How Dryer Fabric Conditions Affect Paper Dimensional Stability

Paper shrinkage is a common and complex physical phenomenon in the dryer section of the fabric.

It is mainly influenced by the fiber’s own dehydration shrinkage and by external constraints during drying. These factors will also affect the final paper quality.

Causes of Paper Shrinkage

The root cause lies in the moisture removed from the paper on the dryer fabric.

Internal Reasons

As moisture evaporates on the drying net, the fiber cell walls within the brain undergo irreversible deformation due to water loss, generating contractile stress.

Of this, the removal of external water and interfiber water each contributes approximately half of the total shrinkage.

Meanwhile, the drying speed of the paper web will also affect its shrinkage. Uneven moisture distribution in the dryer fabric CD direction also leads to uneven lateral shrinkage.

Dryer Section Affects

Controlling the traction force on the paper sheet and the tension of the dryer fabric during drying is an important means of controlling the shrinkage of the paper sheet in the dryer section.

Dryer fabric tension: It refers to the tension of the fabric that presses the paper sheet against the surface of the dryer cylinder. The greater the dryer fabric tension, the stronger the restraint on the lateral shrinkage of the paper sheet.

Traction force: refers to the tensile force exerted on the paper sheet in the longitudinal direction. The greater the traction force, the more restricted the longitudinal shrinkage of the paper sheet, and the paper may even be stretched.

The Effect of Paper Shrinkage on Paper Properties

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

Strength Performance

The shrinkage rate of paper has an approximately linear relationship with its tensile strength, elongation at break, and breaking energy.

Limiting paper shrinkage usually increases tensile strength but reduces elongation at break.

Dimensional Stability

Paper that undergoes greater shrinkage during drying on the dryer fabric has poorer dimensional stability and is more prone to deformation when exposed to changes in humidity.

Paper Banner Tension

The longitudinal stretching during the drying process creates an uneven stress field inside the paper sheet, which usually manifests as a tight middle section and looser edges.

Methods to Control Paper Shrinkage

Adjusting Traction Force and Dryer Fabric Tension

This is the most direct method. Increasing the traction force limits paper longitudinal shrinkage but exacerbates lateral shrinkage. Increasing the dryer fabric tension limits lateral shrinkage. In production practice, a balance must be struck between the two based on the target paper type.

Therefore, when producing specific types of paper, we can achieve their different characteristics by controlling the traction force and the tension of the wire fabric.

For example, when producing semi-transparent paper, a greater traction force and dryer fabric tension should be applied so that the paper cannot shrink sufficiently in either the longitudinal or transverse directions. In this way, the shrinkage due to water loss will occur in the thickness direction of the paper, thus making the paper thinner and improving transparency.

Zoned Control Dryer Fabric

During the stage of most severe paper shrinkage (eg., when the dryness is about 58%), the dryer fabric can be loosened and the traction force reduced to allow the paper to shrink freely. During the stage of insignificant shrinkage, the dryer fabric can be tightened to improve heat transfer efficiency.

Partial Dryer Fabric Coverage 

If the drying capacity of the section is sufficient, the dryer fabric can be left uncovered on the dryer cylinders with the highest shrinkage rate, allowing the paper sheets to shrink freely.