Forming advances promote better clothing: ever higher machine speeds are set to push forming fabric designs to the limit.

The forming section is a critical part of any paper machine. As such, papermakers focus a great deal of attention on improving the performance of this part of the papermaking process. Indeed, such is the importance of the forming section that practically every other characteristic of the final product is affected in some way or other if things are not running optimally in this machine section.

In recent years, paper machine speeds have gotten a lot faster and much of this progress can be traced back to massive strides undertaken in gap forming technology. But, as machines get faster, ever greater pressure is placed on the forming fabrics to perform to equivalent--or even higher--standards at increased machine speeds.

Voith's DuoFormer TQv or Metso's OptiFormer typify the upward trend in machine speeds witnessed in recent years. While hybrid formers perform well up to some 1200 m/min, today's gap formers continue to push that barrier well beyond the 1500 m/min mark. Speeds of 2000 m/min and upward are now being targeted as future production speed standards.

Clearly, this has a profound impact on the performance required of forming fabrics. In a classic fourdrinier design, the number of wire revolutions per minute might be 16, for example. But for the top wire of a gap former, the number of revolutions per minute jumps to 64. According to Voith's engineers, that means that dewatering times have dropped dramatically, from some 1.5 seconds to 0.17 seconds today.

Clearly, if forming fabrics are to meet the challenges imposed by such changes, further improvements are crucial. Voith, for example, has responded by targeting design improvements on a number of performance characteristics that are critical in increasing formation quality at increased machine speeds. According to Voith, some of the main areas to be tackled include:

* reducing water carrying and misting parameters

* creating a finer surface that provides more support points and helps reduce sheet marking

* increasing the level of mechanical retention

* providing better cross-direction profiles.

With this in mind, Voith recently developed a new forming fabric called Horizon, a thin, triple-layer product that helps reduce water carrying and misting, as well as generating a higher fiber support index.

SPEEDING UP

Metso's senior product manager lot forming, Antti Leinonen, is also well aware of the increased pressure on forming fabrics to perform under these more rigorous conditions. As he explained, "At a certain point, fourdriniers and hybrid formers are out of scope as speeds increase and you can't get the stability, so obviously gap formers are the future.

"But, there are still bottlenecks for certain grades," he continued. "In printing grades the surface characteristics are critical and marking becomes very important. That comes from different sources, such as the suction rolls and the felts, and as you go to higher speeds it becomes more and more difficult to deal with."

Metso does not have an in-house paper machine clothing unit, but the group works closely with Tamfelt and Albany International on R&D matters. The company is working with its partners to tackle misting, reduce sheet marking, boost retention, and optimize profiles.

But as Leinonen noted, whatever area you attempt to optimize, there is always a knock-on effect on other characteristics of the forming fabric. "In gap formers at high speeds you need higher tension in the forming fabric and that creates a big challenge to get the needed stability. We're working in the design range of more than 2000 m/min for gap formers.

"That pushes the tension much higher, which means you need more strength with the minimum of elevation," Leinonen continued. "At the same time, it's also important to keep the fabric clean and open. It's very demanding and you can't always expect to achieve it all at the same time, so there is often an element of compromise in the design."

KEEPING UP

One of Metso's partners, Tamfelt, highlights the challenges inherent in trying to match the ever-improving performance demands generated by new developments in the forming section. As the vice president of forming fabrics at Tamfelt, Martti Heinola, explained, "High speed forming requires good drainage characteristics, especially for fast initial dewatering in the gap. Fabric structure needs to be very stable for even paper profiles, but at the same time fine enough for good mechanical retention. And in order to get the full benefit From the loadable blade units, good flexibility of the fabric in the machine direction is essential. High wire tension, high vacuums, and high dynamic forces also require very good stability properties from fabrics."

Heinola pointed out that forming fabrics are becoming finer, more uniform, and more stable to match the developments seen to date in high speed forming and he fully expects that trend to continue. He also added that developments have taken place at slightly different rates, depending on the grade. For example, fine mesh triple layer and sheet support binding (SSB) fabrics were first used on supercalendered (SC) papers, which are prone to marking. From there though, the use of such fabrics has spread to the fastest lightweight coated and newsprint machines.

IN-HOUSE ADVANTAGE?

Away from the technology itself, changes are taking place on the business side of the industry. Voith intends to merge its fabrics and paper technology development into a single function with the goal of optimizing design efficiencies. As the company's marketing services manager, Mark Hodson, pointed out, "Instead of being Voith Paper and Voith Fabrics, we will work together as Voith Paper Technology and any developments in the machine part will automatically be linked to developments on the fabrics side."

The idea is that rather than develop the forming technology and the fabrics in separate "boxes," the supplier will pursue a grade-focused business whom the fabrics and machinery teams will report to the same boss.

The opposite perspective is that machine clothing and machine building are two very different businesses, and that traditional partnerships between the two industries work well.

As Heinola at Tamfelt said, "What we do is very different from what Voith or Metso do. We have very good relations with Metso on the R&D level. For example, we'll get involved on trial runs at pilot machine level. Even on the new machine at Langerbrugge in Belgium, we'll meet with Metso and the mill people to work out what their requirements are, but of course we still sell directly to the customer."

That view was echoed by Leinonen when asked whether Metso would contemplate acquiring its own paper machine clothing supplier one day. "I would not say we would never do it, but we manage very well with the systems we already have. We can select and develop a forming section with different fabrics and after that it's up to the customer to choose from the different fabrics available. A lot of customers prefer to have that control, especially those that have contracts with certain clothing suppliers. Of course, working this way we can't get all the confidential information on specific customers, but we already get all the information we need from the fabric suppliers." S!

IN THIS ARTICLE YOU WILL LEARN

* Why, as machines get faster, ever greater pressure is placed on forming fabrics to perform to higher standards.

* How technology "bottlenecks" in certain grades are being addressed.

* How the structure of the machine clothing industry is affecting R&D.

ADDITIONAL RESOURCES

* "Papermaking in 2035: What will the paper machine look like?" by Jim Atkins, Solutions!, March 2003, p. 25.

* "Tracking the trends in machine clothing: A combined future," by Alan Rooks, Solutions!, January 2003, p. 53.

* Go to www.tappi.org and type in either "Forming Fabrics" or "Paper Machinery" in the search engine

About the author: Jim Kenny is contributing editor/Europe, for Solutions! magazine, and is based in Brussels, Belgium. He is the former vice president of editorial for Paperloop and today heads his own company, DSI. Contact him by phone at [phone], or by email at [e-mail].