GB2024724A - Reed head assembly for ink jet printing - Google Patents

Abstract

A reed head assembly for high speed ink jet printing, the assembly comprising a clamped metal reed (20) carrying a capillary tube (30), and a coil (45) and permanent magnet (40) to produce transverse vibrations of the tube to provide scanning. An ultrasonic transducer (25) produces longitudinal vibrations of the tube to assist the formation of ink droplets (60.5). <IMAGE>

Description

SPECIFICATION Reed head assembly for ink jet printing.

This invention relates to reed head asembliesfor ink jet printing.

According to the present invention, a reed head assembly for ink jet printing comprises a capillary tube having a discharge outlet at one end, a reed joined to the tube, and means for inducing a macroscopic oscillation in the joined reed and capillary tube in a direction substantially transverse to the axis of the capillary tube.

Preferably the assembly includes a clamp located intermediate the length of the capillary tube and fixing the capillary tube to the reed and a mount for the tube located on the side of the clamp remote from the discharge outlet. The reed head may include means for inducing in the joined reed and capillary tube a microscopic longitudinal vibration, such means possibly being located between the mount and the clamp and comprising an ultrasonic transducer yoked 10 the reed.

These and other features of the invention will be further considered in the following discussion.

Ink jet printing is a printing method involving projection of ink onto a recording medium. Several currently used methods of ink jet printing employ a scanning nozzle. Emission of ink is produced by the action of a pump which forces ink through the scanning nozzle, and image formation is effected by creating a sinusoidal trace of the projected ink. The latter result is obtained by vibrating the nozzle at a given frequency and moving a recording medium in a direction transverse to the line of vibration. The rate of character generation is generally proportional to the frequency oi nozzle vibration, and hence higher frequency nozzle oscillations require a greater amount of ink flow for images of uniform quality.

Nozzle vibration has been achieved by a variety of methods. The Heriz system of ink jet printing produces nozzle o:;cillation by use of a galvano meter. The galvan3meter, however, is both expensive and incapable of consistently producing high speed vibrations. Alternative methods of achieving nozzle vibration rely upon solenoid drives in place of a galvanometer. These methods are also incapable of effecting high speed printing and, in addition, are mechanically complex and costly. Still another method of ink jet printing employs a vibrating reed to produce nozzle vibration. Although this method is both less expensive and less complex than galvano meter or solenoid drive methods, it is unable to effect a high quality printing at speeds greater than 100 characters per second.Moreover, even at that speed, print quality is inconsistent. Furthermore, printing of acceptable quality with a vibrating reed system requires the recording medium to pass within about 3 mm of an electrode assembly which is used to control the flight of the ink drops. As a result the printing of curved or recessed surfaces is difficult and often impossible.

In the vibrating reed system, the problems of inconsistent print quality and the low effective range of the ink jet stream may both be traced to the haphazard separation of the ink stream into individual drops. Various devices have been incorporated into prior systems with a view to regulating ink drop formation; more particularly, these devices represent an attempt to control the situs of ink stream breakup, the size of the ink drops, and the spacing of the drops. A method generaily employed in this regard involves inducing a pulsation of the ink steam at a controlled frequency. This has sometimes been accomplished by means of a transducer which is incorporated into the capillary structure, and placed in intimate contact with the ink stream, perhaps separated by a membrane.These devices, however, possess the shortcoming that the vibrations by which the ink stream pulsation is produced have transverse as well as longitudinal components with respect to the axis of the capillary tube. They therefore interfere, to some extent, with the transverse oscillations of the nozzle by which the scanning pattern is created for printing. This effect is especially pronounced at higher printing speeds (higher frequencies oftransverse nozzle oscillation).

The ability to print at higher speeds is correlative to a higher attainable frequency of character generation, which in turn is dependent on the resonant frequency of the reed. The speed limitations mentioned above with respect to the prior ink jet systems employing a scanning nozzle may be identified with a characteristic upper frequency limit for nozzle oscillation of around 1 KHz. For a vibrating reed system, a higher resonant frequency demands a shorter metal reed. Furthermore, to avoid high current demands in an electromagnetic means for inducing reed oscillation, a thin reed is desirable.

Prior vibrating reed systems have not incorporated a reed with these properties, as such a system necessitates a more compact design, and imposes more rigorous construction tolerances in avoiding spurious resonances which might interfere with the high frequency reed oscillation.

Note: the term "nozzle" has heretofore been used in the sense of the entire tube through which ink is projected; the discussion of the reed head assembly of the present invention, however, assumes the also accepted usage of a structure capping the end of the tube.

It is believed that the present invention makes possible ink jet printing at an increased rate of speed and at an increased rate of character generation. The vibrating reed ink jet printing system may incorporate a short, thin vibrating metal reed.

It is also probably possible to achieve ink jet printing with improved quality of print at higher printing speeds. Certain features of the invention make it possible to increase the effective range of the ink jet stream and thus to obtain printing of acceptable quality upon materials at a greater distance from the ink jet nozzle. Thus, it is easierto print curved or recessed materials.

The invention may be carried into practice in various ways but one reed head assembly embody ing the invention will now be described by way of example with reference to the accompanying draw ings, in which: Figure 1 is a plan view of the reed head assembly; Figure 2 is a view of the coil area of the reed head assembly to a larger scale; Figure 3A is a plan view of the ultrasonic transducer area of the reed head assembly, as seen from the side; and Figure 3b is a plan view of the ultrasonictransduc- er area of the reed head assembly as seen from below.

The reed head assembly 10 is comprised of a metal reed 20, a reed clamp 22, an ultrasonic transducer 25, a capillary tube 30, a capillary tube mount 32, a nozzle 35, a permanent magnet 40, and a coil 45. The capillary tube 30 is secured at one end by the capillary tube mount 32 and is capped at the opposite end by the nozzle 35, which is in this case a drawn out portion of the capillary tube. A filter (not shown) is placed in the capillary tube to prevent infiltration by unwanted particles. The capillary tube 30 passes'through the reed clamp 22 which is located at its midsection, and is yoked along the remainder of its length to a metal reed 20, which reed in turn is secured by the reed clamp 22.The use of a separate capillary tube mount and reed clamp, structures which have hitherto characteristically been combined, facilitates the use of a short vibrating metal reed (that is, a reed which is substantially shorter than the capillary tube). Ink 60 passes through the capillary tube 30 and is emitted through the nozzle 35. The ink 60 is forced through the capillary tube 30 due to the fluid pressure produced by a pump (not shown).

Avertical vibration of the reed 20 and attached capillary tube, shown by the arrows 30.5 in Figure 2, is created by the interaction of a magnetization of its reed 20 along its length with a magnetic field (not shown). The reed 20 is magnetized by a coil 45, through which the reed 20 and the capillary tube 30 pass. The magnetic field is produced by a permanent magnet 40. The interaction between the magnetic moment induced in the metal reed 20 and the permanent magnet's field creates a torque on the reed 20, forcing it to bend towards one of the poles, 40+ or 40-, of the permanent magnet 40, as shown in Figure 2. When the current in the coil 45 is reversed, an opposite torque is created and the reed is forced towards the other pole.When the coil current is alternating at a frequency near the lowest resonance of the reed. viewed as a cantilever beam anchored at the reed clamp 22, a large amplitude motion (of the order of several millimetres) is produced.

In addition to this macroscopic vertical vibration 30.5 a microscopic longitudinal vibration, shown by arrows 20.5 in Figure 2, is created along the length of the reed 20 by the ultrasonic transducer 25. The ultrasonic transducer 25 is attached to a stationary portion of the reed 20 at a point beyond the reed clamp 22. The microscopic longitudinal vibration 20.5 of the reed 20 creates a corresponding longitudinal vibration 35.5 of the attached capillary tube 30 and nozzle 35. The longitudinal vibration 35.5 of the nozzle 35 determines the point at which the stream of ink 60 emitted from the nozzle 35 breaks up into drops, as shown at 60.5 in Figure 2. This vibration also ensures uniformity of drop size, which in turn increased the effective distance over which the ink stream 60 will travel.The permissible distance between the reed head assembly 10 and the object to be imprinted (not shown) is thereby increased.

The longitudinal vibration is produced by a piezoelectric ceramic device attached to the stationary section of the metal reed beyond the reed clamp 22, in order that the transverse oscillation 30.5. of the reed does not hinder the operation of the device.

Such a device may take a number of forms, but all share the characteristic that they impart strictly longitudinal vibrations to the metal reed 20, vibrations which are microscopic in amplitude as compared with the typically several millimetre transvere oscillation 30.5 of the joined metal reed and capillary tube. Thus, the longitudinal vibrations 20.5 and 35.5.

do not significantly interfere with the transverse vibrations 30.5.

The ultrasonic transducer 25 is illustrated in Figures 3A and 3B and includes a piezo-electric ceramic of cylindrical form. The side view, Figure 3A, displays a relatively thick transducer 25 appended to a thin metal paddle 27 at the end of the metal reed 20. The disproportionate thickness of the transducer 25 (which is advantageously eight to ten times the thickness of the metal reed 20 and paddle 27) causes the paddle 27 to readily pick up any vibrations in the horizontal plane (the plane of interface) of the transducer 25. The vibrations of the transducer 25 result from the piezo-electric effect of a voltage (not shown) which is applied between the paddle 27 and the bottom surface of the transducer 25. This causes a radial expansion and contraction of the cylindrical transducer 25.

The bottom view, Figure 3B, illustrates the process of transmittal of vibrations. The piezo-electric ceramic 25, round in cross-section, is roughly the same size as the paddle 27, which is square in crosssection with truncated corners. The radially expanding and contracting oscillations of the transducer 25, shown by arrows 25.5, are transmitted to the coupled metal paddle 27 to produce similar oscillations. The metal reed 20, connected to the middle of one side of the metal paddle 27, picks up only the oscillations of the paddle 27 which are oriented along the axis of the reed. This results in almost purely longitudinal vibrations 20.5 of the metal reed 20. The partially shown reed clamp 22 is preferably constructed of a plastics material which will allow the microscopic longitudinal vibrations 20.5 to pass through unabated, without any spurious resonances added (these would be present, for example, with a metal clamp).

Printing occurs when selected ink drops 60.5, having been screened by their passage through an electrode structure (not shown), reach the object to be imprinted and form the desired pattern.

Claims (16)

1. A reed head assembly for ink jet printing comprising a capillary tube having a discharge outlet at one end, a reed joined to the tube, and means for inducing a macroscopic oscillation in the joined reed and capillary tube in a direction substantially trans verse to the axis of the capillary tube.

2. A reed head assembly as claimed in Claim 1 in which the reed is of metal.

3. A reed head assembly as claimed in Claim 2 in which the reed is joined along its length to the capillary tube.

4. Areed head assembly as claimed in Claim 1 or Claim 2 or Claim 3 in which includes a clamp located intermediate the length of the capillary tube and fixing the capillary tube to the reed.

5. A reed head assembly as claimed in Claim 4 in which the clamp is of plastics material.

6. A reed head assembly as claimed in Claim 4 or Claim 5 which includes a mount for the tube located on the side of the clamp remote from the discharge outlet.

7. A reed head assembly as claimed in Claim 6 which includes means for inducing a microscopic longitudinal vibration of the capillary.

8. A reed head as claimed in Claim 7 in which the means for inducing a microscopic longitudinal vibration is located between the mount and the clamp.

9. A reed head assembly as claimed in any of Claims 1 to 5 which includes means for inducing in the joined reed and capillary tube a microscopic longitudinal vibration.

10. .A reed head assembly as claimed in Claim 7 or Claim 8 or Claim 9 in which the means for inducing a microscopic longitudinal vibration comprises an ultrasonic transducer yoked to the reed.

11. A reed head assembly as claimed in Claim 10 in which the ultrasonic transducer is yoked to the reed at an end remote from the outlet of the capillary tube and is formed by a metal paddle extension of the reed having the same thickness but a greater width than the remainder of the reed, and a piezoelectric ceramic element of greater thickness than the paddle extension attached to one broad face of the paddle extension, and means for applying a potential difference between the paddle extension and the surface of the piezo-electric ceramic element remote from the paddle extension to produce a microscopic oscillation of the element with a longitudinal component ihat is transmitted through the paddle extension -:o the reed.

12. A reed head assembly as claimed in Claim 11 in which the piezo-electric ceramic element is round in cross-section.

13. A reed head assembly as claimed in Claim 11 in which the piezo-electric ceramic element is rectangular in cross-section.

14. A reed head assembly as claimed in any of the preceding claims in which the means for producing the macroscopic oscillation of the joined reed and capillary tube comprises: a magnetic coil which surrounds the joined reed and capillary tube near the outlet end of the capillary tube, means for passing an alternating current through the coil at the lowest resonant frequency of the reed considered as a cantilever beam to induce a magnetic moment within the reed, and a permanent magnet, having poles which straddle the magnetic coil to caused a torque on the metal reed by an interaction of its field with the magnetic moment induced in the reed.

15. A reed head assembly as claimed in any of the preceding claims in which the reed is joined to the capillary tube by a clamp of plastics material.

16. A reed head assembly for ink jet printing, the assembly being constructed and arranged to operate substantially as described herein with reference to the accompanying drawings.