EP0283226A2

EP0283226A2 - Nozzle assembly for an ink jet printer

Info

Publication number: EP0283226A2
Application number: EP88302195A
Authority: EP
Inventors: Mark Smith
Original assignee: Willett International Ltd
Current assignee: Willett International Ltd
Priority date: 1987-03-17
Filing date: 1988-03-14
Publication date: 1988-09-21
Also published as: EP0283226A3; GB8706338D0

Abstract

The present invention provides a nozzle assembly for discharging a fluid as a jet through a nozzle outlet to the assembly, which jet is to be broken up into discrete substantially uniformly sized and spaced apart droplets, characterised in that the assembly comprises a body member (2) having an axial fluid chamber (1) therein which chamber (1) has a generally axially directed fluid outlet (6) located at one end of the chamber (1) and a closed other end, said body member (2) having one or more transverse inlets (7) to the axial fluid chamber (1) whereby fluid can flow through the axial chamber (1) for discharge through the outlet (6); the other end of the body member (2) having affixed thereto through a vibration transmitting mounting a transducer (10), notably a piezoelectric crystal (10), which is adapted to impart axial vibration to the assembly, notably by axial expansion and contraction of the piezoelectric crystal (10) upon the application of an electrical potential thereto; the nozzle assembly having a resonant frequency under the conditions of intended use which is closely adjacent to the frequency of excitation of the transducer (10); the assembly being adapted to be mounted at a point intermediate the two ends of the body member (2).

The invention also provides a print head incorporating a nozzle assembly of the invention.

Description

The present invention relates to a nozzle assembly for an ink jet printer, notably one in which a jet of ink issues from the nozzle and is broken up into discrete uniformly sized and spaced apart droplets.

BACKGROUND TO THE INVENTION:

In continuous ink jet printing, ink is caused to flow throughout the printing period through a fine orifice nozzle so as to form a jet of ink. This jet is broken up into a stream of substantially uniformly sized and spaced apart droplets which are used to form the printed image on the paper or other substrate. The jet can be broken up by a number of methods.
In one form of apparatus, the jet of ink is broken up by means of pressure waves within the ink. Thus, a piezoelectric crystal in the form of a rod extends axially within an ink chamber towards and immediately adjacent the nozzle orifice serving as the outlet to that chamber. The crystal is caused to extend and contract axially by apply electrical pulses to the crystal and this causes the jet to break up. In order to achieve maximum extension of the rod at the desired frequency of operation, the length of the rod is tuned so that its length is that of a half wave length at its resonant frequency. In another example of such an apparatus, a wall of the ink chamber serving the nozzle orifice is caused to flex under the influence of a piezoelectric crystal so as to generate pressure pulses within the ink flowing through the nozzle orifice. In another example of such an apparatus described in US patent 3512172, the nozzle is provided with a pair of piezoelectric crystals mounted on either side of an elongated nozzle chamber and bearing against the mounting of the nozzle assembly. Expansion and contraction of the crystals causes the chamber to extend and contract, thus altering the radial dimensions of the chamber so as to expel ink from the nozzle orifice. In a further form of such a device, as described in US patent 3683396, a piezoelectric crystal is mounted around an elongated tubular nozzle chamber to cause the ink jet issuing from the nozzle orifice located at one end of the tube to be broken up into discrete droplets. However, in order to achieve optimum power transfer through the fluid in the chamber to the jet, the nozzle tube has to be an odd multiple of a quarter of a wave length of sound through fluid in the chamber.
Such nozzle assemblies require accurate design and construction of their components if the resonance criteria are to be achieved. Furthermore, once made, such a nozzle assembly cannot be used in applications where other frequencies of operation are required.
In an alternative form of apparatus, the nozzle assembly itself is caused to vibrate axially so as to cause the jet of ink issuing from the nozzle orifice to break up into droplets. Such an apparatus is described, for example, in US patent 4290074 and comprises a pair of piezoelectric crystals mounted between two parts of a nozzle assembly at the nodal point of the assembly. The excitation of the crystal causes the free end of the nozzle to vibrate axially and the shape of the nozzle is selected so as to maximise movement of the nozzle tip at which the orifice is located. However, such an apparatus is complex and requires accurate construction and assembly of the components thereof. Once an optimum construction has been achieved with a given nozzle assembly, that nozzle cannot readily be used in another application requiring a different frequency of operation. Furthermore, a considerable portion of the input energy will be dissipated through the axial ink inlet and through the mountings of the apparatus.
In those forms of apparatus where the nozzle itself is vibrated by the action of a piezoelectric crystal or other transducer, the nozzle tip vibrates at the frequency of excitation of the crystal or transducer. In order to optimise the vibration of the nozzle, this frequency is selected so as to be as close to the resonant frequency of the nozzle assembly as practical. This can pose manufacturing problems, not only in securing the required accuracy of manufacture on a commercial scale, but also in ensuring that numbers of nozzles can be manufactured with consistent vibration characteristics. This latter is important where the nozzle assembly is to be used in combination with other nozzles in a single piece of apparatus and/or where the nozzle is to be operated in synchronisation with other components of an ink jet printer. For example, in many forms of continuous ink jet printer, the jet of ink passes through a charge electrode so that the individual droplets are given an electric charge. These then pass through another electric field which causes the charged droplets to be deflected from their straight line of flight. The size of the charge on the droplet and/or the size of the deflection field is adjusted for each of the droplets so that that droplet is deposited at the desired position on the paper or other target on which the ink is being printed.
We have now devised a form of nozzle assembly which reduces the above problems and provides a simple form of construction which can be readily and simply tuned to a range of resonant frequencies. Furthermore, the nozzle assembly can readily be cleaned in situ or in a solvent bath without the need for a separate ultrasonic cleaner as currently considered essential, which results i a saving in costs and equipment to the user.

SUMMARY OF THE INVENTION:

Accordingly, the present invention provides a nozzle assembly for discharging a fluid as a jet through a nozzle outlet to the assembly, which jet is to be broken up into discrete substantially uniformly sized droplets, characterised in that the assembly comprises a body member having an axial fluid chamber therein which chamber has a generally axially directed fluid outlet located at one end of the chamber and a closed other end, said body member having one or more transverse inlets to the axial fluid chamber whereby fluid can flow through the axial chamber for discharge through the outlet; the other end of the body member having externally mounted thereon a transducer, notably a piezoelectric crystal, which is adapted to impart axial vibration to the nozzle outlet, notably by axial expansion and contraction of the piezoelectric crystal upon the application of an electrical potential thereto; the nozzle assembly having a resonant frequency under the conditions of intended use which is closely adjacent to the frequency of excitation of the transducer; the assembly being adapted to be mounted at a point intermediate the two ends of the body member.
We have found that the assembly can be readily manufactured using conventional metal working or other techniques with sufficient accuracy to enable assemblies with consistent resonant frequencies to be made directly on a commercial scale. Such nozzle assemblies can be deemed to comprise a resonant rod having an axial bore extending part of the length of the rod. It is particularly preferred preferred that the assembly carry a counter mass upon the exposed end of the transducer so that tuning of the assembly can be carried out merely by adding or removing material from the counter mass. Such a construction enables tuning of the assembly to be carried out simply and accurately without the need to dismantle the assembly, as would be required with prior art designs.
The assembly of the invention can be mounted at or adjacent a nodal point of its vibration so that little or no energy is lost through its mounting, thus optimising the usage of the power input and reducing the risk of overheating of the assembly. It is also preferred that the fluid inlet or inlets be located at or adjacent the nodal point so that little or no energy is lost through the ink supply or supply line assembly, as occurs when ink is fed axially into the chamber through an open end of the ink chamber as with previous designs.
It is particularly preferred that the transducer be mounted wholly externally of the body member, notably upon the closed end face of the body, and that the transducer be a piezoelectric crystal.
We have found that the nozzle assembly of the invention can readily be cleaned in a suitable solvent by exiciting the transducer which simulates the action of an ultrasonic cleaner. The nozzle assembly can thus be cleaned without the need for a separate cleaner mechanism.
The invention is of particular use with ink jet printers in which the fluid flowing through the nozzle assembly is a conductive ink so that it can be charged and deflected as described above. However, the nozzle assembly of the invention can be used with non-conductive ink compositions or for the application of other fluid compositions. For convenience, the invention will be described hereinafter in terms of the use of an ink.

DESCRIPTION OF THE DRAWINGS:

To aid understanding of the invention, it will be described with respect to a preferred form of the nozzle assembly as shown in the accompanying drawing which is a vertical diagrammatic view through the nozzle assembly.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION:

The nozzle assembly comprises an elongated ink chamber 1 within a body member 2. The chamber 1 has a transverse ink inlet 3 located adjacent the nodal point of the assembly for feeding ink into the chamber from a source of pressurised ink 5. In a preferred form, an ink outlet 4 is also provided opposite inlet 3 so that the nozzle can be purged during start up by passing ink through the chamber. The inlet and outlet can be provided with suitable fittings, eg. pipes 7 and 8, for connecting them to the ink flow circuit of the print head in which the nozzle assembly is mounted.
The chamber has at one end thereof a conventional jewel nozzle orifice 6 for discharging ink axially from the chamber. Suitable jewel orifices are available commercially and the optimum bore diameter and bore length for the orifice can be selected in known manner having regard to the operating requirements for the nozzle assembly.
The other end of the chamber is closed by a transverse wall member. Preferably, this wall is formed integrally with the remainder of the body 2. Typically, the wall is provided as the blind end of an axial bore into a rod of the material forming the nozzle body member 2. Preferably, the axial bore or chamber extends to what will be the nodal point of the nozzle assembly with the inlet 3 and outlet 4 being located at the blind end of the chamber.
The body 2 can be formed from a wide range of materials, for example ceramics or rigid plastics. However, it is preferred that the body be made from a material which is a good acoustic conductor in which the speed of sound is at least 6 Km per second, such as those used for the acoustic components in ultrasonic welding equipment, eg.a metal such as steel, stainless steel, aluminium brass or titanium.
At the opposite end of body 2 to the jewel nozzle orifice 6 is mounted the transducer for driving the nozzle assembly. As indicated above, this is preferably a piezoelectric crystal 10. Such crystals are readily available commercially and a suitable one can be selected to cause the nozzle assembly to vibrate at the desired frequency having regard, as is known in the art, to the operating conditions required of the printer in which the nozzle assembly is to be mounted. Typically, the crystal will operate at frequencies of from 20,000 to 200,000, eg. 70 to 80,000, Hertz, with a nozzle orifice 70 micrometres in diameter.
As indicated above, the crystal 10 is mounted externally on the end face of body 2 via a mounting which transmits vibration well so that the maximum energy is transmitted from the crystal to the body 2 and distortion is minimised at the interface between the crystal and body 2. We have found that a particularly suitable form of mounting comprises a direct body/crystal contact through a material through which sound travels at a speed closely similar to that at which it travels through the material of the body.
Thus, the crystal can be clamped securely upon the free end of body 2 by mechanical means to provide a direct body/crystal contact with the free end of the crystal exposed. Alternatively, the body can be formed with ridges or other protrusions which provide point contacts between the crystal and the body, the crystal being secured in place by clamping or by a suitable adhesive. In a further alternative, an interface gauze 11 of the body material can be provided between the crystal and the body to provide the electrical coupling between the crystal and the body, and the crystal secured upon the body by a suitable adhesive in which the gauze is imbedded. If desired, the crystal can be partially inset into the end face of the body to assist location and retention of the crystal on the body.
In order to excite the crystal 10, it is necessary to apply an electric potential across the crystal as is known through electrical contacts 12 and 13 on the end faces of the crystal. The mounting between the crystal and the body provides an electrical connection to the body whereby a charge can be applied to the jet of ink issuing from the nozzle orifice. Alternatively, the contact 13 can be to the body rather than to the crystal, as shown.
The above nozzle assembly can be readily manufactured from commercially available materials using conventional techniques. For example, it can be made by drilling the desired axial bore substantially co-axially into a cylindrical rod of metal so that the body 2 possesses radial symmetry about its longitudinal axis; forming the transverse ink inlet and outlet bores and providing ink inlet and outlet fittings, eg. simple metal tubes 7 and 8, welded or otherwise fixed into those bores; and securing the jewel nozzle orifice 6 into the open end of the bore. As will be appreciated, such a form of the nozzle requires the machining of one main component, the nozzle body, and this can be carried out with a high degree of accuracy using conventional metal working techniques. Jewel nozzle orifices manufactured to a high degree of accuracy are commercially available. The nozzle assembly can thus be manufactured reproducably and consistently on a commercial scale without specialised requirements.
The above composite assembly will have a resonant frequency and the assembly can be designed so that this is close to, eg. within ± 5% of, the required operating frequency for the nozzle assembly. However, such a nozzle assembly would be useful for a narrow range of frequencies. It will therefore usually be preferred to design and construct the assemblies of the invention so that material can be readily added to or removed from the assembly, usually from the body 2 or a countermass attached to the exposed end of the piezoelectric crystal, so as to vary the resonant frequency of the assembly. In this way a standard nozzle assembly can be manufactured for use at a given frequency. However, by adjustment of the countermass the nozzle assembly can be readily modified for use at other frequencies without the need to use special components or to disassemble the nozzle in order to interchange one component for another.
As shown, the counter mass 20 is preferably mounted on the exposed end face of crystal 10 and material is added to or removed to tune the assembly to the desired resonant frequency. By mounting the mass 20 beyond the resonating body 2 so that it does not have to transmit vibration between the crystal and the body or to vibrate as part of the body 2, the acoustic properties of the mass 20 and its mounting on crystal 10 have comparatively little effect upon the behaviour of body 2. Thus, mass 20 can be made from any suitable material, for example an epoxy resin.
We have also found that the shape of the nozzle end of body 2 can affect the width of the frequency range over which the assembly resonates. We prefer to form the nozzle end of body 2 with a taper or bevel 30. Typically, the taper is a 30 to 60° circumferential bevel as shown.
The assembly is preferably mounted by clamps or other suitable means acting at or adjacent a nodal point intermediate the transducer and the nozzle end of body 2 so as to minimise the energy lost through the mounting. Although each assembly of the invention can be tuned so that it will have substantially the same resonant frequency as other assemblies, the exact location of this nodal point may vary from assembly to assembly. This would require minor and accurate variation of the exact position of the mounting to compensate for this variation and to ensure that the mounting was always at the optimum position on the assembly.
We have found that this problem can be reduced if the body of the assembly is held in a mounting made from a material in which sound travels at a speed substantially different from that at which it travels in the material of the body. This provides an acoustically reflective discontinuity at the interface between the body and the mounting and reduces the transmission of energy between the body and the mounting. Typically, the material of the mounting at this interface has a speed of sound transmission which is less than, preferably half to one third, that of the material of the body. Thus, it is particularly preferred to mount the nozzle assembly as a push fit into a hard plastics clip or socket 50. Since the socket 50 does not interfere to a significant extent with the axial vibration of the assembly, it can support the assembly at virtually any point. However, it is preferred that the assembly be mounted in the vicinity of its nodal point.
We believe that the use of such mountings for nozzle assemblies in ink jet printers is novel and the invention therefore further provides a nozzle assembly for an ink jet printer characterised in that the assembly is provided with a mounting in which that portion of the mounting in contactn with the assembly is made from a material which conducts sound at a speed which differs significantly, preferably by at least ± 33%, from the speed at which sound is conducted in the material from which that portion of the assembly in contact with the mounting is made.
The invention further provides an ink jet print head comprising a nozzle assembly which is to be vibrated, preferably by means of a transducer, said assembly being mounted in the said print head by means of a mounting in which that portion in contact with the assembly is fabricated from a material which conducts sound at a speed which differs significantly from the speed at which sound is conducted in the material from which that portion of the assembly in contact with the mounting is made.
The nozzle assembly of the invention is used in an ink jet printer in essentially the same manner as a conventional nozzle assembly. Thus, the assembly can be used in a conventional continuous ink jet printer in which ink is fed under pressure, typically at pressures of from 0.1 to 10 bar, from a reservoir 5 to the inlet 7 to the chamber 1. Upon operation of the piezoelectric crystal 10, the jet of ink issuing from the nozzle orifice 6 is broken up into discrete droplets in the region of the charge electrode 40 to provide charged droplets 41. The charged droplets pass deflection electrode 42 which causes charged droplets to be deflected by an amount corresponding to the charge carried by the droplet. Uncharged droplets are not deflected and are caught in gutter 44 for recycle to reservoir 5. The deflected droplets 41 form a desired print image on substrate 43 moving past the print head.
The nozzle assemblies of the invention can be fitted to new machines or as a replacement for nozzle assemblies in existing machines and can be used in the same manner as conventional nozzle assemblies. However, as indicated above, the nozzle assemblies of the invention readily lend themselves to ultrasonic cleaning to remove adherent dried ink or other contaminants. The magnitude of the amplitude of the vibration of that end of the body 2 carrying the nozzle orifice 6 can be increased by increasing the applied potential to the piezoelectric crystal. Thus, when the orifice tip is immersed in a cleansing solvent, either in situ or by removing the nozzle and immersing it in a suitable container, and the crystal excited, the tip will vibrate at an ultrasonic speed. The effect is substantially the same as when an ultrasonic signal generator is immersed in the solvent, as is done for conventional ultrasonic cleaning of the nozzle orifice. The nozzle assembly of the invention can thus be cleaned without the need for a separate ultrasonic generator, enabling savings in cost and equipment to be made.

Claims

1. A nozzle assembly for discharging a fluid as a jet through a nozzle outlet to the assembly, which jet is to be broken up into discrete substantially uniformly sized droplets, characterised in that the assembly comprises a body member (2) having an axial fluid chamber (1) therein, which chamber (1) has a generally axially directed fluid outlet (6) located at one end of the chamber (1) and a closed other end, said body member (2) having one or more transverse inlets (7) to the axial fluid chamber (1) whereby fluid can flow through the axial chamber (1) for discharge through the outlet (6); the other end of the body member having externally mounted thereon a transducer (10) which is adapted to impart axial vibration to the nozzle outlet (6); the nozzle assembly having a resonant frequency under the conditions of intended use which is closely adjacent to the frequency of excitation of the transducer (10); the assembly being adapted to be mounted at a point intermediate the two ends of the body member (2).

2. A nozzle assembly as claimed in claim 1 characterised in that the transducer (10) is a piezoelectric crystal adapted to impart axial vibration to the nozzle outlet (6) by axial expansion and/or contraction of the piezoelectric crystal (10) upon the application of an electrical potential thereto.

3. A nozzle assembly as claimed in either of claims 1 or 2 characterised in that an adjustable counter mass (20) is carried upon the transducer (10).

4. A nozzle assembly as claimed in any one of the preceding claims characterised in that the assembly has radial symmetry.

5. A nozzle assembly as claimed in any one of the preceding claims characterised in that the body member (2) is provided by a length of a cylindrical rod and the axial chamber (1) is provided by a blind bore located substantially co-axially within said rod and extending axially from one end of the rod, the nozzle outlet (6) being provided at or adjacent the open end of said bore (1) and the transducer being mounted externally upon the closed end of the rod (2).

6. A nozzle assembly as claimed in any one of the preceding claims characterised in that the fluid inlet (7) to the said chamber (1) is located adjacent the nodal point of the assembly.

7. A nozzle assembly as claimed in any one of the preceding claims characterised in that that end of the body member (2) adjacent the nozzle orifice (6) is provided with an external taper or bevel (30).

8. A nozzle assembly as claimed in any one of the preceding claims characterised in that the assembly is provided with a mounting (50) in which that portion of the mounting (50) in contact with the assembly is made from a material which conducts sound at a speed which differs by at least 33% from the speed at which sound is conducted in the material from which that portion of the assembly in contact with the mounting (50) is made.
mounting.

9. A nozzle assembly according to claim 1 substantially as hereinbefore described with respect to and as shown in the accompanying drawing.

10. An ink jet printer incorporating a nozzle assembly as claimed in claim 1.

11. An ink jet print head comprising a nozzle assembly which is to be vibrated, preferably by means of a transducer, said assembly being characterised in that it is mounted in the said print head by means of a mounting (50) in which that portion of the mounting (50) in contact with the assembly is fabricated from a material which conducts sound at a speed which differs by at least 33% from the speed at which sound is conducted in the material from which that portion of the assembly in contact with the mounting (50) is made.

12. A method for cleaning a nozzle assembly as claimed in claim 1 characterised in that it comprises immersing at least the nozzle outlet portion (6) thereof in a suitable solvent and exciting the transducer (10).