How Does The Paper Machine Forming Section Forming The Paper Sheet? Part Ⅰ

The Mechanism of Paper Sheet Formation on Paper Machine:

The forming process of paper sheet on a paper machine involves the dewatering and deposition of fiber suspension on the forming fabric surface, primarily constituting a fluid mechanics process. Chemical forces and colloidal chemical interactions also exert certain influences, particularly on the fine substances within the paper stock, which are relatively significant. The fluid dynamic process primarily encompasses three stages: drainage, directional shear, and turbulence.

Drainage Process

Drainage refers to the flow of water through forming fabric, with the direction of water flow being mostly, but not entirely, perpendicular to the plane of the wire. It is characterized by a variation in flow velocity over time. The main function of drainage is to dewater the fiber suspension, causing the fibers in the pulp to deposit onto the forming fabric and form a layer.

Water release occurs through two mechanisms: filtration and concentration. Filtration occurs when the fibers in the suspension are moveable or can move freely without interfering with each other. During water release through the filtration mechanism, there is a clear boundary between the deposited fiber layer and the thin suspension near the layer, and the concentration of the suspension above the layer that has not been released is essentially constant.

1. The drainage of the filtration mechanism has two effects on paper formation: uniform distribution and layer-by-layer deposition.

Because the flow of discharged water always has the maximum flow rate along the path of least resistance, it carries more easily movable fibers to this place of minimal resistance, where the fiber layer is relatively thin. This results in a uniform growth trend of the layer, which is the uniform distribution effect.

Due to the drainage mechanism of filtration, fibers primarily in a monomeric state are deposited layer by layer on the forming fabric. As a result, the interweaving and interpenetration of fibers in the accumulated structure are relatively less, and the layered nature is more pronounced. This is the layer-by-layer deposition effect.

2. Concentration occurs when the fibers in the suspension become intertwined and continuous, forming a mesh state where the fibers are not easily movable. At this point, there is no clear boundary between the layered sediment and the suspension, and the concentration of the suspension increases closer to the layered sediment. Like the layered sediment, the fiber mesh in the suspension is gradually compressed as the concentration process progresses. The main mechanism of water release from the suspension is filtration. However, both filtration and concentration, as two mechanisms of water release, actually coexist, with water release following the concentration mechanism being localized and temporary.

During the drainage process, fibers are mechanically blocked by the forming fabric and accumulate on it, resulting in a layering. This layering grows and is compressed as the drainage progresses. The friction between fibers within the layering and between fibers and the forming fabric gradually increases during the drainage process, stabilizing the fiber structure of the layering and enabling it to resist flow disturbances in the un-drained suspended matter above the layering and the backwash from the water below the forming fabric. The growth and compression of the layering increase the accumulation of fibers and fine substances.