Segmented flexible heater for drying a printed image

Abstract: Described herein is a segmented flexible heater (50) disposed adjacently to a paper path in a printing apparatus (10) for heating a recording medium (16) both before printing and during printing. The heater (50) includes a curved first portion forming a preheat zone (52) for preheating the medium (16) and a substantially planar second portion forming a heat zone (54) for heating the medium (16) in a print zone (38) wherein the second portion generates heat energy having a temperature greater than the heat energy generated by the first portion. The heater (50) is fixed at one end (58) to a mounting member (60) and is maintained in a curved configuration in the preheat zone (52) by means of inner and outer guide sections (30,32). A tensioning mechanism (64) is provided at the other end (62) of the heater so that the heat zone (54) is supported between first and second support portions (76,78) of a heater support (56).

Description: This invention relates generally to a printing apparatus in accordance with the generic clause of claim 1. Such a printing apparatus is already known from US-A-5 406 321 which will be discussed lateron in more detail. Many liquid inks and particularly those used in thermal ink jet printing, include a colorant or dye and a liquid which is typically an aqueous liquid vehicle, such as water, and/or a low vapor pressure solvent. The ink is deposited on the substrate to form an image in the form of text and/or graphics. Once deposited, the liquid component is removed from the ink and the paper to fix the colorant to the substrate by either natural air drying or by active drying. In natural air drying, the liquid component of the ink deposited on the substrate is allowed to evaporate and to penetrate into the substrate naturally without mechanical assistance. In active drying, the recording medium is exposed to heat energy of various types which can include infrared heating, conductive heating and heating by microwave energy. Active drying of the image can occur either during the imaging process or after the image has been made on the recording medium. In addition, the recording medium can be preheated before an image has been made to precondition the recording medium in preparation for the deposition of ink. Preconditioning of the recording medium typically prepares the recording medium for receiving ink by driving out excess moisture which can be present in a recording medium such as paper. Not only does this preconditioning step reduce the amount of time necessary to dry the ink once deposited on the recording medium, but this step also improves image quality by reducing paper cockle and curl which can result from too much moisture remaining in the recording medium. It is the object of the present invention to provide a printing apparatus which has a heater arrangement that heats the recording medium more uniformly and that at the same time shows a simpler and more economic construction. This object is met by the features of claim 1. For a better understanding of the present invention, reference will now be made, by way of example only, to the accompanying drawings in which:- FIG. 1 is a schematic elevational side view of an ink jet pnnter of the present invention: FIG. 2 is a schematic elevational view of a ink jet printhead, a segmented flexible heater, and a structural support therefore; FIG. 3 is a fragmentary perspective view of the ink jet printer of FIG. 1; and FIG. 4 is a plan view of a segmented flexible heater used in the FIG. 1 ink jet printer. Although the present invention discussed herein may be used for drying any image which is created by a liquid ink printer, the description of the present invention will be described in the environment of an ink jet printer such as that shown in FIG. 1. FIG. 1 illustrates a schematic representation of a thermal ink jet printer 10 in a side elevation view. A translating ink jet printhead 12 printing black and/or colored inks is supported by a carriage 14 which moves back and forth across a recording medium 16, for instance, sheets of paper or transparencies, on a guide rail 18. Multiple printheads printing different colors are also within the scope of this invention. The recording medium 16 is moved along a recording medium path through the printer in the direction noted by the arrow 20. Single sheets of the recording medium 16 are fed from a tray 22 by a document feed roll 24. The document tray 22 is spring biased by a biasing mechanism 26 which forces the top sheet of the stack of recording sheets held by the tray 22 into contact with the feed roll 24. A top recording medium 16 in contact with the feed roll 24 is transported by the feed roll 24 into a chute 28 which is defined by an outer guide member 30 spaced from an inner guide member 32, each of which are curved to thereby reverse the direction of the recording sheets 16 for printing by the printhead 12. Once the recording medium exits the chute 28, the recording medium 16 is driven into the nip of a drive roll 34 cooperating with a pinch roll 36 to advance the recording sheet 16 into a print zone 38. The print zone 38 is the area directly beneath the printhead 12 where ink droplets 40 are deposited by an array of ink nozzles printing a swath of information and arranged on a front face of the printhead. The front face of the printhead is substantially parallel to the recording medium. The carriage 14, traveling orthogonally to the recording medium 16, deposits the ink droplets 40 upon the recording medium 16 in an imagewise fashion. The printhead 12 receives ink from either an attached ink tank or from an ink supply tube (not shown). The image deposited upon the recording medium 16 can include text and/or graphic images, the creation of which is controlled by a controller (not shown), known to those skilled in the art, in response to electrical signals traveling through a ribbon cable 42 coupled to the printhead 12. Before the recording medium 16 has completely left control of the drive roll 34 and the pinch roll 36, an exit drive roll/pinch roll combination (not shown) or other known means captures the leading edge of the recording medium 16 for transport to an output tray 44 which holds printed recording medium. To fix the liquid ink to the recording medium 16, the moisture must be driven therefrom. While it is possible to dry the ink by natural air drying, natural air drying can create certain problems such as cockle or curl and can also reduce the printing throughput of the printer. Consequently, active drying by the application of heat energy to the printed recording medium is preferred. To improve printing quality, the present invention includes a segmented flexible heater 50 which is located along the inside of the chute 28, in contact with and supported by the inner guide section 32, and which extends through the print zone 38. The segmented flexible heater 50 is located within the printer 10 such that the side of the recording medium opposite the side to be printed comes into direct contact with the flexible heater. Heat energy is delivered primarily through conduction. The inner guide section 32 can include apertures, such as round holes, diagonally placed slots, or raised areas to thereby shorten warm-up times. The segmented flexible heater 50 is arranged within the printer 10 such that a first preheat area 52 is defined to preheat the paper before it enters the print zone area 38. A second heat area 54 is defined in the print zone 38 to thereby apply heat energy to the backside of the recording medium 16 during printing. As can be seen in FIG. 1, the portion of the flexible heater 50 located within the print zone 38 is substantially flat thereby providing a substantially flat uniform contact surface for supporting the recording medium in the print zone 38 during printing. Surface flatness is required to insure adequate paper to heater contact as well as to maintain the critical spacing between the printhead 12 and the recording medium 16. To adequately maintain the flatness of the flexible heater within the preheat zone 52 and the heat zone 54, the flexible heater 50 is supported by a print zone area heater support 56. A first end 58 of the flexible heater 50 is attached to a mounting member 60 which, in turn, is mounted to the inner guide member 32. A second end 62 of the flexible heater 50 has attached thereto a spring or tensioning mechanism 64 (here schematically represented) for applying tension to the flexible heater 50. The tensioning mechanism 64 not only maintains the necessary surface flatness in the print zone 38, but also maintains the critical spacing between the printhead 12 and the recording medium 16. By stretching the flexible heater 50, the heater 50 is placed in tension to reduce or eliminate buckling or unevenness which can result from thermal expansion that takes place when the flexible heater is activated. Conceptually, the mechanism for maintaining the sheet flatness within the print zone is illustrated in FIG. 2. The flexible heater 50 is placed under tension by the tension mechanism 64 which stretches the flexible heater over a first strut 70 and a second strut 72 positioned perpendicular to the process direction as indicated by arrow 20 and which are parallel with respect to one another. The first strut 70 and the second strut 72 are positioned at either side of the print zone 38 and run across the width of the flexible heater 50. The flexible heater 50 is pulled to tension in the dimension perpendicular to each of these support surfaces. Since the flexible heater 50 is not physically attached to the surface of the first strut 70 or second strut 72, the heater is free to expand and to contract without creating buckling or corrugation of the heater. The heater 50 has the end opposite the tensioning member attached to a rigid member 74, or for instance, the frame of the printer. Returning to FIG. 1, the print zone support 56 incorporates the features illustrated in FIG. 2 for tensioning the flexible heater 50. The support 56 includes a first portion 76 corresponding to the first strut 70 and a second portion 78 corresponding to the second strut 72. The so called "struts" in the present invention are simply elevated sections of a plastic extrusion that run the width of the print zone from one side of the frame to the other. The flexible heater 50 thereby contacts accurately controlled parallel regions of the support 56 at or before and after end of the print zone 38 to form a flat contact surface. In addition to forming a flat contact surface within the print zone 38, the flexible heater defines a gap 80 between the bottom surface of the flexible heater and a third portion 82 of the support 56 which extends between the first portion 76 and the second portion 78. The gap 80 extends across and beneath the entire width of the recording medium being printed. This gap not only maintains the flatness of the flexible heater within the print zone since the support member 56 does not contact the tensioned flexible heater at this location, but also improves the overall heating efficiency and minimizes the warm-up time of the heater. FIG. 3 is a partial schematic perspective view of the printer 10 for illustrating additional features of the heater 50. The segmented heater 50 includes a first portion 83, corresponding to the first preheat area 50 of FIG. 1, and a second portion 84, corresponding to the second heat area 54 of FIG. 1. The first portion 83 is supported by the inner guide member 32 which creates a curved heating surface used to preheat the recording medium during passage through the chute 28 of FIG. 1. The first portion 83 preheats the recording medium and is primarily necessary when printing on stress papers, since the absorption and desorption of water relaxes the internal stresses of the paper and can result in deformation, such as cockle. The preheating removes excess moisture from the paper and results in a more dimensionally stable sheet as well as improving ink absorption into the paper. Transparencies and certain coated papers do not require preheating and, in fact, can be damaged by excess preheating because of softening. The preheat area 83 is, therefore, separately controllable and can either be turned off or can have the heat produced thereby significantly reduced when these type of media are being printed. The second portion 84 of the flexible heater 50 generates heat energy having a temperature greater than the first portion 83. The second portion 84 provides the primary drying function for driving the liquid from the ink deposited by the printhead 12 in the print zone 38. As can be seen, the distance travelled by the recording medium 16 over the second portion 84 is much less than the distance travelled by the recording medium 16 over the first portion 83. The long dwell time experienced by a recording medium 16 in the first portion 83 drives any excess moisture from the paper. The function of the second portion 84 is primarily to add heat energy to the recording medium while printing to draw excess liquid from the ink. According to the present invention, therefore, a short dwell time of high heat energy follows a long dwell time of low heat energy thereby providing accurately controlled drying for all types of recording medium. The first portion 83 of the flexible heater 50 includes a first aperture 85 and a second aperture 86. The first aperture 85, as well as the second aperture 86, provide for the continued advancement of the recording medium through the print zone by enabling contact of the pinch roller to the drive roller for picking up the lead edge of the recording medium. By providing apertures 85,86 for accommodating the pinch roll and drive rolls 34,36, the amount of heat area provided by the present invention for a preheat zone is increased for the present printer configuration, since an area between the rollers and the print zone heating area can still be heated. FIG. 4 illustrates the two segment flexible heater 50 of the present invention. The construction of the flexible heater 50 consists of thin circuit wire runs of NiChrome resistive heat generating material. The thin layer is sandwiched between a top and a bottom layer of heat resistant polymer such as Kapton(polyimide) or polyester. The selection of materials is based primarily on the operating temperature of the heater. Portions of the heater are organized to supply heat energy according to specific voltages and watt densities. The thin configuration, for instance, preferably having a thickness of less than 2.54cm (0.1in), allows the heater to follow the radius of the preheat section. Since, the core of the heater must be thin, for instance, less than 25.4µm (0.001 in), serpentine shaped NiChrome elements perform well. Orientation of the heat runs in the construction are parallel to the motion of the printhead and perpendicular to the paper motion. The layer sandwiching the NiChrome element provides support for the heater core. The choice of sandwiching materials is primarily driven by the operating temperatures. Kapton (polyimide) is used for high operating temperatures, for instance, greater than 148.9°C (300°F). Polyester, on the other hand, can be used for lower operating temperatures, for instance, approximately 93.3°C (200°F). The overall thickness of this construction is less than 127µm (0.005in) and being made primarily of a plastic, the heater is very flexible. The described flexible heater is also known as a foil heater. Other types of flexible heaters, however, are within the scope of the present invention. For instance, any flexible heater having a low thermal mass is acceptable, such as wire resistive elements traversing a support medium, carbon loaded film, metal film photopatterned with runs of graphite material, or conductive material sprayed or doctor bladed on a support medium. Attached to the heater 50 is a connecting member 88 which includes electrical connecting leads for connecting to the first portion 83 and the second portion 84. Since the first portion 83 generates heat energy for preheating the paper, a current is applied to the first portion 83 by a controller/current supply module 90. The current applied to the first portion 83 is set to a predetermined level such that the heat energy generated by the first portion 83 is approximately 3.875kWm -2 (2.5Win -2 ). The temperature of this region is not sensed, and consequently, there are no feedback signals for maintaining the temperature of the heat generated by the first portion 83. This portion, however, could include temperature sensing. The second portion 84, on the other hand, preferably includes an embedded thermistor 92 for sensing the temperature of the second portion 84. The embedded thermistor 92 is connected to the controller/current supply module 90 which receives electrical signals indicating temperature. A current control signal is returned to the second portion 84 through the connecting member 88. The electrical signals received from the thermistor indicate the sensed temperature and help maintain the temperature of the second portion 84 at approximately 27-42 degrees Celsius (50-75 degrees Fahrenheit) hotter than the temperature of the first portion 83. The heat energy generated by the second region 84 is approximately 7.75Kwm -2 (5Win -2 ). A linear distance A of the first first portion 83 in the paper path direction is approximately 6.35cm (2.5in). A linear distance B of the second portion 84 in the paper path direction is approximately 2.54 cm (1in). Other distances are possible depending on swath width, paper length, heating temperatures, and other know factors affecting drying. Ideally, the second portion 84 is at least twice as wide as the distance covered by the printhead printing a single swath of information perpendicular to the paper path direction. By making the second portion 84 one swath wider than the amount of area covered by the printhead, the ink deposited by the printhead does not experience thermal shock during printing which could occur when transitioning from one print area to the next. The one swath advance print zone located before the actual print zone of the printhead provides for a more accurately controllable heat energy presence on the recording medium. A linear distance C is sufficiently wide to adequately heat the width of the paper deposited with ink. While the embodiments of FIGS. 1, 2 and 3 have illustrated anchoring the flexible heater at the edge 58 and applying tension to the edge 62, it is also possible to apply tension to all four edges of the flexible heater. Consequently, spring loaded mechanisms could be applied not only the second end 62, but also to the first end 58, to a first side edge 94 and a second side edge 96. These edges could include perforations as shown for attaching spring loaded devices thereto. In recapitulation, there has been described a segmented flexible heater for drying a printed image on a recording medium moving along a paper path. The segmented flexible heater is disposed adjacently to the paper path and includes a first portion for preheating the paper and a second portion for heating a portion of the paper located within a print zone. It is therefore apparent that there has been provided in accordance with the present invention a segmented flexible dryer that satisfies the aims and advantages hereinbefore set forth. While this invention has been described in conjunction with the specific embodiment thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. For instance, a segmented flexible heater is not limited to having two portions but can include any number of portions greater than two. Likewise, the temperature selected for the preheat portion and the print area portion can be selected as a function of print mode and media type. For example, a reduced temperature setpoint could be used for non-paper backed transparencies. In another instance, a higher temperature setpoint and improved intercolor bleed performance could be achieved in a high quality print mode which utilizes multi-pass printing where the image is built in two or more passes. These temperature setpoints would be optimized for the particular ink being used.

Claims: Printing apparatus (10) for printing on a recording medium (16) moving along a path through a print zone (38), comprising: a printhead (12), adapted to deposit ink (40) on the recording medium (16); and a heater (50, 83, 84), disposed adjacently to the path, for heating the recording medium (16); wherein the heater (50, 83, 84) is segmented and flexible and includes a first portion (83) defining a first heat region (52) preheating the recording medium (16) and a second portion (84) defining a second heat region (54), characterized in that the second portion of the flexible segmented heater is located within the print zone (38) for heating the recording medium (16) in the print zone (38). Apparatus according to claim 1, wherein the first portion (83) generates heat energy having a first temperature and the second portion (84) generates heat energy having a second temperature which is greater than the first temperature, the first portion (83) being located along the path before the second portion (84) in the direction of movement of the recording medium (16), the second portion (84) of the heater (50) being aligned with the print zone (38). Apparatus according to claim 1 or 2, further comprising at least one drive roller (34), the heater (50) defining at least one aperture (85, 86) to enable each drive roller (34) to contact the recording medium (16) therethrough to advance the recording medium (16) along the path. Apparatus according to any one of the preceding claims, further comprising a control module (90), coupled to said second portion (84), for controlling the heat energy generated thereby. Apparatus according to claim 4, wherein the second portion (84) includes a temperature sensing device (92), coupled to the control module (90), for sensing the temperature of the second portion (84) in accordance with signals received from the temperature sensing device (92). Apparatus according to claim 4 or 5, wherein the control module (90) is coupled to the first portion (83), for supplying a predetermined current thereto. Apparatus according to any one of the preceding claims, further comprising a heater support (56, 60, 64, 76, 78, 82) for supporting the heater (50, 83, 84) adjacently to the path. Apparatus according to claim 7, wherein the heater support (56, 60, 64, 76, 78, 82) includes a first support member (76) and a second support member (78), the second support member (78) being spaced a distance from the first support member (76), the first support member (76) and the second support member (78) contacting the heater (50, 83, 84). Apparatus according to claim 8, wherein the heater support (56, 60, 64, 76, 78, 82) further includes a tensioning member (64), coupled to the heater (50, 83, 84), which places it in tension across the first and second support members (76, 78). Apparatus according to claim 8 or 9, wherein a section (82) of the heater support (56, 60, 64, 76, 78, 82), located between the first support member (76) and said second member (78), is in non-contacting relation with the heater (50, 83, 84), the section (82) being aligned with the print zone (38).

Inventors: LOONEY JOHN H, SZLUCHA THOMAS F, LOONEY, JOHN H., SZLUCHA, THOMAS F.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1996
Main classification: B41J11
IPC codes: B41J29, B41J11, B41J2