WO1990014233A1 - Liquid jet recording process and apparatus therefore - Google Patents

Abstract

A liquid jet recording apparatus comprises a cavity (4) having at least one exit orifice (2) and means (5) for causing pressure variations within a liquid in the cavity whereby to cause the liquid meniscus at the orifice to oscillate. Means (8) are provided for enabling ejection of a liquid from the oscillating orifice.

Description

Liquid Jet Recording Process and Apparatus Therefor

The present invention relates to a liquid jet recording process. More particularly, the invention relates to a liquid jet recording process in which recording liquid (eg: ink) is caused to travel in the form of droplets onto the surface of a recording medium (eg: paper).

Known liquid jet recording processes include so-called "drop-on-demand" recording processes in which liquid is contained in a cavity having a nozzle and a droplet separation arrangement, such as an electrostatic arrangement, is used to separate droplets from the liquid meniscus.

Processes are also known in which a continuous stream of droplets is generated from a nozzle and the direction of travel of the droplets is controlled according to demand.

A desirable result in developing liquid jet recording processes and apparatus therefor is to achieve greater frequency of operation and simplicity of construction.

The present invention provides liquid jet recording apparatus comprising a cavity having at least one exit orifice, means for inducing pressure variations in a liquid in the cavity whereby to cause the liquid meniscus at the orifice to oscillate and means for enabling a droplet of liquid to separate from the meniscus.

Thus in the apparatus of the present invention, the liquid meniscus is continuously oscillated and droplets are separated from the meniscus at selected times as it oscillates. Using the apparatus of the present invention, extremely high speeds of operation have been achieved.

The present invention also provides a method of ejecting a droplet from a volume of liquid contained in a cavity having an exit orifice, the method comprising inducing pressure variations in the volume of liquid whereby to cause oscillations of the liquid meniscus at the orifice and enabling a droplet of liquid to separate from the meniscus.

The means for enabling droplet separation may take a variety of forms as will become apparent from the following description.

In order that the present invention may be more readily understood, several preferred embodiments thereof will now be described by way of example with reference to the accompanying drawings in which:

Figure 1 is a schematic diagram illustrating the basic principle of ink jet recording apparatus according to this invention;

Figure 2 is a schematic diagram of part of the apparatus of claim 1 including a first type of droplet separation arrangement;

Figure 3 is a schematic diagram similar to figure 2 showing a second type of droplet separation arrangement;

Figure 4 is a schematic diagram similar to figure 2 showing a third type of droplet separation arrangement;

Figure 5 is a schematic diagram similar to figure 2 showing a fourth type of droplet separation arrangement;

Figure 6 is a schematic diagram similar to figure 2 showing a fifth type of droplet separation arrangement;

Figure 7 is a schematic diagram showing the basic principle of multi-orifice ink jet recording apparatus according to this invention; and

Figure 8 is a schematic diagram of a part of the apparatus of figure 1 having a droplet separation arrangement within the cavity.

Referring first to figure 1 which is a diagram explaining the basic principle of the present invention, a recording liquid 1 to be discharged from a discharge orifice 2 is fed from a supply arrangement (not shown in figure 1 ) into a cavity 4 and is allowed by capillary force to flow out of a nozzle 3 forming a meniscus at the discharge orifice 2. One wall of the cavity 4 contains a transducer arrangement 5, preferably a piezoelectric device, connected to an electronic drive circuit 6. When the transducer 5 is energised with an oscillating signal by the drive circuit 6, oscillating pressure waves can be generated in the cavity 4. These pressure waves will during the positive cycle cause the meniscus 7 formed by the recording liquid at the discharge orifice 2 to move outwards.

During the negative cycle of the pressure waves, the meniscus will return back and retract into the nozzle 3. It is possible by adjusting the amplitude of the signal supplied by the drive circuit 6 to get the meniscus 7 to oscillate with a considerable amplitude without breaking off and forming a droplet. This amplitude can be several times the diameter of the discharge orifice 2 and is a function of liquid parameters as well as the amplitude of the drive voltage. If the frequency of the drive signal corresponds to transducer resonances and/or resonances in the cavity and/or resonance in the liquid column in the nozzle, the meniscus can be made to oscillate at very high frequencies with very small electrical input power requirements. Oscillating frequencies of several hundred kilohertz have been observed.

In order to stabilise and control the oscillation amplitude of the meniscus a feed back control loop can be incorporated. Either the pressure in the recording liquid in the cavity 7 or the amplitude of the oscillating meniscus can be measured and used to regulate the amplitude of the voltage delivered from the drive circuit 6.

In the region outside the discharge orifice 2 is placed a droplet separation arrangement 8. When this arrangement is energised, by an electronic drive circuit 9, additional energy is added to the recording medium in the oscillating meniscus causing a droplet to be discharged. The transducer drive circuit 6 and the droplet separation drive circuit 9 can be synchronised to assure that the droplet separation arrangement adds the energy to the meniscus at the correct time within the oscillating cycle of the meniscus.

Once a droplet has been separated from the oscillating meniscus it will pass through the droplet separating arrangement and deposit on the recording substrate 10 to form a dot.

The total energy Erpot required to eject a droplet from a small orifice consists of three components:

where

E_v : is the energy required to overcome the viscous drag in the nozzle

E_s : is the energy required to overcome the surface tension and break the droplet of the meniscus

Ejς : is the kinetic energy contained in the flying droplet.

In operation the pressure waves generated in the cavity 4 by the transducer 5 are adjusted to provide an energy input Ep to the nozzle 3 of:

E E_v + E_s

That is, Ep is not high enough to separate and move the droplets away from the oscillating meniscus. Only the droplet separation arrangement 8 is able to provide this additional energy component to the oscillating meniscus and discharge a droplet.

In principle Ep can also be adjusted to be higher than the sum of E_v, E_s and E]_ζ that is the energy generated by the transducer 5 is sufficient to release a droplet from the oscillating meniscus during each cycle. In this case the droplet separation arrangement has to provide a negative energy component preventing droplets from being discharged and only allow a droplet discharge when a dot needs to be formed on the recording substrate 10. In practice however, this arrangement has been found to be more difficult to implement.

For simplicity reasons the recording apparatus described above has been a singe orifice arrangement. In most practical applications multi- orifice arrangements are required. Since the same drive transducer 5 and cavity 4 can be used to supply all nozzles 3 it is very simple to produce large arrays of droplet discharge orifices where the distance between individual orifices is very small.

There are several different ways the droplet separation arrangement 8 can be implemented to provide the additional energy required to separate a droplet from the oscillating meniscus. Some of these are explained in figure 2 to figure 6. In all of the drawings appended hereto, like items are identified with like reference numerals.

Figure 2 shows an arrangement in which a light emitting device 11 such as a laser device is used to provide a thermal energy pulse to the oscillating meniscus 7. Where the device is a laser, the energy in the laser beam 12 is absorbed in the recording liquid and causes a local heating. As a result the surface tension drops and reduces the energy threshold at which the droplet will be released from the meniscus. It is .also possible with a short heat pulse to boil the ink locally. This phase change produces a small bubble which can separate the droplet from the meniscus.

Figure 3 shows an arrangement in which a small heating element 13 is situated just outside the orifice. When energised by a drive circuit 14, the heating element 13 provides a thermal energy pulse which can separate the droplet from the meniscus in the same way as described above.

Figure 4 shows an arrangement where two electrodes 15 and 16 are situated on opposite sides of the orifice. The electrode 16 is connected to a ground potential and the other electrode 15 is connected to a voltage drive circuit 17. When a sufficient high electrical field is applied across the electrodes and thus across the oscillating meniscus a droplet can be ejected.

Two possible ways in which a droplet can be ejected with the arrangement of figure 4 are as follows: A high electric field may be applied across the electrodes sufficient to cause a spark to be generated. This would cause local heating of the liquid leading to droplet separation in the manner described above. Alternatively, if the ink is a polar liquid, the electrodes can be used to generate an electric field in the region of the meniscus which will impart extra kinetic energy to the field at the meniscus leading to droplet separation.

Figure 5 shows an arrangement in which an electrode 18 is placed at a distance away from the orifice which is longer than the amplitude of the oscillating meniscus. The ink is connected to ground potential and when an electrical voltage applied from a drive circuit 19 is connected to the electrode 18 an electrostatic force can be generated pulling a droplet away from the meniscus.

Figure 6 shows an arrangement where two electrodes 20 and 21 forming a capacitor are placed in front of the orifice. When an electrical field is placed across the capacitor the ink, which in this case should have a high dielectric constant , will be drawn into the capacitor causing a droplet to be released.

Figure 7 illustrates multi-orifice orifice liquid jet recording apparatus according to the invention. In this arrangement a single cavity 4 is provided with an array of nozzles 3. As with figure 1 , one wall of the cavity contains a transducer arrangement 5, preferably a piezoelectric transducer arrangement, connected to an electronic drive circuit 6. The transducer produces oscillating pressure waves which cause each meniscus at each orifice to oscillate. Reference numeral 8 denotes a droplet separation arrangement which may be any of the types mentioned above. Preferably, the droplet separation arrangement should be operable to discharge a droplet from a selected one of the orifices. For example, the droplet separation arrangement may comprise an array of heaters of the type shown in figure 3, one corresponding to each orifice 3. Selected ones of the heaters may be energised to eject droplets from selected ones of the orifices to produce spots at desired locations on the substrate. Any of the droplet separation arrangements described above can be adapted for multi-orifice apparatus.

In the arrangement of figure 7 the cavity has an inlet and an outlet connected to the supply arrangement so that liquid can be continuously circulated through the cavity to prevent the formation of air bubbles. This feature may be included in any of the arrangements described above.

A droplet separation arrangement comprising several individual elements may be operated by the same drive circuit, if desired. The individual elements may be separately wired to the drive circuit or they may be separately addressable from the drive circuit.

In all of the arrangements described above the droplet separation arrangement is positioned outside the cavity 4. This is advantageous since it simplifies the construction of the cavity itself and enables it to be made very small. However, it is also possible to provide droplet separation means inside the cavity and one example of such an arrangement is illustrated in figure 8.

In the arrangement of figure 8 a resistor 24 is positioned inside a nozzle. When energised by a drive circuit 23, the resistor provides a thermal energy pulse which can separate a droplet from the meniscus in the manner described above.

Any type of internal droplet separation arrangement can be used in apparatus according to this invention. In a single-cavity-multi-orifice arrangement a separation device can be provided for each orifice.

Alternatively it would be possible to generate a travelling pressure wave in the cavity causing the meniscuses to oscillate at different times, or asynchronously. A single droplet separation arrangement could then be used instead of an array, and the operation of the arrangement would be timed with the standing waves to separate a droplet from the desired orifice.

Claims

CLAIMS:

1. A liquid jet recording apparatus comprising a cavity having at least one orifice, means for inducing pressure variations in a liquid in the cavity whereby to cause the liquid meniscus at the orifice to oscillate and means for enabling a droplet of liquid to separate from the meniscus.

2. Apparatus as claimed in claim 1 in which the means for enabling droplet separation are arranged to supply additional energy to liquid at the meniscus.

3. Apparatus as claimed in claim 1 or 2 in which said means for inducing pressure variations comprise a piezoelectric transducer.

4. Apparatus as claimed in claim 1, 2 or 3 in which the means for enabling droplet separation comprise a light emitting device for providing a thermal energy pulse to the oscillating meniscus.

5. Apparatus as claimed in claim 1, 2 or 3 in which the means for enabling droplet separation comprise a heating device.

6. Apparatus as claimed in claim 1", 2 or 3 in which the means for enabling droplet separation comprise a pair of electrodes arranged to produce an electric field across the meniscus.

7. Apparatus as claimed in claim 1, 2 or 3 in which the means for enabling droplet separation comprise an electrode arranged to generate electrostatic force for pulling a droplet away from the meniscus.

8. Apparatus as claimed in claim 1, 2 or 3 in which the means for enabling droplet separation are arranged to form a capacitor for attracting a droplet away from the meniscus.

9. Apparatus as claimed in any preceding claim in which the means enabling droplet separation are arranged outside the cavity.

10. Apparatus as claimed in any of claims 1 to 6 in which the means enabling droplet separation are arranged inside the cavity.

11. Apparatus as claimed in any preceding claim in which the cavity includes a nozzle terminating in said exit orifice along which the liquid may travel by capillary action.

12. Apparatus as claimed in any preceding claim including means for monitoring the pressure in the cavity or the amplitude of the meniscus and means for regulating the pressure variations in the cavity in response to said monitoring means.

13. Apparatus as claimed in any preceding claim in which the cavity has an array of exit orifices the meniscuses of which are all caused to oscillate by the same means.

14. Apparatus as claimed in claim 13 in which the droplet separation arrangement is operable to enable droplet separation from a selected one or selected ones of said orifices.

15. Apparatus as claimed in any preceding claim in which the pressure variations in the liquid are sufficient to enable droplet separation at the orifice and said means for enabling droplet separation are arranged to prevent droplet separation except when required.

16. A method of ejecting a droplet from a volume of liquid in a cavity having an exit orifice, the method comprising inducing pressure variations in the volume of liquid whereby to cause oscillations of the liquid meniscus at the orifice and enabling a droplet of liquid to separate from the meniscus.

17. A method as claimed in claim 16 in which the pressure variations are sufficient to cause a droplet of liquid to separate from the meniscus and droplet formation is prevented except when required.

18. A method as claimed in claim 16 in which droplet separation is enabled by supplying additional energy to the liquid at the meniscus.

19. A method as claimed in claim 16, 17 or 18 in which the pressure variations are induced by a transducer arrangement at a frequency corresponding to a resonant frequency of the transducer arrangement and/or the cavity and/or a column of liquid adjacent the meniscus.

20. A method as claimed in any of claims 16 to 19 for use with a cavity having a plurality of orifices in which travelling pressure waves are generated in the cavity whereby to cause the meniscuses at the respective orifices to oscillate asynchronously.