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Materials 2 > Casting > Flashcards

Casting Flashcards

1
Q

What are the advantages of casting in general?

A

Complex part geometries
Can create internal and external shapes
Can create very large shapes
Often suited to mass production

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2
Q

What are the disadvantages of casting in general?

A

Limitations on mechanical properties
Poor dimensional accuracy and surface finish for cheaper methods
Molten metals are unsafe
Environmental crap

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3
Q

Where is casting done, and who does it?

A

Foundrymen, in a foundry

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4
Q

How do we account for shrinkage in a mold?

A

We make the mold slightly enlarged, or we use a riser.

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5
Q

What are the general parts of a closed mold?

A

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

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6
Q

What are expendable molds made of? Permanent one?

A

Expendable: sand or plaster + binders
Permanent: metal, or sometimes ceramic

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7
Q

When sand casting, what are cores usually made of?

A

Sand. Go figure.

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8
Q

What are the factor that determine success when pouring the metal into a mold?

A

Temperature
Rate
Turbulence

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9
Q

What are common casting defects?

A

Trapped gas in the part because the mold didn’t have enough porosity
Shrinkage
Hot tear, because the mold was to stiff

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10
Q

What are the two stages of solidification?

A

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

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11
Q

What are the three cooling zones in a cast structure

A

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

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12
Q

What are chemical reactions that can cause problems in casting?

A

Metal oxides forming when molten metal reacts to oxygen

Dross/slag being poured into the mold and affecting surface finish and mechanical properties

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13
Q

How can gas porosity problems be avoided?

A

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)

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14
Q

How do the centers of pure metal parts differ from alloyed metal parts?

A

Pure metals have columnar growth all the way to the center,

Alloys separate in the center

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15
Q

What is Chvorinov’s Rule?

A 
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)
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16
Q

What does the mold constant Cm depend on?

A

Mold material
Thermal properties of casting material
Pouring temp relative to melting point

17
Q

How do we design an effective riser?

A

Use Chvorinov’s Rule to design one that solidifies slower than the casting
Tr = 1.25 Tc (when n = 2)

18
Q

What metal doesn’t shrink?

A

Cast iron with high carbon content

19
Q

What is pattern shrinkage allowance?

A

The amount that a mold is enlarged to allow for shrinkage.

20
Q

How is metal inserted into a sand mold?

A

Gravity flow

21
Q

What are the different pattern materials for sand casting?

A

Wood (cheap but not dimensionally stable)
Metal (expensive but stable and durable)
Hard plastic

22
Q

What are the different types of patterns?

A

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.

23
Q

What are the four requirements for good sand?

A

Refractoriness - able to withstand high temps
Cohesiveness - able to retain shape
Permeability - allows gasses to escape
Collabsibility - Accommodate shrinkage and part removal

24
Q

What determines the properties of sand?

A

Size of particles
Amount of bonding agent
Moisture content
Additives

25
Q

What is green sand?

A

88% silica, 9% clay, 3% water
All grains are evenly coated with additives.
Cheap, but produces rough surface finish, low strength, and poor dimensional accuracy

26
Q

What is the disadvantage of silica sand?

A

Expands at 585C, so it creates surface defects on large flat surfaces, and can trap gasses.

27
Q

What is sand penetration?

A

When the grains become embedded in the part surface.

28
Q

What are the two methods of making a sand mold?

A

Hand ramming - small runs, slow, expensive, nonuniform

Machine - consistency, cheap, large runs

29
Q

What are the physical methods of packing sand into a mold?

A

Slinging
Jolting (whole flask is lifted and dropped to compact sand)
Squeezing

30
Q

What is dry sand?

A

Dry sand. Go figure.

Long storage life, but takes long time to dry.

31
Q

What is a skin dried mold?

A

Torches used to dry only the sand next to the cavity. Used for large steel parts. Binders used to enhance strength of skin dried layer.

32
Q

What is sodium silicate used for?

A

Remains soft and moldable until exposed to CO2. Better shaping.
But less collapsibility and permeability.
Heating makes it stronger.

33
Q

Describe no bake molding.

A

Resin binders are mixed with the sand, so curing begins immediately. Is more expensive, but has high dimensional precision and good surface finish.

34
Q

Describe no bake sands.

A

Are compacted by light vibrations.
Good hot strength.
High resistance to mold defects.
Good collapsibility.

35
Q

Describe shell molding.

A

A mold of created from cured sand, and then the cope and drag are made of rough sand, gravel, etc.
Very expensive.
High level of precision, smooth surfaces.

36
Q

Describe vacuum molding.

A

Vacuum serves as dry sand binder. Eliminates moisture defects and binder costs. Sand is reusable. Better surface finish and collapsibility.
Slow.
Small to medium runs only.

37
Q

What are the different coring methods when using sand?

A

Dump-core box : Baked sand is packed into mold cavity
Single piece cores : core box is close around the core
Core-oil process : Sand is blended with oil, then rammed into simple core box
Hot-Box : Sand is blended with a thermosetting binder
Cold-Box : Gas or catalyst polymerizes the resin added to sand

38
Q

What is a common sand alternative for expendable molds?

A

Plaster of paris
Antioch Process - 1/2 plaster, 1/2 sand
Ceramics -higher temps

39
Q

Describe investment casting.

A

A wax pattern is created, then a mold is created around it, the wax is melted out, and the actual part is then cast.
Used for complex shapes.

Materials 2 (12 decks)

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