinov's Rule where TTS = total solidification time; V = volume of the casting; A = surface area of casting; n = exponent with typical value = 2; and Cm is moldconstant.
