Casting - Process Flashcards
What factors affect choice of technique?
Cost, shape limitations, material, quality (tolerances, finish, integrity), quantity (automation)
What’s the outer fountained of a mould called?
Flask
The upper half of a mould
Cope
Lower half of mould
Drag
What gives hollow features of a mould
Cores
Purpose of a spruce
Pouring molten material
Purpose of gate
Narrow to ensure uniform flow
How to allow gases to escape a mould
Risers or vents
Purpose of risers
Allow gas escape, indicate full,
Issues in filling a mould may be caused by;
Low fluidity, low solidification time, gases not escaping
Preventing gases entering the mould
Flushing with an inert gas or melting and pouring under a vacuum
Why is porosity an issue with casting?
When materials are molten, gases are more soluble or may react
What materials are used in casting?
Metals, polymers or ceramics
What are the general steps in casting?
Pour molten metal into mould → solidify → remove
Advantages to casting
Complex shapes (+internal cavities), large singular parts, inexpensive, competitive (sometimes exclusive) near net shape
Disadvantages to casting
Dangerous, skill intensive, error prone, does not provide some properties
Preventing material shrinkage
Cavity is slightly oversized
Riser supplies extra material
If a mould causes too much restraint on a casting, this can lead to;
Hot tears as material shrinks as strength is low at high temperatures
When grains nucleate at cool mould walls, what 3 characteristics are formed?
Chill zone, bulk zone, solidification fronts
Describe the chill zone
Fine multi-axed grains
Describe the bulk zone
Columnar grains, slowing cooling rate
Describe the solidification front
During cooling they move away from the wall with time, the skin is therefore thinner at concave sections
When can solidification fronts (during cooling) be utilised for casting
Slush casting (pouring it out before it’s set)
{hollow decorative objects}Effect on grain structure with fast cooling
Finer grains
Cored dendrites cause micro (conc surface to centre) and macro (tend to form at centre) segregation
Effect on grain structure with slow cooling
Courses grain structure
What affects cooling rate?
Mould material (chill metal), alloy used
As grain size decreases
^strength and ductility, microporosity, hot tears decrease
Reducing grain anisotropy when cooling materials
Use inoculates
Reducing chance of hot tears
Placing exothermic compounds at critical areas
Porosity during cooling can be caused by
In larger regions the surroundings solidify first
Reduce porosity during cooling
Using chills to increase cooling rate at points (permanent or removable)
What is a rough casting
Initial part removed from mould
What is removed from the rough casting?
Sprue, runners, risers, gates and flash
What can speed up post casting shrinkage?
Annealing
Chovinov’s rule
T = C(V/A)ⁿ
1.5
What helps with casting removal
Draft angles
