Abstract:
An ink reservoir ( 1 ) for maintaining ink ( 10 ) at a negative pressure by using the suction provided the printhead and then regulating the negative pressure with a pressure regulator ( 16 ) that allows air into cartridge at a specified pressure difference. The pressure regulator may be a porous member such as a membrane or mesh filter, or a simple pressure relief valve. The membrane ( 16 ), mesh or foam is selected such that its bubble point equates to the specified pressure difference.

Description:
FIELD OF THE INVENTION 
   The present invention relates to inkjet printers. In particular, the invention relates to the supply of ink to inkjet printheads. 
   CO-PENDING APPLICATIONS 
   The following applications have been filed by the Applicant simultaneously with the present application: 
   
     
       
             
             
             
             
             
           
         
             
                 
             
           
           
             
               7,637,588 
               11/482,970 
               11/482,968 
               7,607,755 
               11/482,971 
             
             
               11/482,969 
               7,530,663 
               7,467,846 
               11/482,962 
               11/482,963 
             
             
               11/482,956 
               11/482,954 
               11/482,974 
               11/482,974 
               11/482,987 
             
             
               11/482,959 
               11/482,960 
               11/482,961 
               11/482,964 
               11/482,965 
             
             
               7,510,261 
               11/482,973 
               11/482,982 
               7,637,602 
               11/482,984 
             
             
               7,530,446 
               11/482,990 
               11/482,986 
               11/482,985 
               11/482,967 
             
             
               11/482,966 
               11/482,988 
               11/482,989 
               11/482,980 
               7,571,906 
             
             
               11/482,953 
               11/482,977 
             
             
                 
             
           
        
       
     
   
   The disclosures of these co-pending applications are incorporated herein by reference. 
   CROSS REFERENCE TO RELATED APPLICATIONS 
   Various methods, systems and apparatus relating to the present invention are disclosed in the following U.S. patents/patent applications filed by the applicant or assignee of the present invention: 
   
     
       
             
             
             
             
             
             
             
           
         
             
                 
             
           
           
             
               09/517,539 
               6,566,858 
               6,331,946 
               6,246,970 
               6,442,525 
               09/517,384 
               09/505,951 
             
             
               6,374,354 
               09/517,608 
               6,816,968 
               6,757,832 
               6,334,190 
               6,745,331 
               09/517,541 
             
             
               10/203,559 
               10/203,560 
               10/203,564 
               10/636,263 
               10/636,283 
               10/866,608 
               10/902,889 
             
             
               10/902,833 
               10/940,653 
               10/942,858 
               10/727,181 
               10/727,162 
               10/727,163 
               10/727,245 
             
             
               10/727,204 
               10/727,233 
               10/727,280 
               10/727,157 
               10/727,178 
               10/727,210 
               10/727,257 
             
             
               10/727,238 
               10/727,251 
               10/727,159 
               10/727,180 
               10/727,179 
               10/727,192 
               10/727,274 
             
             
               10/727,164 
               10/727,161 
               10/727,198 
               10/727,158 
               10/754,536 
               10/754,938 
               10/727,227 
             
             
               10/727,160 
               10/934,720 
               11/212,702 
               11/272,491 
               10/296,522 
               6,795,215 
               10/296,535 
             
             
               09/575,109 
               6,805,419 
               6,859,289 
               6,977,751 
               6,398,332 
               6,394,573 
               6,622,923 
             
             
               6,747,760 
               6,921,144 
               10/884,881 
               10/943,941 
               10/949,294 
               11/039,866 
               11/123,011 
             
             
               6,986,560 
               7,008,033 
               11/148,237 
               11/248,435 
               11/248,426 
               10/922,846 
               10/922,845 
             
             
               10/854,521 
               10/854,522 
               10/854,488 
               10/854,487 
               10/854,503 
               10/854,504 
               10/854,509 
             
             
               10/854,510 
               10/854,496 
               10/854,497 
               10/854,495 
               10/854,498 
               10/854,511 
               10/854,512 
             
             
               10/854,525 
               10/854,526 
               10/854,516 
               10/854,508 
               10/854,507 
               10/854,515 
               10/854,506 
             
             
               10/854,505 
               10/854,493 
               10/854,494 
               10/854,489 
               10/854,490 
               10/854,492 
               10/854,491 
             
             
               10/854,528 
               10/854,523 
               10/854,527 
               10/854,524 
               10/854,520 
               10/854,514 
               10/854,519 
             
             
               10/854,513 
               10/854,499 
               10/854,501 
               10/854,500 
               10/854,502 
               10/854,518 
               10/854,517 
             
             
               10/934,628 
               11/212,823 
               10/728,804 
               10/728,952 
               10/728,806 
               6,991,322 
               10/728,790 
             
             
               10/728,884 
               10/728,970 
               10/728,784 
               10/728,783 
               10/728,925 
               6,962,402 
               10/728,803 
             
             
               10/728,780 
               10/728,779 
               10/773,189 
               10/773,204 
               10/773,198 
               10/773,199 
               6,830,318 
             
             
               10/773,201 
               10/773,191 
               10/773,183 
               10/773,195 
               10/773,196 
               10/773,186 
               10/773,200 
             
             
               10/773,185 
               10/773,192 
               10/773,197 
               10/773,203 
               10/773,187 
               10/773,202 
               10/773,188 
             
             
               10/773,194 
               10/773,193 
               10/773,184 
               11/008,118 
               11/060,751 
               11/060,805 
               11/188,017 
             
             
               11/298,773 
               11/298,774 
               11/329,157 
               6,623,101 
               6,406,129 
               6,505,916 
               6,457,809 
             
             
               6,550,895 
               6,457,812 
               10/296,434 
               6,428,133 
               6,746,105 
               10/407,212 
               10/407,207 
             
             
               10/683,064 
               10/683,041 
               6,750,901 
               6,476,863 
               6,788,336 
               11/097,308 
               11/097,309 
             
             
               11/097,335 
               11/097,299 
               11/097,310 
               11/097,213 
               11/210,687 
               11/097,212 
               11/212,637 
             
             
               11/246,687 
               11/246,718 
               11/246,685 
               11/246,686 
               11/246,703 
               11/246,691 
               11/246,711 
             
             
               11/246,690 
               11/246,712 
               11/246,717 
               11/246,709 
               11/246,700 
               11/246,701 
               11/246,702 
             
             
               11/246,668 
               11/246,697 
               11/246,698 
               11/246,699 
               11/246,675 
               11/246,674 
               11/246,667 
             
             
               11/246,684 
               11/246,672 
               11/246,673 
               11/246,683 
               11/246,682 
               10/760,272 
               10/760,273 
             
             
               10/760,187 
               10/760,182 
               10/760,188 
               10/760,218 
               10/760,217 
               10/760,216 
               10/760,233 
             
             
               10/760,246 
               10/760,212 
               10/760,243 
               10/760,201 
               10/760,185 
               10/760,253 
               10/760,255 
             
             
               10/760,209 
               10/760,208 
               10/760,194 
               10/760,238 
               10/760,234 
               10/760,235 
               10/760,183 
             
             
               10/760,189 
               10/760,262 
               10/760,232 
               10/760,231 
               10/760,200 
               10/760,190 
               10/760,191 
             
             
               10/760,227 
               10/760,207 
               10/760,181 
               10/815,625 
               10/815,624 
               10/815,628 
               10/913,375 
             
             
               10/913,373 
               10/913,374 
               10/913,372 
               10/913,377 
               10/913,378 
               10/913,380 
               10/913,379 
             
             
               10/913,376 
               10/913,381 
               10/986,402 
               11/172,816 
               11/172,815 
               11/172,814 
               11/003,786 
             
             
               11/003,616 
               11/003,418 
               11/003,334 
               11/003,600 
               11/003,404 
               11/003,419 
               11/003,700 
             
             
               11/003,601 
               11/003,618 
               11/003,615 
               11/003,337 
               11/003,698 
               11/003,420 
               6,984,017 
             
             
               11/003,699 
               11/071,473 
               11/003,463 
               11/003,701 
               11/003,683 
               11/003,614 
               11/003,702 
             
             
               11/003,684 
               11/003,619 
               11/003,617 
               11/293,800 
               11/293,802 
               11/293,801 
               11/293,808 
             
             
               11/293,809 
               11/246,676 
               11/246,677 
               11/246,678 
               11/246,679 
               11/246,680 
               11/246,681 
             
             
               11/246,714 
               11/246,713 
               11/246,689 
               11/246,671 
               11/246,670 
               11/246,669 
               11/246,704 
             
             
               11/246,710 
               11/246,688 
               11/246,716 
               11/246,715 
               11/246,707 
               11/246,706 
               11/246,705 
             
             
               11/246,708 
               11/246,693 
               11/246,692 
               11/246,696 
               11/246,695 
               11/246,694 
               11/293,832 
             
             
               11/293,838 
               11/293,825 
               11/293,841 
               11/293,799 
               11/293,796 
               11/293,797 
               11/293,798 
             
             
               10/760,254 
               10/760,210 
               10/760,202 
               10/760,197 
               10/760,198 
               10/760,249 
               10/760,263 
             
             
               10/760,196 
               10/760,247 
               10/760,223 
               10/760,264 
               10/760,244 
               10/760,245 
               10/760,222 
             
             
               10/760,248 
               10/760,236 
               10/760,192 
               10/760,203 
               10/760,204 
               10/760,205 
               10/760,206 
             
             
               10/760,267 
               10/760,270 
               10/760,259 
               10/760,271 
               10/760,275 
               10/760,274 
               10/760,268 
             
             
               10/760,184 
               10/760,195 
               10/760,186 
               10/760,261 
               10/760,258 
               11/293,804 
               11/293,840 
             
             
               11/293,803 
               11/293,833 
               11/293,834 
               11/293,835 
               11/293,836 
               11/293,837 
               11/293,792 
             
             
               11/293,794 
               11/293,839 
               11/293,826 
               11/293,829 
               11/293,830 
               11/293,827 
               11/293,828 
             
             
               11/293,795 
               11/293,823 
               11/293,824 
               11/293,831 
               11/293,815 
               11/293,819 
               11/293,818 
             
             
               11/293,817 
               11/293,816 
               11/014,764 
               11/014,763 
               11/014,748 
               11/014,747 
               11/014,761 
             
             
               11/014,760 
               11/014,757 
               11/014,714 
               11/014,713 
               11/014,762 
               11/014,724 
               11/014,723 
             
             
               11/014,756 
               11/014,736 
               11/014,759 
               11/014,758 
               11/014,725 
               11/014,739 
               11/014,738 
             
             
               11/014,737 
               11/014,726 
               11/014,745 
               11/014,712 
               11/014,715 
               11/014,751 
               11/014,735 
             
             
               11/014,734 
               11/014,719 
               11/014,750 
               11/014,749 
               11/014,746 
               11/014,769 
               11/014,729 
             
             
               11/014,743 
               11/014,733 
               11/014,754 
               11/014,755 
               11/014,765 
               11/014,766 
               11/014,740 
             
             
               11/014,720 
               11/014,753 
               11/014,752 
               11/014,744 
               11/014,741 
               11/014,768 
               11/014,767 
             
             
               11/014,718 
               11/014,717 
               11/014,716 
               11/014,732 
               11/014,742 
               11/097,268 
               11/097,185 
             
             
               11/097,184 
               11/293,820 
               11/293,813 
               11/293,822 
               11/293,812 
               11/293,821 
               11/293,814 
             
             
               11/293,793 
               11/293,842 
               11/293,811 
               11/293,807 
               11/293,806 
               11/293,805 
               11/293,810 
             
             
               09/575,197 
               09/575,195 
               09/575,159 
               09/575,123 
               6,825,945 
               09/575,165 
               6,813,039 
             
             
               6,987,506 
               09/575,131 
               6,980,318 
               6,816,274 
               09/575,139 
               09/575,186 
               6,681,045 
             
             
               6,728,000 
               09/575,145 
               09/575,192 
               09/575,181 
               09/575,193 
               09/575,183 
               6,789,194 
             
             
               6,789,191 
               6,644,642 
               6,502,614 
               6,622,999 
               6,669,385 
               6,549,935 
               09/575,187 
             
             
               6,727,996 
               6,591,884 
               6,439,706 
               6,760,119 
               09/575,198 
               6,290,349 
               6,428,155 
             
             
               6,785,016 
               09/575,174 
               09/575,163 
               6,737,591 
               09/575,154 
               09/575,129 
               6,830,196 
             
             
               6,832,717 
               6,957,768 
               09/575,162 
               09/575,172 
               09/575,170 
               09/575,171 
               09/575,161 
             
             
                 
             
           
        
       
     
   
   The disclosures of these applications and patents are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   The inkjet printheads in the above cross referenced documents have an array of nozzles, each nozzle having an associated ink ejection actuator within a nozzle chamber. Ink from a cartridge or other reservoir is fed to the chambers where the ejection actuators force drops of ink through the nozzle for printing. As printers predominantly use removable cartridges, the invention will be described with specific reference to ink cartridges. However, it will be appreciated that the invention equally applies to any fluid reservoir for supplying a printhead. 
   During periods of inactivity, the ink is retained in the chambers by the surface tension of the ink meniscus that forms across the nozzle. If the meniscus bulges outwardly, it can ‘pin’ itself to the nozzle rim to hold the ink in the chamber. However, if it contacts paper dust or other contaminants on the nozzle rim, the meniscus can be unpinned from the rim and ink will leak out of the printhead through the nozzle. 
   To address this, many ink cartridges are designed so that the hydrostatic pressure of the ink at the nozzles is less than atmospheric pressure. This causes the meniscus across the nozzle openings to be concave or drawn inwards. Paper dust or other particulate contaminants are less likely to contact the meniscus when it is inverted into the nozzle. Furthermore, a positive pressure in the ink chamber helps to drive the flow of ink leaking from the chamber once the meniscus is compromised by paper dust. 
   The negative pressure in the chambers is limited by two factors. It can not be strong enough to de-prime the chambers (i.e. suck the ink out of the chambers and back towards the cartridge) and it must be less than the ejection pressure generated by the ejection drop ejection actuators. However, if the negative pressure is too weak, the nozzles can leak ink if the printhead is jolted or shaken. While this can happen during use, it is more likely to occur during the shipping and handling of the primed printheads. 
   To establish a negative pressure, some cartridges use a flexible bag design. Part of the cartridge has a flexible bag or wall section that is biased toward increasing the ink storage volume. U.S. Ser. No. 11/014,764 and U.S. Ser. No. 11/014,769 (listed above in the cross referenced documents) are examples of this type of cartridge. These cartridges can provide a reliable and constant negative pressure, but the design is relatively complex, bulky and costly to make. Also the ratio of ink used for printing, to the total volume of ink in the cartridge is typically low. Unless the cartridge is refillable, much of the ink is wasted when the cartridge is discarded. 
   Another way of generating a negative pressure in the ink chambers is shown in  FIG. 1 . A piece of foam or porous material  2  is placed in the cartridge  1  over the outlet  3 . The foam  2  has a section that is saturated with ink  4 , and a section  5  that may be wet with ink, but not saturated. The top of the cartridge  1  is vented to atmosphere through the air maze  7 . Capillary action (represented by arrow  6 ) draws the ink from the saturated section  4  into the unsaturated section  5 . This continues until it is balanced by the weight of the increased hydrostatic pressure, or ‘head’ of ink drawn upwards by the capillary action  6 . The hydrostatic pressure at the top of the saturated section  4  is less than atmospheric because of capillary action into the unsaturated section  5 . From there, the hydrostatic pressure increases towards the outlet  3 , and if connected to the printhead (not shown), it continues to increase down to the nozzle openings (assuming they are the lowest points in the printhead). By setting the proportion of saturated foam to unsaturated foam such that the hydrostatic pressure of the ink at the nozzle is less than atmospheric, the ink meniscus will form inwardly. 
   This is a much simpler and cheaper design, but the amount of ink retained in the foam when the cartridge is discarded is still high. The need for an unsaturated section of foam, and the foam itself, makes the volumetric efficiency quite low, i.e. the ratio of ink volume to total cartridge volume is low. Furthermore, the negative pressure at the nozzle will increase as the ink level in the cartridge drops. As the negative pressure must be established at the nozzles when the cartridge is first installed, and the negative pressure increases as the ink in the cartridge is used, there are practical limits on the volume of ink that can be supplied by cartridges of this type. As previously discussed, the negative pressure at the nozzles can not be stronger than the ejection actuators or greater than the de-prime threshold. 
   One attempt to address this is schematically shown in  FIG. 2 . The cartridge  1  essentially has two chambers  8  and  9 ; one holding the foam  2  and the other holding ink  10  only. The chambers are connected by a narrow passage  11  at the floor  12  of the cartridge. The hydrostatic ink pressure below the balance line  13  is the same in each chamber for corresponding heights. The negative pressure in the sealed air space  14  above the ink in the second chamber  9  can be expressed as follows:
 
 P   air =−(ρ. g.H+P   foam )
 
   Where:
         ρ is the density of ink   g is gravity   H is height above the balance line.   P foam  is the pressure at the balance line under the influence of capillary action in the foam.       

   The negative pressure at the nozzles is provided by capillary action  6  to the unsaturated section of the foam  5 . However, the foam  2 , and therefore the printhead, is fed additional ink from the second chamber  9 . As ink drains from the second chamber  9 , tiny bubbles of air  15  form at the opening  11  and rise up to the head space  14 . This arrangement is more volumetrically efficient but still suffers from many of the problems associated with the design shown in  FIG. 1 . A substantial amount of ink remains in the foam when the cartridge is discarded and the second chamber  9  introduces an extra degree of complexity for manufacturing and charging with ink. 
   The present Applicant has developed a range of pagewidth printheads for high speed, 1600 d.p.i. full color printing. High speed pagewidth printheads introduce additional problems for cartridges with foam inserts. Firstly, the cartridge is supplying a much greater number of nozzles than a scanning printhead. In a high speed printer (speeds greater than an A4 page per second) the nozzles have a higher firing rate. Therefore the ink flow rate out of the cartridge is much greater than that of a scanning printhead. The fluidic drag caused by the foam insert can starve the nozzles and retard the chamber refill rate. More porous foam will have less fluidic drag but also much less capillary force. 
   Secondly, pagewidth printheads have a generally elongate structure. By definition they must extend (at least) the width of a page. If one end of the printhead is raised during installation or shipping, the head of ink above the lower-most nozzles can be much greater than when the printhead is horizontal. This increase can overcome the negative pressure at the lower nozzles and cause leakage. 
   OBJECT OF THE INVENTION 
   The present invention aims to overcome or ameliorate at least one of these problems, or provide a useful alternative to the prior art. 
   SUMMARY OF THE INVENTION 
   Accordingly, the present invention provides an ink reservoir for an inkjet printhead, the reservoir comprising:
         a container for maintaining a quantity of ink at a pressure less than ambient pressure;   an ink outlet for sealed fluid connection to the printhead, and, an air inlet with a pressure regulator that allows air into the container at a predetermined pressure difference between the container interior and atmosphere.       

   In some embodiments, the pressure regulator is a valve relief valve. In other embodiments, the pressure regulator is a porous member with a first surface for exposure to atmosphere and a second surface for contacting the ink in the container; wherein during use, air at the first surface moves to the second surface and forms bubbles. 
   Instead of generating the negative pressure in the cartridge with capillary action or biased flexible bags, the present invention uses the suction provided the printhead to drop the pressure in the cartridge to the desired negative pressure and then uses a pressure regulator at the air inlet to keep control the level of negative pressure. The regulator can be a valve member that allows air into cartridge at a specified pressure difference or it could also be porous material with a particular ‘bubble point’. The term ‘bubble point’ is explained below. 
   Using a valve member, such as a simple pressure relief valve in the wall of the cartridge allows the negative pressure inside the cartridge to be closely controlled. By locating the valve so that it is slightly elevated relative to the ink outlet, the hydrostatic pressure of the ink at the outlet remains constant and so the pressure in the nozzles chambers is also constant (ignoring fluctuations from movement or jarring of the printhead). 
   The pressure valve can also provide a convenient point from which the initially charge the cartridge with ink. As discussed above, the nozzles of a pagewidth printhead generate relatively high suction on the cartridge so the threshold pressure difference can be relatively high. The pressure difference should at least be greater than 10 mm H 2 O, but a more practical level would be greater than 300 mm H 2 O. With a high pressure threshold, the negative pressure is strong enough to counter the higher hydrostatic pressures in the lowest nozzles if the printhead is ever angled or held vertically. 
   Even though the pressure relief valve can be relatively simple and inexpensive, a porous member with a suitable bubble point is an even simpler and cheaper form of pressure regulation. The bubble point of porous material is the air pressure applied to one side of the material in order form a bubble on another side that is immersed in ink. Obviously, the bubble point for a given porous material will vary depending on the type of gas and the type of liquid used. The porous material can be in the form of a membrane, mesh or open cell foam. In the case of foam, it is important to note that its function is not to provide any capillary action for generating negative pressure and therefore it is much denser and smaller than the foam inserts used in the prior art cartridges. A foam member used in the present invention absorbs and retains very little ink compared to the foam inserts of the prior art. 
   It will be appreciated that the porous membrane, mesh or foam member can be positioned toward the bottom of the cartridge to maintain a constant hydrostatic pressure at or near the ink outlet. Firing the nozzles will drop the pressure in the cartridge until the bubble point is reached. Continued firing of the nozzles does not further reduce the pressure as tiny air bubbles permeate through the membrane, mesh or dense foam member. 
   Very little ink is retained by the membrane, mesh or foam so the proportion of ink used for printing is much higher. Similarly, the whole cartridge can have a more compact design for a given quantity of ink. Furthermore, it is a simple matter to select a material with bubble point high enough to generate a negative pressure strong enough for pagewidth printheads. The Applicant&#39;s printheads generate about 1200 mmH 2 O nozzle ejection pressure (per color). Therefore, a membrane with a bubble point of approximately 300 mmH 2 O to 600 mmH 2 O is readily available and will generate a negative pressure strong enough to guard against leakage from inclining the printhead or mild jarring. 
   The cartridges have an increasing head space of air as the ink is used. If the internal surface of the air permeable member is exposed to the air in the cartridge it can dry out and become much more permeable to air. If this happens the cartridge will effectively vent to atmosphere and the negative pressure is lost. To safeguard against this, the internal surface of the permeable member must be kept wet. Some ways of achieving this are:
         Using a foam element that absorbs and retains some ink;   Using a second membrane spaced from, but close to, the inner surface of the first membrane so that ink stays between the membranes, even if the cartridge is oriented so that the membranes are in the air of the headspace;   Providing the porous member in a wall of the cartridge and then a hydrophilic wall closely adjacent to the internal surface of the permeable member so that capillary action keeps the internal surface wet (and optionally, putting some wicking material or mesh between the hydrophilic and the internal surface); and,   A series of internal baffles forming an ink trap or maze that maintains ink next to the internal surface.       

   In some embodiments of the invention, the air inlet also has an air maze structure so in the event that ink permeates through the air permeable material, it does not leak to the exterior of the cartridge. The ink outlet may have a filter covering to stop air bubbles from getting to the nozzles. However, the filter should not create a significant flow restriction for the ink. The outlet is not obstructed by a foam insert as it is in the prior art cartridges, and therefore cartridges according to the present invention can supply ink at a high flow rate. As previously discussed, high speed pagewidth printheads require high ink flow rates. 
   Other features and advantages of the present invention will become apparent from the following detailed description of preferred embodiments. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
       FIG. 1  is a schematic section view of a prior art ink cartridge; 
       FIG. 2  is a schematic section view of another prior art ink cartridge; 
       FIG. 3  is a schematic section view of an ink cartridge according to the present invention; 
       FIGS. 4 and 5  are partial schematic section views of alternatives to the ink cartridge shown in  FIG. 3 ; 
       FIGS. 6 and 7  are schematic section views of a double membrane cartridge in different orientations; 
       FIG. 8  is a schematic section view of a cartridge with single membrane and hydrophilic internal wall; 
       FIG. 9  is a partial schematic section view of an alternative to cartridge shown in  FIG. 8 ; and, 
       FIG. 10  is a schematic sectioned perspective of a cartridge according to the invention showing the possible configuration of the tortuous air inlet flow path. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 3  is a simplified sketch of the invention to illustrate the basic operating principles. It uses a membrane  16  positioned near the floor  12  of the cartridge  1  to maintain a negative pressure at the control level  13 . Unlike the prior art cartridges of  FIG. 1 and 2 , the hydrostatic pressure at the control level is set by the bubble point of the membrane. As previously discussed the bubble point of porous material is the gas pressure that needs to be applied to one side to force liquid from the largest wetted pore on the immersed side. 
   When the cartridge is installed, the nozzles can fire into a blotter or the like to lower the pressure in the cartridge. When the pressure at the control level  13  drops to the bubble point, small bubbles  15  will form on the internal surface of the membrane  16  and rise into the head space  14 . This slightly increases the pressure in the cartridge and the bubbles  15  stop forming on the membrane  16 . Once bubbles start forming on the inside of the membrane  16 , the hydrostatic pressure at control level  13  is known. Likewise, if a different pressure regulator is used, once the printhead has initially established the required negative pressure, the control level  13  keeps a constant hydrostatic pressure (equal to the regulator threshold pressure). 
   As ink is consumed by the printhead, the negative pressure at the control level  13  (and therefore at the outlet  3 ) will remain effectively constant. Of course, if the ink level drops below the control level  13 , the membrane  16  is no longer covered by ink and the cartridge vents to atmosphere. To avoid this, the printer should stop printing before the ink level reaches the control level. However, there are methods for keeping the membrane wet when it is exposed to the air of the headspace  14 . These are discussed in detail below. 
   The embodiment shown in  FIG. 4  uses a small block of dense open cell foam  17  instead of the membrane  16  of the previous embodiment. The bubble point of the foam sets the hydrostatic pressure at the control level  13  and the cartridge  1  operates in way as the membrane embodiment. 
   The foam is denser than that used in the prior art cartridges so that the bubble point is high enough to generate the required negative pressure. However, it absorbs some ink and will stay wet (temporarily at least) if it is exposed to the air in the headspace. It will be appreciated that the foam can be easily exposed to the air in the cartridge when the printhead is moved or transported. 
   In these embodiments of the invention, the air inlet  7  has an air maze structure. If ink happens to permeate through the porous material (membrane  16  in  FIG. 3  and foam element  17  in  FIG. 4 ), it does not leak to the exterior of the cartridge. The ink outlet  3  may have a filter  23  covering to stop air bubbles from getting to the nozzles. However, the filter should not create a significant flow restriction for the ink. The outlet  3  is not obstructed by a foam insert as it is in the prior art cartridges and so can supply ink at a high flow rate. As previously discussed, high speed pagewidth printheads require high ink flow rates. 
   The embodiment shown in  FIG. 5  is even simpler in the sense that it does not need an inlet air maze  7  or internal passage covered by a membrane, mesh or foam element. Instead, a pressure relief valve  18  in the wall of the cartridge opens at a threshold pressure which sets the hydrostatic pressure at the control level  13 . Furthermore, if the internal side of the valve is exposed to the air in the cartridge, it does not vent to atmosphere like a dry membrane or foam. It opens when the pressure difference reaches the specified threshold and so maintains a negative pressure in the cartridge even after the ink has dropped below the control level  13  (although the pressure at the outlet  3  will slightly decrease as the level drops below the valve  18 ). 
   The pressure relief valve  18  can be a simple ball-type check valve that is biased into its seat to keep the unit cost to a minimum. It is unlikely to be cheaper than a membrane or foam element however it does provide a convenient means for initially charging the cartridges with ink and allows the cartridge to be very compact. 
   Returning to the membrane embodiment,  FIGS. 6 and 7  show a solution to the problem of membrane drying discussed above. Instead of a single membrane, a pair of membranes  19  and  20  is used. The membranes are closely spaced so that the ink between them does not drain out if the cartridge is positioned such that they are in the air of the headspace  14  (see  FIG. 7 ). As long as the internal surface of the outer membrane  19  stays wet, the cartridge  1  will not vent to atmosphere. 
     FIGS. 8 and 9  show another version of the membrane embodiment that also avoids the membrane drying problem. The cartridge of  FIG. 8  has a membrane  16  in the wall of the cartridge  1 . Closely adjacent the internal surface of the membrane  16  is an internal wall  21 . For water based inks, the internal wall  21  should be approximately 1 mm away from the membrane. The internal wall  21  is made of a hydrophilic material so that ink is held between the wall and the membrane  16  by capillary action when the ink level drops below the membrane. The tiny air bubbles  15  permeating through the membrane  16  rise up through the ink held the wall  21  and into the air space  14 . 
   In  FIG. 9 , wicking material  22  is placed between the wall  21  and the membrane  16  to enhance the capillary action. The wicking material can be fabric, mesh or particulate material. By enhancing the capillary action the ink level can drop further below the membrane before its internal surface dries out. The wicking material also damps any jolts or impacts to the printhead that might otherwise dislodge the ink from between the membrane  16  and the wall  21 . 
   Cartridges according to the invention are particularly suited to use with the Applicant&#39;s range of pagewidth printheads. These printheads will typically generate 1200 mm.H 2 O of suction pressure per color which is much higher than that generated by a scanning type printhead. As the present invention uses the printhead to establish negative pressure in the cartridge, a strong suction allows the threshold pressure of the valve of air permeable material to be relatively high, which in turn allows a stronger negative pressure in the cartridge. A stronger negative pressure in the cartridge makes the nozzles less prone to leakage, particularly the lowest nozzles of a pagewidth printhead that is moved from it horizontal orientation. Furthermore, as discussed above, the unobstructed outlets allow a high ink flow rate to the nozzles. 
     FIG. 10  shows how the air inlet maze might work in practice. The container  8  holds a quantity of ink  10  and encloses the inlet maze  26 , the air expansion chamber  27 , inlet membrane  16  and outlet filter  23 . Inlet opening  25  is open to atmosphere and the outlet  3  forms a sealed fluid connection with the printhead when the cartridge is installed. The cartridge is filled through a sealable fill hole  28  in the top wall. The entire container  8  can be rigid or, parts of the container can be flexible material to lower materials costs. For example the large side walls  30  and  31  can be air and ink impermeable film sealed to the periphery of a rigid wall middle section. 
   The air inlet tube  26  follows a tortuous path to the membrane  16 . The tortuous path has irregular changes in direction so that any ink seeping into the tube  26  is very unlikely to leak out of the inlet opening  25  even if the cartridge is rotated through different orientation during transport. For ink in the lower section of the tube  26  to reach the opening  25 , the cartridge needs to go through a precise sequence of rotations in different directions. The risk of this happening by chance during transport and handling is negligible. 
   The air inlet tube  26  incorporates an air expansion chamber  27 . The cartridge is expected to be exposed to a wide range of temperatures—approximately 35° C. Any ink trapped in the line  26  can be forced to the opening  25  by the increased air pressure. The air expansion chamber  27  is relatively large compared to the tube  26  and so has more capacity to accommodate an expanding gas. 
   The inlet membrane  16  and the associated-chamber  29  is smaller than that of the ink outlet ( 23  and  24  respectively). This accounts for the high rate of ink supply required by the pagewidth inkjet printheads whilst also filtering the ink that leaves through the outlet  3 . The large diameter filter  23  and associated chamber  24  means that the filter surface area is high so that the filter can keep a small pore size to remove all detrimental contaminants, without being an undue flow constriction in the ink supply. 
   The tortuous air inlet path  26  and air expansion chamber  27  effectively prevent ink leakage during transport and handling, with minimal added complexity and cost. The membrane is at the floor of the cartridge so that the negative ink pressure at the outlet  3  will be the bubble point of the membrane regardless of the amount if ink  10  that has been consumed. Furthermore, the vast majority of the ink will be consumed before the membrane is exposed and vents the interior to atmosphere. At this point the cartridge needs to be replaced however, only a small amount of ink will remain in the cartridge when it is discarded. 
   These embodiments are merely illustrative and the skilled worker will readily recognize many variations and modifications that fall within the spirit and scope of the broad inventive concept.