What are the main types of Casting? What are the subsections?
- Temporary Mould: Permanent pattern(sand-casting), Expendable pattern(Investment casting/lost wax)
- Permanent Mould: die casting
What are the advantages/disadvantages of Sand-casting?
Advantages: relatively cheap, no limit to weight or part thickness, it doesnt get much more expensive
Disadvantages: Relatively high porosity, not that dimensionally accurate, poor surface finish, components must be larger than certain size
- What is a pattern?
- What are patterns made of? Advantages/disadvantages?
The shaper of the mould, usually made of wood or plastics. Wood is hella easy to machine, but can be worn down after excessive use
Plastics don’t have to be removed before casting- evaporative casting. Allows for more complex shapes
Note: the pattern must be slightly larger than the final component as the metal shrinks
- What is a pattern?
- What are patterns made of? Advantages/disadvantages?
The shaper of the mould, usually made of wood or plastics. Wood is hella easy to machine, but can be worn down after excessive use
Plastics don’t have to be removed before casting(evaporative pattern casting). Allows for more complex shapes
Note: the pattern must be slightly larger than the final component as the metal shrinks
What is the difference between permanent and non permanent pattern?
Non permanent is single use, like evaporative casting. Permanent patterns need to be tapered so they can be removed easily from the mould
What qualities must the sand in sand casting have?
What can/do we add to the sand to give it these qualities?
- fine, round grain structure
- Good permeability so gases and steam can escape easily
- Strong enough to remain rigid during casting
- Have good collapsibility: can shrink while cooling to avoid defects in component
- We add binding agents, such as clay or coal dust to give more of these qualities
What are cores? What properties must they have?
Cores are solid pieces that produce the cavities in a component. They require similiar properties to the sand in the main cast, but usually tend to have to be stronger, as they must support themselves while metal is casted.
What is investment casting?
A more long-winded process; wax placed into mould, then clay mould formed around hardened wax, wax removed, and finally this is the mould used before being fretted
What’re the advantages/disadvantages of investment casting?
Advantages- can produce very fine detail. Little finishing needed. Relatively simple
Disadvantages: costs more than sand casting- making the die and the wax
What are the six stages of investment casting?
- Make a pattern from wax or polystyrene by injection moulding in a die. The dies are
relatively cheap to machine as they can be made from soft metals such as brass and
aluminium. - The pattern is coated with a refractory material such as ceramic slurry and fine sands to
form a shell. - When the slurry has dried the pattern is melted out in an oven at about 175 0C forming
the hollow mould. - The mould is then fired at about 650–1050 0C to harden and drive out the moisture.
- The metal is then cast in this mould.
- The ceramic mould is then broken away by vibration in a process known as the shakedown.
What is die-casting?
A form of permanent mould casting
- Die formed from metal, before being lined with slurry or graphite to increase lifespan/ ease of component removal
Limiting factors of die casting?
Advantages?
- As the liquid metal must have lower melting point than the die, this limits the particular types of metal that can be die-casted
- hella expensive
Advantages
- Good surface finish
- uniform metallurgical properties=good mechanical properties
- Can be used many, many, many times
- hella accurate
What is pressure die casting?
Difference between hot and cold chamber?
Where metal is forced into the cast under high pressures. This increases the level of detail that can be achieved
Hot chamber- piston sections off portion of molten metal, forcing into gooseneck into die
Cold chamber- molten metal placed into cold piston before being injected: this can achieve higher pressures than the hot chamber method
How are dies made to withstand the enormous physical and thermal pressures?
- High grade metals
- High die mass:component mass ratio, about 1000
- Circulating coolant around die. this also cools component faster, speeding up process
What are the three processes in casting that we must consider from a micro-structural perspective?
- Flow of molten metal into mould: too slow means premature solidification, too slow means increasing porosity
- Solidification of metal: Cools faster at surface, and slowest at center. We have equiaxed structures at center and surface, but more columnal structures near the center. In general, we want equiaxed grain throughout structure so we have uniform and isotropic properties
- Cooling in the mould: Basically, if cools too fast, pores may open up. Additionally, the component may shrink or expand upon cooling
How do we produce equiaxed grain?
What is the equation for calculating time needed?
- We can add chemicals(nucleating additions) to the metal so it cools more homogeneously
- t=c(V/(SA))^2
- note: this is important as we want to minimize time
What type of mould and pattern does sand casting use?
Permanent pattern, temporary mould
What is green sand?
common type of sand used in sand casting. is actually black because of carbon content. Called green because of moisture content
