SSB is now 10 years old and the demands and requirements of the papermakers increased in recent years. The clear trend in forming fabric development was towards finer and finer fabric types. That means finer diameters and higher yarn densities in warp and weft. The target of this development was improved retention and a surface improvement to minimize the marking tendency of the fabric. At the same time the demands of the market increasingly forced customers to run their machines more efficiently.
To meet these new challenges the modern forming fabric concept should be more accommodating regarding different headbox consistencies and wider basis weight ranges. Achieving these aggressive objectives with decade-old technology becomes increasingly difficult given the higher demands in the paper industry.
Why is the SSB structure so successful? The structure offers straight-through drainage, which allows a dramatic increase of fiber support, compared to single and double layer wires, Fig. 2. This results in improved drainage leading to better dryness and excellent retention. With increased fineness (higher weft and warp densities) of the wire, the free surface is more and more reduced leading to higher drainage pressure and lower drainage rates. The operational window gets smaller, vacuum levels increase, drive energy goes up and sheet consistency goes down.
So how can we follow the trend of finer wires without compromising the drainage? To break existing barriers it is necessary to take a closer look at the papermaking process.
The paper furnish consists of fibers, fillers, chemical systems and water. The forming fabric should filter the fibers and fines from the water.
Even the finest paper side structures in the field are not able to mechanically retain a fines particle. We need the first pass retention or the initial fiber matt to help. The forming fabric only directly influences the initial sheet forming. Further sheet forming, retention and drainage performance are controlled by the initial fiber matt. Therefore, the major task of the forming fabric is to build the optimal initial fiber matt, as a basis of the whole sheet structure and PM runnability.
Figure 1 - Forming fabric product history
Out of the box
Forming fabric performance is mainly defined by its behavior in the initial fabric 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.
The optimal initial fiber matt provides:
- Proper sheet formation, due to mobility of the fibers
- Retention of fines and fillers (matt is always finer than the fabric)
- Open structure, to facilitate water removal over the whole former at lower vacuum levels.
Field results showed the initial fiber matt should be open and not too dense. A too dense initial sheet seals the paper and reduces the overall drainage. The flow speed on the paper side of the wire has a significant impact on the porosity of the initial fiber matt. A higher surface open area provides a lower flow speed and keeps the initial matt more open. To provide a more controlled drainage the surface open area on running side needs to be reduced. It is diametrically the opposite of conventional wisdom in the industry!
What does a fabric's optimal drainage channel look like, in detail?
- Most of the fibers coming out of the headbox are MD oriented so it is a known fact that CMD oriented forming fabric meshes provide the highest mechanical fiber retention. So the PS Hole needs to be CMD oriented, providing highest fiber support.
- With regard to drainage capacity and fines and filler retention a high surface open area on paper side is essential. Therefore, a high water amount can pass the forming fabric at relatively low flow speed on sheet-forming level.
- In order to control this flow for optimum and smooth sheet forming, a reduced open area on the running side is necessary.
- The caliper of the wire, and hence its Z-direction hole length, needs to be as short as possible to ensure rapid water removal.
- To guarantee a constant performance throughout the whole fabric life, the caliper change during the run needs to be as low as possible.
Figure 2 - Comparison of dewatering channels
The concept was confirmed by in-house laboratory tests (FRET Tester), but also by Grenoble University, which simulated the flow speeds over different levels through the wire. The surface open areas in Z-direction control or influence the speed of the water flow, Fig. 3.
To apply the right channel Xerium uses an internally-developed tool to simulate the drainage channel, the average mass distribution in Z-direction, and show the porosity or free surface over the whole fabric caliper in MD and CMD direction. This revolutionary approach allows for development focused on the fabric holes not on the meshes.
The exact channel specifications are formed by different ratios between the paper side and running side weft and warp yarns. Changing the yarn diameters and/or densities on paper or running side has an influence the shape of the channel. Application engineers can now build a special channel for each application worldwide, customized on individual customer requirements.
Figure 3 - Real drainage channel of the forming fabric
Xerium offers a product line using EDC to improve papermaking in terms of machine runnability, efficiency and paper quality. The forming fabric portfolio will be sequentially replaced by fabrics having the new EDC characteristics. Following the introduction of Apexx to the newsprint segment, the company recently launched Formexx, which is applied in the manufacture of graphic papers. The next member of the forming fabric product family, Finetexx, to be used for producing magazine and specialty papers, will be launched shortly.
As was the case when it introduced SSB forming fabrics, Huyck Wangner's EDC technology raises the bar to a significantly higher notch. With the new generation of forming fabrics, papermakers now have access to clothing that for the first time ideally combines the two key parameters - top paper quality and cost efficiency - with energy consumption attributes, which gives them a valuable lead in their markets.
Field results prove higher sheet solids on pick-up, lower drag loads due to lower vacuum levels, and improved paper quality. Due to the easier water removal the customers were able to run lower headbox consistencies resulting in improved paper formation at higher or constant retention.
The optimal drainage channel enlarges the operating window providing the clearance for the papermaker to enhance PM performance.