Casting

Question 1

Choice of processes

Answer 1

Cost
Shape
Materials
Quality
Production quantity

Question 2

What is casting?

Answer 2

Pour molten metal into mould, allow it to solidify and then remove the part from the mould

Question 3

Advantages of casting?

Answer 3

Capable of complex shapes with internal cavities
Can make large parts in one piece
Accurate shape - near net shape

Question 4

Factors to consider in casting?

Answer 4

1. The mould
2. Filling the mould
3. Material shrinkage
4. Cast structure
5. Defects
6. Removal and finishing of part

Question 5

What is the flask made up of?

Answer 5

Upper half is the cope

Lower half is the drag

Question 6

Purpose of the riser?

Answer 6

Allow air escape, indicate a full mould and supply molten metal as shrinkage occurs - avoid porosity

Question 7

Fluidity test?

Answer 7

The fluidity index of a metal is defined as the length it will flow in a standard spiral passage before solidifying. Depends on thermal properties of the mould and metal.

Question 8

Purpose of the sprue?

Answer 8

The sprue leads to a narrow gap called the gate and ensures uniform metal flow into the mould. Molten metal is poured into the sprue.

Question 9

What are evolved gases?

Answer 9

Gases are more soluble in liquid metals (becomes trapped gas)

Question 10

Methods of degassing?

Answer 10

Flushing with an inert gas

Melting and pouring in a vacuum

Question 11

Metal cooling (Pure metal)

Answer 11

Grains nucleate against walls - chill zones
The bulk grain structure is columner
Cooling occurs quicker at corners and edges

Question 12

Metal cooling (Alloy)

Answer 12

The solid component forms dendrites (trees) surrounded by the liquid - mushy zone
Takes 2 hours for complete solidification

Question 13

Cooling rates

Answer 13

Slow cooling - coarse grain structure

Higher cooling rates - Fine grain structure

Question 14

As grain size decreases?

Answer 14

Strength increases
Ductility increases
Tendency to crack during cooling decreases (hot tears)
Could lead to formation of cored dendrites

Question 15

Chills and inoculants

Answer 15

Inoculants provide nucleation sites throughout the depth of the liquid metal.
Chills increase cooling rates at critical points

Question 16

Casting defects

Answer 16

Blow
Scar
Blister
Scab
Misrun
Wash
Cold shut

Question 17

Chvorinov’s rule

Answer 17

Depends on volume and surface area and tells us how long it will take to solidify

Question 18

Expendable mould casting

Answer 18

Sand
Investment
Evaporative pattern

Question 19

Permanent mould casting

Answer 19

Gravity die
Pressure die
Centrifugal

Question 20

Sand casting

Answer 20

Green moulding sand - sand with the addition of clay and water
Refractoriness - withstand heat
Cohesiveness
Permeability
Collapsible

Question 21

Sand casting, Advantages and disadvantages

Answer 21

Advantages

1. Wide variety of shapes
2. Relatively cheap moulds

Disadvantages

1. Difficulties meeting exact engineering requirements
2. Surface quality
3. Lower production volumes

Question 22

Investment casting process

Answer 22

Produce master pattern
Produce master die
Produce wax patterns
Assemble wax patterns
Coat the wax patterns
Allow investment to harden
Melt out wax pattern
Pour molten metal into mould cavity
Allow metal to solidify
Remove casting

Question 23

Investment casting characteristics

Answer 23

Only method for producing precision, shaped castings of high melting point materials
Very expensive
Permeability of mould important to allow gases to escape
Minimal finishing
Both high production rates and one-off products
More suited for fairly small products
Great advantage is flexibility, range of alloys processable, very fine surface detail and excellent dimensional tolerances

Question 24

Permanent mould advantages

Answer 24

Mould is reusable
Good surface finish
Dimensional accuracy can be as good as +/- 0.08mm
Mould temperatures can be controlled

Question 25

Permanent mould disadvantages

Answer 25

More suited to casting of low-melting point alloys, e.g. aluminium and zinc alloys
Mould costs can be high
Wear of the mould

Question 26

Gravity die characteristics

Answer 26

Advantages
Relatively long die life, 25,000+ cycles
No expensive machines and equipment required
Dies may be quite cheap - rough cast and minimal machining
Disadvantages
Limited capabilities in part shape, complexity and fine
detail
Production rates quite slow - increased by replicating
number of moulds used

Question 27

Minimum section thickness in gravity die casting?

Answer 27

Aluminum alloys: 3mm
Copper based alloys: 1.5mm
Ferrous alloys: 5mm

Question 28

Pressure die casting process

Answer 28

1. Lubrication of dies
2. Closing and locking of dies
3. Molten metal is forced into the die cavity
4. Held under pressure until it solidifies
5. Die opens
6. Casting is ejected

Question 29

Pressure die casting characteristics

Answer 29

Dimensions accurate (typ. tolerances +/- 0.05mm)
Thin walls (min. section typ. 1mm)
Very little finishing required
Advantages outweigh capital costs if production runs are high

Limited to metals which are of not so high a melting point
Part size limited (to about 10 kg)
High tooling Costs

Question 30

Define misrun

Answer 30

The molten metal solidifies before it enters the mould

Question 31

Solving misrun problems?

Answer 31

High enough temperature?
Add fluidising additives
Ensure walls are sufficiently thick
Remove sharp corners
Run the metal in from different directions

Question 32

Purpose of draft angle?

Answer 32

Draft angles are sloping walls used to enable smooth removal of the ridge. Typically 1-3 degrees

Question 33

Avoiding porosity

Answer 33

Use risers to supply extra metal during casting shrinkage
Incorporate vents to allow escape of gases
Use permeable mould materials
Degas the casting metal
Use chills at thicker sections of the casting
Avoid thickness variation in the casting

Question 34

Size of parts ranking

Answer 34

Largest
  Sand
  Centrifugal
  Gravity, investment and evaporative
  Pressure die
Smallest