1. Casting

2. Casting Among the oldest of manufacturing methods (approx 4000 BC)
Casting process involves pouring molten metal into a mold
Allowed to cool/solidify
Takes shape of mold
Removed from mold
Quickest way to shape metal into a part

3. Casting
Capable of economically producing parts which are large, hollow, and/or intricate in a single piece
Used for prototypes and production parts
Examples of Cast Products
Engine blocks
Crank shafts
Railroad wheels
Transmission housing

4. Basic Casting Process Molten metal poured through a pouring basin
Flows through sprue, runners, and gates into mold cavity
Riser serve as reservoir of molten metal to prevent shrinkage
Vents exhaust air & gas from cavity
Pouring
Basin
Sprue
Vent
Riser
Runner
Gate
Mold Cavity

5. Casting Process Classifications
Expendable Mold/Resuable Pattern
Expendable Mold/Expendable Pattern
Permanent Mold/No Pattern

6. Casting Process Classifications
Expendable Mold/Resuable Pattern
Sand Casting
Plaster-Mold Casting
Ceramic-Mold Casting
Reusable Pattern Materials
Wood
Plastic
metal

7. Casting Process Classifications
Expendable Mold/Expendable Pattern
Investment Casting
Evaporative-Foam Casting
Expendable Pattern Materials
Wax
Plastic
Polystyrene Foam

8. Casting Process Classifications
Permanent Mold/No Pattern
Permanent Mold Casting
Die Casting
Centrifugal Casting

9. Casting Important Factors in Casting Operation These Factors Effect
Flow of molten metal into the mold cavity
Solidification and cooling of metal
Type of mold material
These Factors Effect
Strength
Dimensional accuracy
Surface finish

10. Solidification
During solidification process a series of events occur which affect
Size & shape of grains formed
Influences overall properties
In general casting results in reasonably uniform grain structure
Some difference between
Pure metals
alloys

11. Solidification – Pure Metals
Solidifies at constant temperature
Must give off latent heat of fusion before phase change
Rapid cooling at mold edge
Skin/shell – fine grains
Slower to middle – columnar grains
Like water to ice

12. Solidification - Alloys
Solidify with a temperature range
Produces a mushy or pasty state
Cooling rate effects
Slow – coarse structures/grains
Fast – fine structures/grains
In general as grain size decreases
Strength and ductility increase
Shrinkage voids decrease
Cracking during solidification decreases

13. Fluidity of Molten Metals
Fluidity – capability of molten metal to fill mold cavity
Two basic factors
Characteristics of molten metal
Casting parameters

14. Characteristics of Molten Metal
Viscosity
How runny is it when hot
Surface Tension
Development of film

15. Casting Parameters Mold Design Mold Material Rate of Pouring
Risers, runners, gates, etc.
Mold Material
Thermal conductivity
Roughness of its surface
Rate of Pouring
Degree of superheating
How far above melting point

16. Solidification Time
Is a function of the volume of the casting and its surface area
Where
C – constant (mold material, metal property, temperature)
n – value between 1.5-2, typically 2
Larger surface area equals shorter cooling time

17. Solidification Time Example
Given:
3 parts – same volume, material
Sphere, cube, cylinder
Find:
Relative solidification time of the parts

18. Solidification Time Example
Sphere Formulas
Cylinder Formulas
Cube Formulas

19. Shrinkage Most metals shrink during solidification & cooling process
Causes dimension changes and sometimes cracking
Molten metal contracts as it cools prior to solidification
Metal contracts during solidification process
Metal contracts further as it cools to room temperature

20. Percent Contraction (-)
Shrinkage
Metal
Percent Contraction (-)
Expansion(+)
Aluminum
Zinc
Gold
Copper
Brass
Carbon Steel
Lead
Gray Cast Iron
-7.1%
-6.5%
-5.5%
-4.9%
-4.5%
-2.5-4%
-3.2%
+2.5%

21. Challenges with Casting
Several defects can develop in castings
Most can be avoided with proper design and processing techniques

22. Challenges with Casting
Metallic Projections - fins, flash, rough surface
Too high pressure
Improper mating mold pieces
Cavities - pockets caused by shrinkage or gases
Can be controlled by adding flux
Discontinuities – includes cracks, cold/hot tearing, and cold shunts
Constrained cooling
Molten metal too low temperature

23. Challenges with Casting
Defective Surface – scars, adhering sand layers, oxide scale
Design of gate may improve
Incomplete Casting – premature solidification
Insufficient volume of metal poured
Incorrect Dimensions/Shape – improper shrinkage allowance, warped casting, etc.
Inclusions – form during melting, solidification, and molding
Usually a result of chemical reactions
Reduce strength of casting
Can/should be filtered out

24. Sand Casting Most prevalent form of casting
15 million tons of metal cast by this method annually in the US
Typical sand casting applications
Machine bases
Large turbine impellers
Propellers
Plumbing fixtures
Agricultural and railroad equipment components

25. Sand Casting
Utilizes gravity to feed molten metal into a non-resuseable mold
Sand contains binding materials
Requires a reuseable mold pattern
Produces a parting line on the workpiece
Requires drafts and fillets on pattern
Produces rough textured surfaces
Sprues, risers, and runners must be removed

26. Green Sand Casting Pouring Basin Riser Cope Sprue Runner Drag
Mold Cavity
Gate

27. Green Sand Casting

28. Green Sand Casting Wall Thickness Casting Weight
Typical in.
Feasible in.
Casting Weight
Typical 1 – 50 lb.
Feasible few oz. – several hundred lb.

29. Green Sand Casting Tolerances Surface Finish Typical ±0.125 in.
Feasible ± in.
Surface Finish
Typical μin.
Feasible μin.

30. Factors Affecting Tolerance and Surface Finish
Accuracy of pattern
Dimensional stability of pattern
Casting shrinkage
Pattern smoothness
Pattern wear
Sand compaction
Dimensional stability of casting alloy
Gating and rising system

31. Pattern Type Matchplate – large quantities of small castings
Split – used for large castings
Loose – loose patterns of irregular parting line

32. Design Considerations

33. Other Considerations Draft angle Shrinkage Allowance
Facilitate the removal of the part from the mold
Typically 1º-5º
Shrinkage Allowance
Added to the pattern size so the desired tolerances can be maintained on the part