4.1. Justification of the Need for Individualized Optimization of Process Parameters
The quality of corrugated cardboard is influenced by both the quality of the raw materials and the technological process of its production. In the technological line of the corrugator, it is possible to control this process by setting its parameters from the control panel. In the control systems of modern corrugators, the desired temperature values of raw materials are set, and the automation system controls the adjustable quantities so as to achieve the set parameter values. Regardless of the automatic setting of selected process parameters, the operator has the ability to change them.
In the examined technological line, dependencies of recorded process quantities on the individual habits of the operator conducting production were observed. Temperature distributions at individual measuring points averaged progressively over a two-month period tended to reach fixed values. At points P3P2, P2b, P5P4c, and P3P2c, temperature distributions tended to a single value, independent of the operator. At some measuring points, however, a different behavior of the recorded temperature waveforms was observed. For example, the temperature waveforms at points P5P4 and P4b shown in
Figure 10 and
Figure 11 tend to have a different value in each operator. Since the recorded data relate to the same production conditions, the observed differences suggest the influence of the individual preferences of the operator conducting production on the setting of its parameters. The differences may result from the fact that each operator adopted a different goal and had in mind different product parameters that determined its quality. For one operator, the most important determinant may be efficiency measured by machine speed, while another may pay more attention to maintaining the best possible flatness of the sheets of produced cardboard or the greatest possible energy savings. It is difficult to clearly assess what goal should be achieved in the first place. However, it can be seen that the sets of process parameters given by each operator tend to a constant value as they are averaged, which means achieving the optimum in the subjective understanding of the goal function. It is possible to automate the setting of initial production parameters for a given type of cardboard, either individually for a given operator according to their previous experience, or averaged over all operators, which is a compromise between individual preferences for settings. However, this requires knowledge of the values of individual process variables and, consequently, their continuous measurement. It is desirable that the set parameters be achieved by automatic adjustment of machine settings, e.g., angles of wrapping of heating cylinders by paper or feed steam stream. In the studied technological line, there is no possibility of automatic adjustment of process variables. On the other hand, a visual presentation of desired and currently achieved values can be a valuable indication for the operator.
An additional argument justifying the need for continuous monitoring of process quantities is the possibility of quick detection of emergency situations or the occurrence of problems in the technological line that deteriorate production conditions, e.g., watering of cylinders or heating plates, or uneven web tension. Detection of deviations of process variables from typical for a given production is a good indicator of these situations and allows for quick reaction, often protecting against the production of batches of cardboard with worse properties.
4.2. Algorithm for Optimizing Production Parameters
The essence of the algorithm is to provide the operator with a tool that allows them to optimize the parameters of the production process in accordance with their own experience, enabling the use of data from previously used settings. For this purpose, continuous measurement of the surface temperature of the papers used for production, the webs of the produced single-face board, the board produced, the temperature of the adhesive in the glue pans, and the speed of the machine should be continuously measured in the process line of the corrugator. The measured values are recorded by the measurement data acquisition system, then they are processed in accordance with the developed data analysis algorithm and presented to the operator on an ongoing basis as the desired values during the production of the selected cardboard assortment.
The data analysis algorithm is based on averaging the values of measured quantities from the last several dozen productions carried out by a given operator (the number of productions is a parameter of the algorithm) and for a given type of cardboard. The implementation of the algorithm consists of the following actions:
Entering an individual database of paperboard whose production starts if it has not been previously produced by the operator concerned, establishing and recording in the database the initial production parameters in accordance with the operator’s experience in the production of similar paperboard or the experience of another operator in the production of the board in question, if it has already been produced;
Taking from the database the average value of previously recorded parameters of a certain number of cardboard productions and taking them as the initial parameters of a given production;
Modification of set parameters to improve board quality or process efficiency under given conditions;
Adding modified parameters to the database with automatic removal of the oldest production parameters if the number of production data exceeds the assumed history size.
The operation of the main data processing loop within the proposed algorithm is shown in
Figure 14.
A comparison of the averaging results of the flute temperature distribution in a B-wave single facer during CB67-079N board production by three different operators shown in
Figure 11 reveals a discrepancy in the settings used by individual operators. A similar effect can be seen in
Figure 12, which presents a comparison of the results of averaging the temperature distribution of the liner in a single facer producing wave C. Small discrepancies in the recorded temperatures are also visible at the P3P2c point, i.e., on the surface of the flat layer of one of the cardboard webs after the double facer (
Figure 13). In the last two cases, the observed differences are at the level of one degree Celsius, which in the case of individual measurements could be treated as an accidental error; however, the fact of averaging a large number of measurement data improves the accuracy of the measurement and allows us to assess the actual differences in temperature values during production carried out by different operators. In other places, there are also discrepancies between the average values of temperatures achieved by all operators and the values obtained by each operator, although these differences are smaller.
Different settings are used by each operator in an attempt to achieve different goals in the production process. For example, the combination of machine speed (the transport speed of the cardboard produced) shown in
Figure 15 reveals the desire of operator No. 1 to achieve the highest possible production efficiency. The average machine speed in the analyzed period was 140.5 m/min for the first operator, while the average for other operators was 128.7 m/min. The higher speed used by operator 1 did not cause any deterioration in the mechanical properties of the produced cardboard, as evidenced by the results of the bursting strength and Flat Crush Test (FCT) measurements summarized in
Table 4 and
Table 5. However, differences in the thickness of the cardboard produced by different operators can be seen (
Table 6). The average thickness of the board for operator 1 is 5.82 mm, while the thickness of the board produced by other operators is 6.42 mm. One of the possible reasons for these differences is the viscoelasticity of the paper, whose permanent deformations, and thus the geometrical parameters of the wave, depend on the humidity, temperature, pressure value between corrugating rollers, and duration of this pressure. The behavior of paper exposed to these physical quantities can be described by a four-parameter model, e.g., Burgers [
38]. Models with fewer than four parameters are not able to describe the behavior of paper, and, in practice, rheological models are very rarely used to describe its behavior. The gradual increase in speed achieved by all operators in the final production of the analyzed series was the result of the introduction of a process innovation allowing for increased production efficiency.
The results of laboratory measurements of the properties of CB67-079N are presented in
Table 4,
Table 5 and
Table 6.
The bursting strength values of the cardboard produced by individual operators do not show significant differences. In general, the bursting strength of the cardboard is similar to the sum of the bursting strengths of its individual layers, which in the present case did not change. Similarly, Edge Crush Test (ECT) values can be explained, the differences in which in the examined cases are within the limits of measurement errors.