Casting Processes and Manufacturing Principles

Fundamentals of Metal Casting

• Metal casting is a manufacturing process that involves pouring molten metal into a mould to create a 3D metal part.

• It is a versatile process capable of producing complex geometries with internal cavities that other manufacturing methods might struggle to achieve.

• The cast metal is isotropic, meaning it maintains identical physical and mechanical properties regardless of the direction in which they are measured.

• Necessary tools for casting are generally cheaper and simpler than those for alternative processes.

• It is highly adaptable to mass production; for example, the automotive industry produces massive quantities of engine blocks and transmission cases using these methods.

• It is an economical process with minimal wastage, as extra metal from risers and gates can be re-melted and re-used.

Metal Casting Terminology and Components

• Flask: A container that holds the sand mould intact. It is usually made of wood for temporary applications or metal for long-term use. Depending on its position, it is named differently:

  • Drag: The lower moulding flask.

  • Cope: The upper moulding flask.

  • Cheek: An intermediate flask used in three-piece moulding setups.

• Pattern: An exact replica of the final object to be made, including specific modifications like allowances. It is used to create the mould cavity.

• Parting Line: The dividing line between the two moulding flasks (cope and drag). In split patterns, it is also the dividing line between the pattern halves.

• Bottom Board: A wooden board used at the start of mould making to support the pattern before sand is rammed.

• Moulding Sand: A freshly prepared refractory material made of a mixture of silica, clay, and moisture in specific proportions.

• Facing Sand: Carbonaceous material sprinkled on the inner surface of the cavity to provide a better surface finish.

• Backing Sand: Used and burnt sand that makes up the bulk of the refractory material in the mould.

• Core: A part used to create internal hollow cavities in a casting. These are typically made of sand aggregates to ensure strength and permeability.

• Pouring Basin: A small funnel-shaped cavity at the top of the mould where molten metal is introduced.

• Sprue: Typically a vertical passage through which molten metal travels from the pouring basin to the runner. It often controls the flow rate.

• Runner: Passageways located in the parting plane that regulate the flow of molten metal before it reaches the mould cavity.

• Gate: The actual entry point where molten metal flows into the mould cavity.

• Chaplet: Metallic supports used to hold cores in place inside the mould cavity, counteracting their weight and metallostatic forces.

• Chill: Metallic objects placed in the mould to accelerate the cooling rate in specific areas. This ensures directional solidification and improves the effectiveness of risers.

• Riser: A reservoir of molten metal provided to feed the casting as it shrinks during solidification, preventing internal voids.

Sand Mould Making Procedure

1. Drag Completion: The pattern is placed on the bottom board, and sand is rammed around it to complete the drag.

2. Roll-over Preparation: The drag is prepared to be flipped over.

3. After Roll-over: The drag is inverted, exposing the pattern surface for the cope.

4. Cope Ramming: The upper half of the pattern and gating system are positioned, and the cope is filled with sand and rammed.

5. Mould Ready for Pouring: The pattern is removed, cores are set if needed, and the cope is placed back on top of the drag. Weights are often placed on top to prevent the cope from lifting during pouring.

Advantages and Disadvantages of Casting

Advantages:

• Can produce internal cavities and complex geometries.

• Scales from small parts ($100$s of grams) to very large parts ($1000$s of kilograms).

• Capable of pouring almost any material, both ferrous and non-ferrous.

• Economical with high material re-usability.

• High productivity and adaptability for mass production.

• Simpler and cheaper tooling compared to other processes.

Disadvantages:

• Poor surface finish, often requiring secondary machining operations.

• Susceptible to various casting defects.

• Lower fatigue strength compared to wrought products.

• Challenges with dimensional consistency and accuracy.

• High labor intensity and poor working environment (high temperatures).

• Generally limited to metals with lower melting points (in specific processes).

• Not suitable for low-volume production in specific automated setups.

Pattern Types and Design

• Solid or Single-Piece Pattern: The simplest type, made without sub-parts or joints. It is usually used for large, simple shapes like gear blanks or square blocks. It is placed entirely in the drag.

• Split Pattern or Two-Piece Pattern: Divided into two halves—one for the cope and one for the drag. Dowel pins and holes are used to ensure accurate alignment. Used for complex or large parts like pulleys.

• Match Plate Pattern: A split pattern where the cope and drag halves are mounted on opposite faces of a metallic plate (the match plate). It often includes the gate and runner system, allowing for high output and multiple products in a single box (e.g., pipe fittings).

• Cope and Drag Pattern: Similar to split patterns but used for heavy products (e.g., engine blocks). The cope and drag halves are molded in separate boxes.

• Shell Pattern: Used for curved or hollow parts like pipes. The inner shape forms the core, and the outer shape forms the mould.

Pattern Allowances

• Draft Allowance: Taper provided on vertical surfaces perpendicular to the parting line to facilitate easy withdrawal of the pattern. It typically ranges from $1^{\circ}$ to $3^{\circ}$ for wooden patterns.

• Distortion Allowance: Pre-distortion of the pattern in the opposite direction of expected solid state distortion. Common in shapes like $U$, $V$, $T$, or $L$, which tend to contract at closed ends.

• Machining Allowance: Making the pattern slightly larger so that surface roughness, oxidation, and imperfections can be machined away to achieve required dimensions and finish.

• Shrinkage Allowance: Compensates for the contraction of metal upon cooling. A shrinkage rule is used; for cast iron, this rule is $1/8$ inch longer per foot. A $4$ inch gear blank would be measured as $4-1/2$ inches on the shrinkage rule to ensure the final product is correct.

• Rapping/Shake Allowance: Before removal, a pattern is rapped to slightly enlarge the cavity. This is difficult to quantify as it depends on the operator. It can be ignored for small/medium castings but is critical for large, precision parts.

Molding Sand Types and Properties

• Soil Components:

  • Sand: Coarse particles, size $2.00$ to $0.05\,mm$. No plasticity.

  • Silt: Dust-like sediment, size $0.002$ to $0.06\,mm$. Very low plasticity.

  • Clay: Fine-grained minerals, size lower than $0.002\,mm$. High plasticity.

• Sand Types:

  • Green Sand: Mixture of silica, $18\%$ to $30\%$ clay, and $6\%$ to $8\%$ water. It holds moisture when squeezed.

  • Dry Sand: Green sand moulds that have been baked or dried. Better for large castings.

  • Loam Sand: Equal parts sand, silt, and clay; used for very large castings.

  • Parting Sand: Clean, clay-free silica sand used to prevent the green sand from sticking to the pattern.

  • Facing Sand: High-strength, refractory sand used directly next to the pattern for better finish.

  • Backing Sand (Black Sand): Recycled sand used to fill the rest of the flask.

  • System Sand: Used in mechanical foundries; cleaned and reactivated sand used for the whole flask.

  • Core Sand (Oil Sand): Silica sand mixed with core oil (linseed oil, resin, mineral oil).

Melting Equipment

• Crucible Furnace: Best for small foundries. Types include Pit type (natural draught, in-floor) and Tilting type (forced draught, above-floor).

• Reverberatory (Air) Furnace: Metal is melted on a hearth by heat passing over it. Heat transfer is primarily through radiation from the refractory walls and roof.

• Open Hearth Furnace (OHF): Uses gaseous/liquid fuels. Employs regenerators (checker chambers) beneath the furnace to preheat combustion air using exhaust gas heat.

• Electric Furnace: Uses carbon electrodes to strike an arc with the metal. Capacity ranges from $3$ to $10$ tons. Provides high melting rates and excellent temperature control.

• Cupola Furnace: A vertical cylindrical steel shell lined with fire bricks. Raw materials (metal, flux like $CaCO_3$, and coke) are charged at the top while air is introduced through tuyeres near the bottom.

Specialized Casting Processes

• Gravity Die Casting (Permanent Mold): Uses reusable metal moulds (steel/graphite). Molten metal flows under gravity. Produces high-precision parts like pistons and gear housings.

• Pressure Die Casting: Molten metal is injected at high pressure into reusable dies.

  • Hot Chamber: Used for zinc, tin, and lead alloys.

  • Cold Chamber: Used for aluminum, copper, and magnesium.

• Investment Casting (Lost-Wax): A wax pattern is coated in ceramic slurry to form a disposable mould. The wax is melted out before pouring. Capable of creating high-quality, complex parts like gas turbine blades.

• Plaster Casting: Uses Plaster of Paris instead of sand. Slow cooling allows for high accuracy in thin sections ($30\,g$ to $45\,kg$). Not for ferrous materials.

• Centrifugal Casting: The mould rotates at high speeds. Centrifugal force pushes metal to the outer walls. Used for cylindrical parts like bush bearings and cylinder liners.

• Lost-Foam Casting: Uses polystyrene foam patterns that evaporate when molten metal is poured. Similar to investment casting but simpler for mass production.

• Vacuum Casting: Production occurs under vacuum pressure ($100\,bar$ or less) to eliminate gas entrapment and improve surface quality.

• Squeeze Casting: A hybrid of casting and forging. Metal is poured into a die and then high pressure is applied during solidification. Eliminates porosity.

• Continuous Casting: Molten metal is poured into a water-cooled, open-ended mould. It solidifies from the outside in as it is continuously extracted as a strand (e.g., steel billets/slabs).

• Shell Molding: Sand mixed with resin is hardened into a thin shell around a hot metallic pattern. Provides excellent finish and accuracy.

Casting Defects

Metallurgical and Gas Defects:

• Gas Porosity: Caused by trapped air or released hydrogen. Includes Pinholes (small surface holes), Blowholes (large internal holes detected by X-ray), and Open holes.

• Shrinkage Porosity: Jagged, angular voids caused by volume reduction during cooling. Includes Open shrinkage (caved surfaces) and Closed shrinkage (internal macroporosity).

• Slag Inclusions: Non-metallic materials like sand or oxides trapped in the casting.

• Dross: Metal loss due to oxidation on the surface of non-ferrous metals.

Heat-Related Defects:

• Hot Tears: Irregular cracks caused by thermally induced stresses during contraction.

• Cold Shut: Rounded cracks occurring because two streams of metal were too cold to fuse properly.

Mold Material Defects:

• Cuts and Washes: Low bulges where metal washed away sand.

• Swells: Smooth bulges caused by the mould wall moving back under metallostatic pressure.

• Metal Penetration: A rough surface caused by molten metal entering gaps in coarse sand grains.

• Rat Tails: Irregular lines caused by compression failure and expansion of the sand surface.

Shape and Finish Defects:

• Mismatches: Misalignment of cope and drag (Mold Shift) or cores (Core Shift).

• Flash: Thin sheet of metal leaking through parting surfaces due to insufficient clamping force.

Testing and Inspection of Castings

• Destructive Testing: Specimens are removed to test tensile strength, hardness, torsion, fatigue, and corrosion resistance. Pressure tightness is tested by sealing and pressurizing with air, water, or oil.

• Non-Destructive Testing (NDT):

  • Visual Inspection: Identification of surface cracks, misruns, and sand inclusions.

  • Dimensional Inspection: Using manual tools or Coordinate Measuring Machines (CMM).

  • Liquid Dye Penetrant Inspection (LPI): Dye is applied to the surface; capillary action draws it into cracks. A developer (powdered chalk) then reveals the crack location.

  • Magnetic Particle Inspection (MPI): Used for ferromagnetic materials (Iron, Nickel, Cobalt). Magnetic flux leakage at discontinuities attracts sprayed iron oxide particles.

  • Ultrasonic Testing (UT): Uses high-frequency sound waves. Reflections from internal barriers indicate the location and size of flaws.

  • Radiographic Inspection (X-Ray): Radiation passes through the casting. Variations in density (holes/cracks) show up as dark areas on film. Requires approximately $1000\,V$ per $25\,mm$ of steel thickness.