Types of Materials To Use In The Diverse Methods of Casting
Casting involves pouring liquid metal into a mold cavity, allowing it to solidify, then removing the final part. This manufacturing process allows the efficient creation of complex, detailed metal components at high production volumes. However, cast parts are highly influenced by the mold material and base metal used. Selecting the ideal pairing is crucial for success.
In this comprehensive guide, we’ll examine the most common categories of materials used in major casting processes along with their key characteristics and applications.
Sand Casting Materials
Sand casting utilizes bonded sand molds for simplicity and flexibility. Sand is able to withstand high pouring temperatures, capture fine details, and be recycled many times. There are two main material pairings:
Green Sand
Silica sand mixed with clay and water for bonding. Not heat cured but used in a moist “green” state.
Ideal for:
- High volume production
- Large metal castings
- Low material costs
- Quick molding
Limitations:
- Dimensional accuracy +/- 0.5%
- Surface finish equivalent to 250 grit
Typical metals cast:
- Gray and ductile iron
- Carbon steels
- Stainless steels
- Aluminum alloys
Chemically Bonded Sand
Sand is aggregated using chemical binders that polymerize at room temperature.
Ideal for:
- Intricate, detailed molds
- Low volume production
- Castings under 50 lbs
- Minimal drying time
Limitations:
- Dimensional accuracy +/- 0.2%
- Brittle molds prone to damage
Typical metals cast:
- Non-ferrous metals and alloys
- Magnesium, copper, zinc
- Sterling silver, brass
Investment Casting Materials
Investment casting uses a secondary ceramic mold fabricated on top of an expendable wax pattern. This allows extreme accuracy and a smooth surface finish.
Wax Patterns
Made from injection molded wax or 3D printed from UV curable resins. Used for net-shape patterns burned out during firing.
Ideal for:
- Detailed castings with tight tolerances
- Minimizing finishing
- Cost-effective for small parts
Limitations:
- Pattern costs for injection wax tooling
- Maximum size around 18” x 7” x 9”
- Slow for large volumes
Typical metals cast:
- Stainless steel alloys
- Heat-resistant super alloys
- Titanium-based alloys
- Precious metals
Ceramic Slurry
Built up using silica flour and binders to create a fired ceramic mold capturing imprint of the wax pattern.
Ideal for:
- High-temperature metal pours
- Tolerances to +/- 0.003”
- Castings with thin walls
- Excellent surface finish
Limitations:
- Brittle with minimal collapsibility
- Subject to cracking and tearing
- Limited mold lifespan
Typical metals cast:
- High carbon and alloyed steels
- Nickel and cobalt superalloys
- Reactive alloys like titanium
Plaster Mold Casting Materials
Gypsum-bonded plaster molds offer an economical casting method for low-temperature alloys and non-metals.
Plaster of Paris
Fast-setting calcium sulfate hemihydrate that cures when mixed with water. Inexpensive and readily available.
Ideal for:
- Art/sculpture casting
- Non-ferrous metal prototypes
- Low-cost molds
- Porous molds for trapped air
Limitations:
- Low strength and abrasion resistance
- Dimensional accuracy +/- 1%
- Max temp resistance 120°C
Typical materials cast:
- Zinc and tin alloys
- Lead and pewter
- Wax, plastic, concrete
Silica Sol Plaster
Plaster molds are strengthened using colloidal silica for higher strength and refractory properties.
Ideal for:
- Casting low melt point alloys
- Intricate mold geometry
- Smoother surface finishes
- Cost-effective process
Limitations:
- Still prone to abrasion damage
- Dimensional accuracy +/- 0.5%
- Maximum temps around 300°C
Typical materials cast:
- Magnesium alloys
- White metal die-cast alloys
- Bronze, brass, aluminum
Die Casting Materials
Die casting uses permanent steel molds on hydraulic-powered machines to produce highly precise, rapid castings.
H-13 Tool Steel
Chromium-molybdenum-vanadium alloy tool steel offers the highest die life. Heat treated to 45-51 HRC.
Ideal for:
- High production volumes
- Tens of thousands of cast cycles
- Excellent as-cast finish
- Tolerances to +/- 0.005”
Limitations:
- High tooling costs
- Lead times over 12 weeks
- Limited size to around 75 lbs
Typical metals cast:
- Zinc alloys (ZAMAK)
- Aluminum die-cast alloys
P20 Steel
Nickel-chromium-molybdenum alloy steel is used for lower-cost dies. Heat treated to 30-35 HRC.
Ideal for:
- Prototyping or pilot runs
- Medium production volumes
- Quick tooling delivery
Limitations:
- Lower die life around 10,000 shots
- Finish equivalent to 250 grit
- Tolerances +/- 0.02”
Typical metals cast:
- Magnesium die-cast alloys
- Brass and bronze alloys
Permanent Mold Casting Materials
Permanent mold casting (PMC) uses metal molds to offer excellent dimensional accuracy and surface finish.
Beryllium Copper Alloy
Age-hardened copper beryllium provides excellent thermal conductivity. Polished mold cavity surfaces.
Ideal for:
- Highest as-cast precision
- Mass-producing aluminum parts
- High production volumes
- Gravitational pouring
Limitations:
- High tooling costs
- Mold maintenance required
- Max part size around 25 lbs.
Typical metals cast:
- Wrought aluminum grades
- Magnesium and zinc alloys
- Copper alloys
Cast Iron
Durable, lower-cost iron molds machined from gray cast iron. Used for short-run productions.
Ideal for:
- Low-volume casting runs
- Prototyping cast parts
- Moderately priced tooling
- Low partie counts < 5,000 pieces
Limitations:
- Dimensional accuracy +/- 0.5%
- Part weight under 5 lbs.
- Max production volumes < 1,000 pieces
Typical metals cast:
- Tin, lead, and pewter
- Brass and bronze
- Cast irons
Centrifugal Casting Materials
Centrifugal force is used to contain molten metals against cylindrical mold walls, producing tubular components.
Steel Mold Cans
Cylindrical cast iron or steel molds rotated at high speeds during pouring. Inner mold walls are coated.
Ideal for:
- Repeated production runs
- Cast tubes up to 6 feet long
- Wall thicknesses down to 3/16”
- High material purity
Limitations:
- High tooling costs
- Maximum diameters around 36”
- Difficult design changes
Typical metals cast:
- Stainless steel
- Nickel and titanium alloys
- Reactive alloys like zirconium
Ceramic Lined Cans
Disposable unfired refractory linings containing compacted silica sand. Low cost per mold.
Ideal for:
- Single production runs
- Large diameter molds
- Castings over 500 lbs
- Low costs
Limitations:
- Dimensional accuracy +/- 2%
- Parting line flash common
- Prone to liner cracking
- Surface finish under 63 Ra
Typical metals cast:
- Low carbon steels
- Cast irons
- Aluminum and magnesium
Sand-Resin Shell Molds
A hybrid approach using sand molding covered with a thin resin-bonded layer for accuracy and surface finish.
Silica Sand Mold
Provides bulk containment for the molten pour at low material costs.
Ideal for:
- High production volumes
- Large castings
- Flexible mold geometry
- Low tooling costs
Limitations:
- Limited detail resolution
- Rough surface finish
- Distortion prone
Typical metals cast:
- Carbon and alloy steels
- Stainless steels
- Cast irons
Phenolic Urethane Resin
A thin resin-bonded layer provides high-precision shape and finish replication.
Ideal for:
- Minimizing machining
- Intricate details
- Class A surface finishes
- Casting weights from 2 oz to 5,000 lbs
Limitations:
- Added pattern costs
- Process monitoring for defects
- Resin coatings wear over time
Typical metals cast:
- Aluminum and magnesium
- Copper alloys
- Nickel alloys
In summary, understanding the unique benefits and tradeoffs of the various mold materials and metal alloys used in each casting process is crucial for both quality and cost-effective production. With so many material variables at play, foundries must carefully select and refine the ideal combinations over time for their specific needs and applications. No singular pairing will be optimal across the board. By leveraging the right materials, foundries can consistently cast metal components with maximized efficiency.