A PRINT HEAD AND A METHOD OF PRINT HEAD OPERATION WITH COMPENSATION FOR INK SUPPLY PRESSURE VARIATION

TECHNOLOGY FIELD

[0001] The present print head and method relate to digital printing and particularly to inkjet printing with inkjet print heads.

BACKGROUND

[0002] InkJet printing is a well known in the art printing method. The basics of this technology are described, for example by Jerome L. Johnson "Principles of Nonimpact Printing ", Palatino Press, 1992, Pages 302 - 336. ISBN 0-9618005-2-6. Commercial products such as computer printers, large format graphics printers and others exist.

[0003] An ink-jet print head consists of an array or a matrix of ink nozzles, with each nozzle selectively ejecting ink droplets. The number of operating nozzles and drop volume establish the ink flow from an ink tank, which may be an intermediary ink tank placed in close proximity to the print head or remote tank. When printing average density images the print head on average consumes steady amounts of ink. Sudden changes in ink consumption occur at the beginning and the end of the printing process. Significant changes in ink consumption may take place when sudden shifts from highlights to shadows exist in the printed image. These changes cause air through nozzle ingestion; adversely affect the print head operation and the quality of the printed image.

[0004] European Patent EP 0 956 958 Bl teaches certain techniques of reducing the influence of sudden ink consumption changes.

BRIEF LIST OF DRAWINGS

[0005] The print head structure and operation method are particularly pointed out and distinctly claimed in the concluding portion of the specification. The print head and the method, however, both as to architecture and method of operation, may best be understood by reference to the following detailed description when read with the accompanied 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 method.

[0006] Figures IA - 1C are schematic illustrations of the first embodiment of an mkjet print head.

[0007] Figure 2 is a cross section of the print head of the first embodiment.

[0008] Figure 3 is a schematic illustration of the pressure changes within a conventional print head at the beginning of print head operation.

[0009] Figure 4 is a schematic illustration demonstrating principles of operation of the flexible sidewall.

[0010] Figure 5 is a schematic illustration of the pressure changes within the print head of the first embodiment at the beginning of print head operation.

[0011] Figures 6A and 6B illustrate different geometrical sizes of the flexible cover.

[0012] Figure 7 is a schematic illustration of a printed image with multiple highlight shadow transitions.

[0013] Figure 8 is a schematic illustration of the second embodiment of the inkjet print head.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

[0014] Figure 1 is a schematic illustration of the first embodiment of an inkjet print head. Print head 100 includes a silicon micro machined chip 102 consisting of substrate 104 in which a plurality of ink ejecting channels 108 (Figure IB), each ending by a nozzle 110, is etched. A glass cover 112 covers ink-ejecting channels 108. A plurality of piezo ceramic actuators 116 (Figures IB and 1C) is attached to glass cover 112. Each of the plurality of piezo ceramic actuators 116 is associated with respective ink ejecting channel 108. Epoxy or glue attaches substrate 104 to print head holder 120.

[0015] Holder 120 has a manifold 124 (Figure 2) made as a recess in holder 120, for distributing ink to each of ink ejecting channels 108. One of the sidewalls 128 forming manifold 124, or at least a section of it, is made of flexible material. For protection purposes a rigid cover 130, is placed over flexible sidewall 128. There is a gap 132 between flexible sidewall 128 and cover 130. Cover 130 has one or more air communication ports 136 maintaining atmospheric pressure in gap 132. An ink supply port 140 (Figure 1) with the help of tubing 142 connects manifold 124 with main or interim ink tank 144.

[0016] In a stand-by mode of operation ink ejecting channels 108, ink manifold 124 and tubing 142 are filed with ink 148. When print head 100 becomes operative, which may take place for example; at the beginning of the printing process the drop ejection process depletes ink 148 in print head 100. The process is known as "ink starvation." Tank 144 replenishes ink 148, although the replenishment takes place only after a certain delay. Initially, the pressure of ink in the vicinity of nozzle 110 decreases, and negative pressure front proceeds through the print head and tubing 142 towards ink tank 144. The distance L from nozzles 110 to tank 144 and the speed

of sound C in the ink define the delay (Delay = L/C (second)). Only after this delay, ink 148 begins to flow towards manifold 124 and ink ejecting channels 108 of print head 100. Until replenished ink 148 reaches ink-ejecting channels 108 and nozzles 110 the delay is further increased by the value of the time it takes the ink to travel the distance L. The delay between the start of printing process and the time replenishing ink fills-in the nozzles is 2L/C second. This creates visible flaws in the printed image quality.

[0017] hi conventional print heads having manifold with rigid sidewalls, immediately after the beginning of the operation of print head 100 the pressure, as illustrated in Figure 3, falls abruptly to a very low level. If print head 100 continues to operate in course of this time and drop ejection continues, the pressure in the area of nozzles 110 continues to decrease. If the ink ejection frequency is high, the pressure in the nozzle area can decrease to a level where the ink meniscus in nozzles 110 will not be able to overcome it. At this instant the meniscus breaks and air is ingested into the nozzle, rendering it inoperable. After a delay of about 2L/C second, the pressure, as shown by section 168 of the graph in Figure 3, returns to an equilibrium position and normal print head operation is restored. However, the operation of nozzles 110 that were rendered to be not operable might not be restored.

[0018] Flexible sidewall 128 allows reducing or even eliminating the pressure drop, shown in Figure 3, and the associated with it nozzle failures. Figure 4A is a schematic illustration demonstrating principles of operation of the flexible wall. With the decrease in ink pressure inside manifold 124, air at atmospheric pressure that enters through air communication port 136, deforms and bends inwards flexible sidewall 128 changing the volume of manifold 124 occupied by ink 148. The volume changes such that the pressure variations within the manifold are minimized and steady ink replenishment to nozzles continues.

[0019] Manifold 124 is located at a distance L' from the location of nozzles 110.

Distance L ' is much shorter than the distance to the ink tank L (L '« L) and the time delay between the start of print head operation at the beginning of printing and ink replenishment (LVC) is much shorter than in conventional print heads. Figure 5 is a schematic illustration of the pressure changes within print head having a flexible sidewall at the beginning of print head operation. It shows that the pressure drop is significantly reduced and steady ink pressure 168 establishes itself almost immediately after onset of printing, eliminating potential nozzle failure.

[0020] Flexible sidewall 128 (Figures 2 and 4) should be designed such that it immediately bends to occupy all of the volume released by ink 148 consumed by print head 100 operation in course of the delay 2L/C. As the ink flow gradually replenishes the ink consumed, flexible sidewall 128 restores its original position. For proper functioning flexible sidewall 128 should have a surface being in contact with the ink and a modulus of elasticity sufficient to support sag of sidewall 128 proportional to the amount of ink displaced. These two parameters determine the amount of inward bending 150 (Figure 4) for a given pressure difference across the flexible sidewall 128. Hence, flexible sidewall 128 may cover only a section of manifold 124. Figure 6A illustrates a flexible sidewall 128 completely covering manifold 124 and Figure 6B illustrates a flexible sidewall 128 covering only a section of manifold 124.

[0021] Figure 4B illustrates another embodiment of a print head, where flexible sidewall 128 covers fully or partially both sides of manifold 124.

[0022] Flexible sidewall 128 should be chemically compatible with ink 128 and have low air permeability. It may be of a single layer or multi layer structure. Among materials suitable for the sidewall are ethylene vinyl acetate (EVA) or Mylar®, some types of Polyimide and others.

[0023] Sudden ink pressure changes occur not only at the beginning of printing or print head operation. They may occur at each printed pattern change. Figure 7 shows a printed image 154 that contains multiple transitions from highlights 158 to shadows 162. Such transitions cause abrupt changes in the number of operative ink ejecting channels and associated with it nozzle failures. At the end of print head operation ink continues to flow towards the print head nozzles causing an increase in ink pressure and associated with it ink dripping. Flexible sidewall 128 bends outward 164 (Figure 4A) increasing the volume available for the ink and prevents nozzle plate wetting and ink dripping.

[0024] Figure 8 is a schematic illustration of another embodiment of the inkjet print head having a flexible sidewall. Print head 180 is a micro machined chip consisting of silicon substrate 184 in which a plurality of ink ejecting channels 188 each ending by a nozzle 190 is etched. A glass cover 192 covers ink-ejecting channels 188. A plurality of piezo ceramic actuators 196 is attached to glass cover 192. Each of the plurality of piezo ceramic actuators 196 is associated with respective ink ejecting channel 188. Print head 180 further includes an etched in substrate 184 or otherwise produced manifold 194 for distributing ink to each of ink ejecting channels 188. A sidewall 198 made of flexible material covers manifold 194 or at least a section of it. Another rigid cover 200 is placed over sidewall 198. There is a gap 202 between cover 188 and cover 200. Cover 200 has one or more air communication ports 206 for maintaining atmospheric pressure in gap 202. An ink supply port 210 with the help of tubing 212 connects manifold 194 with main or interim ink tank (not shown). The operation of print head 180 is similar to the operation of print head 100 and was explained supra.

[0025] Relative movement between the print head and substrate, on which the image is printed, enables printing over the whole surface of the substrate. Print head, being lighter than the substrate, usually moves in a type, of reciprocating movement over the substrate. Changes in the print head movement direction, vibrations caused by the

motors and others cause changes in the pressure of the ink in ink delivery system and print head. Use of ink manifold covered by a flexible sidewall and located close to the nozzles of the print head significantly reduces these ink pressure variations and their effect on the print quality.

[0026] A number of embodiments have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the method. Accordingly, other embodiments are within the scope of the following claims: