Casting Processes Overview and Pattern Making

Casting Processes: Necessity and Overview

• Casting is crucial for creating large, single-piece parts and complex shapes with internal cavities.
• It's economically advantageous and suitable for materials difficult to process otherwise.
• The sand casting process uses a disposable sand mold to create the desired shape. The process is relatable to key duplication.

Sand Casting: Pattern Making

• Patterns are replicas of the desired casting, used to create impressions in the sand mold.
• They require modifications like allowances for machining and core prints.
• Materials include wood (suitable for small quantities and large castings, but susceptible to moisture), metal (for mass production, high wear resistance, and tighter tolerances; aluminum is a common choice), and plastic.

Pattern Making: Material Considerations

• Wood: Readily available, lightweight, inexpensive, but absorbs moisture, has a short lifespan, and dimensional changes.
• Metal: Durable, maintains tighter dimensional tolerances, longer lifespan, but heavier (copper, steel) and prone to corrosion. Aluminum is frequently used for its balance of properties.
• Machining allowance is typically 3 mm per surface. Example: A 100mm dimension requires a 106mm pattern.

Draft Allowance

• Draft allowance is included in pattern dimensions to facilitate easy removal from the sand mold.
• Example: A 0.75° draft angle applied to a 109mm external detail adds approximately 1.40mm per side.

Cores

• Cores create internal cavities or holes that aren't part of the main pattern.
• They are produced separately and baked for strength.
• They're placed in core prints within the mold; sand, metal, or ceramics are used.
• Core properties include strength, erosion resistance, permeability to gases, high refractoriness, and a good surface finish.

Chaplets

• Large cores may need additional support provided by chaplets (rigid metal pieces) placed between the core and mold face.
• Chaplet material needs to be compatible with the molten metal.

Gating System

• The gating system channels molten metal into the mold cavity, preventing erosion of the mold bottom.
• Components include: pouring basin, sprue, sprue well, runner, gates, and riser.
• A tapered sprue reduces the momentum of molten metal, minimizing turbulence.

Gating System Components and Functions

• Pouring Basin: Reduces the force of the molten metal stream.
• Sprue: The main vertical channel. A tapered sprue helps regulate flow.
• Sprue Base: Reservoir at the bottom of the sprue to moderate the flow.
• Runner: Horizontal channel distributing molten metal.
• Gate: Smaller channels leading directly into the mold cavity.
• Riser: Reservoir supplying molten metal to compensate for shrinkage during solidification.

Preventing Impurities

• Pouring basins reduce erosion.
• Strainers remove dross (impurities).
• Splash cores reduce turbulence.
• Skim bobs trap heavier and lighter impurities in horizontal gates.

Engineering Analysis of Pouring: Bernoulli's Theorem

• Bernoulli's theorem, considering atmospheric pressure, relates head (height), pressure, and velocity of flowing liquid. The sum of these energies remains constant between two points in the fluid (friction losses are ignored).

• In the context of the sprue, the height of the sprue contributes head, while the velocity differences at top and bottom effect the pressure. The initial velocity at the top of the sprue is assumed zero.

Explore the essential casting processes that enable the creation of complex shapes and large single-piece components. Understand the role of pattern making in sand casting, including material considerations for wood, metal, and plastic. This quiz covers key concepts and advantages of various casting techniques.