In blown film extrusion, one of the most critical parameters influencing the final film properties is the blow-up ratio (BUR) — the ratio of the film’s final bubble diameter to the die diameter. This factor directly affects the film’s thickness uniformity, molecular structure, mechanical performance, and optical quality. Understanding and controlling the blow-up ratio is essential for achieving consistent, high-quality films in packaging and industrial applications.
The blow-up ratio has a significant impact on film thickness distribution. A higher BUR stretches the molten film more in the transverse direction, generally producing thinner films. However, excessive expansion can lead to uneven thickness or film instability if cooling or bubble control is inadequate. Maintaining an appropriate BUR ensures better uniformity across the film width and reduces gauge variation, which is crucial for packaging applications.
As the film is stretched both vertically and horizontally during the blowing process, the molecular chains orient in these directions. A higher BUR increases orientation in the transverse direction (TD), while the draw ratio mainly affects the machine direction (MD). The balance between MD and TD orientation determines the film’s mechanical anisotropy — too high or too low orientation in one direction can lead to poor balance and reduced product quality.
The molecular orientation caused by the blow-up ratio directly influences the tensile strength, tear resistance, and puncture durability of the film. A moderate BUR enhances biaxial orientation, resulting in improved mechanical strength and toughness. However, if the BUR is too high, overstretching may weaken the film and increase the risk of pinholes or breakage during winding.
Optical clarity, gloss, and haze are also affected by the blow-up ratio. A well-optimized BUR promotes uniform cooling and stretching, reducing crystal size and improving film transparency. Conversely, improper ratios can cause uneven cooling or molecular orientation, leading to haze, streaks, or optical distortion. Films used in packaging and printing require optimal optical performance to ensure visual appeal and product visibility.
The blow-up ratio influences the overall film performance balance, including shrinkage, barrier properties, and heat-sealing behavior. A properly chosen BUR contributes to a stable process, consistent film roll quality, and efficient production. It also allows processors to tailor film properties for specific applications such as food packaging, lamination, or agricultural films.
To achieve optimal results, operators must carefully select the blow-up ratio based on resin type, die design, cooling conditions, and product requirements. Common BUR values typically range from 1.5:1 to 4:1, depending on whether a film needs higher strength or better optical clarity. Fine-tuning the BUR in conjunction with frost line height, die temperature, and take-up speed ensures the best combination of film properties and production stability.
A blown film machine is specialized equipment used to produce plastic films by extruding molten polymer through a circular die, inflating it into a thin bubble, and cooling it into a continuous film. The process involves extrusion, inflation (blow-up), cooling, collapsing, and winding. These machines are widely used for manufacturing films from materials like LDPE, LLDPE, HDPE, and biodegradable resins.
Modern blown film machines, such as those from advanced manufacturers like RUIKANG, feature precision temperature control, automatic thickness regulation, and energy-efficient cooling systems. These innovations allow operators to maintain a consistent blow-up ratio and produce films with excellent strength, clarity, and consistency for various industrial and packaging applications.
The blow-up ratio plays a decisive role in defining the mechanical, optical, and structural quality of blown films. By mastering this parameter and leveraging modern blown film technology, manufacturers can achieve superior product performance and high production efficiency.