The document discusses common defects and troubleshooting methods related to the extrusion process, focusing on issues like die swell, blistering, lumpiness, shark skin, melt fracture, black lumps, knitting errors, and plate-out. It explains how these defects occur, their causes, and potential solutions such as adjusting extrusion rates, die geometry, and temperatures. Overall, it emphasizes the importance of careful processing and material handling to minimize defects during polymer extrusion.

1. Extruders: Defects &
Troubleshooting

2. DEFECTS

3. Die swell
 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.

4. 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

5. 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.
 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.
 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).

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

7. Bubbles in Extrudate

8. Surface (Pitting)
Bubbles in Extrudate

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

10. 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.

11. 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

12. Shark-Skin:
 Shark skin tends to be reduced with
increase in temperature
Flow
Direction
 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.

13. 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

14. 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.

15. 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 will occur.
The head pressure may be increased by:
 Increase the die land
 Reducing the cross sectional area at the die
 Lowering the die temperature.

16. 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

20. ABHIJITH K S
MSc PS
CIPET - KOCHI