Common Blow Molding Defects and How to Fix Them

Introduction

In blow molding production, even the most advanced machines can produce defects if the process isn’t optimized. These defects don’t

just affect product appearance — they can also compromise strength, cause leakage, and increase scrap rates. At Link-Tech Machinery,

we’ve helped manufacturers worldwide troubleshoot and solve blow molding issues quickly. In this article, I’ll share the most common

defects in blow molding, their causes, and how to fix them.

1. Uneven Wall Thickness

Symptoms: Some areas of the bottle are too thick while others are too thin.
Causes:

Incorrect preform heating profile.

Poor preform rotation during heating.

Improper stretching speed or ratio.

Solutions:

Adjust heating zones for even temperature distribution.

Check and fix preform rotation mechanisms.

Fine-tune stretch rod speed for balanced stretching.

Link-Tech Advantage: Our intelligent heating systems automatically adjust lamp output for uniform wall thickness.

2. Haze or Poor Clarity

Symptoms: Cloudy or dull bottle surface.
Causes:

Moisture in PET before processing.

Uneven heating or overheating.

Poor-quality raw material or contaminated rPET.

Solutions:

Dry preforms to the recommended moisture level.

Adjust lamp temperature and distance.

Use higher-grade PET or improve rPET cleaning.

3. Blowouts or Holes

Symptoms: Small holes or ruptures in bottle walls.
Causes:

Excessive blow pressure.

Thin wall areas from incorrect stretching.

Material defects in the preform.

Solutions:

Reduce blow pressure to recommended levels.

Ensure even heating and stretching.

Inspect preform quality before loading.

4. Short Shots (Incomplete Bottles)

Symptoms: Bottle doesn’t fully form to mold shape.
Causes:

Insufficient blow pressure or volume.

Leaks in air system.

Poor mold sealing.

Solutions:

Increase blow pressure or volume.

Check and repair leaks in air lines.

Inspect and clean mold sealing surfaces.

5. Sink Marks or Surface Dents

Symptoms: Depressions or uneven surfaces after cooling.
Causes:

Uneven cooling rates.

Excessive wall thickness in certain areas.

Mold cooling channel blockage.

Solutions:

Balance cooling by adjusting water flow.

Redesign preform for more uniform thickness.

Clean and unblock mold cooling channels.

6. Flashing (Excess Material at Seams)

Symptoms: Thin excess material along mold parting line.
Causes:

Worn or misaligned mold.

Excessive blow pressure.

Solutions:

Repair or realign mold.

Adjust pressure to optimal levels.

7. Poor Neck Finish

Symptoms: Threads or sealing surfaces out of spec.
Causes:

Mold wear in neck area.

Preform neck not centered during blowing.

Solutions:

Replace or repair neck insert.

Ensure proper mold clamping and alignment.

Conclusion

Defects in blow molding are common but preventable. The key is quick identification, accurate diagnosis, and effective process adjustments. By understanding the root causes, you can reduce waste, improve product quality, and keep customers satisfied.

At Link-Tech Machinery, we provide more than just machines — we offer technical training, troubleshooting support, and process

optimization to help you run defect-free production. If you’re facing quality issues, contact us today for expert assistance.