EP1038683A1

EP1038683A1 - Direct electrostatic printing apparatus

Info

Publication number: EP1038683A1
Application number: EP00106570A
Authority: EP
Inventors: designation of the inventor has not yet been filed The
Original assignee: Cegumark AB; Array Printers AB
Current assignee: Array Printers AB
Priority date: 1999-03-26
Filing date: 2000-03-27
Publication date: 2000-09-27
Also published as: JP2000272162A; AU4159800A; WO2000058103A1

Abstract

The present invention provides a direct printing apparatus reducing the number of driver IC, contemplating cost reduction, increasing a printing speed and preventing aperture clogging. Each aperture (56) has a size allowing the printing particles (38) for a plurality of dots to pass through it. Control electrodes of each aperture (56) comprises a first electrode (68) positioned at the bearing member side and a second electrode (70) positioned at the backing electrode side. The second electrode (70) is divided into a number equivalent to the maximum number of the dots possible to pass through each aperture (56).

Description

FIELD OF THE INVENTION

The present invention relates to a direct printing apparatus for use in a copying machine, printer, facsimile and the like.

BACKGROUND OF THE INVENTION

U.S. Patents No. 4,860,036 and 5,477,250 disclose a direct printing apparatus. The direct printing apparatus includes a printing station comprising a toner carrier retaining toner on its outer periphery, a backing electrode opposed to the toner carrier and a printing head disposed between the toner carrier and the backing electrode, the printing head having a plurality of apertures and a plurality of electrodes surrounding each aperture. The backing electrode of each printing station is electrically connected to a power source, thereby between the toner carrier and the backing electrode is formed an electric field for attracting the toner on the toner carrier and propelling it toward the backing electrode through the apertures of the printing head. Between the printing head and the backing electrode in each printing station is formed a passage for a sheet.
When an ON voltage is applied to the electrode of the printing head in the printing station, the toner attracting force due to the electric field between the toner carrier and the backing electrode propels the toner on the toner carrier through the apertures toward the backing electrode, causing the toner to adhere to the sheet. When an OFF voltage is applied to the electrode of the printing head, the toner attracting force does not affect the toner on the toner carrier, whereby the toner is never propelled. Thus, when ON and OFF voltage applied to the electrode of the printing head are controlled on the basis of a desired image signal, an image comprising a number of pixels (image elements) with or without dots of the toner is printed on the sheet.
In the above direct printing apparatus, a number of driver ICs are necessary to control the electrodes of the printing head, causing limits to cost reduction and print speed. In order to reduce the number of driver ICs, there has been proposed a Matrix Control. In the Matrix Control, two layers of electrodes are provided around each aperture. The first layer of electrode around the apertures are connected to each other in a main-scanning direction while the second layer of electrode around the apertures are connected to each other in a sub-scanning direction so that the electrodes connected to each other can be commonly controlled. There has been also proposed a Dot Deflection Control (DDC). In the DDC, two layers of electrodes are provided around each aperture. The second layer of the electrode around each aperture is divided into two so that the second layer of the electrode can deflect the toner particles constituting one dot which pass through the first layer of the electrode.
However, in the Matrix Control and the DDC, because one dot is printed through one aperture, it is not possible to increase the print speed. Particularly, in the Matrix Control, when the first layer of electrode is ON while the second layer of electrode is OFF, the toner particles are suspended within the aperture at the position of the first layer of the electrode, causing the aperture to be clogged.

SUMMARY OF THE INVENTION

Accordingly, the present invention has been accomplished to solve the aforementioned disadvantages of the prior arts. An object of the present invention is to provide a direct printing apparatus in which the number of the driver ICs can be reduced, attaining cost reduction, increasing print speed, and preventing the aperture to be clogged.
In order to achieve the aforementioned object, according to the present invention, there is provided a direct printing apparatus for directly depositing printing particles on a print medium to print an image by a plurality of pixels comprising dots of the printing particles, comprising:
a bearing member for bearing printing particles thereon, the printing particles being charged to a predetermined polarity;
a backing electrode opposed to the bearing member, the backing electrode generating electric field which attracts the printing particles;
a printing head disposed between the bearing member and the backing electrode, the printing head having a plurality of apertures through which the printing particles can propel and a plurality of control electrodes disposed around the plurality of apertures; and
a driver for applying the plurality of control electrode with a voltage for allowing the printing particles to be propelled and a voltage for forbidding the printing particles to be propelled in response to an image signal;
wherein each aperture has a size allowing the printing particles for a plurality of dots to pass through it;
the control electrode of each aperture comprises a first electrode positioned at the bearing member side and a second electrode positioned at the backing electrode side; the second electrode is divided into a number equivalent to the maximum number of the dots possible to pass through each aperture;
the divided second electrode of one aperture is electrically connected to the corresponding divided second electrode of the other apertures so that the voltage for allowing the printing particles to be propelled is periodically applied in an electric potential pattern corresponding to a combination of the divided second electrodes; and
the first electrode is applied with the voltage for allowing the printing particles to be propelled at a timing of the electric potential pattern corresponding to the combination of the divided electrode corresponding to the dot or dots necessary to print the mage.
In the direct printing apparatus of above construction, the case that the second electrode is divided into, for example, two will be explained herein after. The aperture of the printing head has a size allowing the printing particles for two dots to pass through it. The second electrode is periodically applied in three electric potential patterns that one and another of the divided electrodes are respectively ON-ON, ON-OFF and OFF-ON. When the first electrode is applied with the voltage for allowing the printing particles to be propelled at the timing that the second electrode is in the pattern of ON-ON, two dots are printed. When the first electrode is applied with the voltage for allowing the printing particles to be propelled at the timing that the second electrode is in the pattern of ON-OFF or OFF-ON, one dot is printed.
As the divided second electrode of one aperture is electrically connected to the corresponding divided second electrode of the other apertures so that the electrodes are commonly controlled, the number of driver ICs can be reduced. As a plurality of dots are printed through one aperture, it is possible to increase the print speed. When the first electrode is ON, any one of the divided second electrodes is necessarily ON. Therefore, the printing particles propelled due to ON of the first electrode necessarily pass through the second electrode and contribute to the print, whereby the toner particles are never suspended within the aperture, reducing the aperture to be clogged. In addition to this, as each aperture has a size allowing the printing particles for a plurality of dots to pass through it, the aperture is hardly clogged, allowing the printing particles to be easily removed at the time of cleaning.
Preferably, in one period of the electric potential pattern applied to the divided second electrodes, a pattern for applying the voltage for allowing the printing particles to be propelled to all of the divided second electrodes may be earlier than a pattern for applying the voltage for allowing the printing particles to be propelled to part of the divided second electrodes.
When print is carried out at a pattern for applying the voltage for allowing the printing particles to be propelled to all of the divided second electrodes, consumption of the printing particles becomes larger than the case that print is carried out at a pattern for applying the voltage for allowing the printing particles to be propelled to part of the divided second electrodes. Thereupon, the former pattern is made temporally earlier than the later pattern in order to extend the time to the next dot forming period, enabling to secure a time for the printing particles to be returned and replenished to the bearing member. Thus, the time required for one printing process can be shortened.
Preferably, a duration of a pattern for applying the voltage for allowing the printing particles to be propelled to all of the divided second electrodes may be longer than that of a pattern for applying the voltage for allowing the printing particles to be propelled to part of the divided second electrodes.
Thus, the dot forming density at the time when the most quantity of printing particles is necessary can be ensured.
Preferably, a time for applying the voltage for allowing the printing particles to be propelled to at least any one of the divided second electrodes may be shorter than that for applying the voltage for forbidding the printing particles to be propelled to all of the divided second electrodes.
Thus, deviation of the print position based on the difference in print time for the dot patterns can be reduced to less than half of the dot pitch and become indistinctive.
Preferably, each aperture may have a shape of ellipse, and the longitudinal axis thereof may be inclined with respect to a main scanning direction.
Thus, deviation in a sub-scanning direction between the print of a plurality of dots and the print of one dot can be made indistinctive.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects and advantages of the present invention will become clear from the following description taken in conjunction with the preferred embodiments thereof with reference to the accompanying drawings, in which:

Fig. 1: is a schematic cross-sectional side elevational view of a direct printing apparatus according to the present invention;
Fig. 2: is a cross-sectional side elevational view of a printing station of Fig. 1;
Fig. 3: is an enlarged cross-sectional view of a printing head of Fig. 2;
Fig. 4: is a cross-sectional view taken along the line IV-IV of Fig. 3 showing a printing head, a developing roller and a backing electrode;
Figs. 5A and 5B: are plane views showing wiring of the first and second control electrodes respectively;
Fig. 6: is a time chart showing control signals to the first and second control electrodes;
Fig. 7: is a time chart showing control signals to the first and second control electrodes when forming 2 dots;
Fig. 8: is a time chart showing control signals to the first and second control electrodes when forming 1 dot;
Fig. 9: is a time chart showing control signals to the first and second control electrodes when forming 1 dot;
Figs. 10A,10B and 10C: are cross-sectional views showing toner-moving states when forming 2 dots;
Fig. 11A: is a time chart showing control signals according to the DDC method;
Fig. 11B: is a time chart showing control signals according to the present invention;
Fig. 12: is a plane view showing deviation of dot positions in a sub-scanning direction;
Fig. 13: is a time chart showing control signals for eliminating the deviation of dot positions;
Fig. 14: is a plane view of printing head having the apertures with the longitudinal axis thereof being inclined with respect to a main-scanning direction;
Fig. 15A: is a plane view showing deviation of dot positions;
Fig. 15B: is a plane view showing a state in which the deviation of dot positions is eliminated;
Fig. 16: is a plane view of printing head with the second control electrode divided into three; and
Fig. 17: is a time chart showing control signals to the control electrodes of Fig. 16.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to the drawings and, in particular, to Fig. 1, there is shown a direct printing apparatus, generally indicated by reference numeral 2, according to the present invention. The printing apparatus 2 has a sheet feed station generally indicated by reference numeral 4. The sheet feed station 4 includes a cassette 6 in which a number of sheets 8 or plain papers are stacked. A sheet feed roller 10 is mounted for rotation above the cassette 6 so that it can frictionally contact with the top sheet 8, thereby the feed roller 10 can feed the top sheet 8 into the direct printing apparatus 2 as it rotates. A pair of timing rollers 12 are arranged adjacent to the sheet feed roller 10, for supplying the sheet 8 fed from the cassette 6 through a sheet passage 14 indicated by a dotted line into a printing station, generally indicated by reference numeral 16, where a printing material is deposited on the sheet to form an image thereon. Further, the printing apparatus 2 includes a fusing station 18 for fusing and permanently fixing the image of printing material on the sheet 8, and a final stack station 20 for catching the sheets 8 on which the image has been fixed.
Referring to Fig. 2, the printing station 16 comprises a developing device generally indicated by reference numeral 24 above the sheet passage 14. The developing device 24 comprises a container 26 which has an opening 28 confronting the sheet passage 14. Inside the container 26 and adjacent to the opening 28, a developing roller 30 as a bearing member of printing particles is rotatably supported in a direction of allow 32. The developing roller 30 is made of conductive material and connected to the earth. A blade 36, preferably made from a plate of elastic material such as rubber or stainless steel, is disposed in contact with the outer surface of the developing roller 30. The container 26 accommodates printing particles, i.e., toner particles 38. In this embodiment, the toner particles capable of being charged with negative polarity are used.
Disposed under the developing device 24, beyond the sheet passage 14, is an electrode mechanism generally indicated by reference numeral 40. The electrode mechanism 40 includes a support 42 made of electrically insulative material and a backing electrode 44 made of electrically conductive material. The backing electrode 44 is electrically connected to a direct power supply 46 which supplies a voltage of predetermined polarity (positive polarity in this embodiment) so that the backing electrode 44 is provided with, for example, a voltage of +1200 volts. Thus, between the backing electrode 44 and the developing roller 30 are formed an electric field E that the negatively charged toner particles 38 on the developing roller 30 are electrically attracted to the backing electrode 44.
Fixed between the developing device 24 and the electrode mechanism 40 and above the sheet passage 14 is a printing head generally indicated by reference numeral 50. Preferably, the printing head 50 is made from a flexible printed circuit board 52, having a thickness of about 100 to 150 micrometers. As shown in Figs. 2 and 4, a portion of the printing head 50 located in a printing zone 54 where the developing roller 30 confronts the backing electrode 44 includes a plurality of apertures 56. Each apertures 56 has an oblong shape lengthened in a direction 64 (hereinafter referred to as main-scanning direction) parallel to the axis of the developing roller 30 so that the toner particles for a plurality of dots (2 dots in this embodiment) can pass through the aperture as shown in one dot chain line in Fig. 3. Each aperture 56 has a minor diameter of about 25 to 200 micrometers and a major diameter of about 50 to 400 micrometers which are substantially larger than an average diameter (about several micrometers to a dozen micrometers) of the toner particles 38.
In this embodiment, as best shown in Fig. 3, the apertures 56 are formed on three parallel lines 58, 60 and 62 which extend in the main-scanning direction 64 and equally spaced in a sheet conveying direction 66 (herein after referred to as sub-scanning direction), ensuring the printing head 50 with a resolution of 300 dpi. The apertures 56 on the lines 58, 60 and 62 are formed at regular intervals of D (504 micrometers in this embodiment), and the apertures 56(56a) and 56(56c) on the lines 58 and 62 are shifted by the distance D/N to the opposite directions with respect the apertures 56(56b) on the central line 60, respectively, so that, when viewed from the sheet transporting direction 66, the apertures 56 appear to be equally spaced. Note that the number N represents the number of line rows and is "3" in this embodiment, however, the number N as well as the interval D can be determined depending upon the required resolution of the print head.
The flexible printed circuit board 52, as shown in Fig. 4, further includes therein oblong doughnut-like first and second electrodes 68 and 70 which surround the apertures 56 and are disposed in an axial direction of the aperture 56. The first electrode 68 is disposed on one side opposing the developing roller 30. The second electrode 70 is on the other side opposing the backing electrode 44. The second electrode 70, as shown in Fig. 5B, is divided into two in the main-scanning direction 64 by the center line in the sub-scanning direction 66 so that it comprises two U-shaped electrodes 70a and 70b.
The first electrode 68 of each aperture 56, as shown in Fig. 5A, is electrically communicated with a respective driver IC of a first driver 72 through a respective printed wiring 74. One electrode 70a in the second electrode 70 of each aperture 56, as shown in Fig. 5B, is electrically communicated with a driver IC of a second driver 76 through a common printed wiring 78a. The other electrode 70b in the second electrode 70 of each aperture 56 is electrically communicated with a driver IC of the second driver 76 through a common printed wiring 78b. The first driver 72 can transmit image signals X to the first electrodes 68. The second driver 76 can transmit control signals Y1, Y2 to the second electrodes 70a, 70b respectively. The first and second drivers 72 and 76, as shown in Fig, 4, are in turn electrically communicated with a controller 80 that feeds out data of image to be reproduced by the printing apparatus.
The control signals Y1, Y2 to be transmitted respectively to the electrodes 70a, 70b of the second electrode 70, as shown in Fig.6, consist of a DC component constantly applied and a pulse component for forming dots on the sheet 8. The control signals Y1, Y2 are periodically repeated by a period of P (for example, 600 sec). The control signal Y1 in the period P includes a 2-dots forming pulse P_LR and an 1-dot forming pulse P_L which are continuously formed in the period P. The control signal Y2 in the period P includes a 2-dots forming pulse P_LR synchronizing with the 2-dots forming pulse of the control signal Y1 and an 1-dot forming pulse P_R delayed from the 1-dot forming pulse P_L of the control signal Y1. The width of the 2-dots forming pulse P_LR (for example, 100 sec) is longer than that of the 1-dot forming pulse P_L, P_R (for example, 70 sec). Thus, for one print process period, the second electrodes 70a, 70b take three states of "Y1-ON, Y2-ON", "Y1-ON, Y2-OFF" and "Y1-OFF, Y2-ON".
The image signals X to be transmitted to the first electrodes 68 is to directly control the formation of dot through the aperture 56. The image signals X, as shown in Fig. 6, consist of a DC component constantly applied and a pulse component in response to the image data from the controller 80 for forming dots on the sheet 8. The image signal X in a period P (for example, 600 sec) includes a dot forming pulse synchronizing with the control signals Y1, Y2. In the case of forming 2 dots in response to the image data, when the control signals Y1, Y2 are in the state of "Y1-ON, Y2-ON", a 2-dots forming pulse P_LR is created in synchronization with the 2-dots forming pulses P_LR of the control signals Y1, Y2. In the case of forming 1 dot, when the control signals Y1, Y2 are in the state of "Y1-ON, Y2-OFF" or "Y1-OFF, Y2-ON", an 1-dots forming pulse P_L or P_R is created in synchronization with the 1-dot forming pulses P_L or P_R Of the control signals Y1, Y2.
In the concrete, in this embodiment, for the first electrode 68, the base voltage V1(B) is about -50 volts, and the pulse voltage V1(P) is about +300 volts. For the second electrode 70a, 70b, the base voltage V2(B) is about -100 volts and the pulse voltage V2(P) is about +200 volts.
Having described the construction of the printing apparatus 2, its operation will now be described.
As shown in Fig. 2, in the printing station 16, the developing roller 30 rotates in the direction indicated by the arrow 32. The toner particles 38 are deposited on the developing roller 30 and then transported by the rotation of the developing roller 30 into a contact region of the blade 36 and the developing roller 30 where the toner particles 38 are provided with triboelectric negative charge by the frictional contact of the blade 36. Thereby, as shown in Fig. 4, incremental peripheral portions of the developing roller 30 which has passed through the contact region bear a thin layer of charged toner particles 38.
In the printing head 50, the second electrodes 70a, 70b are periodically applied with the control signals Y1, Y2 of electric potential pattern as shown in Fig. 6. On the other hand, the first electrode 68 is constantly applied with the base voltage V(B) of about -100 volts. Therefore, the negatively charge toner particle 38 on the developing roller 30 electrically repels against the first electrode 68 and therefore stays on the developing roller 30 without propelling toward the aperture 52.
The controller 80 outputs the image data corresponding to an image to be reproduced to the driver 72. In response to the image data, the driver 72 supplies the pulse voltage V(P) of about +300 volts as shown in Fig. 6. As a result, the toner particles 38 on the portions of the developing roller 30 confronting the electrodes applied with the pulse voltage are electrically attracted by the electrodes 68. This energizes a number of toner particles 38 to propel by the attraction force of the backing electrode 44 into the opposing aperture 56. The converged mass of the toner particles 38 are then deposited on the sheet 8 which is supplied to the printing zone 54 from the sheet feed station 4, thereby forming an image on the sheet 8.
As shown in Fig. 7, at the time when the control signals Y1, Y2 to the second electrodes 70a, 70b are in the state of "Y1-ON, Y2-ON", if a 2-dots forming pulse P_LR as a control signal X is applied to the first electrode 68 in synchronization with the 2-dots forming pulses P_LR of the control signals Y1, Y2, then the mass of the toner particles 38 pass through the whole aperture 56 to adhere to the sheet 8, whereby 2 dots D_LR are formed.
As shown in Fig. 8, at the time when the control signals Y1, Y2 to the second electrodes 70a, 70b are in the state of "Y1-ON, Y2-OFF", if a 1-dot forming pulse P_L as a control signal X is applied to the first electrode 68 in synchronization with the 1-dot forming pulses P_L of the control signals Y1, Y2, then the mass of the toner particles 38 pass through the aperture 56 by the electrode 70a to adhere to the sheet 8, whereby 1 dot D_L is formed by left in Fig. 8 with respect to the aperture 56.
Similarly, as shown in Fig. 9, at the time when the control signals Y1, Y2 to the second electrodes 70a, 70b are in the state of "Y1-OFF, Y2-ON", if a 1-dot forming pulse P_R as a control signal X is applied to the first electrode 68 in synchronization with the 1-dot forming pulses P_R of the control signals Y1, Y2, then the mass of the toner particles 38 pass through the aperture 56 by the electrode 70b to adhere to the sheet 8, whereby 1 dot D_R is formed by right in Fig. 9 with respect to the aperture 56.
It should be noted here that the 2-dots forming pulse P_LR is positioned temporally earlier than the 1-dot forming pulse P_L, P_R in one period P. At the time of forming 2 dots, consumption of the toner particles on the developing roller is more than that at the time of forming 1 dot. Thus, as shown in Fig. 10A, the toner particles 38 are accumulated around the aperture 56 of the printing head 50. Therefore, it takes long time until the toner particles 38 accumulated on the printing head 50 return to the developing roller 30 as shown in Fig. 10B and the head portion of the toner layer reaches the aperture 56 so as to be possible to form next dot as shown in Fig. 10C. In order to eliminate this situation, the 2-dots forming pulse P_LR is positioned temporally earlier as described above, whereby reducing the time taken until the next dot is formed and advancing the print speed.
It should be also noted that the width (duration) of the 2-dots forming pulse P_LR is longer than that of the 1-dot forming pulse P_L, P_R. This is because at the time of forming 2 dots a larger quantity of the toner particles 38 need to be propelled than that at the time of forming 1 dot and same density needs to be obtained as that at the time of forming 1 dot.
Subsequently, the sheet 8 on which the image is formed is transported in the fusing station 18 where the layers of the toner particles 38 are fused and permanently fixed on the sheet 8 and finally fed out onto the final stack station or catch tray 20.
The direct printing apparatus 2 according to the embodiment as described above has three advantages as follows. Firstly, in the direct printing apparatus 2 according to the embodiment, the number of the driver ICs of the control electrodes 68, 70 is reduced. Namely, the driver IC of the first control electrode 68 for one aperture 56 can control the print of 2 dots, whereby the number of driver ICs is reduced to 1/2 in comparison with the conventional direct printing apparatus in which the driver IC of the control electrode for one aperture can control only the print of 1 dot. The divided second control electrodes 70a, 70b for each aperture 56 are commonly controlled. Therefore, the number of driver ICs of them is only two as a whole, which is negligible in comparison with the number of the driver ICs of the first control electrode 68.
Secondary, in the direct printing apparatus 2 according to the embodiment, it is possible to attain low-cost without so highly reducing the print speed. In the aforementioned DDC (dot deflection control) method in which 2 dots are formed through one aperture, the number of driver ICs is reduced to 1/2 similarly to the present invention but conversely double print speed is necessary. This is because time t1, t2 for OFF of the control electrode are necessary as shown in Fig. 11A in order to return the toner, which adheres to the printing head around the aperture after forming dot, to the developing roller to recover the toner layer on the developing roller as described above with reference to Fig. 10A. On the other hand, in the direct printing apparatus 2 according to the embodiment, 2 dots are simultaneously formed. Therefore, as shown in Fig. 11B, time t for OFF of the control electrode 68 can be set to only one time, whereby it is not necessary to reduce the print speed.
Thirdly, in the direct printing apparatus 2 according to the embodiment, the apertures 56 are hardly clogged. In the aforementioned Matrix Control method, when the first electrode is ON and the second electrode is OFF, the toner on the developing roller is slightly detached and propelled toward the aperture but restrained by the second electrode from being propelled. Therefore the aperture is apt to be clogged. On the other hand, when the first control electrode 68 is ON, at least any one of the two electrodes 70a, 70b of the second electrode 70 is ON. Therefore, the toner particles 38 detached and propelled from the developing roller 30 pass through the aperture 56 and contribute to the formation of dot. This means that the apertures 56 are hardly clogged. Further, at the non-printing time, since the first control electrode 68 is not ON, the toner particles 38 are not detached and propelled from the developing roller 30. From further point that the aperture 56 of the printing head 50 has a size allowing the printing particles 38 for two dots to pass through it, the apertures 56 are hardly clogged.
In spite of above described advantages, the direct printing apparatus 2 according to the embodiment has a situation that deflection of the dot position is caused. Namely, when forming 2 dots, the 2-dots forming pulse P_LR is ON at the beginning of period P as shown in Fig. 7. When forming 1 dot, the 1-dot forming pulse PL or PR is ON at a time that the time T_L or T_R elapses after the beginning of period P as shown in Fig. 8 or 9. Therefore, the positions of dots DLR, DL and DR formed on the sheet 8 deviate from each other in the sub-scanning direction 66. This situation will be eliminated by the following two methods.
Firstly, above situation will be practically eliminated by making a time zone Tb within one period P shorter than a remaining time zone Tw as shown in Fig. 13; where Tb is a zone in which the dot forming pulses have a possibility of being ON, and Tw is a zone in which the dot forming pulses have a possibility of being OFF. The maximum deviation of dot position in the sub-scanning direction 66 is less than half of the dot pitch in the sub-scanning direction 66. Actually, the deviation of 1-dot forming position from 2-dots forming position is based on a timing of ON edge of the dot forming pulse. Therefore, when Tb < Tw, the deviation is less than 1/2 dot pitch. Since the normally formed dot is several times a dot pitch, the deviation of less than 1/2 dot pitch would be practically no problem.
Secondly, the deviation of dot position will become indistinctive by inclining the longitudinal axis of both the aperture 56 and the control electrodes 68, 70 around the aperture 56 with respect to the main-scanning direction 64. Namely, as shown in Fig. 14, the major axis of each of the aperture 56 (in this embodiment, elliptical shape), the first and second control electrodes 68 and 70 of the printing head 50 is inclined by degrees (typically, about 20 degree) with respect to the main-scanning direction 64. As a result, as shown in Fig. 15B, the deviations of 1 dot D_L and 1 dot D_R from 2 dots D_LR would be remarkably reduced in comparison with the case of no inclination as shown in Fig. 15A. These three dots are positioned within a short area in the sub-scanning direction 66.
In the direct printing apparatus 2 according to the embodiment, although the second control electrode 70 is divided into two, it can be divided into three as shown in Fig. 16. In this case, the aperture 56 has a size allowing the printing particles for three dots to pass through it. A left side electrode 70a of the second electrode 70 of each aperture 56 is electrically connected to the driver IC of the second driver 76 via a common print wiring 78a. A right side electrode 70b is electrically connected to the driver IC of the second driver 76 via a common print wiring 78b. A central electrode 70c is electrically connected to the driver IC of the second driver 76 via a common print wiring 78c. Fig. 17 is an example of electric potential patterns Y1, Y2 and Y3 of the control electrodes 70a, 70b and 70c in the above embodiment. According to this electric potential patterns, 3 dots, 2 dots and 1 dot can be formed in the same manner as the aforementioned embodiment.
The developing device 24 as shown in Fig. 2 used in the direct printing apparatus 2 according to the aforementioned embodiment is not limited to the aforementioned type but instead any type of developing device used in an electrophotographic type of image forming apparatus can be used.
Although the present invention has been fully described by way of the examples with reference to the accompanying drawings, it is to be noted here that various changes and modifications will be apparent to those skilled in the art. Therefore, unless such changes and modifications otherwise depart from the spirit and scope of the present invention, they should be construed as being included therein.

Claims

A direct printing apparatus for directly depositing printing particles on a print medium to print an image by a plurality of pixels comprising dots of the printing particles, characterized in that the printing apparatus comprises:

a bearing member for bearing printing particles thereon, the printing particles being charged to a predetermined polarity;

a backing electrode opposed to the bearing member, the backing electrode generating electric field which attracts the printing particles;

a printing head disposed between the bearing member and the backing electrode, the printing head having a plurality of apertures through which the printing particles can propel and a plurality of control electrodes disposed around the plurality of apertures; and

a driver for applying the plurality of control electrodes with a voltage for allowing the printing particles to be propelled and a voltage for forbidding the printing particles to be propelled in response to an image signal;
wherein each aperture has a size allowing the printing particles for a plurality of dots to pass through it;

the control electrode of each aperture comprises a first electrode positioned at the bearing member side and a second electrode positioned at the backing electrode side; the second electrode is divided into a number equivalent to the maximum number of the dots possible to pass through each aperture;

the divided second electrode of one aperture is electrically connected to the corresponding divided second electrode of the other apertures so that the voltage for allowing the printing particles to be propelled is periodically applied in an electric potential pattern corresponding to a combination of the divided second electrodes; and

the first electrode is applied with the voltage for allowing the printing particles to be propelled at a timing of the electric potential pattern corresponding to the combination of the divided electrode corresponding to the dot or dots necessary to print the mage.
A direct printing apparatus as claimed in claim 1, characterized in that in one period of the electric potential pattern applied to the divided second electrodes, a pattern for applying the voltage for allowing the printing particles to be propelled to all of the divided second electrodes is earlier than a pattern for applying the voltage for allowing the printing particles to be propelled to part of the divided second electrodes.
A direct printing apparatus as claimed in claim 1, characterized in that a duration of a pattern for applying the voltage for allowing the printing particles to be propelled to all of the divided second electrodes is longer than that of a pattern for applying the voltage for allowing the printing particles to be propelled to part of the divided second electrodes.
A direct printing apparatus as claimed in claim 1, characterized in that a time for applying the voltage for allowing the printing particles to be propelled to at least any one of the divided second electrodes is shorter than that for applying the voltage for forbidding the printing particles to be propelled to all of the divided second electrodes.
A direct printing apparatus as claimed in claim 1, characterized in that each aperture has a shape of ellipse, and the longitudinal axis thereof is inclined with respect to a main scanning direction.