Ink jet printer

In an ink jet printer having a sheet conveying mechanism which changes the speed of conveyance of a recording sheet in accordance with the density of characters or a pattern to be printed to intermittently convey the recording sheet in such a manner that all printed portions are properly dried before the sheet is conveyed to a subsequent position. The recording head of the printer performs a printing operation while pushing the front end portion of the recording sheet against the platen with a sheet retaining lever.

BACKGROUND OF THE INVENTION 
The present invention relates to an ink jet printer in which ink is jetted 
selectively from a number of nozzles. 
An ink jet printer for jetting ink from a selected one or plural nozzles to 
record characters, patterns, etc., on a recording sheet is advantageous in 
that it makes no noise while in operation and it can record data on an 
ordinary recording sheet at low operating cost. 
In general, an ink jet printer of this type uses water-soluble ink in order 
to stabilize the writing operation. As a result, the printer may suffer 
from the difficulty that the water contained in the water-soluble ink may 
make the recording sheet wavy or swell it during a printing operation, as 
a result of which it becomes rather difficult to convey the recording 
sheet to the following work position. 
In order to eliminate the difficulty, Japanese Unexamined Published Patent 
Application No. 156536/1979 has disclosed a device in which hot air is 
blown against the recording sheet on the platen to dry the ink. Also, U.S. 
Pat. No. 4,340,893 has disclosed a device in which hot air is blown 
against the recording sheet directly through the carriage to dry the ink 
on the recording sheet as soon as data is recorded on it. However, those 
conventional devices are still disadvantageous in that, in general, 
characters or patterns recorded on a recording sheet often vary in 
density, and sometimes they may be solid black. Hence, even if a hot air 
dryer is set downstream of the recording section to dry the recording 
sheet, it is rather difficult to sufficiently dry parts of the recording 
sheet having a high pixel density. As a result, wet parts of the recording 
sheet may be brought into contact with the back side of the following 
recording sheet, thus spoiling the recorded image. Also, parts of the 
recording sheet swelled and buckled by the ink may be brought into contact 
with the printing head, thus wearing the latter. 
SUMMARY OF THE INVENTION 
Accordingly, a first object of this invention is to provide an ink jet 
printer in which, even when an image to be printed includes a part having 
a relatively high density requiring a relatively large quantity of ink, 
the recording medium is uniformly dried in its entirety. 
In order to achieve the aforementioned first object of the invention, an 
ink jet printer is provided in which, according to the invention, the 
speed of conveyance of the recording medium is controlled according to the 
image density. 
A second object of the invention is to provide an ink jet printer in which 
the distance between the recording medium and the recording head is 
maintained constant to eliminate the difficulty of a printed image being 
spoiled by contact of the head with the recording medium, whereby the 
resultant image is maintained high in accuracy. 
In order to achieve the second object of the invention, in the ink jet 
printer a pair of sheet retaining means are provided before and behind in 
the direction of travel of recording means, and the one of the pair of 
sheet retaining means which is located before in the direction of travel 
is allowed to push the recording medium against the platen to form a gap 
between the recording medium and the printing head. 
A third object of the invention is to provide an ink jet printer in which, 
according to the invention, the recording medium is conveyed with the 
recorded image unaffected. 
For this purpose, cylindrical rollers relatively small in wall thickness 
and plate-shaped rollers with peripheral teeth are provided downstream of 
the recording means and mounted in such a manner that the rollers are 
confronted with each other.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Preferred embodiments of the invention will now be described with reference 
to the accompanying drawings. 
FIG. 1 shows a typical example of an ink jet printer constructed according 
to the invention, and FIG. 2 shows essential components of the printer of 
FIG. 1 in detail. 
The ink jet printer of FIG. 1 includes a recording medium conveying means 
or mechanism extended between a sheet supplying section 10 and a sheet 
discharging section 60, recording means in the form of a printing head 82 
disposed along the sheet conveying path, and a recording medium drying 
means in the form of a hot air drying unit 90. 
A sheet supplying stand 12 is disposed in the sheet supplying section 10. 
The sheet supplying stand is urged upwardly by a hopper spring 11 in such 
a manner that it is movable vertically, whereby the top of the recording 
medium in the form of a sheet S stacked on the sheet supplying stand 12 is 
held in abutment with a sheet supplying roller 13. A sheet conveying guide 
board 14 extends from the sheet supplying roller 13 to a gate roller 21. A 
separating pawl operating lever 17, which is operated up and down by a 
manual operating lever (not shown), and a separating pawl 16, which is 
operated by the lever 17, are arranged on the guide board 14. A web 
inserting guide board 19 is provided above the sheet supplying stand 12 
confronting the sheet supplying opening 3 formed in the front board 2 of a 
printer body 1 (FIG. 10), so that the recording sheet on the web is 
directly fed into the gate roller 21 with the aid of a tractor 15. 
The guide board 19 is followed by a sheet conveying section 20. In the 
sheet conveying section 20, the gate roller 21 engaged with a drive force 
transmitting mechanism (not shown) is arranged beside a carriage 80 on the 
upstream side of the travel path of the latter. The gate roller 21 is 
designed so that it performs an intermittent feed operation at a speed 
corresponding to the pixel density of a character or pattern recorded on 
the recording sheet S. A driven roller 22 is provided behind a platen 46, 
and a sheet conveying belt 23 is laid over the driven roller 22 and the 
gate roller 21. The gate roller 21 rotates the driven roller 22 through 
the sheet conveying belt 23 at the same peripheral speed. The sheet feed 
belt 23 contacts an idler 25 directly below the platen 46. The idler 25 is 
supported on one end portion of an idler lever 26. A tension spring 27 is 
connected to the idler lever 26 so as to apply a predetermined tension to 
the sheet conveying belt 23. 
A sheet retaining roller lever 29 is supported above the gate roller 21 in 
such a manner that it is swingable about a fulcrum 31. The sheet retaining 
roller lever 29 is designed so as to change the contact pressure between 
the sheet conveying belt 23 and a sheet retaining roller 32 separately 
according to the kinds of recording media to be conveyed, i.e., according 
to whether the recording medium is conveyed by the frictional force 
produced between the sheet conveying belt 23 and the sheet conveying 
roller 32, as in the case of a recording sheet or envelop, or whether the 
recording medium is conveyed by the tractor 15, as in the case of a web. A 
strong retaining spring 33 is connected to one end of the upper arm 29a of 
the sheet retaining roller lever 29 to turn the lever 29 counterclockwise 
in FIG. 1, while a cam 34 is engaged with the upper arm 29a to turn the 
lever 29 clockwise in FIG. 1 against the elastic force of the spring 33. 
When, as indicated by the solid lines, the cam 34 is released to cause the 
strong spring 33 to act directly on the lever 29, the sheet retaining 
roller 32, positioned in an arcuate groove 30 formed in the lower arm 29b 
of the lever 29, is strongly pushed against the sheet conveying belt 23. 
On the other hand, when, as indicated by the two-dot chain lines in FIG. 
2, the cam 34 lowers the lever 29 to release the strong spring 33, the 
sheet retaining roller 32 is weakly pushed against the sheet conveying 
belt 23 by one end portion 35 of a weak coil spring 31, the other end of 
which is connected to the sheet retaining roller lever 29. 
A paper bail 37 provided above the driven roller 22 is moved into or out of 
engagement with the sheet conveying belt 23. The paper bail 37 is composed 
of a shaft 36 and a plurality of plate-shaped rollers with peripheral 
teeth, which are fixedly mounted on the shaft 36. The paper bail 37 is 
supported on one end portion of a paper bail lever 38, which is secured to 
a supporting shaft 39 provided downstream of the driven roller 22. As 
shown in FIG. 3, a sector gear 40 engaged with a pinion 41 is secured to 
the supporting shaft 39. The sector gear 40 is turned clockwise in FIG. 3, 
immediately before the recording sheet S passes through the gate roller 
21, to push the paper bail 37 (supported by the end of the paper bail 
lever 38) against the sheet conveying belt 23 thereby to convey the 
recording sheet S. 
A recording sheet carry-in side guide board 43 is provided upstream of the 
platen 46. The start edge of the guide board 43 is coterminous with the 
meeting point of the web inserting guide board 19 and the sheet conveying 
guide board 14 extended from the sheet supplying roller 13, and the end 
edge is located immediately before the printing section connected to the 
platen 46. At that position, a sheet retaining board 45 is held abutted 
against the guide board 43 by its own elasticity. 
The platen 46 is disposed directly below the travel path of the carriage. 
The base end of the platen 46 is secured to a fulcrum shaft 47 provided 
downstream of the platen 46. The platen 46 is lowered, or retracted, to 
the position indicated by the two-dot chain line in FIG. 2 by a lever (not 
shown) coupled through a link to the manual operating lever adapted to 
operate the separating pawl operating lever 17, thereby to allow the 
passage of a recording medium such as an envelope relatively large in 
thickness. 
A pair of intermediate sheet discharging roller shafts 62 and 65 are 
arranged downstream of the driven roller 22 in such a manner that they are 
adjacent to the driven roller 22. The intermediate sheet discharging 
roller shaft 62 is located below the other intermediate sheet discharging 
roller shaft 65 and below a sheet discharging guide board 61. As shown in 
FIG. 4, a plurality of short intermediate sheet discharging rollers 63 
made of an elastic material are fixedly mounted on the shaft 62 spaced 
from one another. Each of the rollers 63 has cylindrical rollers 64 and 64 
having a small wall thickness at both ends. On the other hand, the upper 
intermediate sheet discharging roller shaft 65 on the side of the head 82 
has a plurality of thin-plate-shaped toothed rollers 66. More 
specifically, the toothed rollers 66 are mounted on the roller shaft 65 in 
such a manner as to confront with the cylindrical rollers 64 of the 
intermediate sheet discharging rollers 63 so that they, together with the 
cylindrical rollers 64, gently nip the wet recording sheet S to discharge 
it. 
The intermediate sheet discharging rollers 63 and 66 may be designed as 
shown in FIG. 5a. A plurality of intermediate sheet discharging rollers 63 
are mounted on the lower intermediate sheet discharging roller shaft 62 
spaced from one other another, and the cylindrical rollers 64 of adjacent 
ones of the sheet discharging rollers 63 are confronted with each other. A 
plurality of thin-plate-shaped toothed rollers 66 are mounted on the upper 
roller shaft 65, which is located on the side of the head 82, in such a 
manner that each toothed roller 66 is located between the adjacent 
cylindrical rollers 64 and 64 extending slightly into the space between 
the cylindrical rollers 64 and 64. This eliminates the difficulty of, in 
the conveyance of the recording sheet, ink on the teeth of the toothed 
rollers 66 sticking to the intermediate sheet discharging rollers 63 and 
thus staining the back side of the recording sheet S. 
A pair of sheet discharging roller shafts 68 and 71 are provided downstream 
of the hot air drying unit 90 (described later in detail). The upper shaft 
discharging roller shaft 68 is disposed below the sheet discharging board 
61. A plurality of sheet discharging rollers 69 made of an elastic 
material are fixedly mounted on the upper sheet discharging roller shaft 
68. Each of the sheet discharging rollers 69, as shown in FIG. 6, has a 
cylindrical roller 70 only at one end thereof. The upper sheet discharging 
roller shaft 71 on the side of the head 82 has a plurality of 
thin-plate-shaped toothed rollers 72. More specifically, the toothed 
rollers 72 are mounted on the roller shaft 71 in such a manner that they 
are spaced from the cylindrical rollers 70 and are slightly overlapped 
with the latter as viewed in the axial direction. These rollers 69 and 72 
strongly wave the dried recording sheet S in the direction of conveyance 
so that the recording sheet S is delivered flat onto a sheet discharging 
tray 74 while being stiffened. 
The carriage 80 is moved while being guided by two guide rails 81 and 81 
which are laid perpendicular to the recording sheet conveyance direction. 
The carriage 80 is provided with the ink jet head 82, which jets ink 
through at least one nozzle onto the recording sheet S. 
As shown in FIGS. 7 and 8a, a sheet retaining means comprises a pair of 
solenoids 88 and 88 are provided on both sides of the carriage 80. A pair 
of sheet retaining levers 86 and 86, which are driven by respective ones 
of the solenoids 88 and 88, are swingably supported on both sides of the 
lower surface of the carriage 80 in such a manner that the end portion 82a 
of the head 82 is positioned between them. When the carriage 80 travels, 
the end portions 86a of the sheet retaining levers 86 are alternately 
moved up an down, as indicated in FIG. 8a, in such a manner that the end 
portion 86a located before the other lever 86 in the direction of travel 
is lowered while the end portion 86a of the other sheet retaining lever 86 
is raised, whereby printing is carried out while the front part of the 
recording sheet S is being pushed against the platen 46 with the end 
portions 86a of the sheet retaining levers 86. 
As shown in FIG. 8b, rollers 87 may be coupled to the end portions 86a of 
the sheet retaining levers 86 so that the end portions 86a of the sheet 
retaining levers 86 can push the recording sheet S against the platen 46 
while smoothly moving on the recording sheet S. 
As shown in FIG. 7, air stream directing means comprises an integral-duct 
type fan 83 with an inverted-V-shaped air blowing outlet is provided 
downstream of the carriage 80 in the sheet conveyance direction. The fan 
83 is designed so that, as shown in FIGS. 9a and 9b, air streams are 
applied to the recording sheet S while being deflected right and left by a 
baffle plate 84 set vertically on one side of the carriage 80, and then 
directed to the other side, thus blowing dust, paper powder or the like 
off the recording sheet. 
As shown in FIG. 2, an upward movement regulating piece 89 is mounted 
upstream of and below the carriage 80 in such a manner that it forms a 
small gap with the end of a retaining board 45. When a relatively thick 
recording sheet S such as an envelope is delivered to the printing 
section, the regulating piece 89 prevents the retaining board 45 from 
being moved upwardly. 
The hot air drying unit 90 operates to dry the recording sheet S, on which 
data have been recorded, by applying hot air to it. As shown in FIG. 10, 
the drying unit 90 is mounted on two side boards 4 and 4 of the printer 
body 1. The drying unit 90 includes a duct 91 confronting a sheet 
discharging guide board 61 (FIG. 2), a heater 93 accommodated in the duct 
91, and a fan 94 (FIG. 11) installed at a suitable location in the printer 
body 1. Air which is introduced into the duct 91 through an air pipe 96 
from the fan 94 is heated by the heater 93, and the air thus heated is 
directed against the recording sheet through a number of small air blowing 
holes 92 formed in the lower board of the duct 91 and directed in the 
sheet conveyance direction. 
A control circuit for the printer is arranged as shown in FIG. 11. 
In FIG. 11, a host computer 50 provides recording data for one page, for 
instance, to be printed. The recording data are applied through an 
interface 51 to a buffer 52 so as to be stored in the latter 52. In 
accordance with an input signal provided by the buffer 52, and a preset 
program, a control means 53 applies control signals to a sheet conveyance 
drive means 55, a head drive means 56, and a heater drive means 57, so 
that a drive motor 58, the printing head 82, and a heater 93 and a fan 
motor 95 perform sheet conveyance, a recording operation, and a recording 
sheet drying operation, respectively. 
In the printer, the recording data stored in the buffer 52 are applied to a 
printing pattern analyzing means 54. The latter 54 determines from the 
input recording data the quantity of ink, or the number of dots, for 
printing each of several regions 1 through 6, as shown in FIG. 12, defined 
along a printing line. More specifically, a detecting means comprising the 
printing pattern analyzing circuit 54 detects the region having the 
largest number of dots in each line, i.e., the region having the largest 
pixel density in each line (in the case of FIG. 12, the fourth region, 
indicated by shading) and, for every line, applies the number of dots 
together with the largest pixel density region, as data, to the control 
means 53. A print drying table indicating relationships between quantities 
of ink per unitary area and corresponding ink drying time periods, as 
shown in FIG. 13, is stored in a ROM (not shown). Referring to the print 
drying table and the largest number of dots of the region i.e. pixel 
density thus inputted, the control means 53 determines an ink drying time 
period (Dx). In this manner, unique ink drying time periods (Dx) for all 
lines i.e. each and every line are obtained. These ink drying time periods 
(Dx) are added successively to determine an estimated ink drying 
completion time instants (Tx) for all the lines previous to and including 
the present line (example as shown as in FIG. 14: D2=2, D3=4, D4=6 . . . , 
T2=88, T3=92, T4=98 . . . ; T4=T2+D3+D4). The ink drying time periods (Dx) 
and the estimated ink drying completion time instants (Tx), defined as a 
drying control table (FIG 14) are written in a RAM (not shown). Referring 
to the drying control table, the control means 53 calculates an estimated 
drying completion time instant Tx, and compares the time instant thus 
calculated with the present time instant (T-elapsed). Based upon this 
comparison, the control means 53 will control the drive motor 58 to 
intermittently convey the recording sheet through the printer. 
Next, the operation of the ink jet printer will be described. 
In a sheet conveying mode, in which a recording sheet is being conveyed, in 
the sheet supply section 10, the separating pawl operating lever 17 is 
positioned as shown in FIG. 1 with a manual operating lever (not shown), 
so that the separating pawl 16 is abutted against the front end of the top 
one of the recording sheets S stacked on the sheet supplying stand 12. On 
the other hand, the lever (not shown) coupled to the manual operating 
lever lifts the platen 46 until the latter 46 becomes flush with the sheet 
conveyance guide board 43, and turns the cam 34 to the position indicated 
by the solid line in FIG. 2 to release the sheet retaining roller lever 
29. As a result, the strong spring 33 acts on the sheet retaining roller 
lever 29 to strongly push the sheet retaining roller 32 coupled to the 
lever 29 against the sheet conveying belt 23. 
When, under this condition, the sheet supplying roller 13 is rotated, the 
top one of the recording sheets S stacked on the sheet supplying stand 12 
is separated from the remaining sheets with the separating pawl 16, and 
moved along the sheet conveyance guide board 14 and the sheet conveyance 
guide board 43 to the gate roller 21. Since the gate roller 21 is strongly 
pushed against the sheet retaining roller 32 through the sheet conveying 
belt 23, the recording sheet S is conveyed to the printing section. 
When the recording sheet S is delivered to the printing section, the 
carriage 80 starts traveling along two guide rails 81 and 81, while the 
sheet retaining lever 86 located forwardly in the direction of travel of 
the carriage is turned by the solenoid 88 so that its end portion 86a 
pushes the recording sheet S against the platen 46. That is, the front end 
portion of the recording sheet S is pushed by the sheet retaining lever 86 
to form a predetermined clearance between the recording sheet S and the 
printing head 82. Under this condition, characters, patterns, etc., are 
recorded on the recording sheet S according to the inputted recording 
data. At the same time, the integral-duct type fan 83 on the carriage 80 
operates to blow air against the recording sheet S through the 
inverted-V-shaped air blowing outlet to form air streams which are 
directed from one side to the other while being deflected right and left, 
thus blowing dust, paper powder or the like off the recording sheet in 
order to prevent the sticking of such foreign matter to the nozzle. 
On the other hand, at the beginning of the sheet conveyance, the paper bail 
lever 38 is turned upwardly by the pinion 41 engaged with the sector 40, 
so that the paper bail 37 coupled to the end of the paper bail lever 38 is 
spaced away from the sheet conveying belt 23. That is, the recording sheet 
S on which dot image has been recorded is moved under the paper bail 37. 
Hence, the recording sheet S is delivered to the sheet discharging section 
60 without spoiling the dot image. 
At the same time, the thin toothed rollers 66 arranged immediately after 
the driven roller 22 are rotated while in contact with the cylindrical 
rollers 64 of the intermediate sheet discharging rollers 63. Hence, the 
recording sheet S wetted through the data writing operation is delivered 
into the hot air drying unit 90 while being in contact with the teeth of 
the thin toothed rollers 66 in a dotted form and being gently held between 
the toothed rollers 66 and the cylindrical rollers 64 of the intermediate 
sheet discharging rollers 63. Therefore, in this operation, the dot image 
on the recording sheet S is not spoiled at all. In the hot air drying 
section 90, the recording sheet S is dried with hot air as required. The 
recording sheet S thus processed is delivered into the sheet discharging 
tray 74 by the sheet discharging rollers 69 and 72. 
When, in the above-described operation, the rear edge of the recording 
sheet S arrives at a position immediately before the gate roller 21, a 
sheet edge detecting sensor (not shown) detects the rear edge to output a 
detection signal. In response to the detection signal, the pinion 41 is 
rotated to swing the paper bail lever 38 downwardly through the sector 40. 
As a result, the paper bail 37 at the end of the paper bail lever 38 is 
pushed against the sheet conveying belt 23, thus holding the recording 
sheet S. Thus, the recording sheet S released from the gate roller 21 is 
forwarded to the hot air drying unit 90 with the recorded image maintained 
unaffected. 
In the case where, in the above-described dot image recording operation, 
pixels forming a pattern, etc., are extremely high in density, the parts 
of the recording sheet S to which a large quantity of ink has been applied 
is greatly wetted to the extent that the part is greatly made wavy or 
swelled. The printing pattern analyzing means 54, receives data to be 
recorded from the buffer 52, detects this phenomenon i.e. areas of 
extremely high pixel density, in advance, detects the region which has the 
largest number of dots in every line, and applies this numerical data, 
indicative of the regions and the numbers of dots, to the control means 
53. The control means 53, refers to the relationships between quantities 
of ink per unitary area and corresponding ink drying time periods (as 
shown in FIG. 13), stored in advance, to form a drying control table (as 
shown in FIG. 14). The controls means 53 compares the present time 
T-elapsed instant with the estimated ink drying completion time instant 
for each line Tx, and determines whether or not to convey the sheet S. 
When the control means 53 determines that the present time instant 
T-elapsed is after the estimated ink drying completion time instant, Tx, 
i.e., Tx&gt;T-elapsed the drive motor 58 is operated, and sheet S conveyed. 
If Tx&lt;T-elapsed the drive motor 58 is held in a standby state. That is, 
the drive motor 58 is operated intermittently at intervals corresponding 
to the printing densities of those high ink density regions in the printed 
lines. 
In the case where the printing operation is carried out for stacked 
envelopes or the like, the separating pawl operating lever 17 is operated 
with the manual operating lever to disengage the separating pawl 16 from 
the envelopes, and the platen 46 is retracted to the position indicated by 
the two-dot chain line in FIG. 2 with a lever which is operated in 
association with the manual operating lever. As a result, the envelope 
taken out of the stack with the sheet supplying roller 13 is delivered to 
the gate roller 21 while being guided by the slope 14a provided before the 
sheet conveying board 14, and then conveyed into the printing section 
while being strongly nipped by the sheet retaining roller 32 and the sheet 
conveying belt 23. Thus, the envelope is placed on the retracted platen, 
whereupon the sealing flap of the envelope is smoothly passed through the 
clearance over the platen 46, so that, similarly as in the case of the 
recording sheet, necessary data are recorded thereon. 
In conveyance of a web with the tractor 15, the cam 34 is turned to the 
position indicated by the two-dot chain line in FIG. 2, and the sheet 
retaining roller lever 29 is turned clockwise in FIG. 2. As a result, the 
strong spring 33 acting on the lever 29 is released. Hence, the sheet 
retaining roller 32 together with the sheet conveying belt 23 supports the 
web with the aid of the weak spring 35, thus smoothly conveying the web 
into the printing section. 
The foregoing description of a preferred embodiment of the invention has 
been presented for purposes of illustration and description. It is not 
intended to be exhaustive or to limit the invention to the precise form 
disclosed, and modifications and variations are possible in light of the 
above teachings or may be acquired from practice of the invention. The 
above embodiment has been described in order to explain the principles of 
the invention and its practical application to enable one skilled in the 
art to utilize the invention in various embodiments and with various 
modifications as are suited to the particular use contemplated. It is 
intended that the scope of the invention be defined by the claims appended 
hereto, and their equivalents.