Continuous inkjet printers

Abstract

The invention discloses various methods of controlling or monitoring the performance of a continuous inkjet printer based on monitoring vacuum levels and/or noise in the gutter line.

Claims

1. A method of controlling the flow of ink and/or air through the gutter line of a single jet continuous inkjet printer using a vacuum pump, said method comprising: identifying transition flow in said gutter line, where transition flow occurs between annular flow where ink flows as an annulus down the gutter line and forms a layer on the inner surface of the line, and slug flow, where ink and air are not mixed but form into, and flow as, individual slugs of ink and air in said gutter line by: running the vacuum pump at a speed sufficient to establish annular flow of ink and air through the gutter line; determining vacuum noise values in the gutter line; comparing vacuum noise values with a predetermined threshold value; and lowering the speed of the vacuum pump until a vacuum noise value is greater than the predetermined threshold value; and controlling said vacuum pump to maintain transition flow in said gutter line by: comparing vacuum noise values with a predetermined threshold value; and if a vacuum noise value is less than the predetermined threshold value, lowering the speed of the vacuum pump; or if a vacuum noise value is greater than the predetermined threshold value, raising the speed of the vacuum pump.

2. A continuous inkjet printer comprising a vacuum pump and a pressure sensor operable to measure vacuum values in a gutter line, and a controller operable to control the vacuum pump to carry out the method of claim 1.

3. A continuous inkjet printer comprising: a gutter line; a vacuum pump operative to draw ink and air through the gutter line; a pressure sensor operable to measure vacuum values in the gutter line; and a controller operable to control the vacuum pump by: identifying the transition between annular flow and transition flow in the gutter line by: identifying transition flow in the gutter line, where transition flow occurs between annular flow, where ink flows as an annulus down the gutter line and forms a layer on the inner surface of the line, and slug flow, where ink and air are not mixed but form into, and flow as, individual slugs of ink and air in said gutter line by: running the vacuum pump at a speed sufficient to establish annular flow of ink and air through the gutter line; determining vacuum noise values in the gutter line; comparing vacuum noise values with a predetermined threshold value; and lowering the speed of the vacuum pump until a vacuum noise value is greater than the predetermined threshold value; and controlling the vacuum pump to maintain transition flow in said gutter line by: comparing vacuum noise values with a predetermined threshold value; and if a vacuum noise value is less than the predetermined threshold value, lowering the speed of the vacuum pump; or if a vacuum noise value is greater than the predetermined threshold value, raising the speed of the vacuum pump.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be described in more detail and by way of example only with reference to the accompanying drawings in which:

(2) FIG. 1: shows a schematic of an ink circuit of a typical continuous inkjet printer suitable for performing the various aspects of the invention;

(3) FIG. 2: shows the different flow regimes that might be observed in the gutter line of a continuous inkjet printer;

(4) FIG. 3: shows plots of gutter line vacuum through the start-up process of a continuous inkjet printer; and

(5) FIG. 4: shows plots of gutter line vacuum, gutter pump control voltage and gutter noise as a function of time.

DESCRIPTION OF WORKING EMBODIMENTS

(6) Referring to FIG. 1 a continuous inkjet printer, in this case a single-jet continuous inkjet printer, is shown in diagrammatic form, the printer drawing ink from ink reservoir 6 and make-up fluid or solvent from reservoir 7. The reservoirs 6 and 7 are topped-up from cartridges 8 and 9 respectively.

(7) Ink is drawn from the reservoir 6 by feed pump 10. The pump 10 pushes the ink through an ink cooler 36 and then through a fine system filter 11. Ink is then directed either to the drop generator 12, through feed line 13, via a damper 14; or through a jet pump 15 and back to the reservoir 6. The ink flow through the jet pump can also be directed through a viscometer loop 16 to enable the viscosity of the ink to be determined. In stand-by mode, when the printer is not printing, all ink is circulated through the jet pump 15 and back to the reservoir 6. In this state the flow of ink is comparatively high while the pressure is comparatively low.

(8) Restrictors are used to balance the flows between the feed path to the printhead and the circulation path back to the reservoir. The drop generator 12 requires a low flow of the order of 5 ml/min at a high pressure of around 3 bar, whilst the jet pump 15 and viscometer loop 16 require a much higher flow of the order of 800 ml/min at a much lower pressure. The pressure at the drop generator 12 is measured by pressure transducer 17 included in the bleed line 18.

(9) In the conventional manner, ink is jetted through the printhead nozzle 20, upon the release of the nozzle valve 21, and the jet is aligned such that it enters the ink catcher or gutter 22 and is returned to the printer via a gutter line 23. A gutter pump 24 draws a vacuum in the gutter line 23, pressure sensor 25 being attached to the gutter line 23, prior to the gutter pump 24, to monitor the vacuum in the gutter line. The ink and air mixture returned by the gutter pump 24 is directed back into reservoir 6, via a gutter filter 26. The gutter pump is preferably an electrically driven variable speed diaphragm pump.

(10) In accordance with a first aspect of the invention, the noise generated in the gutter line is monitored in order to control the operation of the gutter pump 24. This ‘noise’ may comprise pressure fluctuations in the gutter line 23 or fluctuations in the electrical current driving the pump 24.

(11) FIG. 2 shows the different flow regimes that can be found in the gutter line of a continuous inkjet printer. At high flow rates annular flow is observed. Annular flow is characterised by having ink flowing as an annulus down the gutter line and forming a layer on the inner surface of the line, whilst air flows down the centre of the line. At very low flow rates slug flow is observed. Slug flow is characterised by the ink moving slowly and forming into slugs pulled together by surface tension. In slug flow the ink and air are not mixed but form into, and flow as, individual slugs of ink and air. It is the experience of the present applicant that in single-jet continuous inkjet printers, the flow rates at which slug flow is observed are insufficient to clear all the ink as it collects in the gutter.

(12) In between slug flow and annular flow is transition flow, which we interpret as the minimum flow rate that guarantees that all of the ink that enters the gutter line is removed by the pump without overflowing the gutter.

(13) FIG. 3 illustrates vacuum level during the start-up process. Upon start-up of the printer the gutter pump 24 is run and air is sucked down the gutter line. During the period marked A, a high airflow rate is chosen so that flow through the gutter line 23 begins in the annular region, just before the nozzle valve 21 is opened, the value of the gutter vacuum is stored in the operating system as being characteristic of the vacuum level for flowing air down the gutter. The nozzle valve 21 is then opened, ink is emitted from the nozzle 20, and that ink is then collected in the gutter 22. Initially the system has a low vacuum reading with low noise as air is sucked through the gutter line 23. When ink enters the line the vacuum will increase as the pump pulls against the ink, ink having a higher viscosity than air. During period B the printer tests to make sure that the vacuum has increased over the level recorded during period A above. With ink present, once annular flow is established, the noise level in the vacuum line is characterised at point C. Typically this is between 5 and 10 s after opening the nozzle by which time the vacuum should have reached a level at least 10% greater than during period A.

(14) FIG. 4 illustrates how the printer controls the vacuum pump during the start-up process. It should be noted that pump speed is governed by a 0-4V control input, which corresponds proportionately to the vacuum pump speed. FIG. 4 starts at the point marked D on FIG. 3. Between 0 and about 175 secs the pump is run at a high speed, designated by a pump control voltage of 4V, which is a period of time used to prime the gutter or, in other words, a period of time to establish a steady state ink flow and vacuum in the gutter. During this time, a large quantity of air flows down the centre of the gutter, whilst ink is pushed down the edges of the pipe, a flow type known as annular flow. The printer will determine a rolling average vacuum level during this time.

(15) After the gutter has primed, the rolling noise level is characterised and used to control the gutter pump. In a typical implementation the vacuum sensor is sampled at a rate of about 2 kHz, and an average is calculated for every second's worth of data. A value for the vacuum noise is calculated for each sample by comparing each sample to the calculated average vacuum and determining the residual value (i.e. by finding the square of the difference and dividing by average vacuum,). The summation of the residuals for a second's worth of samples is assumed to be representative of the vacuum noise level during that second.

(16) As can be seen in FIG. 4, the value of the residual vacuum noise is compared to a pre-determined threshold level or trigger value and if it is below the trigger value then the vacuum speed is lowered. This is carried out in steps. The trigger value is marked by the horizontal line on FIG. 4 at around 50 on the left hand or vertical scale. This value has been empirically determined over many systems to represent the onset of the transition point between annular flow and transition flow. Alternatively the printer can be put through a calibration regime to determine a transition value. In order to give the control system time to respond to each change in pump speed the printer collects the data for a total of 15 s before changing the pump speed again. The system discards the first three seconds of data as the effects of the pump changing speed compromise the measurements at this time. The next 12 s of worth of data are used and, in themselves, are averaged and compared to the trigger value. The graph of FIG. 4 between 175 secs and about 650 secs illustrates this algorithm well, showing the pump speed being stepped down approximately every 15 secs.

(17) The last section of FIG. 4, between 650 secs and 1400 secs, shows the printer controlling the gutter pump at the transition point. It can readily be seen that the intuitive result, that gutter noise might gradually increase as pump speed lowers, is not the case. Instead there is an abrupt transition in noise level, which is significantly higher than the residual vacuum noise characterising annular flow. The pump speed is moved up and down in response to the residual value being above and below respectively the trigger value. In this way the gutter pump is controlled so that the minimum amount of air is drawn down the gutter in order to clear the gutter effectively.

(18) The flow regime is a characteristic of the system and as mentioned earlier, we have determined a pressure amplitude control threshold, between annular and transition flow, that applies universally for a particular embodiment of printer. Any system tolerance or build-standard variance is automatically compensated for by the control system measuring the true transition from one flow phase to another. Factors affecting gutter flow and vacuum include gutter line internal diameter, gutter line length, ink viscosity (in gutter at ambient temperature), pump efficiency, pump speed, and nozzle diameter (ink flow rate).

(19) By way of example, if we have a weak gutter pump, the system will compensate by driving the pump at a higher speed so as to maintain the pressure amplitude control. If the gutter line length is increased, say from a standard 3 m length to a 6 m length, the system will cause the gutter pump to be operated at a higher speed to maintain the control point.

(20) When the printer system operates in different temperature environments a different pump speed will be required to clear the gutter, as the ink viscosity changes with temperature. As the operating position for the gutter line is based on the transition from annular flow to the transition flow region, which depends on viscosity, the system will find the right point to set the gutter pump so that the gutter is cleared independently of environmental condition.

(21) Accordingly the system is able to find the point that guarantees reliable operation with minimum airflow down the gutter line. As airflow relates directly to solvent consumption, a printer operated according to the invention is therefore able to operate with much reduced solvent consumption.

(22) In another aspect of the invention provides a method of detecting whether the nozzle 20 is correctly aligned with the gutter 22, and thus whether ink ejected from the nozzle has entered the gutter. The most likely scenario for the ink jet to miss the gutter, and soil the substrate, is upon start-up. As mentioned already, at start-up the printer establishes a base line vacuum level and vacuum noise level that characterises air flow through the gutter. Once the system is activated and ink ejected from the nozzle, it is expected that the vacuum level will rise. If this is not detected within a specified period, e.g. 7 seconds, then the printer can deduce that ink has not entered the gutter and shut down the jet, thus preventing further soiling of the substrate. Typically a 10% change is looked for.

(23) In a normal operating mode, the printer will be running with an ink and air mixture passing through the gutter line. According to yet a further aspect of the invention, if the pressure sensor 25 detects a sudden fall in gutter vacuum level, it can deduce that only air is entering the gutter and, for some reason, the ink jet is no longer aligned with the gutter. The printer can therefore be configured to shut down the jet and prevent possible soiling of the substrate. Typically the printer achieves this by running a rolling average of the gutter vacuum level and comparing the currently measured vacuum to the rolling average established a short time before. In the preferred embodiment this is approximately 40 s before. The printer checks that the vacuum level has not fallen by more than 40%.

(24) In still another aspect the invention provides a method of determining if the gutter line is blocked. According to this aspect if the pressure sensor 25 detects a rise in vacuum level then the printer can deduce that the gutter or gutter line is blocked. Typically the printer achieves this by running a rolling average of the gutter vacuum level and comparing the currently measured vacuum to the rolling average established a short time before. In the preferred embodiment this is again approximately 40 s before. The printer checks that the vacuum level has not risen by more than 80%.

(25) In yet another aspect of the invention the printer system uses the measurement of a pump speed and compares this to a vacuum level at start-up to ascertain if the gutter pump is working as intended. If the expected level of vacuum is not observed within a period A as shown in FIG. 3 the printer deduces that the gutter pump is not operating as intended.

(26) Another aspect of the invention concerns the efficient shut down of the printer. After closing off the jet at shut-down, the gutter line must be cleared to ensure that no ink remains in the gutter line which could dry and cause a blockage. The current practice with a continuous inkjet printer is to pump air, ink and solvent through the gutter line for a specified (and long) period of time to ensure the gutter line is cleared. This period of time must be set having regard to the worst-case scenario of the printer being operated at the bottom of its environmental specification and, as a result, shut-down can take a very long time to execute.

(27) According to this aspect of the invention, instead of the printer system being configured to pump the air and ink mixture through the gutter line for a pre-determined period of time, the system is configured to operate the gutter pump while observing the vacuum level in the gutter line using sensor 25. Pumping is continued until the vacuum once again reaches the vacuum level corresponding to air, alone, passing through the gutter. At this point the pump is stopped and the shut-down is completed. A further period of time is run to ensure total clearance.