Patent Publication Number: US-9427994-B2

Title: Apparatus and method of tensioning print media

Description:
An embodiment of the invention provides an ink jet printer comprising: a rotatable drum for supporting a print medium; a motor operable to rotate the drum; an air nozzle directed substantially tangentially to the surface of the drum in a direction that is substantially opposite to the linear direction of the drum; and a printhead operable, in use, to eject ink onto the substrate supported by the drum after the substrate has been treated by the air nozzle. 
     In some embodiments the ink jet printer is a wide-format printer. 
     An embodiment of the invention provides a method of pressing a print medium against a printer drum comprising; placing the print medium on the drum; rotating the drum; and applying a substantially laminar flow of air to the print medium on the drum in a direction that is substantially opposite to the direction of the rotating drum thereby applying a tensioning force to the print medium. 
     An embodiment of the invention provides a method of flattening print media against a printing drum comprising: directing a substantially laminar gas flow across the medium, whilst the medium is on the drum, at a direction that is substantially tangential to the drum. 
     An embodiment of the invention provides a printing apparatus comprising: a rotatable drum adapted to receive a print medium around at least part of the drum&#39;s circumference; and a gas nozzle directed substantially tangential to the circumference of the drum. 
     An embodiment of the invention provides an apparatus comprising: support means for supporting a print medium means; and air flow means for directing air at the print medium means, the air flow means being directed substantially tangentially at the print medium means when the print medium means is on the support means. 
     An embodiment of the invention provides use of an air knife to simultaneously flatten and cool a print medium on a medium carrier. 
     Embodiments of the invention are configured to produce a volumetric flow rate of gas that is equal to or greater than 100 standard cubic feet per minute. Embodiments of the invention are configured to produce a volumetric flow rate of gas that is equal to or greater than 200 standard cubic feet per minute. 
     The medium carrier may be substantially flat or it may be a roller or other rotatable surface. Such a rotatable surface will generally comprise a convex surface for supporting the medium. 
     An embodiment of the invention provides an air nozzle and an attachment for fitting the air nozzle to an ink jet printer so that the air nozzle is substantially tangential to the printing drum of the ink jet printer. 
     Generally this embodiment of the invention will also include instructions on how to fit the air nozzle to the ink jet printer so that the air nozzle is substantially tangential to the printing drum of the ink jet printer. 
     In an embodiment of the invention the nozzle/air knife is directed substantially tangentially to the drum and substantially in the same direction as the linear velocity of the rotating drum. In this embodiment the medium is still flattened against the drum. 
     An embodiment of the invention provides a method comprising transporting a print medium on a support in a first direction and applying an air flow having a major component that is in a direction that is opposite to said first direction so as to apply a tensioning force to the print medium. 
     An embodiment of the invention provides apparatus comprising support means having a surface for supporting a print medium and a gas ejection means for directing gas at the surface of the transport means wherein the gas ejection means is orientated to eject gas at the surface such that in use the print medium is pressed onto the transport means. 
     It should be appreciated that embodiments and aspects of the invention that are defined in a particular category (e.g. a method) then the same embodiment or aspect can also be defined as other categories (e.g. as a printing system or a printer). The skilled person will understand that the features and embodiments of the invention that are described and claimed may be combined in various ways. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, of which: 
         FIG. 1  schematically illustrates a printing system; 
         FIG. 2  schematically illustrates a print medium wrapped on a printing drum; 
         FIG. 3  schematically illustrates, in cross-section, some example drums that can be used according to embodiments of the invention; 
         FIG. 4  schematically illustrates a printing system incorporating an air knife according to an embodiment of the invention; 
         FIG. 5  schematically illustrates an air nozzle and a set of reference axes in relation to a printing drum; 
         FIG. 6  schematically illustrates various orientations of an air nozzle in relation to a printing drum; 
         FIG. 7  illustrates a front view of a large format inkjet printing drum machine; 
         FIG. 8  illustrates a side view of the inkjet printing drum machine illustrated in  FIG. 7 ; and 
         FIG. 9  illustrates a detail of the inkjet printing drum machine illustrated in  FIGS. 7 and 8 . 
     
    
    
     SPECIFIC DESCRIPTION 
       FIG. 1  illustrates a printing apparatus comprising a drum  20  upon which a print medium  10  is wound and a means for applying ink  34  to the medium  10 . In the specific example illustrated in  FIG. 1  the printing apparatus is an ink jet printer in which the ink applying means is a printhead  32  which is supported by a printhead carriage  30 . The carriage  30  moves in relation to the drum  20  so that a printed image may be built up on the medium  20  as the drum  20  rotates. Although a specific embodiment of the invention is described in relation to ink jet printers it should be appreciated that embodiments of the invention can be realised with other types of printer eg dry tone laser printers, liquid electrophotographic printers (eg LEPs and LED printers) to name a few. 
       FIG. 2  illustrates the drum  20  in more detail. In particular, in the example drum illustrated, the drum  20  has a number of vacuum holes  22  so that once the medium  10  has been wound onto the drum  20  the medium  10  may be held onto the drum surface by vacuum forces applied through the vacuum holes  22 . Vacuum holes  22  are often used on large format printers (sometimes also called “wide format printers”). In large format printers the circumference of the drum  20  may be from about half a meter to several meters. 
     Although the drum  20  illustrated is a cylinder having a substantially circular cross-section, embodiments of the invention are not necessarily limited to any particular geometry. The main requirement of the drum is that it is able to transport the medium  10  so as to present the medium to the ink applying means (i.e. the printhead  22  in  FIG. 1 ). As illustrated in  FIG. 3 , the drum  20  may therefore have a non-circular cross-section such as an elliptical cross-section D-shaped cross-section or have a shape/configuration that produces a cam. Generally the drum  20  will have a convex surface for supporting the print medium  10  although embodiments of the invention can use a flatbed medium carrier (described in more detail herein below). 
     The print medium  20  can be any of a wide range of substrates including paper, vinyl, textiles or polypropylene films such as that known YUPO® (sometimes referred to as “synthetic paper”) or other types of polymer film. 
     Large format printing devices, the medium carrier may have a length of several tens of centimetres to several meters (the length being defined in relation to a process direction of the printing apparatus), for example in printers in which the print carrier is a drum  20  the drum  20  may have a circumference of the order of 0.5 meter to several meters. Large format printing devices are generally operated in a controlled environment because small temperature changes can cause significant variations in the size of the printed image and/or degrade image registration. The problem can be severe when flexible printing substrates such as YUPO® are used. 
     The drum  20  may be operable to repeatedly pass under a printhead  32  and a source of drying or curing  40  as illustrated in  FIG. 1 . This processing produces heat and the difference in the temperature of the print medium  10  at the end of the printing process compared to the temperature of the print medium  10  at the start of the printing process may be several Celsius and may be as high as 15 to 20 Celsius. This increase in temperature causes the print medium  10  to swell to produce a deformed area  12  of the medium  10  on the drum  20 . For a drum  20  having a circumference of about 5 meters the ends of the same substrate may expand by a few millimetres. Despite the use of a vacuum to hold the print medium  10  against the drum  20  the difference in size of the print medium  10  from the beginning to the end of the printing process causes sections of the medium  20  to be released from the drum  20 . This causes a loss of image registration and degrades the quality of the printed image. 
       FIG. 1  includes an enlarged view of a section of the print medium  10  in the vicinity of the print head  32 . The swelling of the medium  10  produces print artefacts for example by causing areas different densities of colours than that which were intended (zone A and zone B on  FIG. 1 ).  FIG. 1  shows an area  12  of the medium  10  that has deformed so that it has become detached from the drum  20  so that the printing surface of the medium  10  is at height h above the surface of the drum  20 . When the swelling is such that the height h exceeds the distance d of the printhead  32  above the drum  20  then the ink  34  fired by the printhead  32  will be smeared on the medium  10  and the printhead  32  can become damaged. Typically distance d is about 1.5 mm for large format printers. 
     Referring to  FIG. 4 , according to an embodiment of the invention a nozzle  50  is used to direct gas at the medium  10 . Although other gases may conceivably be used the gas is generally air since it is cheaper than other gases and is not flammable or toxic. In an embodiment of the invention the nozzle  50  produces a high intensity uniform sheet of airflow. Such airflow is often referred to in the manufacturing arts as an “air knife”. The term “air knife” is also commonly used to refer to the nozzle which produces such an airflow. The airflow is directed at the medium  10  on the drum  20  so that the airflow applies a substantially tangential force to the medium  10 . The airflow is generally applied over all or most of the axial length of the drum  20 . This may be achieved using a nozzle  50  which has an opening for producing the airflow wherein the opening has an axial extent which is as long as most or all of the axial length of the drum  20  or longer than the axial length of the drum  20 . 
     The Coanda effect, also known as “boundary layer attachment”, is the tendency of a stream of fluid to stay attached to a surface. For example a stream of fluid may stay attached to a convex surface rather than follow a straight line in its original direction. The Coanda effect keeps the air stream produced by the nozzle  50  attached to the surface of the drum  20 . This is advantageous because it keeps the airflow in the direction required, for example, tangentially to the drum surface and/or in the direction opposite to the linear velocity of the drum surface. Additionally, the Coanda effect causes the jet of air to have a larger area of contact with the medium  10  on the drum  20  thereby flattening and cooling a larger area of the medium  10 . There is a smooth temperature gradient within the airflow attached to the drum  20  so that there is no temperature shock to the medium  10  below the air knife. 
       FIG. 5  illustrates an axis system with reference to the drum  20  in which the T axis is in the tangential direction to the drum&#39;s surface and the R axis is in a direction that extends radially from the drum&#39;s surface (i.e. orthogonal to the T axis). A nozzle directed at an angle ⊖to the tangent to the drum&#39;s surface is operated to produce an airflow with a force F against the surface of a print medium  10  supported by the drum  20 . The force F has a force component in the tangential direction, F T =F cos ⊖, and a force component in the radial direction, F R =F sin ⊖. Preferably the airflow is directed so that most of the force F will act in the tangential direction T. That is, nozzle  50  is directed at the drum surface with an angle of less than 45 degrees so that the major component of the force F produced by the airflow will be in the tangential direction T. 
       FIG. 6  illustrates the nozzle  50  orientated in several different positions (A, B, C, D) relative to the surface of the drum  10 . The nozzle  50  may be orientated substantially tangentially to the drum  20  (position A) so that the airflow exiting the nozzle  50  produces a force F that acts tangentially on the medium  10  on the drum  20 . If the nozzle is position at a slight angle to the tangent to the drum  20  (position B), e.g. 15 degrees, the force F will still be substantially tangential (e.g. F T =F cos15=0.97 F). As the angle, θ, approaches 45 degrees (position C) the tangential component of the force F decrease but it is still the major component (i.e. it is larger than the radial component of the force F R ), At θ=45 degrees the resolved components are equal (F T =F R ) and at θ&gt;45 degrees (eg at position D) the radial component takes over as the major component of the force F. 
     When the tangential component of the force produced by the airflow is acting in a direction that is opposite to the linear velocity, v, of the medium  10  on the drum  20  (at the position that the airflow intercepts the medium  10 ) then there is a relative velocity between the airflow and the medium  10  that is higher than the velocity of the airflow itself. The airflow produces a drag force F D  on the medium  10 . This drag force acts to tension the medium  10  on the drum  20  and, as a consequence flattens the medium  10  against the drum  20 . Higher relative velocities between the airflow and the medium  10  can produce higher forces tending to flatten the medium  10  to the drum  20 . 
     Referring again to  FIG. 4 , a lifting force FL acting on the medium  10  is shown. The lifting force is caused by the airflow over the medium  10  causing a reduced pressure compared to the pressure below the medium  10 . 
     Generally the airflow is substantially laminar however in some embodiments the flow is not laminar but has an overall direction that is substantially opposite to the direction of rotation of the drum  20 . 
     The nozzle  50  may produce an airflow that is substantially laminar across a portion of the airflow and it is this portion that is directed to intercept the drum  20 . In one example the nozzle  50  may have an elongated slot from which the airflow is ejected and the elongated slot is substantially aligned with the axis of the drum  20 . In this case it may be possible that the flow deviates from a substantially laminar flow at the edges of the flow (in the axial direction). In this situation the deviation may be acceptable if the portion of the flow exhibiting the deviation is small compared to the substantially laminar portion of the flow. Alternatively, the slot may have an axial extent that is longer than the axial length of the drum so that at least some of the portion that deviates from a substantially laminar flow does not intercept the drum  20 . 
     The stream of air that passes over the drum  20  involves a large volume of air from the surrounding environment along with the small amount of compressed air from the air knife itself. This large volumetric flow of air has a large cooling effect on the medium  10 . 
     The airflow passing through the nozzle  50  may be cooled or temperature controlled. For example, a cooler may be used to cool the air before it enters the nozzle  50 . The temperature of the airflow may be controlled so that it is cooler than the ambient temperature of the air surrounding the drum  20 . 
     The nozzle  50  and/or cooler can be retrofitted to a printing system to produce the desired airflow over the medium  10  on the drum  20 . The nozzle  50  may therefore be supplied with an attachment for attaching the nozzle to the printing system at the required angle (e.g. substantially tangentially to the printing drum  20 ). The attachment may attach the nozzle  50  at a fixed angle to the drum  20  or may allow for the required angle to be set by a user. A set for retrofitting a nozzle  50  may comprise instructions for fitting the nozzle at the required angle (e.g. substantially tangentially to the printing drum  20 ), the nozzle  50  and the attachment. 
       FIG. 7  is a schematic, frontal view illustration of a large format inkjet printing drum machine  100  and  FIG. 8  is a schematic, side view illustration of the same machine  100 . Machine  100  includes a drum  104  that holds the substrate (print medium)  108 , which may be a vinyl, paper, YUPO type material or other flexible material. Printhead  112  prints successive swathes of the image and progresses from one machine end to the other machine end (from left to right as illustrated in  FIG. 8 ). Associated with the printhead movement is a wide source  116  of curing energy, such as a UV lamp. Arrow  120  indicates drum rotation direction. Under the influence of heat generated by lamp  116 , the substrate  108  changes its size and certain sections of it may even bulge, this is despite the substrate being held down on the drum by a vacuum. 
       FIG. 9  is a schematic expansion of a section of the machine marked A that illustrates a bulge  140  in the medium  10 . 
     Traditional cooling devices, even those providing a large volume of air do not cool sufficiently&#39;substrate  108  or drum  104 , nor are they capable of attaching substrate  108  to the surface of the drum. Use of water-cooling may complicate and would generally be used for cooling the drum  20  rather than directly cooling the substrate  108 . Air knife  124  is installed in such a way that a high intensity, balanced stream of laminar airflow across the entire width of the drum is directed tangential to the drum  104  surface. Such air knife installation generates a strong “laminar” flow in excess of 250 SCFM (standard cubic feet per minute) of air along the drum circumference. The Coanda effect keeps the air stream attached to the drum surface  128 . This develops pressure on the substrate  108  and keeps it attached to the drum surface  128 . The stream involves a large volume of air from the surrounding area along with the small amount of compressed air from the air knife itself. The amount of air involved is more than a magnitude larger than the one produced by conventional cooling means. The method described maintains the temperature of the drum  104  and substrate  108  in the range of ±2.0 Celsius in course of a five minute printing cycle and keeps the substrate  108  firmly attached to drum surface  128 . 
     In an embodiment of the invention the printer is a flat-bed printer that uses a flat-bed medium carriage to transport the print medium with respect to the printing means, In this embodiment the air knife is directed substantially in the direction of the plane of the medium supported on the medium carriage. Flatbed printers generally use less flexible print media than drum based printers however medium expansion can still be a problem and the use of an air knife as described above can be used to improve the print quality of the printed medium. 
     Thus, while the present invention has been described in terms of preferred embodiments, it will be appreciated by one of ordinary skill that the spirit and scope of the invention is not limited to those embodiments, but extends to the various modifications and equivalents as defined in the appended claims.