Casting Flashcards
What are the advantages of casting in general?
Complex part geometries
Can create internal and external shapes
Can create very large shapes
Often suited to mass production
What are the disadvantages of casting in general?
Limitations on mechanical properties
Poor dimensional accuracy and surface finish for cheaper methods
Molten metals are unsafe
Environmental crap
Where is casting done, and who does it?
Foundrymen, in a foundry
How do we account for shrinkage in a mold?
We make the mold slightly enlarged, or we use a riser.
What are the general parts of a closed mold?
The flask holds it together
The drag is the bottom
The cope is the top
The parting line is where cope and drag meet
The cavity is where the part is made
The pattern creates the cavity
The core is an insert for interior geometry
The riser is the added block of molten material to prevent shrinkage
The pouring cup, downsprue, and runner provide the molten material
What are expendable molds made of? Permanent one?
Expendable: sand or plaster + binders
Permanent: metal, or sometimes ceramic
When sand casting, what are cores usually made of?
Sand. Go figure.
What are the factor that determine success when pouring the metal into a mold?
Temperature
Rate
Turbulence
What are common casting defects?
Trapped gas in the part because the mold didn’t have enough porosity
Shrinkage
Hot tear, because the mold was to stiff
What are the two stages of solidification?
Nucleation - solid particles forming from liquid. Undercooling is the difference between Tm and Tnucleation.
Growth - Grains grow when the heat of fusion is removed from the liquid
What are the three cooling zones in a cast structure
Chill Zone - Rapid small nucleation events at the surface of the cavity (randomly oriented)
Columnar Zone - Rapid long thin growth perpendicular to the casting surface
Exquiaxed Zone - Spherical, randomly oriented crystals in the center of the casting
What are chemical reactions that can cause problems in casting?
Metal oxides forming when molten metal reacts to oxygen
Dross/slag being poured into the mold and affecting surface finish and mechanical properties
How can gas porosity problems be avoided?
Prevent gasses form being dissolved in liquid by melting in a vacuum, minimizing turbulence, working around low-solubility gasses, or passing inert gasses through liquid metal (gas flushing)
How do the centers of pure metal parts differ from alloyed metal parts?
Pure metals have columnar growth all the way to the center,
Alloys separate in the center
What is Chvorinov’s Rule?
Total Solidification time Tts = Cm(V/A)^n Where Cm is the mold constant V is the volume A is the surface area of the casting n is an exponent value (often 2)
What does the mold constant Cm depend on?
Mold material
Thermal properties of casting material
Pouring temp relative to melting point
How do we design an effective riser?
Use Chvorinov’s Rule to design one that solidifies slower than the casting
Tr = 1.25 Tc (when n = 2)
What metal doesn’t shrink?
Cast iron with high carbon content
What is pattern shrinkage allowance?
The amount that a mold is enlarged to allow for shrinkage.
How is metal inserted into a sand mold?
Gravity flow
What are the different pattern materials for sand casting?
Wood (cheap but not dimensionally stable)
Metal (expensive but stable and durable)
Hard plastic
What are the different types of patterns?
One piece (solid) patterns - used for simple patterns and small runs
Split patterns (divided into two segments) - used for larger runs
Match plate patterns - cope and drag are attached to each other, and pins ensure alignment.
Cope and drag patterns - have different patterns in the cope and the drag to make multiple parts.
What are the four requirements for good sand?
Refractoriness - able to withstand high temps
Cohesiveness - able to retain shape
Permeability - allows gasses to escape
Collabsibility - Accommodate shrinkage and part removal
What determines the properties of sand?
Size of particles
Amount of bonding agent
Moisture content
Additives
