Mould & Gating System

Mould Design

Mould Components

1. Mould Cavity

• Negative impression of the part
• Defines external shape
• Surface finish critical

2. Core

• Creates internal features
• Hollow sections
• Must be supported by core prints

3. Parting Line

• Interface between cope and drag
• Should be at maximum cross-section
• Affects flash formation

4. Draft

• Taper on vertical walls
• Facilitates pattern removal
• Typical: 1-3°

5. Gating System

• Channels for metal delivery
• Controls flow rate and direction

6. Riser (Feeder)

• Reservoir for feeding shrinkage
• Must solidify last

7. Vents

• Allow gas escape
• Prevent gas porosity

Gating System Design

Purpose of Gating System

1. Deliver molten metal to mould cavity
2. Control flow rate and filling time
3. Minimize turbulence and air entrapment
4. Trap slag and dross
5. Regulate thermal conditions
6. Provide directional solidification

Gating System Components

1. Pouring Basin (Pouring Cup)

Function:

• Receives molten metal from ladle
• Reduces turbulence
• Prevents slag entry
• Maintains constant metal head

Design Features:

• Adequate capacity (2-3 times sprue entrance area)
• Offset sprue entrance (prevents vortex)
• Conical or cylindrical shape
• Strainer/filter (optional)

2. Sprue

Function:

• Vertical channel from pouring basin to runner
• Primary metal delivery path

Design:

• Tapered (larger at top, smaller at bottom)
• Prevents aspiration (air suction)
• Maintains full flow

Sprue Taper Calculation:

Based on Bernoulli's equation and continuity:

A₁/A₂ = √(h₂/h₁)

Where:

• A₁ = Top area
• A₂ = Bottom area
• h₁ = Height at top
• h₂ = Height at bottom

Sprue Base Well:

• Enlargement at sprue bottom
• Reduces turbulence
• Dissipates kinetic energy
• Prevents erosion

3. Runner

Function:

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

Design:

• Trapezoidal or circular cross-section
• Smooth transitions
• Adequate size for flow

Slag Trap:

• Extension beyond last gate
• Traps dross and slag (lighter, flows on top)
• Prevents slag entry to cavity

4. Gate

Function:

• Entry point to mould cavity
• Controls metal entry rate and direction
• Regulates filling

Gate Types:

a) Top Gate

• Entry from top of cavity
• Simple, easy to make
• Disadvantages: Turbulent, erosion, oxidation
• Used for: Non-critical castings, large castings

b) Bottom Gate

• Entry from bottom
• Smooth filling, less turbulence
• Advantages: Better quality, less oxidation
• Disadvantages: More complex mould
• Used for: Quality castings, non-ferrous

c) Parting Line Gate

• Entry at parting line
• Most common
• Balance between top and bottom
• Easy to remove

d) Step Gate (Multiple Gates)

• Multiple entries at different heights
• Progressive filling
• Reduces turbulence
• Used for: Tall castings

e) Horn Gate

• Curved entry
• Smooth flow direction change
• Reduces turbulence

Gate Location:

• At thickest section (for feeding)
• Avoid direct impingement on cores
• Minimize metal travel distance
• Consider solidification sequence

Gate Size:

• Smaller than runner (choke)
• Controls flow rate
• Typical: 0.5-0.8 times runner area

5. Riser (Feeder)

Function:

• Provide liquid metal to compensate solidification shrinkage
• Act as reservoir

Requirements:

1. Must solidify after the casting
2. Adequate volume
3. Proper connection to casting
4. Located at hot spots (thickest sections)

Riser Types:

Open Riser (Top Riser):

• Open to atmosphere
• Atmospheric pressure aids feeding
• Easy to place
• Heat loss from top

Blind Riser (Side Riser):

• Enclosed in mold
• Less heat loss
• Better feeding efficiency
• May use exothermic compounds

Riser Design - Chvorinov's Rule:

For riser to solidify after casting:

(V/A)riser > (V/A)casting

Where:

• V/A = Modulus (volume/surface area ratio)

Riser Volume:

• Must compensate for shrinkage
• Typical: 1.5-2 times shrinkage volume

Riser Neck:

• Connection between riser and casting
• Must be adequate for feeding
• Should freeze after casting but before riser

Riser Aids:

• Exothermic compounds: Generate heat, keep riser liquid longer
• Insulating sleeves: Reduce heat loss
• Breaker cores: Facilitate riser removal

Gating Ratio

Gating Ratio = Asprue : Arunner : Agate

Pressurized System (1:2:4):

• Gate area largest
• Maintains full runners
• Reduces turbulence
• Back pressure in system
• Used for: Non-ferrous alloys (Al, Cu)

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

• Sprue smallest (choke)
• Faster filling
• Runners may not run full
• Used for: Ferrous alloys (steel, cast iron)

Filters and Strainers

Purpose:

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

Types:

• Ceramic foam filters: Porous ceramic, high efficiency
• Mesh strainers: Wire mesh, simple
• Slot filters: Narrow slots, directional flow

Placement: In pouring basin, runner, or gate

Mould Design Considerations

1. Parting Line Selection

Criteria:

• At maximum cross-section
• Simplifies moulding
• Minimizes core complexity
• Facilitates pattern removal
• Affects flash location

2. Draft Angles

Purpose: Easy pattern removal

Typical Values:

• External surfaces: 1-2°
• Internal surfaces: 2-3°
• Deep pockets: 3-5°

3. Fillet Radii

Purpose:

• Reduce stress concentration
• Improve metal flow
• Prevent hot tears

Typical: 3-5 mm minimum

4. Section Thickness

Uniform Thickness Preferred:

• Avoids hot spots
• Uniform solidification
• Reduces shrinkage defects

Thick-to-Thin Transitions:

• Gradual changes
• Avoid abrupt steps

5. Core Design

Considerations:

• Adequate strength
• Proper venting
• Core prints for support
• Collapsibility (to prevent hot tears)

6. Venting

Purpose: Allow gas escape

Methods:

• Mould permeability
• Vent holes
• Parting line vents

Solidification and Feeding

Directional Solidification

Principle: Metal solidifies progressively from casting toward riser

Methods:

1. Proper riser placement
2. Chills: Accelerate local cooling
3. Padding/Insulation: Slow cooling at riser
4. Tapered sections: Gradual thickness reduction

Chills

External Chill:

• Placed on mould surface
• High conductivity (iron, copper)
• Accelerates cooling locally

Internal Chill:

• Placed inside cavity
• Becomes part of casting
• Must be same material

Padding

Insulating Material:

• Placed around riser
• Slows heat loss
• Keeps riser liquid longer

Gating System Design Steps

1. Determine casting weight and volume
2. Calculate pouring time (Chvorinov's rule)
3. Determine metal flow rate (Q = V/t)
4. Calculate sprue area (from flow rate and velocity)
5. Design sprue taper (continuity equation)
6. Determine gating ratio (pressurized or unpressurized)
7. Calculate runner and gate areas
8. Design riser (modulus method)
9. Locate gates and risers (at thick sections)
10. Add filters/strainers (if needed)

Common Gating Defects

Erosion:

• High velocity erodes mould
• Prevention: Reduce velocity, proper gate design

Aspiration:

• Air sucked into metal stream
• Prevention: Proper sprue taper, avoid turbulence

Cold Shuts:

• Metal streams don't fuse
• Prevention: Adequate temperature, proper gating

Slag Inclusions:

• Slag enters cavity
• Prevention: Slag traps, filters, proper pouring

Misruns:

• Incomplete filling
• Prevention: Adequate flow rate, temperature

Special Gating Systems

Whirl Gate:

• Tangential entry
• Rotational flow
• Reduces turbulence

Fan Gate:

• Wide, thin gate
• For flat castings
• Uniform filling

Ring Gate:

• Circular gate around part
• Symmetrical filling
• For cylindrical parts