Casting Flashcards
Choice of processes
Cost Shape Materials Quality Production quantity
What is casting?
Pour molten metal into mould, allow it to solidify and then remove the part from the mould
Advantages of casting?
Capable of complex shapes with internal cavities
Can make large parts in one piece
Accurate shape - near net shape
Factors to consider in casting?
- The mould
- Filling the mould
- Material shrinkage
- Cast structure
- Defects
- Removal and finishing of part
What is the flask made up of?
Upper half is the cope
Lower half is the drag
Purpose of the riser?
Allow air escape, indicate a full mould and supply molten metal as shrinkage occurs - avoid porosity
Fluidity test?
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.
Purpose of the sprue?
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.
What are evolved gases?
Gases are more soluble in liquid metals (becomes trapped gas)
Methods of degassing?
Flushing with an inert gas
Melting and pouring in a vacuum
Metal cooling (Pure metal)
Grains nucleate against walls - chill zones
The bulk grain structure is columner
Cooling occurs quicker at corners and edges
Metal cooling (Alloy)
The solid component forms dendrites (trees) surrounded by the liquid - mushy zone
Takes 2 hours for complete solidification
Cooling rates
Slow cooling - coarse grain structure
Higher cooling rates - Fine grain structure
As grain size decreases?
Strength increases
Ductility increases
Tendency to crack during cooling decreases (hot tears)
Could lead to formation of cored dendrites
Chills and inoculants
Inoculants provide nucleation sites throughout the depth of the liquid metal.
Chills increase cooling rates at critical points
Casting defects
Blow Scar Blister Scab Misrun Wash Cold shut
Chvorinov’s rule
Depends on volume and surface area and tells us how long it will take to solidify
Expendable mould casting
Sand
Investment
Evaporative pattern
Permanent mould casting
Gravity die
Pressure die
Centrifugal
Sand casting
Green moulding sand - sand with the addition of clay and water Refractoriness - withstand heat Cohesiveness Permeability Collapsible
Sand casting, Advantages and disadvantages
Advantages
- Wide variety of shapes
- Relatively cheap moulds
Disadvantages
- Difficulties meeting exact engineering requirements
- Surface quality
- Lower production volumes
Investment casting process
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
Investment casting characteristics
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
Permanent mould advantages
Mould is reusable
Good surface finish
Dimensional accuracy can be as good as +/- 0.08mm
Mould temperatures can be controlled
Permanent mould disadvantages
More suited to casting of low-melting point alloys, e.g. aluminium and zinc alloys
Mould costs can be high
Wear of the mould
Gravity die characteristics
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
Minimum section thickness in gravity die casting?
Aluminum alloys: 3mm
Copper based alloys: 1.5mm
Ferrous alloys: 5mm
Pressure die casting process
- Lubrication of dies
- Closing and locking of dies
- Molten metal is forced into the die cavity
- Held under pressure until it solidifies
- Die opens
- Casting is ejected
Pressure die casting characteristics
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
Define misrun
The molten metal solidifies before it enters the mould
Solving misrun problems?
High enough temperature? Add fluidising additives Ensure walls are sufficiently thick Remove sharp corners Run the metal in from different directions
Purpose of draft angle?
Draft angles are sloping walls used to enable smooth removal of the ridge. Typically 1-3 degrees
Avoiding porosity
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
Size of parts ranking
Largest Sand Centrifugal Gravity, investment and evaporative Pressure die Smallest
