Technical Field

The present invention relates to an ink jet print head and, more particularly, to an ink jet print head which utilizes the so-called drop-on-demand method of operation.

Background Art

Non-impact printers have recently become very popular due to their quiet operation resulting from the absence of mechanical printing elements impacting on record media during printing. Among such printers, ink jet printers are particularly important as they permit high speed recording on plain untreated paper.

Various ink jet printing methods have been developed over the past years. In the so-called contin¬ uous ink jet method the ink is delivered under pressure to nozzles in a print head to produce a continuous jet of ink emitted through each nozzle. The ink jet is separated by vibration into a stream of droplets which are charged, and the flying droplets are either allowed to impact on a record medium or are electrostatically deflected for collection in a gutter for subsequent re- circulation.

In another method, which is known as the drop- on-demand method, the droplets are emitted by means of volumέ displacement brought about in an ink chamber or channel of the print head by means of energization of a piezoelectric element. The volume displacement gener¬ ates a pressure wave which propagates to the nozzles causing the ejection of ink droplets.

The drop-on-demand method has several advan¬ tages over the other above-mentioned method. Ink jet printers using this method have a simpler structure re¬ quiring neither deflecting means for controlling the flight of the droplets nor the provision of an ink re¬ covery system.

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An ink jet print head operating in a drop-on- demand manner is disclosed, for instance, in U.S. Patent No. 4,158,847. Since single drop-on-demand transducers have limited performance potential, several transducers are used in this patent to form a multiple nozzle print head. The nozzles are arranged in a straight line which lies at right angles to the line of printing and the head is arranged to be moved along the line of printing to print characters in a dot matrix manner. As the nozzles must be spaced closely together to provide adequately close spacing for the printed dots, the physical dimen¬ sions of the piezoelectric elements which form sleeves around the individual ink channels create a design dif¬ ficulty, which is overcome according to this patent by arranging the ink channels to extend in a radiating pat¬ tern from the nozzles. The converging channels arrive at the nozzles at different angles and a nozzle plate having parallel bores therein is provided to redirect the ink and determine the required parallel flight path for the ejected ink droplets.

One of the disadvantages of this arrangement resides in the difficulty of accurately aligning the bores of the nozzle plate with the ink channels, par¬ ticularly with the outermost ink channels -whose angle deviates considerably from the perpendicular.

Disclosure of Invention

It is an object of the present invention to provide a multiple nozzle print head of a design which eliminates the above—mentioned alignment problem while still providing close enough dot spacing for dot matrix printing.

Brief Description of the Drawings

. Embodiments of the invention will now be des¬ cribed, by way of example, with reference to the accom- panying drawings, in which:

Fig. 1 is a sectional view of a known type of transducer element used in the print head of the present invention;

Fig. 2 is a view of a cluster of transducers of Fig. 1 in two inclined rows thereof;

Fig. 3 is a front view of a print head for housing a cluster of transducers;

Fig. 4 is a right side view of the print head of Fig. 3; Fig. 5 is a bottom view of the print head of

Fig. 3;

Fig. 6 is a top view of the print head of Fig. 3;

Fig. 7 is a sectional view taken along the plane 7-7 of Fig. 5;

Fig. 8 is a side view of a cluster of trans¬ ducers in one inclined row;

Fig. 9 is an end view of the cluster of trans¬ ducers of Fig. 8 in one inclined row; Fig. 10 is an end view of a cluster of trans¬ ducers in three inclined rows;

Figs. 11A, 11B and 11C show a variation of the inlet end of the transducer ink chamber; and

Fig. 12 is a time-displacement wave diagram of the phenomena of Fig. 11.

Best Method of Carrying Out the Invention

Referring now to the drawing. Fig. 1 illus¬ trates a transducer element of the pulse-on-demand type as disclosed in U.S. Patent No. 3,683,212. This kind of transducer permits a relatively fast loading or filling with ink, it permits reliably purging of any air bubbles in the ink and .it shows good performance of 2,000 drops or more per second in operating rates.

The transducer element .20 of Fig. 1 includes an inlet tube 22 fitted over one end 24 of a glass tube

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26 which is reduced or necked down at the other end to form a nozzle 28 for ejection of droplets 30 of ink onto record media 32 which is normally spaced a relatively small distance from the nozzle. The glass tube 26 serves as an elongated ink chamber around which is provided a piezoelectric crystal sleeve 34 which has an electrical lead 36 connected thereto. An electrical lead 38 is connected to a tinned region 27 of the glass tube 26 so as to provide electrical contact to the inner wall of the piezoelectric sleeve 34. When the piezo¬ electric crystal 34 is electrically pulsed, a droplet 30 of ink is ejected from the nozzle 28 by reason of the sudden constriction of the crystal 34 and the compression of the walls of- the tubing 26. The inlet tube 22 car- ries ink from a supply (not shown) and the tube 22 is made of a pliable grade of elastomer such as silicone rubber to provide for absorption of upstream propagating pressure pulses and to prevent these pulses from inter¬ fering with the ink drop generation process. By reason of the small diameter of the tubular type pulse—on-demand transducer, it is possible to cluster a number of these transducers in an arrangement or pat-tern .so as to form a matrix print head in a com¬ pact area. Since the dot spacing in matrix printing is normally about 0.015 inch-vertically and since the small diameter of 0.050 inch for the individual transducer allows for such compact construction, a grid or matrix of one dot_ per 0.015 inch vertical spacing and two dots per 0.030 inch horizontal spacing provides for small clustered units, as exemplified by the folded pattern shown in Fig. 2. It is seen that for a seven nozzle print head the grid is made up of three transducers being indexed to the right so as to interleaf with the upper four transducers. In other words, the transducers in one row are laterally offset with respect to the trans ducers in the other row. In an arrangement wherein the seven transducers are in a single row, as in an echelon formation, such arrangement is appropriate for printing -^ϊ&E

an N x 7 character matrix having a cell size of 0.015 x 0.015 inch. It is sufficient to point out that the transducers in the cluster of Fig. 2 are separated by typical dimensions as shown for both horizontal and vertical directions in a regular modulus relative to the matrix cell dimension.

Since it is necessary for full width printing that all printing elements have a requirement to sweep or be moved past the first column of dots in a line and the last column of dots, it can be seen that the folded cluster of printing elements, as seen in Fig. 2, reduces the required stroke, thereby reducing the cycle time of operation, increases printer thruput capability and subtracts directly from the printer width requirement. In the seven nozzle folded pattern, the required over- travel for a full width print line is 0.180 inch. The technology for providing firing pulses coordinated with carriage or print head motion and for selecting elec¬ trical channels to be actuated in accordance with data flow for printing with such folded cluster of printing elements is presently within the realm of conventional or well-known logic.

Figs. 3, 4, 5, 6 and 7 show a print head 40 wherein the seven transducers 20 of Fig. 1 are packaged in a housing 42 having mounting lugs 44 and 46. An inlet connection 48 for the ink and a connection or port 50 for electrical leads are provided at the top and the ■ ^' right-side, respectively, of the housing 42. A chamber 52 in Fig. 7 is formed as an ink plenum in the upper portion of the housing 42 and a cover 54 fits over the walls of the chamber. The transducers 20 are positioned within the housing 42 of the print head 40 with the ends of the glass tubes 26 extending through a bulkhead 56 and into the ch-amber 52. A cement-type sealant 58 is applied in a thin layer on the bulkhead 56 and around the glass ends of the tubes 26 to provide a tight en- ^' closure and to hermetically bond the bulkhead 56 to the housing 42 and the glass

In the assembly of the print head 40, the transducers 20 are placed into the plastic housing 42 with the nozzle ends of the transducers extending through holes in the bottom wall of the housing corresponding to the slanted or inclined row pattern as seen in Fig. 5.

The bulkhead 56 which has a matching hole pattern is set in place over the inlet ends of the glass tubes 26 and onto the shoulder provided in the wall of the housing to maintain the transducers in correct registration. The sealant 58 is then applied and the cover 54 is attached by bonding to the housing 42. The electrical leads 36 and 38 from each transducer 20 are brought out through the connection or port 50.

Fig. 8 shows the seven transducer echelon cluster as briefly mentioned above which is appropriate for printing the N x 7 character matrix and- arranged in a single inclined row as seen in the projection view of Fig. 9. These transducers are made up of the inlet tube 22, the glass tube 26, the nozzle 28 and the piezo- electric crystal 34, and are spaced at typical dimen¬ sions as shown.

A modified array or pattern is shown in Fig. 10 wherein eighteen transducers are arranged in three inclined rows for use in higher resolution printing, and are spaced at typical dimensions. When the triple arrangement of Fig. 10 is compared with the single row of Fig. 9, it is seen that the overall width of the pattern is.0.360 inch for both the single and the triple pattern in a typical spacing of 0.060 inch between transducers in the same row. In the double row of Fig. 2 and the triple row pattern of Fig. 10, a typical spacing of 0.030 inch between adjacent transducers in adjacent rows or 0.060 inch between adjacent transducers in the same row. provides an overall width of 0.180 inch between the centers of right and left transducers in the double row pattern and an-overall width of 0.360 inch between centers of right and left transducers in the triple row pattern.

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Figs. 11A, 11B and 11C show a portion of a glass tube ^* 60 which is provided with an inlet end 62 in a necked down configuration or reduced diameter aperture 64 for the purpose of reducing wave. reflection during operation. The abrupt electrical pulsing of the piezo¬ electric crystal element 34 of the transducer 20 and the sudden reduction in volume within the ink chamber result in a system of elastic waves being generated in the fluid ink. This wave system not only causes a droplet of ink 30 to be expressed from the nozzle 28 (to the left in the view shown in Fig. 11) but members of the system also cause undesired disturbances to be propa¬ gated upstream against the supply of ink.

One such of the elastic waves to be considered is the leading upstream propagating wave A in the plane near the end of the tube as seen in Fig. 11A. When such wave A reaches the open end of the tube, the wave is reflected in opposite sign,, that is a compression wave is reflected as an expansion wave of equal strength and en expansion wave is reflected as a compression wave of equal strength. If the wave is incident on the closed end of a tube, the wave reflects in kind and it is readily seen that the reflected wave could disrupt the ink droplet generation process. Some alleviation or comforting of this con¬ dition can be gained or obtained by necking down or reducing the diameter of the inlet port or aperture 64 of the glass tube 60. The incident wave A is shown approaching the inlet port or aperture 64 at the time t ^* = t.. The incident wave A leaves the glass tube 60 at time ^' t = t_ at which time a reduced strength wave B is ' reflected by the change in cross-sectional area of the ink channel or chamber of the transducer and then starts to propagate downstream toward the nozzle or to the left in Fig. 11B.

As the original wave A passes or travels out of the tube 60 into a virtual open space or volume, the wave rapidly is weakened in that the initial pressure

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change, as rise or fall across the wave, abruptly re¬ duces to a much smaller level. An adjustment in equilibrium for this reduction in pressure must be made in the channel of the tube 60 wherein the adjustment takes the form of a pressure wave C of the same family of waves as the wave B but opposite in strength or amplitude. It is thus seen that if the diameter ratio of D to D- is suitably determined and adjusted for the pressures utilized in the operation, the waves B and C will be equal and opposite in strength or value. By reason that the waves B and C are separated slightly in matter of time and space, such waves cannot merge with and cancel each other so that the result is a doublet or double wave across which a pressure change P.-P. will be small and ideally zero compared to the original pressure change.

Fig. 12 is a plot of wave front displacement vs. time which is commonly called a wave diagram and shows the above-described phenomena or operating con- ditions in summary form.

In proceeding with an analysis of the wave reflection problem, it is assumed that the waves are plane waves, that the fluid is compressible and inviscid or non-sticky and that the fluid flow is described as being one—dimensional. Wave strength may be character¬ ized by either ^* the pressure change or the velocity change occurring across the wave and these relationships for waves A, B and C, - are respectively:

Equation 1

^P 2~ ^P 1 ⁼ ~ ^dC ^ ^V 2~ ^V 1^ Equation 2

P ₃ -P ₂ *= -dc (V ₃ -V ₂ ) Equation 3

wherein P=pressure

V ^* =fluid velocity d=density c=speed of sound in the fluid . >^tlS.E_

and the subscripts denote the corresponding region or area of the ink chamber within the transducer.

In the manner of the aperture 64 at the inlet end 62 of the glass tube 60 acting as a throat, the steady flow conditions apply between and within regions 3 and 4 after the time t . Using a superscript asterisk to denote these throat conditions, the following momen¬ tum and continuity equations may be applied.

P -P* = - d (V* ² -V ₃ ² ) Equation 4

. dV*D ₂ ² = <i ₃ D ₁ ² Equation 5

Equations 4 and 5 can be combined to provide Equation 6.

1 2 π P -P* = — d V —± 4 *3 2 ^α 3 D ₂ -1 ^" Equation 6

The pressure at the throat P* is the same static pressure as in the region 4.

P* = P 4. * Equation 7

Since the emitted wave A* is relatively weak,-

P = P _Q (approximately) ^• Equation 8

so that Equation 6 may be replaced by

_ _ 1_ ., . 2 , ^■ .

^F 3 0 ^" 2 3 D ₂ -1 Equation 9

A second relation between P_ and V ₃ may be obtained from Equations 1, 2 and 3. Noting that V = 0,

P -P. = -dc(V ₃ -V,) Equation 10

But the condition to be imposed to insure no net change across the doublet is

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P ₃ = P , (by imposition) Equation 11

therefore.

V ₃ = V^ _^ Equation 12

Now making these substitutions in Equation 9 and using Equation 1 to substitute for V- in terms of P.. -P.. , Equation 9 becomes

P ₁ -P ₀ = (P ₁ -P _Q ) ² D- _j^ ⁴ -l Equation 13

This result yields for the required diameter ratio,

D 1 _ 2d ^■ c 2 ± 4- Equation 14

~2 ^{' P} l- ^P 0

In a typical case for water/glycol based ink, d ■ ^*** ■ ^* 1100 kg/m ² c ■ ^** = 1500 m/s P -P _Q = 100,000 pa

^D l

7T- ~ 14-9.

^D 2

O P