ExtrusionThe Center of all polymer processing

1
Extrusion-The Center of all polymer processing
2
Extrusion

• The basic and most common component of all
  polymer processing equipment
• Injection molding
• Blown film
• Profile extrusion

3
Extrusion

• Profile shapes e.g. rods, fibers, tubes, etc.
• Films and sheets.
• Covering on wire and cables.
• Profile Extrusion
• Note Die Swell, Orifice design and Post
  forming
• Pipe Extrusion
• Sheet Extrusion
• Film Extrusion
• gt 0.25mm( 0.01 in) sheet
• lt0.25mm - film

4
Extrusion

• Has many functions
• Melt
• Mix
• Compound
• Pressurize

5
Extrusion

• Operation Principle five steps
• The extruder plasticated forced out through
  the die
• The die The hot molten of soft plastics takes
  shape.
• Forming The hot material is further shaped.
• Post-forming The material is cut or further
  shaped.
• Secondary processing

6
Extrusion

• Is basically a feed screw with heated a heated
  barrel

7
Extrusion

• Why
• Plug heating is not effective because the thermal
  conductivity of most plastics is too
  low-proportional to electrical conductivity
• Heating from the outside-in is not efficient and
  results in high thermal gradients

8
Extrusion

• Thermal conductivity
• The amount of heat conducted through a sample
• Fourier Law of Conduction

9
Heat flow
.

• Heat flux (qx) through a wall

?1
?2
?1gt ?2
qx
dx
10
Heat flow

• Heat flux (qx)

.
qx
qxdx
dy
dx
11
Heat flow

• Heat flux (qx)

.
qx
qxdx
dy
dx
12
Solution to heat flow

• Thin films
• Long slabs
• Particles
• Melt removal
• Pressure-induced melt removal
• DRAG-INDUCED MELT REMOVAL

13
Melt removal

• Pressure induced removal

F
Heated tool
14
Melt removal

• Drag induced melt removal

V
Heated tool
15
Extruder

• Extruder
• Hopper
• Screw The heart of the extruder
• Three-zone screw is the most used type
• (1) Feed zone
• greatest channel depth
• (2) Compression zone (transition zone)
• decreasing channel depth
• (3) Metering zone
• Assures proper delivery amount

16
Extrusion zone
17
The Screw is a melt drag design
18
Screw flow

• Need drag at barrel

19
Melt profile

• Classical melt cross section

20
Functions of a screw

• Convey
• Mix
• Plasticating (melting)
• Metering
• Venting

21
Transition (Compression) zone

• Promote both the compression and heating of the
  plastic granules.
• Uniformly tapered,
• Increasing root diameter
• Reduces the available volume between flights
• Compressing the granules.
• Air is purged back through the hopper.
• Heating,
• partly by conduction (15)
• mainly by friction from rotary shear (85)
• Mixed into a homogenous melt.
• one-fourth to one-third the entire screw length

22
Metering section

• Accurately controls amount of melt
• Assures smooth melt flow

23
Check valve

• Prevent back flow during injection
• Ball check valve
• Ring check valve
• Located at tip of screw
• Screen pack maybe at final section to trap
  contaminants

24
Two stage

• Release of entrapped volatiles moisture
• Better metering
• Better appearance, uniformity and properties

25
Twin Screws

• More is better (but at a cost)

26
Twin screws
From SPE
27
Twin Screw

• Use with reactive extrusion
• Often a modular design
• Relatively expensive
• More difficult to operate
• Good melting
• Good venting
• Can overload motor

28
Twin screws

• Modular design

29
Twin screws
Better conveying charteristics
30
Twin screws
Fully-intermeshing-co-rotating
31
Twin screws
No wiping
Wiping
No wiping
From SPE
32
Screw output

• Flow (Assuming no back flow!)

0
VbzMelt velocity in Z-Direction Fd and FpShape
factor WChannel width DScrew diameter NScrew
speed (Hz) hChannel depth LScrew
length PPressure ?Pitch ?Clearance flleakage
33
Screw sizes
15 in dia.!
34
Die swell

• Exit flow is larger than die opening
• D/Do
• Typically 1.12

Do
D
35
Die Swell
Die orifice Extruded profile
36
Die swell

• As a function of viscosity

?
Faster flow, more molecular alignment in flow
channel
D/Do
.
?
37
Die swell

• As a function of viscosity

Longer chains fold back on each other once
outside die
MW increase
D/Do
.
?
38
Die swell

• As a function of die temperature

Decreasing temperature T
Lower temperature reduce folding possibility
after existing
D/Do
.
?
39
Die swell

• As a function of die design (length/diameter)

D/Do
Chains recover random orientation with flow
L/D
40
Die swell

• Die design

More die swell
Less die swell
41
Melt fracture

• Shear stress at 105 N/m2
• Irregular flow
• Limit flow
• Limit production
• Limit profits

42
Melt fracture

• Two driving forces
• Slip and stick-Melt sticks to wall then breaks
  free causing pulsation in pressure
• Skin rupture-Die swell causes pressure build up
  in melt at exist, then with sudden cooling the
  surface breaks

43
Melt fracture
Skin rupture
Slip and stick
44
Melt fracture-reductions

• Decrease entrance angle
• Increase temperature
• Reduce viscosity
• Reduce shear stress
• Increase die diameter (reduce stress)
• Reduce molecular weight

45
Die Design

• Three major parts

Manifold
Inlet channel
Land
46
Die Design
47
Die Flow (Newtonian flow)

• For a circular die
• For a rectangular die

RPipe radius µViscosity LPipe
length ?PPressure drop
WPipe width HHeight of opening µViscosity LPip
e length ?PPressure drop
48
Cross over of screw and die
Large die opening
Flow of typical screw
Q
Small die opening
?P
49
Blown film
50
Calendering

• 95 of sheet film products are PVC.
• A series of heated, revolving rollers
  progressively squeezed thermoplastics stock to
  the desired thickness in the forms of sheet or
  film.
• Products handbags, shoes, and luggage.
• Advantages minimum of cleaning
• Disadvantages expensive process

51
Troubleshooting

• Melt Fracture
• Streamlining the flow channel
• Reduce shear stress
• Increase die temperature
• Opening die at land region
• Reduce extrusion rate
• Change die wall material (Ceramic insert)
• Change material (add processing agents)

52
Troubleshooting

• Voids
• Volatiles
• Degradation
• Not enough venting
• Cooling too fast

53
Trouble shooting

• Vent flow-Material coming out of vent
• Root cause is imbalance between stages
• Starve feeding
• Reduce screw speed
• Cool first stage
• Increase temperature in second stage
• Open die gap
• Check screen pack or use low mesh

54
Troubleshooting

• Air entrapment
• Root cause-air does not get out of screw in time
• Use large sized pellets
• Use high compression screw
• Shorten feed
• Use vented extruder
• Vacuum on hopper

55
Troubleshooting

• Gels-cross linked particles-two sources
• P-Gels from polymerization-call supplier
• E-Gels from extrusion
• Particles sticking to screw
• Look for dead spot
• Clean screw and look for scratches

56
Troubleshooting

• Poor mixing
• Add mixing section to screw