On-page inkjet printhead spitting system

An on-page inkjet printhead spitting system purges the printhead nozzles across the printed page, and occasionally in the conventional service station spittoon. In addition to the image-forming droplets, extra purging droplets are fired to maintain pen health. To determine when to purge each nozzle, the number of times each nozzle is fired to print the image is counted or estimated, or printhead characteristics are monitored. The purging dots may be located on the page, in the spittoon, or both. On the page, the purging dots may be hidden from view, in the image background or over the image dots, or hidden in plain sight adjacent image dots, in speed bars, in a watermark type design, or in a repeating pattern. Use of this on-page spitting system conserves ink and improves throughput of the printing mechanism, without requiring any sacrifice in the print quality appearance to the naked eye.

FIELD OF THE INVENTION 
This invention relates generally to an inkjet printing mechanism, and more 
particularly to a method of periodically purging the inkjet printhead 
which enhances the mechanism throughput, e.g. pages per minute output, 
while maintaining a high print quality. 
BACKGROUND OF THE INVENTION 
Inkjet printing mechanisms use pens which shoot drops of liquid colorant, 
referred to generally herein as "ink," onto a page. Each pen has a 
printhead formed with very small nozzles through which the ink drops are 
fired. To print an image, the printhead moves back and forth across the 
page shooting drops as it moves. Typically, a service station is mounted 
within the printer chassis to clean and protect the printhead. For 
storage, or during non-printing periods, the service stations usually 
include a capping system which humidically seals the printhead nozzles 
from contaminants and drying. Some caps are also designed to facilitate 
priming, such as by being connected to a pumping unit that draws a vacuum 
on the printhead. 
During operation, clogs in the printhead are periodically cleared by firing 
a number of drops of ink through each of the nozzles in a process known as 
"spitting." In the past, the waste ink was collected in a reservoir 
portion of the service station, which is often referred to as a 
"spittoon." After spitting, uncapping, or occasionally during printing, 
most service stations have an elastomeric wiper that wipes the printhead 
surface to remove ink residue, as well as any paper dust or other debris 
that have collected on the printhead. 
To improve the clarity and contrast of the printed image, recent research 
has focused on improving the ink itself. To provide faster, more waterfast 
printing with darker blacks and more vivid colors, pigment based inks have 
been developed. These pigment based inks have a higher solid content than 
the earlier dye based inks. Both types of ink dry quickly, which allows 
inkjet printing mechanisms to use plain paper. Unfortunately, the 
combination of small nozzles and quick drying ink leaves the printheads 
susceptible to clogging, not only from dried ink and minute dust particles 
or paper fibers, but also from the solids within the new inks themselves. 
Partially or completely blocked nozzles can lead to either missing or 
misdirected drops on the print media, either of which degrades the print 
quality. Thus, spitting to clear the nozzles becomes even more important 
when using pigment based inks, because the higher solids content 
contributes to the clogging problem more than the earlier dye based inks. 
Unfortunately, while spittoons were suitable for the earlier dye based 
inks, they suffer a variety of drawbacks when used with newly developed 
pigment based inks. 
For example, during spitting the inkjet pens are positioned over the 
spittoon, which consumes valuable printing time, not only to spit, but to 
position the printheads over the spittoon, and then return the printheads 
to the page for printing. This time consumption decreases the throughput 
of the printing mechanism, which is a rated characteristic, often measured 
in pages per minute. Consumers desire faster printing mechanisms, and 
those with a lower throughput rating are considered less desirable. As a 
design compromise, to minimize the loss of page throughput, less time 
could be devoted to spitting. Unfortunately, this compromise often results 
in poor quality printed images, from the omission of dots being printed 
due to clogged nozzles. Thus, in the past there has been an unsatisfactory 
trade-off between throughput and print quality. 
Thus, a need exists for an improved inkjet printhead servicing system, 
which is directed toward overcoming, and not susceptible to, the above 
limitations and disadvantages. 
SUMMARY OF THE INVENTION 
According to one aspect of the invention, a method is provided of purging 
an inkjet printhead used in an inkjet printing mechanism. The method 
includes the step of firing selected nozzles of the printhead to deposit 
image ink droplets on a print media page to print a selected image. In a 
purging step, selected nozzles are purged by firing to deposit purging ink 
droplets on the page. In the illustrated embodiments, black ink purging 
dots are scattered randomly over the page, or in the background areas. For 
color ink spitting, preferably the nozzles are fired over black print 
areas, toward the interior to maintain the crisp outline of the black 
image. 
According to another aspect of the present invention, an inkjet printing 
mechanism is provided, including a chassis and a printhead mounted to the 
chassis for reciprocal movement across a print zone. The printhead has 
plural nozzles that are selectively fired to deposit image ink droplets on 
a print media page to print a selected image in response to a control 
signal. A controller generates the control signal and monitors the number 
of image droplets fired from each nozzle. In response to the monitored 
number of image ink droplets fired, the controller adjusts the control 
signal to fire purging ink droplets from selected nozzles to deposit 
purging ink droplets on the page. 
An overall goal of the present invention is to provide an inkjet printing 
mechanism which uses less down-time for servicing to increase throughput, 
while providing a high quality hardcopy output. 
A further goal of the present invention is to provide a method of purging 
an inkjet pen mounted in a printing mechanism without using the 
conventional spittoon.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 illustrates an embodiment of an inkjet printing mechanism, here 
shown as an inkjet printer 20, constructed in accordance with the present 
invention, which may be used for printing for business reports, 
correspondence, desktop publishing, and the like, in an industrial, 
office, home or other environment. A variety of inkjet printing mechanisms 
are commercially available. For instance, some of the printing mechanisms 
that may embody the present invention include plotters, portable printing 
units, copiers, cameras, video printers, and facsimile machines, to name a 
few. For convenience the concepts of the present invention are illustrated 
in the environment of an inkjet printer 20. 
While it is apparent that the printer components may vary from model to 
model, the typical inkjet printer 20 includes a chassis 22 and a print 
medium handling system 24 for supplying sheets of print media to the 
printer 20. The print media may be any type of suitable sheet material, 
such as paper, card-stock, transparencies, mylar, foils, and the like, but 
for convenience, the illustrated embodiment is described using paper as 
the print medium. The print medium handling system 24 moves the print 
media into a print zone 25 from a feed tray 26 to an output tray 28, for 
instance using a series of conventional motor-driven rollers (not shown). 
In the print zone 25, the media sheets receive ink from an inkjet 
cartridge, such as a black ink cartridge 30 and/or a color ink cartridge 
32. The cartridges 30, 32 are also referred to as "pens" by those in the 
art. The illustrated color pen 32 is a tri-color pen, although in some 
embodiments, a group of discrete monochrome pens may be used, or a single 
monochrome black pen 30 may be used. While the color pen 32 may contain a 
pigment based ink, for the purposes of illustration, pen 32 is described 
as containing three dye based ink colors, such as cyan, yellow and 
magenta. The black ink pen 30 is illustrated herein as containing a 
pigment based ink. It is apparent that other types of inks may also be 
used in pens 30, 32, such as paraffin based inks, as well as hybrid or 
composite inks having both dye and pigment characteristics. 
The illustrated cartridges or pens 30, 32 each include reservoirs for 
storing a supply of ink therein, although other ink supply storage 
arrangements, such as those having reservoirs (not shown) mounted along 
the chassis may also be used. The cartridges 30, 32 have printheads 34, 36 
respectively. Each printhead 34, 36 has bottom surface comprising an 
orifice plate with a plurality of nozzles formed therethrough in a manner 
well known to those skilled in the art. The illustrated printheads 34, 36 
are thermal inkjet printheads, although other types of printheads may be 
used, such as piezoelectric printheads. The printheads 34, 36 typically 
include a plurality of resistors which are associated with the nozzles. 
Upon energizing a selected resistor, a bubble of gas is formed ejecting a 
droplet of ink from the nozzle and onto a sheet of paper in the print zone 
25 under the nozzle. 
The cartridges or pens 30, 32 are transported by a carriage 38 which may be 
driven along a guide rod 40 by a conventional drive belt/pulley and motor 
arrangement (not shown). The pens 30, 32 selectively deposit one or more 
ink droplets on the print media page in accordance with instructions 
received via a conductor strip 42 from a printer controller, such as a 
microprocessor which may be located within chassis 22 at the area 
indicated generally by arrow 43. The controller may receive an instruction 
signal carded via conductor 44 from a host device, which is typically a 
computer, such as a personal computer 45, illustrated schematically FIG. 
1. The printhead carriage motor and the paper handling system drive motor 
operate in response to the printer controller 43, which operates in a 
manner well known to those skilled in the art. The printer controller may 
also operate in response to user inputs provided through a key pad, which 
may be located on the exterior of the chassis in the region generally 
indicated by arrow 46. A monitor coupled to the computer 45 may be used to 
display visual information to an operator, such as the printer status or a 
particular program being run on the computer 45. Personal computers, their 
input devices, such as a keyboard and/or a mouse device, and monitors are 
all well known to those skilled in the art. 
Located at one end of the travel path of carriage 38, the printer chassis 
22 defines a chamber 48 that is configured to receive a service station 
50. Preferably, the service station 50 is constructed as a modular device 
capable of being unitarily inserted into the printer 20, to enhance ease 
of initial assembly, as well as maintenance and repair in the field. The 
illustrated service station 50 has a frame 52 that may be slidably 
received within the chassis chamber 48. However, it is apparent that the 
service station 50 may also be constructed with the station frame 52 
integrally formed within the chassis 22. 
The service station 50 has a tumbler portion 54 mounted to frame 52 for 
rotation when driven by a motor through an optional gear or belt assembly 
(not shown) that engages a drive gear 60. The tumbler 54 includes a main 
body 62 which may support a black ink cap 64 and a color ink cap 65. The 
main body 62 may also support black and color ink wipers 66 and 68 for 
wiping the respective black and color printheads 34, 36. The wipers 66, 68 
may be of an elastomeric material, for instance a nitrile rubber, ethylene 
polypropylene diene monomer (EPDM) elastomer, or other types of 
rubber-like materials known to those skilled in the art. The wiping action 
is usually achieved by moving the printheads 34, 36 across the wipers 66, 
68. Other functions may also be provided on the main body 62, such as 
primers and the like, which are known to those skilled in the art. 
The service station 50 may also include an ink collecting chamber or 
"spittoon" portion 70, which may comprise one or more spittoon chambers. 
In the illustrated embodiment, the spittoon portion 70 has black and color 
spittoon chambers 72, 74 to receive ink that is selectively ejected or 
"spit" from the respective black and color pens 30, 32 when they are 
positioned above spittoon 70. An absorbent liner material, known as a 
"diaper" 76, may be placed near the bottom of the spittoon 70 to retain 
the spit ink while it is drying. Typical liquid absorbent materials may be 
of a felt, pressboard, sponge, or other comparable materials known to 
those skilled in the art. Indeed, the diaper 76 may extend under the other 
components of the service station 50, to absorb any ink leakage, and to 
provide a larger capillary path for liquids from the spittoon 70 to travel 
before evaporating. The spittoon 70 may be separated from drive gear 60 by 
a wall member 78, which may also serve as a side wall for the color 
spittoon chamber 74. 
It is apparent that other arrangements may be used to index the pen 
capping, wiping, etc. functions rather than the tumbler main body 62. For 
example gears or linkages (not shown) known to those skilled in the art 
may be used for selectively engaging the service station equipment 64, 65 
and 66, 68 with the respective printheads 34, 36. For instance, suitable 
translating or floating sled types of service station operating mechanisms 
are shown in U.S. Pat. Nos. 4,853,717 and 5,155,497, both assigned to the 
present assignee, Hewlett-Packard Company. 
The illustrated printer 20 produces a hard copy output, shown in FIG. 1 as 
a print media page 78, upon which has been printed an image 80. While it 
is apparent that the image 80 may take a variety of forms, including text, 
graphics, photographic images, or other designs, for the purposes of 
illustration image 80 has several of these components. First, image 80 
includes a title portion 82, textual portions 84, 85, a photographic image 
86, and a graphic chart, here illustrated as a multi-colored pie chart 88. 
Preferably, the color pen 32 includes the colors cyan, yellow and magenta 
("CYM") which may be combined to provide a full gamut of colors on image 
80, such as the pie chart 88 which has varying cross hatching shown to 
represent the colors purple, blue, gray, and green in FIG. 1. The image 80 
also includes a border 90 which extends around the periphery of the page 
78. As with most hard copy outputs, the image 80 also includes some blank 
regions, such as at 92, where no text, title, graphs, border, or other 
images have been printed. 
Rather than always returning the pens 30, 32 to the spittoons 72, 74 for 
spitting, the illustrated embodiments of the on-page spitting system, 
implemented in accordance with the present invention, may take several 
forms. In one illustrated embodiment, the black pen 30 is used, whereas in 
the another embodiment, the color pen 32 is used. Several methods may be 
used with both black and color pens. These illustrated methods concern two 
factors, first, determining when purging is required, and, second, 
selecting where purging will take place. 
Methods for Determining When to Purge Nozzles 
These when and where decisions may be made by the firmware, software, 
hardware alone or in combination (referred to collectively herein as the 
"control system" or the controller 46) of either the cartridge 30, 32, the 
printer 20, or both. Alternatively, the host device, such as the computer 
45, may determine when spitting is needed, and select the nozzle spitting 
firing scheme to determine where spitting will occur. Thus, either the 
controller 46, the host computer 45, or a combination of the two generates 
a control signal that is delivered to printheads 34, 36 via the conductor 
strip 42 to determine when and where the purging ink droplets are fired. 
First, several embodiments dealing with the question as to when to purge 
are described. 
In one embodiment, the printer controller 46 or the host computer 45 may 
monitor various printhead characteristics to determine when the nozzles 
require purging. For example, the printhead temperature may be monitored, 
with a rise in temperature indicating a possible nozzle blockage or 
occlusion which needs to be cleared by spitting. As another example, the 
energy efficiency or cogation of the printhead 34, 36 may be monitored 
over time, with a drop in efficiency indicating a need for nozzle purging. 
The temperature, energy efficiency or cogation may be measured using a 
variety of different monitoring devices known to those skilled in the art. 
For example, the resistor, which is energized to heat the ink at each 
nozzle for ejection, may be monitored to determine this temperature, and 
the energy delivered to this resistor may also be measured. It is apparent 
that the exact thresholds for temperature or efficiency that trigger a 
need for purging vary with the types of inks used, the pen design, 
selected print quality (e.g., draft, normal or presentation quality), and 
other factors known to those skilled in the art. 
In another embodiment, the illustrated controller 46 or the host computer 
45 may monitor and count the number of times each nozzle is fired for each 
pass of the carriage 38 over the print zone 25. The timing of each pass of 
the printhead 34, 36 scanning across the print zone 25 may also be 
monitored. This monitoring may be done on a predictive basis, by analyzing 
the image data before the image droplets are fired. This allows image and 
purging droplets to be laid down during the same pass if desired. Either 
the controller 46 or the host device 45 may monitor the number of image 
ink droplets designated for each nozzle to generate the firing control 
signal sent to printheads 34, 36 via conductor 42. 
An image firing rate for printing image 80 is determined based upon the 
monitored number of firings of each nozzle, and the timing of each pass. 
The image firing rate is then compared, on a per nozzle basis, to a target 
firing rate required to maintain pen health. For example, the target 
firing rate may be set to ensure that each nozzle has been fired a target 
number "N" times for every "M" seconds. Preferably, for a pigment based 
ink, the spitting of each nozzle is conducted at a rate of ten drops 
(N=10) per nozzle every five seconds (M=5) to maintain pen health, whereas 
dye based inks may require a purging rate often drops (N=10) per nozzle 
every ten seconds (M=10). When the image firing rate is less than the 
target rate, the decision is made to fire purging ink droplets at a rate 
to make up the difference between the target rate and the image firing 
rate. 
As a further embodiment of the present invention, the nozzles may be purged 
on the page 78 by firing all of the nozzles or groups of nozzles on a 
regular basis, such as at selected time intervals, without monitoring 
printhead characteristics. However, identifying only nozzles in need of 
purging, then spitting only these identified nozzles, is preferred to save 
ink. It is apparent that the hardware for the individual timers required 
for each nozzle may require the use of a large area of semiconductor, such 
as silicon, as well as more processor band width to manage the individual 
control for firing each nozzle. However, advances in integrated circuit 
technology continually render the cost of manufacturing such timers to be 
more economical. 
Having now described several embodiments dealing with when to purge, the 
next question to be determined by the control system 46 and/or computer 45 
is where to purge. Indeed, the determination of where to purge each nozzle 
may be applied to select various on-page spitting routines, or a 
spittoon-only spitting system, or to a combination on-page/spittoon 
purging system, each in accordance with the present invention. Several 
on-page purging routines, where the spitting pattern overlays the image 
80, are described below for selecting where to spit on page 78. 
"Hide in the Background" 
On-Page Spitting System 
In accordance with the illustrated embodiment of the present invention, 
while printing image 80 the black pen 30 performs spitting on the page 78 
to hide the purging droplets from view. During recent product 
improvements, pen servicing requirements have gone from eight drops per 
nozzle every fifty seconds for the dye based inks used several years ago, 
to approximately ten drops every five seconds with the current pigment 
based black ink. While the pigment based inks provide darker more vivid 
images on the printed page, by their very nature they require more 
frequent purging. For example, the illustrated printer 20 is expected to 
spend five to ten percent of the total page print time performing spits in 
the spittoon 72 to maintain pen health. This represents a significant 
decrease in the possible throughput of the printer 20 over that achieved 
with the earlier printers. 
It was discovered that small dots, for example, those having a diameter on 
the order of 0.05 mm (0.002 inches), were very difficult to observe when 
randomly placed upon a page. It is apparent that the ability to hide such 
a random scattering of drops over a page improves as the drop size 
decreases. Thus, as higher resolutions are achieved through developments 
in printing technology, the methods illustrated for hiding purging 
droplets on a page will be even more successful in practice. 
The throughput of printer 20 may be significantly increased by spraying 
maintenance drops randomly on the page 78, instead of hindering throughput 
by moving the printhead over the spittoon 72 for spitting. It is also 
apparent that for presentation quality printing, it may be preferable to 
return to a spittoon-only spitting routine. Some spittoon spitting may 
still be useful for handling large amounts of purged ink from either pen 
30 or pen 32, such as after priming at pen initiation, or following a 
period of printer inactivity. 
In one preferred embodiment, the randomly fired health maintenance drops 
are spit at locations which are preferably at least three dots away from 
any "real" dots used to form image 80, if such spacing is possible. To 
hide the purging ink droplets in the background 92, they are preferably 
spaced at least three dots away from each other if possible. This scheme 
of randomly placing the health maintenance drops away from image 80 
ensures that the purging droplets blend into the background portion of the 
blank region 92, and do not appear as bumps along the boundaries of the 
printed text 84, 85, graphics 88, etc., which form image 80. 
The effect of hiding purging droplets was tested by spraying a random 
pattern of dots on a page, and then printing text over the top of the 
sprayed test sheet. The purged dots were found to be unnoticeable to the 
naked human eye. However, one preferred implementation preferably monitors 
a characteristic of each nozzle to minimize the amount of ink used for 
health maintenance of the pen 30. This hide in the background purging 
scheme is preferred for printhead resolutions of 600 dots per inch (dpi) 
or finer resolutions. While illustrated with respect to black ink 
spitting, it is apparent that this random spit-in-the-background scheme 
may also be used with the color pen 32. 
"Hide in the Black" 
On-Page Spitting System 
In accordance with the present invention, while printing image 80 the color 
pen 32 performs spitting on the page 78 in the black areas of image 80, to 
hide the purging droplets from view. For example, the illustrated 
tri-color pen 32 uses dye based inks which require spitting in the range 
of approximately ten drops every ten seconds. Using a conventional 
spitting system, this servicing rate requires many time consuming trips to 
the spittoon 74 while printing documents. Rather than continually tracking 
the printhead 32 to the spittoon 74, it was discovered that small volumes 
of color ink droplets that are fired in black print areas, such as title 
82, text 84, 85 or the border 90 of image 80, are unobservable to the 
naked eye. This method of hiding the color spitting increases the 
throughput of printer 20, without effecting the print quality of the hard 
copy output. 
Preferably, the color purging drops from printhead 36 are placed in black 
printed regions of image 80 which are wide enough to assure that all of 
the color droplets indeed land within the black area, rather than along 
the edge of a black area. For example, in printer design, there is usually 
a known misalignment between the color and black pens, such as up to three 
dot widths. The color purging droplets are then located at least this 
misalignment distance, here, three drop widths, from a black border drop 
to hide the purging droplets from view. This hide in the black purging 
scheme is useful for any resolution. While illustrated with respect to 
color spitting, it is apparent that the black nozzles of pen 30 may also 
be purged using this spit-on-the-black scheme. 
This hide-in-the-black spitting routine may be implemented in a variety of 
different ways. One preferred method purges during a shingling printing 
routine. The concept of shingling the print swaths is well known in the 
art, and is used to alleviate several printing difficulties, including the 
prevention of banding in the finished image and saturation of the print 
media with wet ink. Typically, shingling entails partially overlapping the 
print swaths, like shingles on a house, by first printing only a portion 
of the dots required for a given swath on a first pass of carriage 38 
across the print zone 25. Then, rather than advancing the print media 78 a 
full swath width, only an incremental advance is made, such as a quarter 
or half step. The next printhead pass lays down additional dots over the 
first deposited set of dots, as well as laying down a portion of the dots 
in the next swath. For instance, in a half step or 50% shingling routine, 
for each pass of the printhead carriage 38, one swath is being completed, 
and the next swath is being started. 
In the illustrated embodiment, the purging droplets may be laid down in the 
black image areas during any pass of a shingling routine. Preferably, for 
purging the color pen 32, the purging droplets are fired to be deposited 
over already printed black dots. It is apparent that shingling may be used 
to deposit purging droplets in any of the patterns described herein. 
Indeed, the printing methods described herein are equally applicable to 
other printing mechanisms, including page width array printing mechanisms. 
It is also apparent that black and color dots may be hidden in color 
patterns as well, although not always as well as they can be hidden in 
black print patterns. 
"Hide in Plain View" 
On-Page Spitting Systems 
It is also possible to hide the purging droplets in plain view on the page 
78, in accordance with the present invention. Preferably, the purging dots 
are placed on the page 78 in an esthetically pleasing pattern. For 
example, the controller 46 or host computer 45 may make the printed images 
slightly wider by placing purging dots along the borders of image dots. 
For instance, when printing text this system of widening the image may 
have the pleasing effect of appearing as bolder printed characters. 
In another embodiment, a speed printing mode is made available, in addition 
to the conventional selections of draft, normal and presentation print 
quality modes. This speed printing mode may purge the nozzle in a specific 
location on the page 78. One pleasing pattern may be two speed bars 94 and 
96 located along each edge of the print zone 25. At the end of each print 
swath, before returning across the page, the nozzles may be purged at the 
speed bars 94, 96. The speed bars 94, 96 may be single or double lines, or 
other patterns, such as intertwined lines, diamonds, vines, etc. 
In a further embodiment, the nozzles may be purged on the page 78 in a 
pattern, similar to a watermark on bond paper, placed in a central 
location. Alternatively, the purging image may be distributed over the 
entire sheet, such as the background pictures may consumers select for 
their personal checks. This pleasing pattern may take the form of common 
objects, such as geometric shapes, like a star or diamond, or wildlife 
images, like an eagle, deer or bear, or person's initials, a logo or a 
trademark, such as the "hp" trademark pattern 98, which is owned by the 
Hewlett-Packard Company, assignee of this patent application. 
Alternatively, rather than limiting the pattern to a central location on 
page 78, a repeating pattern of purging droplets may be located over the 
entire page. A selection of patterns may be supplied with the printer 20, 
or the patterns may be defined by the user and input through the host 
computer, for instance. These patterns, whether centrally located or, more 
preferably distributed over the entire page, give the impression that a 
specially printed bond has been used to print the image 80. 
This hide in plain view purging scheme may be particularly useful for 
printhead resolutions on the order of 300 dpi, which are not as easily 
hidden from view using the hide in background or hide in the black 
schemes. However, this hide in plain view scheme is also applicable to 
finer resolutions, including 600 dpi or greater. 
Advantages 
Thus, using the black and color on-page spitting systems illustrated above, 
printer throughput is increased in most printing modes, such as normal and 
draft printing modes. It may still be preferable to limit spitting to the 
spittoon area 70 for generating presentation quality, that is very high 
quality, hard copy outputs from printer 20. This increase in throughput 
rating for normal everyday use is more attractive to some consumers, who 
base their purchasing decisions upon printer speed. Thus, the faster 
throughput is accomplished without sacrificing the quality of the printed 
image 80. 
As illustrated by several of the claims appended below, the method steps 
illustrated above may also be combined to purge an inkjet printhead used 
in an inkjet printing mechanism. For instance, the steps of firing 
selected nozzles of the printhead to deposit image ink droplets on a print 
media page to print a selected image, and of purging selected nozzles by 
firing to deposit purging ink droplets on the page may be used where the 
printed image includes at least one black printed area or a color printed 
area. Here, the purging step may comprise locating the purging ink 
droplets in at least one of the black or color printed areas of the image. 
For example, the locating step may comprise locating the purging ink 
droplets away from edges of any printed area, or the purging step may 
comprise locating color ink droplets in the black areas of the image. 
An advantage of the purging methods illustrated, is that depending upon the 
implementation, less ink is consumed. Conventional spittoon-only spitting 
systems typically purge all of the nozzles simultaneously, whether or not 
the individual nozzles need to be spit. This conventional spitting system 
wastes ink. If the duty cycle of all the nozzles is tracked, by counting 
dots fired or monitoring a printhead characteristic, and only the nozzles 
which have been used less frequently to print image 80 are purged, then 
total ink consumption for spitting is decreased. This method conserves ink 
and increases the effective utilization of each cartridge 30, 32. Whether 
the spitting occurs on the page, in the spittoon only, or in both 
locations, by only firing the nozzles which need purging, ink is 
conserved. 
Another significant advantage of the on-page spitting systems illustrated 
herein is that less ink residue is accumulated in the spittoons 72, 74. 
For instance, while some spittoon spitting may likely still be required, 
such as after priming the pens 30, 32, overall, less ink accumulates in 
the spittoon region 70. As a further advantage, the diaper 76 lasts longer 
before requiring replacement. For example, in the illustrated printer 20 
using the on-page spitting system described above, one-third less ink is 
accumulated in the spittoons 72, 74 than when using a spittoon-only 
spitting routine.