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Paper machine

A paper machine is an industrial apparatus that manufactures paper and paperboard by processing a watery suspension of fibrous pulp into a continuous sheet through sequential stages of formation, pressing, and drying.^[1] This mechanized process, which largely replaced labor-intensive hand papermaking in the 19th century, enables high-volume production of uniform paper products ranging from newsprint to cardboard.^[2] The core mechanism typically involves depositing pulp slurry onto a rapidly moving wire mesh screen, where water drains away to form a wet web, followed by mechanical compression and heat drying to yield the final sheet.^[3]^[4] The invention of the paper machine marked a pivotal advancement in the Industrial Revolution, originating with French engineer Louis-Nicolas Robert's 1799 patent for a device that produced an endless sheet of paper on a moving belt.^[2] Robert's prototype, developed while working for the Didot printing family, aimed to mechanize the labor-intensive hand papermaking process.^[5] In 1801, British stationer John Gamble secured an English patent (No. 2487) for an improved version of Robert's design, adapting it for wider sheets suitable for wallpaper and other uses.^[6] This laid the groundwork for further refinements by brothers Henry and Sealy Fourdrinier, who, collaborating with engineer Bryan Donkin, invested heavily—over £60,000—to build and patent enhanced machines between 1806 and 1807 (Patents Nos. 2951 and others).^[6]^[2] The resulting Fourdrinier machine, named after its primary backers, became the dominant model, with Donkin constructing 191 units by 1851 across Europe and beyond.^[2] In the United States, adoption followed closely, with the first imported Fourdrinier machine installed in Saugerties, New York, in 1827, and the nation's inaugural domestically built unit operational in Connecticut by May 1829.^[2] Philadelphia inventor Thomas Gilpin patented an early American variant in 1816 and activated it at the Brandywine Paper Mill in 1817, accelerating the shift from handmade to machine-produced paper along regional waterways like the Brandywine Creek and Wissahickon River.^[7] By the 1830s, these machines had transformed the industry, reducing production costs dramatically—and enabling paper output at speeds up to 58 feet per second on modern iterations.^[6]^[1] Today, Fourdrinier-style machines remain central to papermaking, processing wood pulp or recycled fibers into rolls up to 33 feet (10 meters) wide and miles long, monitored by computerized systems for quality control.^[8]

Historical Development

Early paper production

Papermaking originated in China during the 2nd century AD, with the process first documented by court official Cai Lun in 105 AD, who refined earlier techniques using materials like mulberry bark, hemp, and rags suspended in water.^[9] Archaeological evidence suggests primitive forms of paper existed as early as the 2nd century BCE, but Cai Lun's method standardized production by beating plant fibers into a pulp, forming sheets on molds, and drying them.^[10] From China, the technology spread westward along trade routes, reaching the Islamic world by the 8th century and Europe by the 12th century, initially through Spain and Italy where it replaced parchment for writing and printing.^[11] In 18th-century Europe and Asia, manual sheet formation relied on vat papermaking, a discontinuous process using molds and felts to produce individual sheets. The key steps included preparing pulp by sorting and beating rags or plant fibers in water to create a fibrous suspension, then dipping a rectangular mold—typically a wooden frame with a fine wire or bamboo screen—into a vat of this stock. The vatman shook the mold horizontally and vertically (known as "promener" and "serrer" in European practice) to evenly distribute the fibers as water drained through the screen, forming a thin wet sheet. This sheet was then couched, or transferred face-down onto a damp woolen felt by flipping the mold; multiple sheets were stacked alternately with felts by a coucher and layered for pressing under heavy screw presses to remove excess water, followed by drying on lines or boards.^[12] In Asia, similar techniques persisted, often using bamboo molds and natural fibers like rice straw or mulberry, with couching and hand-pressing adapted to local materials.^[9] These manual methods were highly labor-intensive, requiring teams of skilled workers—such as vatmen, coucheurs, and layers—who handled each sheet over 30 times during formation, couching, pressing, and drying, often working 9-12 hour shifts in damp, physically demanding conditions.^[12] Output was limited to 1,500-4,000 sheets per day for an efficient team of three in a European mill, depending on paper size and quality, far below the demands of growing print industries.^[12] Quality inconsistencies arose from variations in rag sorting, fiber beating duration, worker skill, and environmental factors like retting times, leading to uneven thickness, lumps, or up to one-third of sheets being spoiled and discarded.^[12] This paved the way for mechanized innovations to address these constraints.

Invention of continuous machines

The invention of continuous paper machines marked a pivotal shift from labor-intensive, sheet-by-sheet papermaking to mechanized production, addressing the limitations of manual methods that restricted output to mere hundreds of sheets per day. In 1798, French mechanical engineer Louis-Nicolas Robert, working as an inspector at the Didot family's paper mill in Essonnes near Paris, developed the first prototype for a machine that formed paper on a continuous web using a moving wire mesh belt to receive pulp slurry and produce an unbroken sheet.^[13] Robert applied for a French patent on September 9, 1798, which was granted on January 18, 1799, under the title "machine à faire le papier, d'une très grande étendu," describing a device capable of producing paper up to 12 meters long continuously.^[13] This innovation stemmed from Robert's observations of the inefficiencies in traditional vat-based processes, where skilled workers manually scooped and pressed pulp into individual sheets, limiting scalability during the post-Revolutionary era's demand for printed materials.^[14] Despite its promise, Robert's machine faced immediate hurdles in France due to financial disputes and political instability; after selling rights to mill owner Saint-Léger Didot for 25,000 francs, Robert regained control in 1801 when Didot defaulted, but development stalled amid the Napoleonic Wars.^[14] To circumvent these issues, Didot's brother-in-law, English papermaker John Gamble, acquired the patent and emigrated to England in 1800, securing a British patent (No. 2487) on October 20, 1801, based on Robert's design, thus transferring the technology across the Channel without Robert's personal involvement.^[14] Robert remained in France, later teaching and living in relative obscurity until his death in 1828, receiving minimal recognition or compensation for his foundational contribution.^[13] In England, London stationers Henry and Sealy Fourdrinier recognized the commercial potential and invested heavily—up to £60,000 over two decades—to refine the concept, engaging engineer Bryan Donkin to construct the first viable prototype.^[2] Donkin, a skilled machinist known for food preservation innovations, built an improved version incorporating Robert's wire belt principle with enhanced pulp distribution and pressing mechanisms; this prototype, installed at Frogmore Mill in Hertfordshire, successfully produced continuous paper sheets.^[2] Funded entirely by the Fourdriniers, who held no initial royalties due to a patent oversight, Donkin's machine represented the first industrial-scale continuous former, laying the groundwork for widespread adoption despite its rudimentary state.^[2] Early prototypes, including Donkin's, encountered significant operational difficulties that hindered reliability and output quality. Machines frequently suffered breakdowns from the fragile wire mesh tearing under pulp load and mechanical components jamming due to inconsistent slurry flow, often requiring constant manual intervention and limiting runtime to hours rather than days.^[15] Moreover, the paper produced exhibited poor uniformity, with uneven fiber formation causing thick spots, weak areas, and visible flocculation—clumps of fibers that compromised strength and printability—stemming from inadequate mixing and drainage on the nascent web.^[15] These issues underscored the need for iterative engineering, but they validated the continuous process's feasibility, spurring further investment in durable materials and flow controls.^[2]

Fourdrinier machine and alternatives

The Fourdrinier machine, patented in 1806 by English stationers Henry and Sealy Fourdrinier, represented the first commercially viable continuous papermaking apparatus, building on earlier concepts to enable efficient production of uniform paper sheets.^[16] Engineer Bryan Donkin played a pivotal role in its development, constructing the initial prototype around 1803 at Frogmore Mill in Hertfordshire, England, after the Fourdrinier brothers secured rights to Nicholas-Louis Robert's 1799 design.^[17] By 1807, further patents refined the machine's components, allowing for its widespread adoption in mills across Britain and later the United States, where the first unit operated in Saugerties, New York, in 1827.^[2] In operation, the Fourdrinier machine deposits a dilute pulp slurry from a headbox onto an endless, moving wire mesh belt, typically woven from bronze or stainless steel wires, which travels horizontally at speeds up to several hundred feet per minute.^[18] Initial dewatering occurs primarily through gravity as free water drains through the mesh, forming a wet fiber mat; this process is enhanced by table rolls and foils that create hydrodynamic forces to shear and consolidate the fibers, followed by vacuum boxes that apply suction to remove additional water, achieving a consistency of about 18-20% solids before transfer to the press section.^[19] This continuous forming mechanism eliminated the labor-intensive hand-molding of individual sheets, dramatically increasing output for thinner grades like newsprint and writing paper.^[2] A key contemporaneous alternative was the cylinder machine, invented and patented in 1809 by English paper manufacturer John Dickinson, which used rotating wire-covered cylinders immersed in pulp vats to form sheets in a semi-continuous manner.^[20] In Dickinson's design, each cylinder dips into a vat of pulp slurry, allowing fibers to deposit on the wire surface as water drains inward; the formed web is then couched onto a moving felt, with multiple cylinders enabling layered or thicker structures by sequential deposition.^[21] This system proved particularly effective for producing heavier stocks, as the cylindrical molds facilitated better control over fiber orientation and thickness uniformity compared to flat-wire methods.^[22] Early variants of twin-wire formers emerged as adaptations of the Fourdrinier principle, sandwiching the pulp between two wires to improve dewatering and sheet symmetry, though they remained secondary to the dominant flat-wire and cylinder designs in the early 19th century.^[23] Comparatively, the Fourdrinier excelled in high-speed production of lightweight, single-layer papers due to its endless belt's efficiency in handling thin slurries, while the cylinder machine offered advantages for board and multi-ply products by allowing independent vats for different fiber types, resulting in stronger, bulkier outputs suited to packaging materials.^[24] These designs coexisted into the mid-19th century, with the Fourdrinier dominating newsprint applications and cylinders persisting for specialty boards.^[2]

Modern advancements

The 20th century marked a period of rapid innovation in paper machine technology, driven by the need for higher production speeds and improved efficiency. Early in the century, the introduction of suction press rolls in 1919 enhanced dewatering in the press section by removing more water through vacuum action, allowing for better sheet consolidation before drying.^[25] By 1920, paper machines achieved speeds exceeding 1,000 ft/min (305 m/min), a significant leap that enabled mass production capabilities far beyond 19th-century limits.^[25] The Yankee dryer, a large steam-heated cylinder first developed in the mid-1800s, became integral to tissue paper production by the late 19th century, providing high-heat drying for thin, soft sheets and achieving moisture reduction from about 42% to 6-7%.^[26] Multi-cylinder dryer configurations, standard since the 19th century, saw optimizations in the 1880s and beyond with improved steam distribution for uniform drying.^[27] Key milestones further propelled these advancements. In the 1950s, speeds surpassed 2,000 ft/min (610 m/min) through innovations like suction pick-up rolls, which transferred the wet web without breakage at high velocities.^[25] The 1960s brought synthetic forming fabrics, replacing traditional metal wires around 1967, which offered greater durability, reduced downtime, and support for faster operations due to their finer mesh and corrosion resistance.^[25] By the 1970s, gap formers—such as twin-wire designs introduced in the 1960s and widely adopted by 1985—improved formation quality for multi-ply papers by creating a controlled gap for better fiber distribution and drainage.^[25] These developments collectively boosted output, with machines reaching over 4,000 ft/min (1,219 m/min) by the late 1990s.^[25] Entering the 21st century, focus shifted to energy efficiency, automation, and sustainability amid stricter environmental regulations. Quality Control Systems (QCS), originating in 1951 with early basis weight measurements, evolved into digital platforms by the 2000s, enabling real-time monitoring and adjustment of sheet properties like basis weight, moisture, and caliper across the web.^[28] Energy-efficient designs, including optimized steam systems and heat recovery, reduced consumption by up to 20% in modern setups.^[29] Sustainable features like closed-loop water systems, promoted post-2000 by regulations such as the EU's IPPC Directive, recycle process water to minimize freshwater use—often achieving over 90% closure in white water circuits—and lower effluent discharge.^[29]^[30] These innovations have dramatically increased performance, with contemporary machines operating at speeds exceeding 2,000 m/min for tissue grades and up to 1,800 m/min for board, while cutting energy use per ton and environmental impact through reduced water and emissions.^[31]^[32] As of 2024, tissue machines have achieved design speeds up to 2,200 m/min, with a record speed of 2,222 m/min reached in 2023.^[32]^[33] For instance, closed-loop implementations have decreased the industry's water footprint by recycling up to 80% of process water, aligning production with global sustainability goals.^[34]

Raw Materials and Preparation

Pulp types and sources

Pulp used in paper machines is primarily derived from lignocellulosic fibers, categorized into mechanical and chemical types based on processing methods that affect fiber integrity, yield, and resulting paper properties. Mechanical pulps, such as groundwood and refiner mechanical pulp (RMP), are produced by physically grinding or refining wood logs or chips, typically from softwood species like spruce or pine, achieving high yields of 90-98% by retaining most wood components including lignin.^[35] These pulps yield shorter fibers, averaging 1.5-2.5 mm in length, with lower brightness (around 60-70% ISO before yellowing) and reduced strength, evidenced by tensile indices of 20-40 kNm/kg, making them suitable for newsprint and lightweight printing papers where opacity is prioritized over durability.^[36] In contrast, RMP involves refining wood chips between grooved plates, producing slightly stronger fibers than traditional groundwood but still with limited bonding due to intact lignin, resulting in papers prone to dimensional instability.^[35] Chemical pulps are generated through processes that selectively remove lignin to isolate cellulose fibers, yielding 40-50% and producing stronger, more uniform materials for high-quality papers. The kraft (sulfate) process, dominant globally, treats wood chips from both hardwood (e.g., eucalyptus) and softwood with sodium hydroxide and sodium sulfide at high temperatures, preserving longer fibers (2.5-4 mm for softwood, 1-1.5 mm for hardwood) and achieving tensile indices of 50-100 kNm/kg, which enable robust applications like packaging and tissue.^[35]^[37] Sulfite pulping, using sulfur dioxide and bisulfite under acidic or neutral conditions, targets specialty dissolving pulps with high purity (over 90% alpha-cellulose), offering superior brightness (up to 90% ISO after bleaching) but lower strength than kraft; it is used for both high-brightness specialty papers such as writing, printing, and tissue, as well as chemical derivatives like viscose rayon.^[35]^[38] These pulps generally exhibit better brightness stability and higher tear resistance compared to mechanical variants, though at higher production costs due to chemical recovery needs.^[35] Globally, pulp sources blend virgin and secondary fibers to meet demand, with virgin wood accounting for approximately 42% of total fiber consumption as of 2023, primarily from sustainably managed forests supplying softwood and hardwood for mechanical and chemical processes.^[39] Recycled paper contributes the majority at 58%, with usage rising in the 2020s due to circular economy initiatives, providing shorter, more variable fibers that enhance cost-efficiency but require blending with virgin pulp for strength in machine stock preparation.^[39] Non-wood sources, such as bagasse from sugarcane and straw from wheat or rice, represent about 5% of production, mainly in timber-scarce regions like Asia, offering alternative fibers with lengths of 1-2 mm and moderate strength for cultural or specialty papers, though processing challenges limit wider adoption.^[40]

Stock preparation processes

Stock preparation encompasses the series of processes that transform raw pulp into a homogeneous, low-consistency slurry optimized for delivery to the paper machine headbox, ensuring uniform fiber distribution and enhanced sheet properties. This stage typically follows pulp production and involves mechanical treatment, chemical addition, purification, and dilution to achieve a furnish suitable for forming. For recycled pulp, additional steps like deinking—using surfactants, alkalis, and flotation or washing to remove inks, adhesives, and contaminants—are critical to restore fiber quality and prevent defects, often followed by bleaching for brightness recovery.^[41]^[3] The goal is to fibrillate fibers for better bonding, incorporate functional additives, remove contaminants, and control solids content to approximately 0.5-1% consistency, which facilitates even flow and dewatering on the machine.^[3]^[42] Pulping refinement, often referred to as beating or refining, mechanically processes the pulp to fibrillate and shorten fibers, thereby improving interfiber bonding, tensile strength, and sheet density. This is achieved using devices like the PFI mill, a laboratory-scale refiner that simulates industrial conditions by applying controlled shear forces to develop fiber properties such as freeness and burst strength. In industrial settings, double-disc refiners or conical refiners apply similar mechanical action at consistencies of 3-5%, where rotating elements cut and swell fibers to enhance hydration and flexibility without excessive energy use. Refining intensity is adjusted based on pulp type, with mechanical pulps requiring more treatment to compensate for shorter fibers.^[43]^[44] Additives are integrated during stock preparation to modify pulp characteristics and final paper performance, with fillers and chemical agents added in the mixing chest or approach flow system. Common fillers include kaolin clay and precipitated calcium carbonate, which improve opacity, brightness, and smoothness by scattering light and filling voids between fibers, typically at loadings of 5-30% by weight. Retention aids, such as cationic polyacrylamides, promote flocculation of fines and fillers to increase their retention on the wire, reducing losses in white water and enhancing machine efficiency. Wet-strength agents, like urea-formaldehyde or polyamine-epichlorohydrin resins, are employed for grades requiring resistance to moist conditions, forming covalent bonds between fibers to maintain integrity.^[45]^[46]^[47] Cleaning and screening remove impurities to prevent defects in the finished sheet, utilizing centrifugal cleaners and pressure screens in multi-stage configurations. Centrifugal cleaners generate high centrifugal forces (up to 10³ m/s²) in a hydrocyclone to separate heavy contaminants like sand, staples, or shives based on density differences, operating at 1% consistency and pressure drops of 100-200 kPa with reject rates of 5-20%. Pressure screens employ slotted or perforated baskets (slots 0.10-0.25 mm) and rotating foils to fractionate fibers and remove oversized particles, achieving high throughput at consistencies up to 5% through hydrodynamic shear that allows flexible fibers to pass while rejecting rigid debris. These processes are sequenced, with forward and reverse cleaners targeting heavy and light contaminants, respectively, to achieve over 95% removal efficiency.^[48]^[49]^[3] Consistency control finalizes the stock by diluting the refined and cleaned furnish with white water or fresh water to a target solids content of 0.5-1%, ensuring optimal headbox flow and formation uniformity. This is managed via automated valves and sensors in the approach system, where dilution adjusts for variations in incoming pulp consistency (typically 3-4%) and machine speed, maintaining stable flow rates scaled to production— for example, around 500-2000 L/min for smaller headboxes on machines producing 100-500 m/min. Precise control prevents fluctuations that could lead to basis weight variations, with short circulation loops recycling water to minimize freshwater use while sustaining the low consistency needed for even fiber suspension.^[30]^[42]

Forming and Wet End Operations

Forming section mechanics

The forming section of a paper machine, also known as the wet end, initiates the transformation of a dilute pulp slurry into a continuous wet web through controlled deposition and dewatering. The headbox plays a critical role by accelerating and distributing the slurry uniformly across the machine width via a narrow slice opening, typically at a consistency of 0.5-1.0%. To ensure even fiber dispersion and prevent flocculation, modern headboxes incorporate turbulence generators, such as internal baffles or rectifier rolls, which induce high shear to randomize fiber positions before ejection as a jet onto the forming wire.^[50]^[51] A key parameter in headbox operation is the jet-to-wire speed ratio, defined as the velocity of the exiting slurry jet divided by the speed of the forming wire, typically maintained between 0.95 and 1.05 to optimize sheet formation. This ratio controls the shear induced between the jet and the moving wire; a ratio near 1.0 promotes isotropic fiber distribution and minimizes directional biases, while deviations introduce oriented shear that aligns fibers preferentially in the machine direction (MD), enhancing MD tensile strength but potentially increasing cross-machine direction (CD) variability. Precise control of this ratio, often adjusted via headbox pressure or slice aperture, is essential for balancing formation uniformity and mechanical properties.^[52]^[50] The wire section employs an endless woven mesh belt, known as the forming fabric, which travels at speeds up to several thousand feet per minute to support the nascent web. As the slurry contacts the wire, initial dewatering occurs primarily through gravity, allowing free water to drain through the mesh openings and raising the solids content from the incoming 0.5-1.0% to approximately 6-7%. This gravitational phase relies on the fabric's permeability and the slurry's drainage kinetics, with the web beginning to consolidate as fibers mat together on the wire surface.^[50]^[53] Further dewatering in the wire section utilizes vacuum boxes positioned beneath the fabric to apply suction, accelerating water removal and increasing solids content to 15-20% by the end of the forming zone. These boxes, often operating at vacuum levels of 10-40 kPa, draw water through the fabric without excessive fiber compaction, while foil boxes—equipped with angled blades or hydrofoils—generate dynamic pressure pulses to enhance drainage and prevent wire marking on the sheet by minimizing fabric imprinting. The transition from gravity to vacuum dewatering ensures the web achieves sufficient strength for transfer to subsequent sections, with stock consistency influencing the overall efficiency of this process.^[53]^[50]^[51] Formation quality in the forming section is quantified by metrics such as basis weight variance, which measures local mass uniformity across the web, ideally kept below 5-10% standard deviation for high-quality sheets to ensure consistent printability and strength. Shear forces arising from the jet-to-wire differential and wire motion significantly influence fiber orientation, with higher shear promoting MD alignment that can reduce CD tear strength by up to 20-30% in anisotropic webs. Optimal formation minimizes flocs and achieves random fiber networks, directly impacting end-product properties like opacity and dimensional stability.^[52]^[50]

Variations in forming designs

While the Fourdrinier machine relies on a single horizontal wire for sheet formation, alternative forming designs employ multiple wires or specialized geometries to enhance dewatering, fiber orientation, and sheet symmetry, particularly for demanding paper grades like board, fine papers, and tissue. These variations address limitations in traditional forming by enabling higher speeds, better retention of fines, and reduced two-sidedness in the final product.^[54] Twin-wire formers utilize two converging wires to sandwich the pulp stock, creating a three-dimensional fiber structure through hydraulic forces and vacuum-assisted dewatering in the gap. This design promotes symmetric sheet properties and improved formation by allowing drainage from both sides, which minimizes basis weight variability and linting compared to single-wire systems. Commonly applied in board production, the Delta Former exemplifies this approach, where an inclined twin-wire configuration supports thicker, more uniform webs for containerboard and specialty boards at speeds up to 800 m/min.^[19]^[55] Gap formers, a subset of twin-wire technology, feature a narrow initial gap between wires or rolls to accelerate dewatering via intense hydraulic shear, enabling operation at high speeds of up to 1,500 m/min for fine and lightweight papers. By reducing two-sidedness through balanced drainage, these formers produce papers with enhanced printability and strength uniformity; for instance, Valmet's OptiFormer Gap integrates shoe and blade elements to optimize fiber alignment in recycled fiber-based linerboard. Voith's DuoFormer TQv further refines this with vertical roll-blade arrangements, achieving superior sheet quality in fast-running machines for printing and packaging grades.^[54]^[56]^[57] Inclined wire and top-wire formers are tailored for lightweight papers such as tissue, where an angled wire path or upper wire overlay improves fines retention and web uniformity by countering fiber flocculation under gravity. The inclined wire design facilitates gentle dewatering for soft, bulky sheets, while top-wire configurations, like those in crescent formers, combine a lower forming wire with an upper felt to capture stock efficiently at speeds of 800–1,300 m/min, yielding high tensile strength and low basis weight deviation. Valmet's IntelliFormer Crescent Former, for example, minimizes web breaks and enhances softness in tissue production through precise vacuum control.^[58]^[59] Hybrid designs, such as the Bel Baie former, integrate inclined multi-layer capabilities for multi-ply products by layering stock via multiple headboxes on a single wire section, promoting interlayer bonding and customized properties like barrier layers in packaging boards. This approach, developed by Beloit (now part of Valmet), supports efficient production of complex structures with good ply adhesion, though it requires precise control to avoid delamination.^[60]^[61]

Pressing and Drying Processes

Press section functions

The press section of a paper machine mechanically removes water from the wet web emerging from the forming section, typically increasing solids content from 15-22% to 40-55% to prepare it for drying.^[62] This dewatering occurs through compression in a series of nips, where the web is squeezed between rolls or other components, consolidating the fibers and enhancing web strength to improve runnability.^[63] Configurations often feature 2-4 nips to progressively extract water while minimizing sheet damage.^[64] Nip presses are central to this process, with two primary types: single-felted roll presses and extended-nip shoe presses. Single-felted presses compress the web between a solid roll and a felt-supported roll, applying line loads of 150-450 kN/m over a short nip width of about 75 mm, achieving up to 40-45% solids.^[64] In contrast, extended-nip shoe presses use a flexible belt over a concave shoe to create a longer nip zone of 250 mm or more, allowing higher loads up to 1,500 kN/m and increasing solids by an additional 4-7% to 45-50% through extended dwell time and optimal pressure distribution.^[62]^[65] These shoe presses, first widely adopted in the 1980s, reduce peak pressures on the web while enhancing dewatering efficiency.^[62] Felt rolls, or press felts, play a critical role by absorbing expressed water through their void volume, typically carrying it away to uhle boxes for removal.^[63] These permeable fabrics, often 5 mm thick in shoe press applications, must balance water transport with minimal rewetting of the web; doctor blades scrape excess water, oil, or contaminants from the felts to maintain performance and prevent buildup.^[65] Dewatering efficiency in the press section is quantified by the expression ratio, or press impulse, which measures water removed per unit area and time—extended nips achieve 15-21 kN·s/m² compared to 3-5 kN·s/m² in conventional rolls, leading to 5-10% higher dryness levels.^[64] This mechanical removal saves significant energy downstream, reducing steam consumption in drying by up to 15% or 0.5-2.0 GJ per ton of paper.^[65]^[64] Web transfer between nips relies on vacuum-assisted pickup rolls and felts to adhere and stabilize the delicate web, preventing breaks or wrinkles during high-speed operation.^[63] Typical setups include open draws minimized by felt-to-felt transfers, ensuring smooth progression through 2-4 nips without interrupting production.^[64]

Dryer section operations

The dryer section of a paper machine removes remaining moisture from the web after pressing, typically entering at around 50% solids content, through thermal evaporation to achieve a final moisture level of 5-7%. This process relies on heat transfer from steam-heated surfaces to vaporize water, with the web supported by fabrics to prevent breakage at high speeds. The section accounts for a significant portion of the machine's energy use, as evaporation is more costly than mechanical dewatering in the press.^[66] Multi-cylinder dryers form the core of most configurations for printing papers and boards, consisting of a series of steam-heated cast-iron cylinders with diameters of 1.5 to 2.2 meters, commonly 1.8 meters. These cylinders operate at surface temperatures of 120-150°C using low-pressure steam (up to 5 bar), achieving drying rates of 20-40 kg H₂O/m²/hour through contact drying where the web wraps around the cylinders for 180-270 degrees. The web is threaded and supported by felts or fabrics in single- or double-felted arrangements, enabling runnability at speeds exceeding 2,000 m/min with ropeless tail threading systems to minimize downtime. Heat transfer from steam to the cylinder shell occurs via condensation, with coefficients ranging from 4,260 W/(m²°C) for plain surfaces to 5,730 W/(m²°C) for ribbed designs that enhance flow by up to 35%.^[66]^[67] The Yankee dryer, a large single cylinder typically 4.5-6.4 meters in diameter, is specialized for tissue and towel grades, combining conductive drying from internal steam heating with convective drying from high-velocity hoods. Operating at speeds up to 2,000 m/min, it achieves high evaporation rates of around 200 kg H₂O/m²/hour, with the web pressed against the hot surface (heated to 100-150°C) for adhesion via sprayed coatings before creping—a doctor blade scrapes the dry sheet to impart bulk and softness. Condensate removal systems, such as syphons and outriggers, maintain uniform shell temperatures, while thermal coatings like Infinikote optimize heat transfer and crepe quality.^[66]^[68]^[69] Evaporation dynamics in the dryer section involve phase change driven by temperature gradients, reducing web moisture from 50% post-pressing to 5-7% equilibrium content, governed by vapor pressure differences and mass transfer through the porous sheet. Heat transfer to the web occurs primarily via conduction in multi-cylinder setups, with contact coefficients around 800 W/m²°C in the constant-rate period where surface evaporation dominates, transitioning to internal diffusion in the falling-rate phase. Overall efficiency is influenced by sheet basis weight, initial moisture, and air humidity, with ribbed cylinders and impingement improving transfer rates.^[66]^[70]^[67] Pocket ventilation systems enhance drying rates in multi-cylinder sections by introducing conditioned air into the enclosed spaces (pockets) between cylinders, preventing web flutter, stabilizing the sheet at high speeds, and removing evaporated moisture to maintain low humidity. These systems use blow boxes and exhaust hoods to direct airflow, tailored to paper grades with variable volumes and temperatures, potentially increasing evaporation by 10-20% while profiling cross-machine moisture uniformity. Energy recovery is achieved through heat exchangers in exhaust streams, recapturing up to 70% of thermal energy for reuse in the mill.^[67]^[69]^[66]

Finishing and Rewinding

Size press and calendering

The size press is a critical finishing unit in paper machines that applies a thin layer of starch or other sizing agents to the dried paper web, enhancing its surface properties and overall performance. Typically positioned after the dryer section, it involves passing the web through a nip formed by two rolls where the sizing solution is transferred, with pickup limited to about 20% of the base sheet weight depending on factors like base paper porosity and solution viscosity (25-40 cp). This process improves water resistance by increasing the sizing degree and reducing sheet porosity, while also boosting internal bond strength, surface strength, tensile strength, and stiffness.^[71]^[72] Two primary types of size presses are used: the traditional pond (or puddle) type and the metering type. In the pond size press, the web passes through a pool of sizing agent held between two rolls, allowing variable but less controlled application suitable for speeds up to 3100 feet per minute; however, it can lead to uneven pickup and turbulence at higher speeds. The metering size press, developed in the 1980s, employs a blade, rod, or gate roll to precisely meter the sizing film onto the applicator roll before transfer to the web, offering better uniformity, higher solids content, reduced drying demands, and operation at elevated speeds without pond instability—making it ideal for modern high-speed machines. Both types commonly use cationic starch for superior pickup and penetration into the porous base sheet, further decreasing opacity and whiteness while increasing roughness for enhanced printability.^[73]^[74]^[71] Following sizing, calendering refines the paper's surface by compressing the web between a stack of rolls under controlled pressure and temperature, primarily to achieve desired smoothness, gloss, and thickness uniformity. Calender stacks typically consist of 2 to 12 rolls arranged vertically or horizontally, with hard-nip configurations using all-metal rolls for basic caliper control, while soft-nip designs incorporate polymer or filled rolls (e.g., fiber-reinforced epoxy) against a heated steel roll to promote plastic deformation without excessive bulk loss. Heated rolls, often at 100-150°C, enhance the web's plasticity, enabling better fiber bonding and surface leveling; for instance, soft-nip calenders can increase gloss by up to 20% compared to hard nips. Supercalenders, featuring alternating hard and soft rolls in multiple nips, provide the highest finish for specialty grades like supercalendered (SC) papers, often operated off-line for precise adjustments.^[75]^[76]^[77] The effects of calendering are quantified through metrics like Bekk smoothness, where values exceeding 40 seconds indicate high-quality surfaces suitable for printing, as achieved in on-line soft-nip setups at loads of 40-50 kN/m. This process reduces surface roughness, improving ink absorption uniformity and print gloss by minimizing fiber show-through, though excessive pressure can lower bulk and increase density—necessitating balanced configurations for coated papers, where on-line calendering is standard to integrate with coating stations for enhanced holdout and reduced ink set-off. Off-line calendering, common for supercalenders, allows greater flexibility for high-gloss coated grades like lightweight coated (LWC) papers, with speeds up to 2200 m/min and widths to 11 m.^[75]^[78]^[79]

Reel and winder sections

The reel section of a paper machine collects the dried and finished web from the dryer section, winding it under controlled tension onto large cores to form parent reels. The Pope reel, a common type, uses a motor-driven drum to apply sufficient tension, ensuring the web wraps tightly and uniformly around the core without slippage or defects. This process maintains basis weight uniformity across the web by monitoring and adjusting tension profiles, which helps prevent variations that could lead to quality issues in downstream converting.^[80]^[81] Winder sections convert these parent reels into saleable rolls by slitting the web to precise customer-specified widths while rewinding. Slitter-rewinders employ razor or shear blades for edge trimming and slitting, removing defective margins and dividing the web into narrower strips before rewinding onto individual cores under controlled tension to achieve consistent roll density. Turret winders facilitate continuous operation by featuring multiple rewind shafts on a rotating turret; as one roll completes, the turret indexes to start a new one without stopping the line, enabling high-speed production up to 2,500 m/min for sensitive grades.^[82]^[83]^[84] Quality control in these sections focuses on preventing web breaks and achieving optimal roll hardness profiles to ensure runnability and storage stability. Web breaks are minimized through automated systems like tail fixation, which uses high-pressure water jets or adhesive to secure the web tail during reel changes, combined with precise tension monitoring to avoid excessive strain. Roll hardness is controlled via nip load adjustments and tension distribution, with profiles measured using tools like Schmidt hammers to verify uniformity; deviations can cause telescoping or bursting during transport.^[82]^[85]^[86] The output consists of massive parent reels weighing up to 160 metric tons and measuring up to 4.6 meters in diameter, which are then slit in the winder to produce rolls tailored to customer widths, typically ranging from 0.7 to 3 meters. This slitting process accommodates diverse end-use requirements, such as packaging or printing, while minimizing waste through automated cut-to-length controls.^[82]^[87]

Machine Design and Materials

Structural materials

Paper machines are constructed using a variety of robust materials to withstand the mechanical stresses, high temperatures, and corrosive environments inherent in papermaking processes. The primary structural components, such as frames and rolls, are typically fabricated from cast iron or steel alloys to provide the necessary rigidity and durability. Cast iron is favored for its excellent vibration damping properties and cost-effectiveness in large-scale machine beds and roll shells, ensuring stable operation under continuous loads. Steel alloys, including high-strength variants, are employed for frames to offer superior tensile strength and weldability, allowing for modular designs that facilitate maintenance and upgrades.^[88] In the wet end of the paper machine, where exposure to moisture, chemicals, and pulp slurries is prevalent, corrosion-resistant materials like stainless steel are essential. Austenitic stainless steels, such as grades 304 and 316, are commonly used for rolls, frames, and piping in this section due to their high resistance to pitting and crevice corrosion from chlorides and acidic conditions. These alloys maintain structural integrity over extended periods, reducing downtime and extending service life in harsh environments.^[89]^[90] Wires and felts, critical for forming and dewatering the paper web, are made from synthetic polymers to achieve high strength, abrasion resistance, and dimensional stability. Forming wires are primarily constructed from polyester monofilaments, which provide excellent hydrolysis resistance and permeability for efficient water drainage. Historically, phosphor bronze wires were used for their durability, but modern synthetic alternatives like polyester and nylon have largely replaced them for better performance in high-speed operations. Felts, serving as press fabrics, are typically non-woven structures of polyester or polyamide fibers, designed to handle compressive forces while allowing water expulsion without fiber embedding.^[91]^[92] Doctor blades, which scrape residues from rolls and dryers, require materials that balance flexibility, wear resistance, and edge sharpness. Ceramics, often in composite forms with epoxy resins, are utilized for their superior hardness and low friction against heated surfaces, minimizing blade wear and sheet breaks. Fiber-reinforced composites, incorporating carbon or glass fibers, offer enhanced stiffness and chemical resistance, particularly in dry end applications where temperatures exceed 100°C. These materials ensure precise cleaning without damaging roll covers.^[93]^[94] Since the early 2000s, advancements in lightweight composites have been integrated into paper machine components to improve energy efficiency and reduce operational costs. Carbon fiber-reinforced polymers (CFRP) are increasingly applied in rolls and structural elements, offering a high strength-to-weight ratio that lowers inertia and vibration, thereby decreasing energy consumption in drive systems compared to traditional metal rolls. These composites also enable smaller diameters and faster startups without pre-heating, enhancing overall machine productivity.^[95]

Variations for specialty papers

Paper machines are adapted for specialty papers through targeted modifications to forming, drying, pressing, and finishing sections, enabling production of diverse grades like tissue, board, and coated products that differ from standard printing or writing papers. These variations optimize properties such as bulk, strength, absorbency, or surface quality while maintaining efficient throughput. For tissue production, machines incorporate through-air drying (TAD) systems to achieve high bulk and softness without excessive compaction. In TAD, hot air permeates the wet web on a perforated cylinder, evaporating moisture non-compressively and forming a porous structure that enhances absorbency and hand feel, contrasting with conventional wet-pressing methods.^[96]^[97] High-bulk dryers, often Yankee cylinders combined with TAD units, further support this by providing even drying and creping for textured surfaces in premium hygiene tissues.^[98] Advanced TAD technologies can yield tissues with bulk indices up to 10-12 cm³/g, significantly higher than the 5-7 cm³/g from standard processes.^[99] Board machines employ multi-layer headboxes to form multi-ply structures, allowing distinct fiber compositions in each layer for tailored strength and stiffness. These headboxes, such as layering Fourdrinier designs, deliver separate stock streams to create inner layers from recycled fibers for cost efficiency and outer layers from virgin pulp for surface quality.^[100] Thicker forming wires, often multi-layer meshes with optimized drainage, support the heavier grammages (up to 500 g/m²) and thicker webs in multi-ply board, preventing web breaks during formation.^[101] Cylinder formers complement this by enabling up to 10 plies in specialty boards like linerboard, enhancing interlayer bonding.^[102] Coated paper lines integrate in-line coaters after the dryer section to apply pigments like clay for improved printability and opacity in grades such as magazine stock. These coaters, including blade or curtain types, deposit uniform layers (typically 5-15 g/m² per side) directly on the moving web, minimizing handling and defects.^[103]^[104] Extended calenders, often soft-nip or supercalender stacks with 8-12 rolls, follow to compact and polish the coating, achieving gloss levels of 50-70% for high-end coated woodfree papers used in publications.^[105] Niche designs for hygiene products feature low-grammage configurations, such as compact machines producing 15-25 g/m² tissues for napkins or wipes. Integrated napkin converters, often modular attachments to tissue lines, fold, emboss, and cut the web in-line, streamlining production of folded products like table napkins.^[106]^[107] These setups reduce waste and enable quick format changes for various hygiene formats, supporting outputs up to 1 ton per day in smaller facilities.^[108]

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[103]
Dingchen 787mm Napkin Paper Making Machine
Rating 5.0 · Review by MIC_BUYER1. Output paper: Tissue paper & toilet paper & napkin paper; · 2. Raw material: Waste paper or wood pulp; · 3. Capacity: 1 t/d; · 4. Net paper width: 950mm; · 5.
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End-Use Products - TISSUE CONVERTING MACHINES - OMET
Paper napkins represent the widest and most common range of products of OMET tissue converting machines. These products are known for their utility and ...Missing: grammage | Show results with:grammage
[105]
Tissue Converting Machines: The Future of Hygiene Product ...
Jul 20, 2023 · Tissue converting machines are designed to convert rolls of tissue into different forms of hygiene products, such as toilet paper, facial tissue ...Missing: specialty grammage