Orifice plate cleaner for hot melt ink jet
BACKGROUND OF THE INVENTION 
     This invention relates to devices for cleaning ink from the orifice plates in ink jet systems and, more particularly, to a new and improved cleaning device for an ink jet orifice plate in a hot melt ink jet system. 
     In ink jet systems, the orifice plate from which ink drops are ejected tends to accumulate a quantity of ink because of accidents during operation, spattering of ink as a result of tails or satellites in the ink drops, and purging of air from the ink supply lines. In hot melt ink systems, the ink jet head is held at a high temperature so that the ink which accumulates on the orifice plate remains liquid during operation of the system. Consequently, drops may be released from the surface of the orifice plate during printing and deposited on the print medium, producing a defective print, or into the mechanism, causing reliability problems. 
     Heretofore, wiper blades have proposed for scraping molten hot melt ink from the surface of an orifice plate in a hot melt ink jet system, but once the ink has been removed, it solidifies on the wiper blade, presenting difficulty when the blade is to be used again. Other cleaning systems using replaceable ink-receiving media such as paper or cloth have been proposed or used. In one case, for example, a D-shaped roller made of resilient material is normally oriented with its flat surface facing the printhead so as to avoid contact with the orifice plate. When cleaning is required, a cloth web held against the D-shaped roll is advanced, causing the roll to rotate so that the web is pressed against the orifice plate during three-quarters of the rotation of the roll. In that case, the pressure applied to the orifice plate varies depending on the compliance of the resilient material of which the roll is made and the tension in the cleaning web, causing variations in the pressure and permitting relatively high pressures to be applied to the orifice plate which can result in abrasive deterioration of the plate. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a new and improved cleaning device for the orifice plate in an ink jet system which overcomes the above-mentioned disadvantages of the prior art. 
     Another object of the invention is to provide an orifice plate cleaning device in which the pressure applied to the orifice plate can be carefully controlled. 
     A further object of the invention is to provide an orifice plate cleaning device which cooperates in the purging of air from ink in the hot melt ink jet system. 
     These and other objects of the invention are attained by providing an ink-receiving medium which is movable with respect to the orifice plate and a resiliently supported pressure member for urging the movable medium against the orifice plate with a selected pressure. In one embodiment, the medium comprises a web which is moved across the surface of the orifice plate and a pair of pressure bars selectively movable toward the orifice plate at spaced positions, one position being beneath the orifices in the orifice plate and the other being in line with the orifices in the orifice plate, so as to retain the medium in contact with the orifice plate at a controlled low pressure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further objects and advantages of the invention will be apparent from a reading of the following description in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a schematic view in longitudinal section illustrating a representative embodiment of an orifice plate cleaning device in accordance with the invention in the retracted condition; 
     FIG. 2 is a view of the device shown in FIG. 1 with the cleaning mechanism in partly extended condition; 
     FIG. 3 is a view of the device shown in FIG. 1 with the cleaning mechanism in fully extended position; 
     FIG. 4 is a schematic view similar to FIG. 3, showing an alternative embodiment of an orifice plate cleaning device in accordance with the invention; and 
     FIG. 5 is a fragmentary cross-sectional view illustrating the spacing of the cleaning web from the ink jet head in the embodiment shown in FIG. 4. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     In the typical embodiment of the invention shown in FIG. 1, a cleaning device 10 includes a web 11 of paper which is conveyed during operation from a supply roll 12 to a take-up roll 13 in the direction indicated by the arrows. The supply roll 12 and the take-up roll 13 are mounted on corresponding spindles 14 and 15, respectively, which extend from one sidewall 16 of the device 10 to an opposite sidewall not shown in the sectional view of FIG. 1, and the take-up spindle 16 is driven as required by a drive motor (not shown) to move the paper web in the direction of the arrows. 
     The supply roll 12 initially contains about 500 inches of any conventional paper of a type capable of absorbing molten hot melt ink readily and, during operation, the paper is driven from the supply roll to the take-up roll at a rate of about 2 to 4 inches per second. To prevent the web 11 of paper from becoming loose in its path between the supply roll 12 and the take-up roll 13, the supply roll spindle 14 is tensioned in an appropriate manner. In addition, a leaf spring member 17, mounted in a fixed crossbar 18 extending between the sidewalls at the bottom of the device 10, assists in preventing the web 11 from becoming slack between the supply roll 12 and the take-up roll 13. The sidewall 16 and the opposite sidewall are also connected by a rear wall 19 and a fixed front crossbar 20 which has a rounded surface 21 shaped to guide the web 11 smoothly around the front end of the device 10. 
     Two longitudinal bars 22, only one of which is visible in the drawings, extend in corresponding grooves 23 in the sidewalls and are connected at their ends by a rear crossbar 24. At the front end of the device, the bars 22 have downward extensions 25 by which they are connected to a movable front end portion 26 of the device. The movable front end portion includes a crossbar 27 to which the downward extensions 25 are connected and two pressure bars 28 and 29 which are resiliently mounted on the movable crossbar 27 by spring members 30 and 31, respectively. As with the crossbar 20, the forward end of the movable crossbar 27 is shaped with a curved surface 32 to guide the web 11 smoothly from the front to the bottom of the device 10. One of the pressure bars 28 is supported behind the paper web 11 at a location in line with the orifices 43 in an orifice plate 44 in an ink jet head 45, and the other pressure bar 29 is positioned behind the web at a lower position in line with the lower part of the orifice plate 44. 
     Thus, the entire assembly consisting of the crossbar 24, the longitudinal bars 22 with their extensions 25, the crossbar 27 and the resiliently mounted pressure bars 28 and 29 is movable toward and away from the orifice plate 44. Moreover, as shown in FIG. 1, the front end of the lower pressure bar 29 is positioned closer to the orifice plate 44 than the front end of the upper pressure bar 28 to cause the lower pressure bar to force the web 11 against the orifice plate before the upper pressure bar reaches the orifice plate. 
     Preferably, the front surface of the upper pressure bar 28 is positioned about 0.02 inch behind the front face of the lower pressure bar 29 so that it does not engage the web 11 in the partially extended condition illustrated in FIG. 2, but engages the web with the desired pressure in the fully extended position illustrated in FIG. 3. In the condition shown in FIG. 2, the web 11 is preferably spaced at least  0.01 inch away from the surface of the orifice plate 44 in the region of the orifices 43. 
     In order to control the position of the movable front end portion 26 of the cleaning device, the crossbar 24 is connected by a shaft 34 to the plunger 35 of a solenoid 36 which is normally retained in the rearmost position shown in FIG. 1 by a spring 37. In order to permit limited forward motion of the assembly 31 when the solenoid 36 is actuated, a movable stop assembly 38 has a stop arm 39 which may be moved downwardly into the path of the crossbar 24, limiting the forward motion of the bar and the corresponding front end portion 26 when the solenoid 36 is actuated, as shown in FIG. 2. When the movable stop assembly 38 is moved upwardly out of the path of the crossbar 24 in the manner shown in FIG. 3, actuation of the solenoid 36 moves the crossbar 24 farther forward against a fixed stop 40, permitting full forward motion of the front end portion 26. 
     In the operation of the ink jet system, the ink jet head 45 is displaced from a home position adjacent to the cleaning device 10 and is transported close to the surface of a record member to project ink drops onto the record member to form a desired image or pattern during which ink may accumulate on the surface of the orifice plate 44. Accordingly, the head 45 is periodically restored to the home position adjacent to the cleaning device as shown in FIG. 1. When the ink jet head is in the home position, the cleaning device can be actuated to remove any ink accumulated on the orifice plate or it may cooperate in purging of air from the ink jet head in the manner described hereinafter. 
     In one mode of operation, the ink jet head is restored to the home position periodically, for example, after printing about five or ten pages, and the front end portion 26 is fully advanced in the manner shown in FIG. 3 so that both bars 28 and 29 urge the paper web 11 against the orifice plate at and below the region of the orifices 43. In this case, the paper is kept stationary and held for a short time, such as one or two seconds, against the orifice plate to blot any ink on the surface of the orifice plate. Thereafter, the front end portion 26 is retracted to the position shown in FIG. 1 and the paper web 11 is advanced just enough, for example, one-quarter inch, to move the portion containing blotted ink out of the immediate region of the orifices. 
     Upon initial start-up of an ink jet system after ink has been solidified, the cleaning device is automatically set to facilitate the purging of any air trapped in the system by cross-flow purging, in which the ink containing trapped air is conveyed from the ink jet head to an internal air-purging device of the type described, for example, in the Hoisington et al. Application Serial No. 043,372, filed April 28, 1987 now Patent No. 4788556. 
     For this purpose, the cleaning device is set in the condition illustrated in FIG. 3, with both pressure bars 28 and 29 urging the paper web 11 against the orifice plate 44 and an internal pressure of about 1 to 3 psi is applied to cause the ink to flow from the head to the internal deaeration device. In this case, to reduce abrasion, the web 11 is advanced between the supply roll 12 and the take-up roll 13 at a relatively low rate, such as less than two inches per second, in order to prevent any ink from spreading to a region on the surface of the orifice plate above the orifices. To make certain that pressure applied internally to transfer the ink within the head does not separate the web from the orifice plate, the upper pressure bar and its resilient support spring 30 provide a force of about one-half pound against the paper. With a face contact area of about 0.05 inch by 3 inches, this is sufficient to resist internal ink pressures on the order of 1 to 3 psi. After such crosspurging is completed, the web 11 is driven a short distance to remove any ink from the orifice plate before the front end portion 26 of the device is retracted by de-energization of the solenoid 36. 
     Such cross-flow purging may also be initiated by the operator if a deterioration in print quality is detected during operation. If the print quality is not improved by cross-flow purging, the operator may set the device to the condition shown in FIG. 2 for outflow purging. To permit such outflow purging and to clean the lower portion of the orifice plate 44, the stop assembly 38 is positioned so that the stop bar 39 will limit the forward motion of the assembly front end portion 26 as shown in FIG. 2. In this position, the lower pressure bar 29 urges the paper web 11 against the lower part of the orifice plate 44, leaving the web spaced from the orifices 43 in the orifice plate. A higher pressure, such as about 10 psi, is then applied to the ink so that the ink is ejected from the orifices toward the spaced web 11 to positively force any trapped air from the ink jet head. 
     During this operation, the paper web is driven in the direction of the arrows at a high enough rate, such as about two to four inches per second, to spread the ink in a thin layer, such as 10 to 30 mils thick, on the paper, and it is then carried with the web onto the take-up roll 13. After outflow purging has been accomplished in this manner, the motion of the web 11 is stopped and the stop assembly 38 is released, permitting the front end portion 26 to move farther forward so that the upper pressure bar 28 urges the stationary paper web 11 against the orifice plate 44 in the region of the orifices 43 in the manner shown in FIG. 3. The web is then moved at a slower rate of, for example, less than two inches per second, for a short time to remove any ink remaining on the orifice plate in the vicinity of the orifices, after which the front end portion 26 is retracted. 
     In a representative example of an orifice plate cleaning device of the type shown in FIGS. 1-3, the pressure bar support springs 30 and 31 have a spring constant of about 10 pounds per inch of deflection so that, when each of the bars 28 and 29 is deflected about 0.05 inch, the force applied by the spring to the pressure bar is about one-half pound. Since a variation of about 20% in the force produced by the spring is permissible, the dimensional tolerances may be large enough so that the structure is convenient and commercially viable. 
     FIGS. 4 and 5 illustrate an alternative embodiment of the invention. In these figures, the reference numerals of FIGS. 1-3 are used to identify the same components described with respect to those figures. In this embodiment, only one pressure bar 28 is provided, and the movable front end portion 26 is arranged to engage the surface of a face plate 46 of the ink jet head. Moreover, since the position of the paper web 11 is defined by the position of the movable front end portion 26 adjacent to the face plate 46, the second pressure bar 29 and support spring 31 of the embodiment of FIGS. 1-3 are omitted, as is the fixed stop 40 of the embodiment of FIGS. 1-3. 
     In order to hold the web 11 against the lower portion of the orifice plate 44 in this embodiment, the crossbar 27a of the movable front end portion 26 has a faceplate-engaging projection 47 at each end and a web guide surface 32a, which is curved to guide the web 11 from the region adjacent to the faceplate 45 to the path extending beneath the device 10 to the take-up roll 13. In addition, the front surface 32a of the crossbar 27a terminates at an angle of about 45° to the plane of the faceplate and is spaced approximately 0.006 to 0.010 inch rearwardly of the front end of the projections 47. 
     With this arrangement, the beam strength of the paper web 11 urges the web into engagement with the orifice plate in the region between the pressure bar 28 and the movable crossbar 27a, providing an urging pressure similar to that of the spring-biased lower pressure bar 29 in the embodiment of FIGS. 1-3 so that the web absorbs and removes any ink on the portion of the orifice plate 44 below the orifices 43. Moreover, the urging pressure is normally sufficient to provide such engagement even when the movable front end portion is in the partialy extended position corresponding to that shown in FIG. 2, and the projections 47 of the movable crossbar 27a are not in engagement with the faceplate 46. 
     Although the invention has been described herein with reference to specific embodiments, many modifications and variations of the invention will readily occur to those skilled in the art. Accordingly, all such variations and modifications are included within the intended scope of the invention.