SHAPE CASTING PROCESSES Casting Processes

Common Shape Casting Processes

Basic Casting Terminology

Pattern and Cores

Sand Casting

Sand Casting Patterns Patterns may be made from a variety of materials: –Wood –Metal –Hard Polymers –EPS (Styrofoam)

Types of Sand Casting Patterns One-Piece

Types of Sand Casting Patterns One-Piece (Using a Follow Board)

Types of Sand Casting Patterns Split Pattern

Types of Sand Casting Patterns Match-Plate

Types of Sand Casting Patterns Cope-and-Drag Split Patterns

Types of Sand Casting Patterns Loose Piece Pattern

Types of Sands Sands can be comprised of: ordinary silica (SiO2), zircon, olivine, or chromite and may be compounded with additives to meet four requirements: –Refractoriness –Cohesiveness –Permeability –Collapsibility

Green Sand Contains bonding agents and water Typical “green sand” is: –88% silica –9% clay –3% water

Mullers ContinuousBatch

Sand Parameters Grain Size – measured by sifting sand through sieves Moisture Content – measured with moisture meter Clay Content – measured by weighing a sample of sand before / after washing Permeability – AFS permeability number measured using “standard rammed sample” (Green) Compressive Strength – measure of mold strength before pouring Hardness – resistance of packed sand to penetration

Permeability Tester

Sand Hardness

Desirable Properties of Sand- Based Molding Materials Inexpensive in bulk quantities Retains properties through transportation and storage Uniformly fills flask or container Can be compacted or set by simple methods Sufficient elasticity to remain undamaged during pattern removal Can withstand high temperatures and maintain dimensional stability until solidification Sufficiently permeable to allow gases to escape Sufficiently dense to prevent metal penetration Sufficiently cohesive to prevent wash-out of mold material into the pour stream Chemically inert to the metal being cast Can yield to solidification and thermal shrinkage, preventing hot tears and cracks Recyclable

Sand Casting Defects Sand Expansion Defects – occur on large, flat portions of castings where large expansion must occur in one direction. Can be alleviated by: –Careful selection of sand geometry (some sands “slide” past each other more easily –Use of low-expansion sands (zircon or olivine) –Additional clay added to absorb expansion –Volatile additives in sand mixture (burn off and create space) Voids or Blows – due to trapped gas and low sand permeability Penetration – overly fluid pour traps sand particles in melt Hot tears or cracks – high solidification shrinkage resisted by mold with poor collapsibility

Sand-Cast Parts

Green Sand Casting Summary

SodiumSilicate-CO 2 Molding Sand has 3%-6% sodium silicate (“water glass”) added as binder. –Mixed in a standard muller –Addition of high-concentration CO2 gas causes binder to harden in a matter of seconds: –Very strong mold, but poor collapsibility

No-Bake / Air Set / Chemically Bonded Sands A variety of other binders that cure at room temperature can be used to hold sand together Provide greater mold strength than green sand. Added cost and time.

Shell Casting Individual grains of sand are pre-coated with a phenolic resin and heat-sensitive liquid catalist. Sand spread on top of a heated (450 – 600 deg. F) metal pattern. Heat bonds the material near the pattern. Excess sand is poured off. Thin shell is removed and placed in an oven for further curing.

Shell Casting

Shell-Mold Casting Summary

V-Process (Vacuum Molding)

Cores and Core-Making

Cores to Make Holes

Dump-type Core Box (Dry Sand Cores)

Chaplets

Three-Segment Flasks