Ink jet printer

An ink jet printer comprises a plurality of print heads 23, 23' respectively communicating with interiors of mutually different ink tanks Tk, Ty, Tm, and Tc and each having an ink nozzle for ejecting ink accommodated in each of the ink tanks Tk, Ty, Tm, and Tc onto a printing surface, wherein the position of a distal end of the ink nozzle having a higher velocity of an ink droplet ejected from the distal end of the ink nozzle is disposed at a position apart farther from the printing surface than the position of a distal end of the ink nozzle having a lower velocity of the ink droplet, so as to reduce variations in a timing when the ink droplet ejected from each of the ink nozzles reaches the printing surface.

BACKGROUND OF THE INVENTION 
The present invention relates to an ink jet printer, and more particularly 
to an ink jet printer having a plurality of print heads. 
The following techniques are conventionally known as the ink jet printers 
of the aforementioned kind. Technique disclosed in (J01) the Unexamined 
Japanese Patent Application Publication No. Hei. 2-198850: 
This publication shows a carriage on which at least two ink jet heads can 
be mounted. In its embodiment, a print head for ejecting ink of two or 
four colors is incorporated in one ink jet head. 
Technique disclosed in (J02) the Unexamined Japanese Patent Application 
Publication No. Hei. 4-353467: 
In this publication, a technique for increasing the density of black 
characters by setting the size of droplets of black ink to a large size. 
Problem of (J01) 
With the technique of (J01) described above, since the size and weight of 
the ink droplet vary for each color, there has been a problem in that 
color offset occurs in a case where the distance between the paper and the 
print head which ejects ink at the same timing is fixed. 
Problem of (J02) 
With the technique of (J02) described above, if the size of the ink droplet 
is enlarged with respect to the ink of the other color(s), the kinetic 
energy generally increases and the ink droplet ejection velocity increases 
by the portion in which the weight of the ink droplet increases. 
Accordingly, since the ink droplet ejection velocities of the black ink 
and the color ink do not coincide, there has been a problem in that color 
offset is liable to occur. 
As another method, it is conceivable to increase the density of black 
characters by increasing the density of the dye of the black ink or 
changing the composition of a solvent. In this case, the ink droplet 
ejection velocity changes in many cases. Therefore, since the ink droplet 
ejection velocities of the black ink and the color ink do not coincide, 
there has been a problem in that color offset is liable to occur. 
To prevent the color offset, it is necessary to optimize the printing 
timing, the drive voltage, the pulse width, values of resistance of 
heating elements, and the like. 
As still another problem, there has been a drawback in that if the 
temperature of the print head rises, the size of the ink droplet becomes 
large, and the ink droplet ejection velocity also increases. In the past 
experience, a relationship that the portion of an increase in the ejection 
velocity=about +1%/.degree.C. has been obtained. Accordingly, there has 
been a problem in that even if the printing timing is set in such a manner 
that the color offset becomes minimal at a certain temperature, the color 
offset becomes large with an increase in the temperature of the print 
head. 
SUMMARY OF THE INVENTION 
In view of the above-described circumstances (and as a result of study), 
the object of the present invention is stated in the following paragraph 
(O01). 
(O01) To prevent the occurrence of color offset even in a case where a 
plurality of print heads with different ink droplet ejection velocities 
are used. 
Next, a description will be given of the arrangement of the present 
invention devised to overcome the above-described problems, and 
constituent elements of the present invention are indicated by the 
reference numerals of constituent elements of the embodiments in 
parentheses so as to facilitate the correspondence with the constituent 
elements of the embodiments which will be described later. It should be 
noted that the reason for the fact that the present invention is described 
in correspondence with the reference numerals used in the embodiments 
which will be described later is to facilitate an understanding of the 
present invention, and should not be construed as limiting the scope of 
the present invention to the embodiments. 
First Invention 
To overcome the above-described problems, the ink jet printer in accordance 
with the first invention of this application is characterized by 
comprising the following requirements: 
A plurality of print heads (23, 23') respectively communicate with 
interiors of mutually different ink tanks (Tk, Ty, Tm, and Tc) and each 
has an ink nozzle for ejecting ink accommodated in each of the ink tanks 
(Tk, Ty, Tm, and Tc) onto a printing surface. 
The print heads (23, 23') are arranged such that the position of a distal 
end of the ink nozzle having a higher velocity of an ink droplet ejected 
from the distal end of the ink nozzle is disposed at a position spaced 
apart farther from the printing surface than the position of a distal end 
of the ink nozzle having a lower velocity of the ink droplet, so as to 
reduce variations in a timing when the ink droplet ejected from each of 
the ink nozzles reaches the printing surface. 
Second Invention 
In addition, the ink jet printer in accordance with the second invention of 
this application is characterized by comprising the following 
requirements: 
A plurality of print heads (23, 23') respectively communicate with 
interiors of mutually different ink tanks (Tk, Ty, Tm, and Tc) and each 
has an ink nozzle for ejecting ink accommodated in each of the ink tanks 
(Tk, Ty, Tm, and Tc) onto a printing surface. 
The print heads (23, 23') are arranged such that the ink nozzle of one of 
print heads (23, 23') which are adapted to effect printing in 
reciprocating strokes and are arranged in such a manner as to be mutually 
spaced apart in a reciprocating direction has a shorter time duration from 
the time an ink droplet is ejected until the ink droplet reaches a 
printing surface than the ink nozzle of another one of the print heads 
(23, 23'). 
A following one of the ink nozzles ejects the ink droplet after the lapse 
of a predetermined time lag from an ejection timing of a preceding one of 
the ink nozzles, such that the ink droplet, ejected from the ink nozzle of 
the print head (23, 23') which follows by being disposed on a backward 
side in a moving direction, impacts against an impact position of the ink 
droplet ejected from the ink nozzle of the print head (23, 23') which 
precedes by being disposed on a forward side in the moving direction, 
during the reciprocating movement. 
An ink-ejection controlling device is arranged such that, in a case where 
the ink nozzle for which the time duration from the time the ink droplet 
is ejected until the ink droplet reaches the printing surface is set to be 
short precedes, the time lag is set to be shorter than in a case where 
that ink nozzle follows. 
Operation of the First Invention 
Next, a description will be given of the operation of the present invention 
which is provided with the above-described features. 
In the ink jet printer in accordance with the first invention of this 
application provided with the above-described features, the plurality of 
print heads (23, 23') respectively communicate with interiors of the 
mutually different ink tanks (Tk, Ty, Tm, and Tc) and eject the ink 
accommodated in the respective ink tanks (Tk, Ty, Tm, and Tc) onto the 
printing surface. 
As for the print heads (23, 23'), the position of the distal end of the ink 
nozzle having a higher velocity of an ink droplet ejected from the distal 
end of the ink nozzle is disposed at a position spaced apart farther from 
the printing surface than the position of the distal end of the ink nozzle 
having a lower velocity of the ink droplet. As the positions of the distal 
ends of the ink nozzles are thus arranged, it is possible to reduce 
variations in the timing when the ink droplet ejected from each of the ink 
nozzles reaches the printing surface. 
Accordingly, it is possible to set the ejection timing of the ink droplet 
from each ink nozzle without taking into consideration the difference in 
the time duration until the ink droplet ejected from each ink nozzle 
reaches the printing surface, and without causing color offset of a 
printed image which is ascribable to variations in the timing when the ink 
droplet reaches the printing surface. 
Operation of the Second Invention 
In the ink jet printer in accordance with the second invention of this 
application provided with the above-described features, the ink nozzles of 
the print heads (23, 23') respectively communicate with interiors of the 
mutually different ink tanks (Tk, Ty, Tm, and Tc), and eject the ink 
accommodated in the respective ink tanks (Tk, Ty, Tm, and Tc) onto the 
printing surface. 
In a case where the print heads (23, 23') effect printing in the 
reciprocating strokes, the ink nozzle disposed on the backward side in the 
moving direction (the ink nozzle which follows) ejects after the lapse of 
a predetermined time lag from an ejection timing of a preceding one of the 
ink nozzles. 
The ink-ejection controlling device is arranged such that, where the print 
heads (23, 23') are arranged in such a manner as to be mutually spaced 
apart in the reciprocating direction, the print head (23, 23') for which 
the time duration from the time the ink droplet is ejected until the ink 
droplet reaches the printing surface is set to be short precedes, the 
aforementioned time lag is set to be shorter than in a case where that ink 
nozzle follows. Consequently, it is possible to reduce the positional 
offset during the reciprocating movement between the impact position of 
the ink droplet ejected from the ink nozzle of the print head (23, 23') 
which precedes in the reciprocating direction and the impact position of 
the ink droplet ejected from the ink nozzle of the print head (23, 23') 
which follows.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
First Form of the First Invention 
The first form of the ink jet printer in accordance with the first 
invention of this application is characterized by being provided with the 
following requirements in the above-described first invention: 
Print heads (23, 23') have ink nozzles respectively communicating with the 
interiors of the ink tanks (Tk, Ty, Tm, and Tc) which accommodate black 
ink and color ink other than black. 
The print heads (23, 23') are arranged such the ink droplet velocity of the 
black ink is higher than the ink droplet velocity of the color ink, and 
the black-ink ejecting nozzle is disposed in such a manner as to be spaced 
apart farther from the printing surface than the position of the color-ink 
ejecting nozzles. 
Operation of the First Form of the First Invention 
In the first form of the ink jet printer in accordance with the first 
invention of this application, the print heads (23, 23') have ink nozzles 
respectively communicating with the interiors of the ink tanks (Tk, Ty, 
Tm, and Tc) which accommodate black ink and color ink other than black. 
Since the black-ink ejecting nozzle has a higher ink droplet velocity than 
the ink droplet velocity of the color ink ejecting nozzles and is disposed 
in such a manner as to be spaced apart farther from the printing surface 
than the position of the color-ink ejecting nozzles. Thus, variations in 
the timing when the ink droplet ejected from each of the ink nozzles 
reaches the printing surface are reduced. Accordingly, it is possible to 
set the ejection timing of the ink droplet from each ink nozzle without 
taking into consideration the difference in the time duration from the 
time the ink droplets are ejected from the ink nozzle for black ink and 
the ink nozzles for color ink other than black ink until they reach the 
printing surface during the reciprocating strokes of the print heads (23, 
23'). 
First Form of the Second Invention 
The first form of the ink jet printer in accordance with the second 
invention of this application is characterized by being provided with the 
following requirement in the above-described second invention: 
An ink-ejection controlling device is provided. If it is assumed that the 
(1); reciprocating velocity of two print heads (23, 23') spaced apart in 
the reciprocating direction is V, (2); the distance between the two print 
heads (23, 23') along the moving direction is L, (3); the distance from 
the distal end of the ink nozzle of one head chip to the printing surface 
is h1, (4); the velocity of the ink droplet ejected from the ink nozzle 
thereof is V1, (5); the distance from the distal end of the ink nozzle of 
another head chip to the printing surface is h2, and (6); the velocity of 
the ink droplet ejected from the ink nozzle thereof is V2, in a case where 
the one head chip precedes during the reciprocating movement, the time lag 
is set to be (L/V)+(h1/V1)-(h2/V2), whereas in a case where the other head 
chip precedes, the time lag is set to be (L/V)-(h1/V1)+(h2/V2). 
Operation of the First Form of the Second Invention 
In the first form of the ink jet printer in accordance with the second 
invention of this application, in the ink jet printer for effecting 
printing during the reciprocating strokes, it is possible to cause the ink 
droplets to impact at the same position on the printing surface 
irrespective of the positions of the ink nozzles of the print heads (23, 
23') arranged to be mutually spaced apart in the reciprocating direction, 
as well as the ejection velocities of the ink droplets. The reason for 
this will be described in the embodiments. 
Referring next to the drawings, a description will be given of examples of 
the forms (i.e., embodiments) of the ink jet printer in accordance with 
the present invention, but the present invention is not limited to the 
embodiments described below. 
Incidentally, to facilitate an understanding of the following description, 
the X-axis, the Y-axis, and the Z-axis of the rectangular coordinate 
system are defined as the directions of arrows X, Y, and Z which are 
perpendicular to each other in the drawings, wherein the direction of 
arrow X is set as the forward direction, the direction of arrow Y is set 
as the leftward direction, and the direction of arrow Z is set as the 
upward direction. In this case, the opposite direction (-X-direction) to 
the X-direction (forward direction) is the backward direction, the 
opposite direction (-Y-direction) to the Y-direction (leftward direction) 
is the rightward direction, and the opposite direction (-Z-direction) to 
the Z-direction (upward direction) is the downward direction. 
In addition, it is assumed that, in the drawings, the symbol with a dot (.) 
placed in a circle (.largecircle.) means an arrow which is directed from 
the reverse side toward the obverse side of the plane of the drawing, 
while the symbol with a cross (X) placed in a circle (.largecircle.) means 
an arrow which is directed from the obverse side toward the reverse side 
of the plane of the drawing. 
First Embodiment 
FIG. 1 is a schematic front cross-sectional view of a first embodiment of 
an ink jet printer in accordance with the present invention. FIG. 2 is a 
perspective view of a head carriage for supporting and carrying a print 
head unit which is used in the first embodiment. FIG. 3 is an exploded 
perspective view of the print head unit of the first embodiment. FIG. 4 is 
a front cross-sectional view of the print head unit in the first 
embodiment. FIG. 5 is a front cross-sectional view of a print head in the 
first embodiment. FIG. 6 is a diagram explaining an ink-channel forming 
member of the print head in the first embodiment, in which FIG. 6A is a 
top view, FIG. 6B is a cross-sectional view taken along line VIB--VIB of 
FIG. 6A, and FIG. 6C is a cross-sectional view taken along line VIC--VIC 
of FIG. 6A. FIG. 7 is a diagram explaining an ink-tank mounting member of 
the print head in the first embodiment, in which FIG. 7A is a front 
cross-sectional view, and FIG. 7B is a view (bottom view) taken in the 
direction of arrow VIIB in FIG. 7A. FIG. 8 is an exploded perspective view 
of a black ink tank and a print head for black in the first embodiment. 
FIG. 9 is a detailed explanatory diagram of the ink tank. FIG. 10 is a 
diagram explaining a method of mounting the ink tank in the head carriage 
and the print head unit. FIG. 11 is a diagram explaining the printing 
position of the ink droplet on the printing surface during the advance and 
return strokes of the print head unit H. FIG. 12 is a diagram explaining 
the printing timings in the first embodiment, and is an explanatory 
diagram in a case where the time durations until the ink droplets ejected 
from different head chips reach the printing surface are set to be 
identical. FIG. 13 is a diagram explaining the printing timings in a 
second embodiment, and is an explanatory diagram in a case where the time 
durations until the ink droplets ejected from different head chips reach 
the printing surface are set to be different. 
In FIG. 1, an ink tank Tk for black (K) and ink tanks Ty, Tm, and Tc for 
yellow (Y), magenta (M), and cyan (C) are mounted on a print head unit H 
of the ink jet printer. 
In FIG. 2, a head carriage C is a member which is reciprocatingly driven to 
the left and the right with the print head unit H mounted thereon, and the 
head carriage C has a carriage body 1 which is integrally molded from a 
resin. The carriage body 1 has a bottom wall 2, a left side wall 3, a 
right side wall 4, and an upper wall 5. 
The bottom wall 2 is provided with a tank-supporting surface 2a at its 
front (X-side portion) and four resilient tank-front-end retaining members 
2b. Further, a head-unit supporting surface 2c is provided at the rear of 
the bottom wall 2. A head-unit mounting opening 2d is formed in the 
head-unit supporting surface 2c. 
Head-unit pressing projections 3a and 3b are provided on the inner surface 
of the left side wall 3. Meanwhile, a head-unit pressing projection 4a is 
provided on the inner surface of the right side wall 4. 
Four resilient tank-rear-end retaining members 5a are provided at a free 
end on the front side of the upper wall 5 along the Y-axis direction 
(left-and-right direction). 
A lower rear end of the head carriage C is supported by a guide rod R in 
such a manner as to be slidable in the left-and-right direction, and a 
belt-connecting member 6 provided at a lower front end of the head 
carriage C is connected to a timing belt B for the traversing of the 
carriage. The timing belt B is arranged to be reciprocated in the 
left-and-right direction by means of a motor, pulleys and the like which 
are not shown. Also, the head carriage C is moved in a sliding manner in 
the left-and-right direction along the guide rod R in correspondence with 
the reciprocating movement of the timing belt B. 
The head carriage C has a connecting-substrate mounting portion 7 on the 
rear surface of the carriage body 1. A connecting substrate 8 having 
connectors 8a and 8b for connection to a connector substrate of the print 
head unit H which will be described later is arranged to be detachably 
mounted on the connecting-substrate mounting portion 7. A flexible cable 9 
is connected to the connectors 8a and 8b, and image data for printing from 
an unillustrated image processing circuit is transmitted via the flexible 
cable 9. 
In FIGS. 1, 3, and 4, the print head unit H which is mounted on the head 
carriage C has a housing 11. The housing 11 has a black ink print-head 
mounting portion 12 (see FIG. 3) at its left end portion. The black ink 
print-head mounting portion 12 has a head substrate front-end supporting 
portion 13, a head substrate rear-portion supporting groove 14, and a head 
mounting hole 15. The head substrate rear-portion supporting groove 14 has 
a lower-end supporting groove 14a, and a rear-edge supporting groove 14b. 
In addition, the housing 11 of the print head unit H has a color ink 
print-head mounting portion 16 at its right side portion. The color ink 
print-head mounting portion 16 has a head substrate front-end supporting 
portion 17, a head substrate rear-portion supporting groove 18, and a head 
mounting hole 19. The head substrate rear-portion supporting groove 18 has 
a lower-end supporting groove 18a and a rear-edge supporting groove 18b. 
Projections 21a and 21b for being supported (see FIG. 4) are provided on 
the left outer side of the housing 11, while a projection 21c for being 
supported is provided on the right outer side of the housing 11. In a 
state in which the print head unit H is mounted on the head carriage C, 
the projections 21a and 21c for being supported and the lower surface of 
the rear portion of the housing 11 are supported on the head-unit 
supporting surface 2c (see FIG. 2) of the head carriage C. 
In a case where the print head unit H is mounted on the head carriage C, a 
lower portion of the housing 11 of the print head unit H is inserted in 
the head-unit mounting opening 2d of the head carriage C, and the 
projections 21a and 21c for being supported and the lower end of the rear 
portion of the housing 11 are supported on the head-unit supporting 
surface 2c of the head carriage C. In this state, if the print head unit H 
is slidingly moved in the backward direction, the projections 21a, 21b, 
and 21c for being supported are respectively pressed downward by the 
head-unit pressing projections 3a, 3b, and 4a (see FIG. 2). 
A print head 23 for color ink (see FIG. 5) which is mounted in the color 
ink print-head mounting portion 16 on the right-hand side of the housing 
11 has an ink-tank mounting member 24 for color ink. In FIGS. 3, 5, and 7, 
the ink-tank mounting member 24 has three hollow cylindrical portions 25 
for forming ink inlet channels. An ink inlet channel 25a is formed in each 
of the hollow cylindrical portions 25 for forming ink inlet channels. The 
ink inlet channels 25a are channels through which ink of the respective 
colors, yellow (Y), magenta (M), and cyan (C) flows in. 
In FIG. 7B, three channel-forming grooves 26 are formed on the lower 
surface of the ink-tank mounting member 24. The channel-forming grooves 26 
are connected to the respective ink inlet channels 25a. A protrusion 27 
surrounding each channel-forming groove 26 is formed around each 
channel-forming groove 26. 
In addition, the ink-tank mounting member 24 is provided with two resilient 
projecting pieces 28 for retention which project downward. Further, two 
connecting holes 29 are formed in the ink-tank mounting member 24. 
A circular plate-shaped filter 31 is disposed at an upper end of each of 
the hollow cylindrical portions 25 for forming ink inlet channels, and a 
hollow cylindrical resilient member 32 is disposed around each of the 
hollow cylindrical portions 25 for forming ink inlet channels. 
As shown in FIG. 6, channel-forming grooves 34 are respectively formed on 
an upper surface of an ink-channel forming member 33, which is connected 
to the lower surface of the ink-tank mounting member 24, at positions 
opposing the three channel-forming grooves 26 (see FIG. 7B) on the lower 
surface of the ink-tank mounting member 24. A small concave groove 35 
surrounding each channel-forming groove 34 is formed around each 
channel-forming groove 34, as shown in FIG. 6. 
The protrusion 27 (see FIG. 7) is fitted in the concave groove 35, and is 
so arranged that ink does not leak from the ink channels formed by the 
channel-forming grooves 26 and 34. A head-communicating hole 36 is formed 
at a left end of each ink channel (26+34) formed by the channel-forming 
concave grooves 26 and 34. 
In addition, two connecting projections 37 (see FIGS. 6A and 6B) are 
provided on the upper surface of the ink-channel forming member 33. These 
connecting projections 37 are projections which are respectively fitted in 
the connecting holes 29 (see FIG. 7B) in the ink-tank mounting member 24. 
Further, two retaining holes 38 (see FIG. 6A) are formed in the 
ink-channel forming member 33, and these retaining holes 38 are holes 
through which the projecting pieces 28 for retention (see FIG. 7) of the 
ink-tank mounting member 24 are passed and are retained. 
Substrate-connecting projections 39 (see FIG. 6A) are provided on a left 
side surface of the ink-channel forming member 33. 
A positioning projection 42 is formed on a front end surface of a head 
substrate 41 which is connected to the ink-channel forming member 33 by 
means of holes (see FIG. 3) to which the substrate-connecting projections 
39 are fitted. Further, printed wiring is formed on the head substrate 41, 
and an IC, a head chip 43, and the like are mounted thereon. A plurality 
of nozzles (not shown) for projecting ink are formed at a lower end 43a of 
the head chip 43. Ink is arranged to be supplied from the 
head-communicating holes 36 to the unillustrated nozzles. It should be 
noted that, as for the arrangements of the head substrate 41 and the head 
chip 43, it is possible to adopt various arrangements which are 
conventionally known. 
Outer peripheral portions of lower ends of the head substrate 41 and the 
ink-channel forming member 33 are connected together by being fitted in a 
hole 44a (see FIG. 3) of a connecting frame member 44. 
The print head 23 for color ink is constituted by the elements denoted by 
the reference numerals 24 to 44. 
In FIG. 4, the head chip 43, the connecting frame 44, and the like of the 
print head 23 for color ink are passed through the head-mounting hole 19 
in the housing 11 (see FIG. 3) from above to below, and are disposed below 
the lower surface of the housing 11. Further, the positioning projection 
42 of the head substrate 41 and its rear end are positioned and supported 
by the head substrate front-end supporting portion 17 and the head 
substrate rear-portion supporting groove 18 of the housing 11. 
As shown in FIG. 4, a positioning member 46 having a U-shaped cross section 
is fitted between the rear-edge supporting groove 18b and an, upper end of 
a rear side of the head substrate 41 which is fitted in the rear-edge 
supporting groove 18b of the head substrate rear-portion supporting groove 
18, and the upper end of the head substrate 41 is thereby fixed to the 
housing 11. The lower surface of the ink-channel forming member 33 of the 
print head 23 for color ink is bonded to the upper surface of the bottom 
wall of the housing 11. 
While the above-described print head 23 for color ink is arranged to eject 
ink of three colors, yellow (Y), magenta (M), and cyan (C), a print head 
23' for black ink which is mounted in the black ink print-head mounting 
portion 12 at the left end portion of the housing 11 is arranged to eject 
ink of only one color of black. The print head 23' for black ink has 
constituent elements similar to those of the print head 23 for color ink, 
so that the constituent elements of the print head 23' which correspond to 
the constituent elements of the print head 23 for color ink will be 
denoted by the same reference numerals to which primes (') are appended. 
Accordingly, as shown in FIGS. 3, 4, and 8, the print head 23' for black 
ink has an ink-tank mounting member 24', a circular plate-shaped filter 
31', a hollow cylindrical resilient member 32', an ink-channel forming 
member 33', a head substrate 41', and a connecting frame 44'. 
Thus the print head 23' for black ink is constituted by elements denoted by 
the reference numerals 24' to 44'. 
In FIG. 4, the head chip 43', the connecting frame 44', and the like of the 
print head 23' for black ink are passed through the head-mounting hole 15 
in the housing 11 (see FIG. 3) from above to below, and are disposed below 
the lower surface of the housing 11. Further, a positioning projection 42' 
of the head substrate 41' and its rear end are positioned and supported by 
the head substrate front-end supporting portion 13 and the head substrate 
rear-portion supporting groove 14 of the housing 11. 
As shown in FIG. 4, a positioning member 46' having a U-shaped cross 
section is fitted between the rear-edge supporting groove 14b and an upper 
end of a rear side of the head substrate 41' which is fitted in the 
rear-edge supporting groove 14b of the head substrate rear-portion 
supporting groove 14, and the upper end of the rear side of the head 
substrate 41' is thereby fixed to the housing 11. The lower surface of the 
ink-channel forming member 33' of the print head 23' for black ink is 
bonded to the upper surface of the bottom wall of the housing 11'. 
In FIG. 4, the position of a lower end 43a' (the position where ejection 
holes of black-ink ejecting nozzles are formed) of the head chip 43' of 
the print head 23' for black ink fixed to the housing 11 is set .DELTA.h 
higher than the position of the lower end 43a (the position where ejection 
holes of the color-ink ejecting nozzles are formed) of the head chip 43 of 
the print head 23 for color ink fixed to the housing 11. 
Accordingly, in FIG. 1, if it is assumed that the distance from the 
printing surface to the lower end 43a (the position where the ejection 
holes of the color-ink ejecting nozzles are formed) is h1, and that the 
distance from the printing surface to the lower end 43a' (the position 
where the ejection holes of the black-ink ejecting nozzles are formed) is 
h2, then h2=h1+.DELTA.h. 
In FIG. 3, a connector substrate 47 which is mounted on the rear surface of 
the housing 11 has a pair of left and right retaining pawls 48 which 
project in the forward direction (X-direction). The retaining pawls 48 are 
respectively arranged to engage a pair of retaining holes 11a (see FIG. 4) 
which are formed in a rear wall (not shown) provided at left and right end 
portions of the rear surface of the housing 11. The connector substrate 47 
is arranged to be mounted on the housing 11 by means of the retaining 
pawls 48 which engage the retaining holes 11a (see FIG. 4). 
Two connectors 49 and 49' are provided on the front surface of the 
connector substrate 47. A cable 50 for connection to the head substrate 41 
for color ink is detachably connected to the connector 49 disposed on the 
right-hand side, while a cable 50' for connection to the head substrate 
41' for black ink is detachably connected to the connector 49' disposed on 
the left-hand side. 
The print head unit H in this embodiment is constituted by the elements 
denoted by the reference numerals 11 to 50 and the like. 
In FIGS. 1 and 3, the ink tank Tk for black (K) and the ink tanks Ty, Tm, 
and Tc for yellow (Y), magenta (M), and cyan (C) are used to supply ink to 
the print head unit H. As can be appreciated from FIG. 1, the ink tanks 
Ty, Tm, and Tc for color are of the same size, but the ink tank Tk for 
black is slightly wider than the other ink tanks Ty, Tm, and Tc. However, 
since the ink tanks Ty, Tm, Tc, and Tk are arranged similarly in the other 
aspects, the ink tank Tk for black will be described with reference to 
FIGS. 8 and 9. 
In FIGS. 8 and 9, the ink tank Tk has a tank body 56, a lid 57 for closing 
an upper opening of the tank body 56, and a double-bottom-wall forming 
member 58 which is fitted to a lower end of the tank body 56. 
A front-side projection 56a for retention is provided on a lower portion of 
the front-side surface of the tank body 56, and a knob 56b is provided on 
an upper portion of the front-side surface. Further, a rear-side 
projecting piece 56c for retention is provided on an upper portion of the 
rear-side surface of the tank body 56. A hole 56d for retention is 
provided in the rear-side projecting piece 56c. 
An auxiliary ink chamber A (see FIG. 9) is formed between the lower surface 
of the tank body 56 and the double-bottom-wall forming member 58. 
A hole 59 communicating with the auxiliary ink chamber A is formed at a 
lower end of the inner surface of the tank body 56, and a meniscus-forming 
member 61 is provided at an upper end of the hole 59. A wick 62 for 
constantly wetting the meniscus-forming member 61 is disposed so as to 
secure a space portion 63 and come into contact with a bottom surface at a 
lowermost end of the auxiliary ink chamber A. 
An ink-impregnated member 64 which is impregnated with ink is disposed in 
the tank body 56 (main ink chamber), and an atmosphere-communicating port 
57a for communicating with the atmosphere is formed in the lid 57. The lid 
57 is disposed in spaced-apart relation to an upper surface of the 
ink-impregnated member 64 so as not to compress the ink-impregnated member 
64. A meniscus-forming member 67 for preventing ink leakage from the 
auxiliary ink chamber A is disposed at an ink-supplying port 66 which is 
formed in a lower end of the double-bottom-wall forming member 58. 
In addition, a seal 68 (see FIG. 8) for displaying the color of the ink and 
the like is attached to the right-hand surface of the tank body 56. 
The ink tank Tk for black ink is constituted by the elements denoted by the 
reference numerals 56 to 68. The ink tanks Ty, Tm, and Tc for other colors 
are arranged similarly except that their widths are slightly smaller. 
Operation of First Embodiment 
Next, a description will be given of the embodiment of the ink jet printer 
of the present invention which is provided with the above-described 
arrangements. 
The print head unit H constituted by the elements denoted by the reference 
numerals 11 to 50 is mounted on the carriage C in the operating procedure 
which will be described below. Namely, the lower portion of the housing 11 
of the print head unit H is inserted in the head-unit mounting opening 2d 
of the head carriage C, and the projections 21a and 21c for being 
supported and the lower end of the rear portion of the housing 11 are 
supported on the head-unit supporting surface 2c of the head carriage C. 
In this state, if the print head unit H is slidingly moved in the backward 
direction, the projections 21a, 21b, and 21c for being supported, which 
are shown in FIGS. 3 and 4, are respectively pressed downward by the 
head-unit pressing projections 3a, 3b, and 4a, which are shown in FIGS. 2. 
The print head unit H is then secured on the head carriage C. 
Cross-sectional views of the print head unit H and the head carriage C in 
this state are shown in FIG. 10A. 
In the state of FIG. 10A, the ink tank Tk is mounted in the print head unit 
H and the head carriage C, and the state shown in FIG. 10B is thereby 
obtained. Namely, the front end projection 56a of the ink tank Tk is 
retained at the tank-front-end retaining members 2b (see FIG. 2), and the 
rear-side projecting piece 56c is retained at the tank-rear-end retaining 
members 5a. As for the method of mounting the ink tank Tk in this case, 
the lower surface of the front end of the ink tank Tk is allowed to be 
supported on the upper surface of the tank-front-end retaining members 2b 
(see FIG. 2), and the rear end of the ink tank Tk is inclined downward to 
cause the rear-side projecting piece 56c to be placed below the 
tank-rear-end retaining members 5a. In this state, if the lower surface of 
the front end of the ink tank Tk is slid downward along the surfaces of 
the tank-front-end retaining members 2b, the front end of the ink tank Tk 
moves downward, and the rear end thereof moves upward, and the state shown 
in FIG. 10B is obtained. 
In this manner, the print head unit H and the ink tank Tk are mounted on 
the head carriage C. The mounting of the ink tanks Ty, Tm, and Tc for 
color ink is similar to the above-described case of the ink tank Tk. 
In the print head unit H mounted on the head carriage in the 
above-described manner, the position of the lower end 43a (the position 
where the ejection holes of the color-ink ejecting nozzles are formed) of 
the head chip 43, shown in FIG. 4, of the print head 23 for color ink 
fixed to the housing 11, as well as the position of the lower end 43a' 
(the position where the ejection holes of the black-ink ejecting nozzles 
are formed) of the head chip 43' of the print head 23' for black ink fixed 
to the housing 11, are respectively at the distances of h1 and h2 from the 
printing surface, as shown in FIG. 1. Incidentally, .DELTA.h shown in FIG. 
4 is provided such that .DELTA.h=h2-h1. 
FIG. 11 is a diagram illustrating the operation in a case where the print 
head unit H effects printing while traversing. 
In the above-described print head unit H, in a stationary state, if it is 
assumed that the ink droplet velocity of the color ink is V1, that the 
distance between the paper surface and the distal end of the head chip 43 
for color ink is h1, and that the time until the color ink droplet impacts 
against the paper surface is t1, then t1=h1/V1. 
Similarly, if it is assumed that the ink droplet velocity of the black ink 
is V2, that the distance between the paper surface and the distal end of 
the head chip 43' for black ink is h2, and that the time until the black 
ink droplet impacts against the paper surface is t2, then t2=h2/V2. 
In the print head unit H of this embodiment, V1 and V2 are not equal. 
Therefore, values of h1 and h2 are set such that t1=t2. 
Namely, in this embodiment, the ejection velocity of color ink is set to 
8.5 m/s, and the ejection velocity of black ink is set to 11.5 m/s. 
Further, the distance h1 between the paper surface and the head chip 43 
for color ink is set to 1.5 mm. Accordingly, under these conditions, the 
distance h2 is set such that h2=h1.times.(V2/V1)=2.03 mm. 
In this embodiment, printing is effected at 7 kHz at intervals of 300 dpi. 
In this case, the print dot interval .DELTA.L becomes as follows: 
##EQU1## 
The velocity V of the carriage C in this case becomes follows: 
##EQU2## 
In FIG. 11, it is assumed that the distance between the color-ink impact 
position and the black-ink impact position in a stationary state is L, and 
that the distance between the color-ink impact position and the black-ink 
impact position in the leftward traversing state shown in FIG. 11A is L1. 
Further, .alpha. in FIG. 11A denotes the distance between the color-ink 
ejecting position and its impact position along the printing surface, 
while .beta. denotes the distance between the black-ink ejecting position 
and its impact position along the printing surface. In this case, 
EQU .alpha.=V.times.(h1/V1) 
EQU .beta.=V.times.(h2/V2) 
In this case, L1 becomes as follows: 
##EQU3## 
Next, in FIG. 11B, if it is assumed that the distance between the color-ink 
impact position and the black-ink impact position in the rightward 
traversing state is L2, then 
##EQU4## 
During both advance and return strokes, it is natural that a setting be 
provided such that L1=L2. In the above-described case, 
##EQU5## 
In this embodiment, V1, h1, V2, and h2 are provided such that t2-t1=0 as 
described above, so that L2-L1=0. In this case, L1=L2=L. 
In this case, during the advance stroke, ink ejection by the head chip 43' 
for black ink is effected when the head chip 43 for color ink has moved by 
the distance L after ejecting the color ink, whereas, during the return 
stroke, ink ejection by the head chip 43 for color ink is effected when 
the head chip 43' for black ink has moved by the distance L after ejecting 
the black ink, thereby making it possible to effect printing with the 
black ink and the color ink without positional offset. 
The time T required for the head chips 43 and 43' to move by the distance L 
can be expressed by 
EQU T=L/V 
In this case, as shown in FIG. 12, the order of printing by the head chips 
43 and 43' is reversed between the advance stroke and the return stroke, 
but since the ink ejection by a later head chip is effected with a lag of 
the time T, the printing with the black ink and the color ink can be 
effected without positional offset. 
Second Embodiment 
In the ink jet printer in accordance with the second embodiment of the 
present invention, in FIG. 11, if it is assumed that the moving velocity 
of the print head unit H is V, that the distance from the printing surface 
to the lower end 43a (the position where the ejection holes of the 
color-ink ejecting nozzles are formed) of the head chip 43 for color ink 
in the case where the head chips 43 and 43' are disposed at the distance L 
in the moving direction is hi, that the ink droplet ejection velocity of 
the color ink is V1, that the distance from the printing surface to the 
lower end 43a' (the position where the ejection holes of the black-ink 
ejecting nozzles are formed) of the head chip 43' for black ink is h2, and 
that the ink droplet ejection velocity of the-black ink is V2, then a 
setting is provided such that h1=h2, V1&lt;V2, hence, t1 (=h1/V1)&lt;t2 
(=h2/V2). In this case, .alpha.&lt;.beta., and L1&lt;L&lt;L2. 
In this case, an ink-ejection controlling device provided for controlling 
the ink ejection timing is arranged such that in a case where the head 
chip 43 having a shorter ink-droplet reaching time duration t1 (=h1/V1) 
precedes as shown in FIG. 11A, the ink-ejection controlling device delays 
the ink-droplet ejection timing of the following head chip 43' by 
(L1/V)=(L+.alpha.-.beta.)/V as compared to the preceding head chip 43. On 
the other hand, in a case where the head chip 43' having a longer 
ink-droplet reaching time duration t2 (=h2/V2) precedes as shown in FIG. 
11B, the ink-ejection controlling device delays the ejection timing of the 
following head chip 43 by (L2/V)=(L-.alpha.+.beta.)/V. 
Operation of Second Embodiment 
During the advance stroke shown in FIG. 11A, when the time T=(L/V) elapses 
after printing (ink ejection) by the preceding head chip 43 disposed at a 
forward position in the moving direction, the following head chip 43' 
moves to the same position as the printing position of the head chip 43. 
At the same position, when the head chip 43' effects printing (ink 
ejection), the impact position of the ink droplet on the printing surface 
is offset by (.beta.-.alpha.). To ensure that this positional offset does 
not occur, it suffices if the head chip 43' effects printing (ink 
ejection) at a point of time when a time T1=(L1/V) has elapsed after the 
printing (ink ejection) by the head chip 43. 
In other words, if the head chip 43' effects printing (ink ejection) at the 
point of time when the time T1=(L1/V) =(L+.alpha.-.beta.)/V has elapsed 
after the printing (ink ejection) by the head chip 43, it is possible to 
prevent the positional offset of the printing positions of the head chips 
43 and 43'. 
Similarly, during the return stroke shown in FIG. 11B, if the head chip 43' 
effects printing (ink ejection) at the point of time when the time 
T2=(L2/V)=(L-.alpha.+.beta.)/V has elapsed after the printing (ink 
ejection) by the head chip 43, it is possible to prevent the positional 
offset of the printing positions of the head chips 43 and 43'. 
That is, in order to ensure that the positional offset of the print dot 
does not occur during the advance and return strokes of the head carriage 
C, it suffices if, as shown in FIG. 13, during the advance stroke (see 
FIG. 13A), the ink ejection by a following head chip is delayed by the 
time T1=(L1/V)=(L+.alpha.-.beta.)/V and, during the return stroke (see 
FIG. 13B), the ink ejection by a following head chip is delayed by a time 
T2=(L-.alpha.+.beta.)/V. 
In addition, as can be seen from FIG. 11A, if .alpha.&lt;.beta., the time 
duration t1 (=h1/V1) from the time the ink droplet is ejected from the 
head chip 43 until it reaches the printing surface is shorter than the 
reaching time t2 (=h2/V2) of the ink droplet from the head chip 43'. Thus, 
the time lag T1 (the elapsed time from the ejection timing of a preceding 
head chip until the ejection timing of a following head chip with respect 
to the same printing position) in a case where the head chip 43 precedes 
whose time duration from the ejection timing of the ink droplet until 
reaching the printing surface is set to be short (see FIG. 11A) is set to 
be shorter than the time lag T2 in a case where the head chip 43 follows 
(the setting is provided such that T1&lt;T2). Consequently, it is possible to 
reduce the positional offset of the impact positions of the ink droplets 
which are ejected from the head chips 43 and 43' disposed in mutually 
spaced-apart relation in the reciprocating direction. 
Namely, as the time (time lag) by which the ink ejection by the head chip 
which ejects ink later is delayed is changed for the advance stroke and 
the return stroke, it is possible to reduce the positional offset of the 
ink droplets. 
Modification 
Although the embodiments of the present invention have been described above 
in detail, the present invention is not limited to the above-described 
embodiments, and various modifications are possible within the scope of 
the gist of the present invention stated in the claims. 
(H01) The present invention may be applied to an ink-jet recording 
apparatus in which three or more print heads for printing while 
reciprocating are disposed in mutually spaced-apart relation in the moving 
direction. 
The above-described invention offers the following advantage: 
(E01) It is possible to ensure that color offset does not occur in a case 
where a plurality of print heads having different ejection velocities are 
used.