Plaster and Investment Casting Processes

Plaster Mould Casting

• A specialized casting process producing nonferrous castings with high dimensional accuracy, smooth surfaces, and fine detail.
• Similar to sand casting but uses plaster of Paris.
• Four recognized plaster mould processes: conventional, match plate pattern, Antioch, and foamed plaster.
• Applications include near-net shape castings like compressor wheels, impellers, electronics gear components, tire molds, and aerospace fuel pump systems.
• Machining costs are significantly higher for these parts cast in other moulds.
• Plaster mould castings usually weigh less than 9 kg (20 lbs) but substantial quantities of castings up to 34 kg (75 lbs) are made.
• An aluminum alloy casting of 1815 kg (4000 lb) has been successfully produced.

Investment Casting

• A ceramic slurry is applied around a disposable pattern (usually wax) to create a casting mold.
• The pattern and mold are disposable.
• Two distinct processes: solid investment (solid mold) process and ceramic shell process.
• The ceramic shell process is more common in engineering applications.

Permanent Mould Casting

• Also known as gravity die casting.
• A metal mold (two or more parts) is repeatedly used to produce many castings.
• Simple cores are metal; complex cores are sand or plaster (semi-permanent).
• Used for high-volume production of castings with uniform wall thickness and limited intricate coring.
• Can produce complex castings, but production volume must justify mold costs.
• More uniform castings, closer tolerances, and improved surface finish are features compared to sand casting.
• Limitations: Not all alloys are suitable, low production quantities are expensive due to tooling, some shapes are not possible.
• Suitable metals include aluminum, magnesium, zinc, copper alloys, and hypereutectic gray iron.

Die Casting

• Characterized by rapidly injecting molten metal into a metal die using hydraulic energy.
• The die absorbs stresses, dissipates heat, and facilitates part removal for the next cycle.

Centrifugal Casting

• Molten metal is poured into a revolving mold, solidified by centrifugal force.
• Used for mass production of circular castings which are free from impurities, and produce high-density, strong castings.
• Castings experience directional solidification (colder metal outside, molten inside).
• Suitable for cylindrical parts and pipes.

Centrifugal Casting Types

• True centrifugal casting
• Semi-centrifugal casting
• Centrifuged casting

Plastics Moulding Processes

• Methods for creating components from granular, powder, or other forms of plastic materials.
• Processes: compression molding, transfer molding, injection molding, cup molding, extrusion molding, calendaring, thermoforming, casting.

Injection Die Moulding

• Thermoplastic materials soften when heated and re-harden when cooled (no chemical change).
• Granular material is fed into a heating cylinder which fills the mold.
• The injection ram pushes the material into a cylinder, forcing it through a nozzle and into mold cavities.
• The mold then closes and cools the part.
• Material heating temperatures are typically 180-280°C.
• Automated systems maintain mold temperature.
• Primarily for thermoplastics but adaptable to thermosets and rubbers.

Extrusion Moulding

• Thermoplastic materials are suitable for extrusion into rods, tubes, sheets, film, pipes, and ropes.
• Thermosets aren't suitable.
• Continuous process with a low initial cost.

Sand Casting Process Flow

• Pattern creation, mold preparation, metal melting, pouring, mold breaking, cleaning, inspection (fettling).
• Sand moulds are created using patterns and cores.
• Cores are shaped pieces made of sand or rubber to form internal features.
  • Mold halves (drag and cope) hold the core.
• Molten metal is poured into the mold cavity, then solidified.
• Castings are removed, and unwanted parts (risers, gates) are removed.

Furnace

• Used to heat metal above its melting point.
• Types: electric (impurity-free, expensive), oil-fired (metal picks up impurities), coal-fired (metal picks up impurities).

Cupola

• Used for melting cast iron.
• Economical and convenient.
• Uses coke as fuel.

Induction Furnaces

• Commonly used to melt iron and non-ferrous alloys due to excellent metallurgical control and relatively pollution-free operation.
• Coreless and channel induction furnaces exist in varying sizes to suit the metal needed for the production to fit.
• Large high-powered induction furnaces are operated at 60Hz.
• Modern units use medium frequencies due to efficiency and quick start-up/shutdown capability.

Coreless Induction Furnace

• Refractory-lined crucible surrounded by water-cooled copper coil.
• All metal in crucible is the secondary.
• Commonly used for melting and superheating.

Channel Induction Furnace

• Coil surrounds only an inductor, which the molten metal forms a loop.
• Surrounding copper coil is the primary.
• Primarily for superheating, duplexing, and holding. Often used for copper alloys.
• Has limitations compared to the coreless induction furnace.

Explore the specialized techniques of plaster mould and investment casting. Understand their unique processes, applications, and the dimensional accuracy they provide in nonferrous castings. This quiz will challenge your knowledge on these essential manufacturing methods.