Traditional forming fabric structures involve a series of design compromises in which fabric life, fabric stability, sheet formation, fiber retention, and fabric drive-load are traded off, one for another, to achieve a specific end result.
This article describes a new approach by Cristini Design Engineers, in which all these properties can be maximized without coincidental negative trade-offs.
Beginning with the paper making surface, all yarns should be identical in size for the most uniform paper surface possible, but this is usually not the case. This new family of fabrics is the result of a technology that begins with this as a premise. These same-sized yarns are then spaced in such a manner that the drainage holes are square in shape, accommodating fibers in all possible orientations, to maximize retention, thereby allowing for reduced headbox consistency, a squarer sheet, and consequent better formation. In the past, structures like these were thought not possible, because of fabric manufacturing considerations and the need for various mechanical properties in the fabric.
Historically, this type of embodiment would automatically result in loss of overall fabric life, but, in the case of these new designs, a binding yarn is incorporated which is simultaneously in the plane of the papermaking surface to enhance FSI, and conversely out of the wear plane on the machine side of the fabric.
As will be shown in the paper, this then allows for a larger diameter wear surface yarn opposite the papermaking side of the fabric. These larger yarns can be made from a super-tough material that maximizes abrasive wear while sliding easily over stationary elements.
A little addressed cause of total energy usage is the extra drag load created as large diameter wear yarns pass over stationary elements, ie, the yarns used to increase life simultaneously increase the drag load and thereby require a larger horsepower demand. This obstacle has been significantly impacted by the usage of yarn materials which have inherently lower coefficients of friction and a weave pattern with low contact area to further reduce drag load and, therefore, drive amperage.
This is accomplished by reducing the "warp coverage" on the back side of the fabric. Warp coverage is the sum of the number of yarns X the size of the yarns. We reduced the number of back side yarns and alternated a smaller yarn with the larger wear absorbing yarns.
FABRIC DESIGN CONSIDERATIONS
It should be intuitively obvious that the forming fabric upon which a sheet of paper requiring a high level of smoothness or a surface upon which to print, should be as monoplanar as possible. As the fibers mold themselves around the yarns which comprise the surface of the fabric, a certain amount of non-uniformity is introduced into the sheet because of the cylindrical shape of the component yarns. Larger diameter machine direction yarns contribute to stretch resistance, but disrupt the smoothness of the mat of paper fibers which are laid upon them. Similarly, larger diameter CMD yarns will reduce the manufacturing cost of the fabric and contribute to wear resistance, stability and drainage, but also disrupt the planarity of the sheet. In typical structures, one of these considerations (MD vs. CMD yarn size) will win out, and a fabric is produced in which certain properties will be maximized and others sacrificed. The first main difference in the forming fabric structures described in this paper, is that yarns in both directions are of equal or nearly equal size to eliminate trade-offs in the papermaking surface. Figure 1 is a visual representation of a fabric Design Matrix, which highlights, on a scale of 0-100 per property, the relative benefits of the best of modern multi-layered forming designs. This will allow a new type of fabric design to be compared mathematically to existing types of structures.
Forming Fabric Design Matrix
Fig. 1 is the Fabric Design Matrix which enables one to give each fabric type a cumulative "score".
Recognizing that not all applications require the maximum relative score for each fabric property, there are times when some features are simply not that important. In most cases, however, "more is better". If the major cause for fabric removal is accidental damage, for instance, then properties like stability and life potential may not be paramount at that given time, recognizing, though, that when causes for those damages are addressed, then life potential moves up in its relative importance.
The key point is in recognizing that sacrifices need not be made unless extraordinary circumstances exist. There is an intrinsic benefit in supplying the best possible fabric in all regards possible to take advantage of benefits in as many areas as possible, whether problems are known to exist in those areas or not.
MONOPLANARITY AND SQUARE DRAINAGE HOLES
Starting with the aforementioned design consideration of same sized yarns on the sheet side, it also makes sense to weave the papermaking surface in a plain weave. For those not familiar with the art, a plain weave structure allows each MD yarn to pass over one CMD yarn, before going under one CMD yarn to complete the repeat pattern. This puts support points or "knuckles" as close as possible to one another, compared with other, more complex weaves. Using same or virtually the same sized yarns in both directions of the paper forming surface and weaving them with close to identical spacing per linear unit of length or width, produces a surface that maximizes the property of monoplanarity, while creating square-shaped drainage holes.
It may not seem immediately obvious why square holes represent an advantage. Because of the pressure drop that occurs when the sheet is extruded from the headbox, paper fibers are preferentially aligned in the machine direction. This may be mitigated to a degree by adjusting the jet-to-wire ratio, but the fact remains that there will be a predominant fiber orientation within the formed sheet. When drainage holes are longer in one direction than the other, the tendency for preferential fiber orientation can be exaggerated because non-retained fibers will fall through the "long side" of the hole preferentially. Square holes give the maximum sheet "squareness" effect, as it relates to that portion of MD/CD tensile ratio impacted by forming fabric design.
Additionally, squarer holes allow better cleaning and a more uniform sheet trim off the wire.
"Balanced" Sheet Side Surface
Fig. 2 highlights the same sized MD and CMD yarns arranged in a plain weave on the papermaking surface. Note the high level on monoplanarity.
There are exceptions, however, when maximum drainage and sheet monoplanarity are not desired features. If the sheet drainage needs to be retarded on heavyweight sheets, for instance, to prevent sheet sealing on the wire, the weave pattern on the sheet side can be modified to a weave with longer "floats" which will effectively slow the rate of drainage. Similarly, on tissue sheets maximum bulk is a desired feature, so smoothness features are disrupted by using MD and CMD yarns of differing diameters and weave patterns. This results in a fabric surface with pronounced "hills and valleys" to induce non-monoplanarity and a resulting higher sheet caliper.
"Unbalanced" Sheet Surface Provides Texture and Bulk
Fig. 3. An "unbalanced" weave on the sheet side induces texture and additional caliper to the sheet.
Figure 4 shows how these differing weave patterns manifest themselves in the sheet surface. The top picture is a low light image of the surface of a sheet made on a standard 2.5 layered forming fabric. The bottom picture is of a sheet made with the plain weave surface with same sized yarns in both the MD and CMD.
Weave Pattern Comparison<
Figure 4. "Standard" 2.5-layer surface (top) vs. pain weave surface (bottom)
VOID VOLUME, DRAINAGE, AND RETENTION
Until recently, the premier line of Sheet Support Binder (SSB) forming fabrics described in the literature required that they be woven on 24 or 20 harness (shafts) looms. In the last year fabric designers in Italy have developed a unique weave pattern which repeats on only 16 harnesses. This results in a much denser fabric with surprisingly low void volume. That water which resides inside the fabric, has momentum of rest until it passes over a dewatering element. The less water contained in the fabric, the lower the energy required to move water from the sheet into the fabric and subsequently, out the back side.
Internal Voids in "Loosely" Vs. "Tightly" Woven Fabrics
Figure 5. There is a high level of internal void volume in 20 or 24 shaft weaves vs. 16 shaft weaves
The Effect of Void Volume on Drainage Vector
Figure 6. High void volume SSB designs (red) will retard drainage vs. 16 harness version (blue). Drainage resultant shown by large blue arrow. The more downward the drainage vector, the more water is removed from the sheet and fabric.
UNIFORMITY OF DRAINAGE CHANNELS
The advent of modeling tools such as Computerized Tomography (CT) Scans can give volumetric measurements of the fabric structure and how those relate to the volume of water flow allowable, the size and number of the drainage channels, and how the uniformity of those drainage channels has changed as we develop new structures. This tool also allows us to examine why designs of the recent past exhibit certain deficiencies. Since these images can be manipulated in 3 dimensions, it is easy to measure the uniformity of the fabric's topography and the resulting imprint it will make upon the sheet. Similarly, it is not sufficient to know only the drainage capacity and rate of the fabric structure, but also its uniformity. If the structure is composed of few large drainage areas vs. many small areas for water flow, then water must move larger distances to get through the fabric, carrying with it a load of fines and fillers. If the object is to create a uniform distribution of fines in all 3 directions in the sheet, it is critical to develop structures which provide many small drainage areas, vs. fewer large drainage holes, as is predicted by Darcy's Equations.
Drainage Hole Uniformity as Measured by Computerized Tomography (CT)
Figure 7. This shows a comparison of the many, closer spaced drainage holes and super smooth sheet side topography of the 16-harness weave, compared to the older SSB style of forming fabric which has been state of the art for the last 5 years. The more infrequent, larger holes in older SSB structures induce the migration of fines and fillers toward the holes, while forcing water to a longer path length before exiting the sheet.
Forming and retaining the fibers delivered from the headbox is, after all, the primary purpose of a forming fabric. Higher retention values can allow lower headbox consistency and therefore improved formation. So, in addition to the relatively minor advantages relating to machine cleanliness, improved retention can have a direct impact on sheet appearance, smoothness, and quality. High FSI numbers can be reinforced by forming the sheet as high as possible from the central plane of the fabric. Keeping the fibers up and level on the surface of the fabric creates a dense smooth mat, with wire side as similar as possible to the top side of the sheet, minimizing two-sidedness.
Figure 8. The combination of a highly planar surface and uniform drainage holes to maximize retention potential.
WEAR RESISTANCE AND DRAG LOAD REDUCTION
It is not just the sheet side and center of these styles of fabrics that have been re-engineered. The primary function of the back side of the forming fabric is to resist the abrasive wear of stationary elements while not constraining the flow of water through the structure. This will always be the case, but in previous fabric embodiments, the large diameter yarns used to resist wear have the negative side effect of increasing drag load from the coarseness of the pattern of those same yarns. This new structure has been developed in a pattern which, although it uses large diameter wear yarns (Fig. 9), has placed these yarns in a more widely spaced pattern which reduces the contact area and results in lower drag loads.
Back Side View of the 16 Harness Construction
Figure 9. The back side of these fabrics have a special lacing pattern and are made of modifiable components
The pattern and spacing of these yarns allow the fabric to "skate" over stationary foil and vacuum units. On multiple applications, drive load reductions of up to 7% have been noted. On typical high speed machines of average width, a 7% reduction in total drive load (the combination of couch and wire turning roll amperage) will result in annual savings of over $100,000.
- A newly designed weave pattern has been developed, capable of repeating on only 16 harnesses, which produces a significantly reduced void volume for higher drainage rates.
- This weave pattern displays high Fiber Support Index numbers without a concurrent reduction in permeability.
- CT Scans show the fabric to have more frequent and more uniform drainage holes for higher levels of retention.
- Differing machine-side materials can be utilized to reduce drag load, improve guiding, and retard wear.