Introduction to Casting

What is Casting?

Casting = Manufacturing process where molten metal is poured into a mold cavity and allowed to solidify into the desired shape

Basic Steps

1. Pattern making - Create pattern (replica of final part)
2. Mold making - Create mold cavity using pattern
3. Melting - Melt metal to liquid state
4. Pouring - Pour molten metal into mold
5. Solidification - Allow metal to cool and solidify
6. Shakeout - Remove casting from mold
7. Cleaning - Remove excess material, finish surface

Advantages of Casting

1. Complex shapes - Intricate internal and external geometries possible
2. Size range - From few grams to several tons
3. Material versatility - Most metals can be cast
4. Near-net shape - Minimal machining required
5. Economical - For complex parts and mass production
6. Hollow sections - Internal cavities easily created using cores
7. Material properties - Can cast materials difficult to machine or form

Limitations of Casting

1. Porosity - Gas porosity, shrinkage porosity possible
2. Surface finish - Generally poorer than machining
3. Dimensional accuracy - Lower than machining or forming
4. Mechanical properties - May be inferior to wrought products (grain structure)
5. Defects - Various casting defects possible
6. Tooling cost - Pattern and mold costs for low volumes

Classification of Casting Processes

Based on Mold Type

1. Expendable Mold Casting

• Mold destroyed to remove casting
• New mold needed for each casting
• Examples: Sand casting, investment casting, plaster casting

2. Permanent Mold Casting

• Mold reused multiple times
• Metal or ceramic molds
• Examples: Die casting, permanent mold casting, centrifugal casting

Based on Pressure

1. Gravity Casting

• Molten metal flows by gravity
• Examples: Sand casting, permanent mold casting

2. Pressure Casting

• External pressure applied
• Examples: Die casting, squeeze casting

Sand Casting

Sand Casting = Most widely used casting process using sand as mold material

Sand Casting Process

Steps:

1. Pattern placement - Place pattern in flask
2. Ramming - Pack sand around pattern
3. Pattern removal - Remove pattern, leaving cavity
4. Core placement - Insert cores for internal features
5. Mold assembly - Assemble cope (top) and drag (bottom)
6. Pouring - Pour molten metal through sprue
7. Solidification - Allow to cool
8. Shakeout - Break mold, remove casting
9. Cleaning - Remove gates, risers, clean surface

Mold Components

Flask: Frame holding sand mold

• Cope - Upper half
• Drag - Lower half
• Cheek - Middle section (if needed)

Pattern: Replica of part (with allowances)

Core: Sand shape for internal cavities

Gating System: Channels for metal flow

• Pouring basin - Receives molten metal
• Sprue - Vertical channel
• Runner - Horizontal channel
• Gate - Entry to mold cavity
• Riser - Reservoir for feeding shrinkage

Parting line: Interface between cope and drag

Molding Sand Properties

Requirements:

1. Refractoriness - Withstand high temperatures without melting
2. Permeability - Allow gases to escape
3. Strength - Hold shape during handling and pouring
4. Collapsibility - Break down after solidification
5. Reusability - Can be reconditioned and reused

Sand Composition:

• Silica sand - Base material (85-95%)
• Clay (bentonite) - Binder (4-10%)
• Water - Activates clay (2-5%)
• Additives - For special properties

Types:

• Green sand - Moist sand with clay binder (most common)
• Dry sand - Baked after molding
• Skin-dried sand - Surface dried
• CO₂ sand - Sodium silicate binder, hardened with CO₂

Pattern Allowances

1. Shrinkage Allowance

• Metal shrinks during solidification and cooling
• Pattern made larger to compensate
• Typical: 1-2% for steel, 1.3% for cast iron, 1.6% for aluminum

2. Draft Allowance

• Taper on vertical surfaces for easy pattern removal
• Typical: 1-3° depending on depth

3. Machining Allowance

• Extra material for subsequent machining
• Typical: 1.5-6 mm depending on size

4. Distortion Allowance

• Compensate for warping during cooling
• Based on part geometry

5. Shake Allowance

• For loose pattern (negative allowance)
• Pattern made slightly smaller

Cores

Core = Sand shape placed in mold to create internal cavities or complex external features

Core Making:

• Made separately from mold
• Use core boxes (patterns for cores)
• Stronger binders needed (oil sand, resin sand)
• Must be permeable for gas escape

Core Prints: Extensions on pattern that create recesses in mold to support cores

Gating System Design

Purpose:

• Deliver molten metal to mold cavity
• Control flow rate
• Minimize turbulence
• Trap slag and dross
• Feed shrinkage

Gating Ratio: Sprue : Runner : Gate area ratio

• Example: 1:2:4 (pressurized system)
• Example: 1:2:2 (unpressurized system)

Choke: Smallest cross-section controlling flow rate

Risers (Feeders)

Purpose: Provide additional molten metal to compensate for solidification shrinkage

Requirements:

• Must solidify after casting
• Adequate volume
• Proper placement (at heavy sections)

Types:

• Open riser - Open to atmosphere
• Blind riser - Enclosed in mold

Caine's Method: Riser design based on modulus (V/A ratio)

Melting and Pouring

Melting Furnaces:

• Cupola - For cast iron (continuous)
• Electric arc furnace - Steel, cast iron
• Induction furnace - Clean melting, precise control
• Crucible furnace - Small batches, non-ferrous

Pouring Temperature:

• Above liquidus temperature
• Superheat: 50-150°C above melting point
• Too low → cold shuts, misruns
• Too high → excessive shrinkage, gas absorption, mold erosion

Common Casting Alloys

Cast Iron:

• Gray cast iron - Good castability, damping
• Ductile iron - Better strength, ductility
• White cast iron - Hard, wear resistant

Steel:

• Carbon steel
• Alloy steel
• Stainless steel

Aluminum Alloys:

• Lightweight
• Good corrosion resistance
• Excellent castability

Copper Alloys:

• Brass, bronze
• Good corrosion resistance
• Decorative applications

Magnesium Alloys:

• Lightest structural metal
• Aerospace applications