1. Field of the Invention
The present invention relates to a printer head, an ink jet printer and a method for driving the printer head.
2. Description of the Related Art
It has recently become widespread to draw up documents on computers that are used as desktop publishers, especially in offices. And lately demands for outputting not only characters and graphics but also colored natural pictures like photographs together with them has become increased. Therefore, it has been required to print high quality natural pictures, and gradation expression with the expression of halftones has consequently becomes important.
Furthermore, the so-called on demand type printers have been coming into wide use in recent years because they are suitable for miniaturization and reduction in cost. The on demand type printers perform recording by discharging ink droplets from nozzles only when it is required to print according to control signals that are in compliance with recording signals to make the ink droplets adhere on a material to be recorded, such as a sheet of paper or a film.
As aforesaid, although various methods as a method for discharging ink droplets from nozzles have been proposed, a method using a piezoelectric element or a heating element is generally used. The former is a method for discharging ink by applying pressure to it by means of the deformation of the piezoelectric element. The latter is a method for discharging ink by the pressure of bubbles produced by vaporizing the ink in nozzles by the heat generated by the heating element.
In addition, various methods have been proposed as a method for mimetically realizing the aforementioned gradation expression with the expression of halftones on the aforementioned on demand type printer that discharges ink droplets. That is, as a first method, there is a method that expresses the halftone gradation by controlling the sizes of the ink droplets to be discharged by varying the voltage value or the pulse width of a pulse voltage to be supplied to the piezoelectric element or the heating element to make diameters of dots to be printed variable.
However, the above first method has a defect that expressible gradation steps are not many, in particular, the expression of low density is very difficult, because there is a limit to the minimum diameter of the droplets owing to the fact that, if the voltage or the pulse width supplied to the piezoelectric element or the heating element is decreased too much, the ink is not discharged. Consequently, the first method is not satisfactory for a printout of a natural picture.
Besides, as a second method, there is a method that realizes a gradation expression by constituting one pixel with a matrix composed of e.g. 4xc3x974 dots without varying the diameters of the dots, and by performing the picture processing such as the so-called dither method or the error diffusion method of each matrix.
However, although seventeen gradation steps of the density can be expressed by the second method if one pixel is composed of the 4xc3x974 matrix, the resolution of a printed picture deteriorates to one fourth if the picture is printed, for example, at the dot density same as that of the first method. Consequently, the printed picture becomes conspicuous in roughness, and thus the second method is also not satisfactory for a printout of a natural picture.
Accordingly, for principally resolving the problems of the conventional on demand type printers, the inventors of the present invention have proposed such a printer as was described in, for example, JP-A 201024/93 and JP-A 195682/95, which printer mixes ink and diluent, i.e. a transparent solvent, together at a predetermined mixing ratio just before discharging to be diluent ink, and immediately discharges the diluent ink from nozzles to make it adhere on a material to be recorded for recording.
Although, in the following description, the system in which ink is used as a quantification medium and diluent is used as a discharge medium, and in which the ink as the quantification medium is mixed with the diluent as the discharge medium to be the diluent ink, and further in which recording is done by discharging the discharge medium, is called as a carrier jet system among the aforementioned systems, there is no problem in a printer even if the diluent is used as the quantification medium and the ink is used as the discharge medium.
A printer in accordance with such a carrier jet system can control the density of the mixed solution to be discharged by varying the mixing ration of the ink and the diluent by varying the amount of the quantification medium that is either the ink or the diluent, and then the printer can separately vary the density of every dot to be printed. Consequently, the printer can print out a natural picture rich in the halftone gradation thereof without producing the deterioration of its resolution.
As a two liquids mixing type printer as stated above, there is the so-called external mixing type printer as will be shown in the following, for example.
The printer includes a quantification medium pressuring chamber where a quantification medium is introduced and a discharge medium pressuring chamber where a discharge medium is introduced. An opening of a quantification medium nozzle communicating with the quantification medium pressuring chamber and an opening of a discharge medium nozzle communicating with the discharge medium pressuring chamber adjoin to each other. The printer oozes the quantification medium from the quantification medium nozzle through the opening surface of the nozzle to the discharge medium nozzle, and makes the oozed quantification medium contact with the discharge medium plugged in the vicinity of the tip of the discharge medium nozzle to form the mixed solution. And then, the printer discharges the discharge medium from the discharge nozzle to discharge the quantification medium and the discharge medium as the mixed solution.
In such a structure, because the qualification nozzle and the discharge nozzle are separately formed, the quantification medium and the discharge medium do not diffuse while they are waiting for being discharged, and mutual affluxes at the time of mixing and discharging can be prevented.
When printing is done by the printer head shown in FIG. 21, it is done along the following description. The description is now done by making use of a timing chart for the imposition of driving voltages shown in FIG. 22. The so-called laminated type piezoelectric elements are used as a first laminated type piezoelectric element 43 and a second laminated type piezoelectric element 44. There are two types of laminated type piezoelectric elements, one of them utilizes the displacement thereof in the shrinking direction (the so-called d-31 direction), and the other utilizes the displacement thereof in the elongating direction (the so-called d-33 direction). The latter is used as both of the first laminated type piezoelectric element 43 and the second laminated type piezoelectric element 44.
As shown in the timing chart of the imposition of the driving voltages of FIG. 22, at first, at a point of time indicated by reference character (a) in FIG. 22, positive voltages, 10 V for the first laminated type piezoelectric element 43 and 15 V for the second laminated type piezoelectric element 44, are being imposed on them as driving voltages. The horizontal axis of FIG. 22 indicates time, and the vertical axis of FIG. 22 indicates driving voltages for the first laminated type piezoelectric element 43 and the second laminated type piezoelectric element 44. At this time, a quantification medium 45 and a discharge medium 49 are in their waiting state shown in FIG. 23(A).
Next, at a point of time indicated by reference character (b) in FIG. 22, the driving voltage for the first laminated type piezoelectric element 43 starts to be lowered until a point of time indicated by reference character (d) in FIG. 22 to be 0 V over a period of 50 xcexcs. Then, the first laminated type piezoelectric element 43 is elongated to push the touching part of a diaphragm 42 out. As a result, the volume of a quantification medium pressuring chamber 56 decreases. Consequently, at a point of time indicated by reference character (c) in FIG. 22 that is within an intermediate period between the point of time indicated by reference character (b) in FIG. 22 and the point of time indicated by reference character (d) in FIG. 22, the quantification medium 45 is pushed out of a quantification medium nozzle 53 as mimetically shown in FIG. 23(B). In the present case, because the quantification medium nozzle 53 is formed so as to gradually approaches to a discharge medium nozzle 54, the quantification medium 45 is pushed out to the discharge medium nozzle 54.
This state is kept for the period of 50 xcexcs from the point of time indicated by reference character (d) in FIG. 22 to the point of time indicated by reference character (e) in FIG. 22. As a result, at the point of time indicated by reference character (e) in FIG. 22, as shown in FIG. 23(C), the quantification medium 45 becomes a state of being coupled to the discharge medium 49 after the quantification medium being contacted to the discharge medium 49.
From the point of time indicated by reference character (e) in FIG. 22, the driving voltage of the first laminated type piezoelectric element 43 is gradually raised to the initial value. Then, since the first laminated type piezoelectric element again shrinks, the volume of the quantification medium pressuring chamber 56 increases so that the quantification medium 45 begins to be pulled into the quantification medium nozzle 53.
As shown in FIG. 24(A), the meniscus of the ink remaining on the discharge medium nozzle 54 in a state of being swollen thereon after being torn off, as shown in FIG. 24(B), comes into a waiting state by the capillary attraction of a mixed solution 69 that is produced by the mixture of the meniscus and the diluent in the discharge medium nozzle 54.
For a period from a point of time indicated by reference character (f) in FIG. 22, which point is later than the point of time indicated by reference character (e) in FIG. 22, to a point of time indicated by reference character (g) in FIG. 22, the driving voltage for the second laminated type piezoelectric element 44 is lowered from 15 V to 0 V over a period of 10 xcexcs. Then, the second laminated type piezoelectric element 44 is elongated to push the touching part of the diaphragm 42 out. As a result, the volume of a discharge medium pressuring chamber 58 decreases. Consequently, at the point of time indicated by reference character (f) in FIG. 22, the mixed solution 69 begins to be pushed out of the discharge medium nozzle 54 as mimetically shown in FIG. 24(C).
This state is kept for the period of 50 xcexcs from the point of time indicated by reference character (g) in FIG. 22 to a point of time indicated by reference character (i) in FIG. 22. As a result, at a point of time indicated by reference character (h) in FIG. 22 that is within an intermediate period between the point of time indicated by reference character (g) in FIG. 22 and the point of time indicated by reference character (i) in FIG. 22, as shown in FIG. 25(A), the mixed solution 69 becomes a state of being further pushed out of the discharge medium nozzle 54.
On the other hand, since the driving voltage of the first laminated type piezoelectric element 43 continues to rise, the quantification medium 45 is being pulled into the quantification medium nozzle 53 so that the part contacting with the discharge medium 49 is left.
The driving voltage of the second laminated type piezoelectric element 44 begins to rise gradually from the point of time indicated by reference character (i) in FIG. 22. Then, the second laminated type piezoelectric element 44 again starts to shrink so that the volume of the discharge medium pressuring chamber 58 begins to increase. As a result, at a point of time indicated by reference character (j) in FIG. 22 that is a point of time a little later than the point of time indicated by reference character (i) in FIG. 22, as mimetically shown in FIG. 25(B), a constriction begins to be produced between the mixed solution 69 and the discharge medium 49. Incidentally, at the point of time, the driving voltage of the first laminated type piezoelectric element 43 returns to the initial value, 10 V, and then is kept in this state.
At a point of time indicated by reference character (k) in FIG. 22, which point is later than the point of time indicated by reference character (j) in FIG. 22, as mimetically shown in FIG. 25(C), the mixed solution 69 is torn off from the discharge medium 49 to be discharged from the discharge medium nozzle 54, and further the discharge medium 49 is pulled into the discharge medium nozzle 54.
Furthermore, at a point of time indicated by reference character (l) in FIG. 22, which point is later than the point of time indicated by reference character (k) in FIG. 22, the driving voltage of the second laminated type piezoelectric element 44 returns to the initial value, 15 V. Incidentally, the period between the point of time indicated by reference character (i) in FIG. 22 and the point of time indicated by reference character (l) in FIG. 22 is 100 xcexcs. At the point of time indicated by reference character (l) in FIG. 22, as mimetically shown in FIG. 26(A), the mixed solution 69 in a sphere continues to fly to a not shown material to be recorded, and then the mixed solution 69 adheres on the material to be recorded for performing recording.
During the period between the point of time indicated by reference character (j) in FIG. 22 and the point of time indicated by reference character (l) in FIG. 22, the quantification medium 45 is gradually plugged in the quantification medium nozzle 53 by the capillary attraction, and then at the point of time indicated by reference character (l) in FIG. 22, as mimetically shown in FIG. 26(A), the quantification medium 45 is plugged up to the tip of the quantification nozzle 53.
Furthermore, at a point of time indicated by reference character (m) in FIG. 22 later than the point of time indicated by reference character (l) in FIG. 22, as mimetically shown in FIG. 26(B), the discharge medium 49 is plugged in the discharge medium nozzle 54 by the capillary attraction similarly to the quantification medium 45, and at a later point of time indicated by reference character (n) in FIG. 22, as mimetically shown in FIG. 26(C), the discharge medium 49 returns to the waiting state thereof.
It is known that the amount of the projection of the first laminated type piezoelectric element 43 varies linearly to the driving voltage thereof. Besides, since the mixing ratio of the mixed solution is determined in accordance with the amount of the pushed out quantification medium, the reflection density of a dot is also determined under a one-to-one correspondence to the driving voltage. That is, the driving voltage is determined under the one-to-one correspondence to a desired gradation value. We here suppose that the gradation value when the quantification is done under the driving voltage of 10 V is the maximum gradation value to be 255/255, as shown in FIG. 22. Then, as the gradation value at the time of the quantification for a period of points of time indicated by reference characters (n)-(o)-(p)-(q) in FIG. 22 when the driving voltage takes a waveform of 4 V, which period follows to the period when the driving voltage takes a waveform of 10 V as shown in FIG. 22, one value of x that becomes x/255 is determined.
The characteristic of the driving system described above that the mixed solution is discharged after waiting for the quantification medium ink, which has been pushed out on the discharge medium nozzle to be quantified, to be naturally settled in the discharge medium nozzle.
That is, the driving system should perform two operations in a period, one of which is the operation of xe2x80x9cthe quantification of the quantification mediumxe2x80x9d and the other of which is the operation of xe2x80x9cthe quantification medium is settled in the discharge medium nozzle while the quantification medium is mixed with the discharge mediumxe2x80x9d. In particular, in the case where a large quantity of the quantification medium is quantified at the maximum density, it is difficult to increase the driving frequency because it took a long time to settle the quantification medium in the discharge nozzle while the quantification medium is mixed with the discharge medium.
Furthermore, if the amount of ink to be quantified is increased more than a predetermined amount, the quantification medium ink overflows the discharge medium nozzle before the quantification medium ink is pulled into the discharge medium nozzle by the capillary attraction. Such overflow may bring about the change of the discharging direction by the fact that the mixed solution to be discharged is drawn by the overflowed ink, and further may result in no discharge at the worst.
For example, although the present system pushes out the quantification medium from the waiting state under 10 V driving voltage, if the driving voltage of the waiting state is tried to be increased, the quantification medium and the discharge medium are not mixed more than a predetermined mixing ratio as shown in FIG. 27. FIG. 27 shows the results of an experiment using the printer head shown in FIG. 21 and the driving voltages of the waveforms shown in FIG. 22. When the amount of the quantification of the quantification medium is increased in the case where the total consumption amount of the mixed solution is 60 pl per one drop, the operation of the head becomes unstable if the amount exceeds the boundary line shown in FIG. 27.
FIGS. 28(A)-28(C) are drawings mimetically illustrating observations of the states of the quantification medium 45 in the vicinity of the quantification medium nozzle 53 and the discharge medium nozzle 54 by means of a microscope using a stroboscope.
FIG. 28(A) shows a halfway state of the quantification of the quantification medium 45 in the vicinity of the nozzles 53 and 54 in the unstable area. The quantification medium 45 that has already quantified exists on the discharge medium nozzle 54 as if it goes to overflow, as shown in FIG. 28(A). In the unstable area in FIG. 27, the quantification medium 45 continues to be pushed out for being quantified furthermore.
As shown in FIG. 28(B), the quantification medium 45, which could not exist on the discharge medium nozzle 54 after being further pushed out, overflows around both the nozzles 53 and 54.
As shown in FIG. 28(C), the remaining quantification medium 45 shown in FIG. 28(B) pulls the mixed solution 69 to be discharged to a direction being off from the direction normal to a nozzle plate, resulting in the disturbance of the direction of discharging the mixed solution 69, not discharging and the like.
That is, the maximum quantification amount of the quantification medium 45 is the amount of the quantification medium 45 that can exist on the discharge medium nozzle 54 without overflowing in this driving system. If quantification more than the maximum quantification amount is tried, the results shown in FIGS. 28(A)-28(C) may bring about such results as are shown in FIGS. 28(A)-28(C).
Furthermore, such an operation as xe2x80x9caccumulating the quantification medium 45 on the discharge medium nozzle 54xe2x80x9d is strongly influenced by the treatment of the surface of the discharge medium nozzle 54 such as a water repellent treatment. If the effect of the water repellent treatment is strong, the amount of the quantification medium 45 capable of being accumulated or being quantified becomes great in quantity. If the water repellent treatment is not done, the quantification medium 45 becomes easy to overflow because the periphery of the discharge medium nozzle 54 is easy to be wetted, resulting that the amount of the quantification medium 45 capable of being accumulated increases. Furthermore, the force of repelling water is easy to produce differences between each channel because of the wear caused by cleaning operations of the head and the deterioration by aging. Namely, the force of repelling water is easy to be influenced by the variation of the state of the water repellent treatment, and the differences between each nozzle are large at the highest gradation value.
Accordingly, the present invention aims at solving the aforementioned problems to provide a printer head, an ink jet printer and a method for driving a printer head capable of improving the mixing ratio of the quantification medium to the discharge medium, and capable of realizing the desired maximum density by making a high density mixed solution for a short time, and then capable of increasing unnecessary mixtures of colors.
According to a first aspect of the present invention, there is provided a printer head in an ink jet printer, comprises: a quantification medium pressuring chamber where a quantification medium is introduced; a discharge medium pressuring chamber where a discharge medium is introduced; a quantification medium nozzle communicating with the quantification medium pressuring chamber; a discharge medium nozzle communicating with the discharge medium pressuring chamber, the discharge medium nozzle being disposed to adjoin the quantification medium nozzle; and a first pressure generating element pulling the quantification medium pushed out of the quantification medium nozzle into the discharge medium nozzle to form a mixed solution by contacting the quantification medium in the discharge medium nozzle through a surface where the quantification medium nozzle opens, wherein the first pressure generating element then generates a pressure for discharging the mixed solution from the discharge medium nozzle.
According to the first aspect of the invention, since the quantification medium pushed out of the quantification medium nozzle is once pulled into the discharge medium nozzle forcibly, and then the mixed solution of the quantification medium and the discharge medium is discharged, the mixing ratio of the quantification medium and the discharge medium can be enhanced to enable the quantification of a great deal of quantification medium for a period. Furthermore, it is possible to make the mixed solution by mixing a great deal of quantification medium with the discharge medium for a short time.
According to a second aspect of the invention, there is provided an ink jet printer equipped with a printer head, wherein the printer head comprises: a quantification medium pressuring chamber where a quantification medium is introduced; a discharge medium pressuring chamber where a discharge medium is introduced; a quantification medium nozzle communicating with the quantification medium pressuring chamber; a discharge medium nozzle communicating with the discharge medium pressuring chamber; the discharge medium nozzle being disposed to adjoin the quantification medium nozzle; and a first pressure generating element pulling the quantification medium pushed out of the quantification medium nozzle into the discharge medium nozzle to form a mixed solution by contacting said quantification medium in the discharge medium nozzle through a surface where the quantification medium nozzle opens, wherein the first pressure generating element then generates a pressure for discharging the mixed solution from the discharge medium nozzle.
According to the second aspect of the invention, there can be obtained advantages similar to those of the first aspect of the invention.
According to a third aspect of the invention, there is provided a method for driving a printer head, the method comprises the steps of: moving a quantification medium pushed out of a quantification medium nozzle from the quantification medium nozzle to a discharge medium nozzle through a surface where the quantification medium nozzle opens; forming a mixed solution by pulling the quantification medium into the discharge medium nozzle to contact with the discharge medium in the discharge medium nozzle; and discharging the mixed solution from the discharge medium nozzle.
According to the third aspect of the invention, there can be obtained advantages similar to those of the first aspect of the invention.
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