In the manufacturing industry, PET blow moulding machines play a crucial role in producing various plastic containers, from beverage bottles to cosmetic packaging. As a PET blow moulding machine supplier, I have witnessed firsthand the common faults that can occur with these machines. Understanding these issues is essential for ensuring smooth production processes and minimizing downtime. In this blog post, I will discuss some of the most common faults of a PET blow moulding machine and provide insights on how to address them.
1. Heating System Malfunctions
One of the most critical components of a PET blow moulding machine is the heating system. This system is responsible for heating the preforms to the appropriate temperature for blowing. If the heating system malfunctions, it can lead to uneven heating, which results in defective products.
- Uneven Heating: Uneven heating can occur due to several reasons, such as worn-out heating elements, incorrect temperature settings, or blocked air vents. When preforms are not heated evenly, some parts of the bottle may be too thin, while others may be too thick. This can affect the strength and appearance of the final product. To address this issue, it is important to regularly inspect the heating elements and replace them if necessary. Additionally, ensure that the temperature settings are accurate and that the air vents are clear of debris.
- Overheating or Underheating: Overheating can cause the preforms to become too soft, leading to sagging or deformation during the blowing process. On the other hand, underheating can result in preforms that are too rigid, making it difficult to blow them into the desired shape. To prevent overheating or underheating, it is crucial to monitor the temperature closely and adjust the settings as needed. Installing temperature sensors and controllers can help maintain a consistent temperature throughout the heating process.
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Case 1: Uneven Heating Causing Thin Bottle Walls in a 2L Carbonated Soft Drink Line
Background
A medium‑sized beverage manufacturer in Southeast Asia was running a 4‑cavity PET blow moulding machine producing 2L carbonated soft drink bottles. The machine had been in continuous operation for 14 months.
Fault Observed
Bottles showed uneven wall thickness – the shoulder area was consistently too thin (0.18 mm vs. standard 0.28 mm), while the base was excessively thick.
Approximately 8–10% of bottles burst during pressure testing (carbonation simulation at 4 bar).
No error codes were shown on the control panel.
Diagnosis Process
1. Checked temperature settings: all zones set to 95°C as per recipe – appeared normal.
2. Used a handheld infrared thermometer to measure preform surface temperature in the heating oven.
Zone 2 (left side): 97°C
Zone 2 (right side): 82°C (significant imbalance)
3. Opened the oven and found that two infrared heating lamps on the right side had dimmed due to over 6,000 hours of use.
4. Also discovered dust accumulation on the quartz lamp reflectors, further reducing radiant efficiency on one side.
Solution
Replaced all 12 heating lamps in the oven (not just the faulty two) to ensure uniform aging.
Cleaned reflectors with isopropyl alcohol and recalibrated lamp positions.
Implemented a monthly oven inspection checklist including lamp brightness check and reflector cleaning.
Result
Wall thickness variation reduced from ±0.06 mm to ±0.01 mm.
Burst rate dropped below 0.5%.
The customer now replaces heating lamps every 5,000 running hours as preventive maintenance.
2. Air Leakage
Air is used to blow the preforms into the desired shape in a PET blow moulding machine. Any air leakage in the system can significantly affect the quality of the final product.
- Leakage in the Air Supply System: Leaks in the air supply system can occur at various points, such as the air compressor, air hoses, or fittings. These leaks can reduce the air pressure, resulting in incomplete or uneven blowing of the preforms. To detect air leaks, you can use a soap solution to check for bubbles at the connections. Once a leak is identified, tighten the fittings or replace the damaged parts.
- Leakage in the Mould: Air leakage in the mould can also cause problems. If the mould is not properly sealed, air can escape during the blowing process, leading to defective products. Regularly inspect the mould for any signs of damage or wear and tear. Replace any damaged parts and ensure that the mould is properly aligned and tightened.
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Case 2: Air Leakage in Mould Base Causing Incomplete Blowing of 500ml Cosmetic Bottles
Background
A personal care packaging company in Eastern Europe was running a semi‑automatic PET blow moulding machine for 500ml oval‑shaped lotion bottles. The mould had been refurbished twice over three years.
Fault Observed
The bottom corner of each bottle (right side) showed incomplete expansion – material did not fully contact the mould wall, leaving a soft, wrinkled area.
Rejected rate increased from 1.5% to 11% within one week.
The operator reported that the blowing sound seemed "weaker" than normal, but machine pressure gauge read 8 bar (within specification).
Diagnosis Process
1. Performed a soap solution test on the entire air path from the high‑pressure manifold to the mould.
2. Bubbles appeared around the quick‑disconnect fitting at the mould base bottom plate.
3. Removed the fitting – found a worn O‑ring (hardened and partially split).
Further inspection revealed a hairline crack in the aluminium mould bottom plate near the air entry channel, caused by repeated thermal cycling and over‑tightening of mould clamps.
This crack only leaked when internal pressure exceeded 5 bar, which is why the static pressure test passed.
Solution
Replaced the O‑ring and the damaged mould bottom plate (kept the rest of the mould).
Added a weekly leak‑down test procedure: pressurise mould cavity to 10 bar, isolate air supply, and verify pressure drop <0.2 bar over 10 seconds.
Retrained operators on correct mould clamping torque to avoid over‑tightening.
Result
Incomplete blowing defects eliminated completely.
Rejection rate returned to 1.2% (mainly other minor aesthetic issues).
The customer now stocks spare mould sealing kits and performs a full air system leak check every 200 production hours.
3. Moulding Defects
The mould is a key component of a PET blow moulding machine, and any defects in the mould can directly impact the quality of the final product.
- Flash or Burrs: Flash or burrs are excess plastic that forms around the edges of the bottle. This can occur due to improper mould design, worn-out mould parts, or incorrect clamping pressure. To reduce flash or burrs, ensure that the mould is well-designed and maintained. Adjust the clamping pressure to the appropriate level and replace any worn-out parts.
- Warping or Deformation: Warping or deformation of the bottle can occur if the cooling process is not properly managed. When the bottle cools too quickly or unevenly, it can cause stress and result in warping. To prevent this, ensure that the cooling system is working effectively and that the cooling time is appropriate for the size and shape of the bottle.
4. Mechanical Failures
The mechanical components of a PET blow moulding machine are subject to wear and tear over time, which can lead to various mechanical failures.
- Motor and Gearbox Issues: The motor and gearbox are responsible for driving the various components of the machine. If the motor or gearbox malfunctions, it can cause the machine to stop working or operate inefficiently. Regularly lubricate the motor and gearbox and check for any signs of damage or excessive wear. Replace any worn-out parts as soon as possible.
- Belt and Chain Problems: Belts and chains are used to transfer power between different parts of the machine. If they become loose, worn, or damaged, it can affect the performance of the machine. Check the tension of the belts and chains regularly and replace them if necessary.
5. Electrical Problems
Electrical problems can also cause issues with a PET blow moulding machine.
- Short Circuits: Short circuits can occur due to damaged wiring, loose connections, or electrical component failures. A short circuit can cause the machine to shut down or even pose a safety hazard. To prevent short circuits, regularly inspect the electrical system for any signs of damage or wear. Ensure that all connections are tight and that the wiring is in good condition.
- Control Panel Malfunctions: The control panel is used to operate and monitor the machine. If the control panel malfunctions, it can lead to incorrect settings or the inability to control the machine properly. Check the control panel regularly for any error messages or malfunctions. If necessary, consult the machine's manual or contact a professional technician for assistance.
Addressing Common Faults
To minimize the occurrence of common faults in a PET blow moulding machine, it is important to implement a regular maintenance schedule. This includes cleaning, lubricating, and inspecting the machine on a regular basis. Additionally, provide proper training to the operators to ensure that they are using the machine correctly and following the recommended procedures.
If you encounter any problems with your PET blow moulding machine, it is important to address them promptly. Contact the manufacturer or a professional technician for assistance. They can provide you with the necessary support and guidance to resolve the issue.
FAQ
Q 1: How can I distinguish between uneven heating and insufficient blowing pressure by visually inspecting the defective bottle?
A 1: It is a common diagnostic challenge. Here is the practical rule used by field technicians:
| Defect Pattern | Likely Cause | What to Check First |
|---|---|---|
| Thick-and-thin wall pattern that repeats around the bottle circumference (e.g., consistently thin on one side, thick on the opposite side) | Uneven heating – the preform is softer on one side | Infrared temperature scan across the oven width. A difference >5°C between left and right is problematic. |
| Overall bloated or incomplete shape with no clear thick/thin pattern – the entire bottle looks "weak" or "short shot" | Insufficient or leaking air – lack of internal pressure during blowing | Soap solution test on all air fittings and mould sealing surfaces during pressurization. |
| Random thin spots that move from bottle to bottle | Preform quality issue (uneven material distribution from injection moulding) | Check preforms lot number and run a different batch. |
Pro tip: Keep a "defect wall chart" next to your machine. Photograph bad bottles and label them with the diagnosed cause. After 3 months, your operators will diagnose 80% of faults within 30 seconds.
Q 2: My temperature controllers show the correct setpoint, but bottles are still overheating. What else could be wrong?
A 2: This is more common than you might think. The displayed temperature is the sensor reading, not necessarily the actual preform surface temperature. Here is what to check in order:
Sensor placement – A thermocouple that has been pulled back 10 mm from its original position can read 15°C lower than actual preform temperature. Compare against a handheld infrared thermometer.
Lamp aging – Infrared lamps lose spectral output as they age. After 5,000–6,000 hours, they may still emit light but at the wrong wavelength for PET absorption. The controller sees correct oven air temperature, but the preform does not absorb energy efficiently. Result: You increase the setpoint to compensate, leading to surface overheating while the core remains cold.
Recirculated air – If the oven exhaust filter is clogged, hot air builds up inside the oven chamber. The thermocouple reads this air temperature correctly, but the preform sees additional convective heating on top of infrared. Symptoms: Preform surface becomes sticky or hazy, while the controller says temperature is normal.
Field case: A customer in Vietnam spent two weeks replacing thermocouples and controllers. The actual problem was a partially blocked oven exhaust duct that trapped hot air. Cleaning the duct restored normal operation in 20 minutes.
Action checklist when displayed temperature does not match bottle quality:
Measure the preform surface temperature with handheld IR gun (target ±2°C across all zones)
Check oven exhaust filter – clean or replace if dirty
Verify lamp hours – replace all lamps in the oven if >6,000 hours
Inspect sensor mounting depth – ensure tip is 5–8 mm from the preforms path.
Q 3: Why do I sometimes see a "thick ring" or "whitening band" around the middle of the bottle, even when temperature and air pressure are within spec?
A: This defect – often called a "stretch mark" or "hinge line" – is caused by stretch rod speed being too slow relative to the blowing speed.
The mechanism explained:
PET needs to stretch longitudinally (longer) and radially (wider) at roughly the same rate.
If the stretch rod moves too slowly, the air pressure expands the bottle sideways before the rod has pulled the preform to its full length.
The plastic at the mid‑height becomes "trapped" – it cannot stretch further and forms a thick ring.
The whitening appears because local stretching exceeds the natural draw ratio of PET (micro‑voiding).
How to fix (in order of likelihood):
| Adjustment | Effect | Typical value range |
|---|---|---|
| Increase stretch rod speed (pneumatic flow control) | Rod reaches bottom faster, allowing longitudinal stretch before radial expansion | 0.4–0.8 m/s depending on bottle height |
| Decrease pre‑blow pressure | Reduces initial radial expansion, giving the rod more time | 6–10 bar (not the main blow pressure of 30–40 bar) |
| Increase pre‑blow delay | Same principle – delays radial expansion | 50–150 ms from start of blowing |
Pro tip: If the defect appears only on the first bottle after a pause (e.g., Monday morning startup), check the stretch rod temperature. A cold rod (below 40°C) sticks to PET and slows down. Install a rod heater or run 10 sacrificial bottles to warm it up.
Conclusion
As a PET blow moulding machine supplier, I understand the importance of ensuring the reliable operation of these machines. By being aware of the common faults and taking proactive measures to address them, you can minimize downtime and improve the quality of your products. If you are in the market for a PET blow moulding machine, we offer a wide range of high-quality machines, including PP Bottle Blowing Machine, Semi Automatic Blow Molding Machine, and Plastic Blow Moulding Machine. If you have any questions or would like to discuss your specific requirements, please feel free to contact us. We look forward to working with you to meet your manufacturing needs.
References
- "PET Blow Moulding Machine Operation and Maintenance Manual"
- Industry research reports on plastic blow moulding technology