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Review of Stress-strain Relationships

Chapters: 3.1.1, 3.1.4

1

Sheet metalworking

Gagemarks

0A

F

specimenofareaOriginal

forceAppliedstressgEngineerin

====

====

o

o

L

LLstraingEngineerin

====

Review of stress-strain relationships

2

Tensile properties tensile test

Stages of tensile test

Uniform elongationNecking -

localised elongation

Maximumload

Fracture

L

LL

Elongation

o

of ====

3

Elastic region - Elastic deformation

i.e., material returns to its original lengthwhen the load is released

Plastic region - plastic deformation

i.e., material does not return to its originallength when the load is released.

Y = yield strength

TS= tensile strength

TS

Y

4


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Strain-hardening (work-hardening)

Metal becomes stronger as the strain

increases - this property is called strain

hardening or work hardening.

TS

Y

5

Shear properties torsion test

Shear involves application of stresses in opposite

directions on either side of a thin element.

6

Shear stress , Shear strain

A

F=

b

=

Shear stress and strain are commonly tested in

a torsion test.

Torsion test

Shear stress tR2

T

2==== Shear strain

L

R====

R

L

t

A

T

A

tTR

7

Shear strength S 0.7 (Tensile strength TS)

Shear stress-shear strain curve

Shearstrength

Fracture

8


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Overview of metal forming processes

Two main broad classifications: Bulk deformation processes

Sheet metalworking processes

9

Bulk deformationprocesses

Rolling

10

Coil

Slab Plate, sheet

Outline of sheet metalworking processes

11

Rolling

SheetPlate

Shearing

CuttingSawing

PunchingBlankingFine blankingStampingEmbossing

Bending

Roll forming

Deep drawing

Rubber forming

Spinning

Superplastic formingExplosive forming

Magnetic-pulse forming

Deburring

CleaningCoating

Plate, sheetCoil

Examples of parts produced by sheet

metalworking processes

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Cutoff

Difference from conventional shearing:

1. Cut edges not necessarily straight

2. Blanks can be nested - avoid scrap

Nesting - for blanking

Waste

material

13

Cutting line

Strip

Strip

Strip

Scrap

Blank

Scrap

Cutting line

Cutting line

14

Punch with 2 cutting edges that match theopposite sides of the blank - irregular shape.Scrap.

Parting Cutting line

Strip

Scrap

Blank

StripBlank

Cutting line

Scrap

Redrawing F, v

Fh Fh

Drawncup

Reverse drawing (is a redrawing process)

Drawn cup facedupwards on die

Drawn cup is placed

face down on die

Requires lower force

than redrawing

Fh

F, v

Drawncup

Fh

15

Effect of Manufacturing Processes

16

Tolerance


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Effect of Manufacturing Processes

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Surface roughness

18

Hollow profiles such as tubes, straw

Mandrel is used to give hollow shape.

MandrelAir in

Air to maintain hollow

form during hardening

19

Wires and cable coating

20

Production of sheet and film

Die opening is a narrow slit (as narrow as 0.4 mm).

Thermoplastic sheet for thermoforming.

Thin film for packaging e.g., grocery & garbage

bags, product wrapping.


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Blow molding

Produces one-piecehollow parts withthin walls:

plastic bottles,

hollow containers

For thermoplastics only:

PE (most common), PP,PVC

Blow molding

22

Extrusion blow molding

Extruder barrel

Tube die

vv v v

vv

Blow

pin Air in

Molded partMold (closed)

Parison

Mold

23

Compression molding

Widely used for thermosets (thermosetting

plastics) & elastomers - phenolics, melamines,

expoxies, urethanes

Electric plugs, dinnerware plates (dishes)

Cavity

PunchCharge

Heatedmold

Heatedmold

Ejector pin

v, F

Moldedpart

v

vCuring till partsolidified.0.5 to 5 mins

Charge moldingmaterial (powder,pellet form)

24

Compare compression with injection molding

Simpler and less expensive mold- no sprueor runner (less scrap material)

Limited to simple shape

Mold must be heatedLonger cycle time - lower production rate

/ /


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Used for thermosets & elastomers

Modifed from compression molding

Polymer melt enters mold cavity as a fluid

Transfer molding

26

Pot transfer molding

Ejector pin

Cavities

Charge(preform)

Transfer ram

Transferpot

v, F

v

Moldedpart

v

v

Sprue

Transfer molding

Charge heated

until softened

Pressure appliedCuring in heated mold

Cull

27

Compare transfer with compression molding

Can form intricate shape as polymer entersthe cavity as a fluid

Use for encapsulation, molding withmetal/ceramic insert

Leads

Moldedplastic bodyChip

Leadframe

Bonds

IC package 28

Transfer molding with metal/ceramic

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