polymer processing module 3b. module 3b spring 20012isat 430 dr. ken lewis introduction processing...

Polymer Processing

Module 3b

Spring 2001 2ISAT 430 Dr. Ken Lewis Module 3B

Introduction

Processing Methods and Operations Choice is dictated by the product desired and

the quantity desired.» Fiber, film, sheet, tube» Cup, bucket, car bumper, chair.

Fiber manufacture is different, it is continuous. Large quantities usually use extrusion or

injection molding Smaller quantities use compression molding or

transfer molding


Extrusion

This process is fundamental to both metals and ceramics as well as polymers.

Definition Extrusion is a compression process

» Material is forced to flow through a die orifice» Cross-sectional shape determined by the shape of

the orifice» Product is long and continuous


Extrusion2

Rarely used for thermosetting polymers

Products» Tubing, pipes, and hose» Window and door moldings» Sheet and film» Continuous filaments (as we saw in module 3A)» Coated electrical wire and cable.


Extrusion3

The extruder consists basically of a hopper and a barrel and a screw.


Extruder

The die is not part of the extruder

Usually ~ 1 – 6 in. dia.

Up to 60 rpm

Flight clearance of only 0.002 in.


Extruder

Feed section Compact to a solid mass Pre heat

Compression or plastication section Melting progresses, degassing

occurs Metering section

Internal heating from viscous flow

Pressure is developed to extrude the material through the die


Extruder2

The screw is a tight fit in the barrel.

Note how the channel depth. changes in the plastication

section. Is constant in the metering

section.

Channel depth

These section lengths will change depending on the polymer being processed.

Compression section Short for materials that melt

suddenly (nylon) Long for gradually softening

materials (polyvinyl chloride)


Channel depth

Pressure applied to polymer melt is a function of the channel depth, dc. Feed section dc is relatively large

» Allows lots of granular polymer to be added to barrel

Compression section dc gets smaller» Applies additional pressure to metering section

Metering section dc is smallest

Can be carefully designed, but… In general, industry uses general kind of “off the

shelf” extruders.


Screw details

Helical flights with space between them Carries the polymer. Flight land is hardened

and barely clears the barrel.

The Pitch (distance the flight travels in one complete rotation) is usually about equal to the diameter.

pitchtan

DA


Melt Flow in the Extruder

OK, the screw turns, the flights advance, WHY DOES THE POLYMER ADVANCE?

Why doesn’t it just slip and slide back?

DRAG FLOW Friction between the fluid and the two

opposing surfaces» The stationary barrel» The moving channel of the turning screw» RECALL…

xv y

Y

x

zy

0v


Melt Flow in the Extruder Qdr, the volumetric drag flow rate.

If we assume that the velocity v is ½ the flight velocity (the moving plate velocity)

xv y

Y

x

zy

0v

0.5drQ vwd

•Where:

•v = velocity of the plate (m/s)

•D = distance between the plates (m)

•W = width of the plates(m)



Most analyses of extruders unroll the helical shaped channel Leads to a rectangular

channel covered by an infinite plate moving at constant velocity

The fluid motion (or flow) in the channel can be decomposed A cross flow in the x – y

plan An axial flow in the z

direction.



The axial flow in the z direction is responsible for the pumping

The cross flow does most of the mixing.


Extruder Mixing & Melting

The happenings in the channel are complex. Near the leading edge

the polymer has experienced the longest residence time.

Mixing is poor» Flow is laminar» Zero turbulence

Polymer cooking can be a problem.

Direction of travel.


Extruder transport

0.5drQ vwd

•Where:

•v = velocity of the plate (m/s)

•D = distance between the plates (m)

•W = width of the plates(m)

Using the unrolled screw model, we can show that:

cosv DN A

cd d

tan cosc fw w D A w A

or

2 20.5 sin cosdr cQ D Nd A A

We have assumed:

wf is negligible

Note:

sintan

cosA

AA


Extruder transport – back pressure.

This is the maximum possible output for an extruder.

Conveyance of the polymer through Smaller and smaller cross sections the screen pack and die…

Creates a back pressure, Qbp.


3 2sin12c

bp

Dd A dpQ

dl


Extruder transport – back pressure

The back pressure is a function of Barrel dimensions The polymer viscosity The flight angle The pressure gradient dp/dl…

The pressure gradient dp/dl Is a function of the screw shape, the barrel

size, the flight angel. If we assume the pressure profile is linear

along the barrel, then dp/dl becomes p/L

3 2sin12c

bp

Dd A dpQ

dl


Extruder transport3 2sin

12c

bp

Dd A dpQ

dl

Then:

3 2sin12

cbp

p Dd AQ

L

•Where:

•p = the head pressure (Mpa)

•L = length of the barrel (m)


Back Pressure Flow

A misnomer It is not back pressure flow It is resistance to forward flow

So what is the net flow?

net dr bpQ Q Q 3 2

2 2 sin0.5 sin cos

12c

net c

p Dd AQ D Nd A A

L

Qnet is what finally comes out of the die!


Back Pressure

There is some (hopefully) negligible slippage of fluid between the flight and the barrel wall.

Back pressure reduces flow but causes plastication. In the limit, the back pressure can stop the flow

0e dr bpQ Q Q

dr bpQ Q

And the maximum pressure becomes:

max 2

6 cot

c

DNL Ap

d


The Net Flow

There are a lot of parameters in the above equation (relation)

They are of two types

Those we control (design parameters)

Those we don’t control (operating parameters)

3 22 2 sin

0.5 sin cos12

cnet c

p Dd AQ D Nd A A

L


Design Parameters

These we control at conception time and are fixed thereafter. Barrel diameter Flight or Helix angle Channel depth dc

Barrel length L


Operating Parameters

These we can fiddle with to optimize the process. Rotational speed, N The head pressure (change the die, slow the

screw, change the temperature) The hidden variable … TEMPERATURE. The viscosity

» But only to the extent that the shear rate and temperature will allow!


Extruder Characteristics

For a given extruder:

2 20.5 sin coscdrQ D Nd A A N

3 2sin

12c

bp

p Dd A pQ

L

or

e

pQ N


Extruder Characteristics

Extrusion PressurePmax

Extruder CharacteristicCurve

Increasing N orincreasing viscosityE

xtruder Flow Rate

Flow up with Increasing N Decreasing p Increasing

Ignores non-Newtonian flow behavior

Ignores friction

e

pQ N


Extruder Characteristicse

pQ N

Usual Recommended

Cescr Ce

scr

Output in Kg/h 0.006 2.2 0.006 2.3

Output in lb/h 16 2.2 20 2.35

A useful estimate of extruder capacity with a L/D = 24 is:

scre eQ C D

Actual output may ± 20% (good for back of envelope calculations)


A screw extruder has D = 75 mm, dc = 5 mm, A = 17.5°. It rotates at 100 rpm. The plastic has a density of 1 gm/cc.

What is the output for zero back pressure?

22 60min0.5 75 100 5 sin 17.5 cos 17.5

mindr

revQ mm mm

hr

33

3 3

1238,822,278.3 238.8

10 10dr

mm cm gm kg kgQ

hr mm cm gm hr

What is the output expected for normal conditions?

2.30.006 75 123dr

kgQ

hr


Die Characteristics

Flow through a die generates back pressure For a simple cylindrical flow channel the flow

rate is given by the famous Hagen – Poiseuille equation:

4

128d

cl

p DQ

L

D = diameter

= melt viscosity [=]


Die characteristics

So flow increases with p Look at the power of the die diameter! This gives the linear die characteristic

curve. Note: some people write the above

equation as:

4

128d

cl

p DQ

L

c sQ K p


Die characteristics

Where Ks is called the die shape factor Still just equation for laminar flow through

a pipe.

4

128d

cl

p DQ

L

c sQ K p




Die characteristiccurve

IncreassingL, n,decreasingD


xtruder Flow Rate

OperatingPoint

Extrusion Curve


Operating Point

The values of Q and p where the curves intersect is the extruder operating point.

Note the shape factor Ks is the slope of the die characteristic curve.



Die characteristiccurve

IncreassingL, n,decreasingD


xtruder Flow Rate

OperatingPoint


example

Consider an extruder with the following properties: D = 3.0 in L = 75 in N = 1 rev/sec dc = 0.25 in A = 20°

Let the melt have a shear viscosity of = 125 lb-sec/in2 = 103.4 Pa sec


example2

Knowing the above characteristics, calculate Qmax and pmax.


22max 0.5 3 1 0.25 sin 20cos 20

secdr

revQ Q in in

3

max 3.568sec

inQ


example3

Knowing the above characteristics, calculate Qmax and pmax.

max 2

6 cot

c

DNL Ap

d

2

max 2

sec cos 206 3 1 75 0.015

sec sin 20

0.25

rev lbfin in

inp

in

max 22796.59

lbfp

in


example4

These two values define the abscissa and the ordinate for the extruder characteristic.

If we have a circular die with a diameter Dd = 0.25 in, and a length Ld = 1.0 in

What’s the shape factor for the die?

3

max 3.568sec

inQ max 2

2796.59lbf

pin


If we have a circular die with a diameter Dd = 0.25 in, and a length Ld = 1.0 inWhat’s the shape factor for the die?

4

128d

sd

DK

L

4

2

0.25sec

128 0.015 1.0s

inK

lbfin

in

5

0.0063916secs

inK

lbf

remember c sQ K p


example5

Now we can find the operating point for the extruder. We can express the extruder characteristic as the straight

line between Qmax and pmax.

maxmax

maxx

QQ Q p

p 3.57 0.0012765xQ p

And from the die equation 0.0063916xQ p

Setting these equal provides the operating point 465.6p psi

3

2.98secx

inQ


Extruder Characteristic

0.000

1.000

2.000

3.000

4.000

5.000

6.000

7.000

0 500 1000 1500 2000 2500 3000

Pressure (psi)

Flo

w R

ate

(in

3/se

c)

Q ex

Q die


Dies

The polymer is extruded past the breaker plate into the die. Our previous example assumed a cylindrical

die Dies come in many flavors.

The die must take into account several factors Die swell bambooing


Die Swell

On the left is a cylindrical die and on the right is an annular die. Note the Barus bulge Due to release of stored elastic energy obtained in the

die and the radical change in velocity of material close to the die walls.


Die Swell

Note that as soon as the polymer has left the die, its surface is free Stress free Polymer will relax unless it is kept under tension.


Surface Fracture

At high shear rates The polymer in the middle of the round channel is

quiescent while the material near the walls is in high shear.

The energy stored is high enough that upon emerging from the die, the polymer fractures in trying the equilibrate the stresses.


Effect of Die Swell

Knowing that die swell will occur is important After the polymer leaves the die it is rapidly

cooling and becoming fixed in shape For each polymer, if we know

» Viscosity» Temperature» Shear rate

We can account for the die swell in the shape of our die


Die shapes

The dies The finished shapes


Pipe extrusion

The central mandrel is supported by spider legs These disrupt the flow of

polymer The polymer rejoins

itself because » the flow rate is low » The conditions haven’t

changed (temperature) To minimize the effect of

the spiders, the mandrel is tapered.


Pipe extrusion To control the pipe size,

other means are used.

Internal sizing mandrel

External sizing using air pressure

External sizing usingvacuum


Tubing Die

Note the expansion to the spider legs and the reduction afterwards.

If the extrusion is too rapid, the spider leg openings will not heal.


Wire Coating Die

The wire runs straight through

Polymer comes in vertically into a distribution cavity

Used for wire diameters of 1 mm up to submarine cables with diameters of 150 mm.


Wire Coating Die2

Note here the wire is helping draw the polymer from the die!

The taught wire provides rigidity during cooling

The product is usually cooled by passing it through a liquid bath

These system roll, making coated wire at speeds up to10,000 ft/min.

Injection Molding


Injection Molding

Polymer is heated, mixed, the then forced to flow into a mold cavity

Similar to extrusion Hopper, barrel, screw

Screw rotation is the principal motion only in one part of the cycle Mixes, compacts, plasticizes, and heats Pressures may reach 10 – 20 MPa (1450 –

2900 psi)


Injection Molding2

In the injecting stage, the screw is driven axially by a piston to generate the working pressure 150 – 250 MPa (21,756 – 36,260 psi)


Injection Molding Sequences

(1) Close the mold (2) Inject the melt

(3) Retract the screw (4) Open mold – eject part


Two Plate Mold The mold here is closed The mold is position

between two platens One stationary One moveable

Note the water channels for quickly cooling the mold and its polymer load.

Sprue: channel from die nozzle

Into the mold

Runner: channel from SprueInto the cavity

Gates: restrict the polymer

Flow into the cavity


Two Plate Mold2

The mold here is open The ejector pins push

the rather fragile plastic from the mold cavity

The sprue and runners are waste..


Two Plate Mold3

Cooling system Usually water passages in the mold itself

Gas vents Usually about 0.001 in deep and 0.5 wide. Allows the air to escape when the cavity is

filling Too small to let the viscous polymer follow.


Two Plate Mold - Parts

Cavities (shape the part) Distribution channels (get the polymer to

the cavity) Ejection system (safely remove the part) Cooling system (change the polymer from

soup to part) Gas venting facility ( allow the cavity to fill)

Thermoforming


Thermoforming

A flat thermoplastic sheet is softened and deformed into the desired shape. Used for large items

» Bathtubs» Skylights» Freezer interior walls» Bumpers

Two steps» Heating» Deforming / forming


Thermoforming

Three major types of thermoforming

Vacuum» Pressure limit of 1 atmosphere

Pressure » Higher allowable pressures

Mechanical


Vacuum Thermoforming


Pressure Thermoforming


Mechanical Thermoforming

In (1) the polymer is pre stretched In (2) the polymer is draped over the positive mold and

pressure applied to force it in place


Mechanical Thermoforming2

In (1) the polymer is pre heated In (2) the polymer is forced into place in the negative mold.


Product design Considerations

In general Strength

» Plastics are not metals» Should not be used in strength or creep critical applications.

Impact resistance» Good, better than many ceramics

Service temperature» Much less than metals or ceramics

Degradation» Radiation» Oxygen or ozone» Solvents

Corrosion resistance» Better than metals


Extrusion Considerations

Desirable product traits Wall thickness should be uniform Hollow sections seriously complicate the

extrusion process Corners

» Avoid as they cause uneven polymer flow and are stress concentrators


Molded Part Considerations

Economic production Injection molding minimum ~10,000 parts Vacuum etc. usually around ~1,000 parts.

Part complexity Possible, just makes the mold more

complicated Wall thickness

Wasteful and can warp during shrinkage Use ribs for stiffness


Molded Part Considerations2

Corner radii and/or fillets Sharp corners are stress concentrators, bad

Holes OK but complicate the mold

Draft (the taper of the cavity) Should be there to allow easy mold removal Recommended drafts

» Thermosets: ½° - 1°» Thermoplastics: 1/8° - ½°


Molded Part Considerations3

Tolerances Shrinkage will occur but is predictable The more generous the tolerances the

easier the manufacture. Typical dimension tolerances are:

» +/- 0.006 – 0.010 inches

Typical hole tolerances are:» +/- 0.003 – 0.005 inches

polymer processing module 3b. module 3b spring 20012isat 430 dr. ken lewis introduction processing...

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