Blown film is a widely used packaging material, and its quality directly determines the performance of end products—from food wrapping to agricultural mulch. At the heart of blown film production lies the Film Blowing Machine, a system that melts polymer resins, extrudes them into a tube, and inflates the tube to form thin film. A critical parameter governing this process is the Blow-Up Ratio (BUR), which dictates how the film’s dimensions, strength, and appearance evolve. Below is a detailed analysis of BUR, its impacts on film quality, and practical considerations for production.

                                   Film Blowing Machine

 

1. What is the Blow-Up Ratio (BUR)?​

The Blow-Up Ratio (BUR) is a dimensionless parameter that describes the degree of inflation applied to the extruded polymer tube during blown film processing. It is calculated as the ratio of the final diameter of the inflated film bubble to the diameter of the extruder die opening (the small circular gap where the molten resin exits the machine).​

Mathematically, BUR = (Bubble Diameter) / (Die Diameter).​

For example, if a Film Blowing Machine uses a die with a 50mm diameter and the inflated bubble reaches 150mm, the BUR is 3:1. BUR typically ranges from 1.5:1 (for thick, rigid films) to 6:1 (for thin, flexible films), though specialized applications may use higher ratios. This parameter is not arbitrary—it directly controls how polymer molecules orient in the film, which in turn shapes the film’s physical and optical traits.

 

2. Impact on Mechanical Properties​

Mechanical properties (e.g., tensile strength, impact resistance, tear strength) are critical for film durability, as they determine how well the film withstands stretching, puncturing, or tearing during use. BUR exerts a profound influence on these properties by altering molecular orientation—the alignment of polymer chains in the film.​

Tensile Strength: Tensile strength measures the film’s ability to resist breaking under tension. At low BUR (e.g., 1.5:1 to 2:1), polymer chains are primarily oriented along the machine direction (MD, the direction the film is pulled during winding). This leads to high MD tensile strength but low cross-machine direction (CD, perpendicular to MD) strength. As BUR increases, the inflated bubble stretches the film radially, forcing chains to orient more in the CD. A balanced BUR (e.g., 3:1 to 4:1) creates uniform MD/CD tensile strength, ideal for films that need to resist stretching in all directions (e.g., grocery bags).​

Impact Resistance: Impact resistance (ability to withstand sudden shocks) improves with moderate BUR. Low BUR films are brittle in the CD, while excessively high BUR (above 5:1) can cause chain overstretching, leading to reduced impact strength—especially in low-temperature environments.​

Tear Strength: Tear strength follows a similar trend to impact resistance. Moderate BUR (3:1 to 4:1) optimizes CD tear strength, as radial stretching aligns chains to resist tearing across the film. High BUR films, however, may develop weak points due to uneven stretching, lowering tear resistance.

 

3. Impact on Optical Properties​

Optical properties (transparency, gloss, haze) are vital for films used in applications where appearance matters (e.g., food packaging, retail wraps). BUR affects these properties by modifying the film’s crystalline structure and surface smoothness.​

Transparency: Transparency depends on the uniformity of the film’s structure—fewer crystalline regions or defects mean more light passes through. At low BUR, slow radial stretching allows polymer chains to form large, irregular crystal structures, which scatter light and reduce transparency. As BUR increases, rapid radial stretching limits crystal growth, producing smaller, more uniform crystals. This enhances transparency—for example, polyethylene (PE) films with a BUR of 4:1 are significantly clearer than those with a BUR of 2:1.​

Gloss: Gloss measures how well the film reflects light. Higher BUR improves gloss by smoothing the film’s surface. The inflated bubble, when stretched evenly, creates a more consistent surface texture, reducing light scattering and boosting reflectivity. Conversely, low BUR films often have a matte finish due to uneven surface tension.​

Haze: Haze (cloudiness) is the opposite of transparency. Low BUR films have higher haze (15–20%) due to large crystals, while high BUR films (3:1 or higher) have haze levels below 10%—critical for applications like clear food packaging, where product visibility is key.

 

4. Impact on Film Thickness​

Film thickness uniformity is non-negotiable—thickness variations cause weak spots, inconsistent barrier properties (e.g., for moisture or oxygen), and poor printability. BUR directly influences thickness control:​

Thickness Calculation: The film’s final thickness is inversely related to BUR. For example, if a Film Blowing Machine extrudes a molten tube with a 0.5mm wall thickness (from a die) and uses a BUR of 3:1, the final film thickness is approximately 0.5mm / 3 = 0.167mm (assuming no stretching in the machine direction).​

Uniformity Challenges: Low BUR (below 2:1) is easier to control, as the bubble is stable and less prone to distortion. High BUR (above 4:1) increases bubble instability—small variations in air pressure or resin temperature can cause the bubble to wobble, leading to uneven thickness (e.g., thinner spots on one side of the film). To mitigate this, modern Film Blowing Machines use automatic bubble control systems (e.g., infrared sensors) to adjust air flow and maintain BUR consistency.

 

5. Practical Implications​

Selecting the right BUR is not a one-size-fits-all decision—it depends on the polymer type, film application, and production goals:​

Polymer Compatibility: Different resins respond differently to BUR. For example, low-density polyethylene (LDPE) is flexible and can handle high BUR (up to 6:1) without cracking, making it ideal for thin films. In contrast, high-density polyethylene (HDPE) is stiffer and requires lower BUR (2:1 to 3:1) to avoid brittleness.​

Application-Specific BUR:​

Agricultural mulch films: Require high CD tensile strength to resist wind, so BUR is often set to 3:1 to 4:1.​

Food packaging films: Need high transparency and barrier properties, so BUR is typically 4:1 to 5:1 (to minimize crystal size and ensure uniformity).​

Heavy-duty bags: Demand high MD tensile strength, so BUR is kept low (1.5:1 to 2:1).​

Production Efficiency: High BUR can increase output—since a larger bubble produces more film per minute—but it requires more precise control (e.g., stable air pressure, consistent resin viscosity). If the Film Blowing Machine lacks advanced control features, high BUR may lead to more waste due to thickness variations.

 

Conclusion​

The Blow-Up Ratio (BUR) is a cornerstone parameter in blown film production, shaping every critical aspect of film quality—from mechanical strength to optical clarity and thickness uniformity. By understanding how BUR interacts with polymer behavior and the capabilities of the Film Blowing Machine, producers can tailor BUR to their specific applications: low BUR for rigid, high-strength films; moderate BUR for balanced performance; and high BUR for thin, transparent films. Ultimately, precise BUR control is the key to producing consistent, high-quality blown film that meets the demands of modern packaging and industrial uses.

 

FAQ

Q1: Can I use the same BUR for all polymer types?​

A: No. Stiffer resins (e.g., HDPE, polypropylene) require lower BUR to avoid chain breakage, while flexible resins (e.g., LDPE, ethylene-vinyl acetate) can tolerate higher BUR. Always adjust BUR based on the resin’s melt flow rate (MFR) and flexibility.​

 

Q2: How does BUR interact with other Film Blowing Machine parameters?​

A: BUR works in tandem with traction speed (the speed at which the film is pulled and wound). Increasing traction speed stretches the film in the MD, reducing thickness—so if you increase BUR (to boost CD orientation), you may need to lower traction speed to maintain the desired thickness.​

 

Q3: What happens if BUR is too high?​

A: Excessively high BUR causes: (1) uneven thickness (due to bubble instability), (2) reduced impact/tear strength (from overstretched chains), and (3) increased film shrinkage (as oriented chains relax over time).