How Dryer Fabrics Influence Paper Shrinkage and Dimensional Stability

Paper sheet shrinkage is a common physical phenomenon that occurs as the wet web travels through the dryer section on the dryer fabric. This shrinkage is primarily caused by the dewatering-induced contraction of the fibers themselves, combined with various external constraints during the drying process, and it significantly affects the paper’s final properties.

Excessive shrinkage can lead to issues such as dimensional instability, curling, wrinkling, poor printability, and reduced quality.

Understanding the causes of paper shrinkage and the role played by the dryer cylinders and dryer fabrics is crucial for maintaining stable paper machine operation and producing high-quality paper.

How do Dryer Fabric Affect Paper Drying Shrinkage?

The fundamental cause of paper shrinkage lies in the removal of moisture from the paper web as it travels through the drying section on the dryer fabric.

Shrinkage of the Fiber Itself

Generally speaking, the moisture within a wet paper sheet comprises free water external to the fibers, as well as water located between and within the fibers.

As the wet paper web travels through the dryer section, supported by the dryer fabric, moisture evaporates under the influence of the dryer cylinders. The subsequent loss of water causes irreversible deformation of the fiber cell walls, thereby generating shrinkage stresses between the fibers.

Among these forms of moisture, the removal of external water and water situated between the fibers is the primary cause of shrinkage stress generation during the drying of the wet paper web.

Drying Rate and Method

Rapid drying alters the shrinkage and final strength of the paper sheet. Additionally, uneven moisture distribution across the dryer fabric during drying can lead to CD shrinkage defects in the finished paper.

External Constraints

This is a key human factor affecting the direction and extent of paper shrinkage.

Traction

The longitudinal tension is exerted on the paper sheet on the dryer fabric. The greater the traction force, the more restricted the longitudinal shrinkage of the paper sheet, and it may even be stretched. At the same time, the paper’s longitudinal stretching causes the fibers to align longitudinally, exacerbating its transverse shrinkage.

Dryer Fabric Tension

The tension of the drying fabric presses the paper sheet against the surface of the drying cylinder. The greater the tension of the dryer mesh, the stronger the restraint on the lateral shrinkage of the paper sheet.

Impact on Paper Quality

The control of paper shrinkage on the dryer fabric directly determines many of the paper’s physical properties.

Strength Performance

Shrinkage has an approximately linear relationship with the paper’s tensile strength, elongation at break, and rupture attack. Limiting shrinkage (such as applying greater dryer fabric tension) will generally increase the paper’s tensile strength but reduce its elongation at break.

Dimensional stability

A paper that shrinks more during drying has poorer dimensional stability and is more prone to deformation when humidity changes. Paper dried under tension exhibits significantly better dimensional stability than paper dried without constraint.

Web Tension Profile

The longitudinal stretching of the wet paper web on the dryer fabric creates an uneven stress field within the paper sheet, typically resulting in a tighter center and looser edges. This uneven tension can affect paper machine operation and subsequent printing processes.

How Does the Dryer Fabric Affect Paper Shrinkage

Dryer Fabric Provides the Necessary Mechanical Constraint

Once the paper sheet enters the drying section, the fibers naturally shrink as moisture evaporates. Without restraint, this shrinkage can be significant, leading to transverse shrinkage. The dryer fabric continuously supports the paper sheet and provides appropriate tension, limiting excessive shrinkage and improving dimensional stability.

Dryer Fabric Affects the Uniformity of Shrinkage

For most paper mills, the real problem isn’t paper shrinkage itself, but the uneven shrinkage that causes curling, wavy edges, and wrinkling. One of the key factors affecting the uniformity of paper shrinkage is the dryer fabric.

A high-quality dryer wire has uniform air permeability and a stable structure, enabling the paper sheet to receive more even heat and evaporation conditions in the horizontal direction. This reduces shrinkage differences caused by variations in localized drying speeds.

Dryer Fabric Affects the Drying Process

Paper shrinkage is often caused by uneven drying in the paper machine’s drying section. The dryer fabric not only transports the paper sheet but also affects airflow, heat transfer, and moisture removal during the paper web’s transport in the drying section. Therefore, uneven air-permeability distribution on the dryer fabric can lead to localized over- or under-drying of the paper, resulting in uneven shrinkage.

Dryer Fabric Stability Affects Shrinkage Control

A high-quality dryer fabric should exhibit low elongation, high tensile strength, and good dimensional stability, enabling it to consistently provide stable support for paper sheets and control shrinkage during long-term operation. This reduces paper shrinkage problems caused by changes in wire tension, paper sheet support, and shrinkage control.

Moreover, if you still want to know more about dryer fabric and its function during the papermaking process, like how to select the correct one, how to install it, how to clean it, and how to save, etc. Please contact us for more information.