• By admin
  • 2026/5/7
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Common Troubleshooting Solutions for PET Blow Molding Machines

PET blow molding machines are essential equipment for producing plastic bottles and containers across the beverage, food, cosmetic, and pharmaceutical industries . However, like any complex machinery, they can encounter technical issues that affect output quality and production efficiency. Identifying and resolving these problems quickly is crucial for minimizing downtime and maintaining consistent product quality. This troubleshooting guide from YUSHUN covers the most common PET blow molding machine problems and their practical solutions.


1. Uneven Bottle Wall Thickness

Symptoms

Bottles show inconsistent material distribution, with some areas thinner than others. This affects bottle strength and appearance .

Causes

  • Off-center stretch rod position

  • Asymmetrical or mismatched blowing holes

  • Low stretching or expansion ratios

  • Preform not rotating properly in the heating furnace

  • Uneven preform wall thickness or injection density 

Solutions

  • Adjust the stretch rod position to ensure it is centered in the preform

  • Check and adjust blowing hole positions and diameters

  • Increase the inflation or stretching ratio

  • Verify the preform rotation mechanism in the heating oven

  • Use higher quality PET preforms with consistent wall thickness 

2. Poor Bottle Transparency (Haze and Pearlescence)

Symptoms

Bottles appear cloudy, milky, or have a pearl-like sheen instead of being crystal clear. This significantly impacts product appeal and marketability .

Causes

  • Thermostat temperature too high or too low

  • Compressed air contains moisture

  • Excessive heating time

  • Uneven preform wall thickness

  • Poor quality or insufficiently dried PET preforms

  • Crystallization due to slow cooling rates 

Solutions

  • Maintain precise control over heating temperatures to prevent overheating or underheating 

  • Use a dryer to remove moisture from compressed air 

  • Decrease heating time and optimize the heating profile

  • Use higher quality, transparent PET preforms and ensure raw material dryness 

  • Implement a controlled cooling process to avoid crystallization 

  • Increase the inflation rate if insufficient 

3. Neck Finish Deformation (Bent Neck or Swollen Threads)

Symptoms

The neck is tilted or bent to one side, or threads show swelling. The support ring below the thread is deformed and not perpendicular to the bottle body. This defect seriously affects filling and capping operations .

Causes

  • Excessive cooling water temperature in the heater

  • Inadequate cooling water flow

  • Improper cooling plate positioning

  • Excessive temperature in the heating zone near the neck

  • Mechanical issues with preform handling 

Solutions

  • Check and adjust cooling water temperature and flow rate

  • Ensure proper cooling plate distance (about 1.5 mm from preform front/back; 2 mm above the preform support ring plane) 

  • Reduce heating percentage in Zone 1

  • Improve ventilation in the heater area

  • Ensure 0.25 mm clearance between the support ring and mold top groove 

  • Align the blow nozzle with the mold 

4. Incomplete Bottom Formation (Concave Foot)

Symptoms

Poor bottom contour, especially concave bottom corners. While this does not significantly affect filling, it impacts bottle stability .

Causes

  • Insufficient high-pressure air supply

  • Low temperature in the bottom corner area

  • High-pressure blow time too short or pre-blow time too long

  • Pre-blow air intake occurring too late

  • Blocked vent holes in the bottom mold 

Solutions

  • Ensure high-pressure air supply reaches at least 28 BAR 

  • Check the high-pressure filter on the mold side for blockages

  • Clear vent holes in the bottom mold

  • Increase heating percentage or add lamps in the bottom corner area

  • Verify pre-blow and high-blow control component settings 

5. Bottle Bottom Cracking or Bursting

Symptoms

Fine cracks appear in the bottom area, often passing through or near the injection point. In severe cases, bottles may burst during or after filling .

Causes

  • Insufficient preform bottom temperature

  • Low IV (intrinsic viscosity) value causing expansion after filling

  • Bottom too thin due to improper stretching

  • Foreign material or excessive alkali concentration in chain lubricant 

Solutions

  • Check clearance between stretch rod and mold bottom (should be about 2/3 of preform bottom thickness) 

  • Increase heating percentage or add lamps in the bottom zone

  • Optimize blow timing to ensure proper material distribution

  • Monitor and maintain proper IV values for the PET resin

6. Inconsistent Heating and Preform Temperature

Symptoms

Bottles show localized defects such as shoulder collapse, warping, or uneven material distribution .

Causes

  • Localized overheating or underheating

  • Improper lamp power distribution

  • Incorrect preform rotation

  • Blocked or misaligned heating elements 

Solutions

  • Adjust lamp power and placement to ensure even heating, especially at shoulders and bottom of the preform

  • If whitening or blue tint appears, reduce shoulder heating and extend preheat time 

  • For bottom sink or navel effect, lower bottom lamp intensity or increase base cooling 

  • Consider installing infrared sensors for real-time temperature feedback 

7. Off-Center Nozzle at Bottle Bottom

Symptoms

The injection point or gate is not centered in the bottle base, affecting stability and appearance .

Causes

  • Premature blowing before stretch rod fully descends

  • Incomplete stretch rod descent

  • Misaligned stretch rod

  • Uneven preform injection density or wall thickness

  • Inconsistent heating 

Solutions

  • Delay blowing time or increase stretch rod descent speed 

  • Adjust magnetic pole switch position

  • Realign the stretch rod

  • Use higher quality PET preforms

  • Check preform rotation and improve heating conditions 

8. Blowing Pressure and Timing Issues

Symptoms

Poor bottom expansion, wrinkled shoulders, bottle distortion, or incomplete bottle formation .

Causes

  • Low air pressure preventing full expansion

  • Incorrect pre-blow timing

  • Faulty pre-blow pressure settings 

Solutions

  • Set blow pressure between 20 and 40 bar depending on bottle volume 

  • Slightly delay pre-blow to allow shoulder formation before full expansion 

  • Fine-tune pre-blow pressure to achieve balanced stretching across shoulder and base

  • Use pressure sensors or flow meters to monitor response in real time 

9. Cooling System Problems

Symptoms

Bottle shrinkage, neck deformation, low transparency, or inconsistent quality between mold cavities .

Causes

  • High cooling water temperature

  • Poor cooling flow or scale buildup in channels

  • Low water pressure or trapped air 

Solutions

  • Maintain chiller outlet temperature at 7 to 12 degrees Celsius 

  • Monitor flow with flow meters

  • Clean filters and check mold channels regularly to ensure unobstructed flow

  • Use thermal imaging tools to detect uneven mold heat zones 

10. Stretch Rod Malfunctions

Symptoms

Stretch rod fails to descend properly or does not return to home position .

Causes

  • Incorrect magnetic pole switch position

  • Excessive stretching rod speed

  • Worn cylinder seals

  • Air leakage in the pneumatic system 

Solutions

  • Adjust magnetic pole switch position 

  • Reduce stretching rod speed

  • Clean solenoid valves and replace worn seals or broken coils

  • Ensure smooth exhaust from the stretching cylinder 

Preventive Maintenance Tips

To minimize downtime and maintain consistent bottle quality:

  • Regularly calibrate heating equipment to ensure optimal temperature settings 

  • Clean cooling system filters and check mold channels regularly 

  • Inspect and replace worn components such as seals, guides, and bearings

  • Monitor compressed air quality to ensure it is dry and contaminant-free 

  • Establish a preventive maintenance schedule based on production volume

  • Train operators to recognize early warning signs of common issues

When to Contact YUSHUN for Support

While many troubleshooting issues can be resolved in-house using this guide, some situations require professional assistance. Contact YUSHUN technical support when:

  • Problems persist after applying standard solutions

  • Complex mechanical or electrical failures occur

  • You need assistance with mold alignment or machine calibration

  • Spare parts or replacement components are needed

YUSHUN provides comprehensive technical support, including installation guidance, operator training, and ongoing service to keep your blow molding line running efficiently.