Top PET Blow Molding Problems and How to Solve Them (Part 1)

Producing high-quality Pet Bottles isn’t just about the right materials—it also depends on mastering the blow molding process. Even with the best equipment, manufacturers often encounter recurring issues that affect product clarity, durability, and consistency. In this guide, we highlight some of the most common problems in PET bottle blowing, along with proven, easy-to-implement solutions that can streamline your production and reduce costly defects.

1. Poor Bottle Transparency
Possible Causes:
Excessive heating temperature or overly long heating time
Moisture present in compressed air
Low transparency of Preform itself
Improper preform design
Insufficient blow-up ratio

Solutions:
Reduce heating temperature and shorten heating duration
Use an air dryer to remove moisture
Improve preform material quality and ensure proper drying
Adjust preform dimensions and structure
Reduce preform diameter to increase blow ratio

2. Uneven Bottle Wall Thickness
Possible Causes:
Stretch rod misaligned with preform center
Asymmetric or uneven air blow holes
Low stretch ratio or blow-up ratio
Preform not rotating in oven
Uneven wall thickness of preform or inconsistent injection density

Solutions:
Re-align stretch rod and air holes
Increase stretch and blow-up ratios
Check rotation mechanism of oven
Improve preform quality and consistency

3. Thin Bottle Bottom
Possible Causes:
Blow starts too early
Excessive bottom temperature
Preform bottom too thin

Solutions:
Delay initial blow timing
Lower heating at the base
Increase bottom wall thickness of preform

4. Visible Mold Line
Possible Causes:
Inadequate clamping pressure
Premature sealing
Mold-preform misfit, especially at the neck

Solutions:
Increase clamping force and adjust clamping rod angle (ideally <5°)
Adjust clamping switch position
Check and repair mold fitting—especially guide post and tightness
Inspect and repair neck thread matching

Optimize Your PET Bottle Production
Addressing these frequent blow molding issues early on can lead to noticeable improvements in both bottle quality and production efficiency. If you're looking for precision-engineered, high-performance molds to elevate your output, we’re here to help. Our team is always available to provide expert advice and reliable support tailored to your production needs.