Method for servicing an inkjet printhead

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of servicing an inkjet printhead without removing the printhead from a carriage of a printer and in particular to methods of servicing a printhead by the utilisation of a controlled puddle of ink generated on the outside of the nozzle plate of the printhead.

2. Discussion of the Background Art

The present invention relates to the art of inkjet printing mechanisms whether of the thermal or piezo variety which may be included in a variety of different products including copiers and facsimile machines in addition to standalone printers either desktop mounted, portable or freestanding. Herein a freestanding printer will be used to illustrate the present invention. Printers of this type have a printhead carriage which is mounted for reciprocal movement on the printer in a direction orthogonal to the direction of movement of the paper or other medium on which printing is to take place through the printer. The printer carriage of a color printer typically has two or more, usually four, thermal ink jet printheads mounted thereon which may be removable. Each of the printheads contains or is attached to a remote supply of ink which is fed via ink channels within the printhead to an ink ejection mechanism generally in the lower part of the printhead and ejected as drops through a nozzle plate having numerous small orifices or nozzles therethrough. For thermal (as opposed to piezo-electric) inkjet printheads ink channels or conduits lead to firing chambers each associated with heater elements, such as resistors, which are energized to heat ink within the firing chambers. Upon heating, an ink drop is ejected from a nozzle associated with the energized resistor.

To service, that is clean, maintain, protect or recover the correct operation of the printhead, typically a “service station” mechanism is mounted within the printer so the printhead can be moved over to the station for servicing. For storage, or during non-printing periods, the service stations usually include a capping system which hermetically seals the printhead nozzles from contaminants and prevents drying. Some caps are also designed to facilitate priming, such as by being connected to a pumping unit or other mechanism 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,” with the waste ink being collected in a “spittoon” reservoir portion of the service station. After spitting, uncapping, priming 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 has collected on the face of the printhead.

A factor in the servicing of printheads is that, to improve the clarity and contrast of the printed image, recent research has focused on improving the ink itself. To provide quicker, 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, which results in a higher optical density for the new inks. Both types of ink dry quickly, which allows inkjet printing mechanisms to form high quality images on readily available and economical plain paper, as well as on recently developed specialty coated papers, transparencies, fabric and other media. Such new faster drying ink formulations have placed additional demands on the servicing of printheads.

A further factor in the servicing of inkjet printheads is that the lifetimes required of the printheads is increasing, particularly for printheads that are utilised in combination with large volume ink reservoirs which are remote from the printhead (so called “off-axis” systems) and which may be replaced without replacing the printhead. Thus, increased levels of, or more effective servicing of printheads are required and furthermore such servicing must cause very little wear or damage to the printhead if it is to have a long lifetime.

A particular problem, that is exacerbated by the use of inkjet printheads for longer periods of time is that the printheads are very sensitive to contamination by either small air or gas bubbles generated during use of the printhead or by solid particles either left within the printhead from manufacturing processes or entering the printhead together with the ink. While this problem has been attempted to be resolved by for example the use of a filter within the printhead as described in EP 0875385, such a filter only addresses the attempted prevention of these problems and does not provide a solution should they occur.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provided a method for servicing an inkjet printhead, the printhead having a body comprising an ink chamber in fluid communication with a plurality of nozzles in a nozzle plate and firing means associated with each nozzle for ejecting ink drops from said nozzles during printing operations, mounted within a carriage of a printer, comprising the steps of generating a controlled predetermined pressure differential across the nozzle plate of the printhead to cause the formation of a controlled puddle of ink on the outside of the nozzle plate, and causing the printer to actuate the firing means so that ink is ejected from at least some of said nozzles into said puddle of ink. The present applicants have discovered that the firing of drops into a puddle of ink on the nozzle plate of a printhead causes turbulence within the ink of the puddle which is effective in recovering the correct operation of defective nozzles.

Servicing in this manner has been found to be more effective than servicing by means of conventional, sequential spitting and priming operations. Furthermore this servicing or recovery technique has been found to give little wear to the printhead.

Advantageously, the firing means are actuated repeatedly to eject ink into said puddle of ink, and preferably the repetition rate of actuation for each nozzle is lower than the repetition rate utilised during normal printing operations. This has been found to further enhance the recovery of malfunctioning nozzle which may be due to the large drop volumes ejected or to the timescale for response of the malfunctioning nozzles to this firing.

In a preferred embodiment the servicing method comprising the further step of, prior to actuating the firing means, determining which nozzles of the printhead are able to correctly eject drops of ink during normal printing operations and then, during said actuation step, only firing the nozzles which are correctly operating. It has been found that firing neighbouring nozzles can alleviated a problem with a malfunctioning nozzle and that firing of a malfunctioning nozzle in some circumstances, for example when an ink conduit is block by a particle, can worsen a problem with the nozzle.

Alternatively, subsequent to determining which nozzles of the printhead are able to correctly eject drops of ink during normal printing operations, only the malfunctioning nozzles are fired. For some causes of malfunction, for example nozzles clogged by plugs of dried or drying ink, this is found to be effective.

Preferably, subsequent to actuation of the firing means the majority of the ink forming said ink puddle is drawn back into the printhead through the nozzles. In addition to reducing the amount of waste ink during servicing, this has been found to be a very effective recovery technique particularly for problems caused by particulate matter. It is believed that the flow out of the printhead due to creation of the ink puddle and firing of the nozzles in combination with the flow back into the printhead of the ink puddle serves to move internal contaminants whether bubbles or particulate matter.

Although the ink puddle may be generated by a controlled decrease in the pressure external to the nozzle plate of the printhead, preferably the puddle is generated by a controlled increase in the internal pressure of the ink chamber of the printhead. Advantageously, the increased internal pressure causes the volume of the ink fired through each nozzle into the ink puddle to be higher than the volume of ink drops fired under normal printing conditions.

In a preferred embodiment, the printhead comprises a variable volume air chamber coupled to the ink chamber and having a vent which is in gaseous communication with ambient atmosphere. The generating step then comprises interfacing a source of gas to the vent of the air chamber of the printhead, and delivering a predetermined controlled volume of gas from the gas source at a pressure above ambient atmospheric pressure to the air chamber so that the air chamber expands within the printhead body causing an increase in the pressure within the ink chamber and thus a controlled flow of ink through the nozzles of the printhead to generate the controlled puddle of ink on the outside of the nozzle plate. This method of generating the ink puddle has been found to be particularly controllable.

According to a second aspect of the present invention, there is provided a method for servicing an inkjet printhead, the printhead having a body comprising an ink chamber in fluid communication with a plurality of nozzles in a nozzle plate and firing means associated with each nozzle for ejecting ink drops from said nozzles during printing operations, mounted within a carriage of a printer, the method comprising the steps of generating a controlled predetermined pressure differential across the nozzle plate of the printhead to cause the formation of a controlled puddle of ink on the outside of the nozzle plate, maintaining said ink puddle on the nozzle plate of the printhead for a predetermined period of time, and reversing said pressure differential so as to draw the majority of the ink forming said ink puddle back into the printhead through the nozzles. The present applicants have discovered that the controlled generation of an ink puddle on the nozzle plate of an inkjet printhead and its subsequent reabsorption into the printhead through the nozzles (even without the nozzles being fired) is effective in servicing the printhead.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages and objects of the present invention will be appreciated from specific embodiments of the present invention which will now be described by way of example only and with reference to the following drawings in which:

FIG. 1

is a perspective view of a large format printer in which the present invention is useful.

FIG. 2

is a top plan view of the printer with its cover removed to show the automatic priming pump and service station at the right end of the path of travel of the printhead carriage.

FIG. 3

is a front elevation view of the service station and priming pump.

FIG. 4

is a right side elevation view of the service station and priming pump.

FIG. 5

is a cross-sectional elevation view taken at line

5

—

5

in

FIG. 3

, of the mechanism for moving the pump to selected positions to prime selected printheads.

FIG. 6

is a cross-sectional elevation view through the pump.

FIG. 7

is a right side elevation view of the printhead carriage with cover in the closed position.

FIG. 8

is a front elevation view of the carriage showing the printhead cover in the raised position.

FIG. 9

is a top plan view of the carriage with printheads installed in two stalls and the cover in raised position.

FIG. 10

is a plan view of the carriage cover partly broken away showing air passageways therein.

FIG. 11

is a graph plotting air pressure profiles delivered by the pump.

FIG. 12

is a graph of a velocity servo soft bump algorithm implementation.

FIG. 13

is a graph of a velocity servo hard bump algorithm implementation.

FIG. 14

is a perspective view in partial cross-section of a printhead showing a ink and pressure regulation mechanism.

FIG. 15

is a perspective view of the regulation mechanism of

FIG. 14

shown without the air bag.

FIG. 16

is a perspective view showing a first side of a regulator lever of the regulation mechanism of FIG.

14

.

FIG. 17

is a perspective view showing a second side of a regulator lever of the regulation mechanism of FIG.

14

.

FIG. 18

is a cross-section through the printhead body.

FIG. 19

is a perspective view of an accumulator lever of the regulation mechanism of FIG.

14

.

FIG. 20

is a perspective view from below of the crown of the printhead.

FIG. 21

is a schematic cross-sectional view of the printhead showing the regulator mechanism in a first fully closed position.

FIG. 22

is a schematic cross-sectional view of the printhead showing the regulator mechanism in a second partially open position.

FIG. 23

is a schematic cross-sectional view of the printhead showing the regulator mechanism in a third fully open position.

FIG. 24

is a schematic drawing of the nozzle plate of a printhead from below showing a puddle of ink covering both columns of nozzles.

FIG. 25

is a side elevation of the schematic drawing of the nozzle plate of

FIG. 24

showing schematically the firing of ink drops into a puddle of ink.

FIG. 26

is a graph of the volume of waste ink when servicing a printhead for the case of spitting only, spitting while priming with no flowback of ink and spitting while priming with flowback of ink into the printhead.

FIG. 27

is a graph of the volume of ink purged onto the nozzle plate of a printhead during priming as a function of the volume of air injected into the printhead during priming for black and colored ink printheads.

FIG. 28

is a graph of the volume of ink purged onto the nozzle plate of a printhead during priming as a function of the duration of the priming operation for black and colored ink printheads.

FIG. 29

is a graph of the volume of ink purged onto the nozzle plate of a printhead during priming as a function of the volume of air warehoused within the ink chamber of the printhead for black and cyan ink printheads.

FIG. 30

is a graph of the internal pressure of the ink chamber of a printhead measured close to the nozzle plate during a priming operation carried out with different volumes of air as a function of time for different pressures of ink supplied to the printhead.

FIG. 31

is cross sectional enlarged view through the nozzle area of the printhead of FIG.

14

.

DETAILED DESCRIPTION OF THE INVENTION

In describing preferred embodiments of the present invention details of the preferred mechanism for applying a positive air pressure to a printhead in order to prime the printhead will first be described. Subsequently, the structure of a preferred printhead for use with embodiments of the invention will be described and finally the servicing and priming of a printhead according to preferred embodiments will be described.

FIG. 1

shows a large format printer

10

of the type which includes a transversely movable printhead carriage enclosed by a cover

12

which extends over a generally horizontally extending platen

14

over which printed media is discharged into a catcher basket. At the left side of the platen are four removable ink reservoirs

20

,

22

,

24

,

26

which, through a removable flexible tube arrangement to be described, supply ink to four inkjet printheads mounted on the moveable carriage.

In the plan view of

FIG. 2

in which the carriage cover

12

has been removed, it is seen that the printhead carriage

30

is mounted on a pair of transversely extending slider rods or guides

32

,

34

which in turn are affixed to the frame of the printer. Also affixed to the frame of the printer are a pair of tube guide support bridges

40

,

42

from which front and rear tube guides

44

,

46

are suspended. The printhead carriage

30

has a pivotal printhead hold down cover

36

fastened by a latch

38

at the front side of the printer which securely holds four inkjet printheads, two of which is shown in

FIG. 9

in place in stalls C, M, Y, K on the carriage. The front tube guide

44

is angled near the left bridge support

40

to provide clearance for opening the printhead cover

36

when the carriage is slid to a position proximate the left side of the platen

14

so that the printhead hold down cover

36

can be easily opened for changing the printheads.

A flexible ink delivery tube system conveys ink from the four separate ink reservoirs

20

,

22

,

24

,

26

at the left side of the printer through four flexible ink tubes

50

,

52

,

54

,

56

which extend from the ink reservoirs through the rear and front tube guides

44

,

46

to convey ink to printheads on the carriage

30

. The ink tube system may be a replaceable system.

At the right side of the printer is a printhead service station

48

at which the printhead carriage

30

may be parked for cleaning and priming the printheads. The printhead service station

48

is comprised of a plastic frame mounted on the printer adjacent the right end of the transversely extending path of travel of the is printhead carriage

30

. The printhead carriage

30

(

FIGS. 8 and 9

) includes four stalls C, M, Y, K which respectively receive four separate printheads containing colored ink such as cyan, magenta, yellow and black. The service station

48

also includes four separate servicing stalls C, M, Y, K which may be provided on a drawer which is moveable forwardly and rearwardly of the printer. The servicing stalls each include a spittoon to capture any ink that may be discharged by the printheads during servicing. The moveable drawer construction of the servicing station forms no part of the present invention.

A printhead servicing pump

50

is mounted on the upper end of a pump positioning arm

80

. A gear enclosure frame

60

is affixed to the right sidewall of the frame of the service station

48

and is spaced therefrom to provide a pocket containing a speed reduction gear mechanism which positions the arm

80

and thus the pump

50

with respect to the printhead carriage

30

. The positioning arm

80

is mounted for movement on a pivot axis

82

extending between the right sidewall of the service station frame and the gear enclosure frame

60

. An arm positioning electric step motor

90

rotates a drive gear

92

thereon which is engaged with the teeth of a large driven gear

94

connected on a common shaft to a small driven gear

96

having teeth which mesh with an arcuate arm positioning gear

98

formed on the pump positioning arm

80

to move the arm through an angle of slightly less than 90°. Movement of the arm

80

positions the pump at various locations along an arc centered on the pivot axis

82

of the arm to align a pump outlet

52

with the inlet end of one of four air conduits

100

,

102

,

104

,

106

arcuately positioned on the side of a pivotally mounted printhead holddown cover

36

on the printhead carriage

30

.

The four air conduits each

100

,

102

,

104

,

106

are each sized to have a substantially equal volume and extend from the inlet ends at the side of the hold down cover

36

internally of the cover and terminate in downwardly directed (when the cover is closed) fluid outlets

110

,

112

,

114

,

116

on the underside of the printhead holddown cover. The air outlets each have a compliant seal

111

,

113

,

115

,

117

therearound which mates with corresponding air inlet ports on the top surfaces of the four printheads when positioned in their respective stalls in the printhead carriage. Also shown on the underside of the printhead holddown cover

36

are spring loaded printhead positioners

120

,

122

,

124

,

126

. It will be seen that the printhead hoiddown cover is pivotally connected to the carriage and fastened in its closed or printhead holddown position by a finger latch

38

and retainer

39

.

The air pump

50

, which may be removably affixed to the upper end of the positioning arm

80

or permanently attached thereto as desired, comprises an open ended cylinder

51

in which an elongated piston

52

having a pair of spaced piston alignment discs

53

,

54

or collars slideably engageable with the inner wall of the cylinder is received. The piston

52

is biased outwardly of the cylinder by a compression spring

55

which is seated at one end against a spring seat

56

in the pump cylinder and which is seated at its other end against a collar

57

surrounding the inner end of a hollow piston stem

58

having an elongated axial passageway

59

therethrough. A compliant seal

61

is seated against the inner piston alignment disc

54

and slideably engages the inner wall of the cylinder to provide an air seal therebetween. The walls of the seal

61

engage the cylinder

51

at an angle so that the seal

61

unidirectionally holds a positive pressure within the air chamber

68

when the piston

52

moves to the right, but does not hold a vacuum when piston

52

moves to the left. The cylinder is closed by a cover

63

attached to the outer wall of the cylinder by one or more fasteners

65

, the construction of which is not relevant to the present invention. Alternatively, the cover may be threadedly affixed to the cylinder. The piston

52

has an enlarged collar

67

at its outer end on which a compliant gasket

69

is affixed for engaging the side wall of the printhead holddown cover

36

and providing an air seal between the outlet

52

of the piston and the side wall of the printhead holddown cover

36

during positioning of the carriage against the piston at the service station.

Servicing of the printheads on the printhead carriage is accomplished in part by positioning the pump

50

for alignment with the air passageway

102

,

104

,

106

,

108

in the printhead holddown cover which conveys air to the printhead to be serviced. Movement of the carriage

30

into the service station

48

with the pump so positioned causes the carriage to engage the compliant gasket

69

at the outlet of the pump with continued movement of the carriage moving the pump piston

52

to the right into the cylinder to discharge air from the air chamber

68

in the cylinder through the central passageway

59

in the piston to thus provide a source of positive air pressure to the printhead which is utilised to prime the printhead as will be described in greater detail below. The nozzle plates of the printheads C, M, Y, K may thus be primed by means of a positive air pressure supplied by the pump

50

. The air pressure supplied by the pump need not contact the ink in the printheads and in fact should not do so to avoid introducing air which must be warehoused in the printhead body. Accordingly, a printhead configuration in which ink in the printhead is contained in a chamber having a volume which can be reduced by application of air pressure to another chamber in the printhead is preferred and will be described in greater detail below. Travel of the carriage away from the pump

50

as it leaves the service station

48

extracts the air which has been previously forced into the printhead cover. If some of the air introduced under pressure to the printhead has escaped during the process, the pump may apply an undesired amount of vacuum to the printhead. The pump design allows the pressure to be clipped at a small negative pressure of approximately −5.0 inches of water to avoid creating a vacuum before damage is done to the printhead. The seal between the pump outlet and the passageway in the printhead holddown cover is broken after the pump piston has traveled under the bias of the spring

55

to the end of its stroke. Thus any backpressure within the printhead necessary for its correct functioning should remain unaffected by the priming operation.

The pump

50

is arcuately postionable as best seen in

FIG. 5

anywhere between a rest position

0

and a reference position R which are defined by stops

84

,

86

on the gear housing

52

which are engaged by the sides of the positioning arm

80

. Positions of the arm for delivery of air by the pump to the cyan, magenta, yellow and black ink printhead conduits

100

,

102

,

104

,

106

on the printhead carriage holddown cover

36

are shown in

FIG. 5

at positions preferably spaced by approximately 6° degrees from each other.

The stepper motor

90

preferably steps the gear

92

at 3.75°/half-step and the gear train preferably provides a 30:1 reduction between the stepper motor

90

and the gear

98

on the pump positioning arm

80

.

The hard stops

84

,

86

which define the limits of travel of the pump positioning arm are preferably placed at 84° from one another. For each printhead servicing cycle, the pump

50

is moved from the parking or rest position

0

in which the arm

80

engages the parking hard stop

84

to the reference position R in which the positioning arm engages the reference stop

86

. The reference stop

86

is positioned closer than the parking or rest stop

84

to the functional angular positions K, Y, M, C in which the pump

50

engages the cyan, magenta, yellow and black printhead conduits

100

,

102

,

104

,

106

on the carriage holddown cover. After movement of the pump positioning arm from the rest position

0

to the reference position R, the arm is then moved in a reverse (clockwise as seen in

FIG. 3

) direction to the preliminary position P. The stepper motor

90

then moves the pump positioning arm

80

in the original direction (counterclockwise in

FIG. 3

) to position the pump

50

in alignment with the desired functional location C, M, Y or K for connection to the related conduit

100

,

102

,

104

,

106

. This movement is performed to assure that, due to backlash, the same gear tooth face set that is used to move the pump positioning arm against the reference hard stop

86

is used to complete the accurate positioning of the pump

50

in the selected functional position.

The hard stops

84

,

86

are integrally formed with the pump positioner housing

52

. This design sacrifices a small amount of positional accuracy in the nominal position of the pump

50

but decouples the hard stop function from the vertical adjustment of the positioner housing

52

. An over-stepping algorithm is used to ensure that the pump positioning arm

80

has contacted the reference hard stop

86

. The over-stepping algorithm includes margin for both backlash and possible lost steps.

All functional angles are placed at even multiples of the nominal angular resolution. This is done to ensure that there are no pump positioning errors because an odd step total for a half-stepping algorithm is, by definition, less stable than an even step total.

The inlets on the printhead holddown cover to the conduits

100

,

102

,

104

,

106

are placed at angles of 6° from one another and are centered around a vertical line which extends through the axis

82

of rotation of the pump positioning arm

80

and are located at the same radius as the outlet of the pump

50

. The axis

82

of rotation of the positioning arm

80

is placed at a maximum reasonably feasible radius from the inlets to the conduits

100

,

102

,

104

,

106

to minimize the vertical distance (

FIG. 4

) between the inlets to facilitate the design of the holddown cover

36

.

The radial margin around each air inlet is preferably about 2.5 mm to the inner diameter of the pump discharge gasket and 3.5 mm to the outside diameter. In the case that the vertical and horizontal alignment error of the axis of rotation

82

of the positioning arm

80

is 0, this translates to a stepping error of about 16 half-steps before the interface fails.

The stroke length or axial displacement of the pump

50

may be easily selected or adjusted to discharge a controlled volume of air to each of the printheads on the carriage. Design control of the length and cross-sectional area of each of the air passageways

100

,

102

,

104

,

106

in the printhead holddown cover

38

to insure that the total volume of each passageway is substantially the same insures that, for a given pump stroke, the pump delivers the same volume and pressure of air to each printhead regardless of which printhead is being serviced. Each printhead may be primed utilising different priming parameters such as the pump stroke and duration as will be described in greater detail below, and these priming parameters for each printhead are stored in a software controller

300

of the printer for controlling the priming operation. The controller

300

is also connected to an environmental sensor

302

which measures the current ambient temperature and humidity surrounding the printer. These measurements may also be utilised by the controller

300

in determining the appropriate priming parameters for a particular printhead. The printer is able to identify specific printheads which are mounted in stalls within the printer carriage

30

in any manner known within the art, for example by reading a memory chip located on the printhead.

The pressure profile delivered by the pump is shown in FIG.

11

and is dependent upon the volume of the air passageways

102

,

104

,

106

,

108

in the printhead holddown cover, the resting volume of the air chamber

69

in the pump itself and the rest position of the printhead carriage prior to priming. The curves shown in

FIG. 11

are based upon an air passageway volume of 1.8 cc and a resting pump chamber volume of 3.2 cc. Three curves are shown. The 3.5 mm COMP curve shows the pressure profile at 3.5 mm axial displacement of the pump while the 7.0 mm COMP curve shows the pressure profile at 7.0 mm axial displacement of the pump. The third curve demonstrates the curve form when an air leak in the system is present. In this case, the priming pressure delivered to the printheads is slightly diminished but is still adequate to perform the priming function. The design of the pump

50

guarantees that the negative pressure caused when the pump displaced air is extracted (by movement of the printhead away from the pump) clips at a pressure of approximately −5.0 inches of water.

The precise location on the printer of the position of the compliant gasket at the pump outlet is determined by the use of a novel velocity servo bumping algorithm. The algorithm has general application to any two relatively moveable components but is more conveniently described in the context of an inkjet printer with reference to movement of the carriage

30

(a first component) with respect to the pump outlet

52

(a second component) to bump the components together preferably through a number of bumping cycles during which the current drawn by an electric motor used to move the carriage to cause the relative movement between the carriage and pump outlet is measured to establish a pulse width modification (PWM) threshold which is exceeded during the bumping. The deflection of one of the components (the pump outlet) has been characterized when the load power exceeds the threshold value.

Most bumping strategies require that the two contacting components have a minimum rigidity to function correctly. They typically assume that once the parts contact there will be no deformation or at least that the resulting deformation will be less than the precision required by the system. These algorithms, therefore, cannot be applied to systems having flexible components such as the compliant gasket

69

at the pump outlet

52

.

FIG. 13

shows a plot of carriage drive motor load (PWM) against interruptions in milliseconds for printhead carriage measurements for a hard bump environment.

To recognize the contact of a flexible component, the algorithm must react to single impulses in the PWM profile. This is to say that the servo algorithm must respond if the threshold is exceeded for a single processor interruption ({fraction (1/1000)} sec.). Also, the servo parameters must have a very undamped response to velocity error. The algorithm depends on the PWM instability at the point of contact to recognize the flexible component. Because the impact can be somewhat unstable and because there is additional noise in the system due to other sources, several bumping samples must be taken to insure data consistency. This data must pass the following sanity checks to be considered valid:

1. The average reading must not exceed a maximum variation from the nominal value (taken as 4σ of the distribution across many previous printers);

2. The 3σ value of the measurement distribution must not exceed a critical value for mechanism function (reading Cp); and

3. No single reading can vary from each machine's own distribution average by more than a critical value (erroneous data point).

Because of the delay of the servo and the compressibility of the flexible components, an offset should be calculated when determining the bump position. As seen in the PWM evolution shown in

FIG. 12

where the horizontal axis indicates interruptions in milliseconds, time B indicates when the PWM threshold (−28 as shown) was exceeded and time A indicates the point at which the true first contact occurred. The positional offset due to these effects has been characterized and shown to be repeatable. This occurs particularly in the case in which two flexible components are assembled in series (the gasket and the spring) with one of the two having a much higher stiffness and particularly preload.

FIG. 12

also demonstrates the transient noise which occurs due to both inertial and friction/stiction effects while accelerating the carriage and approaching the pump. To reduce the risk that the PWM threshold will be exceeded during this phase, carriage movement is started sufficiently far from the nominal position to ensure that discarding the first half of the PWM profile will both eliminate this noise and ensure the flexible component (the pump) is not touched during the initial movement.

The carriage is repeatedly positioned to deflect the pump outlet and during the bumping procedure. The currently preferred algorithm includes the following:

1. Number of bumping cycles: 12.

2. Offset due to connect gasket compression: 6 encoder units (0.25 mm).

3. Maximum variation of average reading from nominal: 24 encoder units (1.0 mm).

4. Maximum 3σ value: 12 encoder units.

5. Maximum single point deviation from average: 6 encoder units.

It has been found that the position of the pump outlet can vary by up to 1.0 mm during construction of a printer. Use of the above positioning algorithm reduces the error between actual pump outlet position and optimum pump outlet position to a maximum of 0.25 of this amount.

A preferred design of printhead for use with embodiments of the present invention and its operation during normal printing as opposed to during priming will now be described.

Referring to

FIG. 14

, reference numeral

200

generally indicates the printhead that includes a body

201

and a crown

202

that forms a cap to the body and defines an ink chamber

232

with the printhead. Located at a remote end of the body is the tab head assembly

203

or THA. The THA includes a flex circuit

204

and a silicon die

205

that forms the nozzle plate.

FIG. 31

is a cross-section through the THA showing the flow path

310

of ink from the ink chamber

232

of the printhead to the nozzle firing chambers

316

via narrow ink conduits

314

. On the underside of substrate

318

are resistors

320

associated with each nozzle

312

. In operation resistors

320

are energised to vaporise a small quantity of ink adjacent the resistor which causes the ink within the firing chambers

316

to be ejected through nozzles

312

as drops of ink

322

. This ink ejection mechanism is of conventional construction. Also located within the pen body

201

is a regulator lever

206

, an accumulator lever

207

, and a flexible bag

208

. In

FIG. 14

the bag is illustrated fully inflated and for clarity is not shown in FIG.

15

. The regulator lever

206

and the accumulator lever

207

are urged together by a spring

235

,

235

′ illustrated in FIG.

15

. In opposition to the spring the bag spreads the two levers apart as it inflates outward. The bag is staked to a fitment

209

that is press-fit into the crown

202

. The fitment contains a vent

210

to ambient pressure in the shape of a helical, labyrinth path. The vent connects and is in gaseous communication with the inside of the bag so that the bag is maintained at a reference pressure during normal printing operations. The helical path limits the diffusion of water out of the bag and also serves to dampen the response rate of the levers

206

,

207

to changes in the pressure differential between the ink chamber

232

and the ambient pressure.

The regulator lever

206

is illustrated in detail in

FIGS. 16 and 17

. Reference numeral

211

generally indicates the location of the area where the bag

208

directly bears against the lever. The lever

206

rotates about two opposed axles

212

that form the axis of rotation of the lever. The rotation of the lever is stopped when the lever engages the printhead body

201

. The axles are located at the ends of cantilevers

213

formed by deep slots so that the cantilevers and the axles can be spread apart during manufacture and snapped onto place on the mounting arms

214

of the crown

202

as illustrated in FIG.

18

. Perpendicular to the plane of the regulator lever

206

is a valve seat

215

and a valve seat holder

216

. The valve seat is pressed into place on the holder and is fabricated from a resilient material. In response to expansion and contraction of the bag

208

, the regulator lever

206

rotates about the axles

212

,

212

′ and causes the valve seat to open and shut against a mating surface on the crown

202

as described below. This rotational motion controls the flow of ink into the printhead body. There is an optimization between maximizing the force of the valve seat and obtaining sufficient motion of the lever. In the embodiment actually constructed the lever ratio of the distance between the centroid of the lever, generally at point

211

, and the axles

212

and the distance between the centre of the valve seat and the axles

212

is between two to one and five to one with four to one being preferred. The regulator also includes a spring boss

217

and engages the spring

235

, FIG.

15

. The spring boss is protected during manufacture by two shoulders

223

which are not illustrated in FIG.

15

.

The accumulator lever

207

is illustrated in FIG.

19

and includes an actuation area

218

where the bag

208

directly bears against the lever. The lever rotates about two opposed axles

219

,

219

′ that form an axis of rotation of the accumulator lever. The axles are remotely located on cantilevers

220

so that the axles and the cantilevers can be spread apart during manufacture and snapped into place on the mounting arms

221

,

221

′ of the crown

202

as shown in FIG.

18

. The accumulator lever also includes a spring boss

222

that engages the other end of spring

235

.

FIG. 15

Like the spring boss

217

on the regulator, the boss

222

on the accumulator is protected during manufacture by the shoulders

224

. These shoulders are not illustrated in FIG.

15

.

Referring to

FIG. 15

reference numerals

235

generally indicates a helical extension spring that urges the two levers

206

,

207

together. The spring is preloaded and engages the bosses

217

,

222

with a coil loop at each distal end. Each loop is a parallel cross-over, fully closed centred loop. This spring is designed to have the least amount of variation in its force constant over its full range of travel so that the back pressure can be regulated as closely as possible.

FIG. 20

illustrates the bottom side of the crown

202

which includes a valve face

227

and the orifice

228

through which ink enters the ink chamber

232

. The valve face mates with the valve seat

215

,

FIG. 16

on the regulator lever

206

. Ink flows through the fluid interconnect

229

,

FIG. 18

, the ink channel

230

and the orifice

228

. At orifice

228

the ink flow into the ink chamber

232

is controlled by the regulator lever

206

. The bag

208

is attached to a boss

231

which provides a gaseous communication path between the interior of the bag and ambient pressure via the vent

210

of the printhead. The axles

212

,

212

′

FIG. 17

on the regulator lever

206

are snapped into the journals

214

,

214

′ as permitted by the cantilevered construction described above. In like manner the axles

219

,

219

′ on the accumulator lever

207

are received in the journals

214

,

214

′, FIG.

20

. Also located bottom side of the crown is the surface

226

that engages the stop

225

,

FIG. 19

on the accumulator lever

207

. The stop

225

and the surface

226

prevent the accumulator lever from interfering with the regulator lever

206

.

During normal printing the flexible bag

208

, shown in

FIG. 14

, expands and contracts as a function of the differential pressure between the back pressure in the ink chamber

232

and ambient pressure communicated through the vent

210

. The bag is shown inflated in FIG.

14

. The bag is designed to push against the two levers

206

,

207

with maximum contact area through the entire range of travel of the levers.

The accumulator lever

207

and the bag

208

under normal printing conditions operate together to compensate for changes in the ambient atmospheric pressure and thus to maintain a substantially constant negative i.e. below atmospheric pressure within the ink chamber

232

(known as the back pressure). Also the accumulator and bag are able to some extent to accommodate changes in the volume of any air that may be entrapped in the printhead (known as warehoused air).

Although most of the accommodation is provided by the movement of the accumulator lever

207

and the bag

208

, there is additional accommodation provided by the regulator lever

206

in cooperation with the resilient valve seat

215

, FIG.

16

. The valve seat acts as a spring and allows some movement of the regulator lever

206

in either direction while the valve is still shut (and thus preventing entry of ink into the printhead). In other words, as the back pressure in the ink chamber

232

decreases i.e. becomes less negative, the bag

208

exerts less force on the levers and the spring

235

urges the levers together. The motion of the regulator lever compresses the valve seat and the regulator lever shuts a little further. Alternatively, as the back pressure increases (becomes more negative) the bag

208

exerts more force on the levers and pushes them apart, however, due to the compliance of the value seat the regulator lever

206

is able to rotate a little before the valve opens.

It should be appreciated that the boss

222

on the accumulator lever

207

is closer to the axis of rotation of the accumulator lever than the boss

217

.

FIGS. 16 and 17

, on the regulator lever is to its axis of rotation. This difference in distance causes the accumulator lever to actuate before the regulator lever moves.

The accumulator lever

207

rotates about the axles

219

until a stop

225

on the lever engages a surface

226

within the crown

202

as illustrated in

FIGS. 20 and 19

. The stop prevents the lever from moving too close and interfering with the regulator lever

206

when the back pressure in the ink chamber

232

drops. The accumulator lever rotates in the other direction until coming into contact with the printhead body

201

as illustrated in

FIGS. 22 and 23

.

When mounted within a stall of the carriage

30

of the printer as shown in

FIG. 1

, the vent

210

of the printhead is connected to ambient atmospheric pressure via one of the air conduits

100

,

102

,

104

or

106

in the printhead holddown cover

36

. The fluid interconnect

229

of the printhead is connected by means of one of the flexible supply tubes

50

,

52

,

54

or

56

to one of four removable ink reservoirs

20

,

22

,

24

,

26

located on the left hand side of the printer as seen in FIG.

1

. Each ink reservoir is individually pressurised under control of the printer to deliver ink to an associated printhead.

In normal printing operations the accumulator and regulator levers

207

,

206

move within the printhead body

201

as shown in

FIGS. 21

,

22

and

23

dependent on the ambient atmospheric pressure and the speed of printing and thus of supply of ink to the printhead. In

FIG. 21

the two levers are shown fully together, the flexible bag

208

is limp and empty of air—this may be due to a large drop in the ambient atmospheric pressure for example or is the condition of the printhead prior to initial filling with ink. If the atmospheric pressure increases, or the pressure within the ink chamber

232

decreases, for example due to ink being ejected from the printhead during printing, the flexible bag

208

fills with air drawn through the air conduit in the carriage cover via the vent

210

of the printhead. Expansion of the bag

208

causes rotation of the accumulator lever

207

, against the operation of the spring

235

thus maintaining a substantially constant pressure differential (set essentially by the choice of spring

235

) between ambient pressure and the pressure within the ink chamber

232

so as to promote effective operation of the printhead. The accumulator lever

207

is able to rotate until it comes into contact with the inner wall

236

of the printhead body

201

as shown in FIG.

22

and it should be noted that it is only at this point, due to the differences in lever arm distances, that the regulator lever

206

begins to rotate. The regulator lever

206

is able to rotate some small amount prior to the opening of the ink orifice

228

, due to the resilience of the valve seat

215

, whereupon ink flows into the ink chamber

232

from the remote in reservoir under pressure. The regulator lever

206

is able to rotate until it meets the opposite inner wall

236

of the printhead body

201

and is shown in this fully open position in FIG.

23

. Once the pressure differential between the ink chamber

232

and atmosphere has been reestablished the regulator lever

206

rotates back to close the ink valve

227

.

Occasionally normal printing operation is suspended in order for one or more printheads to be serviced by the printer for example by performing spitting, priming and/or wiping operations. This many be initiated by the printer at regular intervals, only when a problem with a printhead is detected by the printer or as a result of a user request following detection by the user of a printing problem or by any combination of these circumstances.

In order to prime a printhead mounted within the printer carriage by the use of a carriage activated air pump

50

, the alignment processes described above are first carried out and the piston

52

of the pump is aligned to the air conduit connected to the vent

210

of the printhead. Then a precise movement of the carriage

30

is implemented by the printer to cause the pump

50

to deliver a predetermined volume of air to the flexible bag

208

within the printhead under pressure. This causes the bag to expand within the printhead body

201

and thus to increase the pressure within the ink chamber

232

causing a priming flow of ink into the nozzles

205

. When the carriage

30

is moved away from the pump

50

the pressure within the bag

208

returns to atmospheric and the bag in cooperation with the accumulator and regulator levers

207

,

206

acts to reestablish the desired pressure differential between the ink chamber and ambient pressure as described above.

The priming operation may be performed with a volume of air delivery to the bag

208

of the printhead which is sufficient to cause movement of the accumulator lever

207

of the printhead but not cause any or insufficient movement of the regulator lever

206

so that the ink valve orifice

228

is not opened and the ink chamber

232

is not exposed to the pressure of the ink supply from the reservoirs

20

,

22

,

24

,

26

. However, it has been found for particular printhead designs and for particular ink types that it is advantageous to deliver a further controlled volume of air during priming so that the bag

208

expands to further increase the pressure within the ink chamber

232

and thus causes the regulator lever

206

to be rotated. It is important in these cases to control the supply pressure of ink from the remote reservoirs so as to prevent a large flow of ink into the printhead. Thus preferably a first step in the priming process is to set the pressure of the ink supply from the remote reservoirs to a level at which an insubstantial amount of ink will flow either into or out from the printhead once the ink valve

228

,

227

is opened. This ensures that any flow of ink to or through the nozzles of the printhead is controlled by the air priming system which can be precisely controlled by the printer since it is actuated by carriage movements and not by the ink supply pressure. In the present embodiment the ink supply pressure is first reduced to zero from the pressure used during normal printing and is then raised to the lower pressure used for priming.

It has also been found that a precisely controlled purge of ink through the nozzles of the printhead to form a puddle of ink on the outside of the nozzle plate which is then drawn back into the printhead is effective in resolving a number of problems with printheads which are difficult to resolve without such flowback of ink. For example, the following problems may be alleviated by this technique:

1) dried ink tends to build up on the nozzles plate of printheads after extended use and may interfere with the correct ejection of ink drops for example causing misdirection of the ink drops. The ink itself is the optimal solvent for dried ink and the formation and maintenance of a puddle of ink around such accumulated dried ink allows the dried ink to dissolve or be removed from the nozzle plate.

2) air bubbles may become trapped within the nozzles or the narrow ink conduits leading to the nozzles. The outward flow and then subsequent backward flow of ink through the nozzles tends to break such bubbles free so that they are able to move either out of the printhead or to an innocuous location inside the ink chamber of the printhead as shown in

FIG. 22

, as so called warehoused air,

238

.

3) particles which may become trapped within the printhead during manufacture or which may be brought into the printhead by ink can clog or partially block the flow of ink to a nozzle. If this occurs the nozzle may fire ink at faster rate that it can be replaced which can cause the nozzle to gulp air from outside the nozzle. While generating an ink puddle, ink flows out of nozzles adjacent the blocked nozzle and as the puddle is drawn back into the printhead, flow may also occur through the blocked nozzle causing the particle to move from the nozzle to a more innocuous position within the printhead.

4) for a nozzle to function correctly a constant supply of ink is required so that ink fired from the nozzle is replaced by ink from the ink chamber flowing along the ink conduits. If this continuous ink line is broken to few nozzles which are then starved of ink this is called a local deprime. If this occurs across all nozzles on a nozzle column (shown in

FIG. 3

) it is called a global deprime. The controlled flow of ink firstly out through the nozzles and then back into the nozzles is effective in providing ink to these dry ink conduits and nozzles.

The volume of air delivered by the pump

50

to the printhead bag

208

is controlled to achieve a desired increase in pressure within the ink chamber

232

of the printhead which is sufficient to cause the formation of a puddle of ink of a predetermined volume on the nozzle plate as will be described in greater detail below. As the carriage

30

moves away from the pump

50

air is withdrawn from the bag

208

, thus generating a negative pressure within the ink chamber

232

and facilitating the required flowback of ink into the printhead through the nozzles. This flowback is further facilitated by the spring

235

of the printhead which acts to compress the bag

208

forcing air out of the vent

210

and reestablishing the desired negative pressure within the ink chamber

232

.

While it is conceivable that the application of a controlled negative pressure to the outside of the nozzle plate of a printhead could be utilised to generate a puddle of ink on the nozzle plate, prior art negative pressure priming systems apply a relatively high vacuum for a relatively short period of time and are thus generally unsuitable. Such high rates of ink extraction generally cause the extracted ink to foam, i.e. the formation of tiny bubbles within the ink, and if such extracted ink is then allowed to reenter the printhead via the nozzles these air bubbles could easily become trapped in the nozzles or ink conduits leading to the nozzles.

The presently described technique for purging small quantities of ink onto the nozzle plate of a printhead in the form of a puddle which is largely recaptured by the printhead should be distinguished from prior art techniques in which large volumes of ink are passed through a printhead in order to remove large volumes of warehoused air.

A further technique which has been found to be effective for alleviating problems with printheads when applied either additionally or alternatively to the techniques described above, comprises the firing, or spiting of ink drops into an ink puddle formed on the nozzle plate of a printhead. Preferably this technique is applied in addition to the positive pressure priming technique described since this is convenient for the generation of a controlled puddle on a nozzle plate. It has been discovered that if nozzles of an inkjet printer are fired into a puddle which is maintained on the nozzle plate of the printhead so as to cover the nozzles the ink ejected is trapped by the puddle. Since the drops do not escape the puddle they create a turbulence within the ink of the puddle around the firing nozzles, which it has been found is effective in recovering the correct operation of defective nozzles. Although the word “drops” has been used to describe the action of firing nozzles into a puddle of ink, it will be appreciated that (since the outside of the nozzle is covered by ink which should be in fluid contact with the ink within the firing chamber) when the nozzle is fired the ink ejected does not normally contact air and thus does not have an ink to air surface. These “drops” can thus be seen to more accurately be described as a flow or jet of ink ejected into a larger reservoir of ink within the puddle.

As shown in the schematic diagram of

FIG. 24

, the puddle

239

formed on the nozzle plate

205

should extend to cover substantially all of the nozzles of the nozzle plate (shown in two nozzle columns

240

,

241

).

FIG. 25

schematically shows the drops

242

being fired into the puddle

239

and being captured by it. While it is preferred that substantially all the nozzles are covered by the puddle during this process, it has been found particularly for lower viscosity inks, that if the nozzles plate is not held substantially horizontal within the printer carriage the puddle may move to one side of the nozzle plate exposing some of the nozzles to air.

In addition to being a convenient method of generating a controlled puddle of ink, the use of a positive pressure within the printhead ink chamber

232

has been found to increase the volume of the drops fired which increases the effectiveness of this technique in recovering non-functional nozzles. Furthermore when this technique is employed in combination with the flowback of the puddle ink into the printhead the volume of ink lost from the printhead compared to prior art spitting or prior art priming techniques is dramatically reduced.

FIG. 26

is a graph showing the volume of ink waste from a recovery operation on a printhead which employs a positive pressure prime technique as described above to generate a puddle of ink and spits ink drops into the puddle. The horizontal curve

245

represents the volume of ink that would be lost due to spitting alone as per prior art recovery techniques. This volume is simply the volume of the drops fired over a given time period and thus remains constant as a function of ink displaced by the priming operation which is plotted on the x-axis of the graph. Here the firing of 512 nozzles 1000 times results in a waste ink volume of approximately 0.019 cc. The upper curve

246

represents the volume of ink that would be lost if none of the ink from the priming process nor from the spiting process were drawn back into the printhead. The lower curve

247

shows the actual ink lost when spitting and priming are performed together so that the controlled puddle formed captures fired drops and the puddle is sucked back into the printhead by for 15 seconds. As can be seen from

FIG. 26

the amount of waste ink is reduced as the ink initially displaced by the priming system increases. This is because as the puddle created by the priming process increases in size so does its ability to trap drops fired and the effectiveness of the flowback into the printhead. This trend is halted when the puddle formed is so large that surface tension forces no longer hold it to the nozzle plate and a very large drop of ink detaches from the puddle and drops into the spittoon of the printer.

The reduction of the quantity of waste ink has a number of advantages. Firstly, it allows more of the available ink to be utilised for printing, secondly it reduces the build up of ink on components of the printer (some of which may be handled by user) for example service station components and thirdly it extends the lifetime of the printers spittoon. A further advantage of spitting into an ink puddle compared to conventional spitting into a spittoon is that aerosol (tiny air borne ink particles generated whenever a nozzle is fired) is substantially reduced since this is also trapped by the puddle.

The firing of ink into the puddle of ink has additionally been found to be very effective in aiding the recovery of the printhead from any small air bubbles which may be trapped within the nozzles or ink conduits. This is believed to be because the drops fired dislodge such contaminants.

While normally all the nozzles of a printhead are fired during the above described spitting while priming process it has been found that in some circumstances it is advantageous to fire only some of the nozzles. It is know in the art to detect by various means the functional and the non-functional nozzles within a printhead. For example by means of a drop detector which is able to detect drops of ink fired from a nozzle as they cross a light beam within the service station of the printer. Alternatively, a test pattern may be printed by the printer in which blocks are printed by ink ejected from a single nozzle. This test pattern may then be scanned either by the printer operator who manually inputs the results to the printer or automatically by a sensor mounted on the printer carriage (as described in EP 0863012 in the name of the present applicant, which is hereby incorporated by reference). In such a manner the printer may determine which of the nozzles of a particular printhead are correctly ejecting ink and which are not.

It is thus preferred that the present printer comprises such a system and that subsequent to determining which nozzles are correctly functioning, only these nozzles are actuated by their associated resistors and firing chambers during the described spitting into an ink puddle process. This is advantageous because, as described above the attempted firing of nozzles the ink conduits of which are blocked or partly blocked by a particle may cause the nozzle to gulp air thus exacerbating problems with the printhead. Firing only working nozzles, which are covered by the ink puddle, around the blocked nozzle and then drawing ink back into the printhead from the ink puddle through the blocked nozzle is an effective technique for clearing particles from the nozzle or its associated ink conduit.

Alternatively, only the nozzles which are not functioning correctly may be fired during the recovery process. This can be effective for example when a nozzle is blocked by a dried plug of ink.

The firing of only some of the nozzles of the printhead during the above described recovery processes also serves to reduce the wear caused by repeated firing of nozzles and reduces the amount of waste ink.

It has been found that the effectiveness with which nozzle malfunctions can be corrected is improved by the repeated firing of nozzles, but that the firing frequency should be lower than that normally used when performing printing operations with the printhead. It is believed that this may be because firing of the nozzles at these lower repetition rates increases the volume of the drops fired and thus increases the flow of ink through the nozzles. Furthermore, lower firing frequencies facilitate the movement of air bubbles from the nozzles and their associated ink conduits which may not be able to move, and may even increase in size, if exposed to very high firing frequencies.

It will be appreciated that the techniques described above for priming and recovering the correct operation of printheads may be applied to many differing designs of printheads and that the parameters necessary for the effective use of these techniques will depend on the design of such printheads and on the characteristics of the ink used with the printheads. As will be apparent to those skilled in the art, a number of tests should be carried out on each design of printhead and ink to be utilised in order to determine such parameters and some of these tests will now be described together with parameters that have been found to be effective when utilised with ambient air regulator printheads designed and sold by Hewlett-Packard so as to provide a further guide to the implementation of, and understanding of, embodiments of the invention.

FIG. 27

is a graph of the volume of ink ejected or purged (during a priming operation having a duration of one second) onto the nozzle plate

205

of a number of different printheads having black, yellow, cyan and magenta ink as a function of the volume of air injected into the air chamber

208

of the printhead from the pump

50

. As can be seen this relationship is well defined and thus a specific predetermined volume of ink can be placed on the nozzle plate of a printhead by appropriately controlling the pump

50

. The particular pump employed (and described above) delivers 0.2 cc of air for each millimeter of movement of its piston

52

and since it is actuated by movement of the printer carriage

30

, which necessarily is capable of extremely accurate movement (typically three hundredths of an inch) in its main function of positioning the printheads for printing, the delivery of air can be accurately controlled. It will be noted that the curve

248

for the black printhead is substantially different from those for the colored inks

249

. This is due partly to a different design of the black printhead and partly due to the different nature, particularly viscosity of the inks. The black utilised for this particular design of printhead employs a pigmented ink which has a higher viscosity than that of the dye inks employed in the cyan, magenta and yellow printheads. Due to this higher viscosity as well as different ink formulation and different printing requirements for the black printhead, the internal architecture of the black printhead is different and in particular has larger diameter ink conduits leading to the nozzles, this architecture (despite the higher viscosity of the ink) has resulted in the steeper curve

248

shown in FIG.

27

.

A further important parameter for priming is the duration for which the positive pressure within the air chamber

208

of the printhead is held.

FIG. 28

is a graph of the volume of ink purged onto the nozzle plate of various printheads against the prime duration for a primed air volume of 0.4 cc and with the printhead isolated from the remote ink supply reservoir. As can be seen the volume of purged ink increases steeply with time at first and then more slowly. Also the curve

258

for the black printhead is again offset from the curves

259

for the colored ones.

It is a known problem, particularly for longer life printheads, that air

238

may accumulate in the ink chamber as shown in FIG.

22

. This warehoused air is a compressible component within the ink chamber

232

of the printhead and thus affects the efficiency with which the air delivered to the air chamber

208

by the pump

50

can increase the pressure within the ink chamber and thus purge ink onto the nozzle plate.

FIG. 29

shows how the volume of ink purged decreases as the volume of warehoused air increases for a black

262

and a cyan

263

printhead. The priming parameters for each printhead are calculated taking into account the average volume of air the printhead is likely to have to warehouse during its life so that a new printhead when primed purges slightly more than the ideal volume of ink and a printhead at the end of its life purges slightly less than the ideal volume of ink. An alternative is to store several priming parameters for each printhead and to change the parameters utilised dependent on the life of the printhead.

FIG. 30

is a graph of the pressure within a black printhead ink chamber

232

measured close to the nozzles during a 2 second prime for different values of the pressure of the ink supplied from the remote reservoirs and for different volumes of primed air. Upper curve

250

was achieved with an injected volume of air of 0.62 cc and an ink supply pressure of 0.4 psi, while curve

251

was achieved with the same volume of injected air but with a slightly negative ink supply pressure of approximately −0.1 psi. As can be seen the initial positive pressure generated within the printhead in both cases is the same (approximately 0.63 psi) but for curve

251

this pressure can be seen to decay more rapidly. From this it can be deduced that the peak in the positive pressure within the printhead is due solely to the air injected and is not substantially affected by the pressure of the ink supply. The rapid pressure decay within the printhead for curve

251

is due to the flow of ink from the printhead towards the remote ink supply. The lower curve

252

was achieved with an injected volume of air of 0.41 cc and an ink supply pressure of 0.9 psi. As can be seen from the peak of curve

252

this ink supply pressure of 0.9 psi is substantially higher than the peak pressure generated within the ink chamber of the printhead (approximately 0.3 psi) and thus flow of ink into the printhead would be expected if these parameters were utilised. The final curve

253

was achieved with an injected volume of air of 0.53 cc and an ink supply pressure of 0.2 psi. This later curve is the most desirable for priming the printhead since it shows little decay in the internal pressure and is likely to represent a good balance of pressures between the ink supply and the priming pressure in order to prevent flow of ink into or out of the printhead from the ink supply reservoirs for this particular printhead. The decay of pressure seen in curve

253

may be due to a loss of air pressure within the positive pressure priming system for example from the piston gasket

69

or from the seal on the printhead holddown cover

36

and/or from the flow of ink onto the nozzle plate of the printhead.

It can also be seen from

FIG. 30

that the pressure within the ink chamber prior to priming (approximately −0.11 psi and know as the backpressure) is accurately reestablished by the flexible bag

208

(operating in cooperation with the levers

206

,

207

of the printhead) after the priming operation when the bag

208

is once again in contact with atmospheric pressure. A further feature that can be seen from curve

251

of

FIG. 30

is that at point

260

of the curve the backpressure within the ink chamber has exceeded i.e. become more negative than the operating point. This is because in this case the ink supply pressure has been set too low i.e. at a slightly negative pressure and this has caused a significant flow of ink out from the printhead towards the remote ink reservoir. Once the operating point of the regulator lever is passed, the regulator valve

227

is opened and ink flows into the printhead until the backpressure returns to −0.11 psi as can be seen in FIG.

30

.

The following represents the presently preferred process parameters for performing a printhead service which includes a controlled prime with positive pressure air, spitting while priming and flowback of ink into the printhead.

perform a cleaning operation on the printhead comprising conventional spitting and wiping

reduce ink supply pressure from remote reservoirs from 2.1 psi to zero then raise pressure to 0.2 psi

position pump to inlet of air conduit on carriage cover

read stored priming parameters for printhead to be primed

apply pulse warming to heat printhead to 60 C. for black printhead and 35 C. for color printheads

actuate pump by carriage movement of 2.67 mm for black printhead to give 0.53 cc of injected air and 0.18 cc of purged ink; and by 2.54 mm for color printheads to give 0.51 cc of injected air and 0.08 cc of purged ink

hold pump in compressed position and thus hold pressure within air chamber of printhead for 1 second

fire nozzles for the first 0.5 seconds of the 1 second pressure hold at a frequency of 2 kHz, thus firing 1000 drops per nozzle

allow flowback of ink puddle into printhead for 15 seconds

perform a second cleaning operation on the printhead comprising conventional spitting and wiping

Cleaning of the nozzle plates of the printheads prior to implementing the present servicing technique in which the ink puddle is drawn back into the printhead is important to avoid contaminants which may be on the outer surface of the nozzle plate from being taken into the printhead together with the ink.

Although the majority of the ink puddle has been reabsorbed into the printhead within about 3 seconds after the pump is removed from the inlet of the air conduit on the carriage cover, a further 12 seconds is allowed to enable any remaining waste ink on the nozzle plate to dissolve any dried ink on the nozzle plate prior to performing the second conventional cleaning operation.

It has been found that heating of the printhead (by for example applying pulses of current to heaters within the printhead as is well know in the art) prior to a priming operation is advantageous for a number of reasons. Heating the printhead to a predetermined temperature reduces the variability of the priming process due to ambient temperature variations (if these are not taken into account via the printer sensor

302

as described below). Also it has been found that heating the printhead seems to aid recovery of the printhead from failures due to air bubbles. Thus heating the ink of the printhead is employed despite the fact that it has also been found in certain cases to reduce the ability of the ink to flowback from the nozzle plate into the printhead due to a reduction in the viscosity of the ink.

As described above the printer comprises a controller

300

which is utilised to control recovery operations for various printheads and which stores the determined optimum parameters for these operations. Since the printer is able to identify specific printheads, different parameters may be stored for example for printheads of different designs or containing ink of different formulations for example dye-based, pigment-based or UV resistant.

Furthermore, the controller in selecting an appropriate set of parameters for a particular printhead may consult a printer mounted sensor

302

to determine the current temperature or humidity and utilise this information to aid in the choice of parameters for the recovery operation.

Persons skilled in the art will understand that the above disclosure of the preferred embodiment of the invention may be modified and that a number of alternatives embodiments are possible within the scope of the invention, for example it will be appreciated that, while the preferred source of gas is a source of a constant volume of gas, the predetermined volume of gas can be supplied from a constant pressure source of gas provided this pressure has been characterised to result in a predetermined increase in the volume of the air chamber of the printhead when said constant pressure source is applied to the air for a characterised period of time.

Number	Name	Date	Kind
4500895	Buck et al.	Feb 1985	A
4558326	Kimura et al.	Dec 1985	A
4609925	Nozu et al.	Sep 1986	A
5185615	Koitabashi et al.	Feb 1993	A
5355158	Inada et al.	Oct 1994	A
6123403	Makino et al.	Sep 2000	A

Number	Date	Country
0 313 204	Apr 1989	EP
0 872 346	Oct 1998	EP
05193141	Aug 1993	JP
WO 9716315	May 1997	WO

Method for servicing an inkjet printhead

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