Stretch Ratio of PET Bottle Preforms

The stretch ratio is one of the most critical parameters in the PET bottle blowing process.
It directly affects the mechanical strength, transparency, wall thickness uniformity, and appearance of the final bottle.

1. Definition

During the PET stretch blow molding process, the preform is heated and stretched in two directions:

• Axial (longitudinal) — along the length of the preform

• Radial (transverse) — around its circumference

Thus, the stretch ratio is described in three parts:

1. Axial Stretch Ratio
   = Effective bottle height (excluding neck and bottom) ÷ Effective preform length

2. Radial Stretch Ratio
   = Bottle diameter ÷ Preform diameter

3. Total Stretch Ratio
   = Axial Stretch Ratio × (Radial Stretch Ratio)²
   (since the radial direction affects area expansion)

2. Calculation Example

Assume:

• Preform inner diameter = 20 mm

• Bottle outer diameter = 80 mm

• Preform effective length = 50 mm

• Bottle effective height = 200 mm

Then:

• Axial Stretch Ratio = 200 ÷ 50 = 4

• Radial Stretch Ratio = 80 ÷ 20 = 4

• Total Stretch Ratio = 4 × 4² = 64

3. Typical Reference Ranges (for PET Bottles)

⚠️ Too low stretch ratio → thick walls, poor clarity
Too high stretch ratio → over-thinned material, internal stress, whitening or deformation

4. Practical Notes

• The optimal total stretch ratio is usually between 10 and 16.

• A good axial-to-radial ratio balance (e.g., 3:2 or 3:2.5) produces the most uniform wall thickness.

• Actual ratios vary depending on bottle capacity, design, and material distribution.