Casting Flashcards
slay the midterm pt. 2
what shape can be casted?
ANY SHAPE!
what are the capabilities/advantages of Casting?
- complexity (any shape)
- net shape (single step process)
- size
- material
What are the 2 types of casts
- expandable mold (can not be reused)
- permanent mold (can be reused)
Disadvantages of casting
- mechanical properties
- poor dimensional accuracy and finish (sand casting)
- shrinkage, porosity, cracks
- permanent mold is expensive
- microstructures hard to control
- not uniform
What are the 6 basic requirements for casting process?
- mold cavity
- melting process
- pouring technique
- solidification process
- mold removal
- cleaning, finishing, inspection operations
What are the two halves of a mold?
- Cope = upper half
- Drag = lower half
What does a riser do?
holds extra material and solidifies last, minimizes shrinkage and creates pressure
what does the down sprue and pouring cup do?
minimizes turbulence (trapped air molecules) and controls flow rate
Purpose of a draft angle
allows the material to be removed from the casting
pouring temperature has to be … (for pure metals)
much higher than melting point, so it doesn’t solidify fast
Local solidification time (for pure metals)
freezing begins to freezing finishes (all at melting point)
Total solidification time (for pure metals)
pouring temperature to freezing temperature finishes
Relation ship between melting points and freeze speed
high melting point freezes first, low melting point freezes last
Do alloys and metals freeze the same?
No, for alloys its a freezing range not point
What happens in alloy segregation
- chill zone, material freezes fast (edges)
- columnar zone, dendrites, has metal with high melting point
- equiaxed zone, center then has very no high melting point metals
what is used to minimize alloy segregation?
cooling rate and nucleating agents, mechanical means; vibration, electromagnetic stirring
(macro easier to minimize than micro)
is it better to have a high or slow cooling rate?
high cooling rate is better because it has a shorter solidification time
what does a slow cooling rate cause?
coarse dendritic structures (large spacing between dendrite arms)
What does a high cooling rate cause?
- small dendrites
- large equiaxed zone
- small grain size result in
- strength and ductility increase
- micro-porosity decreases (shrinkage voids)
- decrease crack on casting
Solidification time, Chorinov’s Rule
TST = Cm(V/A)^n
v= Volume
A= surface area
n=2
Cm= mold constant
Fluidity (fill mold before freezing) is affected by the following 4.
- viscosity
- surface tension (high tension low f)
- inclusion (insoluble)
- freezing range (long range, low f)
How to make sure riser is last to solidify
(V/A)Riser > (V/A)Casting
at what stage does shrinkage happen?
In all stages! when pouring, freezing, and cooling
What is shrinkage allowance?
when making the pattern a little bigger to make the mold bigger.
what is solidification toward in directional solidification?
the riser
What can external chills do?
induce chilling effect creating directional solidification!
velocity of liquid equation
v= sqrt[2gh]
what is volumetric flow rate (Q)
Q = vA
velocity*area
what is time required to fill a mold
T = Mold cavity volume/ volumetric flow rate
T = V/Q
What is aspiration in casting?
as metal goes down sprue and velocity increases, unwanted air goes into the mold
to determine the minimal cross sectional area of the taper at any point what equation is used?
A1/A2 = sqrt[h2/h1]
(assuming pressure on top and bottom are equal)
what is gates in casting?
the passages where the molten material goes into a mold
how should a gate be designed?
- for large parts, multiple gates
- should feed into thick sections of casting
- fillets for where gate and casting meet (less turbulence)
- gate closest to sprue should be far enough to remove
- minimum gate length should be 3 to 5 times the gate diameter, depending on metal being casted
What are the different types of expandable molds?
- sand molds
- shell molds
- vacuum molds
- plaster molds
- investment molds
What are the different reusable molds?
- permanent molds
- die casting
- centrifugal casting
pros and cons of sand casting
pros
- many metals, sizes, shapes, cheap
- high temps
- high volume
cons
- poor finish & tolerance
- automation is not easy
what are casting, sand properties
- cohesiveness (retain shape after packing)
- strength (maintain shape)
- permeability (allow air to escape~ large grain size, bad surface finish)
- refractoriness/ thermal stability (withstand high temps)
- collapsibility (ease of reusing same sand)
- reusability
what is typical mix of sand binders
90% sand, 7% clay, 3% water
silica (siO2)
The 3 types of sand molds
- green sand (sand, clay, water) - contains moisture
- dry sand (organic binders not clay) - baked to make stronger
- skin dried (drying mold cavity surface of a green) - 10 to 25 mm using torches or heat lamp
what is the pattern
full sized model of part (enlarged for shrinkage and machining allowance)
The 3 pattern materials
- wood (moisture, warps)
- metal (expensive , lasts)
- plastic (compromise)
The 4 pattern types
(worst to best)
- solid pattern
- split pattern
- match plate pattern
- cope and drag pattern
what holds a core in place
chaplets and they have a higher melting temp
is sand casting net shape?
no its near net shape because it needs trimming and removal of core (if it has internal structure)
the 2 different types of sand casting
- shell molding (only creating the shell of the moldings pattern,2 halves together, pouring molten, final product) - no porosity, better finish, too expensive
- Vacuum mold - takes time, better finish, expensive, no porosity bc no water is used
Expanded Polystyrene Process
is when u use the polystyrene beads in a die, take out and coat it, then pour molten metal
vary good surface finish, cheap, high volume manufacturing the, porosity problems minimized
Cost of DIE used to make pattern is very expensive
what is investment casting
when a pattern made of wax is coated with a refractory material to make mold, then wax is melted away before pouring molten metal
what is the difference between Expanded Polystyrene Process (lost foam) and Investment casting
polystyrene is using beads (expanding polystyrene) and investment casting is using wax
What type of finish does investment casting give
- no porosity issues
- VERY GOOD SURFACE FINISH
- used for medical tools
- very expensive
- too many steps
What’s the difference between plaster mold vs sand mold
plaster has better finish and accuracy but worst porosity and cant withstand high temps (only used for low melting point materials)
cant use wood patterns b/c too much moisture
pros of ceramic finish?
- good surface finish and porosity
- can with stand high temp
what do u base ur choice on when it comes to sand, plater or ceramic casting?
this is based on the melting point of the material
what material do permanent molds use?
instead of sand the use a metal mold
What happens when u use metal
gives u the wall situation with the chill zone
processes of permanent mold casting
- basic permanent mold process
- die casting
- centrifugal casting
- basic permanent mold process
use metal for 2 sections (easy open and close) mold must with stand the temp
- Die casting
Appling high pressure, throughout processes till it solidifies
- hot chamber (continuous, high production rate, low melting point)
- cold chamber (no continuous source of molten material, low production rate but can be used for high melting point materials)
- centrifugal casting
by rotating mold at high speed, uses centrifugal force to distribute molten material
The 3 groups of inertial forces caused by rotation
- true centrifugal casting (radially symmetric inside is always circular, tubular)
- Semi centrifugal casting (radially symmetric, poor quality in center, density and quality increases in outer)
- centrifuge casting (non radial structures and smaller features)
What are slush castings used for?
used for hollow structures because it doesn’t allow for everything to solidify
The 4 extra General Defects
- Misrun
- Cold shut
- Cold Shot
- Shrinkage cavity
General Defects: Misrun
casting that solidifies before completely filling mold cavity
General Defects: cold shut
2 portions of metal flow togther but lack of fusion due to premature frezzing
General Defects: cold shot
metal splatters during pouring and becomes trapped in casting
General Defects: shrinkage cavity
depression in surface caused by solidification shrinkage that restricts amount of molten metal available in last region to freeze
Foundry inspection methods
- any defects
- does it meet specification (required measurements)
- does it have the metallurgical/chemical/ physical properties
