Wiping print media

An example printing apparatus is described comprising a wiping element before the print zone, for slipping in contact with a print medium and wiping a printing surface of the print medium. An implementation of a wiping roller for wiping the printing surface of a print medium may comprise a rigid core, a layer of elastic material and a driving pinion. An example method for printing is also described, comprising 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.

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.

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 inFIG. 1. Example apparatus may comprise a print zone1where a printing fluid may be deposited on the printing surface5of a print medium4from a printhead2. Input rollers3may cause the advance of the print medium4towards the print zone1, in a direction of print media advance shown by arrow A inFIG. 1.

According to examples disclosed herein, a wiping element10may be provided in the apparatus before the print zone1, 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 medium4when the print medium is advanced towards the print zone1, thereby wiping the printing surface5of the print medium4before 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. 2shows schematically examples of printing apparatus also comprising a print zone1, a printhead2, input rollers3to cause the advance of a print medium4in a direction of print media advance A. The print medium4may be fed from a media roll6.

In examples such as shown inFIG. 2, the wiping element may be a wiping roller20that is provided in the media advance path before the print zone1and slips in contact with the printing surface5of the print medium4to wipe it.

In some examples, the angle through which there is contact between the wiping roller20and the print medium4is 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 medium4in the advance path provides a degree of pressure to apply the medium4against the wiping roller20and 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 inFIG. 3a, the wiping roller20comprises a layer of elastic material21, for example, attached on a rigid tubular core22.

The layer of elastic material21may be compressed when applied against the print medium, so it may allow maintaining the wiping roller20in contact with the printing surface5along all the width of the print medium4even if there is some degree of misalignment, and therefore may allow relatively uniform wiping, avoiding local defects.

In some examples, dimensions for a wiping roller20may be between 50 and 60 mm for the diameter D of the core22, and between 4 and 10 mm for the thickness t of the layer of elastic material21.

In some examples, such as illustrated inFIG. 3b, the wiping roller20comprises a sheath23of textile material covering the layer of elastic material21. The sheath23may be made for example of polyester microfiber or suede.

The presence of a sheath23may improve the mechanical resistance of the wiping roller20. 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 material21may 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. 4for example shows examples in which a motor30with an output shaft31is mounted on the frame40of 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 inFIG. 4, the wiping roller20comprises a driving pinion24and the motor30drives the wiping roller20in rotation through a transmission32between the motor output shaft31and the driving pinion24. In an example such as that ofFIG. 4the transmission32may be a gear transmission and may comprise a transmission pinion33intended to mesh with the driving pinion24.

Also visible inFIG. 4is that in some implementations of a printing apparatus with a wiping roller as disclosed herein, the wiping roller20may be mounted on a pair of idle support rollers41(one visible inFIG. 4). The wiping roller20is provided with a cylinder section25for resting on the support rollers41.

In implementations as disclosed herein, a wiping roller20for wiping the printing surface of a print medium, for example such as disclosed above in relation toFIGS. 3a, 3band4, 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 roller20may comprise as disclosed above a rigid core22, a layer of elastic material21attached to an outer surface of the rigid core, and a driving pinion24. In some examples it may also comprise a sheath23of textile material.

In some implementations it may be foreseen to install a wiping roller20, for example having a layer of elastic material21and a driving pinion24, 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 roller20from 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 element10(FIG. 1) may be stationary. For example, the wiping element10may 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 ofFIG. 5, and may comprise, in block100, 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 block200.

In some examples, the wiping operation in block100is performed with a wiping roller slipping in contact with the printing surface of the medium, such as examples of a wiping roller20as 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 block100the 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 block100, 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 roller20such as shown inFIGS. 2 and 3amay be employed. Further features may be for example as follows:tubular steel core with an outer diameter D=50 mmPU foam rubber layer adhered on the steel core, thickness t=5 mmfoam rubber layer elastically compressible to 50% of original thicknessdynamic friction coefficient of the foam material on vinyl media μk=0.6angle of contact of the wiping roller with the media: 110°back tension of the medium: 30 N/mspeed 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.