Patent Grant
7195327
The present invention relates to a droplet ejection apparatus and its drive method, and in more detail, to a droplet ejection apparatus and its drive method in which, by applying a micro-vibration to a liquid meniscus in a nozzle in a degree in which the droplet is made not to be ejected, an increase of viscosity of the liquid in the nozzle is suppressed.
As a droplet ejection head in which the droplet is ejected from the nozzle when a volume of a channel is made changed, an ink jet recording head to record an inkjet image is well known.
The viscosity of the ink for the inkjet used for this inkjet recording head is, normally, about 2–5 cp (centipoise) in a room temperature. However, recently, accompanied with an offering of the high performance, additives are increased, and the ink with the high viscosity of 5–10 cp in the room temperature is also increased. Such a high viscosity ink is, as far as it is ejected under the normal temperature and normal humidity, when the drive voltage of the recording head is a little increased, it can be ejected, but because the viscosity is increased under the low temperature circumstance and is not smaller than 10 cp, and further, because the volatilization of the ink composition from the surface of the meniscus is fast under the low humidity circumstance, the ink viscosity of the nozzle surface is rapidly increased, and the ejection becomes very difficult.
As described above, it is very difficult to eject stably the ink whose viscosity is increased from the recording head under the low temperature circumstance and low humidity circumstance. For example, by also when the droplet ejection is interrupted for only a short period, when the ejection is re-started, it does not normally eject the ink from the first droplet, and the image quality is very lowered.
Conventionally, as one of countermeasures to suppress the increase of the ink viscosity of the nozzle surface, when the meniscus in the nozzle is applied micro-vibration in a degree in which the ink droplet is not ejected from the surface of the nozzle, a method by which the ink of the nozzle surface with increased viscosity is mixed with the ink in a channel is well known. And in the Patent Documents 1 and 2, when the micro-vibration is given to the surface of the nozzle under a condition in which the print is stopped, descriptions that the ink at the surface of the nozzle is mixed and the ink viscosity is lowered, are written.
Under the low temperature and low humidity circumstance, because an increase of ink viscosity is very fast from the above reason, it is necessary that, after the micro-vibration is given to the meniscus, the ink droplet is ejected at once. Further, in order to conduct the stable ejection, it is necessary that the ink droplet is ejected after the vibration of the meniscus by the micro-vibration is turned down and under the condition that the meniscus position is settled at predetermined position, and when the ink droplet is ejected in a situation other than this, a size of the ejected ink droplet or the flying speed is varied, resulting in a cause of landing position error. Further, in order to effectively stir and mix the ink, it is necessary that the meniscus is largely vibrated, however, when the high frequency drive is conducted, the vibration of the meniscus needs to be attenuated early.
The method by which the meniscus of the ink is effectively swayed in this manner, or the residual vibration is effectively cancelled, can be explained from the acoustic theory as follows.
When a channel is expanded (or reduced), a pressure wave generated in the channel is, while the inversion of the pressure is repeated every 1 AL, gradually attenuated. In the case where a deformation of the channel is restored at the timing when the pressure is reversed after 1 AL after the channel is deformed, an initial pressure and a newly generated one are intensified together, and the meniscus of the ink is largely vibrated. Hereupon, the AL (Acoustic Length) is ½ of the acoustic resonance period of the channel. Accordingly, when the time after the channel is deformed, until it is restored, is made odd times of the AL, the meniscus can be largely vibrated. However, because the residual pressure is not cancelled, and the vibration of the meniscus remains, the ink droplet cannot be ejected at once.
On the other hand, in the case where the deformation of the channel is restored at the timing of 2 AL after, since the pressure is reversed and reversed again, the initial pressure and the newly generated pressure are cancelled out each other, the meniscus is not largely vibrated.
Accordingly, in the case where the time after the channel is deformed until the deformation is restored, is made even times of the AL, because the residual pressure is cancelled out, the vibration of the meniscus is turned down, and the nozzle becomes ready to eject the ink droplet at once, however, the meniscus can not be largely vibrated.
As can be seen from the above-description, in order to eject the high viscosity ink at the high frequency and stably under the low temperature and low humidity, it is necessary that the conflicting two conditions are attained, namely the meniscus needs to be largely vibrated so that the ink of the nozzle surface is effectively stirred, and the residual vibration generated by this vibration needs to be effectively cancelled out.
Both of the technology of the above-described Patent Documents 1 and 2 are technologies by which the micro-vibration pulse by a trapezoidal wave is applied on a nozzle in which the ink ejection is not conducted, and it is made in such a manner that the micro-vibration is given to the meniscus. Therefore, after the micro-vibration is given to the meniscus, because there is a time before the ink is ejected, the ink viscosity is increased again in that time, and particularly, under the low temperature and low humidity circumstance, there is a problem that the normal ejection becomes difficult. Further, in the trapezoidal wave, the circuit structure becomes complicated, and further, because the voltage sensibility becomes low, the necessary drive voltage is increased and the power consumption is increased. Further, when a number of times of applying of the micro-vibration pulse is not increased, the sufficient effect can not be obtained, and as a result, there is a problem that it results in a decrease of the printing speed.
Accordingly, the object of the present invention is to provide a droplet ejection apparatus and its drive method by which, when the liquid in the nozzle is effectively stirred, the improvement effect of the decapping characteristic is high even under the circumstance of the low temperature and low humidity, and even just after the micro-vibration of the meniscus, the droplet can be made to be stably ejected, and the high frequency and stable ejection can be conducted.
Herein, the decapping characteristic indicates a lowered amount of the initial flying speed by the so-called decapping phenomenon that, in the situation in which the nozzle surface is opened, the viscosity of the liquid is increased by the meniscus drying.
The above object can be solved by the following features of the invention.
a drive signal generator for generating a set of drive signals including a plurality of drive pulses;
a drive pulse selector for selecting a set of drive pulses in accordance with a print datum of each pixel; and
a head for ejecting a droplet from a nozzle provided corresponding to a channel, by changing a volume of the channel according to the set of drive pulses selected,
wherein, the drive signal includes a micro-vibration pulse as one of the drive pulses to generate a micro-vibration of meniscus in the nozzle in such a degree that the droplet is not ejected, said micro-vibration pulse being formed of rectangular waves which include at least one micro-vibration pulse having a pulse width of (2n) AL, where AL is ½ of the acoustic resonance period of the channel, and n is an integer not smaller than 1.
a first pulse formed of a rectangular wave to expand the volume of the channel, and 1 AL later, restoring it to an original state; and
a second pulse formed of a rectangular wave to reduce the volume of the channel, and a prescribed period later, restoring it to the original state,
wherein a voltage of the first pulse Von is higher than a voltage of the second pulse Voff.
generating a set of drive signals including a plurality of drive pulses by a drive signal generator;
selecting a set of drive pulses in accordance with a print datum of each pixel by a drive pulse selector;
ejecting a droplet by changing a volume of a channel according to the set of drive pulses selected, from a nozzle of the droplet ejection head, the nozzle being provided corresponding to the channel,
wherein a micro-vibration pulse is applied onto the droplet ejection head to generate a micro-vibration of meniscus in the nozzle in such a degree that the droplet is not ejected,
wherein, the drive signal includes a micro-vibration pulse as one of the drive pulses to generate a micro-vibration of meniscus in the nozzle in such a degree that the droplet is not ejected, said micro-vibration pulse being formed of rectangular waves which include at least one micro-vibration pulse having a pulse width of (2n) AL, where AL is ½ of the acoustic resonance period of the channel, and n is an integer not smaller than 1.
a first pulse formed of a rectangular wave for expanding the volume of the channel, and 1 AL later, restoring it to an original state; and
a second pulse formed of a rectangular wave for reducing the volume of the channel, and a prescribed period later, restoring it to the original state,
wherein a voltage of the first pulse Von is higher than a voltage of the second pulse Voff.
According to the present invention, a conflicting problem which is a fact that the meniscus is largely vibrated, and the liquid on the nozzle surface is effectively stirred, and a fact that the residual vibration generated by this vibration is effectively cancelled out, can be solved, and when the liquid in the nozzle can be effectively stirred, the droplet ejection apparatus and its drive method in which even under the low temperature and low humidity circumstance, the improvement effect of the decapping characteristic is high, and further, even just after the micro-vibration of the meniscus, the droplet can be stably ejected, and the high frequency and stable ejection can be conducted, can be provided.
a) is a general viewed perspective view showing an example of a recording head, and
a)–(c) are views showing a motion at the time of the ink ejection of a recording head.
a)–(c) are explanation views of time division motions of the recording head.
a)–(f) are views respectively showing the waveforms of a micro-vibration pulse and an ejection pulse.
Using the drawings, an embodiment of the present invention will be described below.
Between the conveying roller 31 and the conveying roller pair 32, a recording head 2 is provided so that it is opposite to a recording surface PS of the recording medium P. This recording head 2 is arranged in such a manner that a nozzle surface side is opposite to the recording surface PS of the recording medium P, and mounted on a carriage 5 provided so that it can reciprocally move along X–X′ direction (primary scanning direction) shown in the view, which is almost perpendicular to the conveying direction (sub-scanning direction) of the recording medium P by a drive means, not shown, along a guide rail 4 which is installed across the width direction of the recording medium P, and through a flexible cable 6, it is electrically connected to a drive signal generating section 100 (refer to
Such a recording head 2 moves the recording surface PS of the recording medium P accompanied to the movement of the carriage 5, in X–X′ direction, shown in a view, and in this movement process, by ejecting an ink droplet, a desired inkjet image is recorded.
Hereupon, in a view, numeral 7 is an ink receiving unit provided at a position such as a home position, where the recording head 2 waits at the period of no-recording. When the recording head 2 is at this waiting position, after the ink whose viscosity is increased, is micro-vibrated and its viscosity is decreased, it is conducted that a small amount of the ink droplet is discarded to this ink receiving unit 7. When the recording head 2 stops its operation for a long period of time at this waiting position, it is not shown, but by putting a cap on the nozzle surface of the recording head 2, it is protected. Further, numeral 8 is an ink receiver provided at an opposite position to the ink receiving unit 7 with the recording medium P between them, and in cases where recording is conducted in reciprocal both directions, when the head is switched from a going movement to a returning movement, the discarded ink droplet is received in the same manner as an above description.
The recording head 2 shows herein, as shown in
Here, the drive signal generating circuit corresponds to the drive signal generator in claims of the invention, and the drive pulse selecting circuit corresponds to the drive pulse selector in the claims.
Each of partition walls 27 is structured, as shown by an arrow in
When the ejection pulse is applied on electrodes 29A, 29B, and 29C which are formed with close contact on each partition wall 27 surface, by the control of the drive signal generating section 100, the ink droplet is ejected from the nozzle 23, by the motion illustrated below. Hereupon, in
Initially, when the ejection pulse is not applied on any one of electrodes 29A, 29B, and 29C, any one of partition walls 27A, 27B, and 27C is not deformed, but in the situation shown in
Further, when the electric potential is returned to 0 from this situation, partition walls 27B and 27C are returned from the expansion position shown in
In the case where the recording head 2 having a plurality of channels 28 separated by the partition wall 27 at least whose one portion is structured by the piezoelectric material is driven as described above, when the partition wall of a channel is moved so as to conduct the ejection, because adjacent channel is influenced. Normally, in a plurality of channels 28, it is made to be one group by combining the channels 28, which are separated each other with at least one other channel 28 between them, and the plurality of channels 28 is divided into more than 2 groups, and is driven and controlled in such a manner that the ink ejection motion is successively conducted by time division for each groups. For example, when all channels 28 are driven and a solid image is outputted, so-called three cycle ejection method is carried out, by which the channels 28 are selected every 3 channels to form three groups, and the ink droplet is ejected with being divided into 3 phases.
By using
At the time of the ink ejection, initially, the voltage is applied on the electrode of each channel of group A (A1, A2, A3), and the electrodes of both adjacent channels are electrically grounded. For example, when the ejection pulse of the positive voltage of 1 AL width is applied on the channel of group A, the partition wall of the channel of the group A wanted to be ejected, is deformed outward, and the negative pressure is generated in the channel 28. By this negative pressure, the ink is flowed in the channel 28 of the group A from the ink tank. (Draw).
When this situation is kept for 1 AL, because the pressure is reversed to the positive pressure, when the electrode is electrically grounded at this timing, the deformation of the partition wall is restored to the initial condition, and the high pressure is applied on the ink in the channel 28 of group A (Release). Further, when at the same timing, the negative voltage is applied on the electrode of each channel of group A, the partition wall deforms inside, and the further high pressure is applied on the ink (Reinforce) and the ink column is extruded from the nozzle. After 1 AL, the pressure is reversed and the inside of the channel 28 becomes negative pressure, and when further 1 AL passes, because the pressure in channel 28 is reversed and becomes the positive pressure, when the electrode is electrically grounded at this timing, the deformation of the partition wall is restored to the initial condition, and the residual pressure wave can be cancelled out.
Subsequently, the same motion as above description is conducted on each channel 28 of group B (B1, B2, B3), and further successively onto each channel 28 of group C (C1, C2, C3).
Hereupon, the AL (Acoustic Length) is, as described above, ½ of the acoustic resonance period of the channel. This AL is found as the pulse width in which the flying speed of the ink droplet becomes maximum, when the voltage pulse of rectangular wave is applied on the partition wall 27 which is the electric-mechanical conversion element, the speed of the ejected ink droplet being measured, the voltage value of the rectangular wave being made constant, and the pulse width of the rectangular wave being made to be changed. Further, the pulse is a rectangular wave having a prescribed voltage height, and the pulse width is defined, when the 0 voltage is 0% and the voltage height is 100%, as the time period between at 10% rise from 0 voltage in rising edge and at 10% fall from the voltage height in decaying edge. Further, the rectangular wave herein indicates a waveform in which both of the rise time between 10% and 90% of the voltage, and the fall time between them are within ½ of the AL, and preferably, within ¼.
In the inkjet recording head of such a shear mode type, because the deformation of the partition wall is caused by the difference of the voltage applied on the electrodes provided on both sides of the wall, in place of that the negative voltage is applied on the electrode of the channel to conduct the ink ejection, as shown in
Next, by using
In the present invention, the micro-vibration pulse to micro-vibrate in the degree in which the ink droplet is not ejected from the nozzle, is generated in the drive signal generating section 100 shown in
In the present invention, when the rectangular wave is used for the micro-vibration pulse, the efficiency in which the meniscus is micro-vibrated, is better as compared to a method by which the trapezoidal wave is used, and it can be micro-vibrated by the low drive voltage, and further, there is an effect in which the drive circuit can be designed by a simple digital circuit.
For example, in the example shown in
Successively, when the ejection pulse of the positive voltage of 1 AL width is given to the channel of group A, and successively, the ejection pulses of the positive voltage of 2 AL width are respectively given to channels of group B and group C, the ink is ejected from the channel of group A by the above-described DRR drive method, and the pixel is recorded. When the multi-droplets ejection is conducted, this 2 kinds of pulses are repeated for the number of droplets wanted to be ejected.
In the same manner also in the case where the ejection from the channel of the group A is completed, and the ejection is successively conducted from the channel of the group B, after the electrode of the channel of the group B is electrically grounded, the micro-vibration pulse of the positive voltage of 1 AL width and the micro-vibration pulse of the positive voltage of 2 AL width are respectively applied with 1 AL interval on electrodes of channels of the group A and the group C, and the meniscus is micro-vibrated. After that, the ejection pulse of the positive voltage of 1 AL width is given to the electrode of channel of group B, and when successively the ejection pulse of the positive voltage of 2 AL width is given to electrodes of channels of group A and group C, the ejection from the channel of group B is conducted, and the pixel is recorded. The applying of the micro-vibration pulse and the ejection of droplet of the channel of group C are also conducted in the same manner.
Next, the case where any one of channels of group A, group B, and group C does not conduct the ejection, and the micro-vibration is given to the meniscus in the order of group A→group B→group C, will be described by using
In the same manner as in
Next, the method of selecting the drive pulse for each pixel is explained by using
As described above, when the micro-vibration pulse including the rectangular wave is applied always, also on the printing pixel (in the case of
Particularly, as shown in
Hereupon, the phrase “before the ejection pulse”, indicates, in the ejection of the ink droplet after the micro-vibration, the time in the range in which the effect is seen in the improvement of the decapping characteristic.
In this manner, when the micro-vibration pulse is applied on all print pixels in the image recording region, it is preferable that the maximum extrusive amount of the meniscus which is micro-vibrated in the degree that the ink droplet is not ejected from the surface of the nozzle, is not larger than nozzle radius. When the extrusive amount by the meniscus micro-vibration is large, the meniscus can not return up to the timing of the next ink ejection, and the stable ejection becomes difficult, however, when the extrusive amount of the meniscus in this case is suppressed under the nozzle radius, even just after the meniscus micro-vibration, the stable ejection can be conducted.
Hereupon, the phrase “maximum extrusive amount” is the maximum value of the extrusive amount of the meniscus from the surface of the nozzle in one time of the meniscus extrusive motion. The extrusive amount of the meniscus from the nozzle 23 can be measured by the strobe-synchronization, for example, by using the digital microscope [VH-6300] produced by KEYENCE Co. The extrusive amount is, as shown in
Further, the shape of the opening of the nozzle 23 is not limited to the true circle, but there are various shapes such as the ellipse shape, and herein, the nozzle radius is ½ of the longest diameter on the surface (the surface of the nozzle forming member 22) side of the nozzle 23.
In the present invention, it is well when at least one of the rectangular wave whose pulse width is an even number AL width, that is, (2n) AL, is included in the micro-vibration pulse, and as shown in
Further, herein, because the vibration is made conducted again by the micro-vibration pulse of even number AL width after 1 AL after the meniscus is largely vibrated by the micro-vibration pulse of the odd number AL width, and after the remained pressure wave is cancelled, the ejection is conducted, the decapping characteristic is largely improved and stable ejection can be attained.
Hereupon, when the rectangular wave whose pulse width is (2n) AL, is applied on the last of the micro-vibration pulse, as shown in
In the above example, it is made that n=1, and one micro-vibration pulse whose pulse width is 2 AL is included, however, n may be an integer not smaller than 2.
In the example shown in FIG. 7–
Furthermore, when more than 2 rectangular waves whose pulse width is (2n) AL are included in the micro-vibration pulse, n may be different respectively, and further, in the case where the micro-vibration pulse has 2 and more micro-vibration pulses which includes at least one rectangular wave whose pulse width is (2n) AL, when the interval between former rectangular wave and the latter rectangular wave is an integer times of AL, it is preferable because the meniscus can be effectively micro-vibrated.
In the example shown in
Incidentally, the ejection pulse is not restricted to that used in the embodiment, but may be a drive pulse including a pulse which can expand the volume of the channel and after a certain time restore or reduce the volume of the channel to eject the ink droplet.
The electric-mechanical conversion element in the present invention is, as described above, not limited to one formed of the piezoelectric material which forms the partition wall between adjacent channels and which is deformed in the shearing mode by applying the electric field, but when it is one which gives the recording head the function by which the volume of the channel is changed, any structure may be allowed, however, as shown in the present embodiment, when it is structured by the piezoelectric material which is deformed in the shearing mode, it is preferable because the above-described rectangular wave can be more effectively used, the drive voltage is lowered, and the more effective drive can be conducted.
Further, in the above description, an application of the inkjet recording apparatus is shown as an example of the droplet ejection apparatus, and an inkjet recording head is shown as an example of the droplet ejection head, however, the present invention is not limited to this, but it can be widely applied to the droplet ejection apparatus and to the drive method of droplet ejection head by which the liquid in the channel, whose viscosity is easily increased, is made to be a droplet and is ejected from the nozzle.
(The Evaluation 1 of the Ejection Stability)
Each channel of the recording head (number of nozzles: 256, nozzle diameter is 23 μm) of the shear mode type shown in
Condition
Measuring Method of the Ejection Stability
In respective micro-vibration pulse applying conditions, by changing the voltage Von, Voff, the flying speed of the ink droplet is increased, and the flying situation of the ink droplet is observed. The upper limit of the flying speed in which the curving of the ejection direction and the spattering of the satellite are not caused, is defined as the upper limit of the stable ejection speed.
The Evaluation Standard of the Ejection Stability
Measuring Method of the Decapping Characteristic
In each micro-vibration pulse applying condition, the drive voltage is fixed at the value where the flying speed of the ink droplet becomes 6 m/s in regular drive of the ejection head (Von/Voff=2/1), and the change of initial flying speed at the time of ink ejection is observed while increasing the ejection interval. It is recognized that the smaller the change of initial flying speed, the greater the effect of improvement in the decapping characteristics.
As shown in Table 1, when the micro-vibration pulse formed of only the rectangular wave having a pulse width of AL multiplied with odd number is applied the ejection stability is not sufficient, however when the micro-vibration pulse including the rectangular wave having a pulse width of AL multiplied with even number is applied it is possible to conduct a stable ejection even just after the meniscus is micro-vibrated.
(Evaluation of the Decapping Characteristic)
Each channel of the recording head of the same shear mode type as the case of the evaluation 1 of the ejection stability is divided into 3 groups, and by using the micro-vibration pulse and the ejection pulse shown in
The decapping characteristic is measured for an arbitrary 1 nozzle by the following method. That result is shown in
Condition
Measuring Method of the Decapping Characteristic
About the condition in which the micro-vibration pulse is not applied on the no-print pixel and print pixel, and the condition in which the micro-vibration pulse shown in
The drive voltage is fixed to the voltage whose flying speed at the time of this normal drive is 6 m/s, and while the ejection interval is spread, the change of the initial flying speed when the ink droplet is ejected, is measured.
As shown in Example 3 and Example 4 in Table 2, when the micro-vibration pulse is applied before ejection, it can be confirmed that it is effective in the prevention of the decapping phenomenon under the low temperature and low humidity circumstance. Because of applying the micro-vibration pulse on all pixels, even in a pattern of the ejection of only end portion in the image recording region, the stable droplet formation becomes possible. Further, when the meniscus before ejection is micro-vibrated, the effect of an increase of the drive efficiency (a decrease of the drive voltage) can also be obtained.
(The Evaluation 2 of the Ejection Stability)
Each channel of the recording head of the same shear mode type as the case of the evaluation 1 of the ejection stability is divided into 3 groups, and the ink meniscus extrusive amount from the nozzle when only the micro-vibration pulse in
The result in which the ejection stability is estimated by the following method by using this result and the micro-vibration pulse and ejection pulse shown in the same view, is shown in Table 3.
Condition
Under the condition in which the ejection drive voltage is fixed and the flying speed of the ink droplet is constant (6 m/s), the flying situation when only the micro-vibration pulse voltage is changed, is observed.
Evaluation Standard of the Ejection Stability
Measuring Method of the Decapping Characteristics:
In each micro-vibration pulse applying condition, the drive voltage is fixed at the value where the flying speed of the ink droplet becomes 6 m/s in regular drive of the ejection head (Von/Voff=2/1), and the change of initial flying speed at the time of ink ejection is observed while increasing the ejection interval in a condition that only the micro-vibration pulse voltage is changed. It is recognized that the smaller the change of initial flying speed, the greater the effect of improvement in the decapping characteristics.
As shown in Example 5 and Example 6 in Table 3, when the maximum extrusive amount of the meniscus by the micro-vibration pulse is not larger than the nozzle radius (11.5 μm), it can be seen that even just after the micro-vibration pulse is applied, the ejection stability is good. Accordingly, even at the time of high frequency drive, the micro-vibration can always be given to meniscus.
Hereupon, in both of Comparative Example 7 and Comparative Example 8, the maximum extrusive amount of the meniscus is larger than the nozzle radius. In cases where only the micro-vibration pulse is applied, mistake-ejection will not be generated even when the maximum extrusive amount of the meniscus becomes about three times of the nozzle radius, but in cases where the ejection pulse is applied before and after the applying of the micro-vibration pulse, when the maximum extrusive amount of the meniscus becomes larger than the nozzle radius (measured when only the micro-vibration pulse is applied) mistake-ejection tends to be generated. In Comparative Example 7, mistake-ejections are partially generated among the all nozzles of the recording head, and in Comparative Example 8, mistake-ejections are generated entirely.
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