Centrifugal Casting

Introduction

Centrifugal Casting = Casting process where molten metal is poured into a rotating mold, and centrifugal force distributes the metal

Principle

• Mold rotates at high speed (300-3000 rpm)
• Centrifugal force pushes molten metal outward against mold wall
• Metal solidifies from outside inward
• Denser metal forced to outside, impurities to inside

Types of Centrifugal Casting

1. True Centrifugal Casting

Process:

• Mold rotates about its own axis
• Axis can be horizontal, vertical, or inclined
• No cores needed for hollow parts
• Metal poured while mold rotates

Characteristics:

• Produces hollow cylindrical parts
• Inside surface is free (not controlled by mold)
• Outside surface controlled by mold
• Symmetrical about axis of rotation

Applications:

• Pipes, tubes, cylinders
• Gun barrels, bushings
• Rolls, sleeves, rings

Advantages:

• No central core needed
• Good mechanical properties (dense outer layer)
• Impurities segregate to inside (can be machined away)
• No gating system needed
• High production rate

Limitations:

• Limited to cylindrical/tubular shapes
• Inside diameter not precise
• Inside surface may be rough

2. Semi-Centrifugal Casting

Process:

• Mold rotates about central axis
• Central core or riser used
• Symmetrical parts around vertical axis
• Gating system at center

Characteristics:

• Both inside and outside surfaces controlled by mold
• Better dimensional accuracy than true centrifugal
• Centrifugal force aids feeding and densification

Applications:

• Wheels, pulleys, gears
• Flywheels, impellers
• Symmetrical parts with hub

Advantages:

• Better dimensional control
• Dense outer regions (where strength needed)
• Reduced shrinkage defects
• Central riser feeds effectively

3. Centrifuging

Process:

• Multiple mold cavities arranged around central sprue
• Entire assembly rotates
• Centrifugal force aids mold filling
• Not necessarily symmetrical parts

Characteristics:

• Parts not necessarily symmetrical
• Multiple small parts cast simultaneously
• Gating system at center

Applications:

• Jewelry, dental castings
• Small complex parts
• Investment casting with centrifugal assist

Advantages:

• Multiple parts per cycle
• Better mold filling (thin sections)
• Reduced porosity

Process Parameters

Rotational Speed

Centrifugal Force: G = ω²r / g

Where:

• G = G-force (multiples of gravity)
• ω = Angular velocity (rad/s)
• r = Radius (m)
• g = Gravitational acceleration (9.81 m/s²)

Typical G-forces: 60-80 G for most applications

Speed Calculation: N = √(G × g / r) × (30/π)

Where N = rpm

Effects of Speed:

• Too low: Incomplete filling, porosity, metal draining
• Too high: Mold erosion, segregation, hot tears
• Optimal: Depends on metal, mold size, wall thickness

Pouring Rate

• Must match mold rotation and solidification
• Too fast → turbulence, splashing
• Too slow → premature solidification

Mold Temperature

• Affects solidification rate
• Higher → better surface finish, slower solidification
• Lower → faster solidification, possible cold shuts

Mold Materials

Permanent Molds

Materials: Cast iron, steel, graphite

Advantages:

• Long life (thousands of castings)
• Good dimensional accuracy
• Smooth surface finish
• Economical for mass production

Disadvantages:

• High initial cost
• Limited to simple shapes
• Thermal fatigue

Coatings: Refractory wash to protect mold and improve surface finish

Sand Molds

Used for: Large diameter pipes, one-off castings

Advantages:

• Low cost for large parts
• Flexible

Disadvantages:

• Single use
• Poorer surface finish

Advantages of Centrifugal Casting

1. Superior mechanical properties

   • Dense, fine-grained structure
   • Higher strength in outer regions
   • Directional solidification

2. Elimination of defects

   • Reduced porosity
   • Impurities segregate to inside (removable)
   • No shrinkage cavities in wall

3. Material efficiency

   • No gating system (true centrifugal)
   • Minimal machining required
   • High yield

4. Economical

   • High production rates
   • Low labor cost
   • Minimal material waste

5. Versatility

   • Wide range of metals
   • Various sizes (small to very large)

Limitations of Centrifugal Casting

1. Shape limitations

   • True centrifugal: only cylindrical/tubular
   • Must be symmetrical about rotation axis

2. Inside surface

   • Not precisely controlled (true centrifugal)
   • May require machining
   • Possible roughness

3. Segregation

   • Density segregation (lighter elements to inside)
   • May be undesirable for some alloys

4. Equipment cost

   • Specialized rotating machinery
   • High initial investment

5. Size limitations

   • Very large parts difficult
   • Mold strength limitations

Applications by Industry

Pipes and Tubes:

• Water pipes, sewer pipes
• Oil well casing
• Cylinder liners

Automotive:

• Cylinder liners, brake drums
• Piston rings, bushings

Aerospace:

• Jet engine components
• Rocket motor casings

Industrial:

• Rolls for paper mills, steel mills
• Pressure vessels
• Gun barrels

Marine:

• Propeller hubs
• Stern tubes

Metals Cast by Centrifugal Casting

Ferrous:

• Gray cast iron (most common)
• Ductile iron
• Steel, stainless steel

Non-ferrous:

• Aluminum alloys
• Copper alloys (bronze, brass)
• Nickel alloys

Centrifugal Casting Defects

1. Banding (Segregation)

• Layers of different composition
• Caused by: density differences, excessive speed
• Prevention: Proper speed control, alloy selection

2. Hot Tears

• Cracks during solidification
• Caused by: excessive speed, restrained contraction
• Prevention: Proper speed, mold temperature control

3. Inclusions

• Slag, oxides trapped in casting
• Caused by: turbulent pouring, inadequate skimming
• Prevention: Proper pouring technique, filters

4. Uneven Wall Thickness

• Variation in wall thickness
• Caused by: mold misalignment, uneven rotation
• Prevention: Proper mold setup, balanced rotation

5. Surface Defects

• Rough surface, cracks
• Caused by: mold erosion, improper coating
• Prevention: Proper mold maintenance, coatings

Comparison: Centrifugal vs Static Casting

Design Considerations

1. Wall thickness: Uniform preferred, 5-100 mm typical
2. Length to diameter ratio: Up to 15:1 possible
3. Inside diameter tolerance: ±2-5% (true centrifugal)
4. Outside diameter tolerance: ±1-2%
5. Minimum diameter: ~50 mm
6. Maximum diameter: Several meters possible
7. Draft angles: Minimal (0.5-1°) for permanent molds