How Blow-Up Ratio Affects Film Quality in Blown Film Machines | Film Extrusion Guide


What Effect Does the Blow-Up Ratio Have on Film Quality?

In the plastic film extrusion industry, especially for those operating a film blowing machine, understanding the blow-up ratio (BUR) is critical to ensuring high-quality film. Yet many beginners struggle with the right settings and often end up with films that are too stretched, wrinkled, or difficult to seal. This article explores how the blow-up ratio affects film quality and provides guidance on achieving optimal results.

What Is Blow-Up Ratio?

The blow-up ratio (BUR) refers to the ratio of the final film width (after inflation) to the diameter of the die. It’s calculated as:

Blow-Up Ratio (BUR) = Film Width / Die Diameter

For example, if your die is 200 mm and your final film width is 600 mm, your BUR is 3:1.

How Blow-Up Ratio Affects Film Quality in Blown Film Machines

Why Is Blow-Up Ratio Important?

BUR directly influences key film properties such as:

  • Film width
  • Thickness uniformity
  • Mechanical strength
  • Sealing performance
  • Shrink properties (for shrink films)

Incorrect BUR settings can lead to poor film formation and downstream issues during bag making or packaging.

Common Problems When Blow-Up Ratio Is Incorrect

1. Excessive Longitudinal or Transverse Tension

Novice operators often find that their films are too tight either vertically or horizontally. A very high BUR can cause thin, overly stretched films with weak transverse strength, while a very low BUR can result in overly thick films with weak orientation.

2. Poor Sealing in Bags

If the film is not properly balanced due to improper BUR, the heat-sealing process may fail. Bags may not seal completely or may open during handling.

3. Wrinkling or Uneven Film

Inconsistent tension and stretching from an incorrect BUR can lead to wrinkling, warping, or film bubbles collapsing.

4. Wasted Material

When the film quality is suboptimal, it can result in frequent rework or wasted material, increasing costs and reducing productivity.

Recommended Blow-Up Ratios

Film Type Recommended BUR Range
LDPE General Purpose 2:1 to 3:1
HDPE Thin Film 3:1 to 4.5:1
Shrink Film 4:1 to 5:1
Barrier Films 2.5:1 to 3.5:1

The ideal ratio depends on your application, film thickness, resin type, and end-use requirements.

Tips for Beginners Using a Film Blowing Machine

1. Know Your Die Size

Before determining your desired film width, know the diameter of your die. This is essential for calculating the correct BUR.

2. Start With a Moderate Ratio

If you’re unsure, start with a BUR of 2.5:1 or 3:1. Monitor the film tension and quality as you adjust.

3. Adjust Air Pressure Gradually

Don't overinflate the film bubble. Make small changes and observe the effect.

4. Monitor Film Tension

Ensure both longitudinal and transverse tension are balanced. Use a lay-flat width gauge to verify consistency.

5. Consult Resin Manufacturer Guidelines

Different resins behave differently. Check with suppliers for recommended processing parameters.

Conclusion

The blow-up ratio is a crucial parameter that directly affects the width, strength, appearance, and sealability of your blown film. Especially for beginners, understanding and properly setting the BUR can eliminate many common film defects and reduce material waste. By mastering this one key metric, you’re well on your way to producing high-quality plastic films.