Warp Exchange technology has provided benefits such as improved drainage and couch solids as well as extended wear potential
March 2008
By Blake Hender
The paper machine clothing industry continues to push for technology breakthroughs to help papermakers increase machine efficiencies, reduce fiber and energy consumption, and improve overall paper quality. In recent years, advancements have been made in forming fabric designs that have proved to have a significant impact on papermaking productivity.
Triple layer forming designs have been around for more than 30 years, but it has really been in the last decade, as the pressures on papermakers have mounted, that these fabrics have gained wide spread acceptance in the North American market. Papermakers are finding these more complex forming structures to offer real advantages over earlier generations of forming designs. Advantages include improvement in fiber retention, sheet properties, fabric stability and sheet profiles.
To build on this success, Voith Paper Fabrics recently developed a new triple-layer design using Warp Exchange technology. This fabric has a reduced fabric caliper and void volume for a given set of monofilament diameters. By starting with thinner caliper and lower void volume, the fabric can meet specific customer demands to a much greater degree than with previous triple-layer concepts. The Warp Exchange concept can be used for a variety of paper grades and applications.
Evolution of Triple-Layer Design
The basic triple-layer woven structure has been used in forming fabrics for more than 30 years. This concept involves breaking down the different functions of a forming fabric and engineering separate surfaces, thereby optimizing fabric features to meet these different demands. By separating the papermaking and wear surfaces, both can be improved without negatively affecting the other. A fine mesh papermaking surface, using fine yarn diameters and a plain weave (2-shed) woven structure, give superior properties for papermaking. Properties are measured with parameters such as fiber support index (FSI), support points per unit area (SP), and frame size openings. Using any of these parameters, triple-layer structures generate significantly finer forming surfaces than other forming fabric concepts.
The bottom of the triple-layer fabric can be designed to maximize wear and stability properties. This typically involves larger diameter yarns and 5-shed or 6-shed weaves. As these papermaking and wear layers are independent, they must be bound together to form a single fabric, Figure 1. Most development of triple-layer forming fabric technology over the last decade has involved improvements in this binding.
The first generation of triple-layer fabrics used a single cross-machine (CMD) oriented binder yarn to integrate the two fabric layers. The weakness of this design was the resulting thick fabric caliper and high void volume, and the fact that this binder was not durable enough for demanding applications. At higher speeds and high white water ash loads, this single CD binder, Figure 2, would wear internally, resulting in premature failure of the binder before the full wear potential of the fabric’s true wear surface could be realized.
The second generation of triple-layer forming fabrics also used CMD oriented binder yarns, but in pairs that served a dual function – to bind the fabric’s top and bottom layers together and to enhance the fabric’s papermaking surface, Figure 3. These designs, commonly referred to as SSB fabrics, have gone a long way in eliminating delamination issues and are widely used across all paper grades.
However, the SSB concept does not address the issue of caliper and void volume. These are still relatively high, and the only real way of addressing this is to use smaller diameter yarns. The weakness of this approach is that the use of smaller yarn diameters will have a negative effect on stability and wear potential; to improve retention and sheet properties, fabric life will suffer.
Warp Exchange Design
The third generation of triple-layer forming fabrics eliminates the need for performance tradeoffs. This so-called Warp Exchange technology was developed and made commercially available by Voith Paper Fabrics.
Warp Exchange technology uses machine-direction (MD) oriented warp yarns to bind or integrate the fabric’s papermaking and wear surfaces, Figure 4. This MD integration of fabric layers results in a significant reduction of overall fabric caliper, as much as 15-20% within some product lines, without the need to use smaller yarn diameters. This reduces caliper and void volume, offering many opportunities for improvements in fabric performance.
The MD binding yarns are the same yarns that experience tension as the fabric is loading on the paper machine. The fabric is, in effect, bound together using paper machine operating tension. The 25-40 pli, at which most paper machines operate, bond the fabric layers firmly together and eliminate delamination as a failure mode. Therefore, fabric life is dictated only by the wear volume of the fabric’s bottom surface, and not by CD binder delamination.
Specific Machine Applications
Warp Exchange fabrics are now available for graphics, board and packaging, and tissue grades. After running more than 1,000 of these fabrics, the design has proved to enhance performance in a variety of ways.
The low caliper and void volume benefits were immediately recognized on applications with couch solids limitations. Many high-speed graphic Gapformers are characterized by their low couch solids. On these applications, trials with earlier triple-layer designs have usually failed. The caliper and void volume of these products carried excess water, resulting in poor dewatering efficiencies over the vacuum elements. Warp Exchange technology is ideally suited for these applications, and papermakers have consistently seen improved couch solids, as well as formation and retention benefits. Results include:
- Running on a BelBaie II making newsprint at 4,200 ft/min, the Warp Exchange trial with comparable wear potential gave a 1% increase in couch solids.
- Running on a SpeedFormer making newsprint at 4,800 ft/min, the Warp Exchange increased couch solids by 1-1.5% and improved life potential.
The improved dewatering efficiency of Warp Exchange fabrics opened the door for applications requiring improved mechanical retention and sheet quality. With higher rates of dewatering over the vacuum elements, application engineers now had the option of closing up fabric air permeability and open area for a more controlled early table dewatering. This reduced the early table washing out of fines/fillers and kept the sheet fluid longer to allow table activity and formation gains.
With access to Voith Paper pilot machines and working directly with papermakers, Voith Paper Fabrics has determined that on most applications air permeability values can be lowered by as much as 50-75 cfm. Open areas can be reduced by more than 5% as compared with conventional SSB triple-layer designs. This improves retention and sheet quality, while still maintaining couch solids. Results include:
- Running on a Symformer making woodfree uncoated copy grades at 3,300 ft/min, the Warp Exchange fabric gave improved sheet formation and image deletion.
- Running on a fourdrinier making coated grades at 2,500 ft/min, the Warp Exchange product helped reduce pin holes and lower coating usage.
Many papermakers are trying to control paper machine clothing costs by pushing fabric life – not simply extending life, but doing so predictably and in increments sufficient to reach scheduled machine downs for fabric changes or maintenance. On these positions, Warp Exchange offers the possibility of weaving with a larger diameter wear yarn at fabric calipers consistent with current SSB products. Fabric life can be safely extended without any increase in fabric caliper and associated couch solids concerns. Results include:
- Running on a Symformer making woodfree uncoated copy grades at 3,600 ft/min, the Warp Exchange product gave a 33% increase in life potential.
- Running on a BelBaie II making newsprint at 4,200 ft/min, the Warp Exchange with larger diameter wear yarn gave 25% life increase with no hit to couch solids.
The inherent lower caliper and void volume of Warp Exchange weaving also lends itself well to board and packaging applications. These machines have been slower in moving to triple-layer designs but are now beginning to do so. Many of these machines have historically had limitations with vacuum systems, cleaning and knock-off showers, and guide sensitivities. Triple-layer designs with thick caliper and high void volumes cannot operate under these conditions; however, Warp Exchange designs are ideally suited for these applications. Vacuum dewatering plays a huge role in these formers, and they typically have more flatboxes and higher vacuum levels than other applications.
Excessive fabric caliper and void volume translate into increased drive loads, higher stock-on-stock-off guiding sensitivity, and lower couch solids. On top of this, sheet knock-off is harder. A thinner and lower void volume triple-layer design has the potential to address all these limitations.
Even on machines not dealing with vacuum and cleaning system limitations, the Warp Exchange offers benefits. Today’s energy costs are having a profound effect on many papermakers. Where the situation on the former can be improved is drive loads and couch solids. Warp Exchange products run very successfully on the full range of board and packaging machines, from the highest load applications at 380 in and faster than 3,000 ft/min to sack kraft machines that are tensile/TEA driven. Results include:
- Running on a fourdrinier making liner at 2,000 ft/min, Warp Exchange products run with a 10% reduction in drive loads over other designs. Life potential is as good or better.
- Running on a fourdrinier (380-in.) making liner at 2,500 ft/min, the Warp Exchange offers excellent stability and drainage, as well as record life potential.
- Running on a fourdrinier making liner at 2,200 ft/min, Warp Exchange products have reduced drive loads and increased machine speed by more than 60 ft/min.
As more board and packaging customers focus on sheet basis weights and test values, there is a growing demand for finer forming fabric papermaking surfaces, but without sacrificing fabric stability and wear potential. Fiber bleed and associated sheet breaks and loss of wet end efficiencies are also driving this trend.
Within the Warp Exchange family of products, the technology has been further enhanced to allow weaving with smaller diameter MD paper side yarns. This second generation Warp Exchange provides high fiber support values and offers the improved retention and formation critical for tensile generation and reduced fiber bleed. This is achieved with a coarse bottom layer to ensure competitive stability and life. The caliper benefits are maintained for reduced drive loads and excellent couch solids. Results include:
- Running on a fourdrinier making liner and sack kraft at 2,000 ft/min, the second generation Warp Exchange product gave 7-9% improvement in TEA on sack kraft grade.
- Running on a Gapformer making liner at 3,100 ft/min, the second generation Warp Exchange reduced drive loads and vacuums levels, while maintaining drainage and speeds. Stability was excellent.
The thin caliper of Warp Exchange weaving has made these products the workhorse of high-speed tissue applications. As these machines now regularly exceed speeds of 6,000 ft/min, drainage and fabric stability are paramount as increased jet impingement into the fabric is necessary under these conditions. Crescent formers are beginning to dominate the tissue market, and here fabric stability and rigidity are critical for good performance. Warp Exchange products are well suited for these applications, as they offer the best combination of drainage, thin caliper for water carry, and stability/durability. Results include:
- Running on a Crescent former making tissue at 6,000 ft/min, the Warp Exchange products have set record life with best sheet tensile and formation/pin holes.
- Running on a C-Wrap making tissue/towel at 5,500 ft/min, Warp Exchange products offer the best combination of drainage for towel grades and fiber support for lightweight tissue.
Summary
Warp Exchange technology has taken triple-layer forming fabric performance to a new level. By starting with a significantly thinner caliper for any given set of yarn diameters and densities, Warp Exchange products have the application flexibility to be designed to best meet the full range of papermaking demands. Benefits include improved drainage and couch solids to lower drive loads, improved mechanical retention and sheet properties, and extended wear potential.
Blake Hender is application manager-Forming, Voith Paper Fabrics.
