Complementary Effect in the Web Forming Process
In the SMS spunbond nonwoven production line, the web forming method is classified as air-laid web forming, which relies on suction fans to absorb the fiber-drawing airflow and deposit the fibers from the spinning system onto the conveyor belt (web former), thus forming the nonwoven web. The uniformity of the airflow determines the uniformity of the resulting fiber web.
For nonwoven production lines using closed spinning channels, the relevant airflow primarily includes the cooling and drawing airflow and a portion of infiltrated ambient air, which is then exhausted through the suction fan. For open or semi-open spinning systems, the web-forming airflow consists of cooling air, drawing air, and ambient air.
Due to the random nature of air-laid web forming, variations in web uniformity are inevitable, regardless of the specific web forming method used — the only differences lie in the degree of variation and controllability.
On a multi-spinning system nonwoven production line, when an upstream spinning system has already deposited fibers onto the web and a specific area contains fewer fibers (resulting in a sparse or defective region), this area, due to its lower resistance, experiences greater suction than surrounding areas. As a result, fibers from neighboring regions are drawn by the airflow into this sparse area. When the downstream spinning system begins to lay its fibers, more fibers are deposited in the thinner region, effectively compensating for the earlier defect. Conversely, regions that already contain more fibers have greater resistance and weaker suction, so fewer new fibers are deposited there.
This airflow-driven “leveling and balancing” effect is known as the complementary effect.
The driving force behind this complementary effect is the suction airflow. In the one-step web forming process, where all fibers from multiple spinning systems are deposited on the same web former, the complementary effect significantly improves the uniformity of the final product. The greater the number of spinning systems, the higher the probability and intensity of this complementary effect, leading to more pronounced results. The alternating thick-and-thin regions between webs naturally complement each other, noticeably enhancing product uniformity.
Where local suction is stronger, more newly formed fibers are pulled into the sparse region from neighboring areas, thus compensating for the defect. Meanwhile, areas already rich in fibers experience less suction, resulting in fewer additional fibers, and avoiding excessive layering or “over-thickening.” This mechanism drives the overall fiber web across the full width toward a more even and consistent structure.
The complementary effect is a critical physical phenomenon in the air-laid web forming process. However, two prerequisites must be met to harness this effect:
- The fibers from all related spinning systems must be deposited onto the same web former’s conveyor (i.e., single web-former or one-step web forming process).
- All newly formed fibers must be able to migrate and move randomly with the airflow.
Two-step Web Forming Process (Also Called “Double Web-former”)
In this method, multiple spinning systems are positioned on two separate web formers (Web Former #1 and #2). Each system completes fiber deposition independently, and the webs are then merged and bonded via thermal calendaring. While this setup simplifies control of the suction airflow, it weakens the natural complementary effect that occurs when all fibers are deposited on the same web.
As a result, nonwoven fabric product uniformity, hydrostatic pressure performance, and production stability may decline. Additionally, equipment complexity and maintenance costs tend to increase.
Conclusion
By comparing the fiber web quality formed by five beam spinning systems under different web forming methods, it is evident that the single web-former (one-step web forming) process makes better use of the complementary effect, resulting in a more uniform web. In contrast, the double web-former (two-step web forming) process utilizes the complementary effect to a lesser extent, and the resulting fiber web tends to be less consistent.
Currently, the widely adopted single-web forming process has been continuously optimized through extensive production practice. It is a mature web forming method characterized by simple equipment, straightforward process principles, short process flow, high reliability, and minimal interference with the fiber laying process. It is the mainstream technology for web formation in spunmelt spinning nonwoven fabric production.