by O. Baumann and S. Ernst*

1.1. Evolution of forming fabrics
The first synthetic wires have passed through a number of pioneering stages as they have evolved over the past decades. Initially, the main focus was placed on the life time. It wasn't until 2.5- layer designs were introduced that an increase in fiber support became possible along with a corresponding improvement in the papermaking properties of forming fabrics. The last major step was the development of SSB fabrics, which are based on a technology that has had lasting influence on the forming fabric market. In Western Europe this technology has captured approximately 80% of the market over the past 10 years. (Fig.1)

Fig. 1: Forming fabric developments in the last 30 years

1.2. Market demands
The paper industry must constantly adapt to ever more demanding general conditions: Increasing production costs and the growing global demand for pulp impact the cost of raw materials. The rising demand from Asia - above all China - will certainly make these developments even more dynamic. Energy prices have risen sharply. This development is expected to further intensify in the future. Energy remains a strong cost driver, despite the fact that the production process has been greatly optimized in recent years (less electricity, less water per ton of paper). Pressure is rising on papermakers to further enhance production processes. At the same time, the demands on paper quality are also rising. Clothing has a tremendous influence on both of these developments.

* Oliver Baumann, XERIUM Technologies, Inc., Technical Director Forming Fabrics Stephan Ernst, HUYCK.WANGNER Europe, Product Manager Forming Fabrics

The paper furnish consists of fibers, fillers, chemical systems and water. The major task of the forming fabric is dewatering and forming an initial fiber mat to control drainage and retention properties in the successional sheet forming process (Fig. 2)

Fig. 2: Initial sheet structure

Even very thin fiber mat has a higher resistance to drainage than the wire used. The amount of dewatering is thus primarily determined by the drainage behavior of the fiber mat. However, the wire's properties influence the fiber mat's structure as well as its filtration properties.

Current former concepts (roll-blade formers) actively attempt to intervene in the initial sheet formation process, as demonstrated by the latest developments in shoe and counterblade positioning. Thus, the distance between the forming roll and the activity blades is growing smaller in order to leverage the higher mobility of the fibers to improve formation.

2.1. Definition of development goals
The SSB fabrics used in the market today have seen continuous advances in recent years, and in terms of sheet formation properties their possibilities have been fully exhausted. The goal of the new EDC forming fabric technology from Huyck.Wangner is to remove existing limitations with respect to drainage properties and fiber support.

As previously mentioned, forming fabric performance is mainly defined by its behavior in the initial drainage section of the paper machine. By thinking out of the box, development engineers shifted the focus from the fabric structure to a specially formed drainage channel, which in conjunction with the paper side topography paves the way to optimum sheet formation.

This enables us to infer the following properties provided by the optimum initial sheet structure:

• Improved sheet formation due to controlled drainage
• Retention of fines and fillers
• Open structure to facilitate water removal at all drainage elements

Practical experience shows that open sheet structures result in considerable advantages in total drainage, whereas sheets that are too dense lead to increased drainage resistance. The flow speed on the paper side of the forming fabric has a significant impact both on the porosity of the initial fiber mat and the mobility of the fibers. Greater fiber mobility is reflected in considerable formation improvements. A higher surface open area results in a lower flow speed and retains the initial mat's more open structure. Accordingly the surface open area on the running side of the wire needs to be reduced, to control the drainage.

The key goal of EDC's development was to provide a technology whose drainage channels would optimally regulate the initial sheet formation thereby expanding the paper machine's operating window.

2.2. Implementation in a fabric structure
Description of the ideal drainage channel in a forming fabric:

• Since most of the fibers coming out of the headbox are MD-oriented, the drainage channel on the paper side of the fabric should be as CMD-oriented as possible.
• A high surface open area on the paper side is essential for drainage capacity and fines and filler retention. This ensures that a large amount of water can pass through the forming fabric at relatively low flow speed at the sheet forming level. This gentle drainage process improves retention behavior while minimizing the marking tendency of the fabric.
• To control this flow, the surface open area on the running side must be reduced.
• The caliper of the wire, i.e. its Z-direction drainage channel length, must be as small as possible to ensure rapid water removal.
• To guarantee consistent performance throughout the life of the fabric, the caliper change during the run must remain as small as possible.

The perfect drainage channel described above can only be achieved by making fundamental changes to the structure of the fabric. EDC (engineered drainage channel) designs are thus characterized by a special warp construction (warp = monofilaments in the machine direction), guaranteeing an open structure on the paper side and a closed structure on the running side. In conjunction with various weft ratios, the drainage channels are adapted to the respective applications.

Wire caliper and free pore volume are influenced by the fineness of the monofils used. This means that several different warp designs are required in order to cover the whole range of applications.

2.3. Simulation results
A variety of methods were used in order to make the drainage channel visible. Initially, a computer simulation of the mass distribution across the thickness of a forming fabric was performed, which showed significant differences between the fabric types. This made the influence the channel must have on sheet formation unmistakably clear (Fig.3). As is evident here, an EDC design moves the mass towards the running side; in the example shown, the tightest cross-section changes significantly when comparing both designs. The result is due to a specially shaped drainage channel. This drainage channel can be influenced by various weft densities as outlined below. The developed software makes it possible to simulate any conceivable channel.

Fig. 3: Display of mass distribution

Computer tomography images were created to avoid obtaining purely theoretical values and data on the shape of the drainage channel. The advantage of this method is that it yielded real 3D images of the forming fabrics (Fig. 4). It was then possible to use these images as a basis for a drainage flow model.

Fig. 4: 3D computer tomography of a SSB forming fabric.

The goal of the drainage model was to present the various flow speeds along the drainage channel. Fig. 5 shows a comparison of the SSB and EDC design flow speeds at the sheet forming level. The SSB paper side achieved relatively high speeds (= many red and yellow areas). Conversely, the EDC construction shows lower flow speeds (= many blue and green areas). This translates into gentler and more regulated sheet formation for the EDC design. Furthermore, Fig. 5 illustrates the strong CMD-orientation and the large surface open area on the paper side of the EDC fabric.

Fig. 6. shows how the drainage channel is modeled with the help of computer tomography. This was used as a basis for simulating the flow speed throughout the entire fabric body with a 3D flow model. Fig. 7 illustrates the overall lower dewatering speed across the entire cross-section in EDC designs, thus supporting the "gentler" water removal behavior.

Fig. 5: Mesh orientation and dewatering speed

Fig.6: EDC drainage channel

Fig. 7: Flow speeds across the fabric cross-section

3.1. Range of applications
The ability to combine different monofil thicknesses and densities means there are no limitations with regard to the application possibilities. In general, this technology can be used on all paper machine types and paper grades. The present focus is on medium and high warp densities in the graphical paper segment.

3.2. Initial practical results
A theory is only as good as the practical evidence that supports it. EDC designs have already been successfully tested on a variety of paper machines with different paper grades. The results confirm the theoretical objective, as they simultaneously provided increased drainage performance with an additional efficiency boost and improved paper quality.

The following results were documented in detail:

• Gapformer for LWC: Energy savings in stock preparation with lower-value raw materials while maintaining high paper quality.
• Gapformer for LWC: Depending on the paper grade speed increase of up to 4%, additional tonnage.
• Gapformer for newsprint: Higher dryness at pick-up, improved efficiency, additional tonnage
• Hybrid former for LWC: Depending on the paper grade speed increase of up to 7%, additional tonnage.
• Hybrid former for wood free coating base: Efficiency enhancements and speed increases of up to 9% depending on the grade while at the same time providing improved paper quality (Fig.8)
• Fourdrinier for laminate paper: Improved titan-dioxide retention of over 3%, representing significant cost savings.

Overall, the results obtained to date clearly indicate a gentler initial drainage process, which leads to an optimized sheet structure.

Fig. 9: Comparison of sheet forming properties

The initial results also show that the drainage performance is not the sole parameter in PM performance and paper quality. Much more important is adapting the drainage to application-specific requirements, i.e. balancing out all of the influencing factors in achieving optimum sheet formation. EDC offers the possibility of changing the sheet structure entirely. The result is an extended operating window for the PM without placing limitations on paper quality. Thus EDC introduces a new generation of forming fabrics that overcome existing barriers and open up new possibilities in process optimization (Fig. 9).

In summary, the advantages include:

• Efficient initial sheet forming for optimum formation and lowest possible porosity
  • Higher FSI compared to all other conventional SSB fabrics
  • Highest surface open area (SOA) on the paper side
  • Optimum drainage control thanks to specially designed drainage channels
• Retention properties
  • Higher mechanical retention (fibers) due to increased number of wefts on the paper side (FSI)
  • Improved hydrodynamic retention (fines and fillers) resulting from reduced max. flow speed of the water at the sheet forming level (high SOA on PS, reduced SOA on RS).
• Profile quality
  • Higher rigidity than comparable standard SSBs: optimum cross-profiles.