A conventional ink jet printer has a carriage which can reciprocate in a predetermined direction (primary scanning direction). A recording head is mounted in or on the carriage and has an ink discharge surface facing downward. A recording sheet can be conveyed intermittently at regular intervals in a direction perpendicular to the primary scanning direction. A platen lies below the discharge surface of the recording head and faces the recording head. Ink droplets are discharged from the nozzles of the recording head onto the surface (upper surface) of the recording medium supported on the platen to form an image on the medium.
The recording head can discharge ink droplets in a zone including an image recording region. The printer may perform marginless image recording on a recording medium by starting the discharge of ink droplets before the leading end of the medium reaches the edge of the recording region disposed in downstream in the conveying direction, and by performing ink discharge until the trailing end of the medium leaves or passes through the edge of the recording region disposed in upstream in the conveying direction. In this case, there is a problem that the discharged droplets dropping outside both ends of the recording medium adhere to the top surface of the platen which lies below. As a result, these ink droplets stain the under side of the succeeding recording medium supported and sliding on the platen.
In order to solve this problem, the printer has been devised as disclosed in U.S. Pat. No. 6,239,817 corresponding to Japanese Patent Application Laid-open No. 2000-118058 (FIGS. 3 and 4A-4C). The recording head of this printer can record an image in an image recording region. The platen of the printer has an upstream wall and a downstream wall, which extend in a primary scanning direction. The upstream and downstream walls stand upright at the edges of the recording region that are upstream and downstream, respectively, in the primary scanning direction. The space between the walls is open upwardly, and an ink absorber is placed in the opened space. The upstream and downstream walls have wavy ribs formed on the top ends thereof, and protrude from the top ends, respectively.
According to the structure of the platen disclosed in U.S. Pat. No. 6,239,817 corresponding to Japanese Patent Application Laid-open No. 2000-118058, however, while a recording medium is passing over the platen, the medium needs to be out of contact with the top side of the ink absorber which is placed in the platen. Therefore, it is necessary to place the ink absorber so that its top side does not protrude over the top ends of the wavy ribs. This makes it necessary to insert, to the platen, an ink absorber which size and shape are strictly controlled. As a result, the incorporation of the ink absorber and other work take time.
If the opening space in the platen is large in area, the capacity of ink absorber can be enhanced. Conceivably, it is possible to make this opening space large in area only by increasing the distance between the upstream and downstream walls of the platen in the conveying direction. If the distance between the walls is long, a front end portion of a soft recording medium is likely to hang down when the leading end of the medium has passed over the wavy ribs of the upstream wall and is positioned in the opening space, namely, when the leading end portion is supported in the form of a cantilever by these ribs. Likewise, if this distance is long, a rear end portion of a soft recording medium is likely to hang down when the trailing end of the medium has passed over the wavy ribs of the upstream wall and is positioned in the space, namely, when the rear end portion is supported in the form of a cantilever by the wavy ribs of the downstream wall. In this case, the end portions of the recording medium are likely to come into contact with the top side of the ink absorber in the opening space, so that the under side of the medium is likely to be stained with ink.
In order to keep a recording medium out of contact with the ink absorber in the platen space, the vertical distance from the top side of the absorber to the top ends of the wavy ribs may be long. In this case, when the recording head discharges ink droplets erroneously without a recording medium lying on the platen during image recording, the droplets float in the form of ink mist over the absorber. When a recording medium enters the recording region, one or both sides of the medium may be stained by the ink mist.