Castability is the ease of forming a casting Casting is a manufacturing process by which a liquid material is usually poured into a mold, which contains a hollow cavity of the desired shape, and then allowed to solidify. The solidified part is also known as a casting, which is ejected or broken out of the mold to complete the process. Casting materials are usually metals or various cold.[1] Castability can be thought of as how easy is it to cast a quality part. A very castable part design is easily developed, incurs minimal tooling costs, requires minimal energy, and has few rejections.[2]
Castability can refer to a part design or a material property A material's property is an intensive, often quantitative property of a material, usually with a unit that may be used as a metric of value to compare the benefits of one material versus another to aid in materials selection.[1]
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Part design
Part design and geometry directly affect the castability, with volume, surface area and the number of features being the most important attributes.
If the design has undercuts or interior cavities it decreases castability due to tooling complexity. Long thin sections in a design are hard to fill.[1] Sudden changes in wall thickness reduce castability because it induces turbulence In fluid dynamics, turbulence or turbulent flow is a fluid regime characterized by chaotic, stochastic property changes. This includes low momentum diffusion, high momentum convection, and rapid variation of pressure and velocity in space and time. Nobel Laureate Richard Feynman describes turbulence as "the most important unsolved problem of during filling; fillets should be added to avoid this. Annulars in the path of flow should be avoided because they can cause cold shuts or misruns. A design that causes isolated hot spots decreases castability.[3] An ideal design would have progressive directional solidification Directional solidification is a series of measures applied to control the feeding of castings. As most metals and alloys solidify, changing from the liquid state to the solid state they will undergo an appreciable volume contraction. Without attention to control principles, objects cast will contain internal voids commonly called "shrink from the thinnest section to the thickest.[4]
Location of the mold's parting line A parting line in moldmaking is the place where two or more parts of the mold meet. At times, either because the mold halves do not meet with enough precision or because injection pressure is high, material will creep into the space between the molds. This material is generally called molding flash or simply flashing. At times the parting line can also affects castability, because a non-planar parting line also increases tooling complexity.
If a design requires a high degree of accuracy, fine surface finish Roughness is a measure of the texture of a surface. It is quantified by the vertical deviations of a real surface from its ideal form. If these deviations are large, the surface is rough; if they are small the surface is smooth. Roughness is typically considered to be the high frequency, short wavelength component of a measured surface or defect free surface it reduces the castability of the part.[2] However, the casting process can be very economical for part designs that require intricate contoured surfaces, thickness variations, and internal features.[3]
Quantitative analysis
The castability of a design can be partially quantitatively determined by the following three equations. Better castability is denoted by a larger number.[3]
Where Vc is the volume of the casting and Vb is the volume of the smallest box that the casting could fit in.
Where Vc is the volume of the casting and Ac is the surface area of the casting
Where nf is the number of features (holes, pockets, slots, bosses, ribs, etc.)
Material properties
Material properties that influence their castability include their pouring temperature, fluidity Viscosity is a measure of the resistance of a fluid which is being deformed by either shear stress or tensile stress. In everyday terms , viscosity is "thickness". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity. Viscosity describes a fluid's internal resistance to, solidification shrinkage, and slag Slag is a partially vitreous by-product of smelting ore to separate the metal fraction from the unwanted fraction. It can usually be considered to be a mixture of metal oxides and silicon dioxide. However, slags can contain metal sulfides and metal atoms in the elemental form. While slags are generally used as a waste removal mechanism in metal/dross Dross is a mass of solid impurities floating on a molten metal. It appears usually on the melting of low-melting-point metals or alloys such as tin, lead, zinc or aluminium, or by oxidation of the metal. It can also consist of impurities such as paint leftovers. It can easily be skimmed off the surface before pouring the metal into a mold or formation tendencies.[1][5]
See also
References
Notes
- ^ a b c d Ravi, p. 1
- ^ a b Ravi, p. 2
- ^ a b c Ravi, p. 3.
- ^ Ravi, p. 4.
- ^ Tool and Manufacturing Engineers Handbook (TMEH), 4th Edition, Volume 6, Design for Manufacturability, Society of Manufacturing Engineers, 1992.
Bibliography
- Ravi, B., Design for castability, IIT Bombay, http://web.archive.org/web/20061210224324/http://www.energymanagertraining.com/foundries/pdf/CDA7.pdf .
Categories: Casting (manufacturing)
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