Solving Common Issues in PET Pelletizing

PET flake pelletizing involves managing numerous process parameters, and operators may encounter several common issues or challenges. With proper understanding and adjustments, most of these can be resolved or prevented.

Here, we discuss typical problems encountered in PET pelletizing, along with troubleshooting and optimization tips:

Issue 1: Intrinsic Viscosity (IV) Drop / Low-Strength Pellets

Troubleshooting / Prevention:

• Ensure proper drying/degassing: Always start with flakes as dry as possible (ideally <0.5% moisture, or even <0.1% if feasible). Verify that your vacuum pumps are operating effectively. If you see steam or foaming at the vents, you may need to slow down throughput to increase residence time under vacuum. Check that vacuum exhaust lines are clear of blockages.

• Extrusion temperature control: Excessively high temperatures can cause PET to thermally degrade (signaled by yellow or brown discoloration and IV loss). Follow the temperature profile recommended by your equipment supplier and adjust if needed. For example, if yellowing occurs, try slightly lowering barrel temperatures or reducing screw speed to minimize shear heating. Keep PET melt temperatures around 270–280°C, and avoid overheating.

• Additives: In some cases, adding small amounts of chain extenders can help restore IV. These additives (such as multifunctional epoxies or carbodiimides) react with PET chains to rebuild molecular weight. They can be introduced via side feeders. Use them cautiously according to supplier guidelines.

• Vacuum system maintenance: Regularly inspect seals and vacuum pump performance. Water ring pumps need proper water supply and periodic oil changes. Vacuum leaks or low vacuum pressure can leave moisture unremoved.

• Monitoring: Use melt pressure and melt temperature sensors; abnormal readings can indicate partial blockages or overheating, which may degrade the PET.

With a well-tuned machine, IV drop can be kept minimal (modern lines typically maintain ΔIV ≈ 0.02–0.05). If your pellet IV is too low, systematically check the above factors to identify and correct the cause.

Issue 2: Bubbles or Hollow Pellets

If pellets show tiny bubbles or internal foaming (appearing porous or hollow), this is usually a sign of residual moisture or trapped gases in the melt, which expand during cooling.

Causes:

• Incomplete moisture removal (similar to Issue 1).

• Sudden pressure drop during pelletizing may cause dissolved gases to flash off.

Troubleshooting:

• Optimize vacuum degassing (same checks as in Issue 1): Ensure effective moisture and gas removal.

• Pelletizing temperature and die pressure: For strand pelletizing, ensure the draw-down speed isn’t too fast—excessive stretching of the melt can create voids.

• Check vent flow restrictions: Filters or narrow degassing ports can impair venting. Clean these areas and consider widening them slightly, but only under manufacturer guidance.

• Material contamination: Residues from washing agents may release gas during processing. Ensure flakes are thoroughly rinsed and free from corrosive detergent residues.

• Additional degassing: If problems persist, consider using an extruder with a side vacuum vent or an auxiliary vent port. If one vent is insufficient, some systems allow installing extra degassing zones on the extruder.

Boxin’s twin-screw design eliminates dead zones and reduces hollow pellet formation. Its vacuum system ensures smooth pellet surfaces free from black spots or bubbles.

Issue 3: Yellowing or Discoloration of Pellets

Ideally, PET pellets should be clear or slightly bluish. If they appear yellow or brown, it typically indicates thermal degradation or contamination.

Causes:

• Overheating (excessive temperature or prolonged residence time).

• Contaminants like metals or other plastics (especially PVC) can cause PET to degrade or burn.

Troubleshooting:

• Temperature profile: Lower the barrel temperature, especially in the zone where PET fully melts. Even slight moisture at high temperatures can trigger rapid yellowing. Ensure sufficient vacuum to prevent oxidative degradation (oxygen + heat = discoloration).

• Residue buildup: Inspect for burned material on the screw or die. Charred residue can flake off and cause yellow/brown or black specs. If the machine was previously used for other plastics—or is newly oiled—it needs a thorough purge before startup. Use purging compounds during shutdown to reduce buildup.

• Cleaning: Ensure no foreign plastics are present in the flake feed. PVC, in particular, will degrade and burn when mixed with PET, leading to strong discoloration.

• Twin-screw advantage: If you're using a single-screw extruder and experiencing persistent yellowing, poor mixing or dead zones may be to blame. Twin-screw extruders offer better homogenization and reduced stagnation risk.

• Cooling: Make sure pellets are cooled quickly after pelletizing. Hot pellets can stick together and overheat, leading to haze or slight discoloration. If needed, a crystallizer can help keep them tumbling and prevent thermal buildup.

Solving discoloration improves downstream usability of the pellets—especially important if you're targeting transparent or light-colored end products.

Issue 6: Pelletizer Malfunctions (Blade Issues)

Whether strand or die-face pelletizing, poorly maintained pelletizer blades can cause problems.

Symptoms:

Inconsistent pellet length, fines (dust), or pellets getting stuck in the cutter.

Troubleshooting:

• Blade sharpness: Dull blades crush rather than cut, leading to irregular pellets and fines. Regularly sharpen or replace blades. Many systems have self-adjusting blades (spring-loaded against the die face)—ensure these mechanisms are clean and functioning properly.

• Blade alignment: In die-face systems, the pressure of the blade against the die must be just right. If pellets come out with tails or aren’t cleanly cut, slightly increase blade contact (typically via adjustment screws or pneumatic settings).

• Die hole blockage: A clogged hole can cause uneven loading and pellet size variation. Clean die holes if necessary (this may require disassembly and drilling out any blocked polymer).

• Wear parts: Over time, pelletizer rotors, bearings, and other components wear and cause wobbling, leading to uneven cutting. Keep these in good condition (and have spare parts in stock) to avoid unplanned downtime.

• Feed rate vs. cutting speed: If extruder output changes, you may need to adjust cutter speed to maintain pellet length. A cutter that’s too slow produces long pellets; too fast results in excess fines.

Issue 7: Difficulty Maintaining Continuous Operation

At times, the line may trip or require frequent shutdowns (e.g., for screen changes), affecting efficiency.

Causes:

This may result from dirty input material (clogging filters quickly), overheating leading to motor overload trips, or inconsistent feeding causing vacuum loss.

Troubleshooting:

• Continuous filtration: If contamination levels are high, using a continuous dual-piston filter or backflush system allows for longer operation without stops. If you’re using manual screens, consider switching to an automatic screen changer to extend runtime.

• Overheating / Overload: Check whether the extruder or pelletizer motors are running within their load range. Overload can be mitigated by slightly reducing throughput or checking gearbox lubrication and cooling systems. If motors are overheating, install additional cooling units. If surges are causing trips, consider torque limiters.

• Feed consistency: Using hopper loaders or level sensors in the feeder helps ensure it's never starved, which can prevent surges and related issues.

• Stable process parameters: Monitor melt pressure closely. Fluctuations may indicate problems (e.g., partial blockages or feed inconsistency). A stable process should show relatively steady pressure and motor load readings.

Issue 8: Noise and Safety Concerns

Pelletizing lines can be quite noisy—especially strand pelletizers and cutters.

Noise Troubleshooting:

Ensure all rotating components are balanced (imbalance can cause noise/vibration), and maintain gearboxes and lubrication (squealing may indicate wear). If necessary, install noise-reducing features (enclosures, barriers) to ensure operator comfort.

Always enforce safety protocols: Use guards around moving belts and cutters, and require PPE such as hearing protection and gloves for handling hot components.

Optimization Tips:

• Fine-tune the temperature profile:
  
  Adjusting each heating zone by a few degrees can improve melt quality or reduce energy consumption. Work with technical support to find the "sweet spot" for your specific material.

• Regular maintenance schedule:
  
  Don’t wait for problems—create a schedule for checking/replacing vacuum pump oil, cleaning filters, tightening screws, inspecting electrical connections, and calibrating sensors. Preventive maintenance greatly reduces downtime.

• Documentation:
  
  Keep a log of production runs, including any issues and fixes. Over time, this helps spot patterns—e.g., “We see more black specs after 50 hours, time to clean,” or “Screen pressure rises every 2 tons—time for a change.”

• Training:
  
  Ensure operators are well-trained in machine use and the science behind PET processing. For example, they should know that if it starts raining and humidity spikes, some settings might need adjustment (due to wet flakes), or what abnormal noises might indicate. A vigilant operator can catch many issues early.

• Spare parts on hand:
  
  Downtime while waiting for parts can be costly. Identify and stock critical parts (heating bands, thermocouples, screens, pelletizer blades, vacuum seals, etc.). The manufacturer can suggest a spare parts list.

Good manufacturers provide manuals and possibly troubleshooting guides. Some even offer remote diagnostics. If a puzzling issue arises, don’t hesitate to reach out—they may have seen it before and can suggest fixes (e.g., screw modifications or alternative vacuum pumps).