The blow molding process involves biaxial stretching. During this stage, PET polymer chains orient and align in two directions.

While stretching is crucial for strength, it must be carefully controlled. Key ratios are:

• Radial Blow-up Ratio: Optimal range: 3.5–4.2

• Axial Stretch Ratio: Optimal range: 2.8–3.1
  Preform wall thickness should generally not exceed 4.5mm.

Blowing occurs between the PET’s glass transition temperature (Tg) and its crystallization temperature, typically controlled within 90–120°C. Within this range, PET exhibits a rubbery state, allowing it to be rapidly inflated and then cooled into a transparent bottle. In single-stage systems (like the Aoki machine), this temperature is determined by the cooling time during preform injection molding, requiring precise coordination between injection and blowing stages.

The core blowing sequence involves three rapid, synchronized actions:

1. Stretching

2. Pre-blow (Initial Blow)

3. Final Blow

The first two steps are critical for achieving optimal material distribution and overall bottle quality. Precise adjustment of the following parameters is essential:

• Stretch rod start timing

• Stretch rod speed

• Pre-blow start/end timing

• Pre-blow air pressure

• Pre-blow air volume
  Where possible, controlling the overall preform temperature profile and the temperature gradient between its inner and outer walls further enhances results.

Rapid blowing and cooling induce internal stresses within the bottle wall.  However, for hot-fill bottles, stress relaxation must occur above the glass transition temperature (Tg) before filling. Sokos modern automatic plastic blowing machine provide sophisticated controls to manage these critical process variables.