Extrusion and Sheet Metal Forming
Extrusion
Compression forming process in which the work metal is forced to flow through a die opening to produce a desired cross‐sectional shape • Process is similar to squeezing toothpaste out of a toothpaste tube • In general, extrusion is used to produce long parts of uniform cross‐sections
• Two basic types of extrusion:
– Direct extrusion
– Indirect extrusion
Extrusion
Df
D0
Hot, Warm and Cold Extrusion
Direct, Indirect Extrusion
Analysis of Extrusion
The reduction ratio, it is defined as A0
rx =
Af
Ao = cross‐sectional area of the starting billet; and Af = final cross‐sectional area of the
extruded section.
True strain;ε = ln(rx) Extrusion pressure, −
p = Y f ln(rx )
Kε n
Yf =
1+ n
−
Hot vs. Cold Extrusion
• Hot extrusion ‐ prior heating of billet to above its recrystallization temperature ‐ This reduces strength and increases ductility of the metal, permitting more size reductions and more complex shapes • Cold extrusion – ‐ prior heating of billet to below its recrystallization temperature • generally used to produce discrete parts Direct Extrusion Comments on Direct Extrusion • Also called forward extrusion
• As ram approaches die opening, a small portion of billet remains that cannot be forced through die opening • This extra portion, called the butt, must be separated from extruded product by cutting it just beyond the die exit • Starting billet cross section usually round, but final shape is determined by die opening (a) Direct extrusion to produce a hollow or semi‐hollow cross‐section; (b) hollow and (c) semi‐hollow cross‐sections Indirect extrusion Indirect extrusion to produce (a) a solid cross‐section and (b) a hollow cross‐section
Comments on Indirect Extrusion • Also called backward extrusion and reverse extrusion
• Limitations of indirect extrusion are imposed by the lower rigidity of hollow ram and difficulty in supporting extruded product as it exits die Variation of ram pressure vs. ram stroke Hydrostatic Extrusion
A complex extruded cross‐section for a heat sink
Wire and Bar Drawing
Cross‐section of a bar, rod, or wire is reduced by pulling it through a die opening • Similar to extrusion except work is pulled through die in drawing (it is pushed through in extrusion)
• Although drawing applies tensile stress, compression also plays a significant role since metal is squeezed as it passes through die opening Analysis of Drawing
Change in size of work is usually given by area reduction: A −A
r =
0
f
Af
Ao = original area of work; and Af = final work
A0
1
)
• True strain; ε = ln( ) = ln(
Af
1− r
• Pressure, n
−
−
A
p = Y f ε = Y f ln 0
Af
Kε
Yf =
1+ n
−
Sheet Metal Forming
Bending
Shearing
Sheet : 0.4 – 6 mm (1/64”‐1/4”)
Plate > 6 mm (1/4”)
Drawing
Shearing, Blanking, and Punching
Three principal operations in press working that cut sheet metal: • Shearing • Blanking
• Punching Shearing ‐ sheet metal cutting operation along a straight line between two cutting edges
• Typically used to cut large sheets into smaller sections for subsequent operations
Blanking ‐ sheet metal cutting to separate piece from surrounding stock
• Cut piece is the desired part, called a blank
Punching ‐ sheet metal cutting similar to blanking except cut piece is scrap, called a slug
• Remaining stock is the desired part
Shearing between two sharp cutting edges
(1) just before the punch contacts work
(2) punch begins to push into work, causing plastic deformation
(3) punch compresses and penetrates into work causing a smooth cut surface
(4) fracture is initiated at the opposing cutting edges which separates the sheet
(a) Blanking and (b) punching
Clearance in Sheet Metal Cutting
Distance between the punch and die
• Typical values range between 4% and 8% of stock thickness ‐ If too small, fracture lines pass each other, causing double burnishing and larger force ‐ If too large, metal is pinched between cutting edges and excessive burr results
Clearance in Sheet Metal Cutting
• Recommended clearance can be calculated by:
c = at
where c = clearance (mm); a = allowance; and t = stock thickness (mm)
• Allowance a is determined according to type of metal
Allowance a for Three Sheet Metal Groups
Metal group
a
1100S and 5052S aluminum alloys, all
tempers
2024ST and 6061ST aluminum alloys; brass,
soft cold rolled steel, soft stainless steel
0.045
Cold rolled steel, half hard; stainless steel, half
hard and full hard
0.075
0.060
Punch and Die Sizes for Blanking and Punching
For a round blank of diameter Db: Blanking punch diameter = Db ‐2c
Blanking die diameter = Db
where c = clearance
For a round hole of diameter Dh: Hole punch diameter = Dh
Hole die diameter = Dh + 2c
where c = clearance
Die size determines blank size Db; punch size determines hole size Dh; c = clearance
Angular Clearance
Purpose: allows slug or blank to drop through die
• Typical values: 0.25° to 1.5° on each side
Cutting Forces
Important for determining press size (tonnage)
F = S t L or F= 0.7σ t L
where S = shear strength of metal (MPa)
σ = tensile strength (MPa)
t = stock thickness (mm), and L = length of cut edge (mm)
Bending Straining sheet metal around a straight axis to take a permanent bend
Metal on inside of neutral plane is compressed, while metal on outside of neutral plane is stretched
both compression and tensile elongation of the metal occur in bending
Types of Sheetmetal Bending
• V bending ‐ performed with a V‐shaped die
• Edge bending ‐ performed with a wiping die
V‐Bending • For low production
• Performed on a press brake
• V‐dies are simple and inexpensive Edge Bending • For high production
• Pressure pad required
• Dies are more complicated and costly
Spring back in Bending
Spring back = increase in included angle of bent part relative to included angle of forming tool after tool is removed • Reason for spring back:
‐ When bending pressure is removed, elastic energy remains in bent part, causing it to recover partially toward its original shape Spring back in Bending
A −A
SB =
AB'
'
'
b
Bending Force
Maximum bending force estimated as follows:
K bf wt σ
2
F=
D
where F = bending force (N); σ = tensile strength of sheet metal (MPa); w = part width in direction of bend axis (mm); and
t = stock thickness. For V‐ bending, Kbf = 1.33; for edge bending, Kbf = 0.33
Drawing Sheet metal forming to make cup‐shaped, box‐
shaped, or other complex‐curved, hollow‐
shaped parts • Sheet metal blank is positioned over die cavity and then punch pushes metal into opening
• Products: beverage cans, ammunition shells, automobile body panels Stages in Deformation of the Work piece during Deep Drawing
Drawing of a cup‐
shaped part: (1) start of operation before punch contacts work
(2) near end of stroke
(b) Corresponding workpart: (1) starting blank (2) drawn part
Drawing Ratio DR
Most easily defined for cylindrical shape:
Db
DR =
Dp
where Db = blank diameter; and Dp = punch diameter
• Indicates severity of a given drawing operation
‐ Upper limit = 2.0
Reduction r
•
Again, defined for cylindrical shape:
r=
Db − D p
Db
• Value of r should be less than 0.50
True strain; ε = ln(r) Pressure, Kε n
p = Y f ln(r ) Y f =
1+ n
−
−
Thickness‐to‐Diameter Ratio Thickness of starting blank divided by blank diameter Thickness‐to‐diameter ratio = t/Db
• Desirable for t/Db ratio to be greater than 1%
• As t/Db decreases, tendency for wrinkling increases
Clearance in Drawing
• Sides of punch and die separated by a clearance c given by:
c = 1.1 t
where t = stock thickness
• In other words, clearance = about 10% greater than stock thickness
Other drawing operations: Reverse Drawing
Redrawing
(1) start
(2) finish
Common defects in drawn parts
a) wrinkling in flange
b) wrinkling in walls
c) tearing
d) earing
e) surface scratches