BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be outlined in greater detail with reference to the following drawings wherein,
FIG. 1 is a diagram showing an inkjet printer;
FIG. 2 is a diagram showing an ink chamber assembly and its associated transducer;
FIG. 3 shows a relationship between the electrical pulse and the pressure wave which is induced;
FIG. 4 shows a relationship between the accuracy of ink droplet placement and the ink droplet speed;
FIG. 5 shows a relationship between the reliability of an ink droplet ejection process and the ink droplet ejection speed;
FIG. 6 shows an example of a substrate to be printed with various types of image information; and
FIG. 7 is a block diagram showing a circuit that is suitable for measuring the effect of the droplet ejection in the ink chamber by the application of the transducer as a sensor.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a diagram showing an inkjet printer. According to this embodiment, the printer comprises a roller 1 used to support a receiving medium 2 (receiving substrate), such as a sheet of paper or a transparency which is moved along the carriage 3. The carriage includes a carrier 5 to which four printheads 4a, 4b, 4c and 4d have been fitted. Each printhead contains its own color, in this case cyan (C), magenta (M), yellow (Y) and black (K), respectively. The printheads are heated using heating elements 9, which have been fitted to the rear of each printhead 4 and to the carrier 5. The temperature of the printheads is maintained at a desired level by the application of a central controller 10. This arrangement also includes the necessary components in order to enable the printer to perform the method according to the present invention.
The roller 1 may rotate around its own axis as indicated by arrow A. In this manner, the receiving medium may be moved in the sub-scanning direction (often referred to as the X direction) relative to the carrier 5, and therefore also relative to the printheads 4. The carriage 3 may be moved in reciprocation using suitable drive mechanisms (not shown) in a direction indicated by double arrow B, parallel to roller 1. To this end, the carrier 5 is moved across the guide rods 6 and 7. This direction is often referred to as the main scanning direction or Y direction. In this manner, the receiving medium may be fully scanned by the printheads 4.
According to the embodiment as shown in this figure, each printhead 4 comprises a number of internal ink chambers (not shown), each with its own ejection site (in this case a nozzle) 8. The nozzles in this embodiment form one row per printhead perpendicular to the axis of roller 1 (i.e., the row extends in the sub-scanning direction). In a practical embodiment of an inkjet printer, the number of ink chambers per printhead will be many times greater and the nozzles will be arranged over two or more rows. Each ink chamber is provided with a piezo-electric transducer (not shown) which is adapted to generate a pressure wave in the ink chamber so that an ink drop is ejected from the nozzle of the associated chamber in the direction of the receiving medium. This droplet then travels through the air in the direction of the receiving medium 2. The exact location of placement of the droplet on the receiving medium depends, among other things, on the speed of the droplet. Since the desired speed is known beforehand, it can be calculated when each transducers should be actuated in order for a droplet to arrive at the intended location. The transducers are actuated, image-wise, via an associated electrical drive circuit (not shown) by the application of the central control unit 10. In this manner, an image built up of ink drops may be formed on receiving medium 2.
If a receiving medium is printed using such a printer where ink drops are ejected from ink chambers, the receiving medium, or a part thereof, is imaginarily split into fixed locations that form a regular field of pixel rows and pixel columns. According to one embodiment, the pixel rows are perpendicular to the pixel columns. The individual locations thus produced may each be provided with one or more ink drops. The number of locations per unit of length in directions parallel to the pixel rows and pixel columns is called the resolution of the printed image, for example indicated as 400×600 d.p.i. (“dots per inch”). By actuating a row of printhead nozzles of the inkjet printer, image-wise, when it is moved relative to the receiving medium as the carrier 5 moves, an image, or part thereof, built up of ink drops is formed on the receiving medium, or at least in a strip as wide as the length of the nozzle row.
FIG. 2 shows an ink chamber 19 and a piezo-electric transducer 16. Ink chamber 19 is formed by a groove in base plate 15 and is limited at the top mainly by piezo-electric transducer 16. Ink chamber 19 converges into an exit opening 8 at the end thereof, this opening being partly formed by a nozzle plate 20 in which a recess has been made at the level of the chamber. When a pulse is applied across transducer 16 by the pulse generator 18 via the actuation circuit 17, the transducer bends in the direction of the chamber. This produces a sudden pressure rise in the chamber which, in turn, generates a pressure wave in the chamber. According to an alternative embodiment, the transducer first bends away from the chamber, thus drawing in ink via an inlet opening (not shown), after which the transducer is moved back into its initial position. This also produces a pressure wave in the chamber. If the pressure wave is strong enough, an ink drop is ejected from exit opening 8. After the expiration of the ink drop ejection process, the pressure wave, or a part thereof, is still present in the chamber, after which the pressure wave will fully damp over time. This pressure wave, in turn, results in a deformation of transducer 16, which then generates an electric signal. This signal depends on all the parameters that influence the generation and the damping of the pressure wave. In this manner, as known from European patent application EP 1 013 453, it is possible, by measuring this signal, to obtain information on these parameters, such as the presence of air bubbles or other undesirable obstructions in the chamber. This information may then, in turn, be used to check and control the printing process.
In FIG. 3 a relationship between the electrical pulse and pressure wave induced is shown. For this, three examples of electrical pulses and corresponding pressure waves in the ink chamber are schematically provided in the figure. Firstly electrical pulse 40 is shown, which pulse is schematically represented as a varying voltage V during a time t. When this pulse is applied to the transducer 16 as depicted in FIG. 2, a pressure wave 50 is induced in the ink in the corresponding ink chamber. This pressure wave is schematically represented as a varying pressure P during a time t. Dot 51 indicates the moment when an ink droplet is actually ejected from the nozzle of the ink chamber. This droplet has a speed of 6 meters per second, which speed corresponds to the electrical pulse 40 for this ink chamber.
In the second example the electrical pulse 42 is shown, which pulse is also schematically represented as a varying voltage V during a time t. When this pulse is applied to the transducer 16 as depicted in FIG. 2, a pressure wave 52 is induced in the ink in the corresponding ink chamber. This pressure wave is schematically represented as a varying pressure P during a time t. It can be seen that this pressure wave differs substantially from wave 50 in that the amplitude and frequency are higher. Dot 53 indicates the moment when an ink droplet is actually ejected from the nozzle of the ink chamber. This droplet has a speed of 8 m/sec, corresponding to the electrical pulse 42 for this ink chamber.
A third example is given wherein electrical pulse 44 is shown, which pulse is also schematically represented as a varying voltage V during a time t. When this pulse is applied to the transducer 16 as depicted in FIG. 2, a pressure wave 54 is induced in the ink in the corresponding ink chamber. This pressure wave is schematically represented as a varying pressure P during a time t. This wave differs substantially from waves 50 and 52. Dot 51 indicates the moment when an ink droplet is actually ejected from the nozzle of the ink chamber. This droplet has a speed that corresponds to the electrical pulse 44. In this case, the speed is 5 m/sec.
FIG. 4 shows the relationship between the accuracy of ink droplet placement and the ink droplet speed. In the table, the first column shows a relative indication of the ink droplet placement accuracy, going from “Very high,” through “High,”“Moderate” and “low” to “very Low.” The dot placement accuracy corresponding to these indications is depicted in the second column by giving the droplet placement deviation as a percentage relative to the ink dot size after hitting the receiving substrate. Typically an ink dot has a size of 10 μm in diameter. A very high accuracy in this particular example thus corresponds to an ink droplet placement deviation of 5% of 10 μm which equals 0.5 μm. A very low accuracy in this example corresponds to an ink droplet placement deviation of 1000% of 10 μm which equals 100 μm.
FIG. 5 shows a relationship between the reliability of an ink droplet ejection process and the ink droplet ejection speed. Vertically, the reliability τ for ink droplet ejection process is given, i.e., as an average value for all the ink chambers of an ink jet print head. A reliability of 100% means that ink droplet forming process will always be successful. A reliability of, e.g., 98% means that, on average, two out of one hundred intended droplets will not be adequately formed (i.e., will not be formed in a way that they will hit the receiving substrate).
Horizontally the ink droplet ejection speed is given. For this particular print head it can be seen that with speeds up to 3 m/sec, the reliability is virtually 100%. After that the reliability starts to decrease noticeably, but up to 6 m/sec this will in general not lead to any disturbing print artefacts for regular ink jet prints. At a speed of 9 m/sec, the reliability has decreased to approximately 99%. This value in this example is regarded as a limit for good ink jet printing. Above that speed, the reliability is so low that print artefacts are becoming disturbingly visible. It may be clear for the skilled person that the actual relationship between the reliability and the ink droplet speed depends strongly on the type of ink jet head. This relationship has to be established for each inkjet head. In practice this can be done by varying the ink droplet speed and measuring the number of actual droplet ejections relative to the intended number of ink droplet ejections. Also, which reliability is still acceptable also largely depends on the application. For example, for text printing, less stringent demands will generally apply as compared to CAD drawings.
FIG. 6 shows an example of a substrate to be printed with an ink jet printer according to the present invention. The substrate is divided into parts intended for various types of image information. Substrate 2 is a transparent plastic medium that is being used as a mask in the prochamberion of printed circuit boards. Sub-part 60 is intended for an image that shows the title of the mask. The print quality needed for this type of image information is “Very low.” Sub-part 62 is intended for an image that reflects a technical specification of the actual mask. The print quality needed for this image is “Moderate” with respect to figures in the specification and “Low” with respect to text in the specification. Sub-part 64 is intended to receive the actual print mask. The print quality needed for this part of the substrate is “Very High.” Sub-part 66 is intended for an image that shows the date of prochamberion of the mask and other tracking data. The print quality needed for this type of image information is “low.”
When printing this substrate with the ink jet printer according to FIG. 1, using the method according to the present invention, only sub-part 64 will be printed with very high droplet speeds. The print quality of this part of the complete image, i.e., the print quality with respect to ink droplet placement, will be very high. The chances of ink droplet ejection failure are somewhat higher than for the other parts of the receiving substrate, but still low enough to guarantee an adequate image. The other parts are printed with lower ink droplet ejection speeds. Note that in part 62 two different droplet speeds will be used. A moderate speed with respect to figures to be printed and a low speed with respect to text to be printed.
FIG. 7 is a block diagram showing the piezo-electric transducer 16, the actuation circuit (items 17, 25, 30, 16 and 18), the measuring circuit (items 16, 30, 25, 24, and 26) and control unit 33 according to one embodiment. The actuation circuit, comprising a pulse generator 18, and the measuring circuit, comprising an amplifier 26, are connected to transducer 16 via a common line 30. The circuits are opened and closed by two-way switch 25 which can be devised as a hardware switch or as any other arrangement that electrically mimics the same effect. Once a pulse has been applied across transducer 16 by pulse generator 18, item 16 is, in turn, deformed by the resulting pressure wave in the ink chamber. This deformation is converted into an electric signal by transducer 16. After the expiration of the actual actuation, two-way switch 25 is converted so that the actuation circuit is opened and the measuring circuit is closed. The electric signal generated by the transducer is received by amplifier 26 via line 24. According to this embodiment, the resulting voltage is fed via line 31 to A/D converter 32, which offers the signal to control unit 33. This is where the measured signal is analysed. In this way, clear information can be provided about the circumstances in the chamber during the time the pressure waves run through the chamber. In other words, information can be gathered about the physical effect the droplet ejection step had in the chamber. If necessary, a signal is sent to pulse generator 18 via D/A converter 34 so that a subsequent actuation pulse is modified to the current state of the chamber. Control unit 33 is connected to the central control unit of the printer (not shown in this figure) via line 35, allowing information to be exchanged with the rest of the printer and/or the outside world.
It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim.
Patent Application
20070273721
