Taking a Closer Look: Innovations in topographical analysis
By Pulp & Paper Canada
Pulp & Paper Canada Special Report...
By Pulp & Paper Canada
Pulp & Paper Canada Special Report
This month, Pulp & Paper Canada takes a look at the topic of machine clothing, more specifically, at the issue of analysis of the topographical properties of the fabric itself. As there have been many recent developments that have made it possible to examine the fabric at increasingly better resolutions, we are surveying the industry in order to showcase what the industry’s prominent players are working on.
In order to do this, P&PC approached four companies, Voith Paper, AstenJohnson, Weavexx and Albany International, and posed the following question:
Please outline the methods that are currently being used to scrutinize the fabric, the development of any new technology that is making this process easier and what resolutions can be obtained with the employment of such topographical analysis? Is nanotechnology an issue? Are there any other problems that still remain to be addressed?
Here are their responses.
The realization that past characterization methods used to determine product properties of paper machine clothing were not fulfilling printer’s special requirements, motivated Voith Paper Fabrics to select different, more innovative measuring techniques to determine surface properties of fabrics.
The demands on machine clothing performance characteristics have often conflicted, for example, internal volume vs. running time with forming fabrics or compaction resistance of press fabrics vs. non-marking ability. Papermakers need a fabric that is multi-faceted, a fabric that can obtain one advantage without sacrificing another. Therefore, a complex approach is needed to determine the technical product parametres necessary for advanced papermaking.
In order to do this, research and development requires complex technology in surveying the surface topography of machine clothing, one of the most influential factors on end product surface smoothness. Surface topography of clothing plays a major role in the end product printability, therefore, it must be constantly optimized during the development phase.
The most diverse methods of influencing fabric surface topography, such as sanding of forming fabrics in order to reach monoplane surfaces or the compaction and surface coating of press fabrics to increase surface smoothness, are necessary for the optimization of smoother fabric surfaces. A more even fabric surface leads to smoother end products, i.e., better printability.
Conventional methods of measuring surface topography of fabrics, although reliable, are very limited in the amount of knowledge they provide. Standard techniques such as impression tests or scan methods using analysis software only provide a two-dimensional surface view of the fabric. However, during the papermaking process, the third dimension plays a major role in influencing surface quality of the end product. During the dewatering process, the paper is formed by the fabrics, not just the top layer of the fabric, but the entire fabric structure. Voith Paper Fabrics, therefore, implements a third dimension into surface topography measurement techniques, evaluating and measuring all support layers of the fabric to ensure a smoother surface.
Using three-dimensional surface topography technology, Voith Paper Fabrics is able to establish most critical factors necessary to provide the fibres the ultimate amount of support. To do this, Voith Paper Fabrics Research and Development / Technology Teams work with a modular, optical scanning profilometer to optimize as well as stabilize manufacturing processes. Once the fabrics are scanned using a confocal laser, they are then put into a complex software system and analyzed, displaying required results. Results can be shown using topographic views, profiles (2D), photo-realistic views or 3D projections.
The results of these analyses are shared within the Voith Paper Fabrics product development teams. This sharing of information is done in order to be able to offer the customer a substantially optimized product. The notion that fabric surface topography is a result of the supporting fabric structure can now be proven and fabrics can therefore be modified and enhanced in the development phase and lead to improved, smoother end products for the customer.
With the use of innovative technology, Voith Paper Fabrics is able to characterize, in detail, surface topography of fabric samples upon initial evaluation. This provides a great deal of basis information and, therefore, shortens product development time, while saving money and energy on subsequent tests. With initial, detailed results from fabric samples as a basis, R&D is able to complete more qualitative tests with a higher rate of success.
Thanks to highly-sophisticated development testing and measurement techniques carried out prior to the products going to market, manufacturing processes as well as products in the development stages can be improved, providing customers with mature products and a higher product success rate.
Arved Westerkamp, Innovation Management
A number of techniques and equipment may be used to measure the surface topography of paper and paper machine clothing.
Stylus type smoothness measuring instruments such as the Emveco 210-R may be used to evaluate micro-surface characteristics of thin flexible materials such as paper and board products as well as plastic film. However, it is difficult or impossible to measure fabrics with this device, since it does not have the range suitable to the coarseness of the fabric surface.
One technique for measuring fabric topography involves the use of deformable metal foil or other material to essentially replicate the surface. The height variations (roughness magnitude) can then be digitized with a surface profiler.
Low angle surface lighting combined with FFT (Fast Fourier Transform) of a resulting digitized image may be used to bring out the topography and repeating patterns on a sheet surface which may be related to a fabric structure.
Per cent contact under load can be measured using a Chapman smoothness tester. This gives an indication of the topographical nature of a fabric surface.
Caliper profiles are often measured but the distance resolution is not what people typically associate with topographical analysis.
Tactile pressure measuring film having a spatial resolution of five to 15 microns may also be used to generate a pressure distribution profile of a fabric surface under load.
One of the latest technologies to be applied to this problem is confocal microscopy. Through its controllable depth of field and spatial filtering capabilities, the confocal microscope may be used to generate high-quality three-dimensional images of a fabric sample, with vertical resolution ability down to one micron.
Laser and conventional optically based scanning equipment has also been demonstrated to have the ability to map a fabric surface in significant detail. A WYKO type optical surface profiler can be used to obtain the height data of most smooth fabrics. Its large range of both spatial and vertical resolutions make it a versatile instrument (vertical measurement range: 0.3 – 500 micron).
An atomic stylus device better known as atomic force microscope (AFM), having resolution close to that of a scanning electron microscope (SEM), might be used to get the nano-scale topographic detail of the surface. The advantage of this equipment compared to the SEM is its ability to digitize the surface, providing the opportunity to assess surface statistical quantities, also without coating the sample. However, the maximum vertical displacement is usually limited to about ten micron.
Problems arise in trying to relate static topographical evaluations of paper machine clothing to the resultant paper surface as dynamic and static web consolidation forces are very often quite different. We have also learned that the topography of fabrics and/or the final sheet are not the only influ
encing factors on the final paper and print quality.
Brady Patterson, Advanced Product Development Leader – Press
Weavexx has recently added tools to enable very accurate and detailed representation of the topography of forming fabrics, press felts, and dryer fabrics.
Forming and dryer fabric topography is comprised of segments of yarns, generally oriented in either the md or cmd direction of the fabric, and always with some degree of curvature in a plane perpendicular to the fabric.
Explanation of the performance of a fabric has long been based on simplified models of these yarn segments. These model segments generally have been perfectly straight, oriented exactly in md or cmd, and arranged at an elevation in the fabric that facilitated useful calculations.
Relative locations of these yarn segments was historically represented by a measurement of just one point on the surface of each yarn. This measurement was then used in several ways, including for example, in process management and in arguments used to predict product performance.
Newer digital microscopy tools now enable the locations of the surfaces of these yarn segments to be precisely defined by collecting very accurate x/y/z data. As an example, Weavexx now uses digital microscopy to measure the location of the surface of a yarn segment in more than 200 increments to facilitate very accurate calculation of sacrificial yarn volume in the wear surface of a forming fabric. This volume is one factor used in designing a fabric to meet wear- life expectations in a specific application.
A second example relates to the evaluation of results of manufacturing processes designed to bring the highest points on all yarn segments to the same elevation (as well as batt uniformity in press felts). It is useful to assess this parametre over several multiples of the weave repeat. Automated x/y/z measurements and data capture with the digital microscope make this a practical evaluation today. Papermakers are now assured of superior topography on which to form the fibre mat.
Weavexx is also doing very exciting work today with a laser-scanning tool called Weavexx Surface Analyzer (Fig. 1) capable of measuring elevation of a spot on the surface of a yarn segment, or felt surface, to an accuracy of 0.01 microns. When coupled with extremely accurate x/y location data, the elevation information is transformed by system software to present a digital “map” (Fig.2) showing the shape of each yarn segment and precisely locating each one relative to the others in a three-dimensional space. This three-dimensional, digital “map” is being used to calculate fabric properties, which are then correlated to sheet-forming performance. For example, we now can measure for any array of curved yarn segments, the actual dimensions of fibre support and unsupported area at any elevation in the fabric.
Scanned samples of hand-sheets formed on tested fabric structures are enabling refinement of predictability of sheet-forming results.
Ultimately, the papermaker benefits from the application of this work to the design of more efficient paper machine clothing with the ideal papermaking topography for sheet properties and machine runnability.
Wayne Freeman, Technical Director – Forming Fabrics
Forming fabrics influence two key properties in paper print quality; sheet surface smoothness and sheet density uniformity. The papermaking surface of a forming fabric is a series of knuckles and drainage holes. The initial fibre mat forms around the knuckles and imbeds down into the holes until being supported. Minimizing the topography difference between peaks and valleys and optimizing the knuckle distribution can significantly improve print dot uniformity and “dot skip.” Micro sheet density is influenced by drainage velocity variations through the open structure of the fabric. The sheet surface is washed out of fines and fillers by activity pressure and vacuum pulses from stationary and vacuum elements.
A dynamic sheet former is used to simulate actual paper machine operation. A moving headbox lays down a fibre mat onto a stationary fabric over top of a vacuum box. Pressure and vacuum pulses are created within the box to simulate activity during sheet formation. Next, a table roll passes under the newly formed sheet to backwash through the fabric. Oriented hand sheets with formation approaching paper machines are achieved.
Image analysis is used on the hand sheets to identify knuckle and drainage marks. Marking frequency and intensity may be compared for different fabric structures. These images may also be overlaid with standard print halftone images looking for interference patterns between the knuckles and the print dots. New woven structures may be screened as well as optimization of fabric materials and manufacturing processes may be carried out. This equipment has also been used in the development of new seam patterns to optimize termination distribution uniformity in ultra fine and dense structures.
Pressure uniformity of press fabrics is a key variable that influences dewatering in a press nip as well as sheet quality. For a long time, pressure uniformity was only measured at a macro-scale due to limitations in resolution from measuring equipments and substrates used during the measuring process. In recent years, advancement in both equipments and substrates enabled press fabric suppliers to measure and quantify micro-scale uniformity.
Two methods have been developed by Albany International to quantify press fabrics pressure uniformity. While both methods have demonstrated a direct correlation with sheet dewatering and sheet quality, the equipment, substrate and output are significantly different.
The first method, which uses a high-resolution pressure sensitive film, generates a series of pressure data points from a compressed sample. The impression is converted into stress units with a very fine spatial resolution ranging from 12.5 to 40 microns. The standard deviation of the sample distribution is then calculated and used to predict the influence on sheet quality and sheet dryness. Lower standard deviation of pressure indicates better pressure uniformity which correlates directly to sheet dryness and smoothness values.
The second method which uses wax impression generates a series of roughness data points. The impression is converted into a grey scale image, which is then converted into a roughness measurement (microns). Again, a direct correlation has been found between the roughness measurement and the influence on sheet quality and sheet dryness.
Both tests are being used to develop future press fabric structures with superior pressure uniformity characteristics.P&PC
David McVey, Applications Director North American Forming Fabrics, and Philippe Degani Technical Director, Albany International Corp. North American Press Fabrics.