The Forming Section
To start the papermaking process, pulp is diluted and mixed to form a slurry of one part solids to approximately 200 parts water and pumped to the headbox. The mixture is pumped evenly through the slice — which is a narrow slot in the bottom of the headbox — onto the forming fabric. The fabric filters the water and retains the wood fibers to “form” the sheet. This is a critical part of the process because it affects the sheet’s final properties.
This forming section consists of a long moving belt of fine mesh screen supported by rolls or foils. For many decades, this screen belt was made of bronze and called the “wire.” Today it is made of synthetic polymer — primarily polyester monofilament — and is called the forming fabric. Newer machines (gap formers) trap the slurry out of the slice between two forming fabrics. As the forming fabric moves along the forming section, water drainage is assisted by foils. The fiber then passes over the suction boxes to pull more water from the furnish. Still more water is removed at the couch roll just before the sheet leaves the forming section. At this point, the sheet consistency is about 20%.
Dedicated research has produced synthetic forming fabrics that yield improved drainage, higher production and longer life at a lower cost per ton of paper produced.
AstenJohnson’s forming fabrics are available in a wide variety of meshes, depending on the type of paper produced. Coarse mesh fabrics are used to manufacture heavy paper grades such as those used in the production of corrugated boxes. Very fine mesh fabrics are required for lightweight paper, such as writing paper, book paper and magazine paper. The average life span of a forming fabric, depending on the type of paper, is approximately three months.
The Press Section
The sheet is carried into the press section on soft resilient “felts” or, more correctly, press fabrics. Traditionally, press felts were made of 100% wool because wool has the ability to be compressed in the press nip and spring back without being compacted. Press felts were so named because they were manufactured with the filling or “felting” process, in which a loosely woven cloth is densified into a hard fabric as can be seen in felted hats or pool table covers. The fabrics are now referred to as press fabrics because they are produced through needle punching rather than felting.
In the 1950s, synthetics were developed and incorporated in new textile designs with the same resilient properties as wool. Synthetics wear better and therefore run longer than woolen felts. This greatly reduces the downtime necessary to change fabrics, resulting in higher production and lower costs. The average running life of a press fabric is 40 days.
When the sheet is carried through the presses by the fabric, water is squeezed out and into the fabric, further reducing the sheet’s water content. The paper surface finish is further influenced by the press fabric design. As the sheet leaves the press section, it is typically 40% fiber and 60% water, depending on the sheet grade and the paper machine.
It costs more than 10 times as much to remove water by heat in the dryers than it does to squeeze it out mechanically in the presses. Consequently, research activity to improve press section efficiency has been, and still is, at a very high level. Suction presses that pull water from the fabric when it is under compression in the press are common. Grooved presses and blind-drilled rolls, which require no vacuum source, are more recent developments.
All modern presses are based on the principle of providing space or voids for the water to go when it is pressed out of the wet web in order to avoid crushing the sheet. They all improve or lower the water content of the wet web, thereby reducing the load on the dryer section. Newly developed laminated and seamed press fabric designs have shown superior performance on the modern high-load presses now common in paper and board manufacturing.
The Dryer Section
After the maximum amount of water has been removed in the press section, the sheet travels on to the dryer section, where it is dried to its finished condition of 5% water. This drying is accomplished through the supply of heat to the sheet by large steam-heated cylinders.
The primary function of a dryer fabric is to keep the sheet in contact with the dryer cylinders to provide maximum heat transfer from the cylinder to the sheet. If a wet object is placed against a heat source, the water on the surface will heat up. In the case of paper, the evaporation of the heated water creates a positive pressure that tends to lift the sheet off the dryer. This reduction or loss of contact with the dryer will result in poor heat transfer and poor drying efficiency.
A layer of air between the sheet and the dryer also is carried by the moving sheet and dryer cylinder. The dryer fabric presses the sheet onto the dryer, reducing this air layer. Additionally, the fabric minimizes the effect of centrifugal force, which wants to lift the sheet because of the high machine speeds being run today.
The dryer fabric also functions as a drive belt to turn the majority of the dryers and rolls in recent machines, because only a few of the dryers are driven by the machine’s motors.
Dryer fabrics perform all the preceding functions in a very hot and wet environment. The use of select synthetic yarns and weave patterns allows AstenJohnson to supply fabrics that provide long operating life in these adverse conditions. The fabric surface and seams must be smooth to ensure that the fabric does not cause deterioration of the paper surface.
For many applications, the sheet leaving the dryer section is not sufficiently smooth. A calender is used to compress the sheet to give it the smooth surface required for high speed and high quality printing.
The paper is then wound into large reels. These reels are then slitted into rolls of differing widths and diameters, depending on their end use. For many paper grades, these rolls are used by the printer or converter. Other grades require that these rolls of paper be cut into sheets, such as copier or computer printer paper.