Patent Publication Number: US-10787011-B2

Title: Wiping print media

Description:
BACKGROUND 
     Some printing media may contain substances which over time may migrate to the printing surface. This phenomenon may occur for instance when rolled media is exposed to high temperatures, for example, during transportation or storage, or simply when media is stored for some time before use. 
     For example, print media such as vinyl and PVC banners may contain plasticizers to increase their flexibility, and these additives may tend to migrate to the surface. Other substances that may exhibit a tendency to migrate to the printing surface may be, for example, adhesives or silicones present in adhesive media. 
    
    
     
       BRIEF DESCRIPTION 
       Some non-limiting examples of the present disclosure will be described in the following with reference to the appended drawings, in which: 
         FIGS. 1 and 2  are schematic diagrams illustrating examples of printing apparatus according to implementations disclosed herein; 
         FIGS. 3 a  and 3 b    are schematic section views illustrating examples of wiping rollers according to implementations disclosed herein; 
         FIG. 4  is a schematic perspective view showing examples of the mounting of a wiping roller in apparatus as disclosed herein; and 
         FIG. 5  is a flowchart illustrating examples of a method for printing in accordance with examples disclosed herein. 
     
    
    
     DETAILED DESCRIPTION 
     The presence in some printing media of substances, such as plasticizers or adhesives, which over time may migrate to the printing surface forming micro-droplets or random patterns, may affect the quality of printed images. 
     A substance that has migrated to the printing surface, and/or contaminants present in the form of droplets, micro-droplets or moisture on the surface, may create differences in the surface tension, and it may therefore happen that printing fluid does not deposit uniformly. When printing on such a medium, for example with latex printing fluids, defects such as for example graininess, pinholes or coalescence may appear in the printed image. 
     In implementations disclosed herein the printing quality may be improved by wiping the printing surface of the media before printing, so as to spread over a larger surface area, i.e. more evenly, substances that may be present on the surface in the form of micro-droplets or the like. 
     Examples of a printing apparatus as disclosed herein are illustrated in  FIG. 1 . Example apparatus may comprise a print zone  1  where a printing fluid may be deposited on the printing surface  5  of a print medium  4  from a printhead  2 . Input rollers  3  may cause the advance of the print medium  4  towards the print zone  1 , in a direction of print media advance shown by arrow A in  FIG. 1 . 
     According to examples disclosed herein, a wiping element  10  may be provided in the apparatus before the print zone  1 , i.e. upstream of the print zone, in the direction of print media advance A through the apparatus, so that it slips in contact with the print medium  4  when the print medium is advanced towards the print zone  1 , thereby wiping the printing surface  5  of the print medium  4  before printing. 
     By “slip” or “slipping” it is meant herein that the wiping element is in contact with the printing surface of the print medium, and has a different speed from that of the printing surface in the area of contact, such that during operation there is a non-zero relative speed between the wiping element and the printing surface. The relative speed may be caused for example by the wiping element having a higher speed than that of the print medium, or by the wiping element being stationary or having a lower speed than that of the print medium. 
     The friction caused by wiping may have the effect that substances such as plasticizers that may be present on the printing surface in a non-continuous or uneven distribution, for example in the form of micro-droplets, clots, lumps, or other irregularities, are spread or distributed more evenly. For example, a droplet would be “flattened” on the media and spread over a larger area. 
     This allows reducing the differences in surface tension between different areas of the print medium and reducing potential defects in the printed image that may be associated with these differences. The quality of the printed image may therefore be improved. 
     The effect on the printed image of other contaminants present on the surface of the media, for example small amounts of grease from fingerprints due to media handling, may also be reduced. 
       FIG. 2  shows schematically examples of printing apparatus also comprising a print zone  1 , a printhead  2 , input rollers  3  to cause the advance of a print medium  4  in a direction of print media advance A. The print medium  4  may be fed from a media roll  6 . 
     In examples such as shown in  FIG. 2 , the wiping element may be a wiping roller  20  that is provided in the media advance path before the print zone  1  and slips in contact with the printing surface  5  of the print medium  4  to wipe it. 
     In some examples, the angle through which there is contact between the wiping roller  20  and the print medium  4  is between 10° and 120°. 
     A wiping roller may have a relatively small contact area with the print medium and still provide a wiping action. Consequently it may be fitted in the media advance path taking up a relatively small space and without affecting the apparatus footprint. 
     In some examples the back tension of the print medium  4  in the advance path provides a degree of pressure to apply the medium  4  against the wiping roller  20  and maintains the contact between medium and roller. In examples disclosed herein, the back tension may be between 20 and 40 N/m. 
     In some implementations of printing apparatus as disclosed herein, such as illustrated in  FIG. 3 a   , the wiping roller  20  comprises a layer of elastic material  21 , for example, attached on a rigid tubular core  22 . 
     The layer of elastic material  21  may be compressed when applied against the print medium, so it may allow maintaining the wiping roller  20  in contact with the printing surface  5  along all the width of the print medium  4  even if there is some degree of misalignment, and therefore may allow relatively uniform wiping, avoiding local defects. 
     In some examples, dimensions for a wiping roller  20  may be between 50 and 60 mm for the diameter D of the core  22 , and between 4 and 10 mm for the thickness t of the layer of elastic material  21 . 
     In some examples, such as illustrated in  FIG. 3 b   , the wiping roller  20  comprises a sheath  23  of textile material covering the layer of elastic material  21 . The sheath  23  may be made for example of polyester microfiber or suede. 
     The presence of a sheath  23  may improve the mechanical resistance of the wiping roller  20 . Furthermore, maintenance may be simplified by the fact that once the wiping surface becomes affected by wear and/or by having plasticizer or similar substances adhered thereon, as a consequence of use, it is possible to substitute the sheath. 
     In some implementations of a wiping element such as a wiping roller, both with or without a sheath of textile material, the elastic material may be foam, or a soft rubber. In some examples it may be foam rubber, that is, rubber having an air-filled matrix structure obtained by using a foaming agent. For example, the layer of elastic material  21  may be of polyurethane (PU) foam rubber, which is also wear resistant and compatible with printing fluids. 
     In some examples of implementations disclosed herein, the maximum compressibility of the layer of elastic material, defined as the maximum compression the material may undergo while remaining elastic, is at least of 50%. With a 50% maximum compressibility, for example, a layer of elastic material with a thickness of 5 mm may undergo a deformation of up to 2.5 mm in a direction perpendicular to the contact surface, for adapting to misalignments of the print medium. 
     In some examples, implementations of printing apparatus disclosed herein comprise a motor to drive the wiping roller in rotation.  FIG. 4  for example shows examples in which a motor  30  with an output shaft  31  is mounted on the frame  40  of the apparatus. In some examples the motor may be for example a DC motor controlled with an encoder (not shown). 
     In some examples, such as shown in  FIG. 4 , the wiping roller  20  comprises a driving pinion  24  and the motor  30  drives the wiping roller  20  in rotation through a transmission  32  between the motor output shaft  31  and the driving pinion  24 . In an example such as that of  FIG. 4  the transmission  32  may be a gear transmission and may comprise a transmission pinion  33  intended to mesh with the driving pinion  24 . 
     Also visible in  FIG. 4  is that in some implementations of a printing apparatus with a wiping roller as disclosed herein, the wiping roller  20  may be mounted on a pair of idle support rollers  41  (one visible in  FIG. 4 ). The wiping roller  20  is provided with a cylinder section  25  for resting on the support rollers  41 . 
     In implementations as disclosed herein, a wiping roller  20  for wiping the printing surface of a print medium, for example such as disclosed above in relation to  FIGS. 3 a , 3 b    and  4 , may be provided as a kit, or as part of a kit, to be installed in a printing apparatus. The kit may also comprise a driving motor, and may also comprise a transmission. 
     In some examples, such a wiping roller  20  may comprise as disclosed above a rigid core  22 , a layer of elastic material  21  attached to an outer surface of the rigid core, and a driving pinion  24 . In some examples it may also comprise a sheath  23  of textile material. 
     In some implementations it may be foreseen to install a wiping roller  20 , for example having a layer of elastic material  21  and a driving pinion  24 , in a printing apparatus comprising a motor and transmission, in order to print on some kind of print media such as a vinyl banner, which contain substances that may migrate to the printing surface. 
     It may also be foreseen in some implementations to remove the wiping roller  20  from the printing apparatus and change it with a plain roller that is not provided with a layer of elastic material or a driving pinion, for example in order to print on other kinds of print media, without prior wiping of the printing surface. 
     In examples according to some implementations of a printing apparatus, the wiping element  10  ( FIG. 1 ) may be stationary. For example, the wiping element  10  may comprise a wiping surface, flat or curved, against which the print media slips in order to be wiped. 
     The material of the wiping surface of a wiping element according to examples as disclosed herein may have a dynamic friction coefficient below 0.7 with respect to vinyl print media, in order to avoid affecting the accuracy of the print media advance. 
     Implementations of a method for printing are illustrated schematically by the flowchart of  FIG. 5 , and may comprise, in block  100 , spreading over a larger print medium area, by wiping, amounts of a substance that may migrate through a print medium and that is present on the printing surface of a print medium, before printing on the print medium in block  200 . 
     In some examples, the wiping operation in block  100  is performed with a wiping roller slipping in contact with the printing surface of the medium, such as examples of a wiping roller  20  as disclosed above. Slipping between the wiping roller and the print medium, and therefore wiping, may occur while the print medium is advancing, but also while it is stationary, for example when printing is performed in swaths on a print medium while stationary, and the print medium is advanced between swaths. 
     According to some implementations disclosed herein, in block  100  the wiping roller may be rotated, for example employing a motor, in order to cause slipping of the wiping roller with respect to the print medium at least when the print medium is stationary. It may be rotated for example with an angular speed between 20 and 40 rpm. 
     In some implementations, example methods may involve rotating the wiping roller with an angular speed that causes the relative tangential speed of the surface of the wiping roller with respect to the printing surface of the print medium to be between about 2 in/s and about 5 in/s (between about 50.8 mm/s and about 127 mm/s), for example between about 3 in/s and about 4 in/s (between about 70.6 mm/s and about 101.6 mm/s). 
     During the wiping operation in block  100 , in some examples of the method a tension of the print medium maintains contact between the wiping roller and a printing surface of the print medium. 
     Some implementations of such methods may be performed by printing apparatus as disclosed above. 
     In examples of printing operations in which implementations of this disclosure are put in practice, a wiping roller  20  such as shown in  FIGS. 2 and 3   a  may be employed. Further features may be for example as follows:
         tubular steel core with an outer diameter D=50 mm   PU foam rubber layer adhered on the steel core, thickness t=5 mm   foam rubber layer elastically compressible to 50% of original thickness   dynamic friction coefficient of the foam material on vinyl media μ k =0.6   angle of contact of the wiping roller with the media: 110°   back tension of the medium: 30 N/m   speed of rotation of the wiping roller: minimum 30 rpm       

     Although a number of particular implementations and examples have been disclosed herein, further variants and modifications of the disclosed devices and methods are possible. For example, not all the features disclosed herein are included in all the implementations, and implementations comprising other combinations of the features described are also possible.