WO1985002012A1 - On-demand large character ink jet printer - Google Patents

Abstract

An ink jet printer of the "drop-on-demand" type, for printing of large characters with improved image quality and reliability. Ink is supplied under static pressure to an ink impulse assembly (50), which acts as a positive displacement pump. The ink impulse assembly (50) includes a pressure chamber (60) walled in by an elastomeric membrane (65) having inlet (53) and outlet (56) ports sealed by check valves (54, 57). A solenoid (70)-driven plunger (71) rod (80) displaces the membrane (65) to constrict the chamber (60), causing a prescribed volume to be ejected through a nozzle assembly. The printer enjoys a simplicity of design and is highly adaptable, while avoiding many of the operational difficulties of prior art drop-on-demand ink jet designs. Character height may be adjusted within a certain range using a rotatable nozzle plate, with appropriate adaptations in the character generation electronics.

Description

ON-DEMAND LARGE CHARACTER INK JET PRINTER BACKGROUND OF THE INVENTION The present invention relates to drop-on-demand ink jet printers, and more particularly to ink jet printers for the generation of relatively large characters. Ink jet printers have generally been classified within two broad categories -- "continuous" printers and "drop-on-demand" printers. Printers of the former type include an assembly for continuously emitting a stream of ink, which after breakup into droplets is gated to selectively permit passage of droplets to the substrate to be imprinted. The apparatus for gating the ink droplets and collecting the surplus drops, typically electrostatic in principle, adds a significant degree of complexity to the system. The drop-on-demand printers, such as that of the present invention, eject ink droplets only during print intervals, thus avoiding the need for additional gating apparatus between the ink jet source and the record medium. Ink jet technology has been adapted to a wide range of printing applications. Letter quality printers are designed for character heights on the order of millimeters and must meet relatively rigorous demands as to print quality. In such applications as package imprinting, however, it is more typical to specify a character height on the order of centimeters, and there is more leniency as to character definition. Whereas the former printers might have a resolution on the order of hundreds of pels, the latter applications would more typically involve rasters of 5x7 or 7x9. The latter applications, to which the pointers of the present invention are directed, emphasize such criteria as simplicity of design, reliability, and flexibility of operation; of course, image quality is still a prime concern. A representative patent on a large character drop-on-demand ink jet printer is U.S. Patent No. 4,378,564 to A.R. Willett. This patent discloses a system which incorporates a pressurized ink supply with output manifold, which gates the pressurized ink using a series of solenoid valves. Ink droplets passed by the solenoid valves are propelled from jewelled orifice nozzles to form the character. This system is vulnerable to a number of hazards characteristic of on-demand ink jets, such as air bubbles entrapped within the printer. Such gas contamination reduces the efficiency of such printers in that air bubbles must be purged from the system to achieve a proper ink flow. Such printers also suffer a limited print range, tendency toward ink splattering on the image, and a danger of ink clogging of the nozzles. Additionally, the linear relationship between valve operation and dot size imposes a critical restraint on speed, which can only be increased by raising the input pressure (which raises other difficulties). Another type of drop-on-demand ink jet printer which is the subject of considerable patent art utilizes a "pressure cavity" to which ink is supplied at low input pressure. An electrically actuated transducer mounted in the rear of the cavity produces fluid oscillation in the ink, thereby producing a series of ink drops. Such a transducer may be a radially expansive disk, as in U.S. Patent No. 3,787,884 to Demer, or may be a member coaxially aligned with the supply tube, as in U.S. Patent No. 3,683,212 to Zoltan. This oscillatory technique is, again, subject to a number of practical difficulties. Such ink jet printers are vulnerable to entrapped air bubbles, and various complexities have been adopted in an attempt to minimize the difficulty, as exemplified by U.S. Patent No. 4,323,908 to Lee et al. Another paramount consideration is efficiency of performance, i.e. ensuring that the print actuation energy is not unduly dissipated in producing the jet.

European Patent Application No. 81302728.1, assigned to Nippon Electric Company, discloses a drop-on-demand ink jet printer of the type incorporating a pressure chamber with oscillatory excitation by a piezoelectric element fastened to one wall of the chamber. This printer gates ink inflow and outflow from the pressure chamber using a pair of check valves, illustratively of the disk valve type, in order to control the fluid resistance characteristics at the input and output ports. This apparatus, however, is unsuitable for large character printers inasmuch as it relies on the limited energies associated with the peizoelectric excitation. Accordingly, this apparatus is designed for relatively minute fluid volumes.

Therefore, it is a principal object of the invention to provide a simple, reliable ink jet printer of the drop-on-demand type. Such printer is intended for large character printing applications, with relatively low drop repetition rates and large image elements. A related object is to provide the capacity for outputting relatively high fluid volumes in such an ink jet.

Another object of the invention is the avoidance of air bubble entrapment in ink jet operation. It is a related object to ensure reasonably trouble-free operation of such printers. Another related object is the use of a reasonably simple, elegant design, thereby to reduce maintainance requirements. A further object of the invention is the ability to reliably produce ink jets over a range of droplet repetition rates.

Still other objects relate to image quality; such printers should avoid a tendency toward "drop-out" of droplets in the jet, as well as splattering and other undesirable effects. These printers should permit printing over a reasonable range of distances between nozzle and printing surface. Yet another object of the invention is the design of a flexible, versatile ink jet system. Such a system should accommodate a workable range of ink viscosities, character heights, and image element sizes.

A still further object is to achieve an efficient use of ink in character generation.

SUMMARY OE THE INVENTION In furthering the above and additional aspects, the invention provides an ink jet printer of the "drop-on- demand" type, of a design particularly suited to the printing of large characters. The printer of the invention incorporates a printing head, i.e. drop- projecting device, incorporating an ink-containing pressure chamber, which receives ink from a supply reservoir and intermittently discharges masses of ink to a nozzle assembly. Discharge of ink from the pressure chamber and replenishment of the chamber are effected by an ink impulse assembly, designed on the positive displacement pump principle. An elastomeric membrane forms one wall of the pressure chamber. A moveable rod displaces the membrane to constrict the volume of the pressure chamber, which therefore expels a preselected mass of ink. In the preferred embodiment, ink inflow and outflow from the pressure chamber is controlled by check valves, and the moveable rod is coupled to a solenoid plunger for reciprocating motion.

In accordance with one aspect of the invention, the pressure chamber is configured to contain a volume of ink compatible with a range of masses to be discharged in a given print cycle. In the preferred embodiment, the chamber contains only slightly more than the maximum volume which is to be discharged in a given cycle. The chamber includes input and output ports, and an elastomeric membrane which is sealed to an open face thereof. The output port, which is in fluid communication with the ink feed nozzle, is preferably placed opposite the elastomeric membrane. In the preferred embodiment, the chamber has a radially symmetric shape, such as a frustrum of a cone or spherical cap. The dimensions of the pressure chamber advantageously are chosen to ensure, on the one hand, that a pressure pulse will be effectively transmitted through the output port, and on the other hand, that the discharged ink will not be subjected to excessive perturbations which might lead to formation of "satellite drops" or other fluctuations of the emitted ink jet. In order to provide reasonably high drop production rates, it is desirable to reduce the time required to restore the impulse assembly to its initial state, and to replenish the chamber with ink.

In accordance with another aspect of the invention, the membrane or diaphragm which defines one wall of the pressure chamber consists of an elastomeric material. This membrane is advantageously stretched over an open face of the pressure chamber in a watertight seal. In the preferred embodiment of the invention, the membrane is comprised of a natural or synthetic rubber disk. During a printing cycle, the moveable rod exerts a transverse force at the center of the membrane causing its deformation towards the output port. The stretched membrane exerts an elastic restoring force, which upon removal of the external force on the rod will return these structures to their rest positions in a sinusoidally dampened mannner. This geometry has been observed to enhance ink jet stability, reducing satellite drop formation.

In accordance with a further aspect of the invention, the moveable rod is coupled to a mechanism for intermittently exerting a driving force on the rod and membrane, as described above. In the preferred embodiment, the rod is linked a solenoid plunger. Advantageously, the rod is essentially in contact with the membrane while at rest, so that there will be only a minimal initial impact as the rod is accelerated. In the preferred embodiment, the rod has a cross section approximately corresponding to a central, target area of the opposite chamber wall. The rod exerts a force sufficient to deform the membrane to occupy a volume illustratively between about 25 percent of the original chamber volume, i.e. of a single drop of ink, and essentially the entire chamber volume. The solenoid or other driving mechanism exerts a displacing force over an interval illustratively between less than 1 millisecond and several milliseconds; membrane restoration requires a further interval of several milliseconds.

Stili another aspect of the invention relates to the design of suitable means for controlling the fluid impedance characteristics at the input and output ports of the pressure chamber. In the preferred embodiment of the invention, ink inflow and outflow is controlled by a pair of check valves. During a printing cycle, these valves allow sufficient discharge of ink through the output port while essentially preventing back flow of ink through the input port. During interim periods, the pressure chamber has been at least partially evacuated of ink, thereby causing a pressure differential across the infeed check valve which allows replenishment of the chamber from the ink reservoir.

In the preferred embodiment of the invention, during character production ink droplets are produced at a repetition rate on the order of hundreds of drops per second. Image elements created by such ink drops typically have diameters between 1 and 10 millimeters. The printer, when used for imprinting of a relatively moving object, such as a container, is typically placed at a distance on the order of .5 to 3 centimeters from the printing surface. The drop-on-demand ink jet printers of the invention enjoy substantial operating advantages over prior art printers, particularly those intended for large character applications. The principle of operation of the ink impulse assembly permits the use of printing fluids over a wide range of viscosities, by providing a mechanism for imparting the energy required for producing the jet. This design avoids a problem frequently encountered in similar ink jet printers, i.e. system failures due to entrapped air bubbles. The present ink jet printers provide well formed dot-defined images while enjoying flexibility in various printing parameters such as image element diameter, nozzle to print surface spacing, and print repetition rate. By avoiding the need for a pressurized ink supply, this system eliminates the characterisitc problems of operation at higher supply pressures.

A still further aspect of the invention relates to the design of a multielement printing head for generation of matrix-defined characters. The printing head of the preferred embodiment incorporates a linear array of jet nozzles set in a rotatable nozzle orifice plate. The user may vary the character height by rotating the plate from its orthogonal orientation, to trigonometrically alter the span of the nozzles along the vertical axis. Suitable adjustments are made in the character generation electronics to ensure proper timing for print head actuation.

One particularly advantageous utilization of these ink jet printers is in the marking or coding of articles such as cartons. One or more ink jet printing heads are disposed adjacent a relatively moving conveyor, creating images on detected articles carried thereon. Preferably, an optical encoder monitors relative article motion to coordinate printing head actuation. BRIEE DESCRIPTION OF THE DRAWINGS The above and additional aspects of the invention are illustrated in the following detailed description of the preferred embodiment, with reference to the drawings in which: Figure 1 is a perspective view of a coding system for cartons carried on a moving conveyor, incorporating a plurality of ink jet printing heads in accordance with the preferred embodiment; Figure 2 is a partial exploded view of a single ink jet printing head from the system of Figure 1 ;

Figure 3 is a sectional view of an ink impulse assembly from the printing head of Figure 2;

Figure 4 is a sectional schematic view of the ink-chamber-membrane area of the ink impulse assembly of Figure 3, during deformation of the membrane;

Figure 5 is a plan view of the rotatable nozzle plate from the ink jet printing head of Figure 2, showing alternative orientations of the plate in phantom; Figure 6A is a schematic view of two 5x7 character rasters for the full-height position of the nozzle plate of Figure 5; Figure 6B is a schematic view of two 5x7 character rasters for the 3/4-height position of the nozzle plate of Figure 5;

Figure 6C is a schematic view of three 5x7 character rasters for the 1 /2-height position of the nozzle plate of Figure 5; and

Figure 7 is a partially sectioned view of a given nozzle from the ink jet printing head of Figure 2. DETAILED DESCRIPTION Reference should now be had to Figures 1 -7 for a detailed description of a large character drop-on-demand ink jet printer according to a preferred embodiment of the invention. The ink jet printers of the preferred embodiment have particular utility in the formation of relatively large characters of from 13-50 mm in height. Ink jet printers of this type are well suited to the marking or coding of articles such as the cartons shown in the perspective view of Figure 1. Figure 1 illustrates an ink jet printing system 10 which is disposed adjacent a conveyor belt 150 carrying a series of cartons C. The printing system 10 includes a stanchion 100 in which three printing heads 20A, 20B, and 20C are slideably mounted at an adjustable separation from cartons C. The ink jet printers of the invention are capable of imaging at distances as great as 25 mm, with a preferred separation being on the order of 12 mm. At these distances a printing head 20A produces high resolution dot matrix images with individual dots having diameters on the order of between 1 and 10 millimeters.

Printing system 10 includes a control module 90, illustratively consisting of a multi-character display 92, and a keyboard input panel 91. Controller 90 interfaces with internal electronics 98 such as a microprocessor, which also receives signals from an electrooptical or electromechanical transducer for detecting the movement of cartons C past; the print heads 20. A particularly suitable device for this purpose is an optical encoder (not shown), which translates a mechanical response to carton motion (such as the rotation of a shaft) into interruptions of a light beam which are output as electrical pulses. Such devices provide digital signals of high resolution and reliability, which are employed as timing signals for actuating printing heads 20a-20c in coordination with the progress of cartons C. This encoder compensates for nonlinearities in carton speed relative to print heads 20, thereby avoiding a "bunching" of the dot images. The printing heads of the invention, shown in Figure 1 as incorporated in a stationary in-line coding system, may be adapted to a variety of other configurations such as a hand-held printing gun.

Figure 2 shows an exploded view of an individual printing head 20 from the apparatus 10 of Figure 1. The print head 20 includes a circular cylindrical housing 21 (not shown in this figure) having a nozzle plate 35 at one end. The nozzle plate 35 houses seven nozzles 36a, 36b, etc. which terminate at nozzle orifices 37. Ink is delivered to print head 20 through hoses 115 by an ink supply module 110 (Fig. 1), which includes a tank and filter. Due to the use of internal ink impulse assemblies 50 which pressurize the ink for jet formation, there is no need for a pressurized ink supply. The ink supplied to printing head 20 is subject only to a static head due to ambient air pressure. This avoids a number of significant problems in operation and maintainance, such as the need to purge the system of entrapped air bubbles to ensure efficient jet formation, and clogging of ink within the ink jet nozzles. Ink is routed through supply lines 115 to a manifold plate 45 where it is channelled to seven individual reservoirs 41a - 41g in reservoir block 40 (in a face seal with plate 45).

Plates 25 and 26 house seven ink impulse assemblies 50a - 50g, of which only one is shown in Figure 2. Each of these ink impulse assemblies is in fluid communication with one of the reservoirs 41a - 41 g in reservoir block 40. A given impulse assembly 50 functions to discharge a given volume of ink under pressure to one of the nozzle assemblies 30. The driving force for ink pressurization and ejection is provided by solenoids 70a - 70g, only one of which is shown. In response to an actuating signal from control electronics 98 to solenoid 70a, the ink impulse assembly 50a propels a quantity of ink through output duct 48a to be ejected through nozzle 36a.

Figure 3 shows in section the various elements of an ink impulse assembly 50. This assembly consists of an ink chamber block 51 nested in complementary cavities in plates 25 and 26; a solenoid 70 and plunger 71 housed in plate 27; and a rod 80 fitted to plunger 71. A resilient bumper 85 supported by disk 28 delimits the rearward motion of plunger 71. Ink impulse assembly 50 operates as a positive displacement pump, by intermittently discharging and replenishing ink contained within chamber 60, through the deformation of an elastomeric membrane 65 which forms one wall of the chamber.

Chamber 60 receives ink from reservoir 40 via input port 53 subject to the fluidic control of an input check valve 54, and discharges ink through output port 56 via output check valve 57. The check valve 54 provides a relatively low fluid impedance in the input direction but resists reverse flow. Check valve 57 permits outflow of ink with relatively little reverse flow from the output duct 38 back into chamber 60. As discussed below, the inward deformation of membrane 65 raises the internal pressure of chamber 60 and imparts fluid momentum toward ports 53 and 56. During this period, check valve 57 allows discharge of a certain volume of ink, while check valve 54 prevents a significant backflow through the infeed port 53 which would dissipate energy. As the membrane is restored to its undistorted rest position, a partial vacuum within chamber 60 draws ink from the supply reservoir 41 to replenish the chamber, without a marked reverse flow of ink from the output ducts 38 and nozzle 30. This reduces the extent of retraction of the meniscus at nozzle 30 after ink. ejection, and provides higher drop repetition rates and more reliable jet formation. A particularly preferred type of valve in this apparatus, as illustrated in Figure 3, is the duckbill check valve. This valve characteristically permits controlled flow in the forward direction over a broad range of volumes, i.e. from a single drop to virtually the entire contents of chamber 60.

Chamber 60 desirably has a low volume, i.e. slightly greater than the largest volume of ink which may be discharged therefrom during a given print period. The ink chamber geometry, in such aspects as the ratio of membrane area to chamber volume, and ratio of membrane diameter to depth of the chamber, balances factors such as the efficiency of energy transfer to the discharged ink mass, and the need to avoid secondary perturbations of the ink which might cause nonuniformities in the emitted ink stream. A variety of cross-sections are suitable, but a chamber shape having radial symmetry has particular advantages in combination with a circular membrane which is perpendicular to the axis of symmetry, i.e. ease of machining and superior operational characteristics. The input and output ports may be side-by-side or remotely located, but desirably the output port is located directly opposite a central portion of the membrane so that during membrane deformation the fluid momentum will be directed toward the output valve 57. The illustrated chamber has a flat frustoconical profile, with the input and output ports bordering the central circular section 64. The rod 80 is oriented along the axis of symmetry, and preferably contacts the membrane 65 while at rest. The energizing of solenoid 70 drives the solenoid plunger in direction A (Figure 4), pushing rod 80 against a central portion of membrane 65. The rod 80 advantageously has a cross-section complementing a central portion of the ink chamber. In the preferred, illustrated embodiment, the rod. comprises a circular cylindrical member of a width matching the diameter of the central circular section 64 of chamber 60. This concentrates the energy in the preferred, central region of chamber 60. Contact at rest between rod 80 and membrane 65 is desirable to avoid an initial impact between these structures which would create harmonics in the deformed membrane profile. This in turn has been observed to cause formation of undesirable "satellite drops" in the ink jet.

Solenoid 70 is energized during an interval sufficient to drive rod 80 and membrane 65 at least partially towards the opposite chamber wall 64. Other suitable mechanisms may be employed to displace rod 80, having the requisite response time and force-displacement characteristics. An illustrative range of "on" times for solenoid 70 is between 1 and 3 milliseconds. At the lower end of this range, membrane 65 will be only slightly deformed, sufficiently to expel a single drop of ink through output valve 57. At the other extreme, membrane 65 is forced against the opposite chamber wall, virtually emptying the chamber of ink. During the actuated period, the rod exerts a force on the membrane which depends on such parameters as ink density and viscosity, and plunger displacement. The printing characteristics are considerably more energetic than those of piezoelectric transducers and other prior art oscillatory devices, and afford substantial operating advantages. This system is extremely flexible, permitting a broad range of print fluid volumes, viscosities, drop repetition rates, and other parameters. Such printers provide attractive, high resolution images over a variety of drop diameters and printing distances. These variations are achieved through adjustment of the solenoid actuation characteristics, rather than of an input supply pressure or valve actuation period. By relying on inertial formation of the jet rather than gating of pressurized ink or perturbation of ink under osmotic pressure, this system allows a greater range of effective print distances due to enhanced control over the energy of the jet. This operational principle also permits independent control over dot size and drop repetition rate.

At the end of the solenoid's actuated state, the external force on rod 80 is removed and membrane 65 will rebound elastically (arrow "B" in Figure 4). It has been found preferable to omit any return spring or other external bias, relying only on membrane elasticity to return the membrane to its undistorted profile. This physical arrangement has been observed to provide sinusoidally damped vibration of the membrane, which avoids distortions in jet formation such as satellite drops. This system is thus self-damping, i.e. avoids oscillatory flexture of the membrane 65 by returning energy to the rod 80 and plunger 71. The return stroke of plunger 71 is limited by bumper 85. The membrane 65 advantageously comprises an elastomeric disk or other member which is sealed to the chamber block 51 to prevent leakage of ink (Fig. 3). The method of securing membrane 65 to block 51 preferably should provide a taut condition for the membrane without unduly constraining the membrane at its perimeter. Adhesive bonding has been found unsuitable for this purpose; illustratively, the membrane comprises a natural rubber disk. A suitable thickness for membrane 65 is on the order of 10-30 mils, preferably around 20-25 mils. O-ring 59 ensures that any escaped ink from chamber 60 is contained within the cavity 26-c in plate 26.

The drop repetition rate of print head 20 is largely determined by the on time of solenoid 70, and by the time required to restore the membrane and rod to their rest positions and to replenish the chamber with ink. The solenoid's activated period determines the amount of energy transferred to the ejected ink, and therefore this parameter is dependent on factors such as ink density and viscosity and the volume of ink which is discharged. The time required to restore the system to its initial state is also dependent on these factors as well as hydraulic parameters such as the fluid resistance of valves 54 and 57. Typical figures for solenoid on times and restoration times are on the order of milliseconds; this corresponds to drop repetition rates on the order of hundreds of drops per second.

In the preferred embodiment of the invention, the print head 20a shown in Figure 2 incorporates seven ink impulse assemblies 50a - 50g which propel ink through respective nozzle assemblies 30a - 30g. Figure 5 gives a plan view of a preferred construction of a seven-orifice nozzle plate 35, wherein the orifices 37a - 37g are aligned in a single column at equally spaced intervals. Orifice plate 35 is mounted in an annular frame 39 using lock screws 38, in order to permit rotation from its normal, vertical orientation. Thus, the column of nozzle orifices 37 may be rotated from the vertical position to one of angles θ₁ , and θ₂ with respect to vertical, an adjustment by which the user can reduce character height as explained below.

Figures 6A - 6C give schematic views of 5x7 character rasters which may be produced by the printing head 20a using various orientations of the nozzle plate 35. In the simplest case, in which the plate 35 has a vertical orientation for the production of full height characters, a given character raster I has a spacing X between vertically adjacent dots and a spacing S₁ between adjacent columns. In this instance each raster column corresponds exactly to the printing head column of orifices 37. Figure 6B illustrates the formation of 5x7 rasters for a printing head orientation of angle 6, from the vertical, which is used to generate 3/4-height characters. In this configuration, the column of nozzle orifices 37 correspond to a diagonally adjacent series of raster elements (indicated in Figure 6B by the dotted lines). A particular printed column is formed by successive row-by-row positions of the printing head during which time the printing surface has advanced by the relative distance S₂. Note that the scale of character rasters I and II in Figure 6B correspond to a diagonal separation X between elements. Figure 6C shows three character rasters I-III generated using a printing head orientation of angle from the vertical, for the production of 1/2-height characters. In this case, a given column of printing head orifices 37 correspond to raster elements separated by a horizontal spacing of two elements and vertical spacing of one. This corresponds to a horizontal separation S₃ of adjacent columns, as scaled by a separation X between, for example, column 1, row 7 and column 3, row 6 of character raster I.

Figure 7 shows a partially sectioned elevation view of a nozzle assembly 30, illustrating the profile of a given nozzle 36a. Nozzle 36a includes a bore 31 having a relatively broad portion 32, which narrows to a constricted portion 33 toward the nozzle orifice 37. This venturi nozzle design reduces the influence of ambient conditions such as temperature on the ink jet profile. Taken together with the use of materials for the nozzle and nozzle plate which resist wetting the ink, this nozzle construction resists bleeding of ink and facilitates the formation of individual ink drops from the emerging jet. This nozzle profile encourages the formation of a single umbilicus of printing ink from which a drop will be severed, i.e. reduces the occurence of satellite drops. The invention is further illustrated in the following example, which is intended to give a specific operative embodiment without in any way limiting the scope of the invention. EXAMPLE A print head 20 of the type illustrated in Figures 2- 5 and 7 incorporated as its superstructure a series of cylindrical plates mounted within an elongate circular cylindrical housing. The nozzle plate 35, manifold plate 45, and reservoir block 40 were molded from

Teflon (a registered trademark of E.I. DuPont DeNemours & Co., Wilmington, DE, for tetrafluoroethylene polymers), while the remaining structures included a nozzle backing plate 34 were formed from stainless steel.

The nozzle plate 35 included seven circular apertures were bored at a center spacing of 6 mm. Each aperture housed a stainless steel jet outflow nozzle 36 having a profile illustrated in Figure 7. The nozzle channel 31 had a broad portion 32 of inner diameter .6 millimeter narrowing to a segment 33 of inner diameter .2 millimeter at the nozzle orifice 37. The nozzle was 16 mm in length; the broad diameter segment 10 mm, and the narrow diameter segment and transition region 6 mm. The nozzle plate 35 was rotatably seated in an annular plate 39 and secured thereto with lock screws 38.

The ink chamber block 51, formed of stainless steel, included broad and narrow portions respectively nested in complementary cavities in plates 25 and 26. The ink chamber 60 was shaped as the frustrum of a cone with a major diameter of 4.6 mm, chamber depth of .76 mm, with a slope of 45° for the side of a given axial section. The chamber volume was approximately .02 cubic centimeter. Input and output ports 53 and 56 were centered on opposite endpoints of a given diameter of the central circular section 64. The elastomeric membrane 65 was a natural rubber circular disk 6 mm in diameter and .6 mm thick. Duckbill check valves 54 and 57 were fitted to the input and output ports as shown in Figure 3. A teflon rod 80 having an outer diameter of 3.0 mm included an internal bore and was press-fitted to the solenoid plunger 71. Rod 80 was held in light contact with membrane 65 by limiting the travel of plunger 71 using a soft rubber bumper 85. The solenoid 70 was actuated for membrane displacement using pulsed voltages with a duration of between 1 and 3 milliseconds, depending on the desired diameter of the printed dots. The printing head was employed for the projection of a black printing ink having a viscosity of 2.0 centipoise. Three printing heads as described were incorporated in a carton coding system 10 of the type shown in Figure 1. The nozzle orifice plates 35 were located at a distance of 13 mm from the cartons. Progress of cartons C past the printing heads was monitored using an R2000 optical encoder of Renco Corporation, Goleta, CA, which provided timing signals to control circuit 98. Cartons were coded using 5x7 dot matrix alphanumeric characters. By adjusting the nozzle plate orientation as shown in Figure 5, the system provided character heights of 42, 28, and 21 mm. Individual dots had diameters of between 2 and 8 mm, and were well-formed, with no observable splattering of ink or fall-off of droplets. While various aspects of the invention have been set forth by the drawings and the specification, it is to be understood that the foregoing detailed description is for illustration only and that various changes in parts, as well as the substitution of equivalent constituents for those shown and described, may be made without departing from the spirit and scope of the invention as set forth in the appended claims. Although this apparatus has been illustrated in the context of a drop-on-demand printing system, the principles of the invention are applicable to other liquid jet applications.

Claims

WE CLAIM:

1. Liquid jet apparatus, comprising: a source of liquid; a jet outflow nozzle; a chamber block including a chamber in fluid communication with said liquid source through an input port, and with said jet outflow nozzle through an output port; an elastomeric membrane forming one wall of said chamber; impulse means for intermittently elastically distorting said elastomeric membrane so as to constrict the volume of said chamber; and means for controlling said impulse means to control the flow of liquid from said chamber to said outflow nozzle.

2. Liquid jet apparatus as defined in claim 1 for ink jet printing, wherein the liquid is a printing ink.

3« Apparatus as defined in claim 1, wherein the impulse means comprises a rod, and means for linearly displacing said rod to inwardly displace said membrane.

4. Apparatus as defined in claim 3, wherein the impulse means comprises a solenoid and plunger, a rod coupled to said plunger, and means for energizing said solenoid to displace said plunger and rod.

5. Apparatus as defined in claim 4, wherein the rod contacts said membrane while said solenoid is in an inactive state and said membrane is undistorted.

6. Apparatus as defined in claim 4, wherein after the energized interval of said solenoid the membrane and rod and plunger return to their rest positions solely due to the elastic rebound of said membrane.

7. Apparatus as defined in claim 1, wherein the chamber's shape is radially symmetric, and the membrane is secured to a circular rim of said chamber.

8. Apparatus as defined in claim 1, wherein the membrane is comprised of a material selected from the group consisting of natural and synthetic rubbers.

9. Apparatus as defined in claim 1, further comprising two check valves respectively at said input and output ports, said, input check valve providing relatively high fluid resistance in the chamber outflow direction, and said output check valve providing relatively high fluid resistance in the chamber inflow direction.

10. Apparatus as defined in claim 9, wherein said check valves comprise duckbill check valves.

11. Apparatus as defined in claim 2, for generating matrix-defined characters, comprising a plurality of ink chamber blocks, elastomeric membrane, and impulse means in cooperation with a linear column of jet outflow nozzles.

12. Apparatus as defined in claim 11, wherein the jet outflow nozzles are mounted in a rotatable nozzle plate, said nozzle plate being rotated from a vertical printing axis to produce angularly arrayed printed columns, wherein said control means adjusts the timing of the jet nozzle outputs to produce orthogonal character rasters of reduced height.

13. An ink jet-printing process, comprising the steps of: feeding a printing liquid to a chamber within a chamber block, said chamber being walled in by an elastomeric membrane; intermittently elastically distorting said elastomeric membrane to constrict the volume of said chamber and force at least part of the printing liquid within the chamber to a jet nozzle to be projected therefrom; and permitting the relaxation of said membrane and replenishment of said chamber with printing liquid.

14. An ink jet printing process as defined in claim 13, wherein the step of elastically distorting said membrane comprises inwardly displacing said membrane with a rod.