GB2540111A

GB2540111A - Fluids delivery system

Info

Publication number: GB2540111A
Application number: GB1420903.5A
Authority: GB
Inventors: Marcin Zaba Jerzy; Brian Tomlin Matthew; Michael Fost Ian; Lecheheb Ammar
Original assignee: MATRICODE Ltd
Current assignee: MATRICODE Ltd
Priority date: 2014-11-25
Filing date: 2014-11-25
Publication date: 2017-01-11
Also published as: GB201420903D0

Abstract

A series of printheads used in a piezoelectric inkjet drop on demand (DOD) printer are supplied with ink. The required ink pressure at the printhead is adjusted via a closed loop involving the reading of pressure sensors 7 and 8 and the consequent regulating and adjusting of proportional valves VP1 and VP2 16 and 17 , to obtain the desired ink supply pressure. The negative pressure or vacuum and ink flowrate control and adjustment are achieved by the use of a pressure sensor or transducer 9 in a closed loop with a proportional valve VP2 . Two positive displacement diaphragm pumps 2 and 15 are operated at a constant motor speed, controlling the supply and return of ink respectively. Spring loaded diaphragm dampers 3 and 11 may also be fitted to reduce fluctuations in the pressure and vacuum levels. The ink may be heated using heater 4 and filtered by using filter 6. The system may also incorporate an ink storage tank 1 and a further ink reservoir 14 having a filter 13 therein. Flexible umbilical leads may be used to connect the various components together.

Description

Title: Fluids Delivery System Field of the Invention

The present invention relates to a fluids delivery, supply and management system. More particularly, the invention relates to the management and control of ink delivery system in an inkjet printer. More specifically, the present invention relates to what is called a piezoelectric DOD recirculating ink supply system to maintain the ink supplied to inkjet printhead(s) at optimum operating conditions of stable, accurate pressure and temperature.

Background to the invention

Inkjet Piezoelectric Drop On Demand (DOD) or impulse ink jet systems operate to eject ink drops for printing by the application of pressure pulses to the ink. These pressure pulses are generated by the electrical actuation of a piezoelectric element that is attached to one or more sides of each of the printhead channels in communication with and supplying ink to the various nozzles. Because of this fundamental principle of operation these ink channels must be free of air bubbles to ensure the pressure pulse is not absorbed by these air bubles which would then result in failure of the system to eject printing droplets. Also because of this operating principle the pressure of the ink supplied to these nozzles must be maintained within a desired operating range (normally very narrow) and be stable at all times. The operating pressure range is normally with the cappillary range of the printhead ink channels supplying the nozzles and the desired operating pressure at the nozzles is slightly negative being in the range of -5mb to -25mb +/-2mb. Therefore, the ink supplied to these nozzles must be free of pressure fluctuations or pulsations that are larger than +/-2mb. Pressure pulsations are normally generated by the ink recirculating systems using positive displacement pumps such as gear pumps and diaphragm pumps.

From the foregoing, it is clear that in order to obtain a reliable operation and good quality printing, especially in an ink jet printing system of the piezoelectric DOD type, it is necessary that the ink supplied to one or to multiple printheads, which each incorporates one or more arrays of several nozzles, is maintained stable and within the desired preset pressure and temperature range under all specified operating conditions. If the temperature and or pressure at the nozzles deviates and goes outside of the required operating range, ejected printing drops become non-uniform leading to degraded print quality. Furthermore, ink can then start weeping out of the nozzles leading to eventual failure of the system. The opposite can also occur whereby nozzles can be starved out of ink if the temperature and/or pressure at the nozzles is outside the operating range. In particular, if the pressure at the nozzle goes below the minimum range of the required negative pressure. In such case, the air get ingested into the nozzle channels and the printhead deprimes, as it is known in the ink jet industry. When one or multiple nozzles are deprimed, then these cannot eject printing droplet of ink as the nozzle exit get blocked by the air bubble which absorbs pressure pulses and conterract any ink volume displacement.

Prior art

Various ink supply and management systems have been used or proposed for achieving the above requirements and objectives, but none has proved effective and reliable for industrial multicolour digital piezo electric inkjet DOD printing systems.

Most prior arts teach the use of multiple reservoirs closely coupled to each of the Piezoelectric DOD printheads. This prior art not only adds complexity to the printing system, it also adds costs and difficulty in servicing and maintaining such systems.

Other prior art systems regulate the pressure at the printheads by reading pressure sensors situated at the printheads and adjusting the ink supply and recirculation pums speeds until the desired pressure at the printheads is attained. This method of adjusting the pressure at the printheads does not lead to reliable operation of the printheads because positive pumps generate different pressure fluctuations or pulsations levels and frequencies at different pump speeds. Pressure dampers are normally designed to only suppress or attenuate pressure pulsations of a given amplitude and frequency. It ensues that over the range of pumps speeds variations required to adjust the ink pressure at the printheads, the prior art systems can’t supply stable ink to the printheads under all specified operating conditions. This leads to ink weeping out of nozzles and to printer system reliability issues requiring frequent maintenance.

There is therefore a need for providing that is able to control the ink meniscus pressure at the nozzles accurately, stably and simply. A system that does not use multiple components to achieve the objectives and which easy to maintain and service.

Objectives of the invention

One of the objectives of this invention is to provide a recirculating ink system for use with digital piezoelectric DOD inkjet printing systems. More particularly, the present invention relates to an improved ink supply and management system for a piezoelectric DOD digital printing system capable of printing multi colour using at least CMYK coloured inks.

Still a further objective of this invention is to provide a recirculating Piezo DOD ink supply system whereby the ink supply and ink recirculation pumps motor speeds are set to a given value and then maintained at that constant value. Thereafter, the desired pressure at the printheads is adjusted via a closed loop, such as a Proportional, Integrated Differential (PID) control. The PID control loop involving the adjustment and control of two proportional valves located in the ink system cabinet, reading of pressure sensors located in a printhead, and adjusting, controlling and setting the proportional valves to obtain the desired ink supply and recirculation pressures, or more specifically vacuum at the printheads.

Still a further objective of this invention is to provide a system wherein the ink recirculation function are carried out by two positive displacement diaphragm pumps to ensure no adverse effects on the ink or on the proper operating of the pumps, as it would, for instance, when using other positive displacement pumps such as gear pumps. This is because the gears of gear pumps tend to damage the inks, while the solid pigments, such as in Ti02 based ink formulations, erode the gears of the pump.

Summary of the invention

The present invention provides an ink supply system and an ink recirculation or return system for controlling the ink pressure, or more specifically, for adjusting and controlling the vacuum at the printheads of a piezoelectric Drop On Demand (DOD) printing system stably and accurately. The ink system comprises an ink supply, ink return and pressure control circuits incorporating, among other components, two proportional valves, two pumps, two temperature sensors and two pressure sensors. In use, the motor speed of each pump is kept constant while the proportional valves are regulated and adjusted to control the flow of ink supplied to and returned from the printheads. The pressure sensors located at one of the printheads are read and the proportional valves adjusted accordingly to achieve the required vacuum of the ink meniscus at the nozzles to ensure reliable operation of the printer.

When the printer is switched on at start-ups, the ink is first circulated in a short closed loop path by drawing the ink from the ink supply tank using a diaphragm pump whose motor speed is set to run at constant speed. The ink is then conveyed in a short, small plastic pipe to the inlet port of the heat exchanger to raise the temperature of the ink if necessary. The ink flows through the heat exchanger, through the proportional valve and back to the ink supply tank. When the desired temperature has been reached, the proportional valve is adjusted to start regulating the ink flow to the printhead. Thus, a dc driven electromagnetic solenoid valve is opened to allow ink to flow on the supply line to the printheads. Simultaneously, the proportional valve on the return line is being controlled to regulate the flow of ink in the return line until the desired vacuum is achieved at the printheads. The system’s two pressure sensors located in the printhead manifold and the two proportional valves are used as part of the closed loop Proportional Integral Differential (PID) to control the pressure differential at the printheads and thus ensure optimum conditions of the ink meniscus during idle and operating periods.

The temperature sensor located in the ink supply enclosure in the ink heater circulation loop is used as part of the closed loop Proportional Integral Differential (PID) to control the ink temperature of the ink in the ink supply system. The temperature sensor located in the printhead manifold is used to measure the temperature of the ink and compare this to the set target desired operating temperature.

To ensure that the ink supplied to the printhead is free of pressure fluctuations or pulsations a purposely designed and built spring loaded dampers are provided. One of these dampers is located downstream of the supply diaphragm pump and the other damper is located upstream of the recirculating diaphragm pump.

The constant operating pump speed means that the pressure fluctuations or pulsations are of the same frequency and predetermined maximum magnitude.

Therefore, the damper can be designed to totally eliminate these resulting pressure fluctuations, which would have been difficult to do with one damper if the pump speed were variable as these would impart variable pressure fluctuations frequencies and associated maximum magnitudes

The term “ink” is used herein to refer to any fluid suitable for piezoelectric, thermal or continuous ink jet printing and covers not only coloured inks used for printing monochrome and coloured images but also inks used in printing functional materials and for non-imaging applications.

Detailed description

The various subsystems of a preferred embodiment of the present invention are described below:

The ink supply circuit:

Figure 1 shows an example of the prior art ink supply and recirculation circuit diagram. These prior art systems regulate the pressure at the printheads by reading pressure sensors (7 and 8) situated at the printheads and adjusting the ink supply and recirculation pump (2 and 15) speeds until the desired pressure at the printheads is attained. This method of adjusting the pressure at the printheads does not lead to reliable operation of the printheads because positive pumps generate different pressure fluctuations or pulsations levels and frequencies at different pump speeds. Pressure dampers (3 and 11) are normally designed to only suppress or attenuate pressure pulsations of a given amplitude and frequency.

Figure 2 shows a schematic circuit diagram of a preferred ink supply and recirculation system in accordance with the present invention. The ink supply circuit of the present invention is capable of supplying up to 16 off recirculating or flowthrough printheads per colour at the desired pressure and temperature. Such printheads are for instance the Xaar1001 or the Xaar 1002 printheads manufactured and sold by Xaar Pic of UK. Xaar 1001 and Xaar 1002 are Xaar pic trademark. This Ink Management System here abbreviated to IMS, supplies ink using a positive pump such as a gear or diaphragm pump (figure 2). The preferred pump in accordance with this invention is a diaphragm pump. The desired pressure at the printhead is adjusted via a closed loop involving the reading of pressure sensors (figure 2) and regulating and adjusting the proportional valves VP1 and VP2 (figure 2), to obtain the desired ink supply pressure. To ensure that the ink supplied to the printhead is free of pressure fluctuations or pulsations a purposely designed and built spring loaded pressure damper, shown in figure 2, is provided on the supply line located downstream of the diaphragm pump just before the exit from the ink system enclosure of the ink supply pipe This ink supply subsystem incorporates the following components shown on the ink circuit schematics of figure 2: a. A common ink supply and return ink tank (1) b. A dc motor driven diaphragm pump (2) c. An ink heater(4) d. A temperature sensor at the ink system enclosure (5) e. One ink filter (6) f. One proportional valve located in the ink system enclosure VP1 (16) of figure 2 g. A spring loaded diaphragm ink pressure pulsation damper (3) h. A pressure sensor at the supply side to the printhead (7) i. One temperature sensor at the printhead (9)

The ink recirculation or return circuit

The ink recirculation circuit provides vacuum to the printhead so as to constantly recirculating ink that is not used for printing and to ensure the meniscus pressure is maintained under all operating and idle conditions. To ensure the printhead is primed at all times and the required negative ink pressure at the printhead is maintained at all times to prevent ink weeping from the nozzles or ink starvation at the nozzles. The ink recirculation is accomplished using a dc driven diaphragm pump (15). The negative pressure and ink flowrate control and adjustment are achieved by the use of a pressure sensor (9) in a closed loop with a proportional valve (VP2 of figure 2). Regulation and adjustment of the proportional valve ensure that the required negative pressure and ink flowrate are maintained at the printhead nozzles at all times. A spring loaded diaphragm damper (11) suppresses any pressure fluctuations or pulsations which results from the positive displacement diaphragm pump. This subsystem incorporates the following components as shown in figure 2: a. A pressure sensor (9) at the printhead b. A spring loaded diaphragm ink pressure pulsation damper (11) c. A proportional solenoid valve (17) d. A dc motor driven diaphragm pump (15) e. A common return and supply ink tank (1)

The Ink supply reservoir

The ink supply tank or ink storage tank is used to add ink to the ink supply and return tank when the ink in this latter fall below a certain level which is measured by level sensor(s) which is not shown here. The ink add reservoir circuit comprise the following components as shown in figure 2: a. An ink add reservoir (14) b. A filter (13) c. An ink addition electromagnetic solenoid valve (12) d. A diaphragm ink return and add pump (15) e. A main ink supply and ink return tank (1)

The ink supply umbilical:

The printheads are in fluids communication with the ink supply in the ink system enclosure by the use of flexible umbilical (conduit) of up to 8 m in length. This umbilical houses: a. The plastic ink supply and return pipes b. The electrical cable/wire that transmits the pressure sensor reading to the FMS board c. The electrical wire that transmits the temperature sensor reading to the ink management system printed circuit (not shown)

Drawings:

Figure 1 shows an example of the prior art ink supply and recirculation circuit diagram.

Figure 2 shows a schematic circuit diagram of a preferred ink supply and recirculation system in accordance with the present invention

Claims

1. An ink supply system and an ink recirculation or return system for controlling the ink pressure, or more specifically, for adjusting and controlling the vacuum at the printheads of a piezoelectric Drop On Demand (DOD) printing system stably and accurately. The ink supply and recirculation system comprising: A pressure control circuit incorporating, among other components, two proportional valves, two pumps and two pressure sensors. In use, the motor speed of each pump is kept constant while the proportional valves are regulated and adjusted to control the flow of ink supplied to and returned from the printheads. The pressure sensors located at one of the printheads are read and the proportional valves adjusted accordingly to achieve the required vacuum of the ink meniscus at the nozzles to ensure reliable operation of the printer.

2. An ink recirculation system of claim 1 wherein the ink supply and return pumps are positive displacement diaphragm pumps

3. An ink recirculation system of claim 1 wherein two spring loaded diaphragm pumps are used to suppress pressure fluctuations or pulsations resulting from operation of the supply and return diaphragm pumps respectively.

4. An ink recirculation system of any preceding claims wherein two pressure sensors are provided in a printhead for use to measure the supply pressure and vacuum at the said printhead and to use the results to adjust the proportional valves to set and maintain the desired ink meniscus pressure at the nozzles.

5. An ink circulation system of any preceding claims wherein two pressure sensors are provided. One of these is used as part of the closed loop Proportional Integral Differential (PID) to control and adjust the heater to achieve and maintain the desired ink temperature of the ink in the ink supply system. The other temperature sensor is used to measure the temperature of the ink and compare this to the set target desired operating temperature at the printheads and thus determine whether the heater needs to raise the ink supply temperature further.

6. An ink circulation system of any preceding claims wherein an ink reservoir is provided to add ink to the ink supply tank when the ink level in this fall below a certain predetermined value.