Patent Publication Number: US-6334726-B1

Title: Printing method and printer with platen

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a printing method and a printer with a platen. More particularly, the present invention relates to a printing method and a printer with a platen, in which a thermal head can contact recording material in a reliably stable manner. 
     2. Description Related to the Prior Art 
     A thermal printer with type of a direct thermal recording is constituted by at least one platen roller for supporting thermosensitive recording material, and a thermal head including an array of plural heating elements. The recording material includes at least one thermosensitive coloring layer. The thermal head and the recording material are transported in a manner relative to one another, while the thermal head applies heat to the recording material to develop color on the recording material one line after another. There are various types of platen devices including the platen roller, which is constituted by a rubber roller member, and has a shape of a rod with a diameter of several centimeters. 
     According to the prior art, the platen roller is constituted by a rubber member, and becomes deformed according to a convexly curved surface of the heating element array of the thermal head. There is an area with high contact pressure in the vicinity of the central top of the heating element array of the thermal head. However areas distant from the central top of the heating element array have only small contact pressure. A problem exists in that an amount of conducted heat is likely to change due to changes in the contacting pressure. Coloring density is also changed due to changes in the amount of the conduced heat, to create irregularities in the density or color. 
     It is conceivable to increase hardness of the rubber material for the purpose of increasing pressure of the contact between the recording material and the thermal head. However another problem arises in that a depth of sinking of the heating element array into the recording material is decreased. The area of the contact is decreased, and also becomes subjected to the influence of changes in the pressure due to eccentricity of the platen roller and irregularities on its surface. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing problems, an object of the present invention is to provide a printing method and a printer with a platen, in which a thermal head can contact recording material in a reliably stable manner. 
     In order to achieve the above and other objects and advantages of this invention, a printer includes a recording head for image recording to recording material in application of pressure to the recording material. A platen device supports the recording material during the pressure application of the recording head to the recording material, the platen device including first, second and third plates serially laid on one another and secured to one another, the first plate being rigid, the second plate being resilient, and the third plate having higher rigidity than the second plate, having a smooth surface, and contacting the recording material. 
     In a preferred embodiment, the printer is a thermal printer, the recording head is a thermal head for thermal recording by applying heat energy to the recording material. 
     Furthermore, the first plate is produced from metal, the second plate is produced from rubber, and the third plate is produced from resin. 
     The resin is polyethylene terephthalate. 
     Furthermore, a carriage transports the thermal head and the platen device relative to another thereof, wherein the thermal head affects the thermal recording during transportation of the carriage. 
     The thermal head includes one array of heating elements arranged in a main scan direction, and the carriage relatively transports the thermal head and the platen device in a sub scan direction crosswise to the main scan direction. 
     The carriage transports the thermal head. 
     In another preferred embodiment, the carriage transports the platen device. 
     Furthermore, there is a head support unit on which the thermal head is mounted. At least one guiding roller is secured to the head support unit, rotatable about an axis extending in the main scan direction, pressed against the recording material with the thermal head, for stabilizing orientation of the thermal head transported relative to the recording material. 
     At least one guiding roller comprises first and second guiding rollers disposed upstream and downstream from the thermal head with reference to the sub scan direction. 
     In another preferred embodiment, at least one guiding roller comprises first and second guiding rollers arranged in the main scan direction so that the thermal head is disposed therebetween. 
     The second plate is 0.5-10 mm thick, and the third plate is 0.05-0.5 mm thick. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above objects and advantages of the present invention will become more apparent from the following detailed description when read in connection with the accompanying drawings, in which: 
     FIG. 1A is a section, partially broken, illustrating a platen of the present invention, together with a thermal head and a thermosensitive recording sheet; 
     FIG. 1B is a section, partially broken, illustrating a platen according to a comparative example, together with the same as those of FIG. 1A; 
     FIG. 2 is a vertical section illustrating a thermal printer of the present invention; 
     FIG. 3 is a vertical section illustrating another preferred thermal printer in which a platen plate is movable; 
     FIG. 4 is a side elevation illustrating another preferred embodiment in which guiding rollers are associated with a thermal head; 
     FIG. 5 is a perspective illustrating another preferred embodiment of a guiding roller with a thermal head; and 
     FIG. 6 is a section, partially broken, illustrating a structure of the platen. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) OF THE PRESENT INVENTION 
     In FIG. 1A, relative positions of a thermal head  10 , a color thermosensitive recording sheet  11  and a plate-shaped platen  12  in a thermal printer are illustrated in accordance with the present invention. The thermal head  10  consists of a heating element array  13  having an arc shape as viewed in cross section as known in the art of thermal recording. The heating element array  13  is constituted by a head base plate, a partial glaze layer formed in the head base plate, heating elements or resistor elements arranged on the glaze layer, and a protection glass layer formed on the heating elements. The heating elements are arranged in line in a main scan direction, which is vertical to the surface of the drawing sheet. 
     The platen  12  is constituted by a laminated combination of a first plate  15 , a second plate  16  and a third plate  17 . The first plate  15  is produced from metal, and rigid. The second plate  16  is produced from synthetic rubber, and resilient. The third plate  17  is produced from polyethylene terephthalate (PET), and has a smooth surface and resiliency. The platen  12  has a shape of a rectangular quadrilateral, and has a size greater than the recording sheet  11 . A thickness of the first plate  15  is predetermined to be suitable in consideration of the size of the recording sheet  11  so that the first plate  15  does not flex even when pushed upon by the thermal head  10  at the recording time. The second plate  16  is 2.5 mm thick. The third plate  17  is 0.2 mm thick. Note that the second plate  16  can have a thickness in a range of 0.5-10 mm, and in a preferable range of 2-3 mm. The third plate  17  can have a thickness in a range of 0.05-0.5 mm, and in a preferable range of 0.15-0.25 mm. 
     For the thermal recording, the recording sheet  11  is placed on the third plate  17 . The heating element array  13  of the thermal head  10  is pressed against a recording surface of the recording sheet  11 . The thermal head  10  is relatively transported in a sub scan direction indicated by the arrow “A” in a manner relative to the recording sheet  11 . In synchronism with the relative transportation, each of the heating elements is driven to generate heat in accordance with image data. 
     In FIG. 1B, the recording sheet  11  is being subjected to thermal recording as directly placed on a rubber plate  20  included in a comparative platen. The recording sheet  11  and the rubber plate  20  are deformed by the convex face of the heating element array  13  of the thermal head  10 . There is only a small area in which the heating element array  13  contacts the recording sheet  11 . The heating element array  13  exerts pressing force in such a distribution that the pressure P 1  at the center of the heating element array  13  is somewhat great, but that the pressure P 2  and P 3  in positions before and after the center of the heating element array  13  is remarkably smaller. An amount of conducted heat is likely to change due to changes in the pressure. Consequently it is impossible to apply heat energy to the recording sheet  11  with high precision in view of requirement for coloring. There occur irregularities in the density and colors. 
     In the present embodiment, the third plate  17  is disposed between the recording sheet  11  and the second plate  16  and has higher rigidity than the second plate  16 . See FIG.  1 A. Then the recovering force of the second plate  16  can operate in a comparatively large area by means of the third plate  17 . Thus an enlarged area of the contact between the thermal head  10  and the recording sheet  11  is obtained. In addition of the increase in the central pressure P 4 , the pressure P 5  and P 6  can be increased in positions before and after the thermal head, due to the recovering force of the second plate  16  transmitted by the third plate  17 . Influence due to the changes in the pressure is reduced in comparison with the comparative example in FIG.  1 B. It is possible to prevent the occurrence of irregularity in the density due to changes in the pressure. 
     In FIG. 2, a color thermal printer  30  is generally depicted. A frame  31  is provided with the platen  12  supported stationary in relation to a horizontal direction. Clampers  32  are disposed on ends of the platen  12 . A drive unit  33  shifts the dampers  32  up and down in the vertical direction between a clamping position and an open position. The clampers  32 , when in the clamping position, clamp the recording sheet  11 , and when in the open position, release the recording sheet  11  from clamping. 
     A sliding support  35  is disposed over the platen  12 . A guiding rod  36  is so inserted through the platen  12  that the sliding support  35  is guided and kept slidable in the horizontal direction. A shifter mechanism  34  connects the thermal head  10  to the sliding support  35 . The heating elements of the thermal head  10  are arranged in the main scan direction perpendicular to the guiding rod  36 . The shifter mechanism  34  operates to shift the thermal head  10  between a recording position where the thermal head  10  presses the recording sheet  11 , and a retracted position where the thermal head  10  is away from the recording sheet  11 . 
     A head carriage  37  is connected to the sliding support  35 , and includes a motor  38 , an endless timing belt  39  and pulleys  40  and  41 . The motor  38  is provided with a speed reducing mechanism. The endless timing belt  39  includes teeth. The pulleys  40  and  41  also include teeth, which are meshed with the endless timing belt  39  to circulate the same. A connector  42  connects one portion of the endless timing belt  39  fixedly to the sliding support  35 . Thus rotation of the motor  38  in a forward or backward direction drives the endless timing belt  39 , which in turn causes the sliding support  35  to slide in either direction along the guiding rod  36 . Thus the thermal head  10  is transported in the sub scan direction A. 
     A yellow fixer lamp  45  and a magenta fixer lamp  46  are disposed to extend in parallel with the shape of the thermal head  10 , and arranged adjacent to each other in the sub scan direction A. As is known in the art of the thermal recording, the fixer lamps  45  and  46  are driven to emit ultraviolet rays upon thermal recording of each of the three colors, to fix coloring layers optically immediately after the thermal recording. The coloring layers are kept from developing color further in the course of thermal recording of next color. 
     A top plate  31   a  of the frame  31  is provided with a movable lid  50 , which is rotatable about an axis at a pivot  51 , and supported in an openable manner. When the lid  50  is opened, a user manually sets the recording sheet  11  on the platen  12 . There is a recording sheet sensor (not shown) disposed at an end of the platen  12 . When the recording sheet  11  is set in a suitable position, the sheet sensor detects the recording sheet  11 , so that the dampers  32  are controlled and shifted from the open position to the clamping position. Thus the recording sheet  11  is retained on the platen  12  in a fixed manner. 
     Each of the heating elements of the thermal head  10  is driven by a printing control unit (not shown) known to those skilled in the art, in synchronism with movement of the thermal head  10  in the sub scan direction A. The printing control unit operates for the driving in accordance with image data being input, so as to print yellow, magenta and cyan images to the thermosensitive coloring layers in accordance with the three-color frame-sequential recording. 
     The operation of the thermal printer  30  is described now. At first the lid  50  is opened. The recording sheet  11  is placed on the platen  12 . While the lid  50  is open, the dampers  32  are kept in the open position. When the recording sheet  11  is positioned suitably, the sheet sensor detects the suitably positioned state of the recording sheet  11 . The dampers  32  are shifted from the open position to the clamping position to retain the recording sheet  11  on the platen  12 . 
     When a printing key in an operation panel (not shown) is operated, the thermal head  10  is transported in a forward direction toward the right, to record a yellow image line by line. The yellow fixer lamp  45  is driven to illuminate during the recording, to fix the yellow coloring layer after being heated. After the yellow recording, the yellow fixer lamp  45  is turned off. Then the thermal head  10  is moved in a backward direction toward the left, to record a magenta image line by line. The magenta fixer lamp  46  is driven to illuminate during the recording, to fix the magenta coloring layer after being heated. After the magenta recording, the magenta fixer lamp  46  is turned off. 
     The thermal head  10  is transported in the forward direction next, to record a cyan image. Also in the cyan recording, the magenta fixer lamp  46  is caused to illuminate and bleaches uncolored portions remaining besides colored portions. After the cyan recording, the thermal head  10  is shifted by the shifter mechanism from the recording position to the retracted position, and then is transported in the backward direction to a home position defined as an initial location of the thermal head  10 . Then the lid  50  is opened to remove the recording sheet  11  after the recording. 
     In FIG. 1A, the recording sheet  11  is pressed against the heating element array  13  with the high pressure P 4 -P 6  by the virtue of the recovering force of the second plate  16  through the third plate  17 . The recording sheet  11  is received by the third plate  17  which has higher rigidity than the second plate  16 . Thus there is little influence due to changes in the pressure, to suppress occurrence of irregularity in density. 
     In the above embodiment, the third plate  17  is produced from polyethylene terephthalate (PET). Furthermore, the third plate  17  can be produced from polyethylene naphthalate (PEN), polypropylene (PP), triaryl cyanurate (TAC), cellulose triacetate or any other suitable resin that has higher rigidity than the second plate  16 . 
     In the above embodiments, the thermal head  10  is transported back and forth. In contrast, a full-color thermal printer  61  as illustrated in FIG. 3 may include a plate-shaped platen  60  which may be movable back and forth for the three-color frame-sequential recording. A guiding rod (not shown) can be disposed to support the platen  60  in a slidable manner. A guiding screw  62  of a platen carriage is disposed along the guiding rod. A threaded member  63  with a female helicoid is meshed with the guiding screw  62 , and is fixedly secured to the bottom surface of the platen  60 . A motor  66  of the platen carriage is disposed, and causes the guiding screw  62  to rotate back and forth in engagement with gears  64  and  65 . Elements similar to those in the embodiment of FIG. 2 are designated with identical reference numerals. 
     In the above embodiments, the yellow recording is during the forward transportation of the thermal head  10 . The magenta recording is during the backward transportation of the thermal head  10 . The cyan recording is during the forward transportation of the thermal head  10 . Alternatively, three colors can be recorded during the transportation of the thermal head  10  in one common direction, either forward or backward. For this three-color recording, it is preferable to dispose the two fixer lamps in an upstream position with reference to the recording direction of the thermal head  10 . 
     In the above embodiments, a user manually sets the recording sheet  11  on the platen  12 ,  60 . Alternatively, a sheet supply mechanism (not shown) can be disposed for automatically supplying the platen  12 ,  60  with the recording sheet  11 . 
     In the above embodiments, the dampers  32  are arranged on the ends of the platen  12 ,  60  in the sub scan direction A. Of course dampers may be arranged on ends of the platen  12 ,  60  with reference to the main scan direction for the purpose of retaining the recording sheet  11 . 
     In FIG. 4, a preferred embodiment is depicted, in which guiding rollers  71  and  72  are used with a thermal head  70 . The guiding rollers  71  and  72  are disposed on respectively brackets  68  and  69  of a head support unit, and located beside the thermal head  70  in forward and backward directions with reference to the transportation of the thermal head  70 . Furthermore in FIG. 5, a guiding roller  76  can be disposed in each of two lateral positions beside a thermal head  75 . Also, an adjuster mechanism (not shown) may be disposed between the guiding roller  71 ,  72 ,  76  and the thermal head  70 ,  75 , to adjust an amount by which the heating element array are sunken into the recording sheet. 
     In the above embodiments, the first plate  15 , the second plate  16  and the third plate  17  are unified as the platen  12 . To connect those with one another, various securing manners can be used. For example, adhesive agent  80  may be applied to surfaces of those plates  15 - 17  in contact with one another. See FIG.  6 . Alternatively, end positions of those plates may be provided with through holes, and may be secured to one another by use of bolts and nuts. 
     It is to be noted that the plate-shaped platen  12 ,  60  is used in the thermal printer according to the above embodiments, but can be used in a telefacsimile machine, a printing press, a typewriter, or any machine for producing hard copies in any form. 
     Furthermore, the present invention may be used in a magnetic recording device for use with magnetic tape, disk, or other medium. A pad member for contacting the medium with a magnetic head can have a triple-layered structure. 
     The above platen  12 ,  60  is plate-shaped. However a platen in the present invention may be any of various known types, including a platen drum, a platen roller and the like. 
     In the above embodiments, the printer is a line printer of which the thermal head  10 ,  70 ,  75  is long in the main scan direction and transported in the sub scan direction A. However, the plate-shaped platen can be used in a serial printer in which the thermal head is transported in a twodimensional manner in the main and sub scan directions. 
     In the above embodiments, the recording sheets are color thermosensitive recording material. Furthermore, recording material in the present invention may be widely used simple paper called PPC paper, monochromatic thermosensitive recording material, thermosensitive ink film, photosensitive recording material, or the like. 
     Although the present invention has been fully described by way of the preferred embodiments thereof with reference to the accompanying drawings, various changes and modifications will be apparent to those having skill in this field. Therefore, unless otherwise these changes and modifications depart from the scope of the present invention, they should be construed as included therein.