Common PET Bottle Defects in Blow Moulding and How to Fix Them

For packaged drinking water and beverage producers who make their own bottles, the PET blow moulding machine is a critical part of the line. Bottles are produced by heating preforms and stretching them with air pressure inside a mould. When everything is set correctly, the results are clear, strong and consistent. When settings drift, defects appear, causing rejects, leaks, filling problems and unhappy customers.

How Stretch Blow Moulding Works

A PET preform, which looks like a thick test tube with the finished neck already formed, is heated in an oven to a temperature where it becomes soft and stretchable. The heated preform enters the mould, where a stretch rod pushes it down while pre-blow and high-pressure blow air expand it to fill the mould. The bottle cools against the mould and is ejected.

Good bottles depend on the right balance between preform quality, heating profile, stretch timing and blow pressure.

Common Defects and Their Causes

Pearlescence or White Haze

A milky, pearl-like appearance often occurs when the preform is stretched while too cold. The material is over-stretched, creating micro-voids that scatter light. Increasing heat in the affected zone usually helps.

Crystallinity Haze

Cloudy or whitish areas can also result from overheating, which causes PET to crystallise. This often appears near the neck or base. Reducing heat in the affected zone, or checking preform conditioning, may resolve it.

Thin Walls or Uneven Thickness

Uneven wall thickness weakens bottles and can cause collapse or bulging. Causes include uneven heating, incorrect stretch rod timing or poor preform quality. Adjusting the heating profile zone by zone is the usual remedy.

Off-Centre Gate

The injection point at the bottom of the bottle should be centred. An off-centre gate suggests stretch rod problems, uneven heating around the preform circumference or preform defects.

Incomplete Blowing

If the bottle does not fully take the shape of the mould, details at the base or shoulder may be poorly formed. Causes include insufficient blow pressure, air leaks or the preform being too cold.

Base Problems

A weak or poorly formed base can cause bottles to wobble or crack under pressure. Base cooling, heating near the gate and stretch rod settings all affect base quality.

Neck Deformation

The neck is formed during preform injection and should not be heated. Inadequate neck shielding or cooling can deform it, causing capping problems and leaks.

Bubbles, Black Specks and Contamination

These usually originate in the preform itself. Working with reliable preform suppliers and inspecting incoming lots is the best defence.

The Role of Preform Quality

Many bottle problems start with preforms. Moisture, inconsistent weight, contamination or poor storage can cause defects no machine adjustment can fix. Preforms should be stored in clean, dry conditions and used on a first-in, first-out basis. When changing preform suppliers, re-optimise machine settings.

Systematic Troubleshooting

• Change one variable at a time and observe the result
• Record settings that produce good bottles as a reference
• Check environmental conditions, since temperature and humidity affect heating
• Inspect moulds for damage and cleanliness
• Verify air pressure and air quality from the compressor

Testing Bottle Quality

Simple routine tests help catch problems early:

• Visual inspection for clarity and defects
• Wall thickness measurement at key points
• Top load testing to assess strength
• Leak testing after capping
• Volume and weight checks

Preventive Maintenance

Regular maintenance of heating lamps, reflectors, stretch rods, valves and moulds keeps the machine consistent. Burnt-out heater lamps are a common cause of sudden quality changes.

Conclusion

PET blow moulding combines materials science and machine control. Most defects have recognisable causes, and a systematic approach to troubleshooting, combined with good preform quality and regular maintenance, keeps bottles clear, strong and consistent.

Need help with your water or wastewater project? Aguapuro Equipments designs, supplies and services water treatment, effluent treatment and bottling solutions. Send us an enquiry or contact our team to discuss your requirements.