2 casting forming

IEEM 215: Manufacturing Processes

Traditional Manufacturing Processes
Casting
Forming
Sheet metal processing
Cutting
Joining
Powder- and Ceramics Processing
Plastics processing
Surface treatment

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

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
finish
non-ferrous metals, low
production rate
prototypes of
mechanical parts
Ceramic mold complex shapes, high accuracy,
good finish
small sizes impellers, injection
mold tooling
Investment complex shapes, excellent finish small parts, expensive jewellery
Permanent
mold
good finish, low porosity, high
production 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,
flywheels

Sand Casting
cope: top half
drag: bottom half
core: for internal cavities
pattern: positive
funnel  sprue 
 runners  gate 
 cavity 
 {risers, vents}

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?

Sand Casting Considerations..
(d) taper
- do we need it ?
Mold
cavity
chaplet
Mold
cavity
chaplet
(e) core prints, chaplets
- hold the core in position
- chaplet is metal (why?)
(f) cut-off, finishing

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-shells  mold
- pour metal
- solidify (cooling)
- break shell  part

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
pattern
support
sand
molten
metal
polystyrene
burns;
gas escapespolystyrene
pattern
pattern
support
sand
molten
metal
polystyrene
burns;
gas escapes

Plaster-mold, Ceramic-mold casting
Plaster-mold slurry: plaster of paris (CaSO4
), talc, silica flour
Ceramic-mold slurry: silica, powdered Zircon (ZrSiO4)
- The slurry forms a shell over the pattern
- Dried in a low temperature oven
- Remove pattern
- Backed by clay (strength), baked (burn-off volatiles)
- cast the metal
- break mold  part
Plaster-mold: good finish (Why ?)
plaster: low conductivity => low warpage, residual stress
low mp metal (Zn, Al, Cu, Mg)
Ceramic-mold: good finish
high mp metals (steel, …) => impeller blades, turbines, …

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
(few layers)
(d) dry ceramic
melt out the wax
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)

Vacuum casting
Similar to investment casting, except: fill mold by reverse gravity
Easier to make hollow casting: early pour out

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)

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
(iv) die opens, cores retracted; plunger returns
(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

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 ?)

Casting Design: Typical casting defects

Casting Design: Defects and Associated Problems
- Surface defects: finish, stress concentration
- Interior holes, inclusions: stress concentrations
2a
2b
σ0
σ0
σmax
σmax = σ0(1 + 2b/a)
2a
2b
σ0
σ0
σmax
σmax = σ0(1 + 2b/a)

Casting Design: guidelines
(a) avoid sharp corners
(b) use fillets to blend section changes smoothly
(c1) avoid rapid changes in cross-section areas

(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)

(d) avoid large, flat areas
- warpage due to residual stresses (why?)

(e) provide drafts and tapers
- easy removal, avoid damage
- along what direction should we taper ?

(f) account for shrinkage
- geometry
- shrinkage cavities

(g) proper design of parting line
- “flattest” parting line is best

Forming
Any process that changes the shape of a raw stock
without changing its phase
Example products:
Al/Steel frame of doors and windows, coins, springs,
Elevator doors, cables and wires, sheet-metal, sheet-metal parts…

Rolling
Hot-rolling
Cold-rolling

Rolling
Important Applications:
Steel Plants,
Raw stock production (sheets, tubes, Rods, etc.)
Screw manufacture

Rolling Basics
Sheets are rolled in multiple stages (why ?)
Vo
Vfto
tf
Vo
Vfto
tf
Vo
Vfto
tf
Vo
Vfto
tf
thread rolling machine
stationary die
rolling die
thread rolling machine
stationary die
rolling die
Reciprocating flat thread-rolling diesReciprocating flat thread-rolling dies
Screw manufacture:

Forging
[Heated] metal is beaten with a heavy hammer to give it the required shape
Hot forging,
open-die

Stages in Open-Die Forging
(a) forge hot billet to max diameter
(b) “fuller: tool to mark step-locations
(c) forge right side
(d) reverse part, forge left side
(e) finish (dimension control)
[source:www.scotforge.com]

1. Blank (bar) 2. Edging 3.Blocking 4. Finishing 5. Trimming
Flash
(a)
(b)
(c)
1. Blank (bar) 2. Edging 3.Blocking 4. Finishing 5. Trimming1. Blank (bar) 2. Edging 3.Blocking 4. Finishing 5. Trimming
Flash
(a)
(b)
(c)
Flash
(a)
(b)
(c)
Stages in Closed-Die Forging
[source:Kalpakjian & Schmid]

Quality of forged parts
Stronger/tougher than cast/machined parts of same material
Surface finish/Dimensional control:
Better than casting (typically)
[source:www.scotforge.com]

Extrusion
Metal forced/squeezed out through a hole (die)
Typical use: ductile metals (Cu, Steel, Al, Mg), Plastics, Rubbers
Common products:
Al frames of white-boards, doors, windows, …
[source:www.magnode.com]

hydraulic
piston
chamber
chamber
stock
die
extruded shape
hydraulic
piston
chamber
chamber
stock
die
extruded shape
hydraulic
piston
chamber
chamber
stock
die
extruded shape
Extrusion: Schematic, Dies
Exercise: how can we get hollow parts?

Drawing
Commonly used to make wires from round bars
stock (bar)
F (pulling force)
wire
diestock (bar)
F (pulling force)
wire
die
Similar to extrusion, except: pulling force is applied

Crank Shaft
Also see: http://auto.howstuffworks.com/engine7.htm

Sheet Metal Processes
Raw material: sheets of metal, rectangular, large
Raw material Processing: Rolling (anisotropic properties)
Processes:
Shearing
Punching
Bending
Deep drawing

Shearing
A large scissors action, cutting the sheet along a straight line
Main use: to cut large sheet into smaller sizes for making parts.

Punching
Cutting tool is a round/rectangular punch,
that goes through a hole, or die of same shape
F ∝ t X edge-length of punch X shear strength
Punch
die
sheet
crack
(failure in shear)
clearance
die
piece cut away, or slug
t
F ∝ t X edge-length of punch X shear strength
Punch
die
sheet
crack
(failure in shear)
clearance
die
piece cut away, or slug
t

Punching
Main uses: cutting holes in sheets; cutting sheet to required shape
typical punched part
nesting of parts
Exercise: how to determine optimal nesting?

Bending
Body of Olympus E-300 camera
component with multiple bending operations
[image source: dpreview.com]
component with punching,
bending, drawing operations

Typical bending operations and shapes
(a)
(b)

Sheet metal bending
Planning problem: what is the sequence in which we do the bending operations?
Avoid: part-tool, part-part, part-machine interference

Bending mechanics
R = Bend radius
Neutral axis
α
L = Bend length
This section is
under extension
This section is
in compression
Bend allowance, Lb = α(R + kT)
T = Sheet thickness
R = Bend radius
Neutral axis
α
L = Bend length
This section is
under extension
This section is
in compression
Bend allowance, Lb = α(R + kT)
T = Sheet thickness
Bending Planning  what is the length of blank we must use?
Ideal case: k = 0.5 Real cases: k = 0.33 ( R < 2T) ~~ k = 0.5 (R > 2T)

Bending: cracking, anisotropic effects, Poisson effect
Bending  plastic deformation
Bending  disallow failure (cracking)  limits on corner radius: bend radius ≥ 3T
Engineering strain in bending = e = 1/( 1 + 2R/T)
effect of anisotropic stock Poisson effect
Exercise: how does anisotropic behavior affect planning?

Bending: springback
134
3
+





−





=
ET
YR
ET
YR
R
R ii
f
i
How to handle springback:
(a) Compensation: the metal is bent by a larger angle
(b) Coining the bend:
at end of bend cycle, tool exerts large force, dwells
coining: press down hard, wait, release
Initial
Final R
i
Rf
i
f
αf
αi
T

Deep Drawing
die die die die die
punch punch punch punch
blank
part
blank holder
(a) (b) (c) (d) (e)
Examples of deep drawn parts
die die die die die
blank
part
blank holder
(a) (b) (c) (d) (e)
die die die die die
blank
part
blank holder
(a) (b) (c) (d) (e)
Examples of deep drawn parts
Tooling: similar to punching operation,
Mechanics: similar to bending operation
Common applications: cooking pots, containers, …

Sheet metal parts with combination of operations
Body of Olympus E-300 camera
component with multiple bending operations
[image source: dpreview.com]
component with punching,
bending, drawing operations

These notes covered Casting, Forming and Sheet metal processing
Case study on planning of operations (bending)
Further reading: Chapters 10-16, Kalpakjian & Schmid
Summary

2 casting forming

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2 casting forming