Print head stop mechanism for a postage meter

#The instant invention relates to a digital print head used in a metering device, and more particularly to a stop mechanism associated with the digital print head of the metering device which stop mechanism prevents damage to the digital print head caused by "puffy mailpieces" and ensures consistent good quality printing.

#FIG. 1 shows a known arrangement of a digital print head 1 utilized in, for example, a postage meter for printing an indication of postage value (indicia) on a mailpiece (such as an envelope) 3. For simplicity in explanation, only the only structure of the postage meter being shown is print head 1 and its direction of movement and that structure which relates to the clamping of envelope 3 in the position where printing of the indicia image on a print surface 3a of envelope 3 occurs. The remaining structure of the postage meter is well known in the art and includes the apparatus for moving print head 1 back and forth in the horizontal "X" direction of FIG. 1 and the circuitry for synchronizing firing of the individual print head nozzles 4 with print head 1 movement. An example of the structure of the postage meter not shown herein can be found for example in copending application Ser. Nos. 08/579,504 filed Dec. 27, 1995, and 08/554,179 filed Nov. 6, 1995, which are each hereby incorporated by reference.

#A typical operation of a conventional postage meter is as follows. Envelope 3 is either hand fed or automatically fed into the postage meter. A platen 5, upon which envelope 3 rests, raises upward to force envelope 3 against a registration plate 7 in a conventional manner. This ensures that the top surface of envelope 3 is accurately positioned against the bottom of registration plate 7. The accurate registration of envelope 3 is very important for operation of digital print head 1 because the row of nozzles 4 from which ink droplets are ejected onto the print surface of envelope 3 must be precisely positioned a fixed distance "A" from print surface 3a in order to ensure correct ink dot positioning and formation of a good quality image. Registration plate 7 has an opening 7a in a central portion thereof to expose print surface 3a in direct opposition to the row of nozzles 4. Accordingly, as print head 1 is moved in the X direction from the right to left in FIG. 1, a postage indicia image and an optional advertising slogan are printed on print surface 3a via the coordinated ejection of ink drops from nozzles 4 with the movement of print head 1.

#The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate a presently preferred embodiment of the invention, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain the principles of the invention.

#FIG. 1 shows portions of the printing and registering mechanism of a conventional postage stage meter;

#FIG. 2 shows the postage meter of FIG. 1 with a puffy envelope registered therein;

#Referring to FIGS. 3 and 4, a print head 13 includes a row of nozzles 15 in a nozzle plate 15a. A stop mechanism 17 is fixedly attached to or formed as part of a housing portion 19 of print head 13. Stop mechanism 17 includes a plurality of pointed projections 17a which extend below nozzle plate 15a. Pointed projections 17a are each aligned on center with a corresponding one of nozzles 15 for a purpose to be discussed in more detail below.

#In FIGS. 3 and 4, print head 13 is shown at the point where stop mechanism 17 first makes contact with projecting portion 3b of envelope 3. Thus, the tip of each pointed projection 17a contacts print surface 3a of envelope 3. As print head 13 moves from the right to left in FIG. 3 the pointed projections 17a push protruding portion 3b of envelope 3 downward and away from nozzle plate 15a to ensure that a space 21 always exists between the nozzle plate 15a and print surface 3a during printing of the indicia image. The pointed projections 17a therefore prevent contact between print surface 3a and nozzle plate 15a ensuring that any non wetting coating on the surface of nozzle plate 15a is not damaged during printing.

#Each of pointed projections 17a are aligned on center with a corresponding nozzle 15 in order to permit multi-pass printing. That is, the print head of FIG. 3 prints while moving from right to left. It then returns to a home position by moving from left to right, during which movement no printing occurs. However, where multiple passes of the print head are required to produce the final indicia image, the print head is shifted, for example, by half of the distance between the individual nozzles so that on a second pass from the right to left (of FIG. 3) a new set of ink drops will be ejected in interlaced relationship to the first set of ink drops. If the pointed projections 17a are not disposed on center with the nozzles 15, they will, during the second printing pass, run over the ink dots printed during the first pass and smear the image since the ink deposited during the first pass will not have completely dried. Aligning the pointed projections 17a with the nozzles 15 precludes the smearing problem.