Casting Processes

Casting Process

• Casting processes involves pouring molten metal into a mould cavity.
• This is followed by solidification to create a desired shape.

Learning Objectives

• Identify 4-5 different casting processes.
• Learn the two groups of moulds.
• Understand the purpose, pros/cons, applications and methods of each process.
• Understand sand casting, CO2, and investment casting set up and gating systems.

Overview of Casting Process

• The casting process involves making a mould, pouring molten metal into it, and allowing it to solidify.

Expendable Mould Casting

• Mould is used only once
• Process has a Low production rate.

Multiple-Use Mould Casting

• Moulds are made of metals.
• Moulds can be used many times.
• Process has a High production rate.

Expendable Mould Casting Processes

• Green Sand
• Carbon Dioxide
• Investment

Green Sand Casting

• Mould Material: Sand, Bentonite (clay), and Water.
• Cast Materials: Wide range of metals.
• Surface Finish: Poor.
• Quantity: Low.
• Dimension Accuracy: Low.
• Shapes Complexity: Simple.
• Mechanical Properties: Poor (weak & brittle due to loose sand).
• Capital Cost: Low.

Carbon Dioxide Casting

• Mould Materials: Sand, Sodium Silicate, and CO2.
• The sand is hardened after Silica gel (glue) is formed from the CO2 and Sodium Silicate mixture.
• Recycling is difficult
• Cast Materials: Wide range of metals.
• Surface Finish: Poor.
• Quantity: Low.
• Dimension Accuracy: Low, but better than green sand due to the hardened sand.
• Shapes Complexity: Simple, but can be more complex than green sand.
• Suitable for parts with thin sections e.g., heat exchanges and core fabrication.
• Mechanical properties: better than Green sand
• Capital cost is low

Investment Casting

• Mould Materials: Primary (fine) and Secondary (coarse) refractory material.
• Cast Materials: Wide range of metals and high-temperature alloys.
• Surface Finish: Good.
• Quantity: Low.
• Dimension Accuracy: High.
• Shapes Complexity: Complex.
• Mechanical Properties: Good.
• Capital Cost: High.

Multiple-Use Mould Casting Processes

• Hot Chamber Pressure Die
• Cold Chamber Pressure Die

Hot Chamber Pressure Die Casting

• Mould Materials use Alloy Steels.
• Cast Materials use Zinc.
• Zinc has a lower melting temperature, minimizing iron contamination.
• Surface Finish is Good.
• Quantity: High (mass production).
• Dimension Accuracy is Good.
• Shapes Complexity is Complex.
• Good mechanical properties due to faster cooling.
• High Capital Cost, but economical for mass production.

Cold Chamber Pressure Die Casting

• Mould Materials: Alloy Steels.
• Cast Materials: Aluminum with lower melting temperature but higher than Green Sand, Carbon Dioxide and Invesment processes.
• Surface Finish: Good.
• Quantity: High (mass production).
• Dimension Accuracy: Good.
• Shapes Complexity: Complex.
• Mechanical Properties: Good (smaller grains due to faster cooling).
• Capital Cost: Very high (more economical for mass production).

Gating System Components

• Pouring Basin: Where molten metal is initially poured.
• Sprue: Allows molten metal to descend to the runners.
• Runner: Allows molten metal to flow to various mould cavities through the ingate.
• Gate: Allows molten metal to flow from the runners to the mould cavities.
• Riser: Feeds shrinkage during solidification.
• Core
• Mould
• Vents: Increase permeability of the sand

CO2 Process

• Moulding Mixture: Sand with a solution of sodium silicate.
• Silica gel is formed by reacting with CO2.
• Produces strong cores and moulds without baking.
• Less draft is necessary.

Investment Casting Steps

• Create a wax pattern by injection moulding.
• Assemble the wax pattern.
• Produce a ceramic shell mould by slurry coating and stuccoing.
• Dewaxing and firing.
• Pouring molten metal into the mould.
• Shell removal process and machining.

Applications

• Sand Casting: Machine tool bed and pump casings.
• Die Casting: Aluminum engine blocks and zinc toy cars.
• Investment Casting: Ni-based superalloy turbine blades and stainless steel implants.