The press section is the next part of the paper machine after the forming section. The most important functions of pressing are to increase sheet solid consistency in order to ensure adequate drying capacity, to consolidate the sheet, and enable web runnabiljty in the early dryer sections. Increased sheet consistency increases wet strength and improves sheet consolidation and fiber-to-fiber bonding which increases sheet strength. This typically improves runnability and reduces press to dryer draw. Pressing can also have a significant impact on quality parameters such as smoothness, ink absorption, bulk, and moisture profile. The fabrics in this section are called press felts or press fabrics. The term "fabric" has a broader meaning; "felt" is a type of fabric which is made of individual fibers only, i.e., no yarn in the fabric structure. Nevertheless, the terms "press fabric" and "press felt" are used interchangeably in papermaking. Although it depends on sheet grade and paper machine, typical sheet consistency at the beginning of the press section is 20% fiber and 80% water and at the end of the press section is 45% fiber and 55% water. At the end of the press section, the sheet is transferred to the dryer section.

During pressing, the sheet is compressed between one or two fabrics and either two rolls, or a roll and a mated extended "shoe" in the press nip to squeeze water from inside the web and out of the felt fibers. Figure 1 shows this process in a plain press nip. Increased compression increases water removal.

The main functions of a press fabric are to support and convey the sheet through the press section, press water from the sheet, provide a medium to accept the water, maintain or impart sheet quality properties, and to drive undriven rolls. The fabric should provide proper protection for the sheet to resist crushing, shadow marking and groove marking. The amount of water that the felt can absorb and the water flow resistance are affected by the void volume (volume that is not occupied by fibers or yarns) and air and water permeability of the fabric. Low flow resistance and the ability to maintain void volume under load are important during operation. Important press fabric properties include pressure uniformity, adequate void volume, required permeability, proper compressibility, batt/base ratio, compaction resistance, abrasion resistance, strength, contaminant resistance, heat and chemical resistance.

Many machine variables can significantly impact pressing. A high sheet basis weight increases the water that must be handled by the press. A higher sheet temperature lowers water viscosity and increases sheet solids. High press impulse (kPa.sec, psi.sec), the product of mean nip pressure (kPa, psi) and nip dwell time (usually measured in ms), increases total water removal and press solids. Press impulse is also the quotient of press nip linear load (kN/m) divided by press speed (m/sec), with the unit of kN/m2 x sec = kPa.sec. The type of furnish and amount of refining affect the freeness and dewatering characteristics of the sheet.- For example, higher freeness furnish and/or decreased refining yields higher solids out of the press. For each example, a change in the opposite direction would have the opposite effect.

1. Press Fabric Functions

   Once stock leaves the headbox, the general requirement of the paper machine is to increase the fiber consistency of the sheet from 0.2-1.5% to 92-96%. After the forming fabric, the cost of additional water removal is far less in the press section than the dryer section. Mechanically removing water in the press section by increasing nip pressures is far less costly than consuming energy in the dryer section. Therefore, the value of efficient press fabric performance cannot be overemphasized.

   Water removal is not the only function of the press fabric. In general, the press fabric must:

   1. Accept the water that is expressed from the sheet in the press nip.
   2. Provide the proper protection for the sheet to:
      • Resist sheet crushing (rupturing the sheet due to excessive hydraulic pressures).
      • Resist shadow mark (water flow mark in the sheet caused by the difference in hydraulic pressure between the land area and open area in a vented roll cover).
      • Resist groove mark (water flow mark in the sheet caused by the difference in hydraulic pressure between the land area and open area in a grooved roll cover).
      • Resist base fabric mark (imprint of base fabric).
   3. Present the proper surface to the sheet so that the necessary degree of smoothness or finish requirement is imparted to the grade of paper being manufactured. This is particularly critical on paper or board that is to be printed.
   4. Transport and carry the sheet. In case of closed draws, transfer the sheet from one position to another.
   5. Provide desirable durability in terms of strength, resistance to mechanical abrasion, chemical degradation, and fill-up from contaminants transferred from the sheet.
   6. Drive non-driven rolls in the press section.

2. Construction of Press Fabrics

   Press fabrics, in general, consist of two basic components as shown in Figure 3.5: the base fabric and the batt.

   The Base Fabric

   Press fabrics are made of 100% synthetics, primarily polyamide (nylon) polymers. Base fabrics are constructed with cabled monofilament, single monofilaments (solid or hollow), or plied multifilament yarns (Figure 2). With the increased use of recycled fibers in the paper stock, the use of plied multifilament yarns has greatly been reduced. Multifilament yarns tend to trap contaminants and are therefore more difficult to keep clean.

   The base fabric may have a single-layer construction (one layer of MD yarns and one layer of CD yarns), a woven multilayer construction (multiple layers of MD yarns with only one layer of CD yarns), or a laminated, multiple base construction with multiple layers of both. The advantage of the multiple base construction design is that the base layers can be of different designs. They may vary in yarn count, yarn size, weave pattern, etc. For example, the top base layer can be very fine to impart the desired sheet properties and the bottom layers can be coarser in order to provide the necessary water handling properties. Laminated fabrics allow a wider range of base fabric pressure uniformity and low batt/base ratio, which is critical for open and clean operation. As paper machine speeds increase, the nip residence time decreases and better surface contact between the sheet and press fabric becomes a requirement.

   The fabrics can be endless or joined with a seam. The yarn type selection and weave pattern of the base fabric are engineered to manipulate pressure uniformity, flow resistance, void volume, and compression properties. In practice, the basic classifications of press fabrics are: conventional (endless) designs, stratified (laminated) designs, and seamed fabrics. Figure 3 shows a few of the main types of base fabrics used in press fabrics.

   

   Figure 3   Major Types of Basis Fabric Structure

   It should be noted that single-layer base products are quickly becoming inadequate on most paper machine applications. They simply are not durable enough to withstand the forces generated on modern machines.

   Seamed fabrics have shown a substantial increase in usage since their introduction in the late 1980s. In North America, approximately 75% of all press fabrics used are seamed, and that proportion continues to grow as seam technology has proven suitable for more applications. The use of seamed fabrics in Asia and Europe is less prevalent. The primary factors for the increase in use of seamed products are safety, reduced installation time and, in some cases, improved press fabric performance There are basically two major types of seamed products (Figure 4): conventionally woven base, and laminated, multiaxial base (multiaxial in that the base layers are slightly angled to one another).

   

   Figure 4   Seamed Press Felts