Change of Form Thermal Methods

1
Change of FormThermal Methods

• Chapter 9

2
Casting

• Sand Casting
• Green sand
• Dry sand
• Investment Processes
• Lost-wax
• Shell mold
• Centrifuged

• Permanent Mold Processes
• Gravity feed
• Low-pressure feed
• Hot-chamber die
• Cold-chamber die
• Centrifugal Casting
• Slush Casting
• Continuous Casting

3
Sand Casting Terms

• Pattern
• Mold
• Shrink Rule
• Parting Plane
• Draft angle 20
• Mulling
• Flask
• Cope
• Drag

• Sprue
• Runner
• Parting sand
• Riddled
• Screeded
• Cores
• Vents
• Riser
• Gate

4
Green Sand

• Casting where the sand is dampened with water so
  that it will hold.
• The amount of water is critical.
• Enough to pack
• Too wet no good
• Too dry no good

5
Dry Sand

• Not really dry
• Moisture is not the primary bonding agent
• Usually mulled with oil to act as the bonding
  agent.
• Longer shelf life than green sand
• Reuse is limited

6
Greed vs. Dry comparison

• Green is cheaper and can be reused more readily
• Dry works better for larger castings
• Dry sand works better for finer detail in the
  casting.
• Both will emit steam when pouring the metal.
• The oil in Dry tends to burn off at pouring.

7
Investment Casting Processes

• Investment slurry of refractory material coats
  the pattern.
• Lost Wax
• Dates back 3000 to 5000 years
• Complex shapes
• Pattern of wax
• Coating is set
• Wax is removed by placing in oven to melt.

8
Investment Casting Processes

• Full Mold Casting
• Lost Foam
• Expanded polystyrene
• Evaporative pattern
• Method used by artists to make one of a kind
  works.
• Used for larger objects than Lost wax

9
Plaster Mold Casting

• Plaster of Paris (gypsum) is used for the mold
• Superior finish to sand
• Pattern usually made of metal
• Casting material limited by temperature 1000oF

10
Ceramic Mold Casting

• Variation on the plaster mold
• Allows for higher melt point materials
• Use of a metal pattern gives good finish quality.
• Little or no post casting machining or finishing
  required.
• Used to make dies and molds.
• Known as the Shaw process.

11
Shell Mold Casting

• Investment material is sand with a resin binder.
• Can be used to make larger castings
• Unlimited shelf life for mold

12
Centrifuged Casting

• Not to be confused with Centrifugal casting
• More force is needed to fill all of the voids in
  a mold.
• Can attain 5-Gs
• Very stable dimensions in centrifuged castings.
• Usually very small part are made.

13
Vacuum Casting

• Vacuum is used to draw metal into the mold.

14
Permanent Mold Casting Processes

• Reusable molds
• Used in high volume casting production
• Process cycle see page 178.

15
Types of Permanent Mold Casting Processes

• Gravity Feed
• Low-pressure feed
• Vacuum feed
• Die casting
• Hot chamber
• Cold chamber
• See page 180 181

16
Centrifugal Casting

• The mold is spun and the liquid metal is forced
  to the outside of the mold.
• Cast iron pipe
• Rail car wheels

17
Slush Casting

• Slush material sticks to the sides of the mold.
• Excess material is drained off.
• Good example is ceramic castings, vases, etc.

18
Continuous pour casting

• Continuous casters used in steel mills.

19
Selecting a Casting Process

• What equipment will be available in-house?
• What are the shape-complexity and size of the
  finished part?
• What physical properties must the finished part
  have?
• What surface quality must the finished part have?
• How many parts must be produced, and what
  production rate must be achieved?