The Ultimate Automatic Stretch Blow Molding Machine Troubleshooting Guide: Diagnosis, Fixes, and Maintenance

If you run a packaging line, you know that a stopped machine is burning money. Every minute your automatic blow molding machine sits idle, your cost per unit goes up and your lead times get longer.

We know that established OEM packaging factories focus heavily on Total Cost of Ownership (TCO) and output per hour. You need your machines running at peak efficiency to meet the demands of big CPG brands.

This guide focuses on the Stretch Blow Molding (SBM) process—heating a preform, stretching it with a rod, and blowing it into a mold. This is the technology behind our AQUA Series (for high-speed beverages) and BOTTLER Series (for cosmetic and wide-mouth jars).

I have broken this guide down by machine systems—mechanical, electrical, and process. Use it to isolate the problem quickly.

Brand Note: While this guide covers many standard issues, remember that Leka Machine’s support team is always ready to assist. We offer remote engineer repair guidance and have engineers available for assembly and troubleshooting.

Safety First: Essential Checks Before Troubleshooting

Before you grab a wrench or open a cabinet, we need to talk about safety.

• Lockout/Tagout (LOTO) Procedures: Never bypass this. Ensure the main power is cut and locked before entering the machine guard.
• High-Pressure Air Safety: SBM machines use high-pressure air (often around 3.0-4.0 Mpa) for the final blow. Always bleed the residual pressure from the blowing station before removing hoses or valves.
• Temperature Hazards: The heating oven and hot preforms can cause serious burns. Wear proper heat-resistant PPE when handling lamps or clearing jammed preforms.
• Emergency Stop Buttons: Before restarting after a fix, physically test the E-Stop to ensure it cuts power immediately.

Phase 1: Machine Startup & Power Supply Issues

Sometimes the problem happens before you even make a bottle.

• “Machine Won’t Start”: If the start button does nothing, check the basics first. Is the main breaker on? Is an Emergency Stop button depressed? Also, check the safety door interlocks. If a sensor thinks a door is open, the PLC will prevent the cycle from starting.
• Air Pressure Stability: Your machine needs consistent air. If the low-pressure air (for mechanical actions) drops, cylinders will lag. Check your compressor and receiver tank.
• PLC Screen Blackout: If your HMI (Human Machine Interface) is dark, check the 24V DC power supply unit inside the electrical cabinet. A blown fuse here is a common, easy fix.
• Sensor Misalignment: Proximity sensors can vibrate loose. If the machine waits indefinitely for a “Mold Open” signal, check if the sensor has shifted away from its target.

Phase 2: Preform Feeding & Loading System Glitches

If you can’t get preforms into the oven, you can’t make bottles.

• Hopper Jams: This is often caused by “bridging,” where preforms interlock across the hopper throat. Ensure your hopper vibration settings match the preform weight.
• Unscrambler Orientation Errors: If preforms arrive upside down, your unscrambler rails or air blowers are likely misadjusted.
• Star Wheel Syncing: The transfer from the loading rail to the heating chain must be seamless. If preforms are getting crushed or dropped, you likely need to adjust the timing of the infeed star wheel.
• Preform Damage: Scratches on preforms turn into ugly scars on bottles. Check your guide rails for burrs and ensure the loading speed isn’t too aggressive.

Phase 3: Heating System & Temperature Control Mastery

In stretch blow molding, heat is everything. This is especially true for the BOTTLER Series, which handles high-value, complex shapes.

• Uneven Heating Profile: You need a pyrometer to read the temperature of the preform body. If you find “cold spots,” you won’t get a uniform stretch.
• Infrared Lamp Failures: Visually inspect your oven. A burnt-out bulb creates a cold section on the preform. Also, check the reflectors behind the lamps; if they are dirty, they won’t reflect heat efficiently.
• Oven Ventilation: If the air in the oven gets too hot, the surface of the preform will crystallize (turn white and brittle). Ensure the oven tunnel fans are working to keep ambient heat down while the infrared penetrates the plastic.
• Voltage Fluctuations: Unstable plant voltage can cause heater output to waver, leading to inconsistent bottle wall thickness.

Phase 4: The Stretching Process – Rods, Speed, and Timing

The stretch rod defines the vertical distribution of the material. Our machines often utilize servo motors for this precise movement.

• Stretch Rod Alignment: If the “gate” (the injection point on the bottom of the bottle) is off-center, your rod is likely misaligned. This creates a weak bottom.
• Rod Speed Settings:
  • Too Fast: You might puncture the bottom of the preform before blowing.
  • Too Slow: The material cools down before it hits the bottom, resulting in a thick, heavy base.
• Bent or Worn Rods: Inspect the tips of the rods. A bent rod will always push the material off-center.
• Calibration: Sometimes the servo motor loses its “Zero Position.” You may need to recalibrate the home position of the stretch unit via the HMI.

Phase 5: Defect Detective – Common Bottle Quality Issues & Fixes

Here are the PET bottle defects solutions we see most often:

Pearlescence (Stress Whitening)

Symptom: The bottle looks white or chalky on the inside.
Diagnosis: The material was too cold when it was stretched.
Fix: Increase the overall preform temperature or reduce the pre-blow pressure.

Haze (Heat Whitening)

Symptom: The bottle looks cloudy or milky on the outside.
Diagnosis: The material was too hot.
Fix: Lower the oven temperature or increase the ventilation in the heating tunnel.

Rocker Bottoms (Unstable Base)

Symptom: The bottle wobbles when placed on a flat table.
Diagnosis: The bottom is still hot and bulging outward after leaving the mold.
Fix: Check that mold cooling channels aren’t blocked. You may also need to increase the cooling time or check the exhaust valve timing.

Drop Impact Failure

Symptom: Bottles crack easily when dropped.
Diagnosis: Poor material distribution (IV drop) or crystallized gates.
Fix: Optimize the material distribution by adjusting lamp zones. Ensure the gate isn’t overheating.

Phase 6: Mold & Clamping Unit Maintenance

Whether you are using a smaller FORMA machine or a high-speed AQUA model, the mold must close perfectly.

• Mold Closing Issues: If the mold moves sluggishly, check for hydraulic pressure drops or wear in the toggle linkage.
• Parting Line Gaps: Do you see a thick vertical line (flash) on the bottle? This means the blowing pressure is overcoming the clamping force. You may need to increase the clamping pressure (measured in KN, e.g., 165KN to 300KN depending on the model).
• Cooling Channel Blockages: Scale buildup acts as an insulator. If the mold runs hot, the bottles will deform. Flush your channels regularly.
• Mold Alignment: The left and right halves must meet perfectly. If they are shifted, you will have a “stepped” parting line that ruins the aesthetic.

Phase 7: Pneumatic & Hydraulic System Diagnosis

• Air Leaks: Walk the machine with an ultrasonic leak detector. A hissing valve is leaking profit.
• Solenoid Valve Sticking: If a valve fails to open or close rapidly, the timing of the blowing process will be off. These often just need cleaning.
• Oil Contamination: Check your hydraulic oil. If it looks milky, water has gotten in. If it smells burnt, it has overheated. Dirty oil will destroy your pump and seals.
• Cylinder Seals: If a cylinder “drifts” (moves slowly when it should be holding still), the internal seals are likely shot.

Phase 8: PLC Alarms & Error Code Interpretation

Modern machines like our AQUA F22 rely on complex electronics.

• Common Error Codes: Keep a cheat sheet for codes like “Servo Overload” or “Heater Fault” near the screen.
• Input/Output (I/O) Monitoring: Use the diagnostic screen on the PLC. This allows you to see the real-time status of every sensor. It’s the fastest way to find a broken switch.
• HMI Touchscreen Issues: If the screen freezes or loses calibration, a restart usually fixes it. If not, the screen hardware may be failing.

Phase 9: Routine Maintenance to Prevent Future Failures

The best troubleshooting is the kind you never have to do.

• Daily Checklist: Grease the lubrication points. Drain water from the air filters. Visually inspect the mold for plastic residue.
• Weekly/Monthly Tasks: Check the tension on the drive belts. Clean the infrared heater lamps with alcohol to ensure maximum heat transfer. Inspect grippers for wear.
• Spare Parts Strategy: Keep an essential kit on hand: O-rings, proximity sensors, heater lamps, and solenoid valves. Waiting for parts from overseas causes unnecessary downtime.

Conclusion: Consistent Quality Through Proactive Care

Troubleshooting is a systematic process: Diagnose -> Isolate -> Fix -> Test. By following this guide, you can reduce the time it takes to get your line back up and running.

Training your operators to recognize these signs early is your best defense against downtime. Keep a maintenance log—history often repeats itself.

Next Step: If you are facing a complex issue or looking to upgrade to a more reliable Automatic Stretch Blow Molding Machine, would you like me to connect you with our technical engineering team for a consultation?

Contact Leka Machine Support