Casting Flashcards
What is casting?
Solid material heated to a molten state, then poured into a mold of desired shape to cool
What are the advantages to casting?
Casts save material while creating complex shapes of various sizes
What is a foundry?
Factory that makes molds, handles molten metal, casts, and cleans
What are the steps to casting?
- Make mold
- Melt metal
- pour metal
- let metal freeze
- remove from mold
Can castings make internal shapes?
Yes
What are some disadvantages to casting?
Limitation on mechanical properties, defects during freezing, poor dimensional accuracy, non-uniform cooling
What are some casted parts?
Engine blocks, car heads, machine frames, dental crowns, jewelry, frying pans
What types of metals can be cast?
Both ferrous and nonferrous
What is an expendable mold process?
The mold used must be destroyed to remove cast, usually made of sand/plaster with binders
What is a permanent mold process?
The mold can be reused after casting, usually made of metals or refractory ceramics
What are the advantages to expendable mold processes?
Geometry can be more intricate on account that the mold is destroyed
What are the advantages to permanent mold processes?
Money is saved on remaking molds
What is the cope?
The upper half of the mold
What is the drag?
The lower half of the mold
What is the parting line?
The line where the two halves separate
How do foundrymen work around shrinkage when casting?
Making the mold cavity slightly larger than the actual size.
How is the mold created?
The material the mold is made of is packed around a pattern, which determines the shape of the part
What is a core used for?
Cores are placed within the mold cavity to define the interior geometry of a part
What is a pouring cup?
The area of a closed mold where metal is poured to minimize splash and turbulence, leads to down sprue.
What is a down sprue?
Funnel type part of mold where metal is lead to the runner
What is the runner?
Area where metal flows before reaching the gate to the cast cavity
What is the riser?
Area to store overflow of metals while metal is still flowing into the cavity, helps prevent shrinkage and maintain fluid pressure
How do you find the heat required for casting?
The sum of the heat to raise temperature to melting point, then for fusion from solid to liquid, then heat to raise molten metal to desired temperature
What factors affect whether the metal flows to all parts of the cavity?
Starting temperature, pour rate, and turbulence
Why does metal have to be poured with no turbulence?
Once the metal hits the mold, a thin layer of solid metal immediately forms due to the difference in temperature with the walls. If the metals are poured with turbulence, the gate may be blocked off by this thin shell
What is dendritic growth?
When metal grains grow in the direction opposite to heat transfer from mold (heat transfer out dendrites grow in)
What direction do columnar grains grow in?
Towards center of mold due to dendritic growth.
Where are the randomly oriented grains located for pure metals?
On the surface
Do alloys follow a linear cooling curve?
Since alloys contain multiple metals, some parts change phases at different temperatures
What is the equiaxed zone?
Zone in the middle of a casted alloy containing the metal with a lower melting point
What effect does a slow cooling rate have on a cast?
Courser dendritic structures due to larger spacings between dendrite arms
What effect does a high cooling rate have on a cast?
Smaller dendrites with a larger equiaxed zone.
What effect does smaller grain size do to a cast?
Higher strength and ductility, microporosity decreases, and the tendency for the cast to crack decreases.
What is a micro-segregation-alloy?
When the alloy solidifies, the dendrite arms are primarily made of the material with a higher melting point, as that solidifies first. This segregates the lower melting point material in between the arms of the higher.
What are the different types of macro-segregation
Normal, inverse, and due to gravity
How does gravity cause macro-segregation
The density of the components of the alloy may cause separation
What does normal segregation mean?
The lower melting point component of a solidifying alloy is driven to the center; high concentration of alloying element at centre
What does inverse segregation mean?
Low concentration of alloying element at center
How to reduce segregation?
Creating more grains manually, either through vibration or stirring, or by using nucleation agents
What is fluidity?
Measure of the capability of a metal to flow into and fill into a mold before freezing
What is viscosity?
Resistance to flow
How does surface tension affect fluidity?
Higher tension means less fluidity
What is inclusion?
Insoluble particles included in the liquid alloy
How does viscosity affect fluidity?
Higher viscosity means lower fluidity
How does inclusion affect fluidity?
Negatively (creates points from which solids can grow from)
How does freezing range affect fluidity?
The shorter the freezing range, the higher the fluidity
Why do iron casts with high carbon content negate shrinkage?
Carbon undergoes graphitization, which expands to counteract the phase change shrinkage
How to achieve directional solidification?
Either by using chills, or by placing faster freezing regions further away from risers
What is an external chill?
Materials that induce faster freezing placed in mold at certain areas
What happens to excess metal left in the riser?
The metal is melted down and reused
Why do porosities form in castings?
Either from gases getting trapped due to high solubility in liquid metal, but low in solid, or shrinkage caused voids
What is aspiration?
Air being pushed into runner by metal proceeding down the sprue at higher speeds
What is ingot casting?
Molten metal is poured from a ladle into ingot molds
What is bottom pouring?
Molten metal is sent into an open mold from the bottom
What are soaking pits?
Areas that store large amounts of water that are used for cooling frozen ingots
What is an ingot?
Uniform slabs of metal (rectangular or circular) that are used in other metal making processes
What is a tundish?
An intermediate pouring dish right after the ladle, holds 3 tons of metal
What is a dummy bar?
Used in continuous casting process, is base where metal from tundish is poured, constantly lowered at the pour rate of the metal
What system lowers the dummy bar?
Roller-pinch rolls
How is friction between the dummy bar and mold reduced?
The mold is coated with either lubricants or graphite
What types of sand go into sand molds?
Green, CO2 infused, and resin bonded
What are shell molds made of?
Resin bonded sand
What are vacuum molds made of?
Dry sand
What are some expendable molds?
Sand, shell, vacuum, plaster, and investment
What are some reusable molds?
Permanent molds (graphite/cast iron), die casting (steel), centrifugal
What are the advantages to sand molds?
Cheap, can take many metals and shapes
What are the disadvantages to sand molds?
Poor tolerances and finish
What are the advantages to shell molds?
higher accuracy, finish, and higher production rate
What are the disadvantages to shell molds?
Limited part size
What are the advantages to plaster molds?
Complex shapes, good surface finish
What are the disadvantages to plaster molds?
Must have non-ferrous metals, and low production rate
What are the advantages to investment molds?
More complex shapes, excellent finish
What are the disadvantages to investment molds?
Small parts, expensive
What are the advantages to permanent molds?
Good finish, low porosity, high production rate
What are the disadvantages to permanent molds?
costly, simpler shapes only
What are the advantages to die-cast molds?
High dimensional accuracy, high production rate
What are the disadvantages to die-cast molds?
costly dies, small parts only, non ferrous metals only
What are the advantages to centrifugal molds?
Good for large cylindrical parts, good quality
What are the disadvantages to centrifugal molds?
Expensive, very few shapes that can be made
Which type of cast is the most commonly used?
Sand casting
Why is sand casting most common?
Packing allows it to keep shape, has good permeability, thermal stability, gives way after casting easily, and sand from cast can be reused
What kind of sand is used in foundries?
Silica (SiO2)
What binders are used in green sand molds?
Bonding clay and water, with a 7% and 3% of the total mix respectively
What binder is used in a dry sand mold?
Organic binder + baking
What is a skin-dried mold?
Green sand mold that has top layer dried (10-25mm)
What is a pattern?
Full sized model of the part, used to imprint onto mold
What is a solid pattern?
A single piece is used to make the mold, thus more difficult to find partition line
What is a split pattern?
Pattern is split into two pieces, so partition line is predetermined
What is a match-plate pattern?
A pattern attached to a solid plate, allowing for alignment of cope and drag
What is a cope and drag pattern?
Same as match-plate, but cope pattern has the riser while drag has the gate
What is trimming?
The post solidification process of removing any excess metal
How are cores removed from the cast?
The core will either be shook out or dissolved
What can be done post-casting to enhance some properties of the cast?
Heat treating
What is a machining allowance?
Extra material left from sand casting to allow for machining
What is sand blow?
A defect common to sand casts, a hole appearing near/below the cast surface
What causes sand blow?
Low permeability, poor venting & higher moisture content of the sand mold
What are pin holes?
Many small gas cavities at or slightly below surface of casting
What causes pin holes?
The release of gases during pouring
What is penetration?
When higher fluidity liquid metal breaches the mold borders
What is mold shift?
A defect where the cope and drag are misaligned
What is macroporocity?
Porosity by shrinkage
What is microporosity?
When the liquid metal solidifies and shrinks between dendrites
How is microporosity removed?
Either by flushing, pouring with inert gas, or by melting and pouring in more metal in vacuum, or deoxidizing
How is a shell mold formed?
The pattern is heated above a dump box filled with a resin sand mix. The box is then flipped so that the mixture lands on the pattern and forms a shell. The shell is then cured and stripped from pattern
What is vacuum molding?
The sand mold is held together by vacuum pressure
What is expanded polystyrene process?
Uses a styrofoam pattern that stays in the mold, immediately vaporized upon pouring
What is investment casting?
The pattern is made from wax, used to shape the mold, and melted away before pouring
What is plaster mold casting?
Sand casting but plaster not mold
What is ceramic mold casting?
Sand and plaster mold but ceramic
What is permanent mold casting?
Cast iron is used instead of sand for casting.
What are some specifications for lost foam (expanded polystyrene) processes?
2.5 mm minimum wall thickness
.3% tolerance on dimensions
2.5 micrometer surface finish
400g - several tons for size limit
What are some specifications for shell mold processes?
1.5-6.25 minimum wall thickness (material dependent)
0.5% dimension tolerance
1.25 - 3.75 micrometer surface finish
30g to 12kg size limit
What are some specifications for plaster mold processes?
0.6mm minimum wall thickness (material dependent)
0.2% dimension tolerance
1.25 - 3 micrometer surface finish
30g to 7kg size limit
What are some specifications for ceramic mold processes?
1.25mm minimum wall thickness (material dependent)
0.4% dimension tolerance
2 - 4 micrometer surface finish
60g to 1 ton size limit
What are some specifications for permanent mold processes?
3mm minimum wall thickness (material dependent)
2% dimension tolerance
2.5 - 7.5 micrometer surface finish
50g to 70kg size limit
What is die casting?
Permanent mold casting where molten metal is injecting into the mold cavity under high pressure, which is maintained during solidification
What metals are used for the die mold?
Tool steel, mold steel, or maraging steel for steels, tungsten and molybdenum
How is the cast removed from the mold?
Ejector pins and lubricants
At what pressure range are metals injected into the cast?
.7 - 700 MPa
What is hot chamber die casting?
The molten metal is pushed into the cast via piston, which then retracts after the casted metal is solidified, drawing back any liquid metal
What is production rate?
How many parts are being produced per hour
What metals does hot-chamber die casting cover?
Zinc, tin, lead, and magnesium
What is cold-chamber die casting?
The die is closed and a ram pushes molten metal from the shot chamber to the die. The ram holds pressure until part fully solidifies
What is centrifugal casting?
The mold is rotated at high speeds such that centrifugal force distributes molten metal to outer regions of die cavity
What is true centrifugal casting?
Mold is rotated about axis at high speeds to produce tubular pieces
What speeds are molds rotated at in true centrifugal casting?
300-3000 rpm
What is semi centrifugal casting?
The centrifugal force is used to produce solid casts
Where are the risers located in semi centrifugal casts?
In the middle
What are some specifications for centrifugal mold processes?
2.5 - 125mm minimum wall thickness (material dependent)
2.5 mm Outer diameter
3.8 mm inner diameter
2.5 - 12.5 mm surface finish
50g to 70kg size limit
What is a misrun defect?
Casting solidifies before completely filling the mold
What is a cold shut defect?
Two portions of metal flow together but freeze prematurely, leaving gaps
What is a cold shot defect?
Metal splatters during pouring and form solid globules that become entrapped during casting
What is a shrinkage cavity defect?
A depression/internal void caused by solidification shrinkage that restricts amount of molten metal available in last region to freeze
