Extruders: defects and troubleshooting.pptx

The slide contains details of most common defects and troubleshooting(reason, causes, remedy) of polymer extruders used in industries for various types of polymer product manufacturing.

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  R egularly 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 R educing the cross sectional area at the die L owering 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

17. TROUBLESHOOTING

18. THANK YOU ABHIJITH K S MSc PS CIPET - KOCHI