Expandable Mold Casting Permanent Mold Casting Casting Defects and Design Principles
W. Li
Sp’ 06
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Pure Metals – Solidify at a constant temperature – Planar solidification front – Columnar crystals Eutectics – Solidify at a constant temperature – Planar solidification front – Lamellar crystals Solid Solutions – Solidify over freezing range (solidus – liquidus) – Dendritic structure
Microsegregation
Microsegregation or coring
Macrosegregation caused by compositional difference over long distances within a casting.
Fluidity is used to characterize the mold filling ability, sometimes castability. m Superheat =
(Tmelt –TL)/TL
Expendable Mold • Permanent Pattern mSand Casting mPlaster Molding
• Expendable Pattern mLost Foam mLost Wax (investment casting)
m Permanent Mold
• Die mHot Chamber mCold Chamber mThixotropic
Use gravity to fill the mold m Mold is destroyed to remove casting m Metal flow is slow m Walls are much thicker than in die casting m Cycle time is longer than die casting because of inability of mold material to remove heat
Advantages • Inexpensive mold • Complex geometry • All alloys • Unlimited size • Economical in low
quantities
m Disadvantages
• Cost per part is
higher mLabor intensive mSlower production rate
• Rough surface finish • Loose tolerances • Requires relatively
thick walls (0.120”)
Create Wax Pattern m Assemble Wax Tree m Coat with Ceramic m Melt out wax m Pour in molten metal m Break off ceramic
Advantages • Good dimensional
accuracy • Relatively inexpensive mold • Rapid production rates possible • Complex shapes • Very high temp materials - Titanium
m Disadvantages
• Long production
cycle mleads to high cost per part
• Mold is not reusable
Mold removed, not destroyed m Uses gravity to fill mold m Metal flow is slow m Mold is steel - has comparatively good thermal conductivity m Machines smaller
Liquid metal injected into reusable steel mold, or die, very quickly with high pressures m Reusable steel tooling and injection of liquid metal with high pressures differentiates die casting from other metal casting processes
Hot Chamber • Zn or Zn alloys only • Higher production rate 15 cycles per minute for small parts • Metal injected directly from melt zone
m Cold Chamber
• Al, Mg, Zn • Melt is poured into
cylinder, which is then shot into chamber
Advantages • High production
rates possible, with high level of automation • Very thin walls possible (0.020” for Mg die casting) • Good surface finish • Economical in large quantities • Better control of mold temps
m Disadvantages
• High tooling cost • Long lead times
(months) • Limited size (<25 lbs) • Limited in alloys (low temperature only) mZn mAl mMg mCu
Porosity m Impurities oxides m Cold Shut m Hot Tearing m Dimensional tolerance and part geometry m Warping m Shrinkage
Fill completely with metal m Solidify quickly without defects m Eject readily from the die
Wall thickness should be as consistent as possible Intersections of walls, ribs and gussets should blend with transitions and generous radii • Generous radii and transitions promote metal flow and internal
integrity. Radii and fillets enhance structural integrity. Fillets reduce heat concentration in both the die and castings.
m Specify standard draft
• Draft is highly desirable on surfaces parallel to the direction of die
draw. Recommended draft is determined by the alloy, the length of the dimension parallel to the die draw, and If the dimension is an inside or outside wall
m Avoid undercuts
• May require machining operations or additional die components,
such as retractable core slides
m Dimensions with critical tolerances should relate to only
one die member • Precision is reduced for relationships across the parting line or to
moving components
Jet engine turbine blades
W. Li
Sp’ 06
m Fluid Flow
Heat Transfer • Degree of Superheat • Material Properties • Die Properties • External Heating/Cooling • Part Geometry
• Degree of Superheat • Part Geometry • Material Properties
(viscosity) • Die Properties • Continuity Equation mA0v0 = A1v1
• Chvorinov’s Rule
• Bernoulli Equation • Max Flow without
turbulence
ts
C
. V A
2
mReynold’s Number Re
ρ.v .D µ
Mass ContinuityQ = A1v1 = A2v2 • where Q is the volumetric rate of flow, m3/s,
A is the cross sectional area of the liquid stream, and v is the velocity of the liquid in that location. vDρ Re = η
m Reynolds number
• where v is the velocity of the liquid, D is the
diameter of the channel, ρ is the density, and η is the viscosity. • For casting, Re = 2,000 – 20,000
Under steady well-developed flow conditions, the total energy of a unit volume of material must be a constant at every part of the system. 2 2 ρv0 ρv1 p0 + + ρgh0 = p1 + + ρgh1 + f 2 2
• where p is the pressure, v is the velocity, h
is the height above a reference plane, f is the energy losses due to friction, and ρ is the density.
The solidification time is a function of the volume of the casting and its 2 surface area (Chvorinov’s rule) volume Solidification _ Time = surface _ area