EP0314486A2

EP0314486A2 - Hydraulically tuned channel architecture

Info

Publication number: EP0314486A2
Application number: EP88310139A
Authority: EP
Inventors: Kenneth E. Trueba; William R. Knight; Niels J. Nielsen
Original assignee: Hewlett Packard Co
Current assignee: HP Inc
Priority date: 1987-10-30
Filing date: 1988-10-28
Publication date: 1989-05-03
Also published as: CA1300974C; KR890006394A; EP0314486A3; JPH01152068A; KR920005741B1

Abstract

The use of lumped resistive elements (22, 24, 32) in an ink feed channel (10) between an ink-propelling element, such as a resistor, (12) and an ink supply plenum (16) provide a means of achieving resistive decoupling and meniscus resonance control with a minimum of deleterious side effects and design compromises typical of prior art solutions.

Description

The present invention relates to ink-jet printers, and, more particularly, to a structure for controlling fluid refill of firing chambers, minimizing meniscus travel and minimizing cross-talk between adjacent nozzles in the printhead used to fire droplets of ink toward a print medium.
When designing printheads containing a plurality of ink-ejecting nozzles in a densely packed array, it is necessary to provide some means of isolating the dynamics of any given nozzle from its neighbors, or else cross-talk will occur between the nozzles as they fire droplets of ink from elements associated with the nozzles. This cross-talk seriously degrades print quality and hence any providently designed ink-jet printhead must include some features to accomplish decoupling between the nozzles and the common ink supply plenum so that the plenum does not supply a cross-talk path between neighboring nozzles.
Further, when an ink-jet printhead is called upon to discharge ink droplets at a very high rate, the motion of the meniscus present in each nozzle must be carefully controller so as to prevent any oscillation or "ringing" of the meniscus caused by refill dynamics from interfering with the ejection of subsequently fired droplets. Ordinarily, the "settling time" required between firings sets a limit on the maximum repetition rate at which the nozzle can operate. If an ink droplet is fired from a nozzle too soon after the previous firing, the ringing of the meniscus modulates the quantity of ink in the second droplet out. In the case where the meniscus has "overshot" its equilibrium position, a firing superimposed on overshoot yields an unacceptably large ejected droplet. The opposite is true if the firing is superimposed on an undershoot condition: the ejected droplet is too small. Therefore, in order to enhance the maximum printing rate of an ink-jet printhead, it is necessary to include in its design some means for reducing meniscus oscillation so as to minimize the settling time between sequential firings of any one nozzle.
Previous approaches to the problem of cross-talk, or minimizing inter-nozzle coupling, can be separated into three classes: resistive, inertial, and capacitive. The following is a brief discussion of each method and a critique of the typical embodiments of these methods.
Resistive decoupling uses the fluid friction present in the ink feed channels as a means of dissipating the energy content of the cross-talk surges, thereby preventing the dynamics of any single meniscus from being strongly felt by its nearest neighbors. In the prior art, this is typically implemented by making the ink feed channels longer or smaller in cross-section than the main supply plenum. While these are simple solutions, they have several drawbacks. First, such solutions rely upon fluid motion to generate the pres sure drops associated with the energy dissipation; as such, they can only attenuate the cross-talk surges, not completely block them. Thus, some cross-talk "leakages" will always be present. Second, any attempt to shut off cross-talk completely by these methods will necessarily restrict the refill rate of the nozzles, thereby compromising the maximum rate at which the printhead can print. Third, the resistive decoupling techniques as practiced in the prior art add to the inertia of the fluid refill channel, which has serious implications for the printhead performance (as will be explained at the end of the inertial decoupling exposition which follows shortly).
In capacitive decoupling, an extra hole is put in the nozzle plate above that point where the ink feed channel meets the ink supply plenum. Any pressure surges in the ink feed channel are transformed into displacements of the meniscus present in the extra hole (or "dummy nozzle"). In this way, the hole acts as an isolator for brief pressure pulses but does not interfere with refill flow. The location, size and shape of the isolator hole must be carefully chosen to derive the required degree of decoupling without allowing the hole to eject droplets of ink as if it were nozzle. This method is extremely effective in preventing cross-talk (but can introduce problems with nozzle meniscus dynamics, as will be discussed below).
In inertial decoupling, the feed channels are made as long and slender as possible, thereby maximizing the inertial aspect of the fluid entrained within them. The inertia of the fluid "clamps" its ability to respond to cross-talk surges in proportion to the suddenness of the surge and thereby inhibits the transmission of cross-talk pulses into or out of the ink feed chan nel. While this decoupling scheme is used in the prior art, it requires considerable area ("real estate") within the print head to implement, making a compact structure impossible. Furthermore, since the resistive component of a pipe having a rectangular cross-section scales directly with length and inversely with the third power of the smaller of the two cross-section dimensions, the flow resistance can grow to an unacceptable level, compromising refill speed. More importantly, however, are the dynamic effects caused by the coupling of this inertance to the compliance of the nozzle meniscus, as will be discussed below.
With regard to the problem of meniscus dynamics, there are apparently no solutions offered in the prior art. Apparently, this is a problem that has only recently surfaced as printhead designs have been pushed to accomodate higher and higher repetition rates. Clearly, any method used to decouple the dynamics of neighboring nozzles will also aid in damping out meniscus oscillations, at least from a superficial consideration. In practice, problems are experienced when trying to use the decoupling means as the oscillatory damping means. These problems can be traced to the synergistic effects between the nozzle meniscus and the fluid entrained within the ink feed channel, as outlined below.
If resistive decoupling is attempted by reducing the width of the entire ink feed channel, the inertia of the fluid entrained within the feed channel increases. When this inertia is coupled to the compliance of the meniscus in the nozzle, it results in a lower resonant frequency of oscillation of the meniscus, which requires a longer settling time between firings of the nozzle. The inertial effect and the resistive effect are hence deadlocked, with the net effect being that settling time cannot be reduced.
Capacity decoupling has been proven effective at droplet ejection frequencies below that corresponding to the resonant frequency of the nozzle meniscus coupled to the feed channel inertia. However, its implementation at frequencies near meniscus resonance is also complicated by interactive effects. Specifically, the isolator orifice acts as a low impedance shunt path for high frequency surges. Hence, the high frequency impedance of an ink feed channel terminated at its plenum end with an isolator orifice will be lower than an equivalent channel without an isolator. This means that during the bubble growth phase, blow-back flow away from the nozzle is increased by the isolator orifice. This robs kinetic energy from the droplet emerging from the nozzle, which results in smaller droplet size and lower droplet velocities and thus lower ejection efficiency. During the bubble collapse phase, the isolator orifice meniscus pumps fluid flow back into the refill chamber, which excites a resonant mode in which the two menisci trade fluid between themselves via the ink feed channel. Since these two menisci are for most practical designs similar in size, and since they are effectively "in series", the equivalent compliance of the coupled system is roughly half of that with only one orifice in it. The two-orifice system will thus resonate at a higher frequency, which is a benefit from a settling time point of view, but the energy stored in the resonating system still needs to be dissipated and therefore constrictive damping will be necessary in such an implementation. While the effects of these resonances is poorly understood at this time, the efficiency decrease may be severe enough to prevent the printhead from working.
It is clear that what is needed is a printhead structure that accomplishes both (1) isolation of any given nozzle from its neighbors and (2) reduced oscillation of the meniscus caused by refill dynamics from interfering with the ejection of subsequently fired droplets, while limiting the severity of any side effects incurred in the implementation of the desired structure.
According to the invention there is provided an ink jet printhead comprising a plurality of ink propelling elements and a plurality of nozzles associated therewith for firing a quantity of ink toward a print medium, wherein ink is supplied to an ink propelling element from a plenum chamber by means of an ink feed channel characterised in that there is at least one constriction in the ink feed channel.
In accordance with the invention, a localized constriction (also referred to as a lumped resistance element) is introduced into the feed channel connecting each nozzle's firing chamber with the main ink supply plenum. The fact that the resistive aspect of each nozzle is localized permits these constrictions to be useful in cross-talk control, since then quantity of inertia they introduce into the feed channels is minimal. This overcomes the aforementioned problem of parasitic inertance present in the prior art in which the resistive aspect is distributed along, and thereby scales directly with, the length of the feed channel. The use of lumped resistance elements allow the printhead designer to vary the relative amounts of resistance and inertance present in the feed channel substantially independently of each other and thereby "tune" the feed channel for an optimum combination of inertance and resistance.
In one embodiment, the lumped resistance element comprises a pinch point between two opposed projections in the ink feed side walls. Since these feed walls are commonly patterned in photoresist, the pinch points are easily implemented by including them in the photomask which defines the ink feed channel geometries. The degree of "pinch" possible is sensitively determined by the photochemical characteristics of the resist film. In practical terms, when using commerically common resist films and light sources, the ratio of film thickness (i.e., wall height) to pinch width ranges up to about 1.2.
This amount of pinch may not be sufficient for all applications. Therefore, another embodiment comprises one or more sharp bends in the ink feed channel. Each sharp bend acts as a lumped resistive element to generate a pressure drop equivalent to that present in an equivalent length of ink feed channel of from about 5D to 10D, where D is the cross-sectional dimension of the channel. This resistive enhancement is accomplished without a proportional increase in the inertia of the feed channel, and without violating the height-to-width limits of the film. This concatenation of serial lumped resistive elements is applicable in principle to the pinch point embodiment as well, although care must be exercised to avoid including any features which might behave as bubble traps or interfere with photoresist development and subsequent washing.
In another embodiment, the ink-propelling element (resistive heating element, piezoelectric element, etc.) is placed below the level of the feed channel, again introducing a sharp bend in the ink feed channel.
In yet another embodiment, a pinch point is introduced into the feed channel by partially obstructing the channel with a dike or "speed bump" lying across the width of the channel. This feature can be photolithographically defined and deposited upon the print head substrate using a film thinner than that used to form the side walls, or it may be affixed to the underside of the orifice plate which forms the "ceiling" of the ink feed channel. In this case, the dike can consist either of photoresist film or of electrodeposited metal. In the latter instance, the electrodeposition can be an operation separate from that used to create the orifice plate, or, in the case of electroformed orifice plates, it can be an integral part of the electroforming process. It is also possible in principle to electrodeposit a metallic dike onto the printhead substrate, provided that the substrate is compatible with the electrodeposition baths.
It should be noted that none of the speed bump implementations described above are required to completely span the full width of the ink feed channel; free standing structures are permissible which act as lumped resistive elements and are limited in their application only by the practical considerations outlined in the previous sections.
These lumped resistive elements can be used singly or in combination with elements of the same or of different types, depending on the details of the application and are not strictly limited to the shapes, materials, and layouts offered above as examples.
The novel printhead structures of the invention accomplish both (1) isolation of any given nozzle from its neighbors, i.e., cross-talk reduction, and (2) reduced oscillation of the meniscus caused by refill dynamics in any individual nozzle. This prevents meniscus displacements from interfering with the ejection of subsequently fired droplets, while limiting the severity of any side effects incurred in the implementation of the desired structure. The new printhead structures have the additional advantage of being easy to implement and easy to "tune" for maximum effectiveness. These structures are directly applicable across the full range of ink-jet printheads.

FIG. 1 is a top plan view of a prior are resistor and ink feed channel configuration;
FIG. 2, on coordinates of distance in µm and time in µsec, is a plot of meniscus damping of an active nozzle and an adjacent nozzle for the prior art configuration of FIG. 1;
FIG. 3a is a top plan view of a resistor and ink feed channel configuration in accordance with one embodiment of the invention;
FIG. 3b is a side elevation view, depicting two alternate embodiments: a "speed bump" structure that can be used in conjunction with the structure depicted in FIG. 3a and a well structure, in which the resistor is placed below the level of the ink channel;
FIG. 3c is an enlarged view of a portion of FIG. 3a;
FIG. 4, on coordinates of volume in p1 and time in µsec, is a plot of meniscus damping of an active nozzle and an adjacent nozzle for the embodiment depicted in FIGS. 3a-b;
FIG. 5 is a top plan view of a resistor and ink feed channel configuration in accordance with an alternate embodiment of the invention;
FIG. 6 is a perspective view of the resistor, ink feed channel and orifice, depicting yet another embodiment of the invention; and
FIG. 7 is a view similar to a portion of FIG. 6, depicting still another embodiment of the invention.

Referring now to the drawings wherein like numerals of reference designate like elements throughout, an ink feed channel 10 is shown, with a resistor 12 situated at one end 10a thereof. Ink (not shown) is introduced at the opposite end 10b thereof, as indicated by arrow "A", from a plenum, indicated generally at 14. Associated with the resistor is a nozzle 16 (such as seen in FIG. 3b), located above the resistor 12. Droplets of ink are ejected through the nozzle (i.e., nornal to the plane of FIG. 1) upon heating of a quantity of ink by the resistor 12.
While the invention is preferably directed to improving the operation of thermal ink-jet printheads, which employ resistors 12 as elements used to propel droplets of ink toward a print medium, such as paper, it will be appreciated by the person skilled in this art that the teachings of the invention are suitably employed to improve the operation of ink-jet printheads in general. Examples of other types of ink-jet printheads benefited by the teachings of the invention include piezoelectric, which employ a piezoelectric element to propel droplets of ink toward the print medium.
Attempts to minimize cross-talk between adjacent nozzles have included lengthening the channel 10, as shown by the dotted lines 10'.
The straight channel 10 does not permit facile damping of the ink. As seen in FIG. 2, damping of the meniscus of ink in the active nozzle takes more than 400 µsec (Curve 16). Simultaneously, the meniscus of ink in a neighboring nozzle is adversely affected by the action of the meniscus of ink in the active nozzle (Curve 20).
In accordance with the invention, a localized constriction 22 (also referred to as a lumped resistance element) is introduced into the feed channel connecting each nozzle's firing chamber with the main ink supply plenum. The localized construction 22 may comprise a sharp bend or a pair of opposed projections, and one or more such constrictions may be present in various combinations.
For example, a pair of opposed projections 24, depicted in FIG. 3a, is employed alone or in conjunction with locating the resistor 12 below the floor 26 (indicated by the dashed lines) of the channel 10, as seen in FIG. 3b.
The use of one or more sharp bends, or constrictions, considerably improves the damping of the fluid motions as seen in FIG. 4. Damping of the meniscus of ink occurs in about 250 µm (Curve 28). Simultaneously, the fluid meniscus in a neighboring nozzle is hardly affected by the action of the meniscus the active nozzle (Curve 30).
Preferably, the length of the channel 10 ranges from close to the resistor to about 60 µm. The height of the channel 10 ranges from about 15 to 30 µm, while the width of the channel ranges from about 20 to 40 µm.
The considerations that govern the channel dimensions relate to the amount of ink that has to be replaced after each firing. This amount is the sum of the quantity of ink that is ejected out through the nozzle 16 plus the quantity of ink that moves back through the feed channel. The latter quantity is re ferred to as the blow-back, and is desirably as small as possible.
To get maximum performance, fast refill time in conjunction with avoiding having to overcome below-back in the ink feed channel 10 is required. While a refill time of 0 µsec with very fast damping (no oscillation) is ideal, it is not possible. Refill times of about 250 µsec and less are found to provide adequate results at a frequency of 4 kHz. For a pen operating at 6 kHz, the corrresponding acceptable refill time is about 167 µsec and less.
The tradeoff is that increased damping implies a slower refill. Since it is desired to maximize both refill and damping, optimizing them is the only possibility.
The shape of the projections 24 in the area of the opening 10a can contribute to the optimization of refill and damping. Specifically, the projections can be sharp, as shown in FIG. 3a, or rounded, as shown in FIG. 3c. The radius R of the rounding may range from about 5 to 10 µm.
The configuration of the projections 24 affects turbulent flow of the ink in the vicinity thereof. In particular, sharper corners increase the turbulence, thus leading to higher resistance, in the ink feed channel 10 during the bubble growth phase. This reduces blow-back and decreases refill time.
Sharp corners are difficult to define lithographically in some resists, such as DuPont's VACREL. However, other resists, such as the polyimides, may permit better definition.
Thus, it can be seen that blow-back can be minimized by constricting the opening to the channel 10 with the opposed projections 24, by constricting the channel (such as providing a "speed bump" 32 across the junction of the channel and the firing chamber on the floor 26′ thereof (FIG. 3b), by placing the resistor 12 in a well below the level of the refill channel (as alternately depicted in FIG. 3b), or by introducing bends 22 in the ink feed channel. In the case of the speed bump 32, it may be placed on the floor or the ceiling, partially or fully extending across the width of the channel 10.
In its broad aspect, the invention contemplates placement of one or more constrictions 22, 24, whether employing projections or one or more sharp bends in the ink feed channel 10, between the plenum 14 and the nozzle 16. Each sharp bend 22 introduces a pressure drop equivalent to that generated by an equivalent length of ink feed channel 10 of from about 5d to 10d, where d is the cross-sectional dimension of the channel, without introducing the inertia enhancement effect. This is because the resistance enhancement swamps out and overwhelms the inertia change. Thus, the lengthening of the channel 10, depicted in FIG. 1, is avoided by adding one or more constrictions 22, 24 therein.
As shown above, the sharp bends may be introduced by setting the resistor 12 below the channel 10 or by adding a speed bump 32 across the floor 26′ or ceiling of the channel. In another embodiment, depicted in FIG. 5, one or more sharp bends 22 may be introduced in the channel 10. In particular, FIG. 5 depicts two constrictions, one at 22a and the other at 22b. FIGS. 6 and 7 depict yet additional embodiments, such as opposed projections 24 (FIG. 6) or a free-standing pillar 22′ in lieu of a speed bump 32 (FIG. 7).
The constricted feed channel or the labyrinth feed channel can be introduced into the printhead architecture without lengthening the feed channel structure and without revising the orifice plate with the addition of isolator orifices.
The mass "seen" by the nozzle meniscus as it oscillates is, for the labyrinth, predominantly the fluid mass in the firing chamber. The resistance of the labyrinth ink feed channel decouples this mass from that entrained in the labyrinth.
The use of lumped resistive elements in the ink feed channel to allow independent adjustment of the feed channel's resistive and inertial parameters is useful in ink-jet printer applications based on thermal and non-thermal ink-jet technologies.
A comparison was made between a straight ink feed channel of the type depicted in FIG. 1 (prior art) and an ink feed channel of the invention as depicted in FIG. 3a. In each case, the resistor was 50 µm x 50 µm square. In the prior art case ("straight"), the ink feed channel was 150 µm long and 70 µ wide. In the configuration of the invention ("opposed projection"), the resistor was placed 25 µm below the bottom of the ink feed channel; the ink feed channel was 50 µm long (from the edge of the resistor to the opening to the reservoir) and had protuberances affording an opening of 35 µm wide.

In the comparison, for a given drop size (in picoliters, pl), the refill time (in microseconds, µsec) and the overshoot volume (in pl) and the blow-back volume (in pl) were measured. The results are shown in Table I below.

Table I.

Barrier Type	Drop Size, pl	Refill Time, µsec	Overshoot Vol., pl	Blow-back Vol., pl
Straight	75	130	36	75
	150	242	38	232
Opp. Proj.	75	135	16	40
	100	150	16	48
	150	156	16	78

Table I shows that the opposed projection configuration of the invention works because the blow-back volume is held in check. The straight barrier with 150 pl drop actually has to refill 382 pl because of the excessive amount of blow-back.

In another example, the W and L dimensions, depicted in FIG. 3c, were varied. The projections all had 5 µm radius (R) rounded corners. The drop volume was 150 pl in all cases. The results are shown in Table II.

Table II.

L, µm	W, µm	Refill Time, µsec	Overshoot Volume, pl	Blow-back Volume, pl
4	22	178	11	71
4	26	172	16	85
4	30	174	20	94
4	34	182	24	104
4	38	200	27	127
4	42	212	29	150
4	30	174	20	94
8	30	184	19	91
12	30	194	18	89
16	30	209	17	92

From the foregoing data, it appears that the dominant contributor to fast refill is the width W provided by the opposed projections, or the amount of constriction. The length L of the straight section should be held to a minimum, since increased length does slow refill.
A study was also made of opposed projections with sharp corners and opposed projections with 5 µm radius rounded corners. The refill time was found to be 20 µsec shorter for the sharp corner configuration, but the blow-back volume was 3 pl less. Of the 20 µsec speed-up, some may be attributed to the reduction down to zero of the equivalent straight pipe section inherent in the rounded corners.
Thus, a feed channel architecture for an ink-jet pen is provided for use in thermal ink-jet printers.
It will be clear to one of ordinary skill in the art that various changes and modifications of an obvious nature may be made without departing from the spirit of the invention, and all such changes and modifications are deemed to fall within the scope of the invention as defined by the appended claims.

Claims

1. An ink-jet printhead comprising a plurality of ink-propelling elements (12) and a plurality of nozzles (16) associated therewith for firing a quantity of ink (A) toward a print medium, wherein ink is supplied to an ink propelling element from a plenum chamber (14) by means of an ink feed channel (10), characterised in that there is at least one constriction (22,23,32) in the ink feed channel.

2. The printhead of Claim 1 wherein the ink-propelling elements comprise resistive heating elements.

3. The printhead of Claims 1 or 2 wherein one end of the ink feed channel is provided with a pair of opposed projections (24), thereby forming a constriction between the plenum and the channel.

4. The printhead of Claim 3 wherein said projections are sharp.

5. The printhead of Claim 3 wherein said projections are rounded .

6. The printhead of any preceding Claims wherein at least one constriction is achieved by setting the ink-propelling element in a plane below that of the channel.

7. The printhead of Claim 6 wherein the ink-propelling element is about 5 to 40 um below the plane of the channel, as measured from the top of the element to the bottom of the channel.

8. The printhead of any preceding Claim wherein at least one constriction is achieved by formation of at least one sharp bend (22) along the length of the channel.

9. The printhead of any preceding Claim wherein said at least one constriction comprises a projection (32) formed across at least a portion of one of the lateral sides of said channel.

10. The printhead of any preceding Claim wherein said at least one constriction comprises a pillar (22′) in said channel.