Patterns

What is a Pattern?

Pattern = A replica or model of the final casting used to create the mold cavity

Functions of Pattern

1. Create mold cavity shape
2. Incorporate allowances (shrinkage, draft, machining)
3. Provide core prints for core support
4. Include gating system elements (sometimes)

Pattern Materials

1. Wood

Advantages:

• Easy to work with
• Low cost
• Good for prototypes and small batches
• Readily available

Disadvantages:

• Absorbs moisture (dimensional changes)
• Low durability (wears quickly)
• Warps with humidity changes
• Not suitable for mass production

Types: Pine, mahogany, teak

2. Metal

Advantages:

• High durability
• Dimensional stability
• Good surface finish
• Suitable for mass production
• Long life

Disadvantages:

• Higher cost
• Heavier
• Difficult to modify

Types: Aluminum, brass, cast iron, steel

3. Plastic

Advantages:

• Lightweight
• Good surface finish
• Moisture resistant
• Moderate cost
• Easy to mold complex shapes

Disadvantages:

• Lower strength than metal
• May deform under pressure

Types: Polyurethane, epoxy, ABS

4. Plaster

Advantages:

• Excellent surface finish
• Good for intricate details
• Low cost

Disadvantages:

• Fragile
• Low durability
• Limited to small patterns

5. Wax

Used in: Investment casting (lost wax process)

• Melts out of mold
• Excellent detail reproduction

Types of Patterns

1. Single Piece (Solid) Pattern

• Simplest type
• Entire pattern in one piece
• Used for simple shapes
• Low production quantities
• Parting line must be flat

Advantages: Simple, low cost Disadvantages: Limited to simple shapes, difficult molding

2. Split Pattern (Two-Piece Pattern)

• Pattern split along parting line
• Two halves: cope and drag
• Most common type
• Dowel pins for alignment

Advantages:

• Suitable for complex shapes
• Easy mold making
• Better for production

Disadvantages: Higher cost than solid pattern

3. Match Plate Pattern

• Pattern halves mounted on opposite sides of a plate
• Plate matches cope and drag
• Gating system included on plate
• High production rates

Advantages:

• Fast molding
• Consistent quality
• Suitable for machine molding
• Good for mass production

Disadvantages:

• Expensive
• Pattern size limited by plate size

4. Cope and Drag Pattern

• Pattern halves mounted on separate plates
• One plate for cope, one for drag
• Used for large patterns
• More flexible than match plate

Advantages:

• Suitable for large castings
• Easier handling of large patterns

Disadvantages:

• Alignment more critical
• Slower than match plate

5. Gated Pattern

• Pattern with gating system attached
• Gates, runners, risers included
• Used for small castings
• Multiple patterns on one plate possible

Advantages:

• Complete mold system
• Consistent gating
• Good for small parts

6. Sweep Pattern

• Partial pattern rotated to create full cavity
• Used for large cylindrical or conical shapes
• Pattern is a board or template

Advantages:

• Low cost for large symmetric parts
• Suitable for one-off large castings

Applications: Large pipes, bells, tanks

7. Skeleton Pattern

• Framework pattern
• Used for large, simple hollow shapes
• Sand filled and shaped around skeleton

Advantages: Economical for very large castings

8. Follow Board Pattern

• Used with irregular parting lines
• Board supports pattern during molding
• Pattern removed from side

9. Loose Piece Pattern

• Pattern with removable sections
• Pieces removed separately to avoid damage
• Used for complex shapes with undercuts

Pattern Allowances

1. Shrinkage Allowance (Contraction Allowance)

Purpose: Compensate for metal shrinkage during solidification and cooling

Shrinkage Values:

• Cast iron: 1.0% (10 mm/m)
• Steel: 2.0% (20 mm/m)
• Aluminum: 1.3% (13 mm/m)
• Brass: 1.5% (15 mm/m)
• Magnesium: 1.3% (13 mm/m)

Application: Pattern made larger by shrinkage percentage

Shrink Rule: Special scale incorporating shrinkage allowance

2. Draft Allowance (Taper Allowance)

Purpose: Facilitate pattern removal from mold without damaging cavity

Draft Angle:

• Wood patterns: 1-3°
• Metal patterns: 0.5-2°
• External surfaces: 1-2°
• Internal surfaces: 2-3° (more draft needed)
• Deep pockets: 3-5°

Factors Affecting Draft:

• Pattern material (rough surfaces need more)
• Depth of cavity (deeper needs more)
• Molding method (machine molding needs less)

3. Machining Allowance (Finish Allowance)

Purpose: Provide extra material for machining operations

Typical Values:

• Small castings (<300 mm): 1.5-3 mm
• Medium castings (300-1000 mm): 3-6 mm
• Large castings (>1000 mm): 6-12 mm

Factors:

• Casting size (larger → more allowance)
• Casting method (sand casting → more allowance)
• Material (steel → more than aluminum)
• Surface finish required

4. Distortion Allowance (Camber Allowance)

Purpose: Compensate for warping during cooling

Application:

• Reverse distortion built into pattern
• U-shaped castings tend to close → pattern made more open
• Long flat sections tend to sag → pattern made with reverse camber

Factors:

• Part geometry
• Material properties
• Cooling rate
• Section thickness variations

5. Shake Allowance

Purpose: Account for pattern enlargement due to rapping (for loose patterns)

Application:

• Negative allowance (pattern made slightly smaller)
• Typically 1-2 mm
• Only for loose patterns that are rapped

Pattern Color Coding (Standard)

Purpose: Indicate different surfaces and features

Standard Colors:

• Unfinished surfaces: Black or red
• Machined surfaces: Yellow
• Core prints: Black stripes on yellow
• Loose pieces: Red stripes on yellow
• Seats for loose pieces: Red diagonal stripes
• Stop-offs (not to be molded): Green

Core Prints

Core Print = Extension on pattern that creates recess in mold to support core

Functions:

1. Support and locate core in mold
2. Provide passage for gases from core
3. Seal mold cavity

Design Considerations:

• Length: Sufficient to support core (typically 10-25 mm)
• Taper: Draft for removal
• Strength: Must support core weight and buoyancy forces

Pattern Design Guidelines

1. Avoid sharp corners: Use fillets and radii
2. Uniform wall thickness: Minimize hot spots
3. Adequate draft: For easy removal
4. Minimize undercuts: Use cores or loose pieces if necessary
5. Proper parting line location: At maximum cross-section
6. Consider shrinkage: In all dimensions
7. Simplify where possible: Reduce cost and complexity
8. Provide adequate core prints: For core support
9. Consider gating: Location for metal entry
10. Mark machining surfaces: Clear identification

Pattern Maintenance

Requirements:

• Regular inspection for wear
• Repair damaged surfaces
• Maintain dimensional accuracy
• Proper storage (controlled humidity for wood)
• Cleaning after use
• Protective coatings (varnish, paint)