Abstract:
Disclosed is an apparatus for degassing a liquid, comprising: a liquid supply and an outlet communicating with the liquid supply via a flow path; a degassing tank, disposed in the flow path between the liquid supply and the outlet, the degassing tank incorporating a gas-permeable vent; a means for moving the liquid along the flow path; and a heater, disposed in the flow path between the degassing tank and the outlet, for heating the liquid and thereby removing gas therefrom. The apparatus is particularly suited to the ink supply system of a thermal ink jet printer where the removal of gases dissolved in the ink improves print quality.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to a liquid degassing apparatus, and more particularly to a degassing apparatus in the ink supply system of a thermal ink jet printing device. 
     A thermal ink jet printer has at least one printhead in which thermal energy pulses are used to produce vapor bubbles in ink-filled channels and so cause droplets of ink to be expelled from the channel orifices towards a recording medium. The thermal energy pulses are usually produced by resistors, each located in a respective one of the channels, which are individually addressable by current pulses to heat and vaporize ink in the channels. As a vapor bubble grows in any one of the channels, ink bulges from the channel orifice until the current pulse has ceased and the bubble begins to collapse. At that stage, the ink within the channel retracts and separates form the bulging ink which forms a droplet moving in a direction away from the channel and towards the recording medium. The channel is then refilled by capillary action, which in turn draws ink from a supply container. It is conventional to provide an arrangement to clean the channel orifices periodically while the printhead is in use and to close-off the orifices when the printhead is idle to prevent ink in the printhead from drying out. 
     One form of thermal ink jet printer is described in U.S. Pat. No. 4,638,337 to Torpey et al. That printer is of the carriage type and has a plurality of printheads, each with its own ink supply cartridge, mounted on a reciprocating carriage. The channel orifices in each printhead are aligned perpendicular to the line of movement of the carriage and a swath of information is printed on the stationary recording medium as the carriage is moved in one direction. The recording medium is then stepped, perpendicular to the line of carriage movement, by a distance equal to the width of the printed swath and the carriage is then moved in the reverse direction to print another swath of information. As an alternative to providing each printhead with its own ink cartridge, the printheads can be supplied with ink from one or more supply tanks which need not be mounted on the carriage. 
     U.S. Pat. No. 4,454,518 refers to the importance of temperature control in an ink jet printer and, in particular, the control of the ink temperature in a printer of the type that utilizes a piezoelectric transducer to cause the discharge of ink droplets from a printhead. U.S. Pat. No. 4,929,063 describes the cooling of the printhead of a thermal inkjet printer by causing ink to flow through the printhead in a volume far greater than that required for printing purposes. Temperature control of thermal inkjet printheads is also discussed in U.S. Pat. Nos. 4,896,172 and 4,980,702. 
     U.S. Pat. No. 5,121,130 to Hempel et al. discloses a printhead assembly for a thermal ink jet printer in which the ink supply path carrying ink to the printhead passes through, and receives heat form, a heat sink adjacent the print heaters of the printhead. The ink then passes to the printhead via a secondary reservoir, the position of which relative to the printhead establishes the ink pressure at the printhead discharge orifices. Capping means is provided to cap the discharge orifices when the printhead is idle and to purge ink from the printhead when required. 
     Problems arise with ink jet printers which are known in the art due to dissolved gases being present in the ink in the ink reservoir. As well as making it difficult to control the temperature of the ink, the dissolved gases can have a significant effect on the amount of ink expelled in a droplet when the ink near a channel orifice is heated, and on the manner of its expulsion, thereby reducing print quality. When a heater resistor near a channel orifice is addressed, dissolved gases in the vicinity expand and even merge with the ink vapor bubble, thereby distorting the vapor bubble, and therefore the ink droplet, from its optimum volume and shape. Also, in ink jet printers it is conventional to maintain a negative pressure at the printhead; the presence of air bubbles may make this difficult. A degree of resistance to flow of the ink may also be produced by dissolved gas bubbles, particularly when collected around filter screens so that capillary refill is impeded or blocked. 
     The present invention seeks to provide a liquid degassing apparatus for removing gases dissolved in a liquid. The present invention further seeks to provide an ink degassing system in the ink supply system of an ink jet printer. The present invention further seeks to provide ink supply system of an ink jet printer in which the ink temperature and the amount of dissolved gas in the ink is precisely controlled. 
     SUMMARY OF THE INVENTION 
     The present invention provides an apparatus for degassing a liquid, comprising: a liquid supply and an outlet communicating with the liquid supply; a degassing tank, disposed between the liquid supply and the outlet, the degassing tank incorporating a gas-permeable vent; a means for recirculating the liquid; and a heater, disposed between the degassing tank and the outlet, for heating the liquid and thereby removing gas therefrom. The liquid may comprise ink (or any other liquid) and the outlet may be connected to the printhead of an ink jet printer. 
     Preferably, the outlet is connected to the degassing tank at the base thereof, and the heater is disposed adjacent the base. Preferably, the degassing tank comprises an overflow tank having an inlet, an overflow outlet and a supply outlet, the degassing tank being arranged with the supply outlet disposed below the level of the overflow outlet. Preferably, the overflow outlet is connected to the liquid supply. Alternatively, the level of the liquid may be maintained by means of a level sensor and control system linked to the recirculating means, such as a pump. Preferably, the overflow tank is mounted near but slightly below the outlet (and therefore the printhead) so as to maintain a negative pressure of, for example, -1 inch (-25.4 mm) of water at the printhead. 
     Preferably, the flow path between the supply outlet of the degassing tank and the outlet is provided by a gas-impermeable conveyance, e.g. a stainless steel or teflon pipe. 
     In one embodiment, the heater comprises metal wire or foil extending around the periphery of the flow path. The wire or foil may be mounted inside or outside the hose. Preferably, the wire or foil extends from the degassing tank for a distance along the flow path, the distance being selected to achieve optimum heating of the ink. In another embodiment, the heater is an immersible heater located in the degassing tank or flow path and upstream from a filter. 
     Preferably, a filter is provided in the flow path, between the heater and the outlet. The filter may comprise any suitable conventional filter. 
     Preferably, the vent includes a membrane which is impermeable to the liquid, but permeable to air or gas. The membrane may be formed of Goretex™ fabric. 
     According to another aspect of the present invention there is provided an apparatus for degassing a liquid, comprising: a liquid supply, an overflow tank and a pump for circulating the liquid from the supply to the tank and back via an overflow outlet to the supply, the overflow tank providing above the liquid an airspace which is vented to the atmosphere and an outlet below the surface of the liquid for delivering degassed liquid, the outlet having associated therewith a heater for heating and degassing the liquid. 
     According to another aspect of the present invention there is provided an ink supply system for an ink jet printer, incorporating a degassing apparatus as described above. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An embodiment of the invention will be described, by way of example, with reference to the accompanying drawing, in which: 
     FIG. 1 is a diagram of a thermal ink jet printer including an ink supply system; and 
     FIG. 2 illustrates a degassing apparatus used in the ink supply system shown in FIG. 1. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The printhead 1 of the assembly, which is shown in FIG. 1 in dashed line, is mounted on electrode board 2 and can be of any conventional type, and a suitable printhead is disclosed in U.S. Pat. No. 5,121,130 to Hempel et al., which is incorporated herein by reference. 
     The ink supply for the printhead 1 (shown in dashed line) is contained in a main supply tank 6. Ink is delivered from the tank by a pump 7 located in a line 8 that extends from the tank outlet 9 and through the heat sink 5 to a secondary supply tank 10, which is described in greater detail below. If the main supply tank is located above the heat sink and the secondary supply tank, the ink may be delivered by gravity. A supply outlet 11 at the bottom of the secondary tank 10 is connected by a line 12 to deliver ink to the printhead adaptor block 4 while a return outlet 13 at the top of the secondary tank is connected by a line 14 to return ink to the main supply tank 6. On the top of the secondary tank 10 there is an air vent 15. 
     Ink delivered to the printhead adaptor block 4 passes, via fill holes (not shown), into the manifold in the printhead. The ink channels in the printhead are filled from that manifold by capillary action in the usual way. 
     A maintenance station 16 for the printhead is connected to a respective port in the top of the main tank 6 by a line 17. A vacuum pump 18, which is associated with operation of the maintenance station 16 as will be described below, is also connected to a respective port in the top of the main tank, by a line 19. 
     It will be understood that the system may include fluid filters as appropriate in the ink flow lines, for example in the supply and return lines 8, 14 and in the printhead adaptor block 4. 
     Operation of the printhead assembly shown in FIG. 1 will now be described and, initially, it will be assumed that the printer of which the assembly forms part is already in operation so that the pump 7 is running and ink is being drawn from the main tank 6. The pump 7 delivers ink to the secondary tank 10 at a rate which, under all conditions, is at least equal to the rate at which ink is withdrawn from the overflow tank for printing (i.e. via the supply outlet 11). Consequently, the ink level within the secondary tank 10 may rise and, if it reaches the return outlet 13, excess ink is returned to the main tank via the line 14. 
     At the printhead 1, drops of ink are discharged from the channel orifices by the formation of vapor bubbles within the ink channels in known manner. Following each discharge, the channel is re-filled by capillary action from the adaptor block 4 which, as already described, receives ink from the secondary tank 10. 
     During its passage from the main tank 6 to the secondary tank 10, the ink passes through the heat sink 5 of the printhead, close to the heater resistors in the ink channels, and is heated. Consequently the ink arrives at the secondary tank 10 with a higher temperature than the ink in the main tank 6 and is, accordingly, more suited to the elevated temperature at which the printhead 1 is operated. 
     The maintenance station 16, the construction of which is conventional, will not be described in detail; for more information on a typical maintenance station refer to U.S. Pat. No. 5,121,130. It will be appreciated that the maintenance station 16 could be connected to a vacuum pump 18 directly instead of through the tank 6 as shown in the drawing. However, the arrangement shown has the advantage that any ink drawn from the printhead during a cleaning operation flows directly into the tank 6 while ensuring that the return outlet 13 of the secondary tank is connected to a source of suction (via the top of the main tank) to prevent overfilling. 
     It is generally accepted that the ink pressure at the channel orifices in the printhead should be maintained at a slightly reduced level, typically in the range of from -0.2 to -2.0 inches. (-0.5 to -5.1 cms) of water. In the arrangement shown in FIG. 1, the location of the secondary tank 10 relative to the channel orifices determines the ink pressure at the latter and is adjusted to ensure that a negative pressure head is established and that the desired ink pressure at the channel orifices is achieved. The possibility of pressure surges occurring in the ink is reduced by the venting of the secondary tank 10 to atmosphere, at 15. 
     The arrangement shown in FIG. 1 enables effective control of the printhead temperature to be achieved. 
     An arrangement as shown in FIG. 1 can be used in a carriage-type printer as described and is also applicable to large, stationary arrays of printheads with large common heat sinks and remote ink supplies such as are found in pagewidth printers. In a color printer, a plurality of printheads (typically four) is provided, each assembly being as shown in FIG. 1 and being used to discharge ink of a particular color. Certain components of the system shown in FIG. 1 can also be shared by some or all of the printheads. For example, a single vacuum pump 18 only is required, even when (as, for example, in a color printer) there is more than one main tank 6. During priming, pump 7 need not be operated and optional valve 20 is closed, so that ink removed by priming is withdrawn from secondary tank 10. 
     Referring to FIG. 2, this shows the liquid degassing apparatus in accordance with the present invention, as used in the ink supply system of FIG. 1. The basic component of the degassing apparatus is the secondary tank 10 which functions as an overflow tank. The tank 10 receives at its inlet 21 a liquid such as ink 34 which is pumped from the liquid supply 6 through heat sink 5 by pump 7. The overflow outlet 13 is at the same height as the inlet 21 so that the ink forms a surface 22 at about the same level; any excess ink flows through the overflow outlet 13 and is returned to the liquid supply 6 via line 14 as indicated by arrow 14A. The tank 10 is mounted relative to the printhead 1 in such a way as to maintain a negative pressure of about -1 in (-25.4 mm) at the printhead. 
     Above the surface of the liquid ink is a space 24 from which gas can escape to the atmosphere 26 through vent 15. A semi-permeable membrane (Goretex™ fabric) 28 is stretched across vent 15; the membrane permits the escape of gas but prevents the escape of liquid ink through the vent 15. 
     A gas-impermeable stainless steel tubing 30 is connected to the supply outlet 11 of the tank 10. At the junction of the tubing 30 with the supply outlet 11 several turns of resistive wire 32 are wound around the tubing 30 and connected to a suitable power supply (not shown), thereby forming a means for heating the ink 34 in the vicinity of the supply outlet 11. The wire 32 extends about 0.25 to 0.75 inch (6.4 to 19 mm) along the tubing 30. In operation, the heating of the ink 34 causes gases dissolved in the ink 34 to come out of solution and rise into the space 24, pass through the membrane 28 into the atmosphere 26, prior to the ink reaching the exit filter 36 (see below). Thus the ink passing to the printhead 1 along tubing 30 is substantially free of dissolved gases and remains that way. Should any trapped air bubbles be introduced downstream from the filter, the degassed ink will absorb it. To provide a further means of ensuring that trapped gases and/or impurities are not passed to the printhead, the filter 36 is arranged in the flow path of the ink 34, downstream of the heater wire 32. 
     The length of tubing 30 between the supply outlet 11 and the filter 36 should be long enough to ensure that any released gas will rise into space 24 in secondary tank 10 rather than travel downstream to the filter. The filter would stop any outgassed bubbles from passing therethrough and, thus, prevent them from entering the printhead, but gas bubbles collected or trapped on the filter surface would impede the flow of ink therethrough. Accordingly, the tubing 30 diameter and length between the outlet 11 and filter 36 must be selected to enable very small gas bubbles to join together to form larger bubbles which would tend to escape into the space 24 in secondary tank 10 prior to being drawn to the filter 36. One method to ensure this gas bubble control is to provide roughened surface sites (not shown) in the heated zone, as on the interior of the tubing 30, so that gas bubbles readily nucleate thereon (i.e., on the irregular surface of the roughened areas). Readily nucleated bubbles tend to grow and join together to form larger bubbles which more quickly move upward towards the space 24 in secondary tank 10. Alternatively, an immersion heater 32A could be used to heat the ink at a location either in the tubing 30 upstream from the filter 36 or in the bottom of the secondary tank near the outlet 11, whereby the ink passing the heated zone and approaching the filter is substantially degassed. The immersion heater could also have the roughened nucleation sites which readily nucleate and grow bubbles. 
     The present disclosure has been made only by way of example and numerous changes in details of construction, as well as different combinations and arrangements of parts, may be made without departing from the true spirit and scope of the invention as hereinafter claimed.