1. Polymer Processing
Module 3b

2. 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 Dr. Ken Lewis

3. 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 Dr. Ken Lewis

4. 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.
Spring 2001
ISAT Dr. Ken Lewis

5. Extrusion3 The extruder consists basically of a hopper and
a barrel and
a screw.
Spring 2001
ISAT Dr. Ken Lewis

6. Extruder Usually ~ 1 – 6 in. dia. Up to 60 rpm The die is not part of
the extruder
Flight clearance of only in.
Spring 2001
ISAT Dr. Ken Lewis

7. Extruder Feed section Compression or plastication 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 Dr. Ken Lewis

8. 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)
Spring 2001
ISAT Dr. Ken Lewis

9. 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 Dr. Ken Lewis

10. 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.
Spring 2001
ISAT Dr. Ken Lewis

11. 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…
Spring 2001
ISAT Dr. Ken Lewis

12. 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)
Where:
v = velocity of the plate (m/s)
D = distance between the plates (m)
W = width of the plates(m)
Spring 2001
ISAT Dr. Ken Lewis

13. 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 Dr. Ken Lewis

14. Melt Flow in the Extruder
The axial flow in the z direction is responsible for the pumping
The cross flow does most of the mixing.
Spring 2001
ISAT Dr. Ken Lewis

15. 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 Dr. Ken Lewis

16. Extruder transport Using the unrolled screw model, we can show that:
Where:
v = velocity of the plate (m/s)
D = distance between the plates (m)
W = width of the plates(m)
Extruder transport
Using the unrolled screw model, we can show that:
We have assumed:
wf is negligible
Note: or
Spring 2001
ISAT Dr. Ken Lewis

17. 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.
Spring 2001
ISAT Dr. Ken Lewis

18. 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
Spring 2001
ISAT Dr. Ken Lewis

19. Extruder transport Then: Where: p = the head pressure (Mpa)
L = length of the barrel (m)
Spring 2001
ISAT Dr. Ken Lewis

20. Back Pressure Flow A misnomer So what is the net flow?
It is not back pressure flow
It is resistance to forward flow
So what is the net flow?
Qnet is what finally comes out of the die!
Spring 2001
ISAT Dr. Ken Lewis

21. Back Pressure And the maximum pressure becomes:
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
And the maximum pressure becomes:
Spring 2001
ISAT Dr. Ken Lewis

22. 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)
Spring 2001
ISAT Dr. Ken Lewis

23. 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 Dr. Ken Lewis

24. 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 Dr. Ken Lewis

25. Extruder Characteristics
For a given extruder: or
Spring 2001
ISAT Dr. Ken Lewis

26. Extruder Characteristics
Flow up with
Increasing N
Decreasing p
Increasing 
Ignores non-Newtonian flow behavior
Ignores friction
Spring 2001
ISAT Dr. Ken Lewis

27. Extruder Characteristics
A useful estimate of extruder capacity with a L/D = 24 is:
Usual
Recommended Ce
scr
Output in Kg/h
0.006
2.2
2.3
Output in lb/h 16 20
2.35
Actual output may ± 20% (good for back of envelope calculations)
Spring 2001
ISAT Dr. Ken Lewis

28. What is the output for zero back pressure?
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?
What is the output expected for normal conditions?
Spring 2001
ISAT Dr. Ken Lewis

29. 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:
D = diameter
 = melt viscosity [=]
Spring 2001
ISAT Dr. Ken Lewis

30. 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:
Spring 2001
ISAT Dr. Ken Lewis

31. Die characteristics Where Ks is called the die shape factor
Still just equation for laminar flow through a pipe.
Spring 2001
ISAT Dr. Ken Lewis

32. Extrusion Curve
Spring 2001
ISAT Dr. Ken Lewis

33. 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.
Spring 2001
ISAT Dr. Ken Lewis

34. 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 = Pa sec
Spring 2001
ISAT Dr. Ken Lewis

35. example2 Knowing the above characteristics, calculate Qmax and pmax.
Spring 2001
ISAT Dr. Ken Lewis

36. example3 Knowing the above characteristics, calculate Qmax and pmax.
Spring 2001
ISAT Dr. Ken Lewis

37. 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?
Spring 2001
ISAT Dr. Ken Lewis

38. If we have a circular die with a diameter Dd = 0
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?
remember
Spring 2001
ISAT Dr. Ken Lewis

39. example5 And from the die equation
Now we can find the operating point for the extruder.
We can express the extruder characteristic as the straight line between Qmax and pmax.
And from the die equation
Setting these equal provides the operating point
Spring 2001
ISAT Dr. Ken Lewis

40. Spring 2001
ISAT Dr. Ken Lewis

41. 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 Dr. Ken Lewis

42. 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 Dr. Ken Lewis

43. 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 Dr. Ken Lewis

44. 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 Dr. Ken Lewis

45. 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 Dr. Ken Lewis

46. Die shapes The dies The finished shapes Spring 2001
ISAT Dr. Ken Lewis

47. 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 Dr. Ken Lewis

48. Internal sizing mandrel
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 Dr. Ken Lewis

49. 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.
Spring 2001
ISAT Dr. Ken Lewis

50. 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.
Spring 2001
ISAT Dr. Ken Lewis

51. 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.
Spring 2001
ISAT Dr. Ken Lewis

52. Injection Molding

53. 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 Dr. Ken Lewis

54. 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 Dr. Ken Lewis

55. Spring 2001
ISAT Dr. Ken Lewis

56. Injection Molding Sequences
(1) Close the mold
(2) Inject the melt
(3) Retract the screw
(4) Open mold – eject part
Spring 2001
ISAT Dr. Ken Lewis

57. 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
Gates: restrict the polymer
Flow into the cavity
Runner: channel from Sprue
Into the cavity
Spring 2001
ISAT Dr. Ken Lewis

58. 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..
Spring 2001
ISAT Dr. Ken Lewis

59. Two Plate Mold3 Cooling system Gas vents
Usually water passages in the mold itself
Gas vents
Usually about 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 Dr. Ken Lewis

60. 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 Dr. Ken Lewis

61. Thermoforming

62. 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 Dr. Ken Lewis

63. Thermoforming Three major types of thermoforming Vacuum Pressure
Pressure limit of 1 atmosphere
Pressure
Higher allowable pressures
Mechanical
Spring 2001
ISAT Dr. Ken Lewis

64. Vacuum Thermoforming
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ISAT Dr. Ken Lewis

65. Pressure Thermoforming
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ISAT Dr. Ken Lewis

66. 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 Dr. Ken Lewis

67. Mechanical Thermoforming2
In (1) the polymer is pre heated
In (2) the polymer is forced into place in the negative mold.
Spring 2001
ISAT Dr. Ken Lewis

68. 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 Dr. Ken Lewis

69. 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 Dr. Ken Lewis

70. 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 Dr. Ken Lewis

71. 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 Dr. Ken Lewis

72. Molded Part Considerations3
Tolerances
Shrinkage will occur but is predictable
The more generous the tolerances the easier the manufacture.
Typical dimension tolerances are:
+/ – inches
Typical hole tolerances are:
+/ – inches
Spring 2001
ISAT Dr. Ken Lewis