Have You Ever Noticed The Waterline On Forming Fabric

What Is The Waterline On Forming Fabric

In the forming section of the paper machine, the pulp is evenly sprayed from the headbox onto the forming fabric. The forming wire supports the movement of the pulp, while a vacuum box and drainage device underneath the wire fabric remove moisture from the pulp, resulting in the initial dewatering and forming of the paper web.

In the forming section of a paper machine, the waterline is a crucial dividing point where the pulp transitions from a liquid suspension to a wet sheet. As the dewatering process continues on the forming fabric, a significant amount of water is removed, and the fiber layer gradually becomes visible. At this point, the optical properties of the fiber layer surface also change: the specular reflection that was present when the moisture content was high gradually transforms into a diffuse reflection effect after dewatering, thus forming a distinct light-dark boundary on the forming fabric. Because this phenomenon directly reflects the initial forming and dewatering state of the paper sheet, the waterline is considered one of the key visual indicators in the operation of the forming section.

Location 

Appears on the surface of the forming fabric between the headbox and the vacuum tank or coil roll.

Physical Significance of Water Line

The appearance of the waterline marks a critical turning point in the dewatering of the paper web on the forming fabric. Before this point, the surface of the paper web is covered by a continuous film of water, and dewatering occurs primarily through the combined effects of the web’s own weight, the forming fabric, and the headbox. Beyond the waterline, however, there is no longer a distinct layer of free water on the surface of the wet paper web, and subsequent dewatering relies primarily on the forced suction of the vacuum suction box to remove moisture from the interstitial spaces between the fibers.

Ideal Water Line Condition on Forming Fabric

Ideally, the water line on the forming fabric should be a relatively stable, clear, and straight line spanning the full width of the fabric. The clarity, straightness, and stability of the water line provide a direct indication of the uniformity of the dewatering process in the forming section, and serve as an important visual indicator for assessing the quality of the paper formation on the forming wire and the distribution of fibers.

Water Line & Paper Quality

Waterline Position of Forming Fabric

Generally, the position of the water line is considered an indicator of the dewatering speed of the forming fabric section. The further forward the water line is, the faster the dewatering speed, and vice versa.

When the water line on the forming fabric is positioned towards the rear of the wire section, it usually indicates insufficient dewatering capacity in the forming section. Water cannot be effectively and promptly removed in the early stages, leading to a slower overall dewatering speed. In this situation, suspended fibers are prone to re-agglomeration due to Brownian motion, forming fiber clumps. This reduces sheet uniformity and affects forming quality. Simultaneously, insufficient wet sheet dryness often results in decreased paper web strength, making it more prone to breakage in the press section or subsequent operations, thus impacting the stable operation and production efficiency of the paper machine.

While a forward-moving water line on the forming fabric indicates a faster dewatering speed, a faster dewatering rate is never necessarily better. If water is extracted too rapidly during the transport of wet paper sheets on the fabric, the fibers cannot be fully dispersed and interwoven on the forming wire, resulting in poor paper uniformity, uneven structure, and even paper defects such as “pulp runners”.

The optimal location of the water line depends on factors such as the type of paper being produced and the speed of the paper machine. Generally, the water line is best located in the middle to later part of the forming fabric, which ensures that the fibers have adequate time to disperse and gradually dehydrate and form the paper under good control.

Therefore, we can conclude that the water line affects the horizontal consistency of the paper, and thus, regular inspection and adjustment of the forming section are necessary to ensure the water line position.

Waterline Status

The straightness of the water lines on the forming fabric can also be used to judge the consistency of the paper web. Ideally, the water lines should be straight or nearly straight. Crooked water lines usually indicate uneven lateral distribution of moisture in the wet paper and poor dehydration.

Straight & Stable 

This indicates that the paper dehydrates evenly across the cross section (CD) on the forming fabric, which is the basis for good cross-section uniformity.

Bending, Undulating & Unclear

A curved or blurred waterline is typically a key indicator of uneven cross-directional dewatering of the paper on the forming fabric. Possible causes of this phenomenon include uneven opening of the headbox lip, localized defects in the forming fabric, and malfunctions in the dewatering elements. These factors disrupt the uniform dewatering of the pulp in the machine direction, leading to inconsistent basis weight and thickness distribution in the sheet. This ultimately results in paper defects such as streaks and cloudiness, severely affecting the paper’s uniformity and forming quality.

Two-Side Difference

If the initial dewatering of the paper on the forming fabric is too intense—that is, if the waterline is positioned too far forward—it can easily cause a large amount of fine fiber components and fillers to be carried away from the fabric side along with the water flow. As a result, the fiber structure on the forming fabric side of the paper is relatively coarse and has a lower filler content, while the opposite side exhibits greater fineness and a higher filler retention rate. This imbalance in dewatering further exacerbates the two-side difference in the paper, affecting the sheet’s surface properties, printability, and overall quality stability.

Paper Defects

During the fabric forming process, you may encounter various paper defects for a variety of reasons, which can become a problem in your papermaking process. Below are some paper defects that can be predicted using the water line.

Pinholes – These may be related to improper dehydration of the paper on the forming wire, air ingress, or vacuum settings. Abnormal water lines sometimes indicate these problems.

Crushing – If the water line on the forming wire is too far back, it indicates that the paper sheet still has too high a moisture content when entering the press section, which may lead to structural damage under subsequent pressure.