Ink jet head and ink jet printer capable of preventing variation of a volume of an ink droplet due to cross talk

An ink jet head that varies a capacity in each of plural pressure chambers arranged in parallel, each of which is defined by side walls, each communicating with an ink supplying path, for ejecting an ink droplet from an ejecting nozzle mounted at one end of this pressure chamber is provided with a dummy nozzle that is provided at one end of the pressure chamber positioned at the non-printing region and is set to have an aperture diameter at the ink ejecting side greater than an aperture diameter of the ejecting nozzle and to have a flow impedance approximately same as that of the ejecting nozzle. When the ink droplet is ejected from the ejecting nozzle communicating with the pressure chamber positioned at the end section within the printing region, the capacity in the pressure chamber positioned at the non-printing region is varied simultaneous with the variation of the capacity in the pressure chamber positioned at the end section within the printing region, thereby preventing a variation in volume of the ink droplet ejected from each ejecting nozzle caused by a crosstalk and preventing the occurrence of a non-uniform density or deterioration in image quality.

CROSS REFERENCE TO RELATED APPLICATION

The present application is based on Japanese Priority Document JP2002-353233 filed on Dec. 5, 2002 the content of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an ink jet head and an ink jet printer performing an image formation by ejecting an ink droplet.

2. Discussion of the Background

In a conventional technique, a shear mode ink jet head has been well-known as disclosed in U.S. Pat. No. 4,879,568 wherein a capacity in a pressure chamber is varied by pressure means that produces a shear strain in accordance with an electrical signal for selectively ejecting ink from an ejecting nozzle provided at each pressure chamber, thereby performing an image formation. This type of shear mode ink jet head has a characteristic that the pressure chamber is easy to be arranged with high density.

However, the above-mentioned shear mode ink jet head has a problem that a phenomena so-called crosstalk occurs in which a pressure fluctuation in some pressure chamber gives a fluctuation in a pressure or a flow velocity of the ink in the other nearby pressure chamber. It is considered that the crosstalk occurs because the pressure of the ink in the pressure chamber displaces a partitioning wall between the pressure chambers to thereby change the ink pressure in the adjacent and nearby pressure chambers.

Pressure chambers at the side of both ends within a printing range receive the crosstalk from only the other pressure chambers positioned at the inside within the printing range, while the pressure chambers positioned at the inside of the printing range receive the crosstalk from the other pressure chambers positioned at both sides. Therefore, the influence by the crosstalk is different between the pressure chambers positioned at both sides within the printing range and the pressure chambers positioned at the inside thereof. This leads to a difference between a volume of the ink droplet ejected from an ejecting nozzle communicating with the pressure chambers positioned at both sides within the printing range and a volume of an ink droplet ejected from an ejecting nozzle communicating with the pressure chambers positioned at the inside of the printing range, thereby being likely to cause a non-uniform density or a deterioration in image quality in a printed matter.

There is an ink jet head ofFIG. 13disclosed in, for example, Japanese Unexamined Patent Application No. 2000-135787 as an ink jet head aiming to establish an equalization of the influence of the crosstalk exerted on each pressure chamber. The ink jet head shown inFIG. 13has three dummy pressure chambers102formed respectively at both sides of plural pressure chambers101arranged in a printing range, each pressure chamber101having a single ejecting nozzle103communicating therewith and each dummy pressure chamber102having plural dummy nozzles104communicating therewith. The “dummy pressure chamber” means herein a pressure chamber from which ink is not ejected even if a driving signal is applied.

When for example, an ink droplet is ejected by changing the capacity in the pressure chamber101apositioned at the edge section within the printing range in the ink jet head shown inFIG. 13, the dummy pressure chamber102asimilarly changes its capacity simultaneous with the ejection of the ink droplet. Further, when an ink droplet is ejected by changing the capacity in the pressure chamber101bpositioned at the edge section within the printing range, the dummy pressure chamber102bsimilarly changes its capacity simultaneous with the election of the ink droplet. Further, when an ink droplet is ejected by changing the capacity in the pressure chamber101cpositioned at the edge section within the printing range, the dummy pressure chamber102csimilarly changes its capacity simultaneous with the ejection of the ink droplet.

This enables to exert the influence of the crosstalk from the other pressure chambers (effective pressure chamber and dummy pressure chamber) positioned at both sides on the pressure chambers101a,101band101cpositioned at the edge sections within the printing range, like the other pressure chambers positioned at the inside of these pressure chambers101a,101band101c.

However, the ink jet head shown inFIG. 13has plural dummy nozzles104communicating with one dummy pressure chamber102in order not to eject an ink droplet from the dummy nozzles104in case where the capacity in the dummy pressure chamber102is charged.

Therefore, a flow impedance of the dummy nozzle104for the dummy pressure chamber102, i.e., a viscosity resistance, inertial resistance or the like of the ink produced at the dummy nozzle104reduces in inverse proportion to the number of the dummy nozzle104. As a result, a main acoustic resonance frequency of the ink in the dummy pressure chamber102differs from that of the ink in the pressure chamber101.

The main acoustic resonance frequency is a frequency in which, when the pressure chamber is driven by applying voltage with the pressure means, a pressure wave occurring in the ink in the pressure chamber is transmitted through the ink in the pressure chamber and is overlapped to thereby become the greatest pressure vibration. This frequency is called a Helmholtz resonance frequency.

Therefore, when a driving signal having a waveform matched to the acoustic resonance frequency of the ink in the effective pressure chamber101is applied to the ink in the dummy pressure chamber102, an extraordinary pressure fluctuation occurs in the dummy pressure chamber102, whereby the crosstalk caused by the extraordinary pressure fluctuation occurring in the dummy pressure chamber102is exerted on the respective three effective pressure chambers101positioned at both end sections within the printing range, thereby rather entailing a problem of bringing non-uniform density or deterioration in image quality depending upon the situation.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide an ink jet head and an ink jet printer capable of preventing a variation of a volume of an ink droplet ejected from each ejecting nozzle caused by a crosstalk, thereby being capable of preventing the occurrence of a non-uniform density or deterioration in image quality.

The object of the present invention can be attained by a novel ink jet head and ink jet printer of the present invention.

According to a novel ink jet head of the present invention, an ink jet head that varies a capacity in plurality pressure chambers arranged in parallel, and respect vely communicating with ink supplying paths, each chamber being defined by side walls, wherein the plurality of the pressure chamber comprise a printing region and a non-printing region, thereby ejecting an ink droplet from an ejecting nozzle mounted at one end of this pressure chamber is provided with a dummy nozzle mounted at one end of the pressure chamber positioned in the non-printing region and set to have an aperture diameter at the ink ejecting side greater than an aperture diameter of the ejecting nozzle and to have a flow impedance approximately same as that of the ejecting nozzle. When the ink droplet is ejected from the ejecting nozzle communicating with the pressure chamber positioned at an end of the printing region, the capacity in the pressure chamber in the non-printing region is varied simultaneously.

Further, according to a novel ink jet printer of the present invention, the ink jet head and a recording medium are relatively moved such that the recording medium passes a print position opposite to the ejecting nozzle in the ink jet head, and pressure means and head driving means at the ink jet head are driven based upon a driving signal in accordance with image data.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

An embodiment of the present invention will be explained with reference toFIGS. 1 to 8.FIG. 1is a longitudinal side view showing an ink jet head, whileFIG. 2is a sectional view taken along a line A—A inFIG. 1.

The ink jet head1in the present embodiment is provided with two piezoelectric members (lower piezoelectric member2and upper piezoelectric member3) polarized in a direction of a plate thickness as shown inFIGS. 1 and 2. Two piezoelectric members2and3are laminated with the same polarity opposed to each other. The laminated piezoelectric members2and3are fixed to a substrate4made of a non-polarized low dielectric constant piezoelectric member.

The substrate and the piezoelectric members2and3fixed to this substrate4have plural channels5arranged in parallel with the same space. The plural channels5are processed by using a diamond cutter or the like.

A top plate frame6is adhered on the top surface of the substrate4. This top plate frame6seals a part of the top surface of the channel5, whereby pressure chambers7(7a,7b,7c. . . ) are formed.

The space between the adjacent pressure chambers7is composed of the lower piezoelectric member2and the upper piezoelectric member3and is partitioned by side walls8(8a,8b,8c. . . ) that form pressure means for varying the capacity in the pressure chamber7in accordance with a driving signal.

The top plate frame6is provided with an ink supplying path9that communicates with all pressure chambers7.

A top plate frame10is adhered onto the top surface of the top plate frame6. This top plate10is provided with an ink supplying opening11communicating with the ink supplying path9. Connected to the ink supplying opening11is an ink supplying pipe (not shown) for supplying ink to the ink jet head1.

Provided at the inner surface of each channel5are electrodes12(12a,12b,12c, . . . ) electrically independent to one another. The electrode12in this embodiment is made by non-electrolysis nickel plating. Each electrode12is connected to a driver IC (not shown) that is driving means via a flexible cable13connected to the rear end section of the substrate4.

A nozzle plate14made of polyimide is adhered onto the front side of the pressure chamber7. Mounted at this nozzle plate14are ejecting nozzles15(15e,15f,15g, . . . ) and dummy nozzles16(16a,16b,16,16d). The ejecting nozzles15and the dummy nozzles16in this embodiment are formed by a laser processing. The laser processing of the ejecting nozzles15and the dummy nozzles16to the nozzle plate14is performed after the nozzle plate14is adhered on the front side of the pressure chamber7.

The ejecting nozzles15are formed at the positions opposite to the pressure chambers7(7e,7f,7g, . . . ) positioned within the printing range. The dummy nozzles16are formed at the positions opposite to the pressure chambers7(7a,7b,7c,7d) positioned at the outside of the printing range.

It is to be noted thatFIG. 2shows only one end section of the ink jet head1, and formed at the other end section of the ink jet head1are also four pressure chambers7positioned at the outside of the printing range and four dummy nozzles16positioned so as to oppose to these pressure chambers7.

Ink is injected through the ink supplying pipe to the ink jet head1from the ink supplying opening11, and then, filled in the ink supplying path9, pressure chambers7, ejecting nozzles15and dummy nozzles16.

When a negative driving signal with respect to the terminals G is applied from the driver IC to the electrode12e, for example, in the ink jet head1having the above-mentioned construction, an electric field perpendicular to the polarizing direction occurs at the side walls8dand8e. The side walls8dand8erespectively bend in the opposite direction for increasing the capacity in the pressure chamber7eas shown inFIGS. 3A and 3Bdue to an inverse piezoelectric effect caused by the electric field perpendicular to the polarizing direction, thereby producing a shear strain. This increases the capacity in the pressure chamber7e(FIG. 3A). Further, when a positive driving signal with respect to the terminals G is applied to the electrode12efrom the driver IC, the capacity in the pressure chamber7eis decreased on the contrary (FIG. 3B). As described above, applying the driving signal to the electrode12eenables the capacity in the pressure chamber7eto be selectively varied. When the capacity in the pressure chamber7eincreases, the pressure of the ink in the pressure chamber7eis reduced, thereby causing a pressure fluctuation starting with a negative polarity in the ink in the pressure chamber. Further, when the capacity in the pressure chamber7edecreases, the pressure of the ink in the pressure chamber7eis increased, thereby causing a pressure fluctuation starting with a positive polarity in the ink in the pressure chamber7e. The ink in the pressure chamber7eis ejected from the ejecting nozzle15eas ink droplets when the pressure fluctuations overlap with each other to thereby increase the pressure of the ink in the pressure chamber7e.

Subsequently, the dummy nozzle16and the ejecting nozzle15are explained.FIGS. 4A and 4Bare sectional views showing shapes of the dummy nozzle16and the ejecting nozzle15. The dummy nozzle16has a shape wherein the diameter of the nozzle is widened toward the ink ejecting direction. The ejecting nozzle15has, contrary to the dummy nozzle16, a shape wherein the diameter of the nozzle is narrowed toward the ink ejecting direction.

An aperture diameter Dod of the dummy nozzle16at the outlet side is set such that it is approximately the same as an aperture diameter Dir of the ejecting nozzle15at the inlet side. An aperture diameter Did of the dummy nozzle16at the inlet side is set such that it is approximately the same as an aperture diameter Dor of the ejecting nozzle15at the outlet side. The ejecting nozzle15and the dummy nozzle16are formed so as to have a symmetrical taper shape with respect to the direction in which the ink droplet are ejected.

One preferable example of sizes of the dummy nozzle16and the ejecting nozzle15is as follows:

Aperture Diameter Dod at the outlet side of the dummy nozzle16: 54 micrometers

Aperture Diameter Did at the inlet side of the dummy nozzle16: 27 micrometers

Aperture Diameter Dor at the outlet side of the ejecting nozzle15: 27 micrometers

Aperture Diameter Dir at the inlet side of the ejecting nozzle15: 54 micrometers

In this case, the ratio of the sectional area of the dummy nozzle16at the outlet side to the sectional area of the ejecting nozzle15at the outlet side is 4:1 since it is in proportion to the square of each diameter. Specifically, the flow velocity of the ink in the dummy nozzle16is one fourth the flow velocity of the ink in the ejecting nozzle15at the position of an ink meniscus m. Accordingly, ink droplets are not ejected from the dummy nozzle16even if the pressure of the ink in the pressure chambers7ato7dincreases.

Moreover, when the diameter at the outlet side increases like the dummy nozzle16, force that the ink meniscus m holds its position by a surface tension of the ink is weakened, but its static negative pressure limit Ps becomes −2222 Pa when calculated by using a formula (1),Ps=-4⁢σDoi(1)
wherein the surface tension (σ) of the ink is 30 mN/m.

An ink hydrostatic pressure at the nozzle surface is required to be maintained within 0 to −2222 Pa, but normally, an ink supplying pressure is adjusted to have the ink hydrostatic pressure at the nozzle surface of −1000 Pa, thus there is no problem.

Further, even if the ink hydrostatic pressure becomes instantaneously less than the negative pressure limit Ps, the nozzle diameter becomes small as the ink meniscus m retreats in the dummy nozzle16, to thereby increase the negative pressure limit, with the result that the force for recovering the ink meniscus m to the original position is strengthened.

Therefore, the ink meniscus m retreats to the inside of the pressure chamber7and an air bubble is caught in the pressure chamber7, whereby the negative pressure limit that causes a malfunction of the ink jet head1is the same as that of the ejecting nozzle15.

The above-mentioned dummy nozzle16and the ejecting nozzle15are easily formed by a process using laser beam L. Specifically, a laser irradiating device having an imaging optical system is utilized, wherein a relative position of a laser projection lens and the nozzle plate14is varied by an xyz stage, and when the dummy nozzle16is formed, a laser converging surface is matched to the bottom surface of the nozzle plate14by the adjustment of the z stage as shown inFIG. 5A, while the laser converging surface is matched to the top surface of the nozzle plate14by the adjustment of the z stage as shown inFIG. 5Bwhen the ejecting nozzle15is formed.

The acoustic characteristics of the dummy nozzle16and the ejecting nozzle15are as follows. When the following definitions are made in the ejecting nozzle15inFIG. 6B:

p(t): ink pressure at the inlet of the nozzle

q(t): ink flow rate at the inlet of the nozzle

M: inertial resistance of the ink in the nozzle

R: viscosity resistance of the ink in the nozzle

(ρ): density of the ink

y(x): radius of the nozzle at the position x

r(y) pressure gradient due to the viscosity per unit flow rate of the ink flowing through a cylinder with a radius y

Ln: length of the nozzle an equation of motionP⁡(t)=M⁢ⅆⅆt⁢q⁡(t)+Rq⁡(l)(2)
relating to the ink in the nozzle is established whereinM=ρπ⁢π⁢∫0Ln⁢1y⁡(x)2⁢⁢ⅆx.(3)

It is understood from the formula (2) that the acoustic characteristic of the nozzle for the pressure chamber7, i.e., the flow impedance is characterized by the inertial resistance M and the viscosity resistance R.

Considering here an inertial resistance M′ and a viscosity resistance R′ of the dummy nozzle16that is opposite in direction to the ejecting nozzle15as shown inFIG. 6A, a following formula (4) is obtained.R=⁢∫0Ln⁢r⁡(y⁡(x))⁢⁢ⅆx(4)M′=⁢ρπ⁢∫0Ln⁢1y′⁡(x)2⁢⁢ⅆx=ρπ⁢∫0Ln⁢1y⁡(Ln-x)⁢⁢ⅆx=ρπ⁢∫0Ln⁢1y⁡(x)⁢⁢ⅆx=M(5)R′=∫0Ln⁢r⁡(y′⁡(x))⁢⁢ⅆx=∫0Ln⁢r⁡(y⁡(Ln-x))⁢⁢ⅆx=∫0Ln⁢r⁡(y⁡(x))⁢⁢ⅆx=R(6)

It is understood from above formulas (5) and (6) that the inertial resistances M, M′ and the viscosity resistances R, R′ of the ejecting nozzle15and the dummy nozzle16each having an opposite shape in direction to each other are the same, which means that the flow impedances of both nozzles are the same.

Accordingly, in case where the outlet diameter Dod of the dummy nozzle16is approximately the some as the inlet diameter Dir of the ejecting nozzle15and the inlet diameter Did of the dummy nozzle16is approximately the same as the outlet diameter Dor of the ejecting nozzle15as disclosed in the present embodiment, the dummy nozzle16and the ejecting nozzle15have approximately the same flow impedance.

This enables to make the pressure vibration characteristic of the pressure chambers7bto7dat the non-printing region approximately equal to the pressure vibration characteristic of the pressure chambers7e,7f, . . . positioned within the printing range, and further enables to make the main acoustic resonance frequency of the ink in the pressure chambers7b,7c, . . . approximately equal thereto.

Further, in case where a suction operation of the ink is performed from the ejecting nozzle15and the dummy nozzle16upon the maintenance of the ink jet head1, more ink than necessary is made to flow from the dummy pressure chamber in the conventional ink jet head provided with plural dummy nozzles in the dummy pressure chamber.

On the other hand, more ink than necessary is not made to flow from the dummy pressure chamber in the present embodiment since the dummy nozzle16and the ejecting nozzle15have the same viscosity resistance. This enables to reduce a waste of ink upon the maintenance.

FIG. 7is a timing chart of a driving signal WW outputted from the driver IC to the electrode12in a black solid printing. The driving signal is not applied to the electrode12awhich consequently has a constant potential. Applied at all times to the electrode12bis a potential some as that applied to the electrode12e. Applied at all times to the electrode12cis a potential same as that applied to the electrode12f. Applied at all times to the electrode12dis a potential same as that applied to the electrode12g. AlthoughFIG. 7shows only one end section of the ink jet head1, the same is applied to the other end section of the ink jet head1.

The driving signal is time-shared in three phases. When ink is ejected from some nozzle15, ink is not ejected from the next-door nozzles on both sides of the nozzle ejecting ink and is not ejected further from the adjacent nozzles of the next-door nozzles.

The driving signal WW is made of seven drop signals W continuously arranged. When this driving signal WW is applied to the pressure chamber7, one ink droplet is ejected from the ejecting nozzle15per one drop signal. In case where the number of the drop signal W is seven in the driving signal WW, for example, seven ink droplets are continuously elected from the ejecting nozzle15for a single driving signal WW. Accordingly, if the amount of the ink droplet adhered on one pixel is intended to be changed, the number of the drop signal W in the driving signal WW may be changed. This construction can perform a printing of 8-tone including the case where ink is not ejected.

The drop signal W is made of an expanding pulse P1for expanding the capacity of the pressure chamber7, a contracting pulse P2for contracting the capacity of the pressure chamber7and a quiescent period between both pulses. The width of the expanding pulse P1, the quiescent period and the width of the contracting pulse P2are respectively 1 AL. The AL means here a time that is half the main acoustic resonance period of the ink in the pressure chamber7, i.e., a time for inverting the average of the pressure of the ink in the pressure chamber7from a positive value to a negative value or from a negative value to a positive value. The main acoustic resonance frequency that is the inverse of the main acoustic resonance period of the ink in the pressure chamber7is called Helmholts resonance frequency. The expanding pulse P1ejects ink from the ejecting nozzle15, while the contracting pulse22has an effect of killing the pressure vibration produced by the expanding pulse P1.

As described before, the present invention can approximately match the main acoustic resonance period of the ink in the pressure chamber7, with which the dummy nozzle16is made to communicate, to the main acoustic resonance period of the ink in the pressure chamber7with which the ejecting nozzle15is made to communicate. Strictly speaking, there may be a possibility that both main acoustic resonance periods are delicately different from each other since the shape in the vicinity of the nozzle (dummy nozzle16, ejecting nozzle15) is different between the pressure chamber7with which the dummy nozzle16communicates and the pressure chamber7with which the ejecting nozzle15communicates. This difference hardly matters in the case of ejecting one droplet. However, in case where plural ink droplets are continuously ejected as in the present embodiment, the timing of the driving signal W may be matched to the main acoustic resonance period of the ink in the pressure chamber4with which the ejecting nozzle15is made to communicate.

In this configuration, the potentials of the electrode12band the electrode12e, the potentials of the electrode12cand the electrode12f, the potentials of the electrode12dand the electrode12gare respectively the same, so that when the shear strain occurs at the partitioning walls8dand Be of the pressure chamber7e, the shear strain simultaneously occurs at the side walls8aand8bof the pressure chamber7b. Further, when the shear strain occurs at the partitioning walls8eand8fof the pressure chamber7f, the shear strain simultaneously occurs at the side walls8band8cof the pressure chamber7c. Additionally, when the shear strain occurs at the side walls8fand8gof the pressure chamber7g, the shear strain simultaneously occurs at the side walls8cand8dof the pressure chamber7d. Even if the pressure chambers7b,7cand7dhave the shear strain, ink is not ejected since the dummy nozzles16b,16cand16dcommunicate with the pressure chambers7b,7cand7d. However, the flow impedances of the dummy nozzle16and the ejecting nozzle15are approximately the same, with the result that the pressure vibration approximately same as that in the pressure chambers7c,7f,7g. . . is produced in the pressure chambers7b,7cand7d. Therefore, the amplitude of the crosstalk leaked from the pressure chambers7b,7cand7dalso becomes the same as the amplitude of the crosstalk leaked from the pressure chambers7e,7f,7g. . . . Accordingly, upon the black solid printing, the pressure chambers7e,7fand7gpositioned at the end of the printing region receive the crosstalk of the same amplitude from both sides like the other pressure chambers7h,7i,7j. . . positioned at the inside of the printing region. Accordingly, the volume of the ink droplet ejected from the ejecting nozzles15e,15fand15gcan be made approximately equal to the volume of the ink droplet ejected from the ejecting nozzles15h,15i,15j, . . . . Consequently, a non-uniform density at the end of the printing region and deterioration in image quality can be prevented.

Although the above-mentioned embodiment makes an explanation taking as an example the ejecting nozzle15and the dummy nozzle16both having the linear taper shape in the inner peripheral surface, the inner peripheral surface of an ejecting nozzle15A and a dummy nozzle16A may be formed like a curved taper shape as shown inFIGS. 9A and 9B. In this case, the ejecting nozzle15A and the dummy nozzle16A are formed such that the taper shape becomes symmetrical with respect to the ink ejecting direction, thereby being capable of making the flow impedances of the ejecting nozzle15A and the dummy nozzle16A approximately equal as described above.

Subsequently, another embodiment of the present invention will be explained with reference toFIGS. 10 to 12. It is to be noted that the same parts as the embodiment 1 are given by the same numerals for omitting the explanation thereof.

FIG. 10is a perspective view showing a part of an ink jet printer of another embodiment according to the present invention. The ink jet printer is provided with a line ink jet head20. The line ink jet head1has plural ink jet heads1arranged in a line and a head holding member21for holding these ink jet heads1. The plural ink jet heads1are arranged along the arrangement direction of the ejecting nozzle and the dummy nozzle at the head holding member21as shown inFIG. 11. The ink jet heads1are arranged alternately with respect to both surfaces of the plate-like head holding member21. This can arrange the printing range of each ink jet head1along the arrangement direction of the ink jet head1without a space.

The ink jet printer has a sheet transporting belt23for transporting recording sheet22such that the sheet23passes the position opposite to the ink jet head1held by the head holding member21. The sheet transporting belt23in the present embodiment has an endless belt shape wound around a pair of rollers24. A driving mechanism such as a motor or the like not shown is connected to at least one of the pair of rollers24. The sheet transporting belt23is rotated by rotatably driving at least one of the rollers24by the driving mechanism to thereby transport the recording sheet22. Upon transporting the recording sheet22by the sheet transporting belt23, the recording sheet22is adsorbed to the sheet transporting belt23by static electricity or airflow, or the edge section of the recording sheet22is held by a holding member not shown, so that the recording sheet22comes in close contact with the sheet transporting belt23. A method for bringing the recording sheet22into close contact with the sheet transporting belt23is a well-known technique, so that its explanation is omitted.

FIG. 12is a block diagram showing various electric circuits provided at the ink jet printer of another embodiment of the present invention and a relationship among these electric circuits. The ink jet printer has an image memory25that stores image data printed on the recording sheet22. A control circuit26reads the image data stored in the image memory25in a predetermined order when the recording sheet22transported by the sheet transporting belt23passes the position opposite to the ink jet head1, and transmits a print signal according to the read-out image data to a driver IC27. The driver IC27outputs the driving signal WW having a predetermined shape to the corresponding ink jet head1. This enables a printing according to the number of the drop signal W or the like in each driving signal WW like the above-mentioned disclosure.