Patent Description:
In the inkjet printing technology, ink is ejected through the nozzles of the inkjet printhead to form ink droplets based on printing image data when the inkjet printhead and the printing medium move relative to each other. The ink droplets fly through the space between the inkjet printhead and the printing medium and strike the printing medium at defined positions, therefore completing formation of the desired print image by controlling formation of each ink drop.

In current digital drum printing practice, a printing medium such as textile or paper is fitted on the drum and printed continuously to effectively improve the printing efficiency. However, the existing digital drum printing apparatus, such as the technical solution disclosed in <CIT>, includes multiple printheads, each of which prints only one color of ink. The printheads are arranged in a straight line, and the entire printing medium has to pass through all the printheads loaded with different color inks to complete a print. The distance that the printing medium passes through the printheads along the straight line is equal to the length of the printing medium in the straight line direction plus the length of all the printheads arranged along the straight line direction. Therefore, the printing speed is determined by the linear motion speed along the straight line and the above distance. In addition, <CIT> only includes a single drum. After finishing printing each part, a unprinted part needs to be loaded and then the printing can be resumed, consequently causing printing delay. In order to shorten the delay due to part loading, <CIT> discloses a method in which one printhead can print on multiple drums. This method allow printing on one drum while loading and unloading printing media on other drums, thereby shortening the delay of parts replacement and improving printing efficiency. However, the printing speed for a single drum has not increased. To improve printing speed on a single drum, <CIT> discloses a drum printing apparatus, in which the printing direction of the printhead (i.e. the printhead motion direction) and the drum rotational axis form an included angle, and the printhead and the drum move continuously and synchronously during printing. However, due to the included angle, after the printhead travels a certain distance, there will be no printing medium underneath it. Therefore, this printing method limits the length of the printing medium and cannot meet the printing requirements when the printing medium is longer. What is still needed to be solved is how to further increase the printing speed of one-drum printing, as well as meeting the needs of printing on media with various lengths.

<CIT> discloses an ink jet printer having a cylindrical drum rotated about its axis and past a translatable print bar located adjacent and parallel thereto. The print bar has equally spaced, identical print heads mounted along the length thereof to print images on the rum or directly on a recording medium mounted on the drum. Each print head has an array of high density nozzles that extend for a predetermined length. The spacing between print heads is equal to the inter multiples of the nozzle array length. The print bar may be translated a distance equal to an integer divisor of the nozzle array length up to one full nozzle array length during each drum revolution. The drum rotation and concurrent print bar translation produce barber pole shaped swath of image on the drum by each print head. Multiple passes are required to print a complete image.

<CIT> discloses a printing system for printing on an object having a curved surface. The printing system includes a support assembly for supporting an object having the curved surface to be printed on. The support assembly includes a gripper configured for holding the object thereon at a predetermined working distance between the curved surface of the object and a printing head unit. Paragraphs <NUM> to <NUM> of the Detailed Description of Embodiments section, as well as FIGS. 20E and 20F more clearly show what is being described in `<NUM>. 20F shows an object <NUM> having a curved outer surface to be printed by printing system <NUM> (see also paragraph <NUM> as well as <FIG> for various configurations of the printing system <NUM> relative to object <NUM>). Paragraph <NUM> states that FIGS. 20E and 20F demonstrate use of a conical expansion mechanism for gripping the object <NUM>. A hollow cylindrical shaped element <NUM> is shaped from an elastic material to permit radial expansion of its shape to provide a gripping mechanism for achieving a firm grip on a hollow object <NUM>. Paragraph <NUM> states that FIG. 20F is a cross-sectional view exemplifying a conical expansion grip mechanism where the tapering ends of two conical elements 46c are introduced into the ends to increase the diameter of cylindrical element <NUM>. The conical elements 46c are gradually moved toward one another so that they become pressed against the end openings 46e of the cylindrical element and apply outwardly directed radial forces until the outer surface of the cylindrical element <NUM> grips the inner surface of the hollow object <NUM>. The conical elements in FIG. 20F are elements of a gripping system. They are not the object to be printed.

According to an aspect of the present invention, there is provided a printing apparatus according to the independent claims <NUM> and <NUM> and a method of printing according to the independent claim <NUM>.

The printing apparatus and printing method proposed in the present invention shorten the relative motion distance between the printheads and the printing medium during printing process by dividing the printing medium into multiple printing areas and using multiple printheads with a variety of different inks to print, thereby reducing the overall printing completion time and increasing the printing speed.

The fastest printing speed can be achieved by setting the pitches of the printheads along the first direction to be substantially equal, where the lengths of the printing areas are equal, and the pitch of the printheads and the length of the printing areas are substantially equal. In addition, when the number of printheads does not match the number of printing areas, the printheads can be used alternately to balance the usage frequency of each printhead, so as to avoid various problems caused by over or under utilization of the printheads.

It is to be understood that the attached drawings are for purposes of illustrating the concepts of the invention and may not be to scale. Identical reference numerals have been used, where possible, to designate identical features that are common to the figures.

In the description of this invention, it should be noted that the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", "top", "bottom", "above", "below", etc. indicate orientation or positional relationship based on the orientation or positional relationship shown in the drawings. They are used to facilitate the description of the present invention and simplify the description, rather than to indicate or imply that the devices or elements referred to must have specific orientations, specific oriented structures and operations. Therefore, they cannot be understood as a limitation of the present invention. The terms "first", "second", "third" are for descriptive purposes only, and cannot be understood as indicating or implying relative importance. In addition, unless otherwise clearly stipulated and defined, the terms "mount", "connect", "connection" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it also can be mechanical connection or electrical connection; it can be directly connected or indirectly connected through an intermediate medium, or it can be an internal connection between two components. For those skilled in the art, the specific meanings of the above terms used herein can be understood in specific circumstances.

In reference to <FIG>, the printing apparatus includes a motion controller <NUM> and a print driver <NUM>. The motion controller <NUM> controls the motion of the printhead <NUM> and the printing medium <NUM> through a transport mechanism <NUM>. The print driver <NUM> controls the printing of the printhead <NUM>. The motion controller <NUM> and the transport mechanism <NUM> can provide rotational and linear motion and control. The image data source <NUM> provides image data, which is translated by the image processor <NUM> into commands for printing. The term "image" is meant herein to include any pattern of dots specified by the image data. It can include graphics or text images. It can also include various 2D or 3D dot patterns suitable for printing functional devices or three-dimensional structures if suitable inks are used in the printing. The motion controller <NUM> feeds back the position information of each printhead <NUM> and the printing medium <NUM> to the print driver <NUM> and the image processor <NUM> in real time. The image processor can provide the image data according to the relative position of the printheads <NUM> to the printing medium <NUM>. The print driver <NUM> sends output signals to the electrical pulse source <NUM> according to the image data translated by the image processor <NUM>. And the electrical pulse source <NUM> sends the electrical pulse waveform to the printhead <NUM>. The printhead <NUM> can also feed back information such as the temperature of the printhead to the print driver <NUM> in real time to adjust print parameters (such as the ink ejection voltage waveform). The printing apparatus includes at least one printhead. According to the positions of the printheads <NUM> and the printing medium <NUM>, the print driver <NUM> instructs the printheads <NUM> when to start printing and when to end printing, and controls each printhead <NUM>, each printing unit therein, and each nozzle therein throughout the entire printing process according to the desired printing image patterns.

As shown in <FIG>, the printing apparatus includes three identical printheads arranged along the direction <NUM>, which are a first printhead <NUM>, a second printhead <NUM>, and a third printhead <NUM>, respectively. The printing apparatus also includes a drum <NUM>, a guide rail <NUM>, a transporter <NUM>, a drive shaft <NUM>, and a motor <NUM>, forming the transport mechanism <NUM> of the printing apparatus in <FIG>. For clarity, the detailed connection between the controller and transport mechanism and mounting structures are omitted in the figures. The guide rail <NUM> extends longitudinally along the direction <NUM>, and the printheads are mounted on the guide rail <NUM> and distributed along the direction <NUM>. The drum <NUM> has a rotational axis G (indicated by a dashed line in the <FIG>) and can be driven to rotate around the axis G. The printing medium <NUM> can be disposed on the surface of the drum <NUM>. The motion controller <NUM> issues commands to the motor <NUM> that drives the drive shaft <NUM> to rotate, and then the drive shaft <NUM> rotates the drum <NUM>. By controlling the drum <NUM>, the rotation of the printing medium <NUM> disposed thereon can be controlled. The drum <NUM> rotates around the rotation axis G, and causes the printing medium <NUM> attached to its surface to rotate around the G axis. The motion controller <NUM> also controls linear motions of the printheads on the guide rail <NUM> through the transporter <NUM>. The transporter <NUM> can be connected to the first printhead <NUM>, the second printhead <NUM> and the third printhead <NUM> respectively, or alternatively can be connected to a common movable mechanical structure on which three printheads are mounted together, so that the three printheads can be moved linearly back and forth on the guide rail along the direction <NUM>.

In the embodiments of the present invention, each printhead includes at least two printing units. <FIG> uses the first printhead <NUM> as an example to illustrate four configurations of the printing units. In the example shown in <FIG>, the first printhead <NUM> includes three printing units <NUM>, which are disposed along the direction <NUM>. The first printhead <NUM> is in fluidic communication with three color inks c1, c2 and c3, and each printing unit <NUM> is in fluidic communication with one color ink. The c1, c2, and c3 inks can be any color, for example, c1 can be magenta ink, c2 can be cyan ink, and c3 can be yellow ink. <FIG> shows the case of four printing units, the four printing units <NUM> being arranged along the direction <NUM>. The first printhead <NUM> is in fluidic communications with four color inks c1, c2, c3, and c4, and each printing unit <NUM> communicates with one color ink. The c1, c2, c3, and c4 inks can be any color, for example, c1 can be magenta ink, c2 can be cyan ink, c3 can be yellow ink, and c4 can be black ink. In the example shown in <FIG>, the first printhead <NUM> includes four printing units <NUM>, where the four printing units are arranged along a direction <NUM> that is inclined at an angle with respect to direction <NUM>, such that the angle is greater than <NUM> degrees and less than <NUM> degrees. The first printhead <NUM> communicates with four color inks c1, c2, c3, and c4, and each printing unit <NUM> communicates with one color ink. The c1, c2, c3, and c4 inks can be any color. For example, the c1 can be cyan ink, the c2 can be yellow ink, the c3 can be magenta ink, and the c4 can be black ink. In the example shown in <FIG>, the first printhead <NUM> includes four printing units <NUM>, where the four printing units are arranged along a direction <NUM> that is perpendicular to the direction <NUM>. The first printhead <NUM> communicates with four color inks c1, c2, c3, and c4, where each printing unit <NUM> communicates with one color ink. The c1, c2, c3 and c4 inks can be any color. For example, the c1 can be yellow ink, the c2 can be magenta ink, the c3 can be cyan ink, and the c4 can be black ink. In other embodiments shown in <FIG>, the first printhead <NUM> can be in fluidic communication with various functional inks, for example, three printing units are respectively connected to magenta ink, cyan ink and yellow ink, and one printing unit is connected to a color fixative. Using the above four printing unit configurations as examples, this invention also includes other printing unit configurations.

Corresponding printing units in each printhead communicate with the same ink. For example, the printing units in the first printhead <NUM>, the second printhead <NUM>, and the third printhead <NUM> can all be arranged in the manner shown in <FIG>, where the c1 printing unit in the printhead <NUM> communicates with cyan ink, and the c1 printing units in the second printhead <NUM> and the third printhead <NUM> also communicate with cyan ink; the c2 printing unit in the first printhead <NUM> communicates with magenta ink, and the c2 printing units in the second printhead <NUM> and the third printhead <NUM> also communicate with magenta ink; the c3 printing unit in the first printhead <NUM> communicates with yellow ink, and the c3 printing units in the second printhead <NUM> and the third printhead <NUM> also communicate with yellow ink.

Each printing unit in a printhead includes a plurality of nozzles <NUM> formed on an orifice plate. <FIG> shows four configurations of nozzles. In the example shown in <FIG>, the printing unit <NUM> includes six nozzles <NUM> marked with <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>, respectively and arranged in a row along the direction <NUM>. The six nozzles are arranged in a row according to their sequential numbers. In the example shown in <FIG>, the nozzles are arranged in a row along the direction <NUM> that is inclined at an angle with respect to the direction <NUM>. In the example shown in <FIG>, the printing unit <NUM> includes two rows of nozzles <NUM> arranged along the <NUM> direction. The two rows of nozzles are spaced apart from each other along the direction <NUM> and are staggered along the direction <NUM> by half of an adjacent nozzle center-to-center distance. In the example shown in <FIG>, the printing unit <NUM> includes a plurality of nozzles <NUM> arranged in a two-dimensional array (dotted line enclosed). The length L of the two-dimensional array along the direction <NUM> is greater than the width W along the direction <NUM> perpendicular to the direction <NUM>. Each column includes four nozzles, where the nozzles in the first column are marked with <NUM>, <NUM>, <NUM> and <NUM>, respectively, the nozzles in the second column are marked with <NUM>, <NUM>, <NUM> and <NUM>, respectively, and the nozzles in the third column are marked with <NUM>, <NUM>, <NUM> and <NUM>. The nozzles are arranged in four rows along the direction <NUM>, each row including three nozzles, where the nozzles in the first row are respectively marked as <NUM>, <NUM>, and <NUM>, the nozzles in the second row are marked as <NUM>, <NUM>, and <NUM>, the nozzles in the third row are labeled <NUM>, <NUM> and <NUM> respectively, and the nozzles in the fourth row are labeled <NUM>, <NUM> and <NUM> respectively. The nozzles with the same last digit in their numbers are aligned in a column, and the column direction is inclined with respect to the direction <NUM>. Beyond the four configurations shown above, those skilled in the art can understand that other nozzle arrangements are also applicable to this invention, and the number of nozzles is not limited to the illustrated examples and can be any other number.

In reference to <FIG>, the printing apparatus includes a printing medium <NUM> disposed opposite to the printheads <NUM>, <NUM>, and <NUM>. The printheads can be on the top with the printing medium <NUM> below, so that the nozzles of the printheads face the printing medium <NUM>. The printing medium <NUM> is attached to the drum <NUM> surface and rotates around the rotational axis G. The cross section of the printing medium <NUM> in a plane perpendicular to the rotational axis G is circular with the rotational axis G passing through the center of the circle (<FIG>) and extending in parallel to the direction <NUM>. The printheads are disposed along the direction <NUM> with the centers of the nozzle array (<FIG> and <FIG>) on the printing units forming a straight line along the direction <NUM>. The straight line through the centers of the nozzle arrays and the rotational axis G are coplanar and define a plane that intersects the curved surfaces of the printing medium <NUM> at the intersection line Q. The intersection line Q is parallel to the rotational axes G and direction <NUM>, and is also substantially parallel to the orifice plate on each printing unit. The vertical distance (the shortest distance) between the intersection line Q and the orifice plates on the printhead is also called the printing distance that can be controlled and adjusted in a mechanical structure (not shown) that the transporter <NUM>, the guide rail <NUM>, and the drive shaft <NUM> are mounted to. In the embodiment shown in <FIG>, the printing distance along the direction <NUM> is constant. The constant printing distance ensures uniform printing quality. For a given printhead, usually the shorter the printing distance, the better the print quality. The printing medium <NUM> is divided, by the number of printheads, into three printing areas along the direction <NUM>, which are labeled as H, J, and K, respectively.

The printing medium <NUM> is fitted as a sleeve on the drum <NUM> from the first end <NUM> to the second end <NUM> of the drum <NUM>, that is the printing medium <NUM> is put on the drum <NUM> starting from the first end <NUM> and moving toward to the second end <NUM> until all the printing medium is fitted on the surface of the drum <NUM>. The printing medium <NUM> can be cylindrical shaped (such as a wine bottle, a metal tube, a plastic tube, a cardboard tube, etc.), or it may not sustain a cylindrical shape in its normal state, for example knitwear (such as socks, pants, some garment materials) and flexible materials (such as textiles, plastic films, paper or leather), but it can be conformally fit on to or wrapped around the surface of the drum <NUM>. The preferred size of the drum <NUM> is determined by the size of the printing medium <NUM>, which ensures no slippage of printing medium on the drum, and has a sufficient surface to allow the flexible materials to cover the surface of the drum without overlapping or being irreversibly deformed. A hard cylindrical medium can be fitted as a sleeve on a suitable drum <NUM>, or it can be directly mounted on other rotational mechanisms without using a drum. The rotational axes of the other rotational mechanisms and the drum <NUM> coincide with the rotational axis G. The length of the printing medium <NUM> along the direction <NUM> is m. In general, the length of the drum <NUM> along the axial direction is greater than m. At the beginning of printing, the distance between the nozzle closest to the first end (for example, nozzle <NUM> in <FIG>) of the first printhead <NUM> and the first end <NUM> of drum <NUM> along the direction <NUM> is a. The printing medium <NUM> is divided into equal sized areas according to the number of printheads, marked as H, J, and K with equal length of c along the first direction <NUM>. The printheads are identical and evenly spaced with pitch of b. This pitch is also the distance between corresponding nozzles (having same relative positions in the respective printheads) in adjacent printheads. For example, combining configurations in <FIG> and <FIG>, each printhead has four printing units, and each printing unit has a two-dimensional array of nozzles. The distance between the first printhead <NUM> and the second printhead <NUM> is equal to the distance between the nozzle <NUM> in the c1 printing unit of first printhead <NUM> and the nozzle <NUM> in the c1 printing unit of the second printhead <NUM> along the first direction (also direction <NUM>), and is also equal to the distance between nozzle <NUM> in the c2 printing unit of the first printhead <NUM> and nozzle <NUM> in the c2 printing unit of the second printhead <NUM> along the first direction <NUM>. Similarly, the distance between the second printhead <NUM> and the third printhead <NUM> is the equal to the distance between the corresponding nozzles of the adjacent printheads. The distance between all the adjacent printheads herein is referring to the distance between the corresponding nozzles of the adjacent printheads. In the embodiment shown in <FIG>, the length m of printing medium <NUM> equals the distance a between the nozzle closest to the first end of the first printhead <NUM> and the opposite first end <NUM> of the drum <NUM>. Also in the <FIG> embodiment the printhead pitch b equals the length c of the equally sized printing areas of printing medium <NUM>. In other words, m = a and b = c. When the medium length m is changed, the distance between the printheads can be adjusted so that m = a, b = c = m / n, where n is the number of printheads.

<FIG> is a top view of the printing apparatus. In order to more clearly display the positional relationship between the printhead and the printing medium, the transporter, the connections between the transporter and the printheads, and the mountings are omitted. In reference to the example shown in <FIG>, the first printhead <NUM>, the second printhead <NUM>, and the third printhead <NUM> are arranged along the direction <NUM>. The printing units <NUM> in each printhead are arranged along the direction <NUM>. The printing medium <NUM> is divided into three printing areas H, J, and K along the direction <NUM> (which is the same direction as the rotational axis G). In the example shown in <FIG>, the first printhead <NUM>, the second printhead <NUM>, and the third printhead <NUM> are arranged along the direction <NUM>. The printing units <NUM> in each printhead are arranged along the direction <NUM>. The printing medium <NUM> is divided into three printing areas H, J, and K along the direction <NUM>. In the example shown in <FIG>, the first printhead <NUM>, the second printhead <NUM> and the third printhead <NUM> are arranged along the direction <NUM>. The printing units <NUM> in each printhead are arranged along the direction <NUM>. The printing medium <NUM> is divided into three printing areas H, J, and K along the direction <NUM>. The above embodiments show three cases of positional relationship between the printheads and the printing medium. Those skilled in the art will understand that the arrangement of the printing units can be arranged in other ways, as long as the arrangement direction of the printhead is parallel to the rotational axis G of the printing medium <NUM>.

When the printing apparatus is printing, the printing medium <NUM> continuously rotates around the rotational axis G at a uniform angular speed, and at the same time, each printhead is printing and advancing synchronously in the direction <NUM> at a constant speed. The motion direction of the printheads is parallel to the direction of the rotational axis G to ensure that the printheads keep a constant distance to the printing medium <NUM> during the motion and not deviating away from the printing medium. In multi-color printing, each printing unit communicates with one color ink. When the printing medium <NUM> completes one full rotation, the printheads advance a distance equal to the length of one printing unit in a printhead along the direction <NUM>. Each printing area is printed by one printhead, for example, the first printhead <NUM> is responsible for printing the K area, the second printhead <NUM> is responsible for printing the J area, and the third printhead <NUM> is responsible for printing the H area. In addition, in the embodiment described above with reference to <FIG> where m = a, b = c = m / n, since the distances b between printheads are equal to their respective printing area lengths c and the printing area lengths are also equal to each other, each printhead starts printing at the same time and finishes printing at the same time. After printing the entire printing area is completed, the relative distance moved along direction <NUM> between the printheads and the printing medium <NUM> is equal to the length of the single printhead plus the length of the printing area c = b along direction <NUM>. Therefore, the larger the number of printheads (n), the smaller the pitch b between adjacent printheads, the smaller the length c = b of corresponding printing area, and the faster the speed of printing a piece. The length of a printhead refers to the distance between the two furthest nozzles in a printhead along the direction <NUM>. Taking the first printhead <NUM> as an example, with reference to <FIG> and <FIG> configuration, the length of the printhead <NUM> is equal to the distance between the nozzle <NUM> in the c1 printing unit and the nozzle <NUM> in the c4 printing unit in the first printhead <NUM> along the first direction <NUM>. The length of any printhead needs to be less than the length of the printing area, preferably less than half the length of the printing area. In addition, the length of the printing unit refers to the distance of the two furthest nozzles in a printing unit along the direction <NUM>. Taking the c1 printing unit as an example with nozzle configuration shown in <FIG>, the length of the printing unit is the distance from nozzle <NUM> to the nozzle <NUM> along the first direction. The length of the printing unit is less than m / n and is also less than or equal to the length of the printhead divided by the number of printing units.

In another embodiment, the distances between adjacent printheads may not be equal. It can be designed that the distance difference between any two pairs of adjacent printheads does not exceed <NUM>% of the distance between any pair of adjacent printheads. For example, the distance difference between the first printhead <NUM> and the second printhead <NUM> and between the second printhead <NUM> and the third printhead <NUM> does not exceed <NUM>% of the distance of the first printhead <NUM> and the second printhead <NUM>, or the distance of the second printhead <NUM> and the third printhead <NUM>. The length difference of any two printing areas, H/J/K, does not exceed <NUM>% of the length of any printing area. If the distance between the first printhead <NUM> and the second printhead <NUM> is equal to the length of the printing area K, the distance between the second printhead <NUM> and the third printhead <NUM> is equal to the length of the printing area J, the distance of the third printhead <NUM> to the first end <NUM> along the direction <NUM> is equal to the length of the printing area H, but K≠J≠H. In this case, it is also possible that three printheads start printing at the same time, but the three printheads do not finish printing at the same time. The printing time depends on the maximum distance between adjacent printheads, which also corresponds to the maximum printing area length. To print the same media, setting the printheads with non-equal distances takes longer time to print a part than for equal distances. Also the usages on the printheads are uneven.

<FIG> shows a configuration where the printing medium <NUM> has a conical frustum shape. It can be a frustum in its natural form such as a paper cup, or a flexible material with the dimensions of the curved surface of a frustum and conforming to the surface to take on the frustum shape when wrapped onto a frustum. For example, the printing medium can be knitwear (such as socks, pants, or other garment parts) and flexible materials (such as cloth, plastic film, paper, or leather). The axis of the conical frustum <NUM> is G (dashed line in <FIG>). Conical frustum <NUM> has a first end <NUM> having a diameter that is larger than that of the opposing end <NUM>. When the conical frustum rotates around the axis, the printing medium <NUM> is driven to rotate around the rotational axis G. The cross sections of the printing medium <NUM> in a plane perpendicular to the rotation axis G are circular shapes with the rotational axis G passing through the center of the circle. Although these cross sections of the conical frustum are circular shapes, the diameters of the cross sections are different at different axis points. If the embodiment shown in <FIG> is used, in which the axis G of cylindrical drum is coplanar with and parallel to direction <NUM>, the intersection line Q of the surface of conical frustum is inclined with respect to the direction <NUM>. In other words, the distance between the printing medium <NUM> and the printheads changes along the direction <NUM> (one end is closer to the printhead, and the other end is farther away from the printhead). That means the printing distance changes along the direction <NUM>, consequently causing a change in the ink drop placement position and resulting in uneven print quality and gradual distortion of the image. In order to solve this problem, the rotation axis G in <FIG> is inclined relative to the <NUM> direction in the same plane, and the inclined angle is equal to <NUM>° - α , where α is the included angle between a line <NUM> that is in the plane of the first end <NUM> and perpendicular to the rotational axis G of the conical frustum <NUM>. After the rotational axis G is inclined, the intersection line Q becomes parallel to the direction <NUM>, so the printing distance is substantially constant along the direction <NUM>. The printing medium <NUM> is also divided into three printing areas along the direction <NUM>. Each printhead prints one printing area. The transporter <NUM> is connected to the first printhead <NUM>, the second printhead <NUM>, and the third printhead <NUM> respectively and controls the printhead linear motion along the guide rail <NUM>. The motor <NUM> drives the drive shaft <NUM> to rotate, in turn, the drive shaft <NUM> rotates the frustum drum <NUM> and controls the printing medium <NUM> to rotate around the G axis. The other features in <FIG> can be the same as those described in <FIG>.

The embodiment described above solves the problem of the distance change along the direction <NUM> between a conically shaped printing medium <NUM> and the printheads. In the direction perpendicular to the plane formed by the axis G and the direction <NUM>, that is also the width direction of the printhead (the direction <NUM> in <FIG>), the distance between the nozzles in the printheads and the printing medium varies along the printhead width direction (direction <NUM>) as shown in <FIG> for some printheads (such as printheads with two-dimensional nozzle arrays), because those printheads extend along a width direction and the surface of the printing medium curves in the width direction. Taking the first printhead <NUM> as an example, the distances of ink droplets ejected from nozzles at different positions along the direction <NUM> to the corresponding positions on the printing media are different. The shortest distance is at the center of the nozzle array, the furthest distance is at the two width ends. If the radius r of the drum cannot be much larger than the width d of the printhead (r / d » <NUM>), the distance variation between the nozzles and the printing medium cannot be ignored. The narrower the printhead width (equal to the distance between the endmost nozzles of the printhead along direction <NUM>), closer to satisfying r / d >> <NUM>, the better for printing quality. However, if the printhead width is too narrow, it will significantly affect the printing speed and resolution. Therefore, it is necessary to find the maximum width of the printheads with acceptable print quality. Assume that the acceptable distance between the endmost nozzles of the first printhead <NUM> and the printing medium <NUM> is i, and that beyond this distance, the printing quality will become objectionable. The distance between the nozzle at the middle of the first printhead <NUM> and the printing medium is f. The radius of the printing medium <NUM> is r. The angle β can be solved from the equation cos β = <NUM> - (i-f) / r. The width of the first printhead is d = 2r * sin β. If the width of the first printhead <NUM> is not greater than the calculated d, the print quality can meet the requirements.

When the printing apparatus shown in <FIG> prints, all the printheads move synchronously and start printing after entering their respective printing areas. <FIG> shows a snapshot during printing when the first printhead <NUM> is printing area K, the second printhead <NUM> is printing area J, and the third printhead <NUM> is printing area H. <FIG> shows the state at the end of printing. After each printhead finishes printing its own area, it leaves the corresponding area and completes one pass. If the same printing apparatus is used to print a higher resolution and / or a higher ink coverage, the printheads can return to the starting positions and repeat the printing process of <FIG>, <FIG>, and <FIG>. This printing process can be repeated multiple times, also known as multiple pass (the number of times the printheads pass over the surface of the printing medium) printing. For multi-color printing (one printhead prints two colors or more), after each complete rotation of the printing medium, the relative displacement of the printhead and the printing medium along the first direction is equal to e/k, where e is the length of a printing unit in the first direction, and k is the number of passes and is a positive integer greater than or equal to <NUM>. For monochrome printing, after each complete rotation of the printing medium <NUM>, the relative displacement of the printhead and the printing medium in the first direction is equal to the length of a printhead in the first direction divided by k, where k (the number of passes) is a positive integer greater than or equal to <NUM>. For clarity, the transport, the connection mechanism between the transport, the printheads, and the motor are not shown in <FIG> and <FIG>. The following figures also omit those parts. It is understood that this will not hinder the understanding of this invention by those skilled in the art.

In practical applications, the distance b between the printheads and the length m of the printing medium <NUM> may not be optimized and matched as shown above in <FIG> and <FIG>. <FIG> show the printing process when the distance b between adjacent printheads is greater than the length c of the printing area.

Specifically, as shown in <FIG>, the lengths of the printing areas are equal (c = m / n), and the printheads are arranged at an equal distance of b, where b > c and each printhead is responsible for printing its corresponding area. So the third printhead <NUM> should be closest to the beginning of the area H at the start of printing. Determined by the distance b between adjacent printheads and the length c of the printing area, the second printhead <NUM> is farther away from the area J, and the first printhead <NUM> is the farthest away from the area K. As a result, the distance a between the nozzle closest to the first end of the printhead and the opposite first end <NUM> of the drum is greater than m (a > m). During printing, the third printhead <NUM> enters the area H first and starts printing while the other printheads advance toward their respective printing areas without printing. The printheads continue to advance. As shown in <FIG>, the second printhead <NUM> enters the area J and starts printing. At this time, the third printhead <NUM> has printed a partial area, and the first printhead <NUM> has not yet entered the area K. The printheads continue to advance in the direction <NUM>. As shown in <FIG>, the first printhead <NUM> enters the area K and starts printing. At this time, the second printhead <NUM> and the third printhead <NUM> are both in the printing process. The printheads continue to move forward. As shown in <FIG>, the third printhead <NUM> completes printing. At this time, the second printhead <NUM> and the first printhead <NUM> are both in the printing process. The printheads continue to move forward to a state shown in <FIG>, when the second printhead <NUM> completes printing, and only the first printhead <NUM> is still printing the area K. The printheads continue to advance to a state shown in <FIG>, when the first printhead <NUM> completes printing, and both the second printhead <NUM> and the third printhead <NUM> have previously finished printing. At this time the printing job on this printing part is completed. In the embodiment shown in <FIG>, the printhead <NUM> that started printing first would end printing first, and the printheads <NUM> and <NUM> that started printing later would end printing later. Each printhead has a different printing start time and a different printing end time. Compared with the embodiment where the positions of the printhead are adjusted so that b = c and m = a (shown in <FIG>), the overall printing time in the <FIG> embodiment is longer.

Similarly, <FIG> show the printing process when b is less than c.

Specifically, as shown in <FIG>, the lengths c of the printing areas are all equal to m/n (c = m / n). The printheads are arranged at an equal distance of b. At the printing start position, the first printhead <NUM> should be the closest to the beginning of the area K, while the second printhead <NUM> is farther away from the area J, and the third printhead <NUM> is the farthest away from the area H. During printing, the first printhead <NUM> enters the area H first and starts printing, while the other printheads advance toward their respective printing areas without printing. The printheads continue to advance to a state as shown in <FIG>, when the second printhead <NUM> enters the area J and starts printing. At this time, the first printhead <NUM> has printed part of area K and continues printing, and the third printhead <NUM> has not entered the area H. The printheads continue advancing in the direction <NUM> to a state shown in <FIG>, when the third printhead <NUM> enters the area H and starts printing, at this time the second printhead <NUM> and the first printhead <NUM> are also in the printing process. The printheads continue advancing to a state shown in <FIG>, when the first printhead <NUM> finishes printing the area K, and at this time, the second printhead <NUM> and the third printhead <NUM> are both in the printing process. The printheads continue moving forward to positions shown in <FIG>, where the second printhead <NUM> completes printing, and only the third printhead <NUM> is still printing the area H. The printheads continue advancing to a state shown in <FIG>, when the third printhead <NUM> completes printing, and both the second printhead <NUM> and the first printhead <NUM> have finished printing before this time. The printing job on this printing part is completed. As shown in <FIG>, the printhead that started printing first completes printing first, and printheads that started printing later would end printing later. Each printhead starts printing at different times, and also completes printing at different times. The overall printing time for a part is longer relative to the optimized configuration shown in <FIG>, <FIG> and <FIG>.

In the printing process of the embodiment shown in <FIG> above, the distance between the adjacent printheads can be unequal, but the variation is preferred not to exceed <NUM>%. The optimal configuration is where the distances between printheads are uniform. The lengths of the printing areas can also be unequal, but the variation is preferred not to exceed <NUM>%. The optimal configuration is where the printing areas have equal length. Under other combinations of different printhead distances and different printing area lengths, the printhead start and complete sequences can be different from those shown in <FIG>, but the process should be similar. Each printhead has a different start time and a different complete time. Compared with the optimized configuration shown in <FIG>, <FIG> and <FIG>, the overall printing time is longer.

<FIG> show an embodiment where the printing apparatus includes two printheads with distance b between the first printhead and the second printhead. The printing medium is divided into <NUM> areas where the lengths of the areas J and K are both c. The length of the area H can be less than or equal to c, where c and b are substantially equal.

At the start shown in <FIG>, the first printhead <NUM> and the second printhead <NUM> move synchronously in the direction <NUM> and start printing at the same time. As the first printhead <NUM> completes printing the area K, the second printhead <NUM> also completes printing the area J (shown in <FIG>). Afterwards, the first printhead <NUM> and the second printhead <NUM> continue moving in the direction <NUM>, but the first printhead is not printing while the second printhead prints the printing area H, until the second printhead <NUM> finishes printing the area H. Then the printing job on the printing medium <NUM> is completed (<FIG>).

After the above printing job is completed, the printheads can print during moving in reverse direction, that is, the printheads move in the reverse direction of the direction <NUM>. At the start, the first printhead <NUM> and the second printhead <NUM> are located at the positions shown in <FIG>, where the first printhead <NUM> prints the printing areas J and K, and the second printhead <NUM> prints the printing area H. The printing process is similar to the method shown in <FIG>, but in the reverse direction.

In the embodiments shown in <FIG>, the length m of the printing medium is less than n * b, and the printing apparatus includes three printheads with distance b between the second printhead <NUM> and the third printhead <NUM>. The printing medium <NUM> is divided into two printing areas, such that the lengths of the printing area H and the printing area J are both c, and c = b.

At the start shown in <FIG>, all printheads move in the direction <NUM> simultaneously. The second printhead <NUM> and the third printhead <NUM> start printing at the same time. The third printhead prints the printing area H, the second printhead prints the printing area J, and the first printhead <NUM> does not print. As shown in <FIG>, the third printhead <NUM> and the second printhead <NUM> complete printing the printing areas H and J, respectively. The first printhead <NUM> does not print during this printing process.

If the printhead <NUM> has not been used for a long time, it will affect its jetting performance and need maintenance, resulting in wasted ink. To avoid that, after multiple prints, the printing method shown in <FIG> can be utilized where the first printhead <NUM> and the second printhead <NUM> are a distance b apart, and b = c. The printheads move in a direction opposite to the direction <NUM>. The first printhead <NUM> prints the printing area J, the second printhead <NUM> prints the printing area H, and the third printhead <NUM> does not print.

In some embodiments the printheads can not only move synchronously, but also each printhead can move independently shown in <FIG>. In <FIG>, the first printhead <NUM> is aligned to the edge of the area K while the second printhead <NUM> has not yet entered the printing area J, and the third printhead <NUM> is in the printing area H. Before printing starts, the third printhead is moved from the position shown in <FIG> to the position shown in <FIG>, and the second printhead <NUM> is moved from the position shown in <FIG> to the position shown in <FIG>. The result is that the configuration is changed to the embodiment shown in <FIG>. In addition, the printheads can be moved according to the length m of the printing medium, so that the printhead distance b is equal to m / n (b = m / n), and the length c of each printing area is equal to printhead distance (c = b), forming the configuration shown in the <FIG> embodiment. Furthermore, the speeds of the printheads can also be different. For example, the resolutions required in different printing areas on the printing medium can be different, or some printing areas need to be left blank. For areas requiring lower resolution or including white space, the printheads scan over the area quickly. The printheads scan slowly in the areas that require higher resolution.

As shown in <FIG>, the printing areas can overlap. The printing area H and the printing area J overlap in area I. The length of the I area is p, where p does not exceed <NUM>% of the length of any printing areas H, J, or K. The overlapping area I is printed jointly by the first printhead <NUM> and the second printhead <NUM>, meaning part of the dots in the overlapping area I are printed by the first printhead <NUM>, and the remaining dots are printed by the second printhead <NUM>. Having the overlapping area I can enable feathering to blend the boundary between the printing area H and the printing area J to be less abrupt and improve the printing quality. For printing in other overlapping areas, the same printing method as described above can be used, which will not be described here.

The embodiments described above in <FIG> include only one row of printheads. <FIG> shows a cross section of an embodiment including multiple rows of printheads. The cross section is perpendicular to rotational axis G. In a multi-row printhead configuration, each printhead is similar to the printheads described in previous embodiments. The printheads in each row <NUM> are aligned along the direction <NUM> that each row extends. Printhead rows <NUM> are parallel to each other and have identical shortest vertical distance h to the surface of the drum <NUM>, and are arranged along tangential lines of the cylindrical surface <NUM> at a distance h from the surface of the cylindrical drum <NUM>. The printhead rows can be staggered or aligned to increase the print resolution or print speed.

<FIG> shows a printing apparatus in which the drum <NUM> rotates in the direction indicated by the arrow. The printing medium <NUM> is wrapped on the surface of the drum <NUM>. The three printheads <NUM>, <NUM>, and <NUM> are arranged along the direction <NUM>. Each printhead includes four printing units <NUM>, which are disposed along the direction <NUM>. The nozzles in each printing unit <NUM> are disposed in four rows along the direction <NUM>. From left to right, the four printing units <NUM> jet black (K), cyan (C), magenta (M), and yellow (Y) inks (the order of ink positions can be interchanged). In a printing unit, the first row (Row <NUM>) of nozzles is paired and aligned with the fourth row (Row <NUM>) of nozzles, and the second row (Row <NUM>) of nozzles is paired and aligned with the third row (Row <NUM>) of nozzles, since the distances of two corresponding nozzles to the surface of the printing medium <NUM> are the same. Each color printing unit has multi-nozzle arrays design, which can increase printing speed or provide nozzle redundancies to improve printhead life. Only the color structure of the first printhead <NUM> is shown in the figure. The second printhead <NUM> and the third printhead <NUM> can have the same structure, but for simplicity, the color order is not shown for them. It is understood that this will not hinder the understanding of those skilled in the art.

<FIG> shows an embodiment of another printing apparatus. The difference from <FIG> is that the printheads are arranged in two rows along the direction <NUM>, such that the printheads <NUM>, <NUM> and <NUM> are in one row and the printheads <NUM>, <NUM> and <NUM> are in another row. The printheads in the two rows are aligned in a way that the first printhead <NUM> and the fourth printhead <NUM> are identical and symmetrically arranged with respect to the rotational axis, the second printhead <NUM> and the fifth printhead <NUM> are identical and symmetrically arranged, and the third printhead <NUM> and the sixth printhead <NUM> are identical and symmetrically arranged. All printheads can have the same color order structure as shown in the first printhead <NUM>. In each printhead, the first nozzle row is symmetrical to the fourth nozzle row of the opposing printhead, the second nozzle row is symmetrical to the third nozzle row of the opposing printhead, and the third nozzle row is symmetrical to the second nozzle row of the opposing printhead, and the fourth row of the nozzles is symmetrical to the first nozzle row of the opposing printhead. The two corresponding nozzles on the opposing printheads have a constant distance to the surface of the printing medium <NUM>. Using multiple rows of printheads can increase the printing resolution and speed. The figure only shows the color structure of the first printhead <NUM> and the fourth printhead <NUM>, other printheads can also have the same structure, but are not shown in the figure for simplicity. It is understood that this will not hinder the understanding of those skilled in the art.

<FIG> shows a printing apparatus different from <FIG>, in which the printheads are arranged in two rows along the direction <NUM>. The printheads <NUM>, <NUM> and <NUM> are in one row and the printheads <NUM>, <NUM> and <NUM> are in another row. The printheads in the two rows are staggered. The fourth printhead <NUM> is substantially aligned with the gap between the first printhead <NUM> and the second printhead <NUM>. The fifth printhead <NUM> is substantially aligned with the gap between the second printhead <NUM> and the third printhead <NUM>. Similarly, the second printhead <NUM> is substantially aligned with the gap between the fourth printhead <NUM> and the fifth printhead <NUM>, and the third printhead <NUM> is substantially aligned with the gap between the fifth printhead <NUM> and the sixth printhead <NUM>. The term substantially aligned means best fit of printheads and corresponding gaps that have substantially equal length along the direction <NUM>. The printheads can be slightly longer or shorter than the gap. The printhead configuration can be any of the configurations described above. During printing, the drum <NUM> rotates, and each printhead moves back and forth along the direction <NUM>. Since the gaps between the printheads in the same row are aligned with the printheads in the other row, each printhead only needs to move a printhead width, without having to move a printhead width plus the distance between the printheads. So the printing efficiency is greatly improved.

<FIG> shows another printing apparatus. It differs from <FIG>, having printheads arranged in four rows along the direction <NUM>. The printheads <NUM>, <NUM> and <NUM> are in the first row, the printheads <NUM>, <NUM> and <NUM> are in the second row, the printheads <NUM> and <NUM> are in the third row, and the printheads <NUM> and <NUM> are in the fourth row. The first and second rows are in the middle, and the third and fourth rows are on outer sides. The printheads of the first and second rows are aligned with each other, and the printheads of the third and fourth rows are aligned with each other. The printheads of the first and third rows are staggered, and the printheads of the second and fourth rows are staggered. Specifically, the seventh printhead <NUM> is substantially aligned to the gap between the first printhead <NUM> and the second printhead <NUM>, the eighth printhead <NUM> is substantially aligned to the gap between the second printhead <NUM> and the third printhead <NUM>, and the second printhead <NUM> is substantially aligned to the gap between the seventh printhead <NUM> and the eighth printhead <NUM>. The gap between the fourth printhead <NUM> and the fifth printhead <NUM> is substantially aligned to the ninth printhead <NUM>, the gap between the fifth printhead <NUM> and the sixth printhead <NUM> is substantially aligned to the tenth printhead <NUM>, and the gap between the ninth printhead <NUM> and the tenth printhead <NUM> is substantially aligned with the fifth printhead <NUM>. Each printhead configuration can be any of the configurations described above. During printing, the drum <NUM> rotates and each printhead moves back and forth along the direction <NUM>. Due to the alignment of the printheads in rows <NUM> and <NUM> and the filling of the gaps between the printheads in the same row with the printheads in the other rows, each printhead can cooperatively print a region of the image with a corresponding aligned printhead, and each printhead only needs to move a printhead width, without having to move a printhead width plus the distance between the printheads. a printhead length plus the distance between the printheads. This is also more efficient than the printing shown in <FIG>.

In other embodiments, the printheads can be stationary, and the printing medium moves in the first direction while rotating. It can be understood for those skilled in the art that this can also apply to cases described above and achieve the same technical results.

Claim 1:
A printing apparatus comprising:
n printheads (e.g. <NUM>, <NUM>, <NUM>) arranged along a first direction (<NUM>), each printhead being in fluid communication with at least two different inks (e.g. c1, c2, c3, c4);
a printing medium (<NUM>) arranged opposite to the printheads, the printing medium including s printing areas (e.g. H, J, K) divided along the first direction such that each printing area is configured to be printed by a printhead, wherein the printing medium has a shape of a conical frustum (<NUM>) including:
a rotational axis (G); and
a first end (<NUM>) having a larger diameter than that of an opposing second end (<NUM>), wherein the rotational axis of the conical frustum is inclined relative to the first direction such that a printing distance between the printheads and the printing medium is configured to be substantially constant as the printing medium is rotated about the rotational axis;
a motion controller (<NUM>) that controls the motion of the printheads and the motion of the printing medium; and
a printing driver (<NUM>) that controls the printing of the printheads wherein s and n are positive integers greater than <NUM>.