casting

casting

o   Liquid metals hold gas – when solidifies it forms bubbles – reduced mechanical properties – to prevent = vacuum

o   Oxide defects during pouring = impurities – results in poor surface finish, machinability + mechanical properties – to prevent = mould design reduced turbulence + vacuum

Sand casting disposable

     Single use mould casting (disposable) very common

     Advantages – cheap, high variety metals, no limit on size, complex shapes, short lead-time

     Limitations – low dimensional accuracy, rough surface finish, high porosity (holes), slow

     Applications – train wheels, ship propellers, pump casings, turbine impellers, engine crankshafts

Investment casting single use

Ø  Master die required to make wax pattern, soluble was cores + ceramic cores

Ø  Advantages – excellent surface finish, high dimensional accuracy, complex shapes, almost all metals, no parting line concerns, fast

Ø  Limitations – expensive, high labour cost, size limited, significant lead-time, high porosity (holes)

Ø  Applications – gears, valves, precision housings, aircraft engine components, knee replacements

Pressure casting permanent

o   Molten metal forced upwards by pressure increase or vacuum

o   Advantages – clean (no oxidisation), good mechanical properties, high dimensional accuracy and surface finish, >85% material yield

o   Limitations – long, expensive, shape + size + complexity limited

o   Applications – high quality automotive items

Slush casting permanent

·       Molten metal into mould, shell forms on inside to desired shell thickness, casting then removed

·       Advantages – hollow casts, efficient material use, good external detail

·       Limitations – hollow casts ONLY, variable wall thickness throughout mould

·       Applications – ornaments e.g. candlesticks, latex + silicone props

Die casting permanent

     Molten metal injected in closed metal die under high pressure – two types: hot chamber, cold chamber

     Hot chamber – gooseneck submerged in heated reservoir (low melting point)

     Cold chamber – molten metal transferred to unheated chamber (high melting point)

     Advantages – fast, excellent dimensional accuracy, good surface finish, thin sections, good mechanical properties

     Limitations – expensive, high quantity only to justify cost, small components only

     Applications – wide variety of non-ferrous castings

Centrifugal casting permanent

Ø  Molten metal in rotating mould, utilises inertial force – three types

true centrifugal casting – axis can be horizontal or vertical

·       Advantages – clean metal, good dimensional accuracy, strong exterior, large parts

·       Limitations – inner surface must be cylindrical, outer surface must be rotational symmetry, expensive

·       Applications – pipes, pressure vessels, cylinder liners, bake drums, bushings + bearings

semi-centrifugal casting – MUST have rotational symmetry, poured into central reservoir

·       Advantages – stacks several moulds, more complex shapes than true

·       Limitations – MUST have rotational symmetry

·       Applications – gear blanks, pulley sheaves, wheels, impellers, rotors for electric motors

centrifuging – any shape is offset from axis of rotation

·       Advantages – wide variety of intricate + thin-walled shapes, clean metal, strong parts

·       Limitations – small parts

·       Applications – wide range of products, investment casting patterns