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 0.10 to 0.60 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. 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 or rapier. 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 Shaft: Number of yarns before the weaving pattern repeats
• Binder: Strand in a triple layer which holds together the top and bottom layers

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.

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 (SL series), double layer (DL and DAL series), and triple layer (SSB series). Single layer fabrics are a compromise between good papermaking characteristics and long life for low speed Kraft & Packaging Grades machine or some high speed Tissue Grades machine. 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.

Adding extra fine cross-directional (CD) support yarns on paper side to a single layer structure creates a 1.5 layer structure (SLA series), which is commonly used in low- to medium-speed Kraft & Packaging Grades and Publication Grades paper machines at speeds up to 500 m/min. This proprietary design provides better fiber retention and sheet support, good drainage, and improved sheet release than single layer design.

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 (also known as 2.5 layer, DLA series) are an extension of double layer developments. Addition of extra yarns on the paper surface enhance fiber support and fines retention. Improved paper surface is obtained without compromising stability or life potential. Currently, double layer (DL series) and 2.5 layer (DLA series) are mostly used in paper machines with special requirements (such as certain Gap Former that are very sensitive to fabric thickness) or producing Publication Grades, Kraft & Packaging Grades, and Tissue Grades on some low- to medium-speed paper machines when considering costs.

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.

Triple layer fabrics have two independent layers. The paper 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.

The currently most advanced design of triple layer is SSB (Sheet Support Binder) which using independent top and bottom layers bound by dedicated binding yarns. SSB fabrics are essential for high-speed machines (>800 m/min), offering maximum fiber support and minimal wire marking for premium kraft & packaging grades and publication grades.

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.
• 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.

	Figure 15: Top illustration- 2/5 shed SSB with 2:1 CD ratio. Middle illustration- 2/10 shed SSB with 3:2 CD ratio. Bottom illustration- SSB with MD binder.

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, AstenJohnson, Huachen and Jinni.

The physical layout of the forming section fundamentally determines drainage mechanics. Whether the machine is a Fourdrinier, hybrid former, or gap former changes how water is removed from the sheet. Table length, foil arrangement, foil angles, spacing, and loading determine the intensity and frequency of pressure pulses applied to the stock. Suction box positions, slot widths, and vacuum levels further influence drainage rate and sheet consolidation.

Fabric design parameters such as caliper, void volume, permeability, weaving pattern structure, and yarn diameter must be matched to this drainage environment. If the fabric is too open for the available vacuum and foil configuration, excessive drainage may occur in the early forming zone, potentially leading to sheet sealing and poor formation. Conversely, insufficient permeability can restrict drainage, limiting machine speed or increasing vacuum demand.

Paper grade characteristics strongly influence fabric structure. Basis weight range affects support requirements, particularly for lightweight grades where fiber support and marking control are critical. Furnish composition- including hardwood, softwood, recycled fibers, and filler content- determines drainage behavior, fines retention sensitivity, and abrasion potential.

Publication Grades and Specialty Grades typically demand improved fiber support and surface smoothness control, requiring finer top-layer designs in multilayer fabrics. Kraft & Packaging Grades may tolerate more open structures but require higher mechanical durability due to higher filler loading and abrasive conditions. Tissue Grades normally need controlled drainage formation, low fiber carryback and reduced pinholes. Pulp Grades need to be determined based on whether it's a pulp forming or a pulp press to ensure the fabric needs to withstand the high pressure of a two-wire press. In short, the fabric must balance drainage, retention, surface quality and running life according to grade objectives.

Machine speed is a primary design driver. As speed increases, drainage time decreases and sheet stability becomes more sensitive to fabric uniformity and structural integrity. Higher speeds also increase dynamic forces on the fabric, making dimensional stability and seam integrity more critical.

Headbox consistency, white water solids load, and temperature also influence drainage efficiency and contamination tendencies. Elevated filler levels can increase abrasion and plugging risk, requiring appropriate yarn materials and structural design to maintain long-term permeability.

The forming fabric does not operate independently, it functions as part of the drainage system. Vacuum levels applied at suction boxes and couch roll must align with the fabric’s air permeability and void volume. A mismatch between fabric openness and vacuum intensity can result in excessive energy consumption, poor retention, or sheet two-sidedness.

Fabric design must therefore be optimized in relation to the actual vacuum profile rather than theoretical values. Understanding how drainage energy is distributed across the forming section is essential for correct selection.

Fabric permeability must be maintained throughout its operational life. Shower type, pressure, alignment, and chemical cleaning regime all influence cleanability. Machines running recycled furnish or high stickies content are particularly prone to contamination.

Fabric design affects how easily contaminants are released. Yarn diameter, surface topology, and weave structure influence debris accumulation. Selection must consider both drainage performance and long-term cleanability.

Abrasive fillers, foil materials, and tension levels contribute to fabric wear. High filler content or aggressive foil loading increases abrasion, especially on the machine side. Edge wear patterns often reflect alignment or tension distribution issues. The manufacturer must evaluate expected service life and select appropriate polymer materials, yarn reinforcement strategies, and structural geometry to ensure dimensional stability and durability without compromising formation performance

In summary, for manufacturers, forming fabric selection is a data-driven engineering exercise. The more complete and accurate the machine and process information, the more precisely the fabric can be designed to meet operational and quality objectives.

Selection begins with clarity about grade requirements. Formation quality, retention performance, smoothness targets, and porosity specifications all influence fabric choice. Lightweight grades such as Publication Grades, Specialty Grades and Tissue Grades demand excellent fiber support to prevent marking and two-sidedness. Heavier grades such as Kraft & Packaging Grade and Pulp Grades may prioritize drainage capacity and durability.

Without clearly defined performance priorities, fabric selection becomes reactive rather than strategic.

Drainage in the forming section occurs in stages: initial gravity drainage over foils, followed by vacuum-assisted dewatering. Excessively rapid early drainage can cause fines migration and poor formation. Insufficient drainage limits speed and increases energy demand.

Paper makers should evaluate whether current drainage distribution supports uniform sheet consolidation. If dryness targets are not achieved at the couch roll without excessive vacuum, the forming fabric design may require adjustment.

Fabric openness directly affects fines retention and white water consistency. Highly open fabrics enhance drainage but may reduce retention if chemical systems are not optimized. Conversely, very tight fabrics may increase retention but restrict drainage.

Selection should therefore consider the interaction between fabric structure and retention aid chemistry. Changes in furnish composition often require reassessment of fabric design.

As machine speed increases, sheet stability becomes more sensitive to fabric uniformity and MD stability. Multilayer forming fabrics are often used in higher-speed applications to improve fiber support while maintaining drainage capacity.

Dimensional stability is essential to prevent tracking issues and edge wear. Fabric stretch characteristics must be appropriate for machine tension conditions

Permeability loss over time indicates plugging or contamination. Shower alignment and pressure should be periodically verified. Chemical cleaning protocols must be compatible with fabric materials.

If permeability declines rapidly despite proper cleaning, fabric design may not be optimal for the furnish characteristics

Systematic monitoring of wear patterns provides valuable diagnostic information. Uneven cross-machine wear often indicates mechanical alignment issues. Premature machine-side wear may suggest excessive foil loading or abrasive furnish conditions.

Fabric life should be evaluated in terms of total performance contribution rather than simply operational days. A fabric that enables higher speed or improved formation may deliver greater overall value even with similar service life.

Effective forming fabric selection requires open communication between mill and supplier. Sharing vacuum profiles, furnish changes, speed increases, and performance concerns allows more accurate recommendations. When fabric selection is treated as a joint engineering task, long-term performance improves significantly.

Forming fabric selection is fundamentally a technical alignment process between machine conditions, grade objectives, and drainage physics.

For manufacturers, accurate and comprehensive machine data is essential to engineer a fabric that balances drainage, retention, support, and durability.

For paper makers, understanding how fabric structure influences sheet properties enables informed decision-making and stronger collaboration with suppliers.

Because every paper machine operates under unique conditions, forming fabric selection must always be customized- not standardized.

Mesh count (yarns per cm or per inch) and yarn diameter shape how a Forming Fabrics performs- and this is where a skilled Forming Fabric Manufacturer earns their keep.

A fine mesh (high count) with thin yarn for a smooth surface, while a coarse mesh (low count) with big yarn for fast drainage and longer life.

Thicker yarns (0.35-0.60mm) enhance forming wire durability but may reduce fiber support; thinner yarns (0.1-0.30mm) improve FSI but wear faster. Choose based on your grade’s priority (e.g., thin yarns for tissue grades, thick for kraft & packaging grades).

A quality Forming Fabric Supplier will help you test combinations, even offering Customized Forming Fabric designs to match your exact grade and speed. China Forming Fabric manufacturers, for example, Huachen, Jinni, TPY, often tweak mesh specs for local pulp types, ensuring a better fit than generic suppliers.

The choice between single layer, 1.5 layer, double layer and SSB triple layer depends on your mill’s speed, grade, and cost goals.

Forming Fabrics is also traditionally known as PM Wires. Formerly, the Fourdrinier wire was a bronze wire mesh. Plastic forming fabric substitutes became available in the 1960s, and although more expensive, have largely displaced bronze wires on modern PMs. Nevertheless, the word "wire" remains in use as a general term for both types. The longer life of fabrics means less downtime for wire changing and thus extra production.

In each case, the desired properties are built into the individual layers. They thus aim to reconcile the conflicting demands of: drainage, retention, paper quality (eg: lack of visible wire marking), web support and release, drive energy, fabric stability, ease of cleaning, and long life. The most influential fabric properties on paper structure and PM performance are: air permeability, mesh count, modulus, fiber support index, drainage index, caliper, percent open area, and void volume.

Permeability (initial gravity drainage) and Caliper (support layer) work in tandem, but striking the right balance takes expertise- something top Forming Fabric Suppliers excel at.

A thick caliper stabilizes the web but slows drainage, a thin caliper speeds drainage but risks sagging. Publication Grades paper mills need a happy medium, while recycled paper mills prioritize permeability to flush contaminants.

Here’s where Customized Forming Fabric makes a difference: A Forming Fabric Manufacturer can adjust wire caliper and pore size for your mill’s unique pulp. China Forming Fabric experts, familiar with diverse regional water chemistry, often recommend tweaks that generic fabrics miss- like slightly thicker caliper for mills with high mineral content in their water.

Fiber Support Index (FSI), as adopted by the paper industry, was originally developed by Beran for use when running single layer fabrics. It is a number that takes into account the support length of the surface of the yarns on which the sheet of paper is formed. Beran also recognized that the cross machine direction support lengths were preferable to machine direction support lengths and, therefore, gave them a double weighting.

Experience has shown that this same Beran FSI can be used to compare the support characteristics of single layer, 1.5 layer, double layer, 2.5 layer and SSB triple-layer fabric structures with good results.

FSI is affected by the weave pattern and mesh of the surface of the fabric on which the sheet is formed but not the yarn diameters. It does not, however, give any indication of the uniformity of support lengths in either direction.

FSI is a calculation used to evaluate fabric design for adequate fiber support (The formula is as follows). The K value is a constant which describes the fiber angle distribution. The a and b coefficients used in this calculation are unique to each fabric style and running attitude. They are based on mesh and count and are derived from a two dimensional model.

For single layer products, the equation is a relatively good estimation of the initial interaction between the fiber web and the forming fabric. This equation does not apply to complex multi-layer structures because it assumes the K value is a constant and does not account for differences in furnish. In addition, the coefficients a and b do not account for secondary fiber support or strand width. Finally, the FSI equation does not provide for the orientation of the fiber support, the distribution of the support, or the size of the openings.

Drainage Index (DI) is a calculated figure which utilizes the same Beran CD coefficient and air permeability. It does not account for any MD contributions because it is believed that the CD support is primarily responsible for controlling the degree of fiber embedment. The use of air permeability as a variable in the Drainage Index calculation is an attempt to describe the initial flow resistance of a forming fabric. The Drainage Index provides a relative comparison between fabrics of a similar design but is not indicative of actual differences in flow observed on paper machines. No individual property of a forming fabric can accurately predict how it will perform on a paper machine. These properties should be used as an aid in understanding and comparing fabrics of the same. structure. In addition, many of the properties are used as a quality check for consistency within a product and as a control parameter over time.

Fiber Support Index (FSI)= K/(K+1)*(aN_m+2bN_c)
Drainage Index (DI)=(bP_aN_c)/1000
K = Fiber angle distribution constant
a = MD support coefficient
b = CD support coefficient
N_c = Number of CD yanrs
N_m = Number of MD yarns
P_a = Fabric air permeability

The percent (%) top surface open area indicates the total open area of the combined drainage holes (orifice) in the top surface of a fabric. The percent open area is calculated using a plan view of the top surface of the fabric and subtracting the area taken up by the yarns from the total area, leaving the area of openness that is usually expressed as a percentage of the total area.

Experience has shown that in multi-layer constructions (double layer, 2.5 layer and triple layer), this figure gives a much better indication of the drainage potential of a fabric than CFM that can be dramatically affected by the center and bottom plains in a multi-layer construction.

Void volume refers to the space inside a fabric that is not occupied by the woven material. It affects rewetting of the sheet after the dry line, amount of water carry back and the amount of water needed for the flooded nip shower.

Forming fabrics are the backbone of the papermaking process, but when they lose shape due to lengthwise elongation or widthwise shrinkage, the consequences are significant: sheet defects, increased downtime, and higher costs. Dimensional stability is a critical quality parameter that impacts both paper makers and forming fabric manufacturers. In this blog, we explore how paper makers can optimize their operations and how manufacturers can design stable fabrics, fostering collaboration to enhance fabric life and paper quality.

• Optimize Weaving Tension: Proper tension or crimp percentage during weaving is the foundation of dimensional stability, particularly for SSB (sheet support binder) fabrics. The top plain weave structure typically requires double the crimp of the bottom twill structure to ensure stability. Precise tension settings during weaving create a balanced structure that resists elongation under machine tension (4-7 kg/cm).
• Perfect the Heat-setting Recipe: Effective heat setting relies on the 3T principle: Temperature, Tension, and Time. Maintain a temperature of 190°C, optimal machine direction (MD) and cross direction (CD) tension, and controlled cooking time (number of rounds with gradual temperature increases). This process stabilizes the fabric structure, minimizing elongation and shrinkage under operational stresses.
• Achieve Optimum Modulus: With proper weaving and heat-setting settings, the fabric’s Modulus (500 / elongation % at 50 N/cm) is optimized, indicating how much the fabric will elongate during operation. A higher Modulus reduces elongation percentage, but an excessively high Modulus can flatten warp crimp, weakening seam strength. Balance Modulus to ensure durability without compromising other fabric properties.
• Determine Optimal Supply Length: Designing the correct supply length is a critical challenge. Obtain precise minimum and maximum paper machine length and stretch margin from the customer. Use these data alongside the fabric’s Modulus and growth (elongation in the first 3-7 days) to calculate the actual supply length, which may vary from the order length, ensuring the fabric runs stably and meets machine requirements.
• Account for Width Contraction: Forming fabrics experience width shrinkage under MD tension, so manufacturers must study and standardize the shrinkage percentage for each design. Adjust this as an allowance during fabrication to ensure the final width meets customer requirements under operational tension, preventing shrinkage-related issues on the paper machine.
• Deliver Customized Solutions: Forming fabrics are tailor-made products, designed to meet the specific requirements of each paper machine. Avoid generalized designs by collaborating with paper makers to understand their machine’s tension, stretch margin, and operating conditions, ensuring a fabric that delivers optimal stability and performance.

• Verify Stretch Margin to Manage Elongation Modern: High-speed paper machines typically accommodate 2-3% stretch, often equipped with automatic stretch rolls, while older machines have less than 1% stretch margin, making them more susceptible to elongation issues. Ensure your machine has at least 1% stretch margin to handle the typical 1% elongation of forming fabrics over their lifetime. If your machine lacks sufficient margin, consider two options: install mechanisms to accommodate elongation or use shorter fabrics with tighter mounting. There are no shortcuts here- verify your machine’s capacity with your supplier to prevent issues.
• Optimize Tension and Drag Load: Maintain fabric tension within the recommended range of 4-7 kg/cm, noting that most faster machines operate at 7 kg/cm. Keep tension slightly below the machine’s drag load to avoid excessive elongation or width shrinkage. Over-tensioning, especially on machines without auto stretch rolls, can exacerbate shrinkage. Work closely with your supplier to fine-tune tension settings and control drag load, ensuring better stability and consistent performance.
• Understand Synthetic Fabrics and Modulus: Unlike rigid metal wires, which offered no elongation but only one-fourth the lifespan, modern synthetic fabrics are flexible and prone to elongation. To choose the right fabric, ask your supplier for the fabric’s Modulus value- a higher Modulus indicates greater resistance to elongation. Compare Modulus values across suppliers, alongside other parameters like weave structure and material, to select the best fabric for your machine’s conditions.