Common Technical Issues with Fully Automatic Wide-Mouth Blow Molding Machines

The blow molding process involves biaxial stretching, during which PET chains undergo bidirectional extension, orientation, and alignment. This enhances the mechanical properties of the bottle wall, improving tensile, tensile-impact, and impact strength while ensuring excellent gas barrier performance. Although stretching helps increase strength, over-stretching must be avoided by controlling the blow-up ratio: the radial ratio should not exceed 3.5–4.2, and the axial ratio should not exceed 2.8–3.1. The wall thickness of the preform should not exceed 4.5mm.

Fully automatic wide-mouth blow molding machines operate between the glass transition temperature and the crystallization temperature, typically controlled within the range of 90–120°C. Within this range, PET exhibits a highly elastic state, allowing for rapid blow molding. After cooling and solidification, transparent bottles are formed. In one-step molding processes, this temperature is determined by the cooling time during injection molding. Therefore, the coordination between the injection and blowing stages is critical for fully automatic blow molding machines.

The blow molding process consists of three rapid sequential actions: stretching, primary blowing, and secondary blowing. Proper synchronization is essential, as the first two steps significantly influence material distribution and overall bottle quality. Key parameters to adjust include the timing of stretching initiation, stretching speed, timing of pre-blowing initiation and termination, pre-blowing air pressure, and airflow. If possible, controlling the overall temperature distribution of the preform and the temperature gradient between its inner and outer walls is also recommended.

Common issues and solutions related to induced stress during rapid blow molding and cooling in fully automatic wide-mouth blow molding machines:

1. Thick top, thin bottom: Delay pre-blowing timing or reduce pre-blowing pressure and airflow.

2. Thick bottom, thin top: Apply the opposite adjustments of the above.

3. Wrinkles below the bottle neck: Pre-blowing is too late, pre-blowing pressure is too low, or inadequate cooling in this area.

4. Whitening at the bottom: Preform is too cold, over-stretching, pre-blowing too early, or pre-blowing pressure too high.

5. Magnifying glass effect at the bottom: Excess material at the bottom, pre-blowing too late, or pre-blowing pressure too low.

6. Wrinkles inside the bottom: Bottom temperature too high, poor cooling near the gate, pre-blowing too late, or pre-blowing pressure and airflow too low.

7. Overall bottle cloudiness or opacity: Insufficient cooling.

8. Local whitening: Over-stretching, localized low temperature, pre-blowing too early, or contact with the stretching rod.

9. Eccentric bottle bottom: Possibly related to preform temperature, stretching, pre-blowing, or high-pressure blowing. Solutions include lowering preform temperature, increasing stretching speed, checking the gap between the stretching rod tip and the bottom mold, delaying pre-blowing, reducing pre-blowing pressure, delaying high-pressure blowing, and verifying preform concentricity.