The document discusses various extrusion problems and defects related to thermoplastics, such as die swell, moisture absorption, trapped air, polymer degradation, and melt fracture. It provides insights into the causes of these issues, remedies to mitigate them, and factors influencing the extrusion process, including die geometry and temperature control. Additionally, it highlights the importance of proper mixing and handling of materials to prevent defects like blistering and lumpiness.

1. Extrusion Problems &
Defects

2.  The profile of extruded material grows in size, reflecting its tendency to
return to its previously larger cross section in the extruder barrel immediately
before being squeezed through the smaller die opening.
 Shape memory: Extruded polymer remembers its previous shape when it
was in the larger cross section of the bar.
Die swell

3. Die swell
 Die swell occurs because the sudden release of pressure causes the polymer
chains to relax
 Polymers are viscoelastic Time dependent stress relaxation
 Die swell =
𝐷2−𝐷1
𝐷1
, where 𝐷1 is the inner diameter of the die and 𝐷2 is the
average outer diameter of the extruded part.
 Die swell can vary from 10% to over 100% increase in dimensions depending on:
1. Material
2. Melt temperature
3. Extrusion speed
4. Die geometry

4. Die swell
 Notes:
1. Short-land dies leads to greater swell.
2. Long-land dies leads to less swell.
3. Higher output rate leads to viscous heat generation (frictional heat).
4. Higher output rate leads to greater swell.

5. Die swell
 How to avoid ?
1. Decrease the extrusion rate (screw speed).
2. Increase the length of the die end (land).
3. Increase the drawdown ratio by improving design factors.

6. Die swell
 Drawdown: The size of the designed die dimensions relative to the final part
dimensions. The dimensional ratio of the die to final part is used to offset the
“swell” of the thermoplastic as it exits the die.
 Land length: The length of the steel (of die) that runs parallel to the polymer
flow. The land-length ratio is the ratio of the land length to the die gap (wall
thickness).

7. Die swell
 Differences between the shape of the die and the extruded section:

8. Die swell
 Effect of die geometry:

9. Bubbles in Extrudate

12. Surface (Pitting)

13. Causes
 Moisture Absorption
 Trapped Air
 Polymer Degradation
 Depolymerization of the Polymer
 Incompatible additive

14. Causes: Moisture Absorption
 Polymers have high moisture absorption. Some
plastics, such as PET, the nylons, and
polycarbonate, can degrade and weaken if even a
tiny amount of moisture is present
 When the polymer pellets are heated, the absorbed
moisture is also heated and turned into steam. It
passes through the extruder and boils when the
pressure is relieved at the die lips.
 The result is a pattern of long bubbles, and pits.

15. Causes: Moisture Absorption
 Remedies:
 To remove moisture, the material must be pre-dry in a hot over
before loading into the hopper.
 Dehumidifying dryers are used to get moisture down to 0.01% or
less which is enough to avoid problems.
 A vent must be used in the extruder to remove the trapped
steam.

16. Causes: Trapped Air
 This is more common if the starting material is powder and not pellets.
 This is not common in pelletized material used with long extruder
barrels. However, some old machines have short barrels, and even a
long machine can be pushed so fast that the air is carried forward into
the product.
 A trapped-air surface shows bubbles and pits.

17. Causes: Trapped Air
 Remedies:
 Such a surface will improve if run more slowly if moisture is not
the problem, so we need to avoid the over-speed of extrusion.
 Vents and vacuum hoppers will eliminate trapped air and are
essential for powders, where passages between the particles are
much smaller; the air cannot escape back through these passages
and is carried forward instead.

18. Causes: Degradation of Polymer
 Overheating may produce degradation of the polymer material that is used.
The degradation varies from one polymer to the other.
For example, PVC is the most susceptible to degradation as its processing
temperature is always close to its decomposition temperature.
 This degradation might cause the polymer to produce gases that might
become trapped inside the material.

19. Causes: Degradation of Polymer
 Remedies:
 Avoid overheating and make sure that the temperature we heat the polymer is safe
and causes no degradation.
 Very accurate temperature control is needed for materials susceptible to
degradation.

20. Causes: Depolymerization of Polymer
 It is the process of converting a polymer into a monomer or a mixture of
monomers. All polymers depolymerize at high temperatures driven by
increase in entropy.
 Some of the products of the depolymerization could be volatile monomers,
that could be entrapped inside the material.
 Remedies:
 Avoid overheating and make sure that the temperature we heat the polymer is safe
and causes no degradation.

21. Blistering:
Blistering may be caused by water either absorbed into the granules or lying on
the surface. The amount of steam produced to give blisters will depend on:
 the amount of water present
 the extrusion temperature.
Polymers are hygroscopic and /or require a high process temperature are
normally carefully dried and stored in sealed tins.

22.  SCREW ROTATION TOO FAST
“A screw rotation speed that is too high will tend to ``whip'' air into the molten
plastic. This excessive air may not be drawn out of the material during the
molding process and pockets of the air may be forced to the surface of the
molded part, forming blisters.”
 LOW BACK PRESSURE
“The back pressure setting controls the density of the melt. A low setting results
in a melt that is not dense enough to push out excessive gases.”
 USING A FLATTER TEMPERATURE PROFILE ALONG THE BARREL.
Blistering: Reasons

23. Relation:
“When there are problem handling fluid polymers it is often advisable to change
to a higher molecular weight grade of polymer”

24. PVC :
 Polymer degrade on subjection to continual heating but some are
more resistant than others.
 Among the important commercial thermoplastics PVC is probably the
most susceptible to degradation since it is processed at temperature
close to its decomposition temperature; Therefore, when extruding
PVC very accurate temperature control will be necessary;

25. Lumpiness :
 Some extrudate may have a glossy finish but are lumpy
and very irregular. This is usually the result of poor mixing
of the melt. Good mixing is necessary temperatures and
also subjected to different shear conditions and will thus
have different viscosity .
 Decreasing the melt temperature in the die will increase
melt viscosity and increase the back pressure. Hence
decreasing the die head temperature may help to reduce
lumpiness

26. Shark-Skin:
 Shark skin tends to be reduced with
increase in temperature
FlowDirection

27. Shark-Skin : Pressure Plateau
 Through the lower region of the curve, melt flow is steady and bottle surfaces are
smooth
 As melt pressure continues to increase, suddenly sharkskin occurs.
 if extrusion pressure is raised still further, the sharkskin disappears. Actually, neither
the parison nor the bottle is quite as smooth as one produced in the steady-flow
region.

28. Melt Fracture:
 Pseudoplastic materials become less viscous with increase in shear rate.
Thermoplastic melts almost invariably show pseudoplastic behavior.
 Above some critical shear rate most thermoplastics exhibit a phenomenon
variably known as elastic turbulence or melt fracture. It is characterized by
various type of distortion which have a helical form .
 Increase the melt temperature increases the critical shear rate for onset of
melt fracture. The effect may therefore be reduced and perhaps even
eliminated by reducing the shear rate and /or decreasing the temperature.
 The critical shear rate for onset of melt fracture is reduced by increasing
molecular weight. Therefore, melt fracture is more likely to occur with high
molecular weight grades than low molecular weight grade

29. Melt Fracture:

30.  Five pictures of a of molten polyethylene flowing out of a pipe, visible at the
top. The flow rate increases from left to right.
 Note that in the two leftmost photographs the extrudates are nice and
smooth, while in the middle one undulations start to develop. As the flow rate
increases even further towards the right, the amplitude of the undulations
gets stronger. When the flow rate is enhanced even more, the extrudate can
break. Hence the name "melt fracture".
Melt Fracture:

31. Melt Fracture:

32. Melt Fracture : Remedy
 The melt fracture effect can be reduced by:
 lowering the shear rate
 Raisingthe temperature
 using the material of lower molecular weight

33. Black Lumps:
 The extruder may show black lumps or flecks.
 These may be due to polymer stagnate some points in the machine and
decomposing. Pieces of decomposed material are then swept away by molten
polymer at irregular intervals.
The problems may be avoided by :
 lowering extrusion temperature
 regularly cleaning dies
 avoiding dead spots. The black particles may occur due to contaminated
compound also.

34. Knitting Error :
 When polymer melts pass round a spider leg and or through a breaker plate
the molten material is separated. unless there is a high pressure on the side
of the spider and /or breaker plate the separated melt will not fully knit
together and lines or planes of weakness willoccur.
The head pressure may be increased by:
 Increased the die land reducing the cross sectional area at the die
lowering the die temperatur

35. Plate-Out :
 When extruding polymers, additives are sometimes deposited from the melt
onto the forward part of the screw and onto the extruder head and die. This
effect is know as plate- out and is most frequently met within PVC. Deposition
occur s most frequently in regions of high temperature and high shear.
Formulation also has an important influence.
This can be reduced by:
 lowering the die head temperature
 reducing extrusion speed
 change the formulation

36. Reference:
 https://www.lyondellbasell.com/globalassets/documents/polymers-
technical-literature/tech-topic-sharkskin.pdf?id=13883