Patent Description:
Inkjet printing devices are widely used in multiple technical fields for printing on different printing supports, for example for printing on paper, ceramics, fabrics, plastic films and more.

Document <CIT> discloses a method for adjusting a plurality of printing heads of an inkjet printing device, the printing heads being arranged in groups each of which prints an ink, the method comprising the steps of: printing a test support, analysing the printed test support and defining a tonal index.

Industrial inkjet printing devices typically comprise a plurality of print bars, each of which includes a plurality of printing heads. Each printing head in turn comprises several dozen, or even hundreds, of nozzles.

Each print bar is configured to apply an ink of a predefined colour to a print support. For example, in the case where the inkjet printing device operates according to the four-colour technique, at least four print bars are provided, which are configured respectively to apply yellow, cyan, magenta and black ink.

The inkjet printing device further comprises a control unit by means of which it is possible to select the printing density of each printing head depending on the printing result it is wished to be obtained.

Occasionally, even though a certain printing density for a print bar has been selected, the printing heads that make up that print bar do not provide a uniform printing result. This can be due to various reasons, such as different wear for the heads, clogging of one or more nozzles, ink characteristics and more. Whatever the reason, the fact that the printing heads, with the same selected printing density, provide different printing results from each other, is a significant drawback that can compromise the quality of the printed images.

An object of the invention is to improve the printing results provided by inkjet printing devices.

A further object is to remedy the drawbacks that inkjet printing devices may have when different printing heads print at different printing densities, with the same printing density selected.

Another object is to provide a method which makes it possible, in a simple and effective manner, to adjust an operating parameter of the printing heads of an inkjet printing device so as to ensure that the printing results are as uniform as possible between different printing heads.

According to the invention, a method is provided for adjusting a plurality of printing heads of an inkjet printing device, the printing heads being arranged in groups each of which prints an ink, the method comprising the steps of:.

The method according to the invention makes it possible to make the printing results provided by different printing heads uniform. Any differences between the amounts of ink delivered by each head are compensated by intervening on the electrical voltage applied to each head.

The invention can be better understood and implemented with reference to the accompanying drawings, which illustrate a non-limiting exemplary embodiment thereof, in which:.

<FIG> shows an inkjet printing device <NUM> for printing images, writings, or other on a printing support <NUM>. The printing support <NUM> may be in the form of a continuous tape, as shown in <FIG>, in which case the printing support <NUM> may be made of paper, fabric, plastic or other materials. In an alternative version, the printing support <NUM> may be a discrete support, for example a ceramic tile, a glass plate, a wood panel or other.

The printing device <NUM> may be positioned above the printing support <NUM>, although other mutual arrangements are in principle possible.

The printing device <NUM> and the printing support <NUM> are movable relative to each other so as to present successive portions of the printing support <NUM> to the printing device <NUM>. This can be achieved by holding the printing device <NUM> in a fixed position and moving the printing support <NUM> in an advancement direction F. It is also possible to hold the printing support <NUM> fixed and move the printing device <NUM> in the advancement direction F, or move both the printing device <NUM> and the printing support <NUM>.

The printing device <NUM> further comprises a plurality of print bars <NUM>, each of which extends transversely, in particular perpendicularly, to the advancement direction F. Each print bar <NUM> defines a group comprising a plurality of printing heads placed side by side, arranged in sequence along a main or longitudinal dimension of the print bar <NUM>. Each head in turn comprises a plurality of nozzles.

Each print bar <NUM> is configured to apply a certain ink on the printing support <NUM>.

In the depicted example, the printing device <NUM> operates according to the four-colour technique and comprises four print bars <NUM>, arranged to apply cyan, magenta, yellow and black ink respectively. The printing device <NUM> may however comprise a number of print bars <NUM> other than four, for example more than four print bars <NUM> or even less than four print bars <NUM>.

The printing heads forming each print bar <NUM> operate by exploiting the piezoelectric effect, since each nozzle comprises a piezoelectric element that deforms following the application of an electrical voltage. When the piezoelectric element deforms, a deformation occurs in the corresponding nozzle, which generates a pressure that expels a drop of ink from the nozzle considered.

It is possible to select, for each head of each print bar <NUM>, the value of the applied electrical voltage, which can be varied between a minimum value Vmin and a maximum value Vmax.

A desired value of the printing density can be selected for each printing head of each print bar <NUM>, which value can vary between <NUM>% if the ink in question is not applied, and <NUM>% for the maximum amount of ink applicable.

Even if the same value of the printing density is selected for each printing head that forms a print bar <NUM>, it may happen that the printing heads of the print bar <NUM> operate unevenly and apply different amounts of ink between the printing heads on the printing support <NUM>. The following describes a method for adjusting the printing heads so as to obtain a printing result that is as homogeneous as possible between the different printing heads, with the same printing density selected.

In an initial step of the method, the printing device <NUM> generates a printed test support <NUM>, shown in <FIG>, on which each print bar <NUM> prints, in sequence, a test strip <NUM> extending transversely to the advancement direction F. Each test strip <NUM> is printed with a single ink, applied by the corresponding print bar <NUM>. Each test strip <NUM> comprises a plurality of rows, each of which extends transversely, in particular perpendicularly, to the advancement direction F. Each row is printed with a selected or nominal constant value of the printing density. The selected value of the printing density gradually increases in a direction parallel to the advancement direction F, passing from one row of a test strip <NUM> to the next. For example, an increase by <NUM>% of the selected printing density can be applied between one row of a test strip <NUM> and the next.

The test strips <NUM> have a width equal to the width of the print bars <NUM>, so that all the nozzles forming the printing heads of a print bar <NUM> print at least one coloured zone or patch on the corresponding test strip <NUM>.

In the example of <FIG>, there are four test strips <NUM> on the printed test support <NUM>, corresponding to the four colours applied by the print bars <NUM>. The test strips <NUM> are arranged one after the other along the advancement direction F. Within each test strip <NUM>, the relative rows (which have a constant nominal printing density) follow each other along the advancement direction F with a gradually increasing or decreasing printing density. For example, the lighter row of a test strip <NUM> may have a nominal printing density of <NUM>%, while the darker row of the test strip <NUM> may have a nominal printing density of <NUM>%.

The test strips <NUM> printed on the printed test support <NUM> may be surrounded by a frame <NUM>, of a known type, which may be of assistance in acquiring the test printed support <NUM> during a measurement step.

In the initial step of the method described here, at least two test printed supports are printed, corresponding respectively to the minimum value Vmin and to the maximum value Vmax of the electrical voltage applied to the print bars <NUM>.

After printing the printed test supports <NUM>, a measurement step is envisaged during which the printed test supports <NUM> are measured with a measurement instrument that can optionally, but not exclusively, be of the spectral type, such as a spectrophotometer. In particular, the measurements are made in each row at constant nominal printing density of each test strip <NUM>.

The measurements thus obtained are processed to calculate therefrom the value of a tonal index, i.e. a parameter indicative of the tone of the ink applied at the point where the measurement was made. The tonal index is related to the amount of ink actually applied at the point where the measurement was made.

The tonal index can be the absorbance, also called optical density. However, it is also possible to use other indicators different from absorbance as a tonal index. For example, in an alternative version, the tonal index could be the brightness, and in this case the measurements of the printed test supports <NUM> could be carried out by means of an RGB scan, i.e. using a non-spectral type measurement instrument.

In this way, for each ink applied, a graph of the type shown in <FIG> is obtained, which illustrates how the tonal index IT varies (shown on the y-axis) as a function of the point of the tow at constant nominal printing density at which the measurement was made (or patch, shown on the x-axis).

The graph of <FIG> was obtained for a determined value of the voltage applied to the print bars <NUM>, for example equal to Vmax or Vmin. Furthermore, the graph of <FIG> refers to a single print bar <NUM>, i.e. it is built with the data obtained by measuring a single test strip <NUM> of the printed test support <NUM>.

A plurality of bends <NUM> are shown in <FIG>, each of which refers to a value of the constant nominal printing density selected for the print bar <NUM> to which the graph refers. For example, the bend <NUM> that is further down in the graph of <FIG> corresponds to a nominal value of the printing density of <NUM>%, the next one of <NUM>%, and so on.

In the graph of <FIG>, a plurality of peaks corresponding to respective overlapping zones between two adjacent printing heads of the print bar <NUM> to which the graph refers are visible in each bend <NUM>.

An average ITm of the tonal index is then calculated for each head of each print bar <NUM>, using the individual values of the tonal index IT obtained by processing the measurements of the points printed by that head in a row at constant nominal printing density. In this way it is possible to obtain a graph of the type shown in <FIG>, which refers to a single print bar <NUM>, i.e. to a single ink applied, and to a determined voltage applied to the print bar <NUM> considered.

The series of points P1, P2, P3, P4, P5,. PN joined by a common line <NUM> in <FIG> correspond to the average value of the tonal index for the points printed by the first head, the second head, the third head, the fourth head, the fifth head and so on, up to the Nth head, of the print bar <NUM> to which <FIG> refers. A series of points P1, P2, P3, P4, P5,. PN is provided for each nominal printing density previously selected for the print bar <NUM>.

For example, the points P1, P2, P3, P4, P5,. PN of the series shown in the further up part of <FIG> correspond to the average value of the tonal index for the points printed by the heads of the print bar <NUM> fed with cyan-coloured ink, with a nominal printing density of <NUM>%.

The points P1, P2, P3, P4, P5,. PN of the series shown in the further down part of <FIG> correspond to the average value of the tonal index for the points printed by the heads of the print bar <NUM> fed with cyan-coloured ink, with a nominal printing density of <NUM>%.

The series of intermediate points correspond to intermediate nominal printing densities between <NUM>% and <NUM>%, increasing from bottom to top in <NUM>% increments.

The calculations that led to the generation of the graph of <FIG> are repeated for all the voltage values applied to each print bar <NUM>, at which it is wished to investigate the behaviour of the printing device <NUM>, for example Vmax and Vmin.

In this way it is possible to calculate, for each printing head of a given print bar <NUM>, a range within which the tonal index of the ink points applied by that head can vary. This is shown in <FIG>, in which, for each head, a variability range <NUM> of the tonal index, comprised between a lower limit <NUM> and an upper limit <NUM> is shown. More specifically, for a determined printing head of a print bar <NUM>, the lower limit <NUM> is the average value of the tonal index calculated taking into account all the nominal printing densities of that head, at the minimum voltage value Vmin. Similarly, the upper limit <NUM> is the average value of the tonal index calculated taking into account all the nominal printing densities of that head, at the maximum voltage value Vmax. An index is thus determined for each individual head which is linked to the amount of ink that that head has discharged onto the printed test support <NUM> depending on whether a voltage equal to the minimum voltage Vmin or to the maximum voltage Vmax is applied.

In the example of <FIG>, the variability range <NUM> of the tonal index of a given head is shown as a vertical segment, the lower end of which coincides with the lower limit <NUM> and the upper end of which coincides with the upper limit <NUM>.

<FIG> shows the values for all the heads of all the print bars <NUM>. In particular, the values relating to the heads of a single print bar <NUM> define a series of values <NUM> represented by respective close vertical segments.

By knowing, for a determined head of a predetermined print bar <NUM>, the average value of the tonal index at the minimum voltage value Vmin (lower limit <NUM>) and the average value of the tonal index at the maximum voltage value Vmax (upper limit <NUM>), it is possible to calculate a function that expresses how the average value of the tonal index ITM varies, for a given print bar <NUM>, as a function of the voltage applied to that print bar <NUM>. Graph of <FIG> is thus obtained.

In the depicted example, the bend of <FIG>, which refers to a determined head of a print bar <NUM>, shows how the average value of the tonal index ITM varies as a function of the applied voltage. The points of the line representing the trend of the average value of the tonal index ITM as a function of the applied voltage are calculated by linear interpolation starting from only two values of applied voltage, namely Vmin and Vmax, for which the test prints and the measurements have been made. However, more than two values of applied voltage and more complex interpolation functions can be used, depending on the degree of precision desired.

A similar graph is built for all the printing heads of all the print bars <NUM>, so as to be able to obtain, for each printing head, how the average value of the tonal index ITM varies as a function of the voltage applied to that head.

At this point, from the information contained in the graph of <FIG> (or from similar graphs built for the other heads), it is possible to derive the voltage value at which each head must work to obtain a desired value of the tonal index.

In other words, after selecting a value of the tonal index considered optimal and obtainable by each printing head for a certain printing process, it is possible to obtain the voltage value to be set in each printing head of each print bar <NUM> so that that printing head produces prints having the desired value of the tonal index.

The printing heads of the printing device <NUM> are thus made capable of printing uniformly, i.e. producing printed zones in which the selected tonal index has a substantially constant value.

<FIG> shows, in table form, an example of results that can be obtained using the method described so far. After choosing a value of the tonal index that is considered optimal, the electric voltage value to be applied to that head is provided for each printing head (in the example shown there are seven printing heads) and for each print bar (in the example shown there are four print bars corresponding to the cyan, magenta, yellow and black inks, respectively).

It has been experimentally verified that, by proceeding as described above, substantially uniform printing results can be obtained between the heads that make up the printing device <NUM>.

From the information obtained for the printing heads as described above, it is also possible to obtain other indications, as will be described below with reference to <FIG>.

<FIG> is a graph similar to the one of <FIG>, referring to a series of values <NUM> relating to the heads of a single print bar <NUM>. <FIG> shows the variability range <NUM>, the lower limit <NUM> and the upper limit <NUM> already defined with reference to <FIG>.

A band <NUM> common to the variability ranges <NUM> of all printing heads of the print bar <NUM> considered was also indicated. To obtain uniform printing results, it is possible to select only values of the tonal index that fall within the band <NUM>, so that all heads of a print bar <NUM> can work with the selected value of the tonal index.

If a printing head has a tonal index that can vary in a variability range <NUM> placed outside the band <NUM>, it means that that printing head works excessively unevenly with respect to the remaining heads, and it is therefore appropriate to intervene on that head, for example by replacing it.

In this way, the method described so far can also serve as a diagnostic tool, to understand when it is necessary to intervene for replacing or repairing a printing head.

It is also possible to calculate the average tonal index, as previously indicated, for the working or current electrical voltage value Vatt with which the printing device <NUM> was actually working before starting the procedure to adjust the printing heads so as to obtain uniform printing results. This value of the tonal index can be referred to as the current average tonal index.

The current average tonal index can be determined by printing a test support <NUM> while the print bars <NUM> work with an electrical voltage equal to the current electrical voltage Vatt, using the same distribution of coloured zones adopted when printing the test supports <NUM> with the maximum and minimum values of the voltage Vmax and respectively Vmin.

The test support <NUM> corresponding to the electrical voltage Vatt is measured and the measurements obtained are processed to obtain the current average tonal index, which has been indicated with the reference number <NUM> in <FIG>.

By proceeding in this manner, for each printing head of each print bar it is possible to determine a current average tonal index <NUM>, which is interposed between the lower limit <NUM> and the upper limit <NUM>.

In the graph of <FIG>, an auxiliary reference line <NUM> can also be inserted, positioned near the current average tonal indices <NUM> of the printing heads analysed, in order to quickly identify, in a visual way, which printing heads work most unevenly, and then quickly intervene on the voltage values applied to those individual printing heads.

More specifically, the auxiliary reference line <NUM> is the straight line drawn at a reference value of the average tonal index, with respect to which the deviations of the current average tonal indexes <NUM> are minimised.

In this way it is possible to highlight the printing heads having an average tonal index <NUM> that deviates more from the reference value of the average tonal index (corresponding to the auxiliary reference line <NUM>), and intervene only on these heads, modifying the electrical voltage, for example using a table similar to that of <FIG>.

This makes it possible to eliminate or reduce any printing defects without completely changing the way the printing device <NUM> operates.

For example, in the case of <FIG>, one could intervene only on the printing head no. <NUM> whose current average tonal index <NUM> deviates more from the value corresponding to the auxiliary reference line <NUM>.

In the examples described so far, reference has always been made to a printed test support <NUM> in which the coloured zones printed by each print bar <NUM> are arranged along test strips <NUM>.

However, this condition is not necessary and, in a version not shown, it is possible to provide a printed test support <NUM> comprising, for each print bar <NUM>, a series of coloured zones arranged according to a predefined distribution, not necessarily in the form of a strip. More specifically, for each print bar <NUM>, there is provided a series of coloured zones for each nominal printing density that it is wished to test.

The predefined distribution can be chosen, for example, to maintain a noncritical ink flow between the different printing heads.

Claim 1:
A method for adjusting a plurality of printing heads of an inkjet printing device, the printing heads being arranged in groups each of which prints an ink, the method comprising the steps of:
- printing at least two test supports (<NUM>) respectively for a first electrical voltage (Vmin) and for a second electrical voltage (Vmax) applied to the printing heads;
- analysing the printed test supports (<NUM>);
- defining a function which expresses how a tonal index varies as the electrical voltage applied to the corresponding printing heads varies, the tonal index being indicative of the amount of an ink applied by each group of printing heads, the step of defining a function being carried out using measurements of the printed supports (<NUM>) performed during the step of analysing;
- selecting a desired value of the tonal index for the printing heads;
- determining, based on said function, an electrical voltage to be applied to at least one printing head in order that said at least one printing head works with said desired value of the tonal index.