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Sand Casting Sand Mould Type of patterns Pattern Materials Pattern allowances

Comprehensive study notes, diagrams, and exam preparation for Sand Casting Sand Mould Type of patterns Pattern Materials Pattern allowances.

Sand Casting: Sand Moulds, Patterns, and Allowances

Definition

Sand casting is a primary metal casting process that uses a mixture of sand to create a mould cavity into which molten metal is poured. Once the metal solidifies and cools, the sand mould is broken away to reveal the final cast part.

Main Content

1. Types of Sand Moulds

  • Green Sand Mould: Composed of sand, water, and clay (bentonite). It is used in its "green" (moist) state and is the most common and cost-effective method for university syllabus projects.
  • Dry Sand Mould: The mould is baked in an oven to remove moisture, which increases strength and prevents gas defects, making it ideal for large, heavy castings.
  • Loam Sand Mould: A mixture of sand and clay with high water content, often used for very large castings, shaped using a "strickle board."

2. Pattern Materials

  • Wood: Easy to work with and inexpensive, making it a favorite for "one-off" or low-volume production. Teak or mahogany is preferred for durability.
  • Metal (Aluminum or Cast Iron): Used for mass production due to high durability, resistance to moisture, and dimensional stability.
  • Plastics/Polymers: Lightweight and resistant to moisture/corrosion, often used for complex shapes in modern automated casting lines.

3. Pattern Allowances

  • Shrinkage Allowance: Metals shrink when cooling from liquid to solid. Patterns are made slightly larger to compensate for this volume contraction.
  • Machining/Finish Allowance: Added extra material on surfaces that need to be machined post-casting to ensure the final dimension is accurate.
  • Draft Allowance: A slight taper added to vertical surfaces to allow the pattern to be withdrawn from the sand mould without damaging the mould walls.
  • Shake/Rapping Allowance: Since the pattern must be "shaken" to loosen it for removal, the mould cavity becomes slightly larger; the pattern is made slightly smaller to account for this.

Working / Process

1. Pattern Making and Placement

  • The design of the component is used to create the pattern, incorporating all necessary allowances.
  • The pattern is placed on a moulding board, and the drag (bottom half of the flask) is placed over it.

2. Moulding and Ramming

  • Sand is packed (rammed) around the pattern.
  • A cope (top half) is placed, and the gating system (sprue, runner, riser) is carved into the sand to allow metal flow.
    [COPE]  (Contains sprue/riser)
    --------------------------
    [DRAG]  (Contains pattern)
    --------------------------

(Simplified cross-section of a sand mould setup)

3. Pouring and Solidification

  • The pattern is withdrawn, the mould is closed, and molten metal is poured into the cavity.
  • After the metal reaches room temperature, the sand is shaken out to retrieve the casting.

Advantages / Applications

  • Versatility: Can cast almost any metal that can be melted, including iron, steel, and aluminum alloys.
  • Low Cost: Sand is an inexpensive material, and the equipment required is relatively simple for exam preparation.
  • Size Range: Suitable for both tiny parts and massive engine components or machine bases.

Summary

Sand casting is a fundamental manufacturing process essential for engineering students. It involves creating a replica of a part using a pattern, forming a cavity in sand, and filling that void with liquid metal. Understanding the nuances of pattern allowances—such as shrinkage, draft, and machining—is vital for solving design problems in university exams or technical interview questions.

Key terms to remember: Pattern, Flask, Cope, Drag, Gating System, and Shrinkage Allowance.

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