11-05-2012, 05:11 PM
FUNDAMENTALS OF METAL CASTING
casting.pdf (Size: 335.25 KB / Downloads: 226)
Introduction
• Dated back 6000 years
• Ingot vs. Shape casting
• Polymers and ceramics are cast as well.
• Issues in casting
– Flow
– Heat Transfer
– Selection of Mold Materials
– Solidification- Nucleation and Growth
• Depending on how we control solidification, these
events influence the size, shape, uniformity and
chemical composition of the grains.
Two Main Categories
1. Expendable mold processes –A mold after process
must be destroyed in order to remove casting
– Mold materials: sand, plaster and similar materials +
binders
– More intricate geometries
2. Permanent mold processes – A mold can be used
many times to produce many castings
– Mold: made of metal and, less commonly, a ceramic
refractory material
– Part shapes are limited
– Permanent mold processes are more economic in high
production operations
Casting Processes
• Forming the Mold Cavity
– Mold cavity is formed by packing sand around a pattern.
– The pattern usually oversized for shrinkage is removed.
– Sand for the mold is moist and contains a binder to maintain shape
• Cores in the Mold Cavity
– The mold cavity - the external surfaces of the cast part
– A core, placed inside the mold cavity to define the interior
geometry of part. In sand casting, cores are made of sand.
• Gating System - Channel through which molten metal
flows into cavity
– A downsprue, through which metal enters a runner
– At top of downsprue, a pouring cup to minimize splash and
turbulence
• Riser - Liquid metal reservoir to compensate for shrinkage
during solidification
– The riser must be designed to freeze after the main casting
solidify.
Fluidity
• Fluidity: A measure of the capability of a metal to flow into and fill the mold
before freezing. (Inverse of viscosity)
• Factors affecting fluidity - Pouring temperature, Metal composition, Viscosity,
Heat transfer to the surroundings, Heat of fusion and Solidification
• Higher Re, greater tendency for turbulence flow
– Turbulence and laminar flow
Reynold’s number: Re=vDr/h
Re ranges 2,000(laminar) to 20,000 (mixture of laminar-turbulence)
greater than 20,000 turbulence resulting in air entrainment and dross
(scum) formation
• Minimize turbulence by avoiding a certain range in flow direction
Pure metals: good fluidity
Alloys: not as good