GB2288889A

GB2288889A - Ink jet printer

Info

Publication number: GB2288889A
Application number: GB9407985A
Authority: GB
Inventors: Thomas J Madden; Richard Sutera; Philip Edward Stephens
Original assignee: Videojet Systems International Inc
Current assignee: Videojet Technologies Inc
Priority date: 1994-04-22
Filing date: 1994-04-22
Publication date: 1995-11-01
Anticipated expiration: 2014-04-22
Also published as: GB2288889B; GB9407985D0

Abstract

An ink jet printer comprises an ink reservoir 1, an ink supply circuit 2, 3, 4, 11, 12 for supplying ink from the reservoir 1 to a printing head 5 and for returning unused ink to the reservoir. A viscosity sensor 18 comprises a vibratory structure (20, 21, 22, Figure 3) immersed within the ink reservoir, means (26, Figure 2) for vibrating the structure and means (27, Figure 2) for detecting the vibrations. A make-up fluid reservoir 14 is connected to the ink reservoir and control circuitry 13 causes make-up fluid to be added to the ink reservoir upon detection of a change, for a given temperature, in vibrations of the structure. <IMAGE>

Description

INK JET PRINTER This invention relates to ink jet printer and in particular to an ink control system for such a printer.

In a continuous ink jet printer ink passes from a reservoir to a printing head where the ink is broken into droplets, typically using a piezoelectric transducer. The droplets are then electrostatically charged and can be deflected by means of an electric field onto the target. Unused droplets are captured in a gutter and returned to the reservoir.

It is important for the operation of such a printer that the viscosity of the ink and other physical parameters are carefully controlled. Solvent evaporation occurs as the ink passes through the printing head and returns to the reservoir via the gutter.

Typically an ink reservoir has an associated make-up fluid reservoir containing appropriate solvents which can be added to the ink reservoir during operation.

One way of controlling the viscosity is shown in EP-A-0142265. A viscometer comprises a vertically disposed tube containing a mobile ball. Every fifteen minutes ink is diverted from the ink circuit to fill the tube urging the ball towards the top. The ink pressure is then removed and the time taken for the ball to fall through the ink gives a measure of the viscosity and an indication of the amount of make-up fluid that needs to be added to the ink reservoir. The fact that the viscosity is measured and adjusted only periodically means that its value can vary. Furthermore its use requires extra valve and control circuitry in order to effect the periodic measurement.

This invention provides an ink jet printer comprising an ink reservoir, an ink supply circuit for supplying ink from the reservoir to a printing head and for returning unused ink to the reservoir, a viscosity sensor comprising a vibratory structure immersed within the ink reservoir, means for vibrating the structure and means for detecting the vibrations, a make-up fluid reservoir connected to the ink reservoir, an ink temperature sensor and contol means for causing make-up fluid to be added to the ink reservoir upon detection of a change, for a given temperature, in the vibrations of the structure.

With an ink jet printer according to the invention, the viscometer can be run continuously and therefore the viscosity can be maintained at a substantially constant value, while the need for providing hydraulic control circuitry for the periodic measurement known within the prior art, is avoided.

A suitable viscometer is described in co-pending patent publication GB-A2,236,591, the contents of which are hereby incorporated into this application. In essence, that sensor comprises a tuning fork arrangement which is caused to resonate by means of piezoelectric transducers. Changes in viscosity of a fluid within which the sensor is immersed dampen the vibrations, changing the mechanical Q-factor of the resonance. The applicants have discovered that, at a given temperature, the bandwidth of the resonance changes in a predictable manner along with the change in viscosity due to solvent evaporation.

In order that the invention may be well understood, an embodiment thereof will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic block diagram of an ink jet printer according to the invention; Figure 2 is a longitudinal section of view through a sensor for use in the printer shown in Figure 1; Figure 3 is a cross-sectional view along the line A-A of Figure 2; and Figure 4 is a graph showing a atypical relationship between resonant bandwidth and the change in ink viscosity.

Referring to Figure 1, an ink jet printer shown in much simplified form comprises an ink reservoir 1 from which ink is conveyed along a line 2 to a filter 3 by means of a pump 4. A printing head 5 of the multi-jet type comprises a manifold 6 incorporating piezoelectric transducers (not shown) by which ink supplied to the manifold can be broken up into a series of uniformly sized droplets, each jet being shown schematically by lines 7. Electrodes 8 are provided for charging the droplets, while further electrodes 9 deflect the charged droplets on to the target (not shown).

Any uncharged or unused droplets are collected in a gutter 10. This ink is returned to the reservoir via the pump 11 and filter 3.

As will be apparent to those skilled in the art, an ink jet printer includes further hydraulic control circuitry, not necessary for the understanding of this invention, disposed between the reservoir and the printing head which is shown schematically by box 12 under control of the microprocessor control circuitry 13.

In order to accommodate changes in viscosity due to solvent evaporation a further reservoir 14 containing make-up fluid, typically a mixture of solvents, is provided together with a solenoid valve 15 which can be actuated by the control circuitry 13. A further ink reservoir or cartridge 16, together with a respective solenoid valve 7, is provided for adding ink to the reservoir 1. A viscosity sensor 18 is mounted within the reservoir 1 together with a temperature sensor 19. Both sensors 18, 19 are connected to the control circuitry 13. As shown in Figures 2 and 3 the sensor 18 is of generally similar construction to that disclosed in British patent publication GB-A2,236,591 and essentially comprises a pair of flat vanes 20, 21 which extend from a circular diaphragm 22.The periphery of the diaphragm is fixed to the inner surface of cylindrical housing 23, forming two compartments 24, 25 separated by the diaphragm 22. Two rectangular plates 26, 27 of a piezoelectric ceramic material are attached to the surface of the diaphragm within compartment 24. Leads 28 extend from the transducers 26, 27 and out through a pipe 29. Holes 30 are provided within the housing wall to allow fluid to flow past the tubes 20, 21. In use, application of an alternating voltage to transducer 26 causes the diaphragm 22 to bend and the free ends of the vanes 20, 21 to move towards and away from each other. The frequency of the alternating voltage is selected to correspond to the resonant frequency of the structure. The other transducer 27 is operative to detect vibrations of the diaphragm 22.Changes in viscosity of the ink dampen the vibrations changing the mechanical Q-factor of the resonance. Figure 4 shows, for such a structure, the variation in bandwidth of the resonance together with the percentage change in ink weight for ink EJ101 at a temperature of 200C. The sensor used had a resolution of 0.79% ink weight change for a change in bandwidth of 0.11 Hz.

In use, the control circuitry 13 continuously monitors the viscosity of the ink taking data from both the temperature and viscosity sensors 18, 19. As soon as the inks' viscosity varies outside of predetermined limits for a given temperature, the appropriate valve 15, 17 is actuated to add make-up fluid or ink as required. The variation in viscosity with temperature for a given ink composition can be determined empirically and data representative of that variation stored within the control circuitry 13.

In an alternative embodiment the temperature sensor may operate as a thermostat together with a heater or heater/cooler to maintain the ink at a constant temperature so as to simplify the data processing requirements.

As shown and described herein, the vanes 20, 21 are mounted such that the relatively narrower edge leads during oscillation. This has been found to provide maximum sensitivity in respect of the viscosity of the ink. In an alternative, or additional sensor, the vanes can be turned through 90O so that the wider face leads during oscillation which provides maximum sensitivity in respect of ink density. In a further alternative the vanes may be of circular cross section, which is sensitive to both viscosity and density information.

As described, the make-up fluid is added in an on/off" way. In an alternative, the microprocessor can be arranged to detect the amount of viscosity off-set and to calculate how quickly the viscosity is changing per unit time. Make-up fluid may then be added proportionally to that rate thus preventing large swings in viscosity.

Claims

1. An ink jet printer comprising an ink reservoir, an ink supply circuit for supplying ink from the reservoir to a printing head and for returning unused ink to the reservoir, a viscosity sensor comprising a vibratory structure immersed within the ink reservoir, means for vibrating the structure and means for detecting the vibrations, an ink temperature sensor, a make-up fluid reservoir connected to the ink reservoir, and control means for causing make-up fluid to be added to the ink reservoir upon detection of a change, for a given temperature, in the vibrations of the structure.

2. An ink jet printer according to claim 1 in which the structure is vibrated at substantially its resonant frequency, the control means together with the detecting means being operative to detect the bandwidth of the resonance and to cause make-up fluid to be added upon detection of a change in the bandwidth.

3. An ink jet printer according to claim 1 or 2, in which the viscosity sensor comprises a tuning fork arrangement, a piezoelectric transducer for vibrating the structure and a further piezoelectric transducer for detecting the vibrations.

4. An ink jet printer according to any of claims 1 to 3 in which the control means is operative to compare the detected viscosity with the desired viscosity at the temperature detected by the sensor therefor, and to add make-up fluid according to the difference between the two viscosity values.

5. An ink jet printer according to any of claims I to 3 in which the ink temperature sensor is operative as a thermostat together with a heater or heater/cooler to maintain the ink at a constant temperature.

6. An ink jet printer according to any preceding claim in which the control means is operative to calculate the rate at which the viscosity is changing and to add make-up fluid in proportion to the calculated rate.

7. An ink jet printer substantially as described with reference to the drawings.