Patent Publication Number: US-6660103-B1

Title: Cleaning process for ink jet printheads

Description:
BACKGROUND 
     Certain printing systems use drop on demand ink jet printheads. These printheads deposit ink through one or more nozzles onto a substrate to create a desired image. For example, in some large-scale applications, ink jet printing systems have been used to print images on substrates, such as banners, museum displays, billboards, sails, and bus boards. 
     Typically, the ink used in these printing systems are made of a dye or pigment to create the various colors of the image, and a carrier liquid, such as water or some other suitable solvent. In addition, the ink contains a polymer that acts as a glue which fuses to a harden state to keep the pigment in place after the ink has been deposited onto the substrate. 
     However, over time, the ink accumulates in the nozzles, as well as around the orifices of the nozzle plates, and in the various channels of the printhead which convey the ink through the printhead to the nozzles. 
     Conventional techniques to clean the printheads include removing the printhead from the printing system, forcing a solvent such as glycol ether DPM acetate through the printhead, and then purging the printhead with high pressure air to blow out the debris from the channels and nozzles. 
     However, forcing glycol ether DPM acetate through the printhead has certain drawbacks. When glycol ether DPM acetate is streamed into the head, the fluid simply takes the easiest route through the printhead, thereby avoiding any blocked channels. The printheads are typically held together by an epoxy that can break apart if a chemical that is too aggressive is introduced, or if a pressure that is too high is used to force solvents or air through the printhead. 
     Glycol ether DPM acetate is commonly used in cleaning operations of printheads, which is not a very aggressive chemical. As such, glycol ether DPM acetate does not properly re-dissolve polymer that has been throughly dried. Furthermore, glycol ether DPM acetate tends to simply break the very dry polymer into chunks which can then flow into the smaller channels of the printheads, thereby exacerbating the problem. Thus, the use of glycol ether DPM acetate is typically effective if the printheads are still wet with ink or if used immediately after blockage is detected. 
     The present invention implements a method of cleaning ink jet printheads without rendering the printheads inoperative by soaking the printheads in a first cleaning solution of acetone and n-methyl-2-pyrolidone, and then flushing the printhead with a gas, such as air. The solution is made of about 70% acetone and about 30% n-methyl-2-pyrolidone. 
     In some embodiments, prior to soaking the printhead the printhead is flushed with a second cleaning solution, such as, for example, glycol ether DPM acetate, and an operator observes the streaming of the second cleaning solution from one or more nozzles of the printhead to determine if the printhead is partially or fully plugged. The printhead can also be flushed with the second solution after being flushed with the first cleaning solution. In some instances, the printhead is discarded if the printhead remains partially or fully plugged. 
     In certain embodiments, a print test is performed. If the print head passes the print test, it is typically returned to service. If not, then steps are taken to determine if the failure is due to an electrical malfunction. If the failure is attributable to an electrical malfunction, the printhead is disassembled to determine the cause of the electrical malfunction. If an electrical malfunction is not the cause of the failure of the print test, then the print head is again soaked in the first cleaning solution, and then flushed with air. If the print head still fails the print test, the printhead is typically discarded. 
     In some embodiments, the process of soaking the printhead in the first cleaning solution and then flushing the printhead with air is performed two to three times or more. The soaking process can occur over a time period of about 15 minutes, and the flushing process can occur over a time period of about 10 seconds. The gas can be at a pressure of about 5 psi. 
     In another embodiment, a method of cleaning a printhead includes soaking the printhead in a solution made of acetone and n-methyl-2-pyrolidine, flushing the printheads with a air, repeating the soaking and the flushing steps two additional times, flushing the printhead with a solution of glycol ether DPM acetate, and observing the streaming of the solution of glycol ether DPM acetate from the nozzles. These steps can be followed by a print test as described above. 
     Embodiments may have one or more of the following advantages. Soaking the printheads in a cleaning solution of n-methyl-2-pyrolidone and acetone for a limited period of time does not cause damage to the printheads, although the cleaning solution is an aggressive chemical. In particular, the cleaning solution does not dissolve the epoxy, which holds the printheads together because the cleaning solution is able to clean out the dried ink before dissolving the epoxy. Also, since the printheads merely soak in the cleaning solution, and the air used to flush the printheads is at a low pressure, the printheads are not subjected to high internal pressures which can damage the printheads. Soaking the printheads in the cleaning solution facilitates capillary action that draws the cleaning solution up into the blocked nozzle orifices. The capillary action of the soaking process of the present invention is an effective means of rewetting and re-dissolving the pigment/polymer plugs that can cause blockage of the printheads. The yield from the cleaning process is higher than that of conventional techniques. That is, of the printheads pulled from service to be cleaned, the cleaning process is able to clean a large percentage (over 90%) of the printheads so that they can be returned to service. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
     FIG. 1A is a perspective view of a series of printheads soaking in a cleaning solution within a tray in accordance with the invention. 
     FIG. 1B is a perspective view of the tray of FIG.  1 A. 
     FIG. 2 is a perspective view of a single printhead of FIG.  1 A. 
     FIG. 3 is a cross-sectional view of the printhead of FIG. 2 along the line  3 — 3 . 
     FIG. 4 is a cross-sectional view of the printhead of FIG. 2 along the line  4 — 4 . 
     FIG. 5A is a flow diagram of step  1  of a sequence of steps for cleaning the printhead of FIG. 2 in accordance with the invention. 
     FIG. 5B is a flow diagram of step  2  of the sequence of steps for cleaning the printhead of FIG.  2 . 
     FIG. 5C is a flow diagram of step  3  of the sequence of steps for cleaning the printhead of FIG.  2 . 
     FIG. 6A illustrates how the nozzles of the printhead of FIG. 2 spray for a good stream test. 
     FIG. 6B illustrates how the nozzles of the printhead of FIG. 2 spray for a bad stream test. 
     FIG. 7A is a flow diagram of an alternative sequence of steps for cleaning the printhead of FIG.  2 . 
     FIG. 7B is a flow diagram of a sequence of steps for a print test. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A description of preferred embodiments of the invention follows. 
     Referring to FIGS. 1A and 1B, there is shown a series of printheads  10  soaking in a cleaning solution  12  contained in a tray  14 . The printhead  10  sits on top of a jig  16  so that the printhead  10  is immersed in the cleaning solution up to a fill-to line  18  (FIG.  2 ). The cleaning process of the present invention is able to remove ink that has partially or fully plugged the various channels of the printhead  10  without dissolving the epoxy which holds the printhead together, nor using a high pressure purging process, which may also break apart the printhead, to blow debris out of the printhead. The cleaning process can be used to clean out solvent-based inks and water-based inks. The cleaning process can also be used to clean out UV-curable inks if the inks have not fully cured. 
     The printhead  10 , shown in greater detail in FIGS. 2A-2C, typifies the type of printhead used in digital drop on demand inkjet printing. Although the cleaning process is described below in conjunction with an ink jet printhead as shown in FIG. 2, the cleaning process of the present invention can be used to clean other types of printheads as well, such as, for example, those made from ceramic material or stainless steel. 
     The embodiment of the printhead  10  illustrated in FIGS. 2A-2B includes a pair of modules  20   a  and  20   b  press fit into a collar assembly  22 . The collar assembly  22  is provided with a base  24  attached to an upper portion  23  of the collar assembly  22 . A rock trap  27  is positioned between the modules  20   a  and  20   b  and the base  24 . The rock trap prevents debris from flowing into the numerous channels of the base  24 . 
     When the printhead  10  is in use, ink flows through an inlet  25  (FIG. 2) of the collar  22  into a manifold  26  of each module  20   a  and  20   b.  A set of channels  28  of each module  20   a  and  20   b  conveys the ink from the manifold  26  to another set of channels  30  of the base  24 . The ink flows through the channels  30  and exit from a set of a set of nozzles  32  with orifices aligned linearly on a nozzle plate  33  through which the ink is emitted and then deposited onto a substrate. The flow of the ink through the modules is indicated by the set of arrows A. 
     For each module  20   a  and  20   b,  there are 128 channels  28  and a corresponding number of channels  30  in the base  24 . The channels  30  and hence the nozzles  32  are interlaced so that there are 256 nozzles aligned in a linear manner. That is, as one nozzle emits ink from either module  20   a  or  20   b,  an adjacent nozzle emits ink from the other module. The printhead  10  is a type of an ink jet printhead manufactured by Spectra, Inc. of Hanover, N.H., and is described in greater detail in the product brochure entitled “Nova JA-256/80 Liquid,” by Spectra, Inc., the entire contents of which are incorporated herein by reference. 
     Typically, the channels  30  have several right angles and have a diameter of about 100 μm, while the orifices of the nozzles  32  on the nozzle plate  33  have a diameter of about 50 μm. Over time, some of the ink accumulates in the channels  30  and the nozzles  32 . For example, solvent-based inks, as well as water-based inks and UV-curable inks, contain a polymer that holds the ink in place when deposited onto a substrate. However, this polymer may build up in the various channels and nozzles and then partially or fully plug up the channels and nozzles. Embodiments of the cleaning process are able to redisolve this polymer so that it can be flushed out and the printhead  10  can be returned to service. 
     Referring now to FIGS. 5A-5C, a process  100  for cleaning the printhead  10  will be described in accordance with the invention. After the process  100  initializes the first part  102  of the process in step  103 , the process performs, in step  104 , a pretest of the printhead  10 . The process flushes the printhead with a solvent such as glycol ether DPM acetate under a pressure of about 5 psi, and an operator observes the streaming of the solvent of glycol ether DPM acetate through the nozzles of the printhead  10 . The operator determines if the streaming of the solvent is a “good stream test,” as illustrated in FIG. 6A by the columnar streams of solvent  900 , or a “failed stream test,” as shown in FIG.  6 B. Typically, after the printhead has been in service for a period of time, the streaming of the solvent from the nozzles appears as shown in FIG. 6B, where certain nozzles  1000  are plugged, while the solvent streams from other nozzles  1002  in a ragged and uneven manner. That is, the streams are not columnar, but rather are cork screwed and/or emit from the nozzles at an angle. 
     If in step  106 , the process  100  determines that if the printhead passes the stream test as illustrated in FIG.  6 A. That is, the solvent streams are columnar. The printhead is returned to service in step  107 . Otherwise, the process  100  proceeds to step  108  and soaks the printhead  10  in a solution of 70% acetone and 30% n-methyl-2-pyrolidone by weight. n-methyl-2-pyrolidone is represented by the formula C 5 H 9 NO and the chemical structure.                    
     The soaking process  108  lasts about 15 minutes, and, As mentioned above, the printhead  10  is immersed in the cleaning solution up to the fill-to line  18 , so that capillary action draws the cleaning solution into the nozzles  32  and up throughout the channels  30  so that the rock trap  27  as well the bottom portions  50  of the modules  20   a  and  20   b  are immersed in the cleaning solution. 
     Next, in step  110 , the process  100  flushes the printhead  10  with a gas, such as, for example air, for about 10 seconds at a pressure of about 5 psi. Next, in step  112 , the process  100  soaks the printhead  10  in the cleaning solution for another 15 minutes, and in step  114 , flushes the printhead  10  again with air. 
     If in step  116 , the process determines that air cannot pass through the printhead  10 , the process labels the printhead in step  117 , for example, with a red sticker, to indicate that the printhead may require disassembly to perhaps remove inorganic debris from the rock trap  27 . 
     If air is able to pass through the printhead  10 , the process  100  proceeds a second part  200  of the process  100 . After initializing the second part  200  in step  202 , the process  100  again soaks the printhead  10  in the cleaning solution for about 15 minutes in step  204 , and then flushes the printhead with air in step  206  for about 10 seconds. In step  208 , the process soaks the printhead again in the cleaning solution. The process in step  210  places the head on a lint-free cloth for example, and then in step  212  flushes the printhead with air for 10 seconds at 5 psi. If the process in step  214  determines that no visible pigment is emitted from the printhead  10 , the process marks the printhead  10  with a blue sticker, for example, and proceeds to a third part  300  of the process  100 . If the pigment is visible, the process determines in step  216  if the pigment is solid or not. If the pigment is not solid, the process marks the printhead with a green sticker, and proceeds to the third part of the process  300 . If the pigment is solid, in step  218  the process  100  marks the printhead with a yellow sticker and repeats the second part of the process  200  one additional time. 
     Part  3  of the process  300  is essentially a testing test. The third part begins in step  301 , and then the process  100  flushes the printhead with a cleaning fluid such as glycol ether DPM acetate in step  302 . Again, an operator observes how the cleaning fluid streams from the printhead in step  304 . It the solvent streams as that shown in FIG. 6B (failed stream test), the process  100  in step  305  discards the printhead since the printhead is essentially unrecoverable. 
     If the printhead passes the stream test, then in step  306 , the process checks the printing capabilities in a print test. If the printhead passes the print test, then the process  100  readies the printhead to be returned to service in step  307 . 
     If the printhead fails the print test, the process  100  determines in step  308  if the failure is due to an electrical malfunction, and if it is, then the operator dissassembles the printhead in step  309  to repair the printhead. 
     If the printhead print failure is not attributable to an electrical malfunction, then the process  100  determines in step  310  if parts  2  and  3  of the cleaning process have been repeated for that particular printhead. If those parts have been repeated, then the operator disassembles the printhead in step  311  to determine the cause of the print test failure. For example, the failure may be due to debris accumulated on the rock trap, in which case, the operator merely has to remove the debris from the trap. Otherwise parts  2  and  3  of the cleaning process are repeated once more as indicated by step  312 . 
     The cleaning process is not limited to that shown in FIGS. 5A-5C. For example, there is shown in FIG. 7A an alternative process  400  with a shortened sequence of steps to clean the printhead  10 . The cleaning process  400  beings in step  402  and proceeds to step  404  where the process  400  flushes the printhead and determines whether or not the printhead passes the streaming test. Thus in step  406 , if the process  400  determines the printhead is not partially or fully plugged, the process returns the printheads to service in step  407 . If the printhead is partially or fully plugged, then in step  408  the process soaks the printhead in a solution of 30% n-methyl-2-pyrolidone and 70% acetone for about 15 minutes, and then in step  410  flushes the printhead with air at about 5 psi for about 10 seconds. This soak/flush sequence of steps is performed one or more times. In the illustrated embodiment, the sequence is repeated three times as indicated by the logic loop  412 . That is prior to the first soak/flush sequence, the counter i is initialized to zero in step  412   a.  Then, after each soak/flush sequence, the counter i is incremented by one in step  412   b,  and when the process  400  determines in step  412   c  that i=3, the process proceeds to step  414 . 
     In step  414 , the process  400  flushes the printhead with glycol ether DPM acetate, and determines in step  416  if the printheads stream freely (FIG. 6A) or not (FIG.  6 B). If they do not, then the process  400  discards the printhead in step  418 . Otherwise the process proceeds to a print test  500  which begins in step  502 . Next, in step  504 , the process determines if the printhead passes the print test. If it does, the process prepares the printhead for shipment back to service in step  505 . Otherwise in step  506 , the process evaluates the printhead  10  to determine if the print failure was attributable to an electrical malfunction. If it is, then the process dissassembles the printhead in step  508  for repair. If the failure is not due to an electrical malfunction, then the process  400  cleans the printhead one more time as indicated by step  510 . If the cleaning process has been repeated once as determined in step  512 , then the process  400  has the printhead disassembled for repair in step  513 . 
     The cleaning processes  100  and  400  can be manual operations. In such cases, a human operator performs each of the above identified and discussed steps. However, in some applications, the processes  100  and  400  are partially automated with some manual intervention, for example, to observe the streaming of the printheads. In such cases, the automated steps of the processes  100  and  400  are under the direction of a controller  1100 . In other applications, the processes  100  and  400  are fully automated with essentially no human intervention, in which case all the steps of the processes  100  and  400  are under the direction of the controller  1100 . 
     It will be apparent to those of ordinary skill in the art that methods disclosed herein may be embodied in a computer program product that includes a computer usable medium. For example, such a computer usable medium can include a readable memory device, such as a hard drive device, a CD-ROM, a DVD-ROM, or a computer diskette, having computer readable program code segments stored thereon. The computer readable medium can also include a communications or transmission medium, such as a bus or a communications link, either optical, wired, or wireless, having program code segments carried thereon as digital or analog data signals. 
     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.