Common Issues and Solutions for PET Injection Molded Preforms
A preform is an injection-molded item. As an intermediate semi-finished product for subsequent biaxial stretch blow molding, the neck of the preform is already set during the injection molding stage, but issues often arise. This article shares common problems with preforms and their solutions.
1. **White Bottom on PET Preform**
Causes:
① Insufficient hot runner temperature
② Insufficient material temperature
Solutions:
① Enhance insulation measures
② Increase the temperature of the hot nozzle in that area
③ Appropriately increase material temperature
④ Increase injection speed
⑤ Reduce holding pressure
⑥ Slow down the cooling water flow rate
2. **PET Preform Not Transparent**
Causes:
① Insufficient drying of the material
② Insufficient plasticization of the material
③ Preform wall thickness too large or insufficient cooling
④ Contamination with other plastics
⑤ Material temperature too low
Solutions:
① Ensure material is dried for more than 4 hours
② Increase material temperature
③ Increase screw rotation speed
④ Check if drying temperature is too low or if injection volume is mismatched; ensure material is dried at 165°C to a moisture content below 0.02%
⑤ Enhance cooling, reduce wall thickness
3. **PET Preform Changes from Transparent to Opaque**
Causes:
① Demolding temperature too high
Solutions:
① Enhance cooling water
② Extend cooling time
③ Adjust injection time
4. **Haze on One Side of PET Preform**
Causes:
① Holding pressure too high
② Eccentric gate hole in the bottom mold
Solutions:
① Reduce pressure
② Correct the mold
5. **Silver Streaks or Yellowing in PET Preform**
Causes:
① Plastic decomposition
Solutions:
① Reduce drying temperature, extend drying time
② Adjust injection speed
③ Reduce material temperature
④ Reduce hot nozzle temperature
6. **Bubbles in PET Preform**
Causes:
① Insufficient drying
Solutions:
① Enhance drying
② Increase material temperature
③ Reduce screw rotation speed
7. **Irregular Ring Grooves Inside PET Preform**
Causes:
① Water vapor condensation on the core and cavity
Solutions:
① Enhance workshop drying
② Increase cooling water temperature
③ Wipe the mold dry
8. **Cracking at the Gate of PET Preform**
Causes:
① Misalignment between hot runner gate and mold gate
② Undercut at the gate
③ Gate area too hot
Solutions:
① Correct the mold
② Modify the gate
③ Extend cooling time
④ Increase injection speed or injection time
⑤ Enhance gate insulation
9. **Short Shot or Weld Lines at the Preform Mouth**
Causes:
① Poor venting
② Eccentric injection
Solutions:
① Enlarge venting slots
② Clean the lip mold
③ Reduce injection speed
④ Correct the gate hole in the bottom mold
10. **PET Preform with One Side Thick and One Side Thin**
Causes:
① Poor mold manufacturing
② Gate hole misalignment
③ Injection pressure too high
④ Core shift
Solutions:
① Correct the mold
② Increase material temperature
③ Reduce pressure
④ Use molds with high strength, high precision, and adjustable core structure
11. **Surface Sinks on Preform**
Causes:
① Material temperature too high
② Injection speed too short
③ Holding pressure too short
④ Injection time too short
⑤ Insufficient cooling
Solutions:
① Reduce material temperature
② Reduce injection speed
③ Increase holding pressure
④ Increase injection time
⑤ Increase water pressure
12. **Stringing at the Gate of PET Preform**
Causes:
① Poor insulation
② Insufficient cooling
③ Pressure inside the hot runner
④ Poor screw check ring function
⑤ Holding pressure too low
Solutions:
① Ensure gate insulation
② Increase cooling
③ Enhance screw backflow
④ Clean the screw and nozzle
⑤ Increase holding pressure
13. **Bending of Preform After Demolding**
Causes:
① Eccentric gate hole
② Uneven cooling
Solutions:
① Correct the mold
② Reduce holding pressure
③ Increase injection speed
④ Extend cooling
⑤ Clean water channels
⑥ Increase cooling water flow rate
14. **Bending of Preform After Heating**
Causes:
① Eccentric preform
② Eccentric heating holes or heating seat
③ Uneven preform density
Solutions:
① Correct the mold
② Adjust heating positioning components
③ Reduce heating temperature, especially in the bending area
15. **Local Whitening in PET Blown Bottle**
Causes:
① Insufficient preform heating
Solutions:
① Increase PET preform temperature and ensure internal and external uniformity
16. **Whitening of Preform After Heating**
Causes:
① Preform heating temperature too high in the whitened section
Solutions:
① Ensure PET preform is blown between 80°C and 110°C
17. **PET Bottle Does Not Inflate Fully**
Causes:
① Insufficient air pressure or volume
② Insufficient preform heating
③ Defects in preform injection molding
Solutions:
① Increase air pressure and volume
② Increase preform temperature
③ Check if the PET preform softens evenly using 100°C water
18. **Bottle Bottom Tearing**
Causes:
① Bottom temperature too high
② Body temperature too low
③ Severe plastic degradation
Solutions:
① Adjust preform temperature
② Check and replace preform material; enhance drying
③ Delay blow timing
19. **Eccentric Preform**
Causes:
① Blowing too early
② Air pressure too high
③ Misalignment between stretch rod and bottle bottom
④ Severe preform eccentricity
⑤ Stretch rod speed too slow
⑥ Stretch rod more than 1.5mm from bottom mold
⑦ Preform bottom temperature too high
Solutions:
① Delay blow timing
② Reduce air speed
③ Correct installation alignment
④ Check and correct preform condition
⑤ Adjust stretch cylinder pressure
⑥ Adjust stretch rod end to 1-1.5mm from bottom
⑦ Lower bottom heating temperature
20. **Bottle Not Fully Inflated in Vertical Direction**
Causes:
① Preform heating temperature too high in the unfilled section
② Cold spots in the preform in the unfilled section
③ Insufficient air pressure or volume
④ Poor venting in blow mold
⑤ Blow mold temperature too high
⑥ Preform wall too thin
⑦ Insufficient longitudinal stretch ratio
Solutions:
① Adjust PET preform temperature
② Avoid touching the heated preform body; check if cold hard spots resulted from contact
③ Increase stored air pressure and volume
④ Enhance venting
⑤ Add cooling water to the blow mold
⑥ Correct the mold
⑦ Increase stretch ratio (longitudinal)
21. **Uneven Wall Thickness in Circumferential Direction of PET Bottle**
Causes:
① Eccentric preform
② Uneven heating
③ Poor venting
Solutions:
① Improve preform mold
② Improve heating equipment
③ Lower the temperature of the problematic part of the PET preform
④ Increase venting
22. **Volcano-Like Protrusion at Bottle Bottom**
Causes:
① Preform bottom too cold or other parts of the preform too hot
Solutions:
① Advance blow timing; adjust heating temperature
23. **Local Whitening After Blowing**
Causes:
① Localized insufficient heating
② Localized excessive stretch ratio
③ Moisture in compressed air
Solutions:
① Increase preform temperature
② Check preform design
③ Enhance water and oil filtration
24. **Poor Vertical Load Capacity of Bottle**
Causes:
① Uneven wall thickness
② Preform heating temperature too high
③ Poor bottle shape design
Solutions:
① Improve wall thickness uniformity
② Form at low temperature and high pressure where possible
③ Improve bottle design
25. **Bottle Easily Broken When Dropped**
Causes:
① Bottom wall thickness not properly stretched
② High-temperature plastic degradation
Solutions:
① Improve bottom stretch ratio
② Enhance raw material drying before injection
③ Reduce production temperature
26. **Speckles or White Haze Inside Bottle**
Causes:
① Blowing with unclean air
② High air humidity
Solutions:
① Install air filtration device
② Reduce humidity of blown air
27. **Bottle Too Soft**
Causes:
① Poor design
② Processing temperature too high
Solutions:
① Improve design
② Perform injection and blow molding at low temperatures where possible
28. **Burnt Smell After Blowing**
Causes:
① Heating temperature too high
② Poor compressed air quality
③ Use of inferior raw materials
Solutions:
① Reduce blow molding temperature
② Maintain air compressor and filters
③ Use materials from reputable manufacturers
29. **Water Develops Off-Taste After Being Stored in Bottle for Some Time**
Causes:
① High acetaldehyde content in the bottle
② Unqualified raw materials
Solutions:
① Use low-temperature injection and blow molding
② Check acetaldehyde content of raw materials before processing; reject inferior materials.
