Common Issues and Solutions in PET Preform Production*

**Common Issues and Solutions in PET Preform Production**

A preform is an injection-molded product that serves as an intermediate semi-finished item for subsequent biaxial stretch blow molding. The neck of the preform is shaped during the injection molding stage, but various issues may still arise during production. This article outlines some common problems encountered in preform production and their corresponding solutions for reference:

1. **PET Preform Bottom Turns White**  

  **Cause:** ① Insufficient hot runner temperature ② Low material temperature  

  **Solution:** ① Improve insulation measures ② Increase hot nozzle temperature ③ Slightly raise material temperature ④ Increase injection speed ⑤ Lower holding pressure ⑥ Slow down cooling water flow rate  

2. **PET Preform Becomes Opaque**  

  **Cause:** ① Insufficient raw material drying ② Inadequate plasticization ③ Excessive preform wall thickness or insufficient cooling ④ Contamination with other plastics ⑤ Too low material temperature  

  **Solution:** ① Ensure raw materials are dried for at least 4 hours ② Raise material temperature ③ Increase screw speed ④ Check if the material has been sufficiently dried to a moisture content of less than 0.02% at 165°C ⑤ Strengthen cooling, reduce wall thickness  

3. **PET Preform Turns from Transparent to Opaque**  

  **Cause:** ① Too high mold release temperature  

  **Solution:** ① Enhance cooling water circulation ② Extend cooling time ③ Adjust injection time  

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4. **PET Preform Has Foggy Areas on One Side**  

  **Cause:** ① Excessive holding pressure ② Off-center gate hole  

  **Solution:** ① Reduce holding pressure ② Correct mold alignment  

5. **PET Preform Shows Silver Streaks or Yellowing**  

  **Cause:** ① Plastic degradation  

  **Solution:** ① Lower drying temperature, extend drying time ② Adjust injection speed ③ Reduce material temperature ④ Lower hot nozzle temperature  

6. **PET Preform Contains Air Bubbles**  

  **Cause:** ① Insufficient drying of raw materials  

  **Solution:** ① Improve drying ② Increase material temperature ③ Reduce screw speed  

7. **Irregular Ring Grooves Inside PET Preform**  

  **Cause:** ① Condensation of water vapor in the core or cavity of the mold  

  **Solution:** ① Improve workshop drying ② Raise cooling water temperature ③ Dry the mold  

8. **PET Preform Cracks at the Gate**  

  **Cause:** ① Misalignment between hot runner and gate ② Incorrect gate orientation ③ Overheating in the gate area  

  **Solution:** ① Correct mold alignment ② Adjust the gate ③ Extend cooling time ④ Increase injection speed or injection time ⑤ Enhance gate insulation  

9. **PET Preform Not Fully Injected or Has Weld Lines**  

  **Cause:** ① Poor venting ② Off-center injection  

  **Solution:** ① Increase vent slots ② Clean mold lips ③ Lower injection speed ④ Correct mold gate hole alignment  

10. **Uneven Wall Thickness in PET Preform**  

   **Cause:** ① Poor mold manufacturing ② Misaligned gate ③ Excessive injection pressure ④ Core shift  

   **Solution:** ① Correct mold ② Raise material temperature ③ Lower injection pressure ④ Use high-strength, high-precision molds with adjustable core structures  

11. **PET Preform Surface Depressions**  

   **Cause:** ① Too high material temperature ② Too short injection speed ③ Too short holding pressure ④ Too short injection time ⑤ Insufficient cooling  

   **Solution:** ① Lower material temperature ② Lower injection speed ③ Increase holding pressure ④ Extend injection time ⑤ Increase water pressure  

12. **PET Preform Gate Draws String**  

   **Cause:** ① Poor insulation ② Insufficient cooling ③ Pressure within the hot runner ④ Poor screw reverse check ⑤ Too low holding pressure  

   **Solution:** ① Ensure proper gate insulation ② Enhance cooling ③ Strengthen reverse check ④ Clean the screw nozzle ⑤ Increase holding pressure  

13. **PET Preform Warps After Ejection**  

   **Cause:** ① Off-center gate ② Uneven cooling  

   **Solution:** ① Correct mold alignment ② Lower holding pressure ③ Increase injection speed ④ Extend cooling time ⑤ Clean water channels ⑥ Increase cooling water flow  

14. **PET Preform Warps After Heating**  

   **Cause:** ① Off-center preform ② Misaligned temperature control holes or seats ③ Uneven preform density  

   **Solution:** ① Correct mold ② Align temperature control components ③ Lower temperature, especially for the bending area  

15. **PET Blow Molded Preform Turns White Locally**  

   **Cause:** ① Inadequate preform temperature adjustment  

   **Solution:** ① Increase PET preform temperature and ensure uniformity both inside and outside  

16. **PET Preform Turns White After Temperature Adjustment**  

   **Cause:** ① Overheating in the white section of the preform  

   **Solution:** ① Ensure PET preform temperature stays between 80°C and 110°C  

17. **PET Bottle Won't Expand During Blowing**  

   **Cause:** ① Insufficient air pressure and volume ② Inadequate preform temperature ③ Defects in the preform  

   **Solution:** ① Increase air pressure and volume ② Raise preform temperature ③ Check if the preform softens evenly at 100°C using hot water  

18. **Preform Bottom Stretches Through**  

   **Cause:** ① Too high bottom temperature ② Too low body temperature ③ Severe plastic degradation  

   **Solution:** ① Adjust preform temperature ② Check and replace raw materials, enhance drying ③ Slow down and delay the blowing time  

19. **PET Preform is Off-Center**  

   **Cause:** ① Blowing too early ② High air pressure ③ Misalignment of the stretching rod with the preform bottom ④ Severe preform eccentricity ⑤ Too slow stretching rod speed ⑥ Stretching rod distance from the bottom mold greater than 1.5mm ⑦ Too high bottom temperature  

   **Solution:** ① Delay blowing time ② Reduce air pressure ③ Correct rod alignment ④ Check and correct preform eccentricity ⑤ Adjust stretching cylinder pressure ⑥ Adjust stretching rod end position to 1-1.5mm from the bottom ⑦ Lower temperature in the lower temperature control zone  

20. **Preform Wall Not Fully Stretched in Vertical Direction**  

   **Cause:** ① Unfilled sections of the preform have too high temperature ② Cold spots in the unfilled sections ③ Insufficient air pressure and volume ④ Poor venting in the blow mold ⑤ Too high blow mold temperature ⑥ Preform wall too thin ⑦ Insufficient vertical stretching ratio  

   **Solution:** ① Adjust preform temperature ② Ensure that heated preforms are not touched and check for cold spots ③ Increase air pressure and volume ④ Increase venting ⑤ Add cooling water to the blow mold ⑥ Correct mold ⑦ Increase vertical stretching ratio  

21. **PET Bottle Has Uneven Wall Thickness in the Circumferential Direction**  

   **Cause:** ① Off-center preform ② Uneven temperature control ③ Poor venting  

   **Solution:** ① Improve preform mold ② Improve temperature control system ③ Lower temperature in defective areas ④ Increase venting  

22. **Preform Bottom Has a "Volcanic" Raised Area**  

   **Cause:** ① Bottom of the preform is too cold or other areas of the preform are too hot  

   **Solution:** ① Adjust the blowing time and temperature  

23. **PET Bottle Turns White After Blowing**  

   **Cause:** ① Insufficient local temperature adjustment ② Excessive stretching ratio ③ Moisture in compressed air  

   **Solution:** ① Increase preform temperature ② Check preform design ③ Enhance filtration of water and oil in compressed air  

24. **PET Bottle Has Poor Vertical Load Bearing Capacity**  

   **Cause:** ① Uneven wall thickness ② Too high preform temperature ③ Poor bottle shape  

   **Solution:** ① Improve wall thickness uniformity ② Form at lower temperatures and higher pressure ③ Improve bottle design  

25. **PET Bottle Breaks Easily**  

   **Cause:** ① Insufficient bottom stretch ② Plastic degradation at high temperatures  

   **Solution:** ① Improve bottom stretch ratio ② Strengthen raw material drying before preform injection ③ Lower production temperature  

26. **PET Bottle Contains Nicks or White Haze**  

   **Cause:** ① Contaminated air during blowing ② High humidity in air  

   **Solution:** ① Install air filtration devices ② Lower air humidity  

27. **PET Bottle is Too Soft**  

   **Cause:** ① Poor design ② High processing temperature  

   **Solution:** ① Improve design ② Form at lower temperatures  

28. **PET Bottle Has a Burnt Smell After Blowing**  

   **Cause:** ① Excessive heating ② Poor compressed air quality ③ Use of low-quality materials  

   **Solution:** ① Lower blowing temperature ② Clean air compressor and filters ③ Use materials from reputable suppliers  

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