This white paper presents a theoretical and experimental investigation into the dynamic behavior of thin web materials, such as plastic films or paper, as they interact with surrounding airflow during high-speed transport. The study focuses on the effects of fluid forces on web flutter and stability, modeling the web as a tensioned beam exposed to cross-flow and pressure gradients. By analyzing how aerodynamic forces influence web motion and instability, the research provides insights that are essential for optimizing processes like printing, coating, and converting, where maintaining web alignment and quality is critical.
A theoretical analysis of the fluid mechanics of the air cushion of the air reversers used in web-handling systems is presented. A two-dimensional model of the air flow is derived by averaging the equations of conservation of mass and momentum over the clearance between the web and the reverser. The resulting equations are Euler’s equations with nonlinear source terms representing the air supply holes in the surface of the reverser. The equations are solved analytically for the one-dimensional case and numerically for the two-dimensional case. Results are compared with an empirical formula and the one-dimensional air-jet theory developed for hovercraft. Conditions that maximize the air pressure supporting the web are analyzed and design guidelines are deduced.
A theoretical analysis of the fluid mechanics of the air cushion of the air reversers used in web-handling systems is presented. A two-dimensional model of the air flow obtained averaging the equations of conservation of mass and momentum over the clearance between the web and the reverser is solved numerically. The effect of the pressure holes in the surface of the air reverser are represented by nonlinear source terms in the modified Euler’s and mass conservation equations. The results of the numerical solution are compared with a 1D analytical solution of these equations, an empirical formula and the one-dimensional air-jet theory developed for hovercraft. Conditions that maximize the air pressure supporting the web are analyzed and design guidelines are deduced.
Rubber covered nip rollers are used in many web handling and processing applications. Successful use of these systems requires an accurate knowledge of the impact of various design and process parameters on key response metrics such as nip pressure and surface speed axial uniformity. These metrics are important since they are directly correlated to operational and functional requirements of nipping processes. Axial variations occur primarily due to roller shell bending, which arise from externally applied end loading forces. The non-linear radial compressive characteristics of elastomeric coverings that are often a part of such systems contributes to system complexity. Previous papers have presented a two-dimensional analytical model that relates force and deformations of rubber rollers in contact with other rollers. In the present paper, a three-dimensional model is presented that extends the previous model by incorporating shell bending deflections. In addition to the non-linearity due to the compressive characteristics of rubber coverings, the model also includes the ability to include other non-linear effects such as roller diameter non-uniformity and misalignment, or skew, of the roller’s rotation axes. The model is used to demonstrate that the use of crowning or skewing must not only account for geometric effects but also for the nearly incompressible nature of rubber coverings to successfully mitigate axial nip pressure variation that otherwise is present in end-loaded nip roller systems.
Nip rollers are used extensively in converting processes; however, correct design and usage of these types of rollers requires a sound understanding of the mechanics of the nip in both the machine and cross-width directions. This behavior is difficult to predict owing to the nonlinear nature of the contact region and the near incompressible material characteristics of typical nip roller coverings. This paper is a companion to a paper presented at IWEB11 where experimental data characterizing web feed rates, nip pressures and wrinkling in nip rollers comprised of single and dual durometer cover systems was presented. In this paper, modeling techniques are developed for predicting nip pressures and nip roller feed rates in both the machine and cross-width directions. The models presented include the ability to analyze roller coverings engineered with the ability to control nip roller feed rate while retaining desirable nip pressure characteristics. Parametric studies demonstrating the influence of many of the material and design parameters of nip rollers on nip roller feed rate and nip pressure are presented. Nondimensionalized parameters are also developed to assist engineers in the design of nip rollers to insure suitable performance.
Nip rollers are used extensively in web handling processes; however, rubber covered rollers have the unwanted and often unpredictable characteristic of unknown surface speed owing to coupling between circumferential and radial strains within the nip. When nip rollers are used to transport continuous webs, this behavior can lead to speed or feed rate variation between the nip roller and the web line in the process direction. Further, variations in feed rate across the width of the web due to roller deflection or other widthwise variations can lead to corrugations and wrinkles. In this paper, a measurement method is described and demonstrated for accurately measuring nip roller feed rates. Data is presented for asymmetrical nip systems consisting of a rubber-covered nip roller loaded against a metal roller. Results are shown for a conventional nip roller covering and a second nip roller covering engineered with the ability to control nip roller feed rate while retaining desirable nip pressure characteristics. Results from a troughing and wrinkling study using two pairs of end-loaded symmetric nip rollers of each design are also presented. These results are used to compare and contrast the performance of nip rollers systems where differences in nip roller feed rate significantly alters system behavior.
Nip rollers are used extensively in converting processes; however, elastomeric-covered rollers have the unwanted and often unpredictable characteristic of unknown surface speed owing to coupling between circumferential and radial strains within the nip. Variations in nip roller feed rate due to width-dependent effects such as roller deflection can lead to web troughing and wrinkling propensity. In this paper, we review measurement methods for accurately measuring nip roller feed rates. Modeling techniques are presented for predicting width-dependent nip roller feed rates due to roller loading and width-dependent bending. Discussion regarding the practical implications of the width-dependent behavior is given. Experimental data from the Optimation Technology Media Conveyance Facility for all aspects of this work will also be presented. This paper, while providing theoretical and experimental details on nip mechanics, will also serve as a useful tutorial for those interested in learning about the practical operational aspects of nip rollers.
Nip rollers are used extensively in converting processes; however, elastomeric-covered rollers have the unwanted and often unpredictable characteristic of unknown surface speed owing to coupling between circumferential and radial strains within the nip. In this paper, a measurement method is described and demonstrated for accurately measuring nip roller feed rates. Modeling techniques are also presented for predicting nip roller feed rate and results compared to experimental data. A novel contribution of this paper is the demonstration of the ability to analyze roller coverings engineered with the ability to control nip roller feed rate while retaining nip pressure characteristics. This is an important capability in light of the known negative influence of non-uniform nip roller feed rate due to cross-direction variations on web troughing and wrinkling propensity.
The traction developed between a thin flexible web, wrapped around a nonvented, rotating cylindrical roller is studied experimentally and theoretically. A series of eight webs representing a wide range of surface roughness characteristics are traction tested against the same roller over a wide speed range. A one-dimensional finite difference model that couples air film pressure (Reynold’s equation), web bending and solid-body contact using an asperity compliance function is used to model the experimental traction data. An optimization technique is used to estimate the asperity compliance function parameters. A new model for computing the asperity engagement height for nonGaussian surfaces is presented when the roughness of both surfaces is taken into account. Results are presented which indicate the viability and utility of the new methods.
Roller conveyance deals with transporting a continuous web over rollers. Successful roller conveyance is achieved when this process does not damage the web. One defect that can be caused by an improperly designed or malfunctioning roller conveyance system is the formation of intermittent scratches. The purpose of this paper is to develop and present simple equations relating roller velocity and wrap angle in terms of scratch length, scratch pitch and web speed. It will be shown how the equations can be used to diagnose potential roller candidates that might be possible sources of scratches. In addition, troubleshooting guidelines will be outlined for the systematic elimination of scratches.
This paper addresses the challenge of detecting and preventing web-to-roller traction failures in web handling systems, which often go unnoticed until product damage has occurred. Traditional detection methods, such as product inspection or roller speed monitoring, typically occur after a failure, resulting in significant material waste. The study proposes a proactive solution using machine-mounted prony brakes and a process monitoring (PM) system to measure excess traction in real-time during non-saleable runs. By trending this data and applying machine learning algorithms, the system can predict traction deterioration and schedule preventative maintenance, ultimately improving product quality and reducing downtime.
This paper examines the challenges associated with butt splicing in web handling, particularly within coating processes where splices are prone to lifting or “hinging” as they pass over rollers. While butt splices are preferred over lap splices for their minimal impact on coating quality, they introduce mechanical discontinuities that can disrupt production. The study introduces a predictive model that quantifies splice hinging based on key process and material variables, validated through experimental data. The model is then used to assess the influence of different parameters on splice behavior and provides design recommendations to reduce hinging and improve coating process stability.
The role of a pull roller in a web machine is identified. The ability of pull rollers, plain or vacuum type, to permit tension differences is explained. Three types of pull rollers are described. Among them is the un-ported roller that utilizes holes and grooves to increase effectiveness. This paper, while providing theoretical and experimental details on pull roller design, will also serve as a useful tutorial for those interested in learning about the practical design and operational aspects of pull rollers.
During thin film winding, rolls will sometimes form uniform width machine direction (MD) buckles. Commonly referred to as tin-canning defects, they may form across the entire width of the roll or appear in local widthwise bands. Tin-canning defects may also vary in intensity or breadth by location across the roll circumference or by radial position in the roll. Severity of the defect is also known to be a function of time within the wound roll. In order to develop an improved understanding of this defect, a study of nipped center winding was undertaken. The formation of tin-canning defects as a function of tension, nip load, nip roller design and pre-wind spreading was studied. This paper summarizes results from this work and discusses process options for reducing or eliminating this defect.
The manufacture of many products involves the winding of continuous thin, flexible webs into wound rolls. In many applications involving the use of plastic webs, it is beneficial to mechanically emboss the edges of the web prior to winding so as to provide a thickening of the web in these areas. During and after winding, radial pressures developed in the wound roll then are concentrated in the localized embossed areas. This reduces the sensitivity to the formation of web distortions due to stress concentrations that would otherwise develop due to lengthwise persistent widthwise thickness nonuniformities. One of the drawbacks of this process is that the wound roll is now more sensitive to buckling-type defects owing to reduced interlayer pressures in the bulk of the roll away from the embossed edges. In this paper, we present results for a particular type of web defect known as a Gentle Wavy Defect (GWD) that forms in the wound roll due to presence of axial corrugations that develop in the wound roll. We begin by presenting product and process information characterizing this situation along with potential theories as to the cause of the corrugations and GWDs. Next, we provide results from numerous experiments that provide guidance on the likely mechanism responsible for the formation of the roll and web defects. Through these experiments and the application of winding models, we then demonstrate the cause of the defects and provide guidelines on how to avoid them in future applications.
The manufacturing of flexible display products has very tight specifications on the dimensional changes between the coating stations or between different passes of a multi-pass coating process. Web stretching and stress relaxation during conveyance and roll storage all contribute to the dimension stability of this product, and therefore are potential concerns. The quality of a flexible display is very sensitive to defects in the films that are used as part of the assembly. Many film components used in the display are very delicate and can be damaged during web handling. To protect the film from damage during the winding process, it can be interleaved with another film, typically one that is softer and significantly cheaper. This paper reports data from a stress relaxation test on a specific polyethylene terephthalate (PET) web, and on the modeling of the stress development, stress relaxation, and dimensional changes of the PET from roll winding and wound roll storage. We also present modeling results and empirical data on the winding of interleaved films.
Wound roll quality is primarily a function of the level and distribution of stresses developed within a roll both during and after winding. Because of this relationship, developing an improved understanding of wound roll stresses has historically been the main driver in the development of wound roll models. It is well known that wound roll stresses are influenced by many factors including process parameters, product parameters, and environmental conditions. Owing to the increasing desire to streamline process and product design, the complexity of wound roll models has increased over the last few years. In particular, researchers have sought to include more of the factors influencing wound roll stresses into the models. The wound roll model presented in this paper seeks to combine physical effects that have been studied in the past. In addition, a discussion is presented noting the limitations of the model along with recommendations for future modeling.
In typical winding processes, webs are rarely (if ever) ideal but instead thickness non-uniformity and distortions (two examples) are the norm. These deviations from ideal have significant ramifications to both predictive model development and to winding equipment design configurations. This paper will present methods for quantitative measurement of some of the important non-ideal web characteristics. Typical results will be presented for film and paper webs. Also presented will be examples of modeling enhancements needed to generate predictions of the winding behavior in systems comprised of non-ideal webs. Finally, examples will be given of hardware modifications that are driven, to a large extent, by the reality that webs are not perfect. The objective of this paper will be to enlighten the winding process development community regarding a very challenging and relevant area for future research.
In web winding processes, a thin layer of air is entrained into rolls. This air reduces the interlayer pressure in the wound roll because the air acts like a sponge between adjacent web layers. Winding models that include the effect of air entrainment have been developed in recent years to provide better prediction of wound-roll stresses and woundroll quality. However, these models have limited predictive success in narrow-web winding, especially when a pressure roller is not used. During winding, and after a roll is wound, the air in the roll leaks out of the sidewalls through narrow gaps between the layers. The amount of air leaking through the sidewall, when the web is narrow and has a rough surface, is significant. When side leakage is not properly considered, the accuracy of the air entrainment model can be greatly affected. In this paper, a new winding model is developed that includes the effects of air entrained during winding and the subsequent air leakage through the sidewalls during and after winding. Some results of this model are presented, together with comparisons to experimental results and predictions from other historical models.
Rolls of films are routinely stored under varying conditions before being unwound into downstream operations. During storage, interlayer pressures can change relative to the pressures generated during winding. These changes can lead to problems such as film blocking (increased interlayer pressure) and roll shifting/cinching (decreased interlayer pressure). To study the storage effect, a nonlinear wound roll stress model including air entrainment is first developed and applied to predict the in-roll stresses during film winding. Thereafter, a thermal stress model is used to study the temperature effect on wound roll stresses. Key inputs to the models are the stack modulus, contact clearance, and air film reference clearance. A method is developed to measure these key model inputs. Results of a parametric study show that among the processing conditions, storage temperature and thermal expansion coefficients of the core and the film are key factors that affect in-roll ·stresses during storage. Limitations of the models will also be discussed along with recommendations for future modeling development.
Historically, wound-roll models have been used to predict stress levels which develop within winding and wound rolls. To gain maximum benefit from these models, stresses must be incorporated into defect or failure models. This paper focuses on the development of a wound-roll dishing model. Caliper nonuniformity in both the width and the length direction is incorporated into the model. The effect of some of the important conveyance issues is also included in the model. An experiment is described and the results compared to the analytical predictions. Finally, the model is used to study the effects of process parameters on the level of wound roll dishing.
A model has been developed which predicts widthwise variability in wound roll radius and stresses resulting from widthwise thickness nonuniformities. This model has been verified experimentally on wound rolls of film and quantitative agreement between the predicted arid measured results was found, The theoretical and experimental techniques used in its verification are described.
