Rapid Prototyping

Casting Processes

• Example Parts

• Requirements

• Terminology

• Cooling Curves

• Solidification Time

• Fluidity

• Design Considerations

• Defects

• Processes

Rapid Prototyping

Pouring Molten Metals

Non-ferrous application: probably zinc

Rapid Prototyping

Example of a large casting

Note all the subsequentmachining operations

Rapid Prototyping

Rapid Prototyping

Rapid Prototyping

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Rapid Prototyping

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

Rapid Prototyping

Terminology

Weir

Core Print

Draft

Core Box

Chills

Insulation Dross

Better surfaces on bottom of casting

Trap

Rapid Prototyping

CoolingCurve

PureEutecticMaterial

Rapid Prototyping

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.

Rapid Prototyping

Crystal Shapes

Small, somewhat uniform crystals form the skin sectionsLarger, directional, columnar crystals “grow” towards the center

Pure Metal Alloyed

Alloys tendto segregateto thecenter ofthe castingduringcooling

Rapid Prototyping

Prediction ofSolidification Time

• Important for strength properties

• Cast part should solidify before the risers

• Most efficient design of risers minimizes surface area with required volume.

Rapid Prototyping

Prediction of Solidification Time

• Chvorinov’s rule:V = volumeA = surface arean = 1.5 to 2B = constant for metal cast, mold material and casting conditions

t BV

As

n

Rapid Prototyping

Sprue, Runner and Riser Design

The art is progressively becominga science with the aid ofcomputer simulation

Rapid Prototyping

Fluidity• Characteristics of the molten metal

– Viscosity: temperature sensitive

– Surface Tension: Oxide Films

– Inclusions

– Solidification patterns of the alloy

Rapid Prototyping

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

Rapid Prototyping

Design Considerations• Parting Line Location

• Section Thickness

• Flat Areas

• Shrinkage

• Draft

• Machine Allowances

Rapid Prototyping

Parting Plane Affects:

• Number of cores required

• Effective and economical gating

• Methods used to support cores

• Final dimensional accuracy

• Ease of Molding

Rapid Prototyping

Section Thickness

• Hot Spots

• Angles

Rapid Prototyping

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”

Rapid Prototyping

ShrinkageProblems

Cavities

Allowances

Rapid Prototyping

Machine Allowances

• Draft: – 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

Rapid Prototyping

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.

Rapid Prototyping

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