Lec3 Flashcards
Types of permanent mould casting x6
Slush Casting Pressure casting die casting centrifugal casting continuous casting single crystal casting
5 Steps of slush casting
1 Molten metal is poured into the metal mould
2 When desired thickness of solidified skin is obtained.
3 Mould is inverted and remaining liquid metal is poured out.
4 Mould halves are then opened.
5 Casting is removed
How does the skin develop
Solidifcation occurs from outside in due to die being cold
develops in the casting and becomes thicker with time
Applications
hollow castings with thin walls
decorative/ornamental objects
Very common in theatre/film industry for making latex/silicone props and costumes
Can be used for metal as well but not as common
Pressure casting process 2 steps
1 Molten metal is forced upward by either gas pressure into a graphite or metal mould
or for higher quality a vacuum (no gasses lower proosity higher quality)
2 pressure maintained until metal has completely solidified
What is pressure casting used for
high quality casting components (due to low porosity)
Advantages of pressure casting
Avoids rapid oxidation of metal
mould cooling is designed to promote directional solidification (top down better quality)
BUT
long cycle time (relative to other casting)
Die casting process
Molten metal injected into closed metal die under high pressure
pressure is maintained during solidification
die is separated and casting removed
Two types of die casting
hot chamber
cold chamber
What is hot chamber die casting
Liquid kept in chamber and forced into die cavity by hydraulic ram
Applications of hot chamber die casting and why
Zinc tin and lead based alloys - lower melting point have to be able to keep hot rapid process
Advantages of hot chamber die casting x4
1 No transfer process of molten metal (melted in chamber from where its injected into cavity)
2 offers fast cycling time
3 good strength product
4 excellent dimensional precision and surface finished
Disadvantages of hot chamber die casting x4
Expensive setup cots dies made from hardened hot work tool steels
Requires high production rate to justify (10,000+)
CANNOT be used for high melting point metals (steel/titanium)
Cold chamber die casting process 3 steps
1 Metal is melted in a separate furnace and is transported to the die casting machine
2 Molten metal is fed into an unheated chamber
3 A mechanical plunger forces the molten metal into the die cavity where the metal solidifies
Applications of cold chamber die casting
Die casting materials that are not suitable for hot chamber - aluminium magnesium copper
4 Adv of cold chamber
Good strength product
Excellent dimensional precision
excellent surface finish
higher melting point metals
3 disadv of cold chamber
Expensive die
High production rate req 10,000+
Need to transport molten metals (can be automated)
Centrifugal casting process 3 steps
1 Pour molten metal into rotating mould
2 Utilises inertial force caused by rotation to distribute molten metal into mould cavities
3 Metal is held against the mould wall by centrifugal force until it is solidified
3 types of centrifugal casting
True centrifugal
semi centrifugal
centrifuging
Part shapes created by true centrifugal casting
pipes with various outer diameters square or polygonal (inner surfaces remains cylindrical)
Pipes gun barrels steetlamp posts
Adv and disadv of true centrifugal casting x3
good quality
dimensional accuracy
limited shape
Shapes created by semi centrifugal casting
parts with rotational symmetry
Centrifuging process
Mould cavities placed at a certain distance from axis of rotation
molten metal is poured from the centre and forced into the mould by centrifugal forces
applications of centrifugal casting
bushes bearing housings
adv and disadv of centrifugal casting x4
wide range of cylindrical parts good dimensional accuracy BUT shape is limited expensive spinning equipment required
Shapes produced by continuous casting
Basic shapes which become feed stock for forming processes rolling and forming
Value of single crystal casting
V high value, v high purity
high strength low thermal creep
single crystal (no large number of grains and therefore interference between grain weakness)
Applications of single crystal
gas turbines aircraft engines
Method of producing single crystal
Czochralski process
