Forming Processes Flashcards
What is Sand Casting
Pattern is placed in a box and sand is packed around it. Pattern splits into two parts and each part put into two separate boxes (which combine to make a whole box mould).
Once mould is made in the sand, the pattern is removed and a Runner is created to pour the molten metal into the mould
Riser to added to prevent shrinkage and allow build up gasses to escape
Advantages:
- Cheap and quick
- Useful for forming tough alloys
- products have no directional properties (equally strong in every direction)
- Expendable
Disadvantages:
- Poor surface finish
- Defects can occur (cracking, blow holes from trapped gas)
- Hard to cast intricate components
Produces stress free equiaxed grain structure
What is Shell Moulding
- Using metal replica of desired part, surrounding it with thermosetting resin sand which is heated and sets, creating a half shell mould of desired part. 2 half shells are made to create a full shell which metal is then cast into
Used for smaller mass production components which require high finish
- Crankshafts, mower engines
-Expendable
What is Investment (Lost-wax) casting
Wax is cast into a very accurate metal mould to produce exact wax replica of desired component
Wax replica is coated in ceramic slurry and powder, allowed to dry and then fired to melt wax and harden ceramic.
Creates perfect casting which allows intricate components to be cast
- Expensive, expendable
- Used for turbine blades and surgical implants
What is Die Casting
Permanent metal mould
- Gravity die casting: molten metal runs into metal mould under gravity, used for simple shapes
- Pressure die casting: molten metal injected into a water cooled metal mould under pressure - Often used with alloys with high fluidity and low melting points
Non-Expendable, Used for mass producing intricate castings
What is Full mould (lost foam) casting
Usses sacrificial polystyrene pattern to form mould cavity in a sand mould.
Polystyrene pattern casted from permanent metal die
- Polystyrene pattern is packed with sand like conventional sand casting
- Molten metal is poured into mould and vaporises the polystyrene, taking its place.
Used for very complex shapes and removes the need for removing pattern
What is forging
Forming of metal by compressive forces, done above recrystallisation temperature
- done below recrystallisation for better dimensional accuracy but more force required
Closed Die Forging:
- Squeezing metal between two shaped dies, excess is called “Flash” which is later trimmed
Drop forging:
- When upper die is dropped onto lower die
Open die forging:
- When metal is hammered or pressed by vertically moving tool onto flat surface
Forging produces grain flow in the metal (grain conforms to shape of the metal) - greater strength
Used for spanners, bolts, cranks
Rolling
Hot Rolled: Refined, unstressed, equiaxed grains, creates grain flow. Black oxide surface layer
Cold Rolled: Elongated grains (Harder, stronger and less ductile product), cleaner, smoother and more dimensionally accurate finish - Grains conform to the shape of metal
- Cold roll screw threads for stronger and harder grain structure
Extrusion
Metal is heated to soften, it is then forced under pressure through a die
Extruded materials need to be ductile and process can be done either hot or cold. Creates grain flow
Direction extrusion is used on more ductile materials while indirect extrusion is used on less ductile
Drawing
Reduces cross sectional are of rods by pulling them through a die
Deep drawing produces seamless components such as cups, sinks, laundry tubs
Powder Forming
1) Compact metal powder into desired shape - weak enough to be broken my hand but holds shape
2) Sintering- shape is heated so particles bond to produce a solid component
- Porous and self lubricating products
- Complex shapes can be created easily (which would require a lot of machining normally)
- Powders and dies are very expensive, only suitable for mass production
Describe the Heat Affected Zone due to welding
- Centre of weld are small equiaxed grains
- Columnar grains grow outwards from the sides of the weld (Only strength in one direction, creates weak points)
- Grains further out recrystallize into large equiaxed grains (Makes HAZ weaker / more ductile)
- High carbon steels shouldn’t be welded as BCT martensite can form due to rapid cooling thus the weld / joint becomes very brittle
List thermosetting forming processes
Compression Moulding
Transfer Moulding
Injection Moulding
List Thermosoftening forming Processes
Injection Moulding Extrusion Blow Moulding Thermoforming Rotational moulding Calendaring
What is Compression Moulding
Polymer is compressed between two parts of the mould (male and female sections) - Heating under pressure initiates polymerization (crosslinking)
-Limited to simple shapes / used for large parts
What is Transfer moulding
Similar to compression moulding
-Polymer is pre heated in a chamber and transferred in a liquid state into a heated mould
- Quicker than compression moulding
- Forms intricate shapes with thick and thin sections as polymer can flow into the mould
