Method for diagnosing the printing process Background of the invention

The invention relates to a method for diagnosing the printing process. In particular, the invention relates to ascertaining a need for maintenance of the rollers, wherein the need for maintenance of the rollers is ascertained on the basis of the cross-directional profile of tension of the web in the machine direction measured from the web.

The invention also relates to a method for aligning the web of a web printing process in which on at least one roller is installed at least one pressure sensitive sensor on the surface or under the coating of the roller. Preferably, said pressure sensitive sensor measures the location of the edge of the web, preferably from both ends independently.

The invention also relates to a computer program into which is fed the information obtained from the method according to the invention and which guides the automatic maintenance and/or adjustment of the printing process.

For a printing press, no method is known for measuring, for example, the dirtiness of a roller or rollers of the printing press, nor to ascertain a need for cleaning and maintenance caused by dirtiness. Neither is it possible to measure flapping of the web.

Offset printing technique is currently the most commonly used printing method, hi offset printing technique, the printing and non-printing surfaces are on the same level, wherein it can also be referred to as a planographic printing method. The printing element (printing plate) is a level surface on which can be discerned neither clearly raised areas (letterpress printing method) nor recesses (gravure printing method). The planographic printing method is based on the mutual relationship between oil and water, i.e. on that they repel one another.

The offset printing method is based on the fact that oil (ink) and water repel one another. The surface of the printing plate is exposed in a plate printer such that the areas to be printed become areas that attract oily printing ink but repel water. The non-printing areas,

on the other hand, attract water but repel the oily ink. Because the printing ink is oily, it attaches only to areas accepting ink.

In the offset printing method, the image to be printed is not transferred directly from the printing plate to the paper rather first as a reversed image to the surface of the rubber cylinder, then from there the right way up to the paper. This has also given rise to the term "offset" because, in this printing technique, the blank to be printed (the printing plate) is never in direct contact with the printing substrate (the paper) rather the blank is transferred from the printing plate to the rubber blanket of the printing unit and then from there to the paper. The material of the paper is too hard and it would quickly wear down and damage the surface of the printing plate.

When printing occurs in the offset printing method, the dampening rollers dampen the surface of the printing plate and, at the same time, the inking rollers spread ink on the printing plate, wherein the dampening water attaches to the non-printing areas and the printing ink attaches to the printing areas. After this, the printing ink transfers from the printing plate to the rubber cylinder and the paper travels between the rubber cylinder and the counter cylinder, wherein the ink transfers to the printing paper.

Offset printing can be done in rotary printing or in sheet-fed printing. In rotary printing presses, an unbroken paper web travelling from a paper roll between the printing rollers is used as the printing paper, which paper web is cut into page sheets only at the end of the presswork. Rotary printing is a faster printing method than sheet- fed printing and, for this reason, it is used when the amounts to be printed are relatively large.

Gravure printing is a typographical printing method in which gravure printing presses are used. In the gravure printing method, the printing portion of the printing surface is lower than the non-printing surface. The printing ink transfers to the material to be printed from the recesses in the printing surface from which the method got its name. Gravure printing does not use as a printing surface printing plates secured around a cylinder, as in the offset printing, rather, on each printing occasion, the content to be printed is engraved

into the copper surfaces of cylinders with an engraving diamond. In addition to a high printing speed and sharp printing quality, the advantage of gravure printing is that relatively cheap mechanical supercalenderized paper can be used.

Gravure printing presses are also generally web printing presses (i.e. rotary printing presses), i.e. presses whose printing material comes from a roll. In the printing process, the gravure printing cylinder is first inked with a liquid printing ink, wherein the recessed cells on the surface of the cylinder are filled. Immediately after inking, a thin steel scraper, i.e. the doctor blade, removes the excess ink from the neck areas between the recessed cells. The material ^' to be printed travels between the impression cylinder and the gravure cylinder, wherein the printing ink is transferred from the recessed cells to the material to be printed with a great compression force. After printing, the printing material travels through a drying device which evaporates the solvent of the printing ink and dries the printing ink.

Traditionally, the operator circles the printing press and observes the behaviour of the web. If there is a problem in some area, then a part of the printing press is left alone, monitored only by automation, and new problems could arise in the process sites left without supervision. In the prior art, no solution to this problem based on measurements is known. Up to this point, operators have made the observations visually.

Currently, web location detectors known and in use in the printing press environment are based on the machine vision. The disadvantages of these are distortions of web locations resulting from the machine vision getting dirty, hi that case, the adjustment system adjusts the web alignment incorrectly, which causes problems in colour superimposition.

Brief description of the invention

The object of the invention is to solve the above mentioned problems. The object of the invention is achieved by a method according to independent claims 1 and 7. The preferred embodiments of the invention are presented in the dependent claims.

In the invention, there are preferably, but not necessarily, used measurement methods of the types in publications WO 2006/075055, WO 2006/075056 and FI 113466 B (METSO PAPER, INC.).

For measuring the location of the edge of the paper web, both the tension profile and profile differences between given process steps can be measured with iRoll technology (METSO PAPER, INC.) or other corresponding technology. In the invention, there is preferably, but not necessarily, used a piezo film. An EMFi film sensor, which is a permanently electrically charged cell plastic film which functions in the same manner as a piezoelectric sensor, can also be used. The voltage signal produced by an EMFi film depends on the sum pressure change exerted on the element such that, when a pressure change occurs, each portion of the element produces an equally large change in the initial voltage. Consequently, the short-term voltage measured with a large-impedance amplifier is the integral of the sum pressure of the sensor. The sensor functions as a dynamic sensor, it does not give a steady result for a sustained applied pressure rather the direct- current voltage component influencing across the sensor changes as a result of leakage currents. Because of this, the sensor functions only when it experiences changes in pressure. A solution according to the invention can also be implemented by means of another sensor, for example, by means of a resistive pressure sensitive film, a piezoelectric PVDF film (polyvinylidene fluoride film) or a PLL film (piezo- pyroluminescent film).

Preferably, the pressure sensor used in the method according to the invention, by means of which the tension profile can be measured, is either a movable film-like pressure sensor that can be installed on the surface of a roller or a roller which contains a fixedly installed film-like pressure sensor and can be installed at some site in the printing process.

Web tension is measured in an iRoll measurement in the machine direction, i.e. lenghtwise in relation to the web. If the measuring sensor is installed cross-directionally, then a measurement result is obtained for the entire length of the web. hi that case, a cross- directional profile is obtained for the lengthwise tension of the web. In practise, the tension of the web is never entirely even throughout the entire length of the web.

Proper printing press diagnostics based on a pressure sensitive sensor, preferably a film, are based on the sensitivity of the film for detecting variations. In a measuring program, these are seen as a vibration exhibiting at a given frequency or as a change of the shape of the tension profiles from a previously known or learned basic profile.

The method according to the invention can be used as a printing press cleanliness and maintenance diagnostic tool. As the rollers get dirty, the diameter of the roller changes, which causes in the web a tension profile having a shape that is deviant in comparison to that of the ordinary. This shape can appear, for example, as a systematic slanting in the profile. A measurement based on a pressure sensitive film is also sensitive enough to detect flapping appearing in the paper web or deterioration of the condition of the roller bearings. Flapping may result from a low web tension or an impulse coming from the frame via the roller/rollers.

In the method according to the invention, one or more iRoll rollers or rollers based on corresponding technology are installed in a printing press. The signal coming from the roller can be monitored by means of long-term data collection. When the roller is clean, from a paper is obtained a given "normal" profile, the basic profile. When the roller is dirty, a distorted profile is obtained which does not correlate with changes of the paper profile.

Preferably, a long-term change or a difference quantity of the long-term trend between a measurement after the unwinder or measurement data that came from the paper factory along with the paper roll and a measurement after the printing unit are compared. These profile differences are calculated by means of two or more measurements. For the difference signal are set allowed deviation limits which indicate a need for cleaning of the rollers or a need for other maintenance of the printing press.

By using the method according to the invention, maintaining of sanitation of the printing press is made easier, and unnecessary cleanings or maintenance actions do not need to be carried out on the rollers rather actions are implemented as needed on the basis of the diagnostic signal.

In another preferred embodiment of the invention, on a tension measurement roller or on one or more other rollers is installed a pressure sensitive sensor, preferably a pressure sensitive film, from which is obtained the information about where the edge of the web is located. This information can be used in aligning the location of the web as the method according to the invention fulfils the requirements set by the printing press for superimposing printing inks. In the preferred embodiment, the localization of the edge is done on the basis of the tension profile measurement.

In the method according to the invention, in a printing press is installed a pressure sensitive sensor on one or more rollers. One or more sensors are installed on the roller such that the information about the location of the side of the web is obtained from each of the ends independently. This information is utilized in the sideways alignment adjustment of the web and to optimise the superimposition of printing inks. In the invention, there is preferably, but not necessarily, used a measurement method presented in the publication FI 113466 B (METSO PAPER, INC.).

On the roller, on the surface or under the coating, is installed one or several units of a pressure sensitive film to accurately measure the location of the edge of the web. There is at least one of these rollers in the printing press, preferably several, or even as many as there are printing units.

Compared to the prior art currently in use, i.e. the machine vision, the technology according to the invention does not get dirty the way the machine vision does nor is it disturbed by paper dust or corresponding impurities floating in the air. Additionally, detection of the web edge can be combined, for example, with tension profile measurements in the same automation system, wherein adjusting can easily be automated.

The use of a pressure sensitive sensor for aligning the edge of a paper web provides very accurate edge alignment values which are required for successful superimposition of printing inks, wherein amounts of waste material can be reduced.

The measurement and control system, to which the invention relates, which significantly improves the prior art, provides new possibilities for applying the pressure sensitive sensor technology in the printing press environment.

Brief description of the figures

FIG. 1 shows a tension profile measured before the printing unit and after the printing unit from a dirty roller.

Detailed description of the invention

The invention relates to a method for ascertaining a need for maintenance of the printing process, wherein the need for maintenance of the rollers is ascertained on the basis of the cross-directional profile of tension of the web in the machine direction measured from the web. Preferably, the measured cross-directional tension profile is compared to a known basic tension profile, the deviation and/or profile difference between the basic tension profile and the measured tension profile is analysed, and the printing process is adjusted on the basis of the deviation and/or profile difference.

Preferably, the basic tension profile according to the invention is a tension profile obtained from the paper factory for the paper roll, the basic tension profile is based on data collected earlier from the same roller, or the basic tension profile is a tension profile obtained from another process site in the printing process, such as a tension profile obtained from a measurement after the unwinder.

According to one embodiment of the invention, the profile difference is outside the deviation limits set for a need for maintenance.

Preferably, a need for maintenance can also be ascertained when a measured tension profile does not correlate with said basic tension profile when vibration is exhibited at a given frequency or when there appears in the shape of the tension profile a slanting or a change that is unlike any previously known or learned basic profile.

In the method according to the invention, the defined need for maintenance is preferably washing a roller, sanding a cylinder, replacing a coating and/or overhauling roller bearings.

The invention also relates to a method for aligning the web of a web printing process in which on at least one roller is installed at least one pressure sensitive sensor on the surface or under the coating of the roller. Preferably, said pressure sensitive sensor measures the location of the edge of the web, preferably from both ends independently.

On the basis of the information obtained from the method according to the invention, the sideways alignment of the web is preferably adjusted, wherein it is preferably possible to optimise the superimposition of printing inks and/or guide the alignment of partial colouring, folding and cutting.

Fig. 1 shows the tension profile before the printing unit and after the printing unit when the roller is covered with printing ink. Graph 1 shows the tension profile before the printing unit and graph 2 is a graph of the tension profile after the printing unit.

The invention also relates to a computer program into which is fed the information obtained from tension profile measurements and deviations and/or profile differences, wherein the program guides the adjustments and maintenance actions made to the printing process.

It is obvious to a person skilled in the art that, as technology develops, the basic idea of the invention can be implemented in many different ways. The invention and its embodiments are not then limited only to the examples presented in the above rather many variations are possible within the scope of the claims.

EXAMPLE 1

A fixedly installed (JPY) iRoll roller was used to measure the tension profile of the printing process before and after the printing unit. Fig. 1 shows a graph of the tension profile in which is seen the shape of the tension profile before and after the printing unit.

At the moment the tension profile was measured, wherein the last roller was covered with printing ink, the tension profiles deviated from one another in the manner shown in Fig. 1. The slanting of the tension profile was reversed.

After cleaning this as much as lmm thick layer of printing ink, the tension profile after the printing unit turned back to the same direction as it was before the printing unit. After cleaning the rollers, the removal of the printing ink collected on the surface of the roller influenced then the diameter of the roller so much that the tension profile turned in the other direction. The printing ink collected on the surface of the roller clearly influences the tension profile. From this it can be concluded that maintaining of sanitation also influences the runnability of the printing press.