Casting Flashcards
advantages and disadvantages of casting over machining
advantages:
- single-step process
- complex geometries are possible
- low cost per component
disadvantages:
- rough surface finish
- poor accuracy
- low strength
- high setup and energy cost
describe sand casting
the pouring of molten metal into a sand mold where it solidifies
how is the sand mould created
the sand is compressed into a cuboid frame split in two (cope & drag) where half a pattern is pressed into it leaving an indentation where the metal will be later poured
components of sand casting
cope & drag = top and bottom layer of sand mold
pouring cop = hole left for metal to be poured through
runner = tunnel connecting the end of the sprue to the riser
riser = liquid metal reservoir, for when the metal shrink, helps slow cooling
gate = to control the rate of flow into the main mould
core = for creating hollow parts in the mould
core print = there to support core
what is the shrinkage rule
allow an error margin for you mould as when the metal cools it will shrink
what materials can be used for the pattern in sand casting
- metal (expensive, robust)
- wood (absorbs water/cracks
- plastic (made for 3D printing)
what is pit moulding
a variation of sand casting, used for producing very large components. A “pit” is dug, and used to form the mould
what are some design considerations when casting
- reducing the need for cores to improve casting strength
- harp corners and angles should be avoided, they are sources of stress concentrations, which may cause hot tearing and cracks in the casting
- changes in thickness should be gradual, as thicker sections will cool more slowly leading to deformation
Expendable Mould Casting Methods
- Shell Moulding
metal pattern is heated, dipped into resin to form thin shell, the two shells are then used as moulds
- Vacuum moulding (V-Process)
a plastic sheet and vacuum are used to maintain the mould shape during casting
- Freeze moulding
Moulds are made from sand, clay and water sprayed with liquid nitrogen forming ice
- Full Moulding (Evaporative Casting)
Polystyrene foam patterns, during pouring the pattern decomposes
- Investment Casting (Lost Wax Process)
a wax pattern produced, the pattern is coated with plaster, the plaster is allowed to harden. The mould is baked melting the wax, the metal is poured into the mould. then plaster is removed with high vibrations.
- Shaw Process
Reusable Mould Casting Methods
- Rubber Mould Casting
Used to cast low melting point materials, Wax, Plastic, metals (70°)
- Gravity Die Casting
- Die casting (Pressure Die Casting)
High pressure is applied throughout solidification, Mould temperature can be lower
- Slush Casting
produces hollow cast without needing cores
- Centrifugal Casting
casting made by spinning molten metal
what is a draft in casting
a slight taper or angle that is added to the vertical walls of a mold or pattern to prevent the cast or molded part from getting stuck
Casting process used for making tube/pipes
Centrifugal casting
Casting process used for making jewellery
Centrifuge casting
Casting process used for making parts with no parting line
Full moulding
Investment casting
Casting process used for casting metal alloys (steel)
gravity die casting
(graphite mould use)
