Abstract:

Objective Melt blowing is a technology that uses high-speed airflow to draw molten fibers for preparing ultrafine fiber nonwoven materials. The performance of melt-blown nonwovens is closely related to fiber diameter, and reducing fiber diameter can directly improve material performance. As the core component of melt-blowing equipment, the geometric structure of the die has a decisive impact on fiber attenuation. This study focuses on the influence of melt-blown die air-slot width on the fiber formation process, combining numerical simulation and experimental methods to analyze the distribution characteristics of airflow fields under different air-slot widths and reveal their impact mechanisms on fiber drawing behavior.

Method The k-ω SST model and Detached Eddy Simulation (DES) model were used to numerically simulate the characteristics of steady-state and unsteady melt-blown airflow fields, respectively. An electronic anemometer was employed to measure the airflow velocity beneath the die. A single-orifice melt-blown spinneret was used for fiber preparation, and an Acuteye-1M-2000CXP high-speed camera was utilized to capture the dynamic trajectories of melt-blown fibers. Fiber whipping motion and fiber diameter were measured by importing high-speed photography images into Image J software. Finally, the porosity of melt-blown fiber materials prepared by dies with different air-slot widths was characterized through oil absorption tests.

Results It was found that as the air-slot width increased, the position where the airflow reaches the maximum velocity moved away from the die. When the air-slot width was 0.5 mm, the airflow velocity reached its maximum at z =2.5 mm, and when the air-slot width increased to 1.5 mm, the maximum velocity was located at z = 6 mm. Consequently, under a large air-slot width, the melt flowing out of the spinneret nozzles could not be effectively drawn by the high-speed airflow, resulting in melt swelling. In contrast, under a small air-slot width, the melt extruded from the spinneret nozzles was able to quickly drawn by the high-speed airflow, thereby avoiding melt extrusion swelling and obtaining finer fibers. Numerical simulation results showed that when the air-slot widths are 0.5 mm, 1.0 mm, and 1.5 mm, the airflow underwent turbulent transition at z = 5 mm, 8 mm, and 17 mm, respectively. This indicates that as the air-slot width decreased, the airflow transitioned to turbulence earlier, promoting the early activation of the fiber whipping and stretching mechanism. The whipping and stretching mechanism was conducive to fiber drawing but causes fibers to enter the turbulent region earlier, leading to intensified nonlinear fluctuations in whipping and stretching and thus deteriorating the uniformity of fiber fineness (at z = 3.97 mm, the CV value of fiber diameter increases from 8.8% to 28.8%). In addition, both simulation and experimental results evidenced that the larger the air-slot width the longer the continuous distance of the maximum airflow velocity. When the air-slot width was 0.5 mm, there was no obvious continuous distance for the maximum airflow velocity; when the air-slot width was 1 mm, the maximum airflow velocity was able to be maintain within a range of 19 mm. And when the air-slot width was 1.5 mm, the stable plateau section of the airflow velocity became even longer, lasting for 29 mm. Oil absorption experiments showed that the oil absorption rates of dies with air-slot widths of 0.5 mm, 1.0 mm, and 1.5 mm were stabilized at 30, 26, and 23 times, respectively. The reason is that as the air-slot width increases, the pressure effect of the airflow on the fiber assembly increases, making the melt-blown fiber material more compact and reducing its porosity.

Conclusion This study investigates the influence of air-slot width in melt-blown dies on fiber attenuation through numerical simulation and experimental validation. By analyzing the airflow field, fiber motion, fiber diameter, and characteristics of fiber assemblies, it is verified that a smaller air-slot width enhances airflow-induced fiber drawing and effectively suppresses melt extrusion swelling. Narrower air-slots promote the whipping motion of fibers, resulting in reduced fiber diameter but deteriorating fiber uniformity. In contrast, wider air-slots prolong the interaction distance between airflow and fibers, leading to denser fiber assembly structures with reduced porosity.

Key words: melt blowing, air-slot width, air turbulence, fiber attenuation, attenuation mechanism, nonwoven material