Optimized Triple-Layer Fabric Improves Paper Machine Efficiency

A corrugated medium mill achieved improved runnability while reducing costs using a new generation of forming fabric from Weavexx

March 2008
By Pierre Brunet and Jason Goins

This paper describes in detail the runnability improvements obtained at a northeastern corrugated medium paper mill. The benefits were recorded with the help of a new generation of forming fabrics supplied by Weavexx. The Titan HTX patented forming fabric generated cost savings of more than $2.5 million annualized for this mill. The production and accounting departments verified the savings. The original objectives of this efficiency improvement project were to reduce wet-end breaks, reduce drive loads and increase mechanical retention. The results far exceeded the targets set.

Table 1 compares the specifications of the fabrics used on the paper machine. A coarse double-layer has been the standard for many years. The need to improve machine efficiency required a fabric design change.

Break Reduction

The most appreciable benefit was the reduction of wet end breaks by 15 hours per month. Wet end breaks are generally caused by stock mottles around the former. Other sources of wet end breaks are fiber carry back, weaker sheet (tensile) and draws. The fabric positively affected break reduction with the following parameters:

Symmetrical fabric surface and topography: The Titan HTX is a triple-layer fabric using SSB technology (Support Shute Binder). The SSB yarns have a double-function. Not only do they structurally bind the layers, they also increase the fiber support by spending more than half of their length at the surface of the fabric. Also, this weave structure contains more yarns of smaller diameter to equal the permeability of the standard fabric. More yarns in a plain weave design maximize the fiber support, as well as increases the number of uniform drainage holes.

In the formation process, the sheet is set higher on the surface of the Titan HTX. Thus, sheet release off the couch is improved, and the chance of sheet breaks at this point is reduced. A consequence of setting the sheet lower into the fabric is partially torn fibers on the sheet surface, causing a “fuzzy” effect. The press felts can pick up these loose fibers causing numerous problems in the press section leading to breaks.

Another benefit of setting the sheet higher is the reduction of fiber carry back. Fiber-carry back increases the need for frequent washups of the machine return run. This is an obvious risk for sheet breaks or fabric damage. During the Titan HTX trial, the return rolls were cleaner, confirming a reduction in fiber carry back.

Figure 1 is a topographical laser scan that shows a sheet surface comparison of the two designs. The scan is a sheet surface view looking into the fabrics to a maximum depth of 300 microns from the highest elevation point. The darker (deeper) areas of the double-layer offer opportunities for fibers to be embedded, whereas the majority of contact points in the Titan HTX are not deeper than 110 microns from the surface.

Figure 1 - Fabric topography

Cleaner running fabric and easier drainage: The SSB concept in the Titan HTX creates many perpendicular channels across the z-direction of the fabric, Figure 2. Dynamically, it facilitates the drainage and the cleaning of the fabric. The direct open area through the Titan HTX is 15%, compared with less than 3% in the double-layer fabric.

Figure 2 - Drainage/cleaning channel

Figure 3 - z-direction

The machine side open area is also greater in the Titan HTX by more than three percentage points. This creates less restriction as water and fiber are drained through the fabric caliper, Figure 3. The result is less energy required for drainage and a higher cleaning efficiency of the machine’s high-pressure showers.

Stronger sheet, better micro-formation: The squared mesh of the fabric combined with smaller sheet side yarns better controls the drainage velocities. More holes of smaller area reduce high velocities that affect micro-formation. The adjacent fiber-to-fiber links will be stronger due to reduced density variation. This phenomenon is better known as “wire mark”, and is common to commercial printing grades. The wire mark is in fact small density/basis weight variations that have an effect on several sheet qualities. Reducing the density variation at the micro-level improves formation, enhances sheet strength and allows better control of the draws. Figure 4 shows the open area properties of both designs.

Figure 4 - Open area

Enegry Savings

The power required to drive the fabric was reduced by 10%. There are two items related to that reduction: structural stiffness and yarn type.

Structural stiffness of the Titan HTX: In general, the MD stiffness of a fabric determines how much it will deflect into the slots or holes of a vacuum-assisted element. The more rigid the fabric, the less it will penetrate into the openings. Reduced deflection equals lower machine horse power at a given speed. Fabric stiffness is measured with the Taber Stiffness Tester. Table 1 shows a 326% increase in MD stiffness over the standard fabric. This large increase in MD stiffness was the main contributor to the reduced drive loads.

Less nylon yarns in the Titan HTX fabric: The Titan HTX fabric was supplied without wear resistant yarns because it has a natural wear volume that is superior to the coarse double-layer fabric. The machine side CMD yarns are 100% polyester, as opposed to the standard fabric that has alternating polyester/nylon CMD yarns.

Speed and Tonnage Increase

The machine sped up an average of 200 ft/min during the fabric life (127 days). That represented an increase of 23 net tons/day of production. The speed increase is related to the following items:

Better drainage – Less sheet sealing: On a long forming table, the drainage is divided into specific zones (initial drainage, pre-mat formation, turbulence/web consolidation, dryness by vacuum drainage). Each zone must accomplish the proper level of drainage to optimize the physical properties of the sheet and wet end efficiency. The higher fiber support and contact points of the Titan HTX slowed down the early initial drainage (which can sometimes be associated to “sheet sealing”). The fiber mat formed gradually, thus enhancing the sheet structure for better transfer to the presses.

Also, the straight-through drainage characteristics, as previously described, increased the sheet dryness by 1% over the vacuum-assisted elements.The fabric parameters that have the most effect on drainage are stated in Table 2.

Considering the equal permeability of the two designs, the difference in drainage performance lies in the surface of the fabric. Figure 5 shows a three dimensional representation of the Titan HTX. The coplanar design is a plain weave structure (each yarn alternating up and down) maximizing these drainage parameters.

Sheet properties maintained at higher speeds: On kraft and board grades, machine speeds may be limited to achieve the required sheet properties. The Titan HTX has allowed other mills to meet these properties at higher speeds. At this mill, Concora is a critical sheet property. During the trial, required Concora values were obtained at higher machine speeds.

Figure 5 - Titan HTX 3D representation

Pierre Brunet is an application specialist, Weavexx, Warwick, QC; Jason Goins is an application engineer, Weavexx, Youngsville NC.