Transcript Polymer Processing - James Madison University

Polymer Processing
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
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
2
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
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
3
Extrusion2

Rarely used for thermosetting polymers

Products
»
»
»
»
»
Spring 2001
Tubing, pipes, and hose
Window and door moldings
Sheet and film
Continuous filaments (as we saw in module 3A)
Coated electrical wire and cable.
ISAT 430
Dr. Ken Lewis
Module 3B
4
Extrusion3

The extruder consists basically of
 a hopper and
 a barrel and
 a screw.
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
5
Extruder
Usually ~ 1 – 6 in. dia.
Up to 60 rpm
The die is
not part of
the extruder
Flight clearance of
only 0.002 in.
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
6
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
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
7
Extruder2


Channel depth
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.


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)
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
8
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.
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
9
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.
pitch
tan A 
D
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
10
v0
Melt Flow in the Extruder
z
x

Y
y
vx   y

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…
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
11

Qdr, the volumetric drag flow rate.

If we assume that the velocity v is ½
the flight velocity (the moving plate
velocity)
Melt Flow in the Extruder
Qdr  0.5vwd
•Where:
•v = velocity of the plate (m/s)
v0
y
z
x
Spring 2001
•D = distance between the plates (m)
•W = width of the plates(m)
Y
vx   y
ISAT 430
Dr. Ken Lewis
Module 3B
12
Melt Flow in the Extruder

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.
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
13
Melt Flow in the Extruder
Spring 2001
ISAT 430

The axial flow in the z
direction is responsible for
the pumping

The cross flow does most of
the mixing.
Dr. Ken Lewis
Module 3B
14
Extruder Mixing & Melting
Direction of travel.

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.
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
15
•Where:
•v = velocity of the plate (m/s)
Extruder
transport
•D = distance between the plates (m)
•W = width of the plates(m)
Qdr  0.5vwd
Using the unrolled screw model, we can show that:
v   DN cos A
d  dc
We have assumed:
w  w c   D tan A  w f  cos A
wf is negligible
or
Note:
Qdr  0.5 2 D 2 N d c sin A cos A
Spring 2001
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Dr. Ken Lewis
tan A 
Module 3B
sin A
cos A
16
Extruder transport – back pressure.
Qdr  0.5 2 D 2 N d c sin A cos A


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.
 Dd c3 sin 2 A  dp 
Qbp 


12
 dl 
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
17
Extruder transport – back pressure
 Dd c3 sin 2 A  dp 
Qbp 


12
dl



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
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
18
Extruder transport
 Dd c3 sin 2 A  dp 
Qbp 


12
dl


Then:
p Dd c sin 2 A
Qbp 
12 L
3
•Where:
•p = the head pressure (Mpa)
•L = length of the barrel (m)
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
19
Back Pressure Flow

A misnomer
 It is not back pressure flow
 It is resistance to forward flow

So what is the net flow?
Qnet  Qdr  Qbp
Qnet
3
2
p

Dd
sin
A
2
2
c
 0.5 D N d c sin A cos A 
12 L
Qnet is what finally comes out of the die!
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
20
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
Qe  Qdr  Qbp  0
Q Q
dr
bp
And the maximum pressure becomes:
6 DNL cot A
p 
d
max
2
c
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
21
The Net Flow
Qnet


p Dd c3 sin 2 A
 0.5 D N d c sin A cos A 
12 L
2
2
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)
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
22
Design Parameters

These we control at conception time and are fixed
thereafter.
 Barrel diameter
 Flight or Helix angle
 Channel depth dc
 Barrel length L
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
23
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!
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
24
Extruder Characteristics

For a given extruder:
Qdr  0.5 2 D2 Ndc sin A cos A   N
p Dd c sin 2 A  p


12 L

3
Qbp
or
Spring 2001
p
Qe   N 

ISAT 430
Dr. Ken Lewis
Module 3B
25
Extruder Characteristics
p
Qe   N 

E
x
t
r
u
d
e
r
Increasing N or
increasing viscosity

Flow up with
 Increasing N
 Decreasing p
F
l
o
w
 Increasing 

R
a
t
e
Extruder Characteristic
Curve
Pmax

Ignores non-Newtonian flow
behavior
Ignores friction
Extrusion Pressure
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
26
p
Qe   N 

Extruder Characteristics
A useful estimate of extruder capacity with a L/D = 24 is:
Qe  Ce Dscr
Usual
Recommended
Ce
scr
Ce
scr
Output in Kg/h
0.006
2.2
0.006
2.3
Output in lb/h
16
2.2
20
2.35
Actual output may ± 20% (good for back of envelope
calculations)
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
27
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?
rev 60 min 
2 
Qdr  0.5   2   75mm   100

  5mm  sin 17.5  cos 17.5
min
hr


3
mm3  cm  1gm
kg
kg
Qdr  238,822, 278.3




238.8

hr  10mm  cm3 103 gm
hr
What is the output expected for normal conditions?
Qdr  0.006   75 
Spring 2001
ISAT 430
2.3
 123
kg
hr
Dr. Ken Lewis
Module 3B
28
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:
p Dd4
Qc 
128 Ll
Spring 2001
D = diameter
 = melt viscosity [=]
ISAT 430
Dr. Ken Lewis
Module 3B
29
Die characteristics
p D
Qc 
128 Ll
4
d




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:
Qc  Ks p
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
30
Die characteristics
p Dd4
Qc 
128 Ll


Qc  Ks p
Where Ks is called the die shape factor
Still just equation for laminar flow through a pipe.
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
31
Extrusion Curve
E
x
t
r
u
d
e
r
F
l
o
w
Increasing N or
increasing viscosity
Die characteristic
curve
Increassing
L, n,
decreasing
D
Operating
Point
R
a
t
e
Extruder Characteristic
Curve
Pmax
Extrusion Pressure
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
32
Operating Point

E
x
t
r
u
d
e
r
F
l
o
w
Increasing N or
increasing viscosity
Die characteristic
curve

Increassing
L, n,
decreasing
D
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.
R
a
t
e
Extruder Characteristic
Curve
Pmax
Extrusion Pressure
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
33
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
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
34
example2

Knowing the above characteristics, calculate Qmax and
pmax.
Qdr  0.5 2 D 2 N d c sin A cos A
2  rev 
Qmax  Qdr  0.5 2   3in   1
   0.25in  sin 20cos 20
 sec 
Qmax
Spring 2001
in3
 3.568
sec
ISAT 430
Dr. Ken Lewis
Module 3B
35
example3

Knowing the above characteristics, calculate Qmax and
pmax.
6 DNL cot A
p 
max
pmax
d
2
c
lbf  sec  cos 20
 rev 

6   3in   1
   75in    0.015

2
in  sin 20
 sec 


2
 0.25in 
pmax
Spring 2001
lbf
 2796.59 2
in
ISAT 430
Dr. Ken Lewis
Module 3B
36
example4
Qmax
in3
 3.568
sec
pmax  2796.59
lbf
in 2

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?

Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
37
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?
 Dd4
Ks 
128 Ld
Ks 
  0.25in 
4
lbf  sec
128  0.015
1.0in
2
in
in5
K s  0.0063916
lbf  sec
remember
Spring 2001
Qc  Ks p
ISAT 430
Dr. Ken Lewis
Module 3B
38
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.
Qx  Qmax 
Qmax
p
pmax
Qx  3.57  0.0012765 p
And from the die equation
Setting these equal provides
the operating point
Spring 2001
ISAT 430
Qx  0.0063916 p
p  465.6 psi
Dr. Ken Lewis
Module 3B
in3
Qx  2.98
sec
39
Extruder Characteristic
7.000
6.000
Flow Rate (in3/sec)
5.000
4.000
Q ex
Q die
3.000
2.000
1.000
0.000
0
500
1000
1500
2000
2500
3000
Pressure (psi)
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
40
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
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
41
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.
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
42
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.
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
43
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.
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
44
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
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
45
Die shapes
The finished
shapes
The dies
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
46
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.
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
47
Pipe extrusion

To control the pipe size, other
means are used.
Internal sizing mandrel
External sizing using
air pressure
External sizing using
vacuum
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
48
Tubing Die
Spring 2001
ISAT 430

Note the expansion to the
spider legs and the reduction
afterwards.

If the extrusion is too rapid,
the spider leg openings will
not heal.
Dr. Ken Lewis
Module 3B
49
Wire Coating Die



Spring 2001
ISAT 430
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.
Dr. Ken Lewis
Module 3B
50
Wire Coating Die2




Spring 2001
ISAT 430
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.
Dr. Ken Lewis
Module 3B
51
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)
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
53
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)

Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
54
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
55
Injection Molding Sequences
(1) Close the mold
(2) Inject the melt
(3) Retract the screw
(4) Open mold – eject part
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
56
Two Plate Mold


The mold here is closed
The mold is position
between two platens
 One stationary
 One moveable

Sprue: channel from die nozzle
Into the mold
Note the water channels for
quickly cooling the mold and
its polymer load.
Gates: restrict the polymer
Flow into the cavity
Runner: channel from Sprue
Into the cavity
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
57
Two Plate Mold2



Spring 2001
ISAT 430
The mold here is open
The ejector pins push the
rather fragile plastic from the
mold cavity
The sprue and runners are
waste..
Dr. Ken Lewis
Module 3B
58
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.
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
59
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)
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
60
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
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
62
Thermoforming

Three major types of thermoforming
 Vacuum
» Pressure limit of 1 atmosphere
 Pressure
» Higher allowable pressures
 Mechanical
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
63
Vacuum Thermoforming
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
64
Pressure Thermoforming
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
65
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
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
66
Mechanical Thermoforming2


In (1) the polymer is pre heated
In (2) the polymer is forced into place in the negative mold.
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
67
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
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
68
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
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
69
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
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
70
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°
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
71
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
Spring 2001
ISAT 430
Dr. Ken Lewis
Module 3B
72