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FUNDAMENTALS OF METAL CASTING
Post: #1

FUNDAMENTALS OF METAL CASTING



.pdf  casting.pdf (Size: 335.25 KB / Downloads: 197)

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
Post: #2
Fundamentals of Casting


.ppt  casting.ppt (Size: 337 KB / Downloads: 290)

Casting Processes

Preparing a mold cavity of the desired shape with proper allowance for shrinkage.
Melting the metal with acceptable quality and temp.
Pouring the metal into the cavity and providing means for the escape of air or gases.
Solidification process, must be properly designed and controlled to avoid defects.
Mold removal.
Finishing, cleaning and inspection operations.

Precision Casting

Plaster Molding

Plaster of Paris (gypsum) is used as the mold material, other components are added to improve the strength and reduce the setting time. The pattern is dried at 400 oF and the two halves are assembled and molten metal is poured into the cavity.
The process produces fine detail with good dimensional accuracy and surface finish. Used to cast aluminum, zinc and copper-based alloys (low-melting temp.)

Ceramic Molding

Similar to plaster molding, except it uses ceramic as a mold which is suitable for high temp. applications. Mixture of zircon (ZrSiO4), aluminum oxide (Al2O3), silica (SiO2) and other bounding agents.

Design Considerations

Wall thickness should be kept uniform if possible. Non uniform wall thickness could lead to warpage and dimensional variations.
Minimum recommended; .025 in or .65 mm, up to .125 for large parts.
Round interior and exterior corners to .01-.015 in radius (min.), prevents an edge from chipping.
 

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