Press Nip Research Articles

Experimental method for investigating the dynamic compression behaviour of fibre-reinforced polyurethane shoe press belts under press nip conditions

An experimental method was developed to examine the dynamic compression properties of structured polyurethane composites used as press belts within a shoe press of a paper machine. The objective was to investigate the influences of the geometrical surface structure and the matrix material composition on the compression properties. Two polyurethane formulations were tested under varying specimen conditions. The results show that the dynamic compression modulus increases with the applied load rate and that temperature and water saturation reduce the influence of dynamic effects on the compression modulus. Furthermore, it was observed that modifications of the matrix material have a more significant impact on the dynamic compression modulus than adaptions in the geometrical structure. This is addressed to the relatively small variations in possible surface designs. Finally, a rate-sensitivity index is introduced to quantify the tested specimens’ rate-sensitive behaviour.

Enhancing tissue wet pressing performance and dry end material efficiency for cost savings

The steadily growing global tissue paper demand lays a foundation for new technologies targeting tissue production optimization, as well as improved material and time efficiency. Tissue making is an especially energy-intensive field of paper industry, creating unique demands for performance in wet pressing and drying processes to cut energy usage. Intelligent roll solutions offer new tools for tissue making to achieve these goals. These tools aim at improving press nip, moisture, and tension profiles; reeling nip and parent roll hardness profiles; rewinder runnability; and end product web handling characteristics in converting. Intelligent rolls can be utilized in all the main processes and positions on tissue production lines. With these tools, production cost reductions and energy savings can be obtained by optimizing the press nip-to-Yankee cylinder contact, avoiding moisture profile errors requiring overdrying with the Yankee hood and reducing reeling/winding broke under low nip load conditions typical to tissue windups. The intelligent roll system consists of a helically mounted force or temperature sensors, roll covers, measurement electronics, digital radio transmission, and a receiver system connected to a user interface or the mill automation system. What distinguishes these implementations is that no layout changes or added external measurement devices are required, helping to fit into compact tissue machine environments, regardless of the equipment type – traditional, hybrid, or through-air drying (TAD) concept. In tissue processing equipment, the optimal positions for these rolls are press nip rollers, reeling cylinders, rewinder or converting line paper lead rolls, or rewinder winding drums. In addition to these, temperature profile measurements are utilized, with the main application being the sheet temperature profile detection after Yankee drying for moisture profile and drying process optimization.

Unique steel belt press technology for high strength papers from high yield pulp

The dry strength properties of hot-pressed moist paper improved as stiff high-yield pulp fibers soften and the sheet density increased. Very high wet strength was also achieved without adding strengthening agents. This research focuses on a new hot-pressing methodology based on a steel belt-based pilot cylinder press with infrared heating. The heated steel belt transports the moist paper into the cylinder nip with two adjacent steel rollers with adjustable nip pressure. The temperature ranges up to 300 °C, maximum speed is 5 m/min, maximum pulling force from the steel belt is 70 kN and the line load in the two press nips is 15 kN/m each. High peak pressures are possible due to the hard press nip between steel rolls and steel belt, allowing a good heat transfer to the paper. The long dwell time allows strained drying of the paper which results to high density and high wet strength. Paper samples from high-yield pulps were tested at different nip pressures, temperatures and machine speeds while the dry content was kept constant at about 63%. High nip pressure showed the largest effect on densification and dry strength. While high temperature and long dwell time seem to be most important in achieving high wet strength.

Wet pressing and product quality: Review of previous pilot machine trials

The purpose of wet pressing is to remove water by mechanical means to consolidate the web and minimize the energy expended in the dryer section. In this process, paper is compacted and densified to degrees that impact end-use performance. Average density is increased by pressing, which has implications for grades where stiffness is important. The z-direction density gradients can affect printing and converting. Lastly, pressing affects surface quality of paper and its response to printing. Broadly speaking, the final press nip dominates the paper surface roughness and the early press nips affect printing performance.

Strong paper from spruce CTMP – Part II: Effect of pressing at nip press temperatures above the lignin softening temperature

Abstract The main objective of the current study was to demonstrate that it is possible to enhance strength properties of sheets from spruce HT-CTMP and CTMP furnishes up to the same level as is common on sheets from softwood kraft pulps by changing conditions in papermaking. To achieve that, sheets of spruce HT-CTMP and CTMP were consolidated at densities close to that of the reference bleach kraft pulp by pressing at press nip temperatures well above the tack and softening temperatures of lignin. On sheets from spruce CTMP (CSF 420 ml), where the fibers were surface treated with cationic starch, it was possible to reach tensile index at the same level as on sheets from the untreated reference kraft pulp. The compression strength (SCT) of CTMP and HT-CTMP sheets, which were achieved at the highest press nip temperature (200 °C) in the study, was equal to or higher than that of the reference kraft pulp sheets. The results show that there is a great yet unexploited potential in papermaking from spruce HT-CTMP and CTMP furnishes, which could be utilized in manufacturing of products where very high requirements upon strength is demanded.

On Mathematical Modeling and Simulation of the Pressing Section of a Paper Machine Including Dynamic Capillary Effects: One-Dimensional Model

This study presents the dynamic capillary pressure model (Hassanizadeh and Gray, Adv Water Resour 13:169–186, 1990; Water Resour Res 29:3389–3405, 1993) adapted for the needs of paper manufacturing process simulations. The dynamic capillary pressure–saturation relation is included in a one-dimensional simulation model for the pressing section of a paper machine. The one-dimensional model is derived from a two-dimensional model by averaging with respect to the vertical direction. Then, the model is discretized by the finite volume method and solved by Newton’s method. The numerical experiments are carried out for parameters typical for the paper layer. The dynamic capillary pressure–saturation relation shows significant influence on the distribution of water pressure. The behavior of the solution agrees with laboratory experiments (Beck, Fluid pressure in a press nip: measurements and conclusions, 1983).

A mechanical model for felt in impulse drying: A cellular materials approach. I. Model development and simulation

This paper presents a physically based model of the densification of a felt in the press nip of a paper machine, running at operating conditions similar to those used in impulse drying. The model incorporates the observed features of wet felts compression such as non-linear elasticity and non-recoverable strains that are rate-dependent. A viscoelastoplastic model, based on the cellular solids theory, was developed to describe this behavior. The governing differential equations of the model were established bearing in mind an assembly of modified Maxwell elements combined in series and then arranged in parallel. This approach resulted in a three-dimensional unsteady-state problem, solved by means of an integrator based in the Gear method. The model was used to predict the evolution of thickness, elastic and viscous moduli and apparent density of the material.

Acoustic emission and tack of heat-set inks during setting on MWC-papers and fountain solution emulsification

This thesis claims a relationship between the film splitting sound emission from the printing press nip and the dynamic interaction occurring there between ink, fountain solution and substrate in offset lithography. The film splitting sound derives from the cavitation formed by the pressure drop in the second half of the print nip flow passage. As the ink film is strained, the cavities expand and eventually implode into breaking filaments at the nip exit, while emitting a partly audible, broadband, high frequency, noisy sound. A free-field microphone, A/D-converter and laptop computer were used to record pressure signals in the frequency range of 10 Hz to 50 kHz emitted by a variety of printing instruments and presses for a range of offset ink and paper types. After signal acquisition and filtering two signal averages of power and frequency were estimated. This average power increased with increasing loads of sheet-fed offset ink on an ink distributor, in accordance with a mass-conservation model developed. The behaviour of average frequency and power over different ink load ranges indicated transitions between different flow regimes. A glossy fine-coated paper gave higher average power than a corresponding matte paper during printing with such inks on a laboratory device, possibly due to an air sealing effect. The sound from tack measurements with the Deltack instrument during setting of heat-set offset inks printed on MWC papers showed a relation between the measured tack rise and average power, reflecting changes in splitting mechanism during the course of setting. With the Hydroscope instrument the interaction between these heat-set inks and fountain solution was studied, with the measured tack and sound emission displaying a clear, but non-linear, correlation. A heat-set offset pilot trial showed that the acoustic response from the printing nip sensitively and systematically detected changes in (LWC) paper type, optical density, ink-fount balance, and press stability. Pilot trials of cold-set offset inks on newsprint by sheet-fed presses indicated a strong correlation between evolution in average power, optical density and fountain solution consumption during the first thousand sheets normally needed for stabilisation. Acoustic measurements of ink film splitting have, aside from the laboratory studies performed by one Japanese group, previously received little attention, with the current study showing that a great deal of information useful to the printer can be accessed from this sound emission. Although the detailed mechanisms for ink film splitting have to be further studied and supported by mathematical simulation, the sensitivity of the acoustic method recommends its implementation for monitoring and control of offset printing.

Suspension arrangement for a roll

A vibration-minimizing suspension mechanism for a press roll is provided. The press roll and an opposing roll each having a rotational axis and cooperate to form a press nip adapted to impart a linear load on a fiber web passing therethrough. The linear load is oriented through the rotational axes of the press roll and the opposing roll. As such, the suspension mechanism comprises a suspension arm having opposed ends and a medially-disposed pivot, wherein the press roll is rotatably engaged at the rotational axis thereof with one of the opposed ends of the suspension arm. The suspension arm is further pivotably and adjustably mounted at the pivot so as to allow the pivot to be adjusted in substantially parallel relation to the linear load. The adjustability of the pivot thereby allowing a mounting line along the suspension arm, the mounting line being defined by the pivot and the rotational axis of the press roll, to be maintained in substantially perpendicular orientation to the linear load to thereby minimize vibration in the press roll.

Roles of Press Felt in Press Section of Paper Machine

The functions required of press felt for paper machines are mainly water removal, web conveyance, and paper surface finish. The water removal performance of felts is closely related to steam consumption during the paper drying process and is a very important factor in determining energy consumption costs.Studies on water removal at the press section have been carried out by many researchers. The mechanism of water removal published by Wahlström in 1960 is the theory that is met with wide acceptance even today. Wahlström studied the process of water removal at the press nip which he divided into four phases and established a press model equation for sheet moisture ratio after the press nip based on peak pressure, flow resistance, evenness of pressure distribution, and rewetting.Nip peak pressure is the force required for pressing the sheet and squeezing the water out and is determined by machine and felt factors. Flow resistance is the resistance which occurred when the water flows at the cell walls, in between fibers, and at the inside of the felt while the pressure is loaded to both the sheet and felt. Even pressure distribution plays an important role in water removal at the press, especially in the case of a low basis weight sheet.Rewetting is water transference from the felt to sheet at the exiting side of the nip, caused by volume expansions of both the felt and sheet, capillary action, and water film splitting. Although water removal performance of felts plays an important role in cost reduction of paper production, felts need to have other critical functions. Therefore, it is very important for us feltmakers to have close discussions with paper makers on felt performance for designing the most suitable felts.

Improvement of thermal and mechanical dewatering of paper in a hot press nip simulator

A laboratory-scale apparatus has been developed to investigate the thermal and mechanical dewatering of paper in the press nip of a high speed paper machine. The apparatus is basically a hammer-and-anvil-type dynamic press simulator modified to also simulate hot nip conditions. Using this device, dewatering experiments have performed on household paper tissue over a wide range of press conditions. The influence of different underlays, maximum pressure and temperature on mechanical dewatering by compression, capillary transfer between porous compressible materials, thermal drying and rewetting of the sheet are discussed.

Analysis of the wet pressing of paper pulp

The equations governing the motion of the paper sheet in a press nip and the anisotropic percolation of the water in the sheet, are derived in invariant form. In order to avoid restrictive assumptions on the configuration of the paper sheet, the system of coordinates is generated by the flow of fibrous material through the nip region. With the corresponding constitutive equations, the formulation consists of a system of partial differential equations for the metric tensor of the coordinate system, and for the water velocity. For practical use, the solution is then mapped back into a cartesian frame of reference. Quantities of industrial interest, such as the residual water content and stresses, as well as the press-induced anisotropy, can be calculated in principle. A Galerkin finite-element approximation is implemented using rectangular linear elements. Two case studies are presented, for large and small permeabilities, and the corresponding differences in water pressure and velocity are in general qualitative agreement with the observations. Finally, the predictive value of the model is demonstrated by the dependence of the solution on the imposed shear stress and its gradient across the sheet.

Device for reducing vibration of a press section constituted by two or more rolls pressed one against the other in a paper manufacturing machine

A mechanism for a press section of a papermaking machine including a press having first and second press rolls forming a pressure loaded press nip therebetween with a support frame and end bearings for the first roll with a support carrying the end bearings on the frame including first and second layers of deformable elastic material with a plate connected to the bearing between the layers of material and plates on the outer surfaces of the layers connected to the frame so that vibrational forces normal to the direction of movement of the web through the nip are passed through the layers of elastic material.

抄紙機プレスパートニップ圧分布の測定

The maximum pressure in the press nip is one of the most important factors on wet paper dewatering in the press nip. By using the pressure-measuring film, the correspondence between the maximum pressure profile and the moisture profile across the machine after the nip was examined at the press part of a paper making machine.There was a good correlation between them. So the pressure-measuring film is able to evaluate the press condition across the machine.