Analysis and Solutions for Common Problems in PET Preform Injection Molding and Blow Molding
I. Overview
PET (Polyethylene Terephthalate) bottles are produced using a “two-step method”. The first step involves injection molding to create preforms, and the second step uses blow molding to form the final bottles. The quality of the preform directly determines the performance and appearance of the final PET bottle. The entire production process demands extremely high control over raw materials, equipment, molds, and process parameters.
II. Common Defects in Preform Injection Molding & Solutions
| Defect Phenomenon | Main Cause Analysis | Solution |
|---|---|---|
| 1. Whitening at Base | 1. Hot runner temperature too low 2. Material temperature too low 3. Cooling too fast (stress whitening) | 1. Increase nozzle temperature for that area 2. Appropriately increase material temperature 3. Increase injection speed 4. Reduce holding pressure 5. Slow down cooling water flow rate 6. Improve insulation measures |
| 2. Preform Not Transparent (Cloudy/Hazy) | 1. Insufficient material drying 2. Insufficient material plasticization 3. Wall thickness too large or insufficient cooling 4. Contamination with other plastics 5. Material temperature too low | 1. Ensure drying for more than 4 hours (≥165°C, moisture content ≤0.02%) 2. Increase material temperature 3. Increase screw speed 4. Check if dryer temperature is too low or shot volume mismatched 5. Enhance cooling, modify design for thinner wall thickness |
| 3. Transparency Turns Cloudy | Demolding temperature too high | 1. Enhance cooling water circulation 2. Extend cooling time 3. Adjust injection time |
| 4. Haze/Shadow on One Side | 1. Holding pressure too high 2. Eccentric gate hole in mold base | 1. Reduce holding pressure 2. Correct the mold |
| 5. Silver Streaks or Yellowing | Plastic degradation | 1. Reduce drying temperature, extend drying time 2. Adjust injection speed (usually reduce) 3. Reduce material temperature 4. Reduce hot runner nozzle temperature |
| 6. Bubbles in Preform | Insufficient drying | 1. Enhance drying 2. Increase material temperature (cautiously) 3. Reduce screw rotation speed (reduce shear heat) |
| 7. Irregular Ring Grooves Inside Preform | Water vapor condensation on core or cavity | 1. Improve workshop air dryness (dehumidify) 2. Increase mold cooling water temperature (above dew point) 3. Wipe dry mold surfaces |
| 8. Cracking at Gate | 1. Misalignment between hot runner nozzle and gate 2. Gate undercut 3. Gate area too hot | 1. Correct the mold 2. Correct the gate design 3. Extend cooling time 4. Increase injection speed or time 5. Improve gate insulation |
| 9. Short Shot or Weld Line at Preform Mouth | 1. Poor venting 2. Eccentric injection | 1. Enlarge venting channels 2. Clean the neck mold (lip mold) 3. Reduce injection speed (if venting is insufficient) 4. Correct mold (gate hole alignment) |
| 10. Uneven Wall Thickness (Side thick/Side thin) | 1. Mold manufacturing defect 2. Gate hole offset 3. Injection pressure too high (causing core deflection) 4. Core shift | 1. Correct the mold 2. Increase material temperature (reduce viscosity) 3. Adjust (reduce) pressure 4. Use high-strength, high-precision molds with adjustable core structure |
| 11. Surface Sinks | 1. Material temperature too high 2. Injection speed too slow 3. Holding pressure/time too low/short 4. Injection time too short 5. Insufficient cooling | 1. Reduce material temperature 2. Increase injection speed 3. Increase holding pressure/time 4. Increase injection time 5. Increase cooling water pressure/flow |
| 12. Stringing at Gate | 1. Poor insulation 2. Insufficient cooling at gate 3. Residual pressure in hot runner 4. Poor screw non-return valve function 5. Holding pressure too low (packing insufficient) | 1. Ensure good gate insulation 2. Increase gate cooling 3. Increase screw decompression (suck back) 4. Clean or replace screw/non-return valve 5. Increase holding pressure/time |
| 13. Warpage after Demolding | 1. Eccentric gate hole 2. Non-uniform cooling | 1. Correct the mold 2. Reduce holding pressure 3. Increase injection speed 4. Extend cooling time 5. Clean mold water lines 6. Increase cooling water pressure/flow |
| 14. Warpage after Reheating (for blow molding) | 1. Eccentric preform 2. Eccentric reheat oven station or pins 3. Inconsistent preform density (crystallinity) | 1. Correct the preform mold 2. Correct reheat positioning components 3. Reduce reheat temperature, particularly in the warping area |
III. Common Defects in Blow Molding & Solutions
| Defect Phenomenon | Main Cause Analysis | Solution |
|---|---|---|
| 1. Local Whitening | Preform heating (conditioning) insufficient | Increase preform temperature and ensure internal/external uniformity |
| 2. Whitening after Reheating | Preform section temperature too high during reheating | Ensure preform temperature is between 80°C – 110°C (Note: Specific range can vary) |
| 3. Bottle Does Not Inflate Fully | 1. Insufficient air pressure/volume 2. Preform temperature too low 3. Preform injection defect | 1. Increase air pressure/volume 2. Increase preform temperature 3. Check preform quality (e.g., test softening uniformity in 100°C water) |
| 4. Pierced Bottle Base | 1. Base temperature too high 2. Body temperature too low 3. Severe material degradation | 1. Adjust preform temperature profile 2. Check/replace preforms/raw material, enhance drying |
| 5. Eccentric Bottle | 1. Blow timing too early (stretches before anchoring) 2. Air pressure too high 3. Misalignment between stretch rod and bottle base mold 4. Severely eccentric preform 5. Stretch rod speed too slow 6. Stretch rod too far from base mold (>1.5mm) 7. Preform base temperature too high | 1. Delay blow timing 2. Reduce air pressure 3. Correct mechanical alignment 4. Check/correct preform condition 5. Adjust stretch cylinder pressure/speed 6. Adjust stretch rod end to 1-1.5mm from base mold 7. Lower reheating temperature at base section |
| 6. Bottle Not Fully Formed Vertically | 1. Section temperature too high (overstretches/thins) 2. Cold spot/hard block in that section (touched?) 3. Insufficient air pressure/volume 4. Poor blow mold venting 5. Blow mold temperature too high 6. Preform wall too thin 7. Insufficient longitudinal stretch ratio | 1. Adjust preform temperature profile 2. Avoid touching heated preforms; identify cause of cold spot 3. Increase air pressure/volume 4. Improve blow mold venting 5. Cool blow mold effectively 6. Correct preform mold design 7. Increase stretch ratio (longitudinal) |
| 7. Uneven Circumferential Wall Thickness | 1. Eccentric preform 2. Non-uniform reheating 3. Poor venting in blow mold | 1. Improve preform mold design 2. Improve reheat oven performance 3. Lower temperature on the thick/uneven side 4. Improve blow mold venting |
| 8. “Volcano” Protrusion at Base | Preform base too cold OR other areas too hot | Advance blow timing, adjust reheat temperature profile |
| 9. Local Whitening After Blowing | 1. Local preform temperature too low 2. Local stretch ratio too high 3. Moisture/oil in compressed air | 1. Increase preform temperature in that area 2. Check preform design (wall thickness distribution) 3. Enhance air filters (dryers, coalescing filters) |
| 10. Poor Vertical Load Capacity (Top Load) | 1. Uneven wall thickness 2. Preform reheating temperature too high (weakens material) 3. Poor bottle design | 1. Improve wall thickness uniformity 2. Use lower temperature and higher pressure during blow molding 3. Improve bottle base/shoulder design |
| 11. Bottle Easily Dropped/Breaks | 1. Base wall thickness not properly stretched (thick, brittle) 2. Material degradation (reduced strength) | 1. Improve base stretch ratio (design/process) 2. Enhance raw material drying before injection 3. Reduce processing temperatures (injection & blowing) |
| 12. Internal Specks/White Haze | 1. Contaminated compressed air 2. High air humidity | 1. Install/maintain air filtration system 2. Reduce humidity of blow air (dryers) |
| 13. Bottle Too Soft | 1. Design issue (insufficient material) 2. Processing temperature too high (affects properties) | 1. Improve bottle/preform design (wall thickness) 2. Use lowest possible processing temperatures (injection & blowing) |
| 14. Burnt Smell After Blowing | 1. Reheat temperature too high 2. Contaminated compressed air 3. Use of inferior raw materials | 1. Reduce reheat temperature 2. Maintain compressor and filters 3. Use certified, food-grade materials |
| 15. Water Develops Off-Taste | 1. Excessive Acetaldehyde (AA) content in bottle 2. Use of inappropriate/regrind material | 1. Use lower temperature injection and blow molding 2. Check AA content of raw material; avoid inferior/off-spec materials |
