Manufacturing Process: Sand Casting

Sand Casting Process

Mold Materials

• **Material Requirements: **
  • Must possess high refractoriness (ability to withstand high temperatures)
  • Can be metallic (e.g., cast iron, mild steel, alloy steels) or non-metallic (e.g., molding sands, plaster of Paris, graphite, silicon carbide, ceramics)
  • Molding Sand: Most commonly utilized non-metallic molding material; should have:
  • High refractoriness
  • Good thermal and chemical stability at high temperatures
  • High permeability and workability
  • Good strength

Types of Molding Sand

• Natural Sand:

  • Sourced directly from natural deposits
  • Requires additives and water for molding
  • Higher clay content allows for replacement of lost sand

• Synthetic Sand:

  • Created by mixing clay-free sand with clay binder, water, and additives
  • Tailored sand grains based on metal being cast

• Chemically Coated Sand:

  • Coated silica grains with a binding agent such as non-thermosetting hydrocarbon resin
  • Maintains moisture content above 3%

Binders Used in Foundries

• Inorganic Binders: Mainly clay-based
  • Fireclay, Kaolinite ($Al2O3 ullet 2SiO2 ullet 2H2O$), Illite ($K2O ullet Al2O3 ullet SiO2 ullet H2O$), Bentonite ($MgO ullet Al2O3 ullet SiO2 ullet H_2O$)

Types of Sand Molds

• Common Molds:
  • Green Sand Molds
  • Dry Sand Molds
  • Skin-Dry Sand Molds
  • Loam Sand Molds
  • Cemented-Bonded Molds
  • Cold-Box Molds
  • CO₂ Molds
  • Resin-Bonded Sand Molds

Green Sand Molds

• Composition: Sand (silica), clay (binder), and water
• Characteristics:
  • Cheapest mold type, used in damp conditions
  • Reclaimed easily for reuse
  • Suitable for small and medium-sized castings

Dry Sand Molds

• Composition: Similar to green sand molds plus 1-2% cereal flour and pitch, baked in oven
• Characteristics: Strong, better surface finish, reduce casting defects

Skin-Dry Sand Molds

• Composition: Partially dried around the cavity
• Characteristics: Combines benefits of green and dry molds

CO₂ Molds

• Composition: Sand mixed with sodium silicate as a binder
• Characteristics: Hardening occurs with CO₂ gas, fast process; ideal for intricate castings.

Resin-Bonded Sand Molds

• Composition: Green sand with thermosetting resins or oils
• Characteristics: Stronger molds; used for making cores, may not need baking.

Properties of Molding Sand

• Permeability:
  • Ability to allow air to pass through;
  • Influenced by clay content, quartz content, moisture, compaction, and grain size;
  • Good permeability prevents casting defects.
• Strength/Cohesiveness:
  • Ability of sand grains to stick together; essential to handle molten metal pressure.
  • Influenced by grain size, moisture, clay content.
• Refractoriness: Ability to withstand heat.
• Plasticity:
  • Ability to flow around patterns; enhanced by clay and water content.
• Collapsibility:
  • Important to accommodate volume reduction during cooling.
• Adhesiveness: Ability to stick to a molding box.

Sand Testing

• Test Types Recommended by BIS:
  1. Moisture Content Test
  2. Clay Content Test
  3. Permeability Test
  4. Fineness Test
  5. Strength Test
  6. Mold Hardness Test

Common Terms in Hand-Molding Process

• Core: Insert used to create cavities.
• Core Prints: Projections on the pattern to hold the core in place.
• Pouring Basin: Reservoir for molten metal.
• Sprue: Vertical channel guiding metal into the mold.
• Runner: Horizontal channel directing metal from sprue to mold cavity.
• Risers: Reservoirs that supply molten metal to prevent shrinkage during solidification.

Hand-Molding Process Steps

1. Drag Half Placement: Position drag on a molding board.
2. Filling with Sand: Pack sand around the pattern.
3. Ventilation: Create vent holes for gas escape.
4. Mold Turning: Flip mold for cope alignment.
5. Cope Alignment: Position cope over the drag.
6. Sprue Placement: Set sprue and riser pins for pouring.
7. Mold Assembly and Clamping: Ensure proper alignment for pouring.