Classification of Casting Defects
1. Metallic Projections
- Metal extends beyond intended boundaries
- Includes: fins, flash, swells, rough surfaces
2. Cavities
- Voids or holes in casting
- Includes: shrinkage cavities, gas porosity, blowholes
3. Discontinuities
- Interruptions in metal continuity
- Includes: cracks, cold shuts, misruns
4. Defective Surface
- Surface irregularities
- Includes: surface roughness, cuts, washes
5. Incomplete Casting
- Missing sections
- Includes: misruns, insufficient metal
6. Incorrect Dimensions
- Dimensional inaccuracies
- Includes: warping, distortion, wrong size
7. Inclusions
- Foreign materials trapped
- Includes: sand, slag, dross, oxides
Major Casting Defects
1. Gas Porosity (Blowholes, Pinholes)
Description:
- Spherical or elongated cavities
- Caused by trapped gases
- Can be surface or subsurface
Causes:
- Gas absorption: H₂, N₂, O₂ dissolved in molten metal
- Mold gases: Moisture in sand, organic binders decomposing
- Poor venting: Inadequate mold permeability
- Turbulent pouring: Air entrapment
- High pouring temperature: Increased gas solubility
Prevention:
- Proper degassing of molten metal
- Dry molds (low moisture content)
- Adequate mold permeability
- Proper venting
- Avoid turbulent pouring
- Use of vacuum or inert atmosphere
- Lower pouring temperature (within limits)
2. Shrinkage Defects
a) Shrinkage Cavity (Pipe)
Description:
- Large void, typically at top center
- Conical or cylindrical shape
- Open to surface or internal
Causes:
- Inadequate feeding during solidification
- Improper riser design
- Isolated hot spots
- Lack of directional solidification
Prevention:
- Proper riser design and placement
- Adequate riser size (modulus > casting modulus)
- Use of chills for directional solidification
- Padding and insulation
- Multiple risers for complex castings
b) Shrinkage Porosity (Microshrinkage)
Description:
- Distributed small voids
- Spongy appearance
- In mushy zone regions
Causes:
- Inadequate feeding in mushy zone
- Wide solidification range (long-freezing alloys)
- Isolated liquid pockets
Prevention:
- Proper feeding
- Directional solidification
- Pressure during solidification
- Alloy modification
3. Hot Tears (Hot Cracks)
Description:
- Irregular cracks with oxidized surfaces
- Occur during final solidification
- Typically at stress concentration points
Causes:
- Thermal stresses during cooling
- Restrained contraction
- Weak grain boundaries at high temperature
- Sharp corners and abrupt section changes
- Rigid cores or molds
Prevention:
- Avoid sharp corners (use fillets)
- Uniform section thickness
- Collapsible cores
- Proper mold design (allow contraction)
- Lower pouring temperature
- Alloy modification (grain refiners)
4. Cold Shuts
Description:
- Discontinuity where two metal streams meet but don't fuse
- Appears as a crack or seam
- Oxide layer between streams
Causes:
- Low pouring temperature
- Slow pouring rate
- Thin sections
- Poor fluidity
- Premature solidification
Prevention:
- Adequate pouring temperature
- Proper pouring rate
- Improved gating design
- Avoid thin sections
- Preheat mold (if appropriate)
5. Misruns
Description:
- Incomplete casting
- Mold cavity not completely filled
- Metal solidified before filling complete
Causes:
- Low pouring temperature
- Insufficient fluidity
- Slow pouring
- Thin sections
- Inadequate gating system
- Mold too cold
Prevention:
- Increase pouring temperature
- Faster pouring rate
- Improve gating design
- Increase section thickness
- Preheat mold
6. Sand Inclusions (Sand Defects)
Description:
- Sand particles embedded in casting surface
- Rough surface texture
- Can be surface or subsurface
Causes:
- Erosion: High velocity metal erodes mold
- Mold wash: Sand washed into cavity
- Loose sand: Poor ramming, weak sand
- Mold damage: Careless handling
Prevention:
- Proper sand strength and hardness
- Adequate ramming
- Proper gating design (reduce velocity)
- Careful mold handling
- Use of mold coatings
- Filters in gating system
7. Slag Inclusions
Description:
- Non-metallic inclusions (oxides, slag)
- Irregular shape
- Lighter than metal
Causes:
- Inadequate skimming of molten metal
- Poor gating design (doesn't trap slag)
- Turbulent pouring
- Oxidation during pouring
Prevention:
- Proper skimming before pouring
- Slag traps in gating system
- Filters
- Smooth, non-turbulent pouring
- Protective atmosphere
8. Penetration (Metal Penetration)
Description:
- Metal penetrates into sand mold
- Rough, sandy surface
- Difficult to clean
Causes:
- High pouring temperature
- Low sand refractoriness
- Coarse sand grains
- High metal pressure (thick sections)
- Inadequate mold coating
Prevention:
- Lower pouring temperature
- Finer sand
- Proper mold coatings (refractory wash)
- Higher clay content in sand
- Adequate ramming
9. Cuts and Washes
Description:
- Irregular surface with sand mixed with metal
- Erosion of mold surface
Causes:
- High velocity metal flow
- Direct impingement on mold surface
- Weak mold
- Poor gating design
Prevention:
- Proper gating design (avoid direct impingement)
- Reduce metal velocity
- Stronger mold (better ramming)
- Mold coatings
10. Swells and Drops
Swell:
- Enlargement of casting due to mold wall movement
- Caused by: insufficient ramming, high metal pressure
Drop:
- Sand from cope falls into mold
- Caused by: weak cope, inadequate support
Prevention:
- Proper ramming
- Adequate mold strength
- Proper cope support (chaplets if needed)
11. Shifts and Mismatch
Description:
- Misalignment between cope and drag
- Step at parting line
Causes:
- Improper mold assembly
- Worn or damaged pins
- Shifting during handling or pouring
- Unequal mold hardness
Prevention:
- Careful mold assembly
- Proper alignment pins
- Secure clamping
- Careful handling
12. Warping and Distortion
Description:
- Dimensional changes from intended shape
- Bending, twisting
Causes:
- Non-uniform cooling
- Residual stresses
- Thin sections
- Restrained contraction
- Premature shakeout
Prevention:
- Uniform section thickness
- Proper cooling rate
- Delayed shakeout
- Stress relief heat treatment
- Proper pattern design (distortion allowance)
13. Cracks (Cold Cracks)
Description:
- Cracks formed after solidification
- Clean fracture surfaces (not oxidized)
Causes:
- Thermal stresses during cooling
- Residual stresses
- Brittle material
- Sharp corners
- Premature shakeout
Prevention:
- Gradual cooling
- Delayed shakeout
- Avoid sharp corners
- Stress relief annealing
- Proper alloy selection
14. Rat Tails and Buckles
Description:
- Long, shallow depressions or folds on surface
- V-shaped or wavy
Causes:
- Expansion of sand mold surface
- Insufficient expansion room
- High moisture content
- Fine sand
Prevention:
- Proper sand composition
- Adequate expansion allowance
- Lower moisture content
- Coarser sand
Defect Detection Methods
1. Visual Inspection
- Surface defects
- Dimensional checks
- Simple and quick
2. Dimensional Inspection
- Calipers, micrometers
- CMM (Coordinate Measuring Machine)
- Verify dimensions and tolerances
3. Radiographic Testing (X-ray, Gamma-ray)
- Internal defects (porosity, cracks, inclusions)
- Non-destructive
- Permanent record
4. Ultrasonic Testing
- Internal defects
- Thickness measurement
- Non-destructive
5. Magnetic Particle Testing
- Surface and near-surface cracks
- Ferromagnetic materials only
- Non-destructive
6. Liquid Penetrant Testing
- Surface-breaking defects
- All materials
- Non-destructive
7. Pressure Testing
- Leak detection
- For pressure vessels, pipes
- May be destructive
8. Destructive Testing
- Sectioning and examination
- Mechanical testing
- Metallographic examination
Summary: Defect Prevention
Mold Design:
- Proper gating and risering
- Adequate venting
- Appropriate mold material
Metal Quality:
- Proper degassing
- Clean metal (skimming)
- Correct composition
Process Control:
- Proper pouring temperature
- Controlled pouring rate
- Adequate superheat
Pattern Design:
- Avoid sharp corners
- Uniform sections
- Proper allowances
Sand Quality:
- Proper strength and permeability
- Correct moisture content
- Adequate refractoriness