Pouring & Solidification

Pouring

Pouring Temperature

Pouring Temperature = Temperature at which molten metal is poured into mold

Superheat = Temperature above liquidus temperature

• Typical superheat: 50-150°C

Effects of Pouring Temperature:

Too Low:

• Cold shuts (metal streams don't fuse)
• Misruns (incomplete filling)
• Poor surface finish
• Oxide inclusions

Too High:

• Excessive shrinkage
• Gas absorption (porosity)
• Mold erosion
• Longer solidification time
• Coarse grain structure

Optimal Temperature:

• Depends on: alloy type, section thickness, mold complexity
• Thin sections → higher temperature needed
• Complex molds → higher temperature for complete filling

Pouring Time

Pouring Time = Time taken to fill mold cavity

Chvorinov's Rule for Pouring Time: t = K × √W

Where:

• t = Pouring time (seconds)
• K = Constant (depends on metal and mold)
• W = Weight of casting (kg)

Pouring Rate:

• Too fast → turbulence, gas entrapment, mold erosion
• Too slow → cold shuts, misruns, premature solidification

Solidification

Solidification Process

Pure Metals:

• Solidify at constant temperature (melting point)
• Sharp solidification front
• Latent heat released at constant temperature

Alloys:

• Solidify over a temperature range
• Liquidus → Solidus temperature
• Mushy zone (liquid + solid coexist)
• Solidification time longer than pure metals

Solidification Time

Chvorinov's Rule: t = B × (V/A)²

Where:

• t = Total solidification time
• B = Mold constant (depends on metal properties, mold material)
• V = Volume of casting
• A = Surface area of casting
• V/A = Modulus (characteristic dimension)

Implications:

• Larger castings solidify slower
• Thick sections solidify slower than thin sections
• Modulus determines solidification sequence

Solidification Shrinkage

Three Stages of Shrinkage:

1. Liquid Contraction

   • Shrinkage in liquid state (cooling from pouring to solidification)
   • Compensated by riser

2. Solidification Shrinkage

   • Volume reduction during phase change (liquid → solid)
   • Typically 3-7% volume reduction
   • Main cause of shrinkage cavities

3. Solid Contraction

   • Shrinkage during cooling in solid state
   • Causes dimensional changes
   • Compensated by pattern shrinkage allowance

Shrinkage Values (Volume %):

• Aluminum: 6-7%
• Cast iron (gray): 2-3% (graphite expansion compensates)
• Steel: 3-4%
• Copper: 4-5%

Directional Solidification

Principle: Metal should solidify progressively from casting toward riser

Purpose:

• Ensure liquid metal available to feed shrinkage
• Prevent isolated liquid pockets (shrinkage cavities)

Methods to Achieve:

1. Proper riser placement - At thickest sections
2. Chills - Metal inserts to accelerate cooling locally
3. Padding - Insulation to slow cooling
4. Tapered sections - Gradual thickness change

Risers (Feeders)

Purpose: Supply liquid metal to compensate for solidification shrinkage

Requirements:

1. Must solidify after the casting
2. Adequate volume to feed shrinkage
3. Proper connection to casting (gate size)
4. Located at thickest sections

Riser Design:

• Riser modulus > Casting modulus
• (V/A)riser > (V/A)casting

Types:

• Open riser - Open to atmosphere (atmospheric pressure)
• Blind riser - Enclosed in mold (may use exothermic compounds)

Riser Aids:

• Exothermic compounds - Generate heat, keep riser liquid longer
• Insulating sleeves - Reduce heat loss from riser

Chills

Chill = High thermal conductivity material placed in mold to accelerate local cooling

Types:

• External chill - On mold surface
• Internal chill - Inside mold cavity (becomes part of casting)

Purpose:

• Control solidification direction
• Reduce hot spots
• Refine grain structure locally

Materials: Cast iron, copper, steel

Solidification Defects

1. Shrinkage Cavity (Pipe)

• Void at top center of casting
• Caused by: inadequate feeding, improper riser design
• Prevention: Proper riser design, directional solidification

2. Porosity

• Small distributed voids
• Gas porosity - Dissolved gases (H₂, N₂, O₂)
• Shrinkage porosity - Micro-shrinkage in mushy zone
• Prevention: Degassing, proper gating, adequate feeding

3. Hot Tears (Hot Cracks)

• Cracks formed during final solidification
• Caused by: thermal stresses, restrained contraction
• Occur in: weak sections, stress concentration areas
• Prevention: Proper design (avoid stress concentrations), collapsible cores

4. Cold Shuts

• Discontinuity where two metal streams meet but don't fuse
• Caused by: low pouring temperature, slow pouring
• Prevention: Adequate pouring temperature, proper gating

5. Misruns

• Incomplete casting (mold not completely filled)
• Caused by: low pouring temperature, insufficient fluidity, thin sections
• Prevention: Higher pouring temperature, proper gating design

Gating System

Functions of Gating System

1. Deliver metal to mold cavity
2. Control flow rate and filling time
3. Minimize turbulence and air entrapment
4. Trap slag and dross
5. Regulate thermal conditions
6. Feed shrinkage (in some designs)

Gating System Components

1. Pouring Basin (Cup)

• Receives molten metal from ladle
• Reduces turbulence
• Prevents slag entry

2. Sprue

• Vertical channel from pouring basin to runner
• Tapered (larger at top) to maintain full flow
• Sprue base well to reduce turbulence

3. Runner

• Horizontal channel distributing metal
• Connects sprue to gates
• May include slag trap

4. Gate

• Entry point to mold cavity
• Controls metal entry rate and direction
• Multiple gates for large castings

5. Riser (Feeder)

• Reservoir for feeding shrinkage
• Must solidify last

Gating Design Principles

Sprue Taper:

• Top area > Bottom area
• Prevents aspiration (air suction)
• Based on continuity equation

Gating Ratio: Asprue : Arunner : Agate

Pressurized System (1:2:4):

• Gate area largest
• Maintains full runners
• Reduces turbulence
• Used for: non-ferrous alloys

Unpressurized System (1:2:2 or 1:4:4):

• Sprue smallest (choke)
• Faster filling
• Used for: ferrous alloys

Pouring Basin Design

Features:

• Adequate capacity
• Offset sprue entrance (prevents vortex)
• Strainer/filter (optional)

Sprue Design

Bernoulli's Equation Application: h = (v₂² - v₁²) / (2g)

Continuity: A₁v₁ = A₂v₂

Sprue Taper: A₁/A₂ = v₂/v₁ = √(h₂/h₁)

Gate Design

Gate Types:

• Top gate - Entry from top (simple, but turbulent)
• Bottom gate - Entry from bottom (smooth filling, less turbulence)
• Parting line gate - Entry at parting line (most common)
• Step gate - Multiple entries at different heights

Gate Location:

• Thickest section (for feeding)
• Avoid direct impingement on cores
• Minimize metal travel distance

Filters and Strainers

Purpose:

• Remove inclusions (slag, oxides, sand)
• Reduce turbulence
• Improve metal quality

Types:

• Ceramic foam filters
• Mesh strainers
• Slot filters

Fluidity

Fluidity = Ability of molten metal to flow and fill mold cavity

Factors Affecting Fluidity:

1. Pouring temperature - Higher → better fluidity
2. Alloy composition - Eutectic alloys → best fluidity
3. Viscosity - Lower → better fluidity
4. Surface tension - Lower → better fluidity
5. Mold temperature - Higher → better fluidity
6. Mold permeability - Higher → better fluidity
7. Section thickness - Thicker → easier filling

Fluidity Test: Spiral test (length of spiral filled)