The basics: what you need to know about forming fabrics.

Editor's Note: This is the first of three articles on the "basics" of machine clothing operation. The next two articles, to be published in later editions of Solutions!, will focus on dryer and press fabrics, respectively.

The primary functions of papermaking forming fabrics are to permit water removed from the sheet to flow through the fabric; support, retain, and form the sheet; and to convey the sheet from the headbox to the press section. The top surface of the forming fabric acts as a filter cloth to create a base for fibers to be deposited to form a fiber mat. Geometry of the forming fabric surface contributes to sheet properties, including wire mark, linting, and sheet smoothness. Better support improves fiber mat quality and retention of fines, fillers, and fibers on the supported side of the mat, while reducing sheet two-sidedness.

The bottom side of forming fabrics contributes conveyor belt characteristics. Most life-reducing wear occurs on the bottom side of the fabric since it contacts wear-producing elements, such as rolls, foils, and flat box covers. Machines with high drag loads require heavy duty fabrics to withstand stretching forces and wear over forming boards, foils, vacuum equipment and rolls. Drag, fabric wear, and life are all related. Mechanically, forming fabrics must have:

* Good wear resistance

* Resistance to stretching, narrowing, skew, puckering, ridging, and wrinkling

* Good ability to guide

* Good capability to be driven

* Resistance to high pressure cleaning showers

* Ability to be cleaned and have the sheet knocked off

Compromises in forming fabric design are made to best meet requirements for each forming section position. Figure 1 shows a single-layer, very open fabric design that would maximize drainage. Figure 2 shows a fabric designed to provide maximum sheet support. Figure 3 shows a fabric with a very coarse yarn to maximize sheet transport and achieve long life.

Papermaking goals for forming fabrics include:

* Good first pass retention

* Good formation

* Low drive power consumption

* Low linting

* Minimal sheet two-sidedness

* Good strength properties

* Proper top surface to achieve desired paper properties

Forming fabrics are woven on textile-type looms using polyester and polyamide yarns. Typical yarn diameters for forming fabrics are 0.11 to 0.50 mm. Forming fabrics can be woven flat and joined (seamed) to make an endless fabric for use on paper machines or woven endless. Most current fabrics are woven flat and seamed.

For flat woven fabrics, warp direction on the weaving loom becomes machine direction (MD) on the paper machine and filling direction on the weaving loom becomes the cross machine direction (CD) on the paper machine. For fabrics woven endless, the warp direction on the loom becomes the CD on the paper machine and the filling direction becomes the MD. CD yarns normally are the wear yarns and MD yarns are load bearing on the paper machine. Higher modulus yarns are used in the MD to reduce fabric stretching on the paper machine.

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Cross machine direction yarns are called "shute" yarns during weaving of seamed fabrics. The term shute comes from shooting the yarn across the weaving loom with a shuttle. A textile industry term for the filling yarn is "weft." Mesh and count characterize forming fabrics. See terminology in Table 1 below.

TABLE 1 Terminology for Seamed Fabrics

* Warp yarns: MD direction
* Weft or shute yarns: CD direction
* Mesh Number of yarns per inch in MD direction
* Count or knock: Number of yarns per inch in CD direction
* Shed or harness: Number of yarns before the weaving pattern repeats

Figure 4 is an illustration of mesh and count. Figure 5 is an illustration of shed in the cross section of a double-layer fabric.

DESIGN IS CRITICAL

Since forming fabrics have a greater effect on final paper properties than press or dryer fabrics, design and manufacturing are critical to paper quality and paper machine performance Basic forming fabric designs are single-layer, double-layer, and triple-layer. Single-layer fabrics are a compromise between good papermaking characteristics and long life. Fine yarns are needed to achieve good quality but are not compatible with good conveying and long life. Single-layer fabrics have one layer of warp yarns and one layer of weft yarns.

Single-layer designs are characterized by long machine direction oriented holes. These holes allow the embedment and loss of relatively long fibers as the initial fiber web is formed, producing a rough fines-poor surface. Production of finer fabrics to reduce these deficiencies results in stability and fabric life losses. Some pictures of single-layer weave patterns are shown in Figures 6, 7, and 8. Figure 9 shows a schematic of a single-layer fabric.

Double-layer fabrics provide two sets of surface characteristics. The top layer is made with smaller diameter yarns to achieve good papermaking characteristics. Larger diameter yarns are used in the bottom layer to provide good wear resistance and longer life. Top layers can migrate to the bottom surface and wear. Double-layer fabrics are more difficult to clean than single-layer fabrics. Advantages over single-layer fabrics include smoother sheet surfaces and better printability. Double-layer fabrics have one layer of warp yarns and two layers of filling yarns.

Double-layer fabrics have machine direction oriented holes similar to single-layer designs. Fiber embedment and fines loss occur. Double-layer weaves provide ability to produce finer weaves without severe losses in stability and life. Extra support double-layer designs are an extension of double-layer developments. Addition of extra strands on the paper surface enhance fiber support and fines retention. Improved paper surface is obtained without compromising stability or life potential. Some pictures of double-layer weave patterns are shown in Figures 10, 11, and 12. Figures 13 and 14 show schematics of double-layer and extra strand double-layer fabrics.

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Triple-layer fabrics have two independent layers. The sheet side has fine yarns for good papermaking characteristics. The fine mesh top surface supports the sheet and increases first pass retention. Large diameter yarns are used on the bottom side to achieve good fabric life. The bottom layer provides resistance to stretch, good cross machine stability, and reduced drive horsepower. The top and bottom layers are stitched together with a binder yarn in the middle. Triple-layer fabrics are easier to clean than double-layer fabrics, but cost about 75% more than single-layer fabrics.

Triple-layer fabrics provide the most uniform holes. The combination of a fine paper surface and relatively coarse wear surface results in excellent sheet surface and stable high life potential. Some triple-layer weave patterns are shown in Figure 15. A schematic of a triple-layer fabric is shown in Figure 16.

Fabric design tools include mesh and count, weave patterns, and yarn diameter and type. Some fabric properties that design application engineers change to optimize forming fabric performance include:

* Total drainage area on the paper side of the fabric is one of the best ways to characterize fabrics. It is expressed as a percentage of the available area.

* Total CD support length expressed in inches per square inch is also considered to be an excellent way to characterize fabrics.

* Open area relates to water handling potential and can be changed by changing yarn diameters and spacing.

* Air permeability is a good indicator of repeatability of manufacturing from fabric to fabric of the same design and is relevant to shower penetration and cleaning.

* Frame length of holes relates to the length of fibers that will be retained.

* Void volume and distribution relates to the amount of water the fabric can handle.

* Plane difference is used to define the difference in height of MD and CD knuckles and relates to wire mark.

* Running attitude relates to MD or CD yarn knuckles dominant on the sheet side, wire mark and fiber support.

* Support points relate to the interface between the top of the fabric and the sheet.

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* Fiber support index is the average number of support points on the fabric surface per unit of fiber length. However, there is low correlation between FSI and formation.

* Drainage index is a calculation to estimate fabric drainage performance. However, there is low correlation between drainage index and actual drainage on paper machines.

* Fabric stiffness is directly proportional to yarn diameter--the bigger the better. However, large yarn diameters compromise papermaking properties.

* Wear resistance is directly proportional to CD yarn diameter on the machine side. Yarn material used and crimp of the bottom CD yarns are also important in fabric life. Fabrics should normally be removed when one-half of the bottom cross direction strand is worn away. Fabric weave may require use of a different wear target. Figure 17 illustrates wear volume.

* Sheet marking is a function of fiber embedment the fabric will permit. It is characterized by CD support of the fabric.

Forming fabric optimization for each position includes many factors, including forming fabric cost, operating life, final sheet quality, forming section configuration, vacuum application, showering, and damage potential, among others

REFERENCES

1. Forming Fabric Design and Optimization, C. Bongers and A. Perfect, TAPPI Wet End Operations Short Course.

2. Forming Fabrics: Practical Applications Considerations, R. Danby and M. Conden, TAPPI Wet End Operations Short Course.

3. All figures in this article are courtesy of Albany International and AstenJohnson.

ABOUT THE AUTHOR

Dick Reese has worked on paper machines for more than 40 years in various production and engineering/technical support roles. He has been a TAPPI member since 1973 and is an active member of the Papermaking and Water Removal committees. He was named a TAPPI Fellow in 1992, received the Manufacturing Division Technical Award in 1997, and the Engineering Division Technical Award in 1999. He is currently an independent papermaking consultant in Norcross, Georgia, USA. He can be reached by phone at [phone] or email at [e-mail].

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WHAT YOU WILL LEARN

* The primary functions of forming fabrics.

* How forming fabrics are made.

* Characteristics of single, double and triple layer fabrics.

* How design application engineers optimize forming fabric performance.

ADDITIONAL RESOURCES

* "Machine clothing strategies keep wraps on cost," Jan Bottiglieri, Solutions!, August 2004. To access this article, go to www.tappi.org and type in the following Product Code in the search field: 04AUGSO26. Or call 800 332-8686 (US), 800 446-9431 (Canada), [phone] (Worldwide)

* "Optimizing PMC operations, Jan Bottiglieri, Solutions!, October 2004. Product Code: 04OCTSO25

* The Machine Clothing Squeeze CD-ROM (TAPPI). Product Code: SQUEEZECD-04, ISBN: 1595100652, Member Price: $55.00, Non-Member Price: $80.00. The Machine Clothing Squeeze CD-ROM is packed with the latest information on machine clothing. It is a custom mixed, searchable CD-ROM containing more than 40 documents related to machine clothing from Solutions!, TAPPI JOURNAL, Conference Proceedings, and Technical Information Papers (TIPs).