Blow-Up Ratio (BUR) in Blown Film Extrusion: A Complete Guide

In Blown Film Extruder Production Line, the Blow-Up Ratio (BUR) is one of the most critical process parameters. It directly affects film width, mechanical strength balance, appearance, and production stability. Understanding how to calculate and properly adjust the blow-up ratio is essential for achieving stable production and consistent film quality.

What Is Blow-Up Ratio (BUR)?

The Blow-Up Ratio (BUR) refers to the ratio between the bubble diameter and the die diameter in a blown film extrusion process.

Formula:

Blow-Up Ratio (BUR) = Bubble Diameter ÷ Die Diameter
The blow-up ratio indicates how much the molten film is expanded in the transverse direction during extrusion.

How to Calculate Blow-Up Ratio

Method 1: Using Bubble Diameter
If the bubble diameter can be measured directly:
BUR = Bubble Diameter ÷ Die Diameter
Example:
Die diameter: 100 mm
Bubble diameter: 250 mm
BUR = 2.5

Method 2: Using Layflat Film Width
In most factories, bubble diameter is difficult to measure. Instead, the layflat film width is commonly used.

Bubble Diameter = (Layflat Width × 2) ÷ π
Then calculate BUR using the standard formula.
Example:
Layflat width: 800 mm
Die diameter: 150 mm
Bubble diameter ≈ 510 mm
BUR ≈ 3.4

Recommended Blow-Up Ratio for Different Materials

LDPE (Low Density Polyethylene): 1.5 – 3.0
HDPE (High Density Polyethylene): 2.0 – 4.0
LLDPE (Linear Low Density Polyethylene): 2.5 – 4.0
Co-extruded Film: 2.5 – 4.5

How Blow-Up Ratio Affects Film Properties

Mechanical Strength Balance

A higher blow-up ratio increases transverse direction (TD) strength but reduces machine direction (MD) strength.
A lower blow-up ratio increases MD strength but reduces TD strength.

Film Thickness Uniformity

Excessively high blow-up ratio may cause bubble instability and uneven thickness.
Too low blow-up ratio may result in thicker center and thinner edges.

Optical Properties

Higher blow-up ratio can reduce transparency due to increased molecular orientation.
Moderate blow-up ratio improves film clarity and surface appearance.

Bubble Stability

Very high blow-up ratio increases the risk of bubble flutter and breakage.
Proper blow-up ratio improves bubble stability and continuous production.

Downstream Processing Performance

Incorrect blow-up ratio can cause poor heat sealing, unstable cutting, and bag-making difficulties.

Common Blow-Up Ratio Problems and Solutions

Bubble instability: Blow-up ratio too high – Reduce blow-up ratio or adjust cooling air.
Film breaks easily: Poor MD/TD balance – Optimize blow-up ratio and haul-off speed.
Uneven thickness: Incorrect blow-up ratio – Adjust blow-up ratio and air ring.

Practical Tips for Setting Blow-Up Ratio

Increase the blow-up ratio gradually and observe bubble behavior.
Adjust blow-up ratio together with haul-off speed and cooling air volume.
Allow sufficient stabilization time after each adjustment.
Reset blow-up ratio when changing raw materials.

Conclusion

The blow-up ratio is not just a number. It directly controls film width, strength balance, appearance, and production stability. A well-optimized blow-up ratio helps reduce material waste, improve film quality, and ensure smooth downstream processing.
Stable blown film production always starts with the correct blow-up ratio.