metal casting processes - vaxxine computer · ppt file · web view ·...
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![Page 1: METAL CASTING PROCESSES - Vaxxine Computer · PPT file · Web view · 2016-12-02©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e. METAL CASTING](https://reader030.vdocuments.us/reader030/viewer/2022021501/5aaf987b7f8b9a25088db404/html5/thumbnails/1.jpg)
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METAL CASTING PROCESSES
Sand Casting Expendable Mold Casting Processes Permanent Mold Casting Processes Foundry Practice In-class Assignment
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Overview of Sand Casting
Most widely used casting process Nearly all alloys can be sand casted Castings range in size and production quantity
A large sand casting weighing over 680 kg (1500 lb) for an air
compressor frame (photo courtesy of Elkhart Foundry)
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Sand Casting Production Sequence
Cavity formed by packing sand around a pattern Gating and riser system Core used for internal geometry New sand mold for each part
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Types of Patterns
Patterns slightly enlarged to account for shrinkagePatterns made of wood, metal or plastic
solid split match‑plate cope and drag
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Core in Mold
(a) core held in place in the mold cavity by chaplets(b) possible chaplet design(c) casting with internal cavity
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Molds & Sands
Desirable Mold Properties Strength Permeability Thermal stability Collapsibility & reusability
Foundry Sands Silica (SiO2) Small grain better surface finish Large grain more permeable Typical mix: 90% sand, 3% water, and 7% clay
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Shell Molding
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Shell Molding
Advantages of shell molding: Better surface finish Good dimensional accuracy
Disadvantages: Expensive metal pattern
Difficult to justify for
small quantities
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Vacuum Molding
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Expanded Polystyrene Process (Lost Foam)
Advantages Pattern need not be removed Simplifies and speeds mold‑making
Disadvantages A new pattern is needed for every casting Economic justification of the process
Application Automotive engines
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Investment Casting (Lost Wax Process)
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Investment Casting (Lost Wax Process)
A one‑piece compressor stator with 108 separate
airfoils made by investment casting (photo courtesy of
Howmet Corp.)
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Permanent Mold Casting
(1) mold is preheated and coated
(2) cores (if used) are inserted and mold is closed
(3) molten metal is poured into the mold
Application – automotive pistons
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Low-Pressure Casting
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Hot-Chamber Die Casting
Casting metals: zinc, tin and lead
Low melting‑point metals that do not chemically attack
mechanical components
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Cold Chamber Casting
Casting metals: aluminum, brass, and magnesium
alloys
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Cold Chamber Die Casting Machine
Mold usually made of tool steel or mold steel
Tungsten and molybdenum (good refractory qualities) used to die cast steel and cast iron
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True Centrifugal Casting
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Cupola For Melting Cast Iron
"charge," consisting of iron,
coke, flux, and possible alloying
elements
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Buoyancy in Sand Casting Operation
During pouring, buoyancy of the molten metal tends to displace the core
Force tending to lift core = weight of displaced liquid less the weight of core itselfFb = Wm ‑ Wc Fb = buoyancy forceWm = weight of molten metal displacedWc = weight of core
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In-class Example
An aluminum‑copper alloy casting is made in a sand mold using a sand core that weighs 20 kg. Determine the buoyancy force in Newtons tending to lift the core during pouring.
Sand core density = 1.6 g/cm3
Al-Cu density = 2.81 g/cm3
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SME Video
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In-class Assignment
A sand core used to form the internal surfaces of a steel casting experiences a buoyancy force of 225.63 N. What is the volume of the sand core in cm3?
Steel density = 7.82 g/cm3
Sand core density = 1.6 g/cm3
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Extra Credit – hand in before test 1
Caplets are used to support a sand core inside a sand mold cavity. The design of the caplets and the manner in which they are placed in the mold cavity surface allows each caplet to sustain a force of 10 lbs. Several caplets are located beneath the core to support it before pouring; and several other caplets are placed above the core to resist the buoyancy force during pouring. If the volume of the core = 325 in3, and the metal poured is brass, determine the minimum number of caplets that should be placed (a) beneath the core, and (b) above the core.
Brass density = 0.313 lb/in3
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Manufacturing Economics
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Time Permitting Content
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Semicentrifugal & Centrifuge Casting
Density of part greater in outer sections
Application: wheels and pulleys
Semicentrifugal Casting Centrifuge Casting
Used for smaller parts
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Crucible Furnaces
Metal is melted without direct contact with burning fuel mixture
lift‑out crucible stationary pot tilting-pot furnace
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Electric‑Arc & Induction Furnaces
Heat generated by electric arc High power consumption for high
melting capacity Primarily for melting steel
Alternating current passing through a coil
Develops magnetic field in metal Induced current rapidly heats
and melts
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Ladles & Additional Steps
Transfer of molten metal to mold
crane ladle two‑man ladle
Trimming Removing the core Surface cleaning Inspection Repair, if required Heat treatment
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Casting Quality - General Defects
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Sand Casting Defects
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Product Design Considerations
Corners on the casting – source of stress concentration
Original design Redesign Draft = 1 for sand casting
Draft = 2 to 3 for permanent mold
Allow 3 mm stock for machining