Production process

Training: the secretes of the wear of the forming fabrics


Precise measurements of the level of wear of forming fabrics enable the paper mill to increase the efficiency of the productive cycle and limit energy consumption and maintenance, thereby reducing all related costs. Entrusting skilled personnel with these tasks is an effective solution to achieve these objectives.

Measuring the wear of forming fabrics is one of the numerous, and yet essential parameters to monitor in a paper mill. The good performance of the paper produced, and the general efficiency of production are at stake. The maintenance of the forming fabric and, in particular, the control of the level of wear normally reached require pre-set downtime machines and the intervention of technicians sent by the forming fabric manufacturer, if necessary. The improvement of these aspects and, above all, the timely intervention with preventive actions might enable to considerably increase the efficiency of the productive cycle.

To achieve this goal, one can act on the way measurements are make. Measurements can be carried out along the line while the machine is working, however this can be done using methods that are more effective than the ones used so far.

Measurements on the forming fabric

In the paper production cycle, the impacts of and on forming fabrics are known, as explained by Stefanie Hänisch, application manager forming di Voith Paper who spoke on these topics at the 2019 Aticelca Congress.

«Some parameters are linked to the quality of production, while others to the performance of the forming fabric». These parameters change, for instance, depending on the phase of the service life of the forming fabric and its runnability, and also determine a variation of energy consumption. There are two main methods normally used to make measurements: the mechanical measurement method, which is carried out while the machine is being run by using a gauge, whereby this method is not very accurate and, above all, it poses risks to the safety of operators; and the measurements carried out by means of optical tools when the forming fabric is not being worked on, whereby this measuring method is more precise, but at the same time more expensive due to the machine downtimes and the fact that it is carried out at predetermined times.

The ideal solution would be to make use of a technology that takes the best from the two methods.

The importance of 3D images

There are measurement systems, which enable to take pictures of the forming fabric and, by analysing these pictures, to gain a better understanding and monitor its wear. A lot of information can be obtained from the images. «The image of a specific area in the forming fabric can, for example, show, if the same fabric is working properly, if there are problems in terms of fluidity or threads that begin to break. All these indications tell us when it is time to act». This method poses difficulties, too, however. According to Hänisch, in order to have clear images the time of exposure must be, first of all, very short. «With a typical speed of 1,000 m/min the forming fabric moves by about 20 micrometres per microsecond. It is, therefore, necessary to have a very short time of exposure to the flash, normally between 1 and 3 microseconds, in order to have snapshots also during PM running process».

Another difficulty lies in the fact that the pictures obtained are normally 2D, and «the two-dimensional nature does not provide all the information; therefore, we need a different model, if we want images to be complete».

That is the reason why Voith, in collaboration with the Baden-Württemberg Cooperative State University (DHBW) in Heidenheim, has developed a system capable of making line measurements of the optical line of the wear of forming fabrics through 3D pictures. «The system is made up of a high-performance standard camera equipped with high-quality optical components for the highest contrast possible, which is essential to highlight very little details, while a high-performance LED flash is the light source used». The system has been tested on various machine types.

Mechanical influences and energy

According to Hänisch, the 3D model that can be obtained with the Voith system is based on the total scan of the forming fabric, which is performed to give highly detailed information on a number of aspects. This essentially means that a computerized tomography of the forming fabric is carried out.

«Understanding the wear process requires information on the microscopic damages caused to the material, i.e. cracks, grooves, waves, holes, etc.».

The evaluations made along the line show the actual wear of the forming fabric. In this way, the system would provide indications also on the way wear works over a long time period and thus enable to understand how and when wear happens during the entire life cycle of the forming fabric.

The measurements made have also enabled to highlight how friction influences energy consumption in the forming section. Considerable savings in terms of energy and costs can, thus, be achieved by optimizing this aspect.

Hänisch goes on and says that «in terms of energy consumption, we noticed close correlation between the latter and wear: the highest energy consumption is registered when the machine is started. For example, we registered higher energy consumption values immediately after the installation of a new forming fabric, whereby they decrease until they become stable. This stabilization coincides with the optimization of machine runnability in the forming fabric section, where lower wear values are also registered».

This information make it possible to better understand what is going on in the PM and «get information about various working conditions, whereby the possibility to replicate them leaves the paper mill more room for manoeuvre to improve its machine», as well as to create prediction models on the service life of the forming fabric.

From theory to practice: the development of Q-Fit

According to the company Sales and Service Manager Luca Perin, based on these studies «Voith has recently developed a new service too, which is to be used by the company technicians, named Q-Fit. This is an innovative tool, which enables to photograph a forming fabric in operating conditions, without causing any disturbance to the PM working phases and making it possible to make high-quality pictures, which enable to evaluate the wear of the forming fabric and, based on that, make the necessary considerations and obtain relevant information».

There is an aspect that Luca Perin would like to stress, i.e. the increased level of safety obtained with this tool: «knowing the actual conditions of the forming fabric enables to avoid accidents and to plan machine downtimes and the changing of the forming fabric with more precision».

Q-Fit is a manual camera, which «enables to make pictures on the edges of the forming fabric, where wear is normally more evident. This makes it possible to have an indication of the wear along the edges and, as a consequence, of the level of wear of the same forming fabric». As Luca Perin explains, the core of the tool is an LED lighting system, which is extremely intensive and rapid, and enables to identify a rapidly moving object in detail. «The forming fabric has to move transversally compared to the camera; it has to move very rapidly and in environments with a high level of humidity and characterized by the presence of water», i.e. exactly like the forming section of a PM. For this reason, the Q-Fit is equipped with an «internal pressurization system of the photography head, which enables to eliminate the water and make sure that the picture is not perturbated. It is therefore possible to safely take pictures both of the wear, and of the paper side of the forming fabric and then evaluate them».

Voith can, thus, make truthful predictions of the residual functioning time of the working fabric.