Extrusión-Rolling and Forming Process.pptx
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Transcript of Extrusión-Rolling and Forming Process.pptx
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Rolling Forging
ExtrusionWire Drawing
Bending Cup Drawing
Shearing
Bulk Deformation Processes Sheet Metal Processes
Metal Forming Processes
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Plastic Deformation Processes
Operations that induce shape changes on the work piece by plastic
deformation under forces applied by various tools and dies.
Bulk Deformation Processes
These processes involve large amount of plastic deformation.
The cross-section of workpiece changes without volume change.
The ratio cross-section area/volume is small.
Performed as cold, warm, and hot working operations
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Cold workingperformed at room temp.
Advantages: better accuracy, better surface finish, high
strength and hardness of the part, no heating is required.
Disadvantages: higher forces and power, limitations to the
amount of forming, additional annealing for some material is
required, and some material are not capable of cold working.
Warm working performed temp. above the room temp. butbellow the recrystallization one.
Advantages: lower forces and power, more complex part
shapes, no annealing is required.
Disadvantages: some investment in furnaces is needed.
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Hotworking involves deformation of preheatedmaterial at temp. above the re crystallization temp.
Advantages: big amount of forming is possible,
lower forces and power are required, forming ofmaterials with low ductility, no work hardening and
therefore, no additional annealing is required.
Disadvantages: lower accuracy and surface finish,
higher production cost, and shorter tool life.
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Rolling
Deformation process in which work thickness is reduced by
compressive forces exerted by two opposing rolls
The rolling process (specifically, flat rolling)
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The Rolls
The rotating rolls perform two main functions:
Pull the work into the gap between them byfriction between work-part and rolls
Simultaneously squeeze the work to reducecross section
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Types of Rolling
Based on work piece geometry : Flat rolling - used to reduce thickness of a rectangular
cross section
Shape rolling - square cross section is formed into a
shape such as an I-beam
Based on work temperature :
Hot Rolling most common due to the large amountof deformation required
Cold rolling produces finished sheet and plate stock
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Some of the steel products made in a rolling mill
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Side view of flat rolling, indicating before and after thicknesses,work velocities, angle of contact with rolls, and other features.
Flat rolling and its analysis
R = roller radius
p = roll pressure
L = contact length
= contact angle
vr= roll speedto = initial plate
thickness
tf= final plate thickness
vo = plate entry speed
vf= plate exit speed
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Flat rolling and its analysis
The work enters the gap between the rolls at a velocity vo and
exits at a velocity vf. Because the volume flow rate is constant
and the thickness is decreasing, vfshould be larger than vo.
The roll surface velocity vr is larger than vo and smaller than vf.This means that slipping occurs between the work and the rolls.
Only at one point along the contact length, there is no slipping
(relative motion) between the work and the roll. This point is
called the Neutral Point or the No Slip Point.
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Flat rolling and its analysis In flat rolling, the work is squeezed between two rolls
so that its thickness is reduced by an amount called
the draft:
where
d: draft
to: starting thicknesstf: final thickness
fo ttd
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Flat rolling and its analysis
Rolling may not be possible (the sheet will not be pulled) if the
draft is large. The maximum draft for successful rolling per pass
is:
Where:
dmax : maximum draft successful rolling per pass
: coefficient of friction
R : roll radius
Rd 2max
R
tt fo2
passesofNumber
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Example:- A series of cold rolling operations is to be used to reduce
the thickness of a plate from 50 mm down to 25 mm in a reversing two
high mill. Roll diameter =700 mm and coefficient of friction between
rolls and work = 0.15. The specification is that the draft is to be equalon each pass.
Determine (a) minimum number of passes required, and
(b) draft for each pass.
Solution
passes43.17
35015.0
25-50passesofnumber
22
R
tt fo
6.25
4
25-50
passesofnumberpassperDraft mm
tt fo
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The Flat-rolling Process:
Roll Force, Torque, and Power Requirements
Rolls apply pressure on the flat strip to reduce its
thickness, resulting in a roll force, F
Roll force in flat rolling can be estimated from
Total power(for two rolls) is
avgLwYFL = roll-strip contact length
w= width of the strip
Yavg = average true stress of the strip
000,60
2kW)(inPower
FLN
)0( hfhR
Roll strip contact length (L ) =
w= Width of the strip
Y avg= Average true stress
N= Speed of the roll in RPM
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Flat rolling and its analysis
From the previous equations we can conclude the following:
1. The contact length decreases by decreasing the roll radius.
2. The roll force depends on the contact length, and therefore,
reducing the roll radius will reduce the roll force.3. The torque and power depend on the roll force and contact
length, and therefore, reducing the roll radius will reduce
both the torque and power.
4. The power also depends on the rotational speed of the rolls,and therefore, reducing the rolls RPM will reduce the
power.
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Various configurations of rolling mills
(a) 2-high rolling mill(b) 3-high rolling mill
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Various configurations of rolling mills
(c) four high rolling mill (d) cluster mill
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Various configurations of rolling mills
(e) tandem rolling mill
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Shape-Rolling Operations
Various shapes can be produced by shape rolling Bars
Channels
I-beams
Railroad rails
Roll-pass design requires considerable experience in
order to avoid external and internal defects
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Stages in Shape Rolling of an H-section part. Various other structural
sections such as channels and I-beams, are rolled by this kind of
process.
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Thread Rolling
Bulk deformation process used to form threads on cylindrical partsby rolling them between two dies
Most important commercial process for mass producing bolts andscrews
Performed by cold working in thread rolling machines
Advantages over thread cutting (machining):
Higher production rates Better material utilization
Stronger threads due to work hardening
Better fatigue resistance due to compressive stresses
introduced by rolling
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Thread Rolling
Thread rolling with flat dies:
(1) start of cycle, and (2) end of cycle
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Ring RollingDeformation process in which a thick-walled ring of smaller diameter
is rolled into a thin walled ring of larger diameter
As thick-walled ring is compressed, deformed metal elongates,causing diameter of ring to be enlarged
Hot working process for large rings and cold working process forsmaller rings
Applications: ball and roller bearing races, steel tires for railroad
wheels, and rings for pipes, pressure vessels, and rotatingmachinery
Advantages: material savings, ideal grain orientation,
strengthening through cold working
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Ring Rolling
Ring rolling used to reduce the wall thickness and increasethe diameter of a ring:
(1) start, and (2) completion of process
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Forging
Deformation process in which work iscompressed between two dies
Oldest of the metal forming operations, dating from about 5000 B C
Components: engine crankshafts, connecting rods, gears, aircraftstructural components, jet engine turbine parts
In addition, basic metals industries use forging to establish basicform of large components that are subsequently machined to finalshape and size
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Classification of Forging Operations
Cold vs. hot forging:
Hotor warmforging most common, due to thesignificant deformation and the need to reducestrength and increase ductility of work metal
Cold forging- advantage is increased strength thatresults from strain hardening
Impact vs. press forging: Forge hammer- applies an impact load
Forge press- applies gradual pressure
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Types of Forging Dies
1. Open die forging- work is compressed between two flatdies, allowing metal to flow laterally without constraint
2. Impression die forging- die surfaces contain a cavity or
impression that is imparted to work part, thusconstraining metal flow -flash is created
3. Flash less forging- work part is completely constrained
in die and no excess flash is produced
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1. Open Die ForgingCompression of work part with cylindrical cross-section between
two flat dies
Similar to compression test
Deformation operation reduces height and increases diameterof work
Common names include upsetting or upset forging
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Limited to simple shapes
Difficult to hold close tolerances
Needs to be machined to final shape
Low production rate
Poor utilization of materialsHighly skilled operation
LIMITATIONS OF OPEN DIE FORGING
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2. Impression Die Forging
Compression of work part by dies with inverse ofdesired part shape
This form of forging is used to make more
complicated parts from Blank bar stock. The Blanks
are compressed between two or more dies to
shape the part. Once the part is shaped, the flash is
removed by either grinding it, trimming, or
machining.
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Stages in Impression-die(Closed-Die) Forging
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Impression Die Forging
Advantages and Limitations
Advantages compared to machining from solidstock: Higher production rates
Conservation of metal (less waste) Greater strength
Favorable grain orientation in the metal
Limitations:
Not capable of close tolerances Machining often required to achieve accuracies and features
needed, such as holes, threads, and mating surfaces that fitwith other components
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3. Flash less Forging
Compression of work in punch and die tooling whose cavity does allow forflash
Starting work part volume must equal die cavity volume within very close
tolerance
Process control more demanding than impression die forging
Best suited to part geometries that are simple and symmetrical
Often classified as aprecision forging process
Flash less forging:
(1) just before initial contact with work piece,(2) partial compression, and
(3) final punch and die closure
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Forging Hammers (Drop Hammers)
Apply an impact load against work part - two types:
Gravity drop hammers - impact energy from falling weight of a heavy
ram
Power drop hammers - accelerate the ram by pressurized air or steam
Disadvantage: impact energy transmitted through anvil into floor of
building
Most commonly used for impression-die forging
Diagram showing details of a drop hammer for impression die
forging
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Forging Presses
Apply gradual pressure to accomplish
compression operation - types:
Mechanical presses - converts rotation of
drive motor into linear motion of ram
Hydraulic presses - hydraulic piston actuates
ram
Screw presses - screw mechanism drivesram
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Extrusion
Compression forming process in which the work metal is forced to flowthrough 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
Direct extrusion
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Comments on Direct Extrusion
Also calledforward 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
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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
Indirect extrusion to produce
(a) a solid cross-section and (b) a hollow cross-section
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General Advantages of Extrusion
Variety of shapes possible, especially in hotextrusion
Limitation: part cross-section must be uniformthroughout length
Grain structure and strength enhanced in coldand warm extrusion
Close tolerances possible, especially in coldextrusion
In some operations, little or no waste of
material
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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 reductionsand more complex shapes
Cold extrusion - generally used to produce
discrete parts The term impact extrusion is used to indicate
high speed cold extrusion
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A complex extruded cross-section for a heat sink(photo courtesy of Aluminum Company of America)
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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 ispulledthrough die in drawing (it is
pushedthrough in extrusion)
Although drawing applies tensile stress, compression also plays a
significant role since metal is squeezed as it passes through die opening
Drawing of bar, rod, or wire
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Wire Drawing vs. Bar Drawing
Difference between bar drawing andwire drawing is stock size
Bar drawing - large diameter bar and rod
stock Wiredrawing - small diameter stock - wire
sizes down to 0.03 mm (0.001 in.) arepossible
Although the mechanics are the same,the methods, equipment, and eventerminology are different
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Drawing Practice and Products
Drawing practice:
Usually performed as cold working
Most frequently used for round cross-sections
Products:
Wire: electrical wire; wire stock for fences, coathangers, and shopping carts
Rod stockfor nails, screws, rivets, and springs
Bar stock: metal bars for machining, forging, andother processes
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Wire Drawing
Continuous drawing machines consisting of multiple draw dies (typically 4
to 12) separated by accumulating drums
Each drum (capstan) provides proper force to draw wire stock through
upstream die
Each die provides a small reduction, so desired total reduction isachieved by the series
Annealing sometimes required between dies
Continuous drawing of wire49
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Bar Drawing
Accomplished as a single draft
operation - the stock is pulled through
one die opening
Beginning stock has large diameter and
is a straight cylinder
This necessitates a batch type operation
Hydraulically operated draw bench for
drawing metal bars