Shape casting process Flashcards
What is the shape casting process?
Manufacture of a complex shape by melting and resolidification of a mould cavity
What is continuous casting?
Liquid metal poured in at same rate as extracted out; makes channel-shaped metals; roll down and extrude afterwards
How was shape casting performed in the middle bronze age?
Used whatever rocks were available to make mould; single block would make a flat-faced object, whilst double block would make a fully shaped object as makes full mould cavity
What shape casts were traded in eastern Mediterranean?
~1000BC; Cu ingots and tangs
What is the very earliest evidence of shape casting?
Otzi (~3300BC); had pure Cu cast, forged (cold worked), polished, and sharpened; mottled due to air bubbles from where gas is rejected from solution during solidification = evidence of casting
What are the basic tools needed for sand casting?
SiO2 and some binding agent
What are naturally bonded sands, and where were they sourced?
Sand and clay mixture + ~5% H2O; found at Mansfield, in deltas etc
What are synthetic bonded sands comprised of?
Selected, washed, and graded grains of sand, selected clays (e.g. bentonites), and water
What is added to synthetic bonded sands for ferrous castings, and why?
Add carbonaceous additive: volatilises when metal is poured into mould; forms gaseous film that protects sand mould from attack/ penetration by the cast iron
How are sand moulds formed?
Clay smeared on sand grain by mixing/mulling process; add water; water causes plates of clay to swell; adjacent clay molecules attracted by Van der Waal’s; clay-bonded sands squeezed and rammed together to make compact mould
What is the general process of moulding?
Liquid metal into pouring basin; flows down downsprue; into a well; along runner bar; flows through ingate into mould cavity; feeder prevents contraction; parting plane separates upper (cope) and lower (drag) halved; patten has contraction allowance so casting is desired size; inclusion either sink or go to top; inclusions machined off
What is the process used in investment casting?
Two aluminium/ wax dies made; form mould when put together; put onto tree (acts as downsprue; wax shape placed in slurry/ ceramic suspension for 24 hours; wax into drying room; repeat; wax melted out; forms green state; placed into furnace for sintering and to make mould hard
How does investment casting allow for smooth surface finish?
Finer slurry = finer detail; often the “facecoat” (initial application of slurry) is made form very fine powder particles
What are the advantages of investment casting?
Renowned for dimensional accuracy; smooth surface finish; complex castings can be made due to patten being melted from shell rather than physically withdrawn
How was CO2 used in moulding?
In WW2, sodium silicate-bonded sands were devised, which were catalysed by reaction with CO2 - alternative to using clay as a binding agent. Desert sand mixed with sodium silicate; levelled; hard base obtained within 24hrs as sodium silicate reacted with atmospheric CO2
