Paper and Board Machines
Since the earliest times of industrial papermaking fourdrinier and mould formers have been used for the production of all kinds of paper and board. In the last century the machines started to be more customized to the special requirements of the individual product grade, based on quality, economy and operational require¬ments. When planning today a new machine or a major rebuild requirements have to be defined and decisions made relating to e. g. capacity, furnish, essential prod¬uct properties, frequency of production changes, multi-ply, multi-layer or single-layer production, machine width and speed, machine equipment, etc. The variety of machines and their layouts are mainly related to the five main classes of paper and board grades with their varying requirements and can be defined as:
. • graphic paper machines
. • packaging paper machines
. • board machines
. • tissue machines
. • specialty paper machines
Graphic Paper Machines
The majority of paper machines worldwide produce graphic paper grades on four¬drinier wire sections or on hybrid formers. These kinds of conventional forming section usually satisfy the quality requirements. Their disadvantages are the limita¬tion in machine speed of about 1200 m min–1 and for some paper grades the nonsymmetry in the web z-direction.
Modern paper machines for graphic paper production are – except for tissue machines – the fastest paper machines. They comprise a gap former, a closed-draw press section and a single tier or combined single/double tier dryer section and, in certain cases, a size press or a coating station. The technological progress as well as increased cost pressures have led to in-line multi-nip calendering positioned be¬tween the dryer or coater section and the reeler in SC and LWC papermaking lines.
The paper machine layout and operating conditions differ according to the spe¬cial requirements of the grades produced as regards
. • woodfree paper such as copy paper or base paper for coated paper
. • wood-containing paper such as newsprint and SC or LWC paper grades
• off-line or on-line coating and
• off-line or on-line calendering.
An example of woodfree paper is copy paper. The most important properties are high bulk at a certain roughness, and good formation. Here the whole paper¬making process has to be considered to meet the high bulk requirements of the
6.11 Paper and Board Machines
market. In particular the wet pressing impact has to be limited in favor of a bulky sheet. The trade-off between dry content, speed potential, energy consumption, and paper quality is a matter of continuous research and development activity. As a consequence, single-nip presses have been recently introduced for copy paper pro¬duction. The single nip press employs one shoe nip and is double-felted. This design enables a no-open-draw web run into the dryer section. Despite the lower web dryness the speed limit is currently well above 1400 m min–1. With increasing machine speeds formation quality decreases. Here the application of lamellas in the headbox nozzle can improve the formation quality to a certain extent. The upcoming tendency for hard fine flocs in look-through is counteracted by optimi¬zation of the lamella geometry.
With the lamellas the MD/CD ratio of strength parameters is reduced, which results in higher strength in CD but also lower strength in MD. These papers are in general single-layer products with one paper machine producing copy paper with a three-layer headbox. Today’s speed limit in copy paper production is about 1600 m min–1. The dryer section is a single tier with a double tier part after the size press or the film coating. Double tier is required in order to produce a paper which does not curl in the copy machine. For coating base paper production a single tier dryer section is possible as the non-symmetrical “frozen tension” during drying in a single-tier dryer section is elimi¬nated during coating by wetting.
Machines for wood-containing papers aim for high dry content after the press section. This not only reduces energy consumption in the dryer section but also improves machine runnability due to the resulting higher web strength at the first free draw. High demands regarding surface smoothness such as for SCA paper grades require a press section where both web sides come into contact with a smooth roll surface without a felt in between. High surface smoothness has to be aimed for, starting from forming and followed during pressing. Smoothness gen¬eration by excessive calendering results in blackening, especially when recycled fibers are used and at lower basis weights. The dryer section is single tier for high runnability at maximum machine speeds which are today up to 1950 m min–1.
Optimized fabrics support the efforts of the paper maker towards high quality and economy. E. g. the wire design has to assist high filler retention in the forming section and the design of the press felts has to ensure good surface smoothness and long lifetime at constant quality production and high machine runnability.
Figure 6.81 shows a schematic of a modern paper machine for LWC paper pro¬duction started up in 2004. It includes a gap former wire section, a press section with a double-felted first press nip and a second press nip with felt and transfer belt, a pre-dryer section followed by a soft calender for preliminary calendering, a coating station, an after-dryer section, a ten-roll multi-nip calender and the reeler. A winder follows which converts the large parent rolls into suitable-size rolls.
Packaging Paper Machines
The main products of packaging paper machines are grades which are the base for corrugated board. Corrugated board consists of two outer layers (testliner or kraftli¬ner) which are glued to a corrugated inner layer (corrugating medium or fluting) to obtain high bending stiffness. Testliner and corrugating medium are made from recycled fibers, kraftliner and fluting from virgin fibers. Corrugated containers must safely transport the packed product e. g. from the origin to the super market. Here they inform the customers, and the staff, about the content and its character¬istics, and therefore should allow an appealing printed display on the surface. So the main quality requirements are the box compression resistance (ability to pile the boxes) and good printability. As a result the raw paper must show sufficient strength, measured as SCT (short span compression test) or RCT (ring crush test) for liner and CMT (Concora medium test) for corrugating medium or fluting.
The various applications require a wide basis weight range of about 70 to 450gm–2, to be run on one single machine. In Europe emphasis is put on 70 to 130gm–2. Here the main furnish is recovered paper. By its adequate treatment in stock preparation and by thorough cleaning of the fabrics and rolls in the paper machine good paper quality as well as good machine runnability at high machine speeds are achieved. Machine speeds were 500 to 800 m min–1 in the 1970s and 1980s. Today the maximum operating speed is about 1450 m min–1, with design speeds of up to 1800 m min–1. The driving force for the speed increase is the emphasis on lowered basis weights and low cost production.
High machine speeds require a twin wire gap former to ensure high uniformity of the product and sufficient dewatering capacity. Twin wire gap formers are availa¬ble in different configurations. One is shown in Fig. 6.82 which depicts a state-of-
6.11 Paper and Board Machines
the-art packaging paper machine with the headbox of the gap former in an upper position. Lamellas in the headbox nozzle help to optimize fiber distribution. A two-layer headbox with different fiber fractions in the two layers is an option to in¬crease paper strength properties.
The press section with three nips ensures high dry content of the web on enter¬ing the dryer section. As the paper web is always supported during its transfer from the wire to the dryer section, without any free draw, machine runnability is high, even at the elevated machine speeds and lowered basis weights. For runn¬ability reasons the pre-dryer section is single tier. Safe web run is supported by air¬flow based stabilizers. For high machine speeds a film press replaces the pond size press as the latter starts to encounter unacceptable pond turbulences at speeds above 800 to 1100 m min–1. Surface sizing increases the strength properties of the final sheet significantly. However the web strength in the machine is reduced dramatically by rewetting during sizing. For low basis weights, high moisture pick up at the size press and high speed production an air-float dryer may be used after the film press, followed by single tier and double tier dryer groups.
In reeling one main requirement is a large paper roll diameter, for two main reasons: First to reduce paper loss. A certain loss at each reel is unavoidable, so less reels with larger diameters for the same production reduce the overall loss. Secondly, to allow one single winder to cope with the paper machine output. This can be reached by reducing the nonproductive time for parent roll change in this noncontinuous process step. Today Jumbo rolls with diameters of up to 4.5 m are produced.
There are other packaging products on the market, e. g. white top liner and sack paper. White top liner has a white surface to cope with the high demands for an appealing display. For its production two webs are formed in two separate forming units – one white and one brown layer and then couched together ahead of the press section.
In the case of sack paper high strength and sufficient elongation at rupture in both directions (MD and CD) of the paper are required. These special properties are important for instance during the filling of the sacks. Other applications are all kinds of wrapping. For the highest elongation of this kind of packaging material the “Clupak process” introduced in the 1960s is applied. Here the still wet web is pressed in a nip where a compressible belt runs with the web. The belt deforma¬tion in the nip and the shape regain of the “extensible unit” after the press nip results in micro-creping with up to about 12 % elongation (MD) at rupture. Micro¬creping can also be obtained by passing the web through a nip with a hard and a soft covered roll.
Cartonboard is a product with a wide range of basis weight of about 160 to more than 600 g m–2. Cartonboards have the highest share, about 80 %, of the total pa¬perboard production. Cartonboard is mainly used for production of carton boxes to pack e. g. food, cigarettes, pharmaceuticals or cosmetics. Additionally a wide range of specialty boards exists like artboard, gypsum board, playing-card board, book¬binders board etc. As the requirements for these board grades vary a lot the furnish and the board machine design may differ accordingly. Some of the basic required properties are:
. • Mechanical properties like bending stiffness (bulk, Young’s modulus), to protect the packed goods against damage.
. • Flatness, plybond, creasability, punching, to ensure good runnability in convert¬ing and packaging lines.
. • Resistance against moisture, gas and flavor, to protect the goods from quality changes.
. • Freeness from impurities like micro-organisms, toxic or mutagenous substrates, taint or odor, to protect the goods against contamination.
. • Brightness, gloss, roughness, printability of the surface, to ensure appealing information, identification and promotion of the packed goods.
Paperboard mainly has a multi-ply structure to achieve the different product re¬quirements in the most economical and environmentally friendly way e. g. by ap¬plying various furnishes in the different plies. The surface plies (top and back plies) have to ensure the strength properties (bending stiffness) and, together with the coating, the required surface quality. With the inner plies (undertop and filler plies) the bulk is optimized (affecting bending stiffness). Most cartonboards are on-machine coated and calendered to obtain appealing surface characteristics.
Depending on the product and the geographic location of the board producer, virgin or recycled fibers or both fiber types are used as raw material. In multi-ply production the individual plies are formed on separate forming units and couched together in wet condition. Forming of two layers in one forming unit is also done. Different forming principles as described below have been and are applied either uniquely or in combination with others.
In suction formers 3 to 11 cylinder formers in series with suction chambers produce webs of about 25 to 100 g m–2 each which are couched on a transfer felt, thus building up the baseboard. The maximum speed is about 350 m min–1, the maximum width is about 5 m. Cylinder formers without vacuum application are limited to a maximum speed of about 150 m min–1.
In fourdriniers and hybrid formers each ply is formed on a separate fourdrinier wire. Hybrid formers are used e. g. to produce the filler ply with the widest basis weight range and highest basis weights which require extended dewatering ca¬pacity. The headbox and wire section have to be adapted to the high throughput ratio and to the high surface quality requirements for coating and printing. The maximum speed of a fourdrinier and of hybrid formers is about 1000 to 1200 m min–1.
As the fourdrinier wire sections have sufficient speed potential gap formers are seldom used in cartonboard production. Nevertheless gap former installations can be found for certain applications of specialty board like gypsum board.
6.11 Paper and Board Machines
The press sections of modern machines for cartonboard are designed for high dewatering capacity, high bulk, smoothness and good runnability. This leads to press concepts with typically three press nips, e. g. a first double-felted suction press nip followed by a double felted shoe-press nip and finally an unfelted smoothing press or a single felted roll press to enhance surface smoothness. Be¬tween the nips a closed web run ensures stable sheet transfer, independent of speed and basis weight.
The dryer section consists of up to 100 drying cylinders in the pre-dryer section and about 16 in the after-dryer section. One or two single-tier groups are followed by double-tier drying groups. For good flatness of the finished product the top and the bottom cylinder rows are heated separately to control curl tendency.
Starch or size can be applied in a film press or a pond size press to enhance surface strength and bending stiffness. In this case an after-dryer section is re¬quired for baseboard drying.
Before coating, the roughness of the baseboard has to be reduced by calender¬ing. In modern board machines this is done by a heated hard-nip calender. As an alternative soft-nip calenders can be used for reduced densification. The latest development is the shoe-nip calender which combines long dwell time, low spe¬cific pressure and high roll surface temperature to achieve minimum roughness at lowest densification. In Europe for machines below about 650 m min-1 the use of a MG (machine glazed) cylinder is state of the art for high bulk, low roughness board production. As the entering web dryness (about 60 to 70 %) and exiting (about 70 to 75 %) web dry content is crucial for smoothness and bulk of the baseboard the MG cylinder is often the bottle neck for machine speed increase. Most likely it will be replaced in future by the shoe-nip calendering technology (Section 6.9).
Virgin-fiber based cartonboard is mainly double coated on the top side and un¬coated or single coated on the back side. White-lined chipboard made of recycled fibers is mainly triple-coated on the top side and uncoated or single-coated on the back side, each layer with special tasks and coating formulations. Coating is done by roll applicators or by free jet applicators for highest surface requirements and machine speeds.
Final cartonboard post-calendering is mainly done with soft-nip calenders. The Pope reeler can be used for reeling up to 3.5 m diameter, reelers with center torque are applied for diameters of up to 4 m. A state-of-the-art board machine (Fig. 6.83) with on-line coating and final calendering has a length of up to 350 m.
The bases for hygienic products are tissue grades with basis weights of 5 to 35gm–2. Consumer demands drive the tissue industry to ever more softness and absorbency of the tissue grades used e. g. in the bathroom (toilet, facial) or in the kitchen (towels). Softness can be increased by the proper selection of fiber furnish and chemical additives or the proper machine concept and operation e. g. by low or no wet pressing. On the other hand sufficient paper strength and economy of production have to be regarded.
In former times tissue was made – and is still made in many places – on four¬driniers, later followed by suction breast rolls (Section 6.3.4) and since the 1970s by gap formers. Since the 1990s the so-called crescent former is the most accepted system. Today’s tissue forming section is a roll former where the headbox jet is deposited in the wedge between two converging fabrics. One-, two- or even three-layer headboxes are used, the latter for stratification of the tissue. The converging fabrics can be either two forming wires or a forming wire and a felt (crescent former). In this case the felt transfers the web directly to the dryer section. Web formation is completed during the contact of the wire(s) on the forming roll. The wire run in the forming section may be designed as a C-wrap or an S-wrap with the headbox placed at the top or bottom position in both cases.
The dryer section in tissue machines consists of either • one large diameter tissue cylinder with an air impingement hood, where the web
is completely dried within less than half a second, and creped
. • a tissue cylinder with air impingement hood where the web is only dried up to about 70 to 80 %, then wet creped and finally dried in a conventional after-drying section or today for high quality tissue production
. • a through air dryer, which dries the web up to about 70 %, followed by a tissue cylinder.
6.11 Paper and Board Machines
In a state-of-the-art conventional tissue machine (Fig. 6.84) the web is formed in a gap former (wire plus felt), transferred (at a dry content of about 12 to15 %) by the felt to the dryer section where it is first pressed and mechanically dewatered (up to about 38 to 40 % dry content) by a shoe press acting against the tissue cylinder. In older machines dewatering is done by one or two press rolls, one of them being a suction press roll. The web is dried, by intense contact and impingement heating, to about 95 % dry content. Here the hood contributes up to about 60 % of the overall drying capacity. The web is then creped by a creping doctor and wound up. During creping the web is reduced in length by about 10 to 20 %.
In a tissue machine for premium soft tissue production (Fig. 6.85) the formed web is first dewatered and pre-dried (to about 70 to 80 % dry content) in a through air dryer without mechanical pressing. Here the through dryer replaces the press nip dewatering in order to preserve the bulk. On the other hand this results in substantially higher energy consumption for drying. The web is then transferred, pressed and glued to the drying cylinder. The pressing impacts the web, mainly locally, according to a special surface pattern of the transferring wire. Coating agent application ensures good sticking of the web to the cylinder surface to obtain the required characteristics at creping. Web length decrease by creping in this case is only about 5 %.
Finally the web is calendered off line for web caliper control and for surface smoothness. The line load is low in order not to lose too much bulk.
Compared to other paper machines tissue machines are compact as regards their widths (maximum 7.9 m) and lengths which are about 45 m for conventional machines and about 60 m for a machine equipped with a through air dryer. Tissue machines are the fastest paper machines with a maximum speed today of about 2200 m min–1 (in the forming section). Maximum production capacity is about 100 000 tpa.
Specialty Paper Machines
The category of specialty papers defines a very wide range of different paper grades with a wide variety of specialized end uses and thus different and particular quality demands. These require special production technologies and specific know-how. Some of the required properties are very common and similar to commodity pa¬pers, but they are usually more distinct or with a closer tolerance. Such character¬istics include profile quality, formation, smoothness, strength, thickness, porosity, and absorption. Some are very particular characteristics, such as wet strength, electrical conductivity, pore size distribution, resistance to certain chemicals, chemical reactivity, light proofness, heat resistance, and cleanliness.
Often specialty paper grades like photo, cigarette or label papers are cut into small formats. Here the basis weight profile as well as the fiber orientation must be as uniform as possible across the width so dilution water technology in the headbox for CD profile control is standard on most paper machines for specialty paper grades. Other important quality characteristics of specialty papers are MD/ CD tensile ratio and dimensional stability. Decor, inkjet and label papers for exam¬ple absorb a large amount of water during laminating, printing or gluing. Fibers, however, swell in width more than in length when wetted. To achieve the best dimensional stability the fibers of the sheet should be randomly oriented to obtain a “square sheet”, i. e. the lowest possible MD/CD tensile ratio is required.
Not only are the quality requirements various but also the production of spe¬cialty papers covers a wide range of speed and basis weights. The basis weights extend from 12 g m–2 for tea bag paper up to 300 g m–2 for specific photo base papers. The speed range extends from 40 m min–1 for banknote papers up to 1400 m min–1 for thermal base paper, for example. Often, different grades are produced on the same paper machine. Further characteristics are small tonnages per grade, many grade changes and a dynamic grade development, i. e. new grades are developed which can cope with new market demands, other grades disappear from the market due to lack of demand. So flexibility is a key word – not only for the production of these papers but also for the paper machine concepts.
The base sheet forming concept for specialty papers is the fourdrinier. It can be used for the whole basis weight range and up to a machine speed of about 1200 m min–1. In order to achieve best results concerning formation and dimen¬sional stability Dandy rolls or hybrid formers and effective wire shaking units are frequently used. New machines however can exceed the speed limit of fourdri¬niers, so gap former concepts are used in this case.
The basic raw material for specialty papers is wood pulp. There are however grades like tea bag, plug wrap or filter paper where the required porosity, absorb¬
6.11 Paper and Board Machines
ency or bulk can only be achieved by using special plant fibers like abaca, sisal or hemp or even synthetic fibers like rayon. These fibers are much longer than wood pulp fibers. Due to their length and the demand for best formation and uniformity, even at lowest machine speeds, the inclined wire sheet forming technology is used for theses grades (Fig. 6.86). Extreme dilutions are necessary in order to avoid reflocculation of the fibers and headbox consistencies even below 0.01 % are run. Overlay papers for high abrasion resistant surfaces such as flooring laminates are produced with embedded fillers (up to 30 % aluminum oxide) on a three-layer former.
Another “old fashioned” well proven sheet forming technology which is found in specialty paper production only is the cylinder mould former (Fig. 6.87). This sheet forming technology is still used for the production of cotton based banknote paper. The paper production on a cylinder mould former enables the application of three key security features:
. • The use of cotton as raw material gives the paper a unique feel.
. • The multi-tone watermark has a wide tonal range going from light through a number of shades to dark.
. • The security thread can be fully embedded or windowed, i. e. the thread appears at intervals in the surface of the banknote.
These security features together with more than a 200 years old tradition and know-how make the cylinder mould former and cotton-based paper still the best choice for banknotes.
The press sections in specialty paper machines need to be highly flexible. For paper grades where only one smooth side is required, multi-roll press concepts with two or three press nips and a closed sheet run can be used. For increased dryness and improved runnability these press concepts can be equipped with a shoe press. For grades like inkjet or bible paper low roughness figures on both sides are required so a bottom-felted straight through press is added in order to reduce the roughness on the top side and to control the roughness two-sided-ness.
Depending on the requirements, all available drying concepts are used for the production of specialty papers. The most common concept is the double tier ar¬rangement. Fast machines are equipped with single-tier dryer groups for improved runnability. When highest smoothness and gloss values on only one side or low CD elongation at rewetting for best dimensional stability are required, the wet web is dried on a Yankee cylinder. If high bulk or high porosity is a must, as it is for filter or tea bag papers for example (Fig. 6.88), the web is dried on a through air dryer.
Specialty papers can be divided into uncoated and coated grades. The uncoated grades are either machine finished, like cigarette papers or base papers for sub-
sequent value adding off-machine process stages such as for photo base paper. Coated grades can be subdivided into two groups:
1. 1. Where the coat is needed for a certain special function, such as thermal or carbonless papers.
2. 2. Where the coat is needed for improved printability, such as label or cast coated papers.
In both cases the base sheet is normally pre-coated on one or on both sides. By pre-coating, the functional coat weight can be minimized or the final surface quality can be improved. The most suitable and economical applicator for the pre-coat is the on line film press. Both sides of the sheet can be coated simultaneously and different applications are possible.
The application of the functional coat is more demanding. For coated commod¬ity grades, the quality generally increases with increasing coat weight. For coated specialty papers like thermal or carbonless paper, however, the quality does not improve further once a certain coat weight for the functional coat is reached at any point. The optimum concerning uniform coverage and minimum coat weight is achieved with the curtain coater: a perfect layer of coating color is applied on the surface without any mechanical impact on the paper. Since the first commercial installation in the mid-1990s the significant advantages of curtain coating have led to more than 20 units being installed within the last 10 years. Due to better flex¬ibility and time efficiency the functional top coat is applied off line.
There is no typical calender concept for specialty papers. Each of the different specialty paper grades has its own demands with regard to surface properties, smoothness, densification, caliper, porosity, transparency, etc. Calendering of spe¬cialty papers, therefore, requires specific customized calender concepts. Thermal base paper, carbonless and label paper are pre-calendered usually with one soft or hard nip calender. For coated inkjet papers, two soft nips are necessary – one for each side. The smooth surface of decor papers is achieved using calenders with two or four soft nips. Here the printed side of the decor paper is in contact with the heated roll surface up to four times, supported by steam application as well. A combination of one hard nip for pre-calendering and a multi-nip calender stack for the final calendering is used for photo base paper. Silicone base paper has the highest demands concerning densification. The paper has an initial moisture of up to 16 % before running through 14 nips of a supercalender, the rolls of which are heated up to 180 °C and loaded up to 400 N mm–1.