Impact of Suction Air Box Structure on Nonwoven Fiber Web Packing Density in Nonwoven Production Line

The structure of the suction air box significantly affects the web density and hydrostatic pressure performance of meltblown nonwoven fabrics. In nonwoven production line, a narrower air inlet leads to higher airflow velocity through the web and conveyor belt, resulting in denser fiber deposition, improved web uniformity, and higher hydrostatic pressure, but requires higher pressure and energy consumption. In contrast, a wider inlet reduces airflow velocity, producing a fluffier web with lower hydrostatic pressure and lower energy demand. Currently, the widely adopted single-step forming process is mature and reliable, and differences in product quality are mainly attributed to the design of the suction air box rather than the forming method itself.

1. Airflow at the Inlet of the Suction Air Box

In nonwoven production systems, the performance of the suction fan must match the width of the inlet of the meltblown system’s suction air box, as it greatly affects product quality. Some meltblown systems are only equipped with a single suction air box and lack auxiliary suction zones. If the fan’s air volume is too high while the pressure is too low, the process adaptability will be poor. The spinning process can be significantly disrupted by ambient airflow, resulting in poor product uniformity and low hydrostatic pressure.

When the inlet is relatively narrow, less ambient air is drawn in, and the airflow entering the suction box is faster. The fiber web accumulates in a smaller and more concentrated area, resulting in higher resistance, and the fan must have higher pressure. In contrast, when the inlet is wider, the airflow speed is slower. Although the fan pressure can be lower, it must support a larger flow rate due to more ambient air being drawn in. Typically, In most nonwoven production lines, the inlet width of a suction air box ranges from 115 to 325 mm, with the maximum width being about three times the minimum.

The energy of a fluid equals the sum of kinetic energy, potential energy, and pressure energy—all of which can be converted into one another. The airflow speed penetrating the nonwoven web and the forming belt, as well as the fiber movement speed before reaching the belt surface, are key factors in determining the packing density of the web. Figure 5-35 shows a schematic diagram of meltblown web forming airflow.

The faster the air and fiber speed, the more kinetic energy is converted into potential energy as the airflow slows due to resistance. When the fiber stops moving upon reaching the belt surface (i.e., relative speed becomes zero), all of its kinetic energy converts to potential energy. Combined with the negative pressure suction from the fan, this results in reduced web thickness, increased density, and smaller average pore size.

Currently, in SMS nonwoven production lines manufactured in China, core equipment such as spinning boxes, spinnerets, and drawing fans used in meltblown systems are mostly sourced from a few common manufacturers with similar technical performance. Differences in product quality mainly arise from variations in equipment configurations and suction air box structures, which in turn lead to differences in the characteristics of the configured fans. However, this has no direct relation to the web forming method of the spinning system.

The widely adopted single-pass web forming process is a mature and optimized technique developed through extensive production practice. It features simple equipment, straightforward process principles, a short production flow, high reliability, and minimal interference with fiber laying—making it the mainstream technology for meltblown-spun nonwoven fabric web formation.

Other web forming methods artificially break a seamless process into multiple stages, complicating what could be simple. Claims that suction airflow disrupts the web structure are far-fetched. In fact, such multi-stage web forming only increases interference with the process and does not contribute positively to web quality, especially since even the basic complementary effect of web formation cannot be utilized.

2. Suction Air Box Design Affects Nonwoven Product Hydrostatic Pressure

In nonwoven production line, under the same melt extrusion output, the forming airflow that the web former must handle is roughly the same. The inlet width of the suction air box determines the cross-sectional area for airflow to pass through the web and forming belt, thereby influencing airflow speed during penetration.

If the suction air box inlet (B) is narrow (≤150 mm), the cross-sectional area is small, and the airflow speed through the web and belt is high. This results in more kinetic energy being converted into pressure energy, increasing web density. The maximum hydrostatic pressure can reach 1.6 to 2.2 times the basis weight. Of course, this also depends on the proportion of the meltblown web layer and requires a suction fan with higher pressure (10–14 kPa). Consequently, fan power and energy consumption are also higher. Figure 5-36 illustrates different suction air box structures in web formers.

If the suction air box inlet (B) is wide (>200 mm), the cross-sectional area is larger, and the airflow speed through the web and belt is lower. Less kinetic energy is converted into pressure energy, resulting in lower web density and a fluffier product. The maximum hydrostatic pressure is approximately 1.2 to 1.6 times the basis weight. The required fan pressure is lower (<6.5 kPa), and under the same flow rate, the motor power needed for the fan is smaller, resulting in lower energy consumption. Many China-made machines and RF equipment use this type of suction box.

Meltblown systems from American company APCO (Nordson) and Germany’s Neumag, as well as some systems produced in China, feature relatively narrow suction inlets. As a result, for SMS products of equal basis weight and structure, the maximum hydrostatic pressure can be about 1.3 times higher than that of machines with wider inlets.

Ultimately, performance differences in nonwoven production line primarily result from variations in suction air box structure rather than differences in web forming methods. Notably, the latest mainstream foreign models, which use spinnerets with higher hole density and finer spunbond fibers, can achieve hydrostatic pressures of 100 cmH₂O for 45 g/m² SMMS products with a 23% meltblown content—while maintaining a production capacity of 236 kg/(m·h).

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