EP3403829A1

EP3403829A1 - Method for opening a clogged nozzle

Info

Publication number: EP3403829A1
Application number: EP18172746.2A
Authority: EP
Inventors: Guido J.M. LEJEUNE; Johannes A.T. Gollatz
Original assignee: Oce Holding BV
Current assignee: Canon Production Printing Holding BV
Priority date: 2017-05-18
Filing date: 2018-05-16
Publication date: 2018-11-21

Abstract

A method is disclosed for actuating a print element in a print head comprising an array of print elements, a print element comprising a liquid, a nozzle, from which a drop of liquid is separable and a piezo-electric transducer for both converting an electric signal into an acoustic wave in the liquid and for converting an acoustic wave in the liquid into an electric signal to determine a status of the print element. The method comprises the steps of: tracing a print element that does not generate a drop of liquid, despite being controlled to do so, and actuating said print element with an electric signal that is adapted to an acoustic characteristic of the print element to deliver acoustic energy in the nozzle.

Description

BACKGROUND OF THE INVENTION

1. Field of the invention

The invention relates to a method for actuating a print element in a print head comprising an array of print elements, a print element comprising a liquid, a nozzle, from which a drop of liquid is separable and a piezo-electric transducer for both converting an electric signal into an acoustic wave in the liquid and for converting an acoustic wave in the liquid into an electric signal to determine a status of the print element.

2. Description of the Related Art

Print systems comprising an inkjet print process are known in a large number of varieties. The generation of a drop of ink from a nozzle of a print element is substantial in these processes, whether a thermal or a piezo-electric type of print head is used, a print head comprising an array of print elements for discharging ink drops resulting in a row of ink dots parallel to the array of print elements when landing on a receiving medium. Furthermore, either a scanning print head or a page wide-array of print elements may be applied. In either case the print elements are moving relative to the receiving medium such that a row of ink dots in a direction of the movement occurs, in general perpendicular to the array of print elements. A further important difference between the various inkjet print processes is in the type of ink that is used, which is e.g. water or solvent based, UV curable, hotmelt or any material that can be applied in a liquid form and solidifies after application, either spontaneously or advanced by a local physical condition, such as ultraviolet or infrared radiation, elevated or lowered temperature, air flow, etc.
All these print systems rely on the fact that a nozzle of a print element is open and sustains an ink flow for an application of ink drops. Occasionally, a nozzle of a print element may become clogged, due to various causes, such as paper dust, paper touches, ink particles, dried ink etc. This results in a situation wherein no drop of ink is generated by the print element, despite being controlled to do so. Various factors contribute to this condition, such as an environment humidity, ink composition variation and other factors. It is known that clogging of a nozzle may be prevented by bringing the ink in a print element into motion without generating a drop of ink. This is accomplished by a so-called non-jetting actuation of a print element and is applied when no ink dot is required at a corresponding print position.
If, despite all precautions, a nozzle still becomes clogged, the associated print element cannot be used in the print process, which may lead to defects in a printed image. During a maintenance process in between two print runs, a print head may be flushed to open the clogged nozzles by increasing the ink pressure in the channels of the print head, but this takes time and wastes ink. Therefore a flushing operation is limited as much as possible.
It is an object of the invention to open a clogged nozzle without flushing a print head.

SUMMARY OF THE INVENTION

In order to achieve this object, the method according to the invention comprises the steps of: a) tracing a print element that does not generate a drop of liquid, despite being controlled to do so; b) actuating said print element with an electric signal that is adapted to an acoustic characteristic of the print element to deliver acoustic energy in the nozzle by using a frequency that is lower than a resonance frequency of the print element in an operable condition, and c) determining if said print element is operable.
During printing, a list is drawn up of print elements that do not generate an ink drop upon actuation to do so. It has been found that a prolonged actuation of a print element on the list with a signal that is not aiming to generate an ink drop, but to increase an amount of acoustic energy in the print element, helps to break down a possible obstruction in the print element. It is noted that this obstruction often affects the acoustic properties of the print element. Therefore, the electric signal is adapted in order to optimally apply acoustic energy in the nozzle. This actuation uses a lower frequency than a resonance frequency of the print element in an operable condition, since the acoustic properties of the print element are affected by an obstruction in the nozzle. In order to increase an acoustic energy in the ink, the amplitude of the actuation signal may be raised. However, diminishing a frequency of the actuation signal, thereby approaching a resonance frequency of the clogged print element advances the provision of acoustic energy at the obstruction and its gradual removal. By regularly auditing said print element, it is determined if further actuation for deblocking is necessary.
In a further embodiment, the tracing of a print element in step a occurs by actuating a number of nozzles according to a predetermined pattern and determining if the actuation has resulted in a dot on a receiving medium. Optical means and image processing for finding a dot on a position associated with an actuated print element provide a reliable method for determining if a print element is operable. If no dot is found in the pattern, the print element is added to the list of failing print elements. The predetermined pattern comprises any image wherein an amount of ink of a single print element can be discerned, including images in a print job.
Alternatively, the tracing of a print element in step a occurs by actuating a nozzle and determining its status from an electric response signal in the piezo-electric transducer as caused by an acoustic wave in the liquid of the print element. Analyzing the electric response pertaining an oscillation time or a damping factor indicates whether or not a nozzle is clogged. Typically, a clogged nozzle increases the damping of an acoustic wave and reduces its oscillation time relative to an open nozzle.
In a preferred embodiment, the actuation of a print element in step b occurs when the print head is capped in a maintenance station. The actuation takes place over a prolonged time in order to gradually break down the obstruction in the nozzle. At some moment a nozzle may open and ink will come out of the print element. If the print head is capped, it will not pollute the print system.
Further details of the invention are given in the dependent claims. The present invention may also be embodied in a print system comprising an inkjet print process and a control unit for controlling electric signals that are applied to a print head in said process, wherein the control unit is configured for executing a method as indicated above.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from the detailed description given herein below and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein:

Figure 1: is a print element wherein a method according to the invention is applied;
Figure 2: shows an actuation signal for a print element;
Figure 3: shows a response after actuation of an open nozzle;
Figure 4: shows a response of a clogged nozzle; and
Figure 5: shows a signal for maximizing an acoustical energy in a clogged nozzle.

DETAILED DESCRIPTION OF EMBODIMENTS

The present invention will now be described with reference to the accompanying drawings, wherein the same or similar elements are identified with the same reference numeral.
First of all, Figure 1 shows a schematic drawing of a print element 53, wherein the invented method is applied. A nozzle 51, from which an ink drop is to be generated, is the end of an ink chamber 52, which is filled with ink. An acoustic wave is generated in the ink when a piezo-electric element 58 is activated by providing an electric signal to electrode 57. If this acoustic wave is sufficiently strong, an ink drop is formed at the nozzle 51. A membrane 56 separates the piezo-electric element 58 from the ink chamber 52. The ink chamber 52 is connected to the ink channel 55 with restrictor 54, for separating an acoustic wave in the ink chamber 52 from the ink channel 55. Print element 53 is one of an array of print elements connected to the ink channel 55.
Figure 2 shows an example of an electric signal 6 that is provided to electrode 57. Starting from a situation wherein the ink is in rest, a voltage across the piezo-electric element is raised with a slope 7 up to a plateau 8. During this time, the ink chamber expands. After a few microseconds the voltage is lowered with a slope 9 back to the rest situation voltage. The height of the voltage, the steepness of the slope and the timing of the electric signal depend on the piezo-electric element and the acoustic properties of the ink chamber. If properly selected, an acoustic wave in the ink will result in an ink drop from the nozzle 51. If the nozzle is clogged by paper dust, paper touches, ink particles, dried ink or anything else, no drop will be generated. Whether or not a drop has emerged from the nozzle may be established by monitoring a corresponding ink dot on a substrate. Preferably, the condition of the nozzle is established by monitoring a response signal from the piezo-electric element 58, after the actuation signal 6 is finished.
Figure 3 shows an example of such response signal 10. As is apparent from the scale on the vertical axis, the response signal is much smaller than the actuation signal. It is a damped oscillation with a frequency close to the natural acoustic resonance frequency of the ink in the ink chamber 52. The damping and frequency of this signal characterize the condition of the nozzle.
Figure 4 shows two response signals 12 and 13. Response signal 12 is the same as the one in Figure 3 and is shown for comparison. Response signal 13 is characteristic for a clogged nozzle and has a lower frequency and a higher damping factor. Thus, a discrimination between the various conditions of a nozzle may be made based on this difference.
Figure 5 shows an actuation signal 15 that can be used to remove an obstruction in a clogged nozzle. The signal has a lower frequency than the actuation signal and is adjusted to deliver a maximum energy at the obstruction in the nozzle. By continuously activating the ink in the ink chamber with this signal, the obstruction is gradually broken down. Preferably, this signal is applied when the print head is capped in a maintenance station (not shown), because in that case no ink will be spoiled when the nozzle opens. However, a signal with a lower amplitude may also be used outside of the maintenance station, although it may take more time to remove the obstruction. In both cases, a regular verification of the nozzle condition is appropriate to determine if further activation is necessary.
The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.

Claims

A method for actuating a print element in a print head comprising an array of print elements, a print element comprising a liquid, a nozzle, from which a drop of liquid is separable and a piezo-electric transducer for both converting an electric signal into an acoustic wave in the liquid and for converting an acoustic wave in the liquid into an electric signal to determine a status of the print element, the method comprising the steps of:
a) tracing a print element that does not generate a drop of liquid, despite being controlled to do so;

b) actuating said print element with an electric signal that is adapted to an acoustic characteristic of the print element to deliver acoustic energy in the nozzle by using a frequency that is lower than a resonance frequency of the print element in an operable condition, and

c) determining if said print element is operable.
The method according to claim 1, wherein the tracing of a print element in step a occurs by actuating a number of nozzles according to a predetermined pattern and determining if the actuation has resulted in a dot on a receiving medium.
The method according to claim 1, wherein the tracing of a print element in step a occurs by actuating a nozzle and determining its status from an electric response signal in the piezo-electric transducer as caused by an acoustic wave in the liquid of the print element.
The method according to claim 3, wherein a damping of the electric response signal is determined.
The method according to claim 3, wherein a frequency of the electric response signal is determined.
The method according to claim 1, wherein the actuation of a print element in step b occurs when the print head is capped in a maintenance station.
A print system comprising an inkjet print process and a control unit for controlling electric signals that are applied to a print head in said process, wherein the control unit is configured for executing a method according to claim 1.