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

©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

METAL CASTING PROCESSES

Sand Casting Expendable Mold Casting Processes Permanent Mold Casting Processes Foundry Practice In-class Assignment


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)


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


Types of Patterns

Patterns slightly enlarged to account for shrinkagePatterns made of wood, metal or plastic

solid split match‑plate cope and drag


Core in Mold

(a) core held in place in the mold cavity by chaplets(b) possible chaplet design(c) casting with internal cavity


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


Shell Molding


Shell Molding

Advantages of shell molding: Better surface finish Good dimensional accuracy

Disadvantages: Expensive metal pattern

Difficult to justify for

small quantities


Vacuum Molding


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


Investment Casting (Lost Wax Process)


Investment Casting (Lost Wax Process)

A one‑piece compressor stator with 108 separate

airfoils made by investment casting (photo courtesy of

Howmet Corp.)


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


Low-Pressure Casting


Hot-Chamber Die Casting

Casting metals: zinc, tin and lead

Low melting‑point metals that do not chemically attack

mechanical components


Cold Chamber Casting

Casting metals: aluminum, brass, and magnesium

alloys


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


True Centrifugal Casting


Cupola For Melting Cast Iron

"charge," consisting of iron,

coke, flux, and possible alloying

elements


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


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


SME Video


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


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


Manufacturing Economics


Time Permitting Content


Semicentrifugal & Centrifuge Casting

Density of part greater in outer sections

Application: wheels and pulleys

Semicentrifugal Casting Centrifuge Casting

Used for smaller parts


Crucible Furnaces

Metal is melted without direct contact with burning fuel mixture

lift‑out crucible stationary pot tilting-pot furnace


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


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


Casting Quality - General Defects


Sand Casting Defects


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

Documents

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