Flashcards in Casting Processes Deck (18):
Three types of casting
Different parts of sand casting
Mould cavity is created using a pattern and cores.
Mould is made up of a cope and drag.
The pattern is removed before assembling the flask to create a cavity.
Metal is poured through the sprue and flows into the cavity through the runner
What is the use of a riser in sand casting?
When the cavity is full molten metal flows up the blind or open riser.
it acts as a store of molten material to flow back into the cavity as the casting cools and shrinks
Patterns in mould making
cont4rsuting a mould starts with a pattern which the sand can be compressed around.
Patterns can be wood, plastic or metal.
The pattern must include tolerances to allow for shrinkage of the metal when it has been cast and a draft angle to allow the pattern to be removed from the sand
Cores in mould making
Cores can be formed in the pattern as green sand cores or separately as dry sand cores.
If they are separate ta core prints will have to be left in the mould for a core to fit into
Sprue and runners in mould making
Sprue and runners will be separate from the pattern as they are removed.
The runners and the gate may be incorporated into the drag pattern with a blind riser often incorporated into the cope pattern
Why would shell moulding be used in place of sand casting?
It is more accurate and gives a better surface finish
Steps in Investment moulding
The required shape is made in wax or polymer (usually through injection moulding)
The investment is coated with a ceramic slurry or plaster then allowed to dry
the wax or polymer is melted out of the coating to leave the cavity
the shell mould can then be fired to give it strength
Why is investment casting used?
It is capable of producing very accurate and intricate parts.
A number of investments will be grouped together on one tree so many components can be made at one time.
Two types of Die casting
they are both permanent mould processes.
Gravity die casting is when hot metal is poured into a metal mould
Pressure casting the metal is forced into the metal mould under pressure
Different types of Pressure Die Casting
Hot and cold chamber
machines are characterised by the load the mould tools can withstand before separating
Dies include cooling channels (reduces cycle time) and ejection system
Process of Pressure Die casting
Metals which can be used are one with low melting point alloys.
A vent is present at eh parting line so the air can escape, this produces a flash which can be trimmed off
Costs and benefits of Pressure Die casting
Dies are expensive (£15K+) and are made from tool steel, due to this large production volumes are required to justify the initial cost
It results in high quality parts with good dimensional accuracy and surface detail
Flaws in casting
Shrinkage cavities / hot tears - occur where differential cooling of restricted contraction of metal produces tensile stresses. This is caused by poor casting design
Cold shut - may occur where two cold streams of metal meet, this is caused by poor mould design
Flaws in sand casting
Sand wash rough irregular surfaces on casting due to sand erosion. this is caused by poor metal flow due to poor mould design
Sand blow - smooth surface cavity arising from the trapped gas in the mould. this is caused by poor venting or high sand humidity
General design rules of casting
All edges must have fillets or radii: promotes laminar flow, avoids columnar solidification problems in dendritic materials, makes the removal easier
Avoid hot spots and large section changes which promote differential cooling
Cores should be avoided to keep costs low
Reinforcing ribs should be used to support large thin webs
The need for subsequent machining should be minimised to keep costs low
Design rules for sand casting
Ensure easy pattern removal
patterns should include allowances for shrinkage and machining
Solidification should move towards the riser - large sections should have risers located next to them