Shape Casting Flashcards
When do the earliest casting processes date back to?
app. 3000 BC
Describe the earliest and simplest shape casting procedure
Chipping a cavity into a stone block, melting the metal and pouring it into the open cavity to solidify
Cast blank would be flat on its upper side, and could be worked into a more desirable shape
Describe the second (stone mould) casting process developed
Cut a mould cavity into the faces of 2 blocks, and clamp them together = double-sided mould.
Channels would have to be cut into the faces to insert the metal.
Still requires further work
What is the modern day equivalent of stone mould casting
Die casting, where steel, cast iron or bronze is the die.
mould can be repeatedly used to make the same object, until the mould or die fails due to thermal fatigue
Describe Sand Casting
Naturally-bonded sands used as the mould
Silica sands with ~ 5-6% clay content, produced naturally
Deposits found through the world
Naturally occurring mixtures need to be washed then mulled/ mixed to smear clay evenly around the sand grains, adding water (5-6%) to activate the clay
Clay plates expand and stick by virtue Van Der Waals force
Makes a very plastic material which can be moulded or squeezed into the desired shape
Ramming the sand together would cause a hard compact or mould to be made: greensand process
Naturally-bonded sands are now rarely used, synthetic sands preferred
Describe the mould making process for sand casting
- Take a pattern
- Fasten to a flat board
- Compact moist sand around the pattern, add to fill the box and level
- Mould removed from pattern
- Mould or cut feeding system, channel for liquid metal to enter the mould, provides fresh liquid metal as the casting solidifies and shrinks
- if internal channel is required, then a core can be used, made out of bonded sand, supported by recesses at the edge of the mould cavity
- Second half mould made separately
- 2 halves clamped or glues together before casting
- Once mould has been cast and L metal solidified the casting can be removed by destroying the mould
What do you have to do if casting iron
Add 2-3% coldest/ carbonaceous material
Volatilises when the metal is poured into the mould to form a gaseous film that protects the sand mould.
In modern iron foundries, greensand is black in colour
What can be used instead of clay as a binding agent
Sodium silicate-bonded sands were devised which are catalysed by a reaction with CO2
Invented in WW2 to make landing strips in the NAfrica desert -> desert sand mixed with sodium silicate, levelled and within 24 hrs a hard base obtained as sodium silicate reacted with the CO2 of the atmosphere
How can moulds develop poor properties
Due to overgrazing with CO2
Limited mould shelf life because of reaction of the sodium silicate with water vapour in the atmosphere
Why Is the sand cast deliberately oversized
Deliberately oversized, so when it cools down and contracts, becomes size needed
When was investment casting invented and what was it used for
Invented by ancient Greeks c. 600 BC, used for statues
Describe the complex process of investment casting
- Make cast out of wax, carved into desired shape, easy because wax has low mp (~130oC) easily locally remelted
- wax assembly invested, dipped into ceramic suspension/ slurry
- coat with coarser refractory powder (stucco) in repeated operation
- until 6-10mm thick built around wax components
- Facecoat (initial application) = initial application, expensive as small amount, allows casting of reactive metals
- Refractory material: material in contact with liquid metal that is poured into the mould
- Wax assembly with encasing ceramic shell placed in furnace at ~300oC
- Wax melts and runs out of the shell, leaving ceramic shell in “green” state
- Fired @ ~1000oC to sinter the refractory materials of which the ceramic shell was made, to create a rigid refractory mould
- l
What is the refractory material made of?
Made from very fine powder particles, so that resulting surface finish is v. good
What are the benefits of investment casting
dimensional accuracy and a very smooth surface finish
very complex castings can be made
reduces need for subsequent working
what was investment casting used for in the renaissance
lizards and amphibians
