The forming section: beyond the fourdrinier.

In the middle of the 19th century, there was an on-going argument among papermakers concerning the advantages of cylinder forming vs. fourdrinier forming. For a little while, it looked like cylinder forming would become the dominant technology, but as we know, that was not to be. Cylinder forming is still used, but almost exclusively for very heavyweight recycled board.

The winds of change are blowing strongly again, and this time it is the fourdrinier that will be left in the archives. You don't have to start the wake yet, since the fourdrinier is still dominant in numbers, but most equipment suppliers are no longer devoting their main research to improving fourdrinier operation. All their effort is focused on the future, and the future is the twin-wire former.

Although many larger suppliers have re-directed their research towards twin-wire forming, others are still developing new fourdrinier technology. One of the main issues in fourdrinier forming is how to generate stock activity without sealing the sheet--pulling fiber into the wire so that the fiber mat impedes further drainage.

Several companies have developed foil units with shaped blades. The entire unit can be moved into or out of the wire on the run for more or less pulsative drainage. These units have been very successful in brown grade applications and have had some success in other grades. More recently, two new devices have been developed that can vary the amount of energy applied to the stock on the fourdrinier wire and thus vary the amount of stock activity.

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One of these is a foil unit with variable frequency and amplitude controlled by a series of air motors (See Figure 1). By applying energy beneath the forming wire, the partially formed sheet is fluidized, allowing the redistribution of fiber flocs, which improves formation and drainage. Improved sheet structure can produce higher couch solids, sheet strength improvement, and the potential for increased machine speeds.

Another new forming unit also uses variable frequency pulsation of the stock by varying the tip-to-tip spacing of foil blades on the run. By varying spacing, the unit can produce a pulse frequency of approximately 100-150 Hz, which produces very fine, medium to high intensity turbulence. If machine speed is changed for a grade change, the foil blades can be moved closer or farther apart from each other to maintain optimum microturbulence.

Headboxes are also being improved. With the development of instruments that can measure tensile stiffness orientation (TSO) either on-machine or off-machine in the mill lab, papermakers are more aware of fiber orientation and its effect on curl and other converting properties.

With a better understanding of fiber orientation, we have also become aware of how poor fiber orientation can be in older headboxes. These units may have a poorly designed manifold, internal cross-flow problems, out-dated slice screw adjustors, or they may be running way over design capacity.

The answer to these problems is a new dilution control, hydraulic headbox. Dilution control headboxes use individually controllable valves to control the cross-direction consistency of the stock in the headbox to correct for heavy and light basis weight areas. New dilution control headboxes are capable of improving CD 2-sigma basis weight variation by 50% or better, while providing TSOs of no more than + or -2 degrees.

If capital for a new headbox is not in the cards, then a rebuild might solve many problems. Depending on the type of headbox, it might be possible to replace the manifold and initial tube bundle of the headbox with a circular distribution manifold with manual or automatic control of consistency in each tube (see Figure 2). Retrofitting existing headboxes can be a very cost effective way to make major improvements in sheet quality.

CYLINDER FORMING

Before we leave "older" forming techniques, it's worth noting that there is still development work continuing in cylinder forming. Traditional vat cylinder machines are used for multiply heavyweight board, but this forming technique is limited by high MD/CD fiber orientation and only minimal control over formation.

Over the years, there have been many developments of headboxes and short forming sections that can be added to cylinder machines. A recent development of cylinder forming combines many modern papermaking advances in a unit that replaces two conventional cylinder vats with one new forming section which can best be described as an "under-felt mini-fourdrinier" (see Figure 3).

This unit uses a circular manifold with dilution control to feed a hydraulic headbox. This technology is right off a modern twin-wire former. The forming section is relatively short, so drainage and stock agitation must be carefully controlled. Initial drainage is over shaped blades, which are adjustable into or out of the forming fabric for more or less drainage and stock activity.

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Final sheet drainage is over pulsative low vacuum boxes where the amount of stock activity can be controlled by the vacuum level in the box. The sheet is then transferred to the bottom of the conveying felt.

As you might expect, applying modern technology to multi-ply cylinder forming can go a long way to correcting some of the limitations of the old cylinder vat forming process. MD/CD fiber ratio is much more controllable, and sheet formation, smoothness, and printability are greatly improved.

TWIN-WIRE FORMING

Twin-wire forming technology is over 50 years old now (and even older from a concept standpoint), and many of the first and even second-generation twin-wire formers have been replaced or rebuilt. Earlier generations of twin-wire formers relied heavily on dewatering over large-radius curved bladed vacuum boxes. In other cases, dewatering was accomplished primarily over large diameter rolls with only limited bladed components following the forming roll.

Blade forming gives good formation but retention suffers due to the sharp pressure pulses of the blades. Depending on the configuration, there can also be a two-sidedness issue as well as a tendency for high MD/CD fiber ratios. Roll forming provides gentler dewatering over a longer time period, which gives good retention and better control over MD/CD ratio, but formation often suffers.

More recently, there has been a convergence of technology in twin-wire forming. Modern twin-wire machines use roll forming to initially drain the sheet and form a mat. Further drainage is over a bladed shoe with opposing blades to provide good formation.

Older generation twin-wire formers can also be rebuilt to incorporate the latest technology, and development in this area continues at a fast pace. Several rebuilds of older Beloit Bel Baie formers are just starting up in Korea, and you can expect a lot more activity in this area.

Top-wire formers are also still evolving based on better understanding of the forming process, and rebuilding of older top-wire units is a good way to improve their overall operation.

TRENDS IN FORMING FABRICS

It is no coincidence that synthetic forming fabrics were introduced at the same time that twin-wire forming first became a commercial success over 50 years ago. Modern twin-wire machines could not exist without the tremendous development in forming fabrics that has occurred over the last decades. And that development is by no means at an end.

Synthetic forming fabrics first were offered in single layer construction similar to the old bronze wire, and a few mills still use single layer forming fabrics due to their low cost. However, most mills have moved on to double layer forming fabrics due to the superior sheet quality these fabrics provide.

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If double layer fabrics provide better sheet properties than single layer fabrics, then triple layer fabrics are another step up. Forming fabrics have a number of performance functions, the main ones being drainage, fiber support, and sheet transport. And there are the overlaying issues of sheet quality and fabric cost and wear life.

Most development work in forming fabrics is now focused on triple layer constructions. That is because triple layer construction allows the top fabric and the bottom fabric to be essentially independent structures, so each can be tailored for the functions that are most important for that part of the overall forming fabric. In other words, the top fabric can be woven for excellent fiber support, good retention, and good sheet smoothness, while the bottom fabric can be engineered for superior wear resistance, good drainage, and good overall forming fabric stability.

To demonstrate the potential difference in sheet quality, Figure 4 shows the image analysis of a sheet made on a twin-wire former using a double layer wire with an extra support strand. Figure 4 also shows how the fabric structure imposes a pattern that affects the structure of the sheet and therefore the printability. Then look at the image analysis of the same sheet made with a standard triple layer forming fabric and the resulting improvement in printability (Figure 5).

If triple layer forming fabrics can improve sheet quality, why aren't all mills using them? There are several reasons, but the main one is cost. Triple layer wires are more complex to manufacture and therefore more expensive. There is also a potential problem with internal wear of triple layers, and triple layer wires traditionally have higher void volume than double layers. Higher void volume means there is more water inside the fabric structure that must first be removed over the flatboxes and couch before water can be removed from the sheet. As a result, sheet solids content may be lower after the couch for triple layer fabrics than for double layers.

Fabric suppliers are addressing these issues with new triple layer constructions. For example, the higher fabric cost can be offset by improved retention and a reduction in retention aid addition. New designs such as the paired MD strands shown in Figure 6 allow for more CD strands and improved sheet support. The paired MD stands also act as the binding strand between the two layers resulting in a lower caliper in the overall forming fabric and lower void volume.

SUCCESS IS KEY DRIVER

In the end, new technology is driven by commercial success in the market place. Since very little capital is being spent to upgrade cylinder and fourdrinier forming sections, equipment suppliers have shifted much of their development efforts to newer technology where capital is being spent--in twin-wire forming. There still are many excellent high-tech products directed to improving sheet quality and machine operations for cylinder and fourdrinier machines, but new technology in these areas in the future will most likely come from smaller suppliers.

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In twin-wire forming, machinery suppliers are rebuilding older twin-wire units to modern standards, and they are producing new twin-wire formers that get better with each generation. Forming fabric development is in many ways in lock step with equipment development. From the very early days of single layer synthetic forming fabrics to the highly sophisticated triple layers we see today, clothing suppliers continue to provide solutions to quality and operational issues. Triple-layer forming fabrics have had good commercial success on new high-speed, twin-wire machines that are being installed in many parts of the world, so that will be where the effort is spent.

Fortunately, this new technology is equally applicable to fourdrinier forming as it is to twin-wire forming. Development effort will always focused on the future. "Nothing endures but change."

LEARN MORE ABOUT FORMING AT THE TAPPI WET END SHORT COURSE

To learn more about new trends in forming as well as how to get more out of the forming equipment you already have, attend the 2005 TAPPI Wet End Operations Short Course, April 4-7, 2005, at the Perdido Beach Hotel, Orange Beach, Alabama, USA. Look over the course content at www.tappi.org or call TAPPI at 1 800 332-8686 in the U.S., 1 800 446-9431 in Canada, or [phone] (International).

WHAT YOU WILL LEARN

* Why machinery manufacturers are directing research efforts away from the fourdrinier.

* New developments in cylinder forming.

* Developments in twin wire forming.

* Trends in forming fabrics.

ADDITIONAL RESOURCES

* TAPPI Wet End Short Course (see box on bottom of page).

JIM ATKINS, SOLUTIONS! EDITORIAL BOARD

ABOUT THE AUTHOR

Jim Atkins is president of Atkins Inc., Flemington, New Jersey, USA, and is a member of the Solutions! Editorial Board. Contact him by phone at [phone] or [e-mail].

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