Casting Processes Example Parts Requirements Terminology Cooling Curves Solidification Time Fluidity Design Considerations Defects Processes

Pouring Molten Metals Non-ferrous application: probably zinc

Example of a large casting Note all the subsequent machining operations

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Requirements Mold Cavity: single or multiple use Melting Process Pouring Technique Solidification Process Mold removal Clean, Finish, Inspection

Terminology Weir Core Print Draft Core Box Chills Insulation Dross Better surfaces on bottom of casting Trap

Cooling Curve Pure Eutectic Material

Metal Properties Metals are a crystalline structure that have a primary bond: metallic bond. Rate of cooling determines the size of the crystal. Size and shape of the crystal determines the mechanical properties.

Crystal Shapes Alloyed Pure Metal Alloys tend to segregate to the center of the casting during cooling Small, somewhat uniform crystals form the skin sections Larger, directional, columnar crystals “grow” towards the center

Prediction of Solidification Time Important for strength properties Cast part should solidify before the risers Most efficient design of risers minimizes surface area with required volume.

Prediction of Solidification Time Chvorinov’s rule: V = volume A = surface area n = 1.5 to 2 B = constant for metal cast, mold material and casting conditions

Sprue, Runner and Riser Design The art is progressively becoming a science with the aid of computer simulation

Fluidity Characteristics of the molten metal Viscosity: temperature sensitive Surface Tension: Oxide Films Inclusions Solidification patterns of the alloy

Fluidity Casting Parameters Mold design: sprue, riser, etc. Surface characteristics of mold materials: heating mold means more fluidity, but slower cool, and less strength Degree of superheat Rate of pouring: slower = lower fluidity

Design Considerations Parting Line Location Section Thickness Flat Areas Shrinkage Draft Machine Allowances

Parting Plane Affects: Number of cores required Effective and economical gating Methods used to support cores Final dimensional accuracy Ease of Molding

Section Thickness Hot Spots Angles

Flat Areas Warping, surface finish, temperature gradients. Solution: break up with ribs, serration Minimums: steel: 3/16” Gray Cast:: 1/8” Aluminum: 1/8” alloys: 0.050” Zinc Alloys: 0.020”

Shrinkage Problems Cavities Allowances

Machine Allowances Draft: Bosses and Spot Faces 0.5 to 2 degrees 1/16”/foot to 3/16”/foot are typical Bosses and Spot Faces Tolerances: 1/32” for small castings, 1/4” for large

Defects Metallic Projections: flash, swells Cavities, blowholes, and pinholes Discontinuities: cracks, tears, cold shuts (knit lines), incomplete runs Defective surfaces: sand layers, oxides Inclusions: form stress risers: slag etc.

Processes Single Use Molds Sand Casting Plaster Mold Ceramic Mold Investment Casting Lost Foam Multiple Use Molds Permanent Molds Die casting Centrifugal casting Continuous casting