Extrusión-Rolling and Forming Process.pptx

7/27/2019 Extrusin-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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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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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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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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(c) four high rolling mill (d) cluster mill


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(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

Documents

Extrusión-Rolling and Forming Process.pptx