Blow-Up Ratio (BUR) Guide: Calculation & Film Quality Impact
In the plastic film extrusion industry, specifically for operators running a film blowing machine, the Blow-Up Ratio (BUR) is one of the most critical process parameters. It determines not just the width of your film, but its physical strength, clarity, and thickness uniformity.
Yet, many beginners struggle to find the "sweet spot." Set the BUR too low, and your film may lack transverse strength; set it too high, and you risk bubble instability. This technical guide explains exactly how BUR affects film quality and provides the formulas you need to optimize production.
1. What Is Blow-Up Ratio (BUR)?
Technically, the Blow-Up Ratio is the ratio between the diameter of the final inflated bubble and the diameter of the die exit. It indicates how much the plastic has been stretched in the transverse (width) direction.
The Practical Formula
Since it is difficult to measure the diameter of a moving bubble, operators typically measure the Layflat Width (LFW)—the width of the film when flattened.
BUR = (0.637 × Layflat Width) / Die Diameter
Example Calculation:
If you are producing a film with a Layflat Width of 600 mm using a 100 mm Die:
BUR = (0.637 × 600) / 100
BUR = 3.82 : 1
2. How BUR Affects Mechanical Properties (MD vs. TD)
This is the most important concept for quality control. Plastic film has two directions of strength:
- MD (Machine Direction): The direction the film is pulled upwards.
- TD (Transverse Direction): The direction the film is expanded outwards.
The Rule of Thumb: Increasing the BUR increases molecular orientation in the Transverse Direction (TD). This makes the film stronger across its width (harder to tear sideways) but may slightly reduce its strength in the Machine Direction (MD). Balancing BUR is essentially balancing MD and TD properties.
3. Common Quality Issues Caused by Incorrect BUR
Low BUR (Below 1.5:1)
- Splittiness: The film has almost no transverse strength. It tears easily in straight vertical lines (like string cheese).
- Thickness Variation: It is harder to distribute thickness variations evenly around a small bubble circumference.
Excessively High BUR (Above 4:1 or 5:1)
- Bubble Instability: The bubble becomes wobbly and difficult to cool, leading to wrinkles and gauge bands.
- Lower Production Speed: A massive bubble takes longer to cool, potentially forcing you to slow down the line speed.
- Port-Lines: Excessive stretching can amplify die lines or imperfections.
4. Recommended BUR Ranges by Material
Different polymers require different stretching ratios to achieve optimal molecular alignment. Below are industry-standard ranges:
| Material Type | Optimal BUR Range | Why? |
|---|---|---|
| LDPE / LLDPE | 2.0:1 – 3.0:1 | Provides a good balance of toughness and optical clarity. |
| HDPE | 3.5:1 – 5.0:1 | HDPE requires a high "stalk" bubble shape and high BUR to achieve impact strength. |
| Shrink Film | 3.0:1 – 4.0:1 | Needs high TD orientation to ensure it shrinks tightly around the product width-wise. |
| Barrier Films (PA/EVOH) | 2.5:1 – 3.5:1 | Moderate ratio prevents delamination of complex multi-layer structures. |
Conclusion
Mastering the Blow-Up Ratio is the first step in moving from a novice operator to a film extrusion expert. By maintaining the correct BUR for your specific material (LDPE vs HDPE), you ensure that your film has balanced strength, uniform thickness, and perfect sealing properties.
If you are experiencing consistent quality issues that adjustment cannot fix, it might be time to evaluate your hardware. Check out our range of high-performance Film Blowing Machines engineered for stable bubble control and precise gauge uniformity.
