Lecture 1-2 Processes Flashcards
Casting is a process whereby
a part is produced by solidification to take the shape of a mould
Casting processes were first seen in the —– ages.
bronze
5 main steps in casting process
- Pattern/mould making
- Melt preparation
- Pour molten metal into the mould
- Allow to solifify
- Remove part from the mould
Net shape means that
the product is ready for direct use
Near net shape means that
the product requires some additional processing/finishing required before it can be used
Why is using metal in casting versatile?
As any metal can be heated to be in the molten state
Casting can be used to form how big of a piece?
Wide range from mm to m in one piece
—– shapes are able to be produced through casting. The shapes can also have —– —–.
Complex, internal cavities
Economically, casting is
a relatively cheap process
Through casting a product, the outcome parts are
net shape or near net shape.
Foundries are places
where casting takes place
Expendable mould examples
- Sand casting
- Investment casting
Permanent mould example
- Die casting
3 considerations regarding the molten material chosen for casting
- Reactivity with air
- Gas solubility
- Safety
Cores are used in casting to produce
internal features
Cope and drag are
the two halves of the sand mould
The entrance and the initial downward “tube” where the molten metal are poured down are called what?
Pouring cup at the top, which leads to the downsprue
The parting line is
the line between the two moulds
A riser is
a reservoir of metal to ensure that the cavity is filled with metal
A riser is used because
A riser reduces the chance of —- occurring in the mould where there isn’t enough metal to go around
when metal is solidifying it shrinks.
voids
The metal after entering the mould, flows down the —- to get to the cavity
Runner
The finished product from sand casting usually requires —– ——. Why?
Post processing, low surface detail
4 Cons of sand casting
- High tolerances = low precision in the finished product
- Flow and shrinkage must be monitored as it is gravity driven
- Safety precautions
- Labour intensive with long cycle time
Pros for sand casting
- Low production volumes
- Cheap moulds
- No size and shape limits to the cast
- Moulds can be changed easily
Die casting uses what sort of pressures? Why?
High pressure, to be able to inject the metals into the die
Die casting produces excess material where the two moulds meet. Name term used to describe this
Flash
Average cycle time for die casting
~ 10 secs
Die casting could be expensive due to (2 points)
- the tool grade steel that is required
- needing the machines capable of closing dies and injecting molten metals
2 pros for die casting
- Process is semi or fully automated
- Very high production rates
Two different options for die casting
- Hot chamber
- Cold chamber
Basic outline of steps for die casting:
- Lubrication of the dies
- Closing and locking of dies
- Molten metal is forced into the cavity
- Dies are held together under pressure til the metal solidifies
- Die opens
- Casting is ejected
In Hot chamber die casting, common metals used are:
Zinc, lead, tin, and magnesium
Disadvantage of cold chamber die casting compared to hot chamber die casting:
extra step of pouring molten metal from a ladle into the piston
Cold chamber compared to hot chamber die casting has a
slightly lower production rate
In Cold chamber die casting metals commonly used are:
Aluminium, brass, magnesium alloys
5 Pros of die casting
- Cheap for large production
- Good accuracy
- Good surface finish
- Able to do thin sections
- Rapid cooling results in small grain size and good strength
Why would you choose cold chamber over a hot chamber die casting method? How does this apply to the difference in process?
The metal you are casting is corrosive - it cannot sit in the hot chamber for long without reacting with the container
Why is the runner an important section of the casting process?
It helps to direct the molten metal and allow it flow at a sensible rate (faster causes turbulence)
The downsprue helps to decrease the
- momentum in the molten metal
- regulates the air going into it
In the Bernoulli equation, what do the letters represent?
p
v
h
g
F
ρ
p - pressure
v - velocity
h - height
g - gravity
F - energy losses due to friction
ρ - density
Defects that can come about due to turbulent flow:
- entrap air from the atmosphere
- air will react with the metal
- voids and air bubbles will form in the metal
A lower Reynolds number means that
the flow is more laminar
A higher Reynolds number means that
the flow is more turbulent
How do we test a metal’s fluidity index
spiral mould test
A higher metal’s fluidity index means that the metal has
a higher fluidity index
In the Chvorinov’s rule, the letters represent
C
V
A
n
C - constant for the alloy being cast
V - volume of the casting
A - the surface area of the casting
n - constant (between 1.5-2.5)
Where does the solidification front occur?
It occurs at the melting point temp
If you are not given an n number for a calculation involving the Chvorinov’s rule, then we assume
that n = 2
Relationship between grain size and cooling rate
Grain size is inversely proportional to cooling rate
In the columnar zone what sort of grain is present? What strength is this?
Longer grain - weaker
In the chill zone what sort of grain is present? What strength is this?
Finer grain - stronger
Where are the two zones found, columnar and chill?
Columnar are more central and chill is on the outer edge
In a cooled down alloy, what feature is seen in the centre of the columnar zone?
Equiaxed zone
What is the main property of the equiaxed zone?
There are no directions to the grains - it is isotropic
Why is a fully equiaxed structure preferred over mixed structures?
Isotropic properties and consistent throughout
Ways to counteract shrinkages
- Use of the riser design
- Remoulding the mould to be slightly larger
A misrun is
solidification occurs before filling the cavity
Cold shuts are
lack of fusion around a core due to cooling
Cold shots are
metal splatter which leads to air entrapment
Shrinkage cavity is
solidification shrinkage which leads to a lack of metal in the mould
Hot tearing or cracking is
contraction during the final stages of cooling due to shape of the mould
8 Main steps in investment casting
- Injection of wax into a cast
- Combining of the wax models onto a larger frame
- Coat in ceramic
- Apply stucco
- Heat to remove and melt internal wax models
- Ceramic shell is fired to strengthen
- Ceramic shell is ready to be used for casting
- Separate the ceramic metal filled casts from the frame
