casting methods Flashcards
what is thermoforming
manufacturing process in which a polymer sheet is heated and then formed to a specific shape using a mould or former
what is plastic memory
when a sheet is gently heated between 160c and 180c and then bent into shape. if the sheet is help in position while it cools it will remain in its new shape until it is reheated and then it will return to its original shape
what is vacuum forming used for
simple trays and containers
what is the mould made out of in vacuum forming
plywood / MDF
what is the method of press forming
a two part mould is to shape a heated sheet of polymer
when is investment casting used
when a complex component is needed that requires a high degree of accuracy and difficulty
process of investment casting
1 - molten wax injected into metal or rubber die
2 - wax pattern removed from die
3 - wax patterns assembled on a tree with feeding and gating system
4 - tree invested with a refactory slurry
5 - slurry stoccoed with refactory powder
6 - investment dried or chemically set
7 - investment de-waxed in steam autoclave or furnace
8 - ceramic mould fired and pre-heated
9 - metal cast into ceramic mould
10 - ceramic shell removed
11 - components are removed from tree
12 - components are finished
what is sand casting
process in which an impression is made in sand using a pattern of a required product
the pattern is slightly oversided to compensate for the shrinkagew3
what is sand casting commonly used for
machine components
car engine blocks
turbines
disadvantages of basic sand casting
the mould has to be remade for every components manufactured
overall the accuracy of the finished product is relatively poor
when is die casting used
to commercially produce engine components and alloy wheels for cars
what materials are used in die casting
aluminium alloy
zinc
magnesium
copper
process of die casting
molten metal is poured into reusable steel moulds called ‘dies’
what type of metals is die casting used on
non-ferrous metals as the higher temperatures needed for casting steel and iron
what type of metal is not suitable for die casting
zinc-based alloys
what does gravity die casting give the metal
sharp and well-defined casting that requires little or no additional machining
what does pressure die casting utilise
pressure
how does the pressure work in pressure dia casting
pressure is applied to the molten metal as it is fed into the dies using a simple plunger
what does die mean
mould
what does the pressure do in pressure die casting
ensure that the molten metal makes good contact with the walls of the casting dies and gives a sharp and well defined casting
What is investment casting used on
Jewellery
Other small batch items
Eight step process of investment casting
Molten wax injected into metal or rubber die
Wax pattern assemblies on a ‘tree’ with a feeding system
Tree coated and ……. With ceramic slurry, and dried and repeated
Tree fired to melt was and harden the ceramic mould
Ingot are melted in a crucible
Melted metal of a particular volume is ladled into the tree and allowed to cool
The ceramic shell is broken and individual component are fettled (removed)
Components are checked for cracks
Components are finished ie. Polishing
Investment casting disadvantages
Slow due to complex process
High operating costs
High labour costs
Investment casting advantages
High quality surface texture
Moderately high flexibility: ease of pattern production
High atom acronym: wax recycled, little metal in feeding system, but ceramic lost
What is sand casting often used for
Machine components
Car engine blocks
Turbines
Sand casting disadvantages
Mould has to be remade for each component
Time consuming
Labour intensive
Overall accuracy of the finished components is poor
Advantages of sand casting often
Low cost
Sand can be reused
What is sand casting only suitable for when it comes to production
One off / batch production
Process of sand casting
- A pattern is produced from wood that is slightly oversized - may be in two parts with connecting dowels
- Pattern placed on a turnover board and a drag placed …..
- pattern sifted with parting powder
- drag is inverted and turnover board removed - other part of the pattern connected
- cope is placed in position - runner and riser are added - step three repeated
- cope and drag are separated and pattern removed
- molten metal ingots are poured into the assembled mould until metal is at the top of the riser
- final piece coded and removed - shrinks slightly -
Gravity die casting advantages
Fast process
Little post-processing required
Can achieve high detail
Gravity die casting disadvantages
Relies on gravity to work
High setup cost
Can only be used with non-ferrous metals
What metal can only be used in die casting
Aluminium alloy
Zinc alloy
MG
CU
High pressure die casting advantages
Create highly detailed components that cool quickly
Very fast process
Little post processing required
What does high pressure die casting rely on?
High pressure input
High pressure die casting disadvantages
Can only be used with non-ferrous metals
Very high setup costs
What is the die made out of in die casting
Tool / die steel
Die casting method
Metal ingots are melted in a large crucible
A 2 part die is made from die steel
Die is set up so the part is stationary and the other can move
A lubrication agent / parting powder may be applied to the mould
Molten metal is ladled in a known volume into the chamber
High pressure forces the metal into the die - cooled
Comp is pushed out with ejector pins and collected
The ….. is removed and the comp - is checked for cracks / air bubbles
- ground and polish
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