CASTING-Traditional Manufacturing Processes
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Transcript of CASTING-Traditional Manufacturing Processes
8/7/2019 CASTING-Traditional Manufacturing Processes
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Traditional ManufacturingProcesses
1RAKESH CHANDER SAINI
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2RAKESH CHANDER SAINI
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Casting
Refractory mold pour liquid metal solidify, remove finish
• VERSATILE: complex geometry, internal cavities, hollow sections
• VERSATILE: small (~10 grams) very large parts (~1000 Kg)
• ECONOMICAL: little wastage (extra metal is re-used)
• ISOTROPIC: cast parts have same properties along all directions
3RAKESH CHANDER SAINI
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Different Casting Processes
Process Advantages Disadvantages Examples
Sand many metals, sizes, shapes, cheap poor finish & tolerance engine blocks,
cylinder heads
Shell mold better accuracy, finish, higher
production rate
limited part size connecting rods, gear
housings
Expendable
pattern
Wide range of metals, sizes,
shapes
patterns have low
strength
cylinder heads, brake
components
Plaster mold complex shapes, good surface non-ferrous metals, low prototypes of
finish production rate mechanical partsCeramic mold complex shapes, high accuracy,
good finish
small sizes impellers, injection
mold tooling
Investment complex shapes, excellent finish small parts, expensive jewellery
Permanentmold
good finish, low porosity, highproduction rate
Costly mold, simpler shapes only
gears, gear housings
Die Excellent dimensional accuracy,
high production rate
costly dies, small parts,
non-ferrous metals
gears, camera bodies,
car wheels
Centrifugal Large cylindrical parts, good
quality
Expensive, few shapes pipes, boilers,
flywheels4RAKESH CHANDER SAINI
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Sand Casting
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Sand Casting
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Sand Casting
cope: top half
drag: bottom half
core: for internal cavities
funnel sprue
runners gate
cavity
{risers, vents}
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Sand Casting Considerations
(a) How do we make the pattern?
[cut, carve, machine]
(b) Why is the pattern not exactly identical to the part shape?
- pattern outer surfaces; (inner surfaces: core)
- shrinkage, post-processing
(c) parting line
- how to determine?
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Sand Casting Considerations..
(d) taper
- do we need it ?
(e) core prints, chaplets
Moldcavity
chaplet
Moldcavity
chaplet
- hold the core in position
- chaplet is metal (why?)
(f) cut-off, finishing
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Shell mold casting- metal, 2-piece pattern, 175°C-370°C
- coated with a lubricant (silicone)- mixture of sand, thermoset resin/epoxy
- cure (baking)
- remove patterns, join half-shellsmold
- pour metal
- solidify (cooling)- break shell part
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Expendable Mold Casting
- Styrofoam pattern
- dipped in refractory slurry dried
- sand (support)
- pour liquid metal- foam evaporates, metal fills the shell
- cool, solidify
- break shell part
polystyrene
pattern
patternsupport
sand
moltenmetal
polystyrene
burns;
gas escapespolystyrene
pattern
patternsupport
sand
moltenmetal
polystyrene
burns;
gas escapes
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Investment casting (lost wax casting)
(a) Wax pattern
(injection molding)
(b) Multiple patterns
assembled to wax
sprue
(c) Shell built
immerse into ceramic slurry
immerse into fine sand
(d) dry ceramic
melt out the wax
(few layers)fire ceramic (burn wax)
(e) Pour molten metal (gravity)
cool, solidify
[Hollow casting:pouring excess metal before solidification
(f) Break ceramic shell
(vibration or water blasting)
(g) Cut off parts
(high-speed friction saw)
finishing (polish) 17RAKESH CHANDER SAINI
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Vacuum casting
Similar to investment casting, except: fill mold by reverse gravity
Easier to make hollow casting: early pour out
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Permanent mold casting
MOLD: made of metal (cast iron, steel, refractory alloys)
CORE: (hollow parts)
- metal: core can be extracted from the part
- sand-bonded: core must be destroyed to remove
Mold-surface: coated with refractory material
- Spray with lubricant (graphite, silica)
- improve flow, increase life
- good tolerance, good surface finish
- low mp metals (Cu, Bronze, Al, Mg) 21RAKESH CHANDER SAINI
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Die casting
- a type of permanent mold casting
- common uses: components for
rice cookers, stoves, fans, washing-, drying machines,
fridges, motors, toys, hand-tools, car wheels, …
HOT CHAMBER: (low mp e.g. Zn, Pb; non-alloying)
(i) die is closed, gooseneck cylinder is filled with molten metal
(ii) plunger pushes molten metal through gooseneck into cavity
(iii) metal is held under pressure until it solidifies
,
(v) ejector pins push casting out of ejector die
COLD CHAMBER: (high mp e.g. Cu, Al)
(i) die closed, molten metal is ladled into cylinder
(ii) plunger pushes molten metal into die cavity
(iii) metal is held under high pressure until it solidifies
(iv) die opens, plunger pushes solidified slug from the cylinder
(v) cores retracted(iv) ejector pins push casting off ejector die 22RAKESH CHANDER SAINI
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Centrifugal casting
- permanent mold
- rotated about its axis at 300 ~ 3000 rpm
- molten metal is poured
- Surface finish: better along outer diameter than inner,
- Impurities, inclusions, closer to the inner diameter (why ?)25RAKESH CHANDER SAINI
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Casting Design: Typical casting defects
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Casting Design: Defects and Associated Problems
- Surface defects: finish, stress concentration
- Interior holes, inclusions: stress concentrations
2a
2b
0
σ0
σmax
σmax = σ0(1 + 2b/a)
2a
2b
0
σ0
σmax
σmax = σ0(1 + 2b/a)
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Casting Design: guidelines
(a) avoid sharp corners
(b) use fillets to blend section changes smoothly
(c1) avoid rapid changes in cross-section areas
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Casting Design: guidelines
(c1) avoid rapid changes in cross-section areas
(c2) if unavoidable, design mold to ensure
- easy metal flow
- uniform, rapid cooling (use chills, fluid-cooled tubes)
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Casting Design: guidelines
(d) avoid large, flat areas
- warpage due to residual stresses (why?)
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Casting Design: guidelines
(e) provide drafts and tapers
- easy removal, avoid damage
- along what direction should we taper ?
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Casting Design: guidelines
(g) proper design of parting line
- “flattest” parting line is best
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