Blown Film Blow-Up Ratio Explained!

If you've ever watched a blown film machine running, you've probably noticed the giant plastic bubble rising above the die head. But have you ever wondered why some bubbles are wider while others stay narrow?

The answer lies in one of the most important parameters in blown film extrusion: Blow-Up Ratio (BUR).

This article explains what BUR is, how to calculate it, and why it matters.

What Is Blow-Up Ratio (BUR)?

BUR is the ratio between the diameter of the inflated film bubble and the diameter of the extrusion die.

Formula: BUR = Bubble Diameter ÷ Die Diameter.
Example: Die 150 mm, Bubble 300 mm, BUR = 2:1.

Why Is Blow-Up Ratio Important?

BUR influences molecular orientation, film strength, thickness uniformity, sealing performance, impact resistance, and overall production stability.

What Happens If BUR Is Too Low?

Low BUR can reduce transverse strength, narrow film width, lower tear resistance, and create unbalanced mechanical properties.

What Happens If BUR Is Too High?

High BUR may cause bubble instability, thickness variation, wrinkles, winding difficulty, and increased risk of bubble collapse.

How to Choose the Right BUR

Consider material (LDPE, LLDPE, HDPE), film application, film thickness, and machine configuration, such as die head, air ring, tower height, IBC, and thickness control.

Typical BUR Ranges

General packaging: 2.0–2.5
HDPE film: 3.0–4.0
Heavy-duty film: 2.0–3.0
High-performance film: depends on formulation.

Tips for Stable BUR Control

Maintain stable bubble diameter, frost line, air pressure, cooling, and film thickness.

Conclusion

Understanding BUR helps improve film quality, reduce waste, increase production stability, and optimize blown film machine performance.

Related Reading

• How Does a Blown Film Extrusion Machine Work?

• Understanding Frost Line Height in Blown Film

• Blown Film Bubble Stability: Common Issues and Fixes

• Guide to Blown Film Die Design and Selection