Casting

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  • Topic: Casting, Sand casting, Melting point
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Manufacturing Technology – II MI 232

Instructor – Inderdeep Singh

Spring 2011-2012

Foundry - Factory that produces metal castings
• Casting:
Process in which molten metal flows by gravity or other force into a mold where it solidifies in the shape of the mold cavity

Steps:
1. Melt the material (metal) 2. Pour it into a mold 3. Let it freeze

Refractory mold  pour liquid metal  solidify, remove  finish

Capabilities and Advantages
• Versatile: complex geometry, internal cavities, hollow sections • Versatile: small (~10 gms)  very large parts (~1000 Kg) – Engine blocks and heads, machine frames, railway wheels, pipes, church bells etc. – Dental crowns, jewelry, small statues, frying pans

• ECONOMICAL: little wastage (extra metal is re-used)

• ISOTROPIC: cast parts have same properties along all directions • Some casting methods are suited to mass production • All varieties of metals can be cast, ferrous and nonferrous • High complexity with few steps (usually)

Disadvantages of Casting
• Limitations on mechanical properties • Poor dimensional accuracy and surface finish for some processes;

e.g., sand casting
• Safety hazards to workers due to hot molten metals • Environmental problems • Expensive and time-consuming patterns/molds/dies • Solidification issues: shrinkage, porosity, ~low strength, brittleness • Some methods require many steps

Casting
Pattern » Mold » Pour » Cool » Remove » Inspect

Types of Casting : Expendable Mold, Multiple Use Mold Sand Casting : Expendable Mold casting process uses sand as molding material

Requirements
• • • • • • Mold : single or multiple use Melting Process Pouring Technique Solidification Process Mold removal Clean, Finish, Inspection

METAL CASTING PROCESSES
Categories
• Expendable mold processes - mold is sacrificed to remove part • Mold (Sand, Shell, Vacuum, Expanded Polystyrene, Investment Casting, Plaster and Ceramic) – Advantage: more complex shapes possible – Disadvantage: production rates often limited by time to make mold rather than casting itself 2. Permanent mold processes - mold is made of metal and can be used to make many castings – Advantage: higher production rates – Disadvantage: geometries limited by need to open mold

Sand Casting
• Widely used casting process, accounting for a significant majority of total tonnage cast • Nearly all alloys can be sand casted, including metals with high melting temperatures • Castings range in size from small to very large (0.1-105kg) • Production quantities from single castings as well as for large-scale casting • Hand moulding and large-scale machine moulding • Cast complicated components • Low initial cost • Poor dimensional accuracy and surface smoothness

680 kg air compressor frame

Sand Casting
Production sequence in sand casting

Steps in Sand Casting
1. Pour the molten metal into sand mold 2. Allow time for metal to solidify 3. Break up the mold to remove casting 4. Clean and inspect casting – Separate gating and riser system

5. Heat treatment of casting is sometimes required to improve metallurgical properties

Mold
• Cavity whose geometry determines part shape – Actual size and shape of cavity must be slightly oversized to allow for shrinkage of metal during solidification and cooling – Molds are made of a variety of materials, including sand, plaster, ceramic, and metal

Sand Casting: Parts of a Sand Mold (expendable mold)
Key terms: Flask, Cope, Drag, Sprue, Runner, Gate, Riser, Mold Cavity, Core, Parting Line

Open Molds and Closed Molds

(a) open mold, simply a container in the shape of the desired part (b) closed mold, in which the mold geometry is more complex and requires a gating system (passageway) leading into the cavity

Making the Sand Mold
• The cavity in the sand mold is formed by packing sand around a pattern, then separating the mold into two halves and removing the pattern • The mold must also contain gating and...
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