How Forming Fabric Speed and Jet Velocity Affect Paper Formation

What Is Jet-to-Wire Ratio

The forming fabric speed and jet velocity are the core parameters of the paper mill forming section. The speed difference between the two (which is also called the jet-to-wire ratio) is the key factor determining paper quality and the presence or absence of paper defects.

The units for both stock speed and forming fabric speed are typically m/min, and the speed ratio is generally defined as J/W (stock speed/wire speed).

Usually, we maintain the jet-to-wire ratio between 0.95 and 1.05, though the specific value in actual production depends on the paper grade and machine speed.

The Importance of The Speed Difference Between Forming Fabric & Jet

Theoretically, the relationship between jet velocity and forming fabric speed determines the orientation of fibers on the forming fabric and, consequently, the uniformity and structure of the paper. The shear force resulting from the jet-to-wire ratio affects drainage and flocculation.

The Bringing Paper Issues

A mismatch between the jet velocity and the forming fabric speed in the forming section of a paper machine leads to a series of paper diseases, which not only affects production efficiency but also increases production costs.

• When the jet speed exceeds the forming fabric speed, fibers undergo intense longitudinal alignment, resulting in an excessively high MD/CD strength ratio (like exceeding 5:1). This leads to cross-directional (CD) sheet shrinkage, uneven strength, and the formation of serrated edges. Additionally, it affects sheet dewatering, causing instability of the waterline and fluctuations in basis weight. In severe cases, curtain-like streaks extending across the width of the forming fabric may occur.
• When the stock velocity is lower than the forming fabric velocity, the fibers are “dragged” outward by the fabric, causing them to align transversely or curl. This disrupts the uniformity of the paper sheet, resulting in a two-sided appearance, and may even lead to excessive dewatering in the forming section, causing the wet sheet to break.
• When the speed difference is negative and its absolute value is large, the fibers on the forming fabric surface are prone to wear, resulting in significant loss of fine fibers and fillers and a low retention rate. These factors lead to a significant decrease in sheet strength, brittleness, low ash content, and a large difference between the two sides of the paper.

The Key Indicators of Forming Fabric Speed & Jet Velocity

Formation—the smaller the absolute difference between the jet velocity and the forming fabric speed, the better the paper formation.

Strength—When the jet velocity is slightly higher than the fabric speed, the machine-direction tensile strength and tear strength of the paper web increase, but this comes at the expense of cross-direction strength.

Dimensional stability —the greater the jet-to-wire ratio is, the more severe the shrinkage of the paper sheet in the wet section. Consequently, after drying, the longitudinal dimensional change is reduced, while the transverse dimensional change increases.

How the Jet-to-Wire Ratio Affects the Service Life of Forming Fabric

During the production process, when the jet-to-wire ratio is high, the wear of the formed fabric will be accelerated. Specifically,

• Excessive jet velocity: Increases impact and abrasion of the stock against the fabric surface, reducing the forming fabric’s service life.
• Excessive fabric speed: Increases frictional shear between the forming fabric surface and fibers, likewise accelerating wear. Moderately reducing the speed difference can extend the fabric’s service life by 10–20%.

Choose the Correct Speed Difference Between the Wire and the Stock Jet

Ideally, the speed ratio between the pulp jet and the forming fabric should be 1. This minimizes relative slippage, allowing the pulp to settle smoothly onto the fabric and causing the fibers to deposit naturally, resulting in a uniform sheet.

In actual manufacturing, the optimal lag or lead ratio for the jet-to-wire is typically determined by the paper type and quality requirements to leverage its properties and achieve better production results.

For example, in the production of newsprint and printing paper, where fibers oriented in the machine direction often require greater tensile strength, the jet rate is typically set slightly above the forming fabric speed (approximately 1%–3%) during manufacturing.

Of course, when producing specialty papers with high manufacturing requirements or papers that require a high degree of uniformity, a 1:1 ratio of wire speed to stock flow is most appropriate.

Here, we provide suitable ratios between the forming fabric speed and jet velocity for different paper manufacturing needs for your reference.