LEP printer, a photo imaging plate for such printer and a method for wiping such photo imaging plate

In an LEP printer having a wiper blade for wiping a photo imaging plate, the photo imaging plate may include a PIP foil with a trailing edge having a zone of a roughening pattern which includes notches that are inclined with respect to a contact line of the wiper blade. The zone of roughening pattern is confined between two opposite margins lacking any roughening.

BACKGROUND

Liquid electro-photographic (LEP) printing involves digital printing using electro-ink, which includes small color particles suspended in imaging oil that can be attracted or repelled to a photoconductive sheet (photo imaging plate) by causing a voltage differential on that sheet.

The first stage of LEP digital printing in such LEP printers involves selective charging of the surface of the Photo Imaging Plate (PIP) using its photo-induced electric conductivity and a laser beam. Next, charged liquid ink is applied to the surface of the PIP. Due to the selective charging ink is attracted to image pixels (at locations on the PIP where surface potential was affected by a laser beam), and rejected from background pixels (where the laser has not discharged the surface potential). This latent image is then transferred from the surface of the PIP to an intermediate transfer media (ITM, also known as “blanket”) in what is known as the “first transfer”. The image is then transferred in what is known as “second transfer” from the ITM to the paper by pressing the paper to the ITM by an impressing drum. In order to evaporate solvents present in the ink liquid prior to the encounter with the paper, the surface of the ITM is maintained very hot, and since the ITM and the PIP are firmly pressed against each other during the first transfer, the PIP foil absorbs heat which is to be dissipated before the next printing cycle. Moreover, since the efficiency of the first transfer is not 100%, some ink and imaging oil residues may remain on the surface of the PIP foil, and these residues may therefore inadvertently affect the next printing cycle if not attended to.

DETAILED DESCRIPTION

In the following detailed description, numerous specific details are set forth. However, it will be understood by those skilled in the art that examples may be practiced without these specific details. In other instances, well-known methods, procedures, and components have not been described in detail so as not to obscure the discussed examples.

Although examples are not limited in this regard, the terms “plurality” and “a plurality” as used herein may include, for example, “multiple” or “two or more”. The terms “plurality” or “a plurality” may be used throughout the specification to describe two or more components, devices, elements, units, parameters, or the like. Unless explicitly stated, the method examples described herein are not constrained to a particular order or sequence. Additionally, some of the described method examples or elements thereof can occur or be performed at the same point in time.

Liquid electro-photographic (LEP) printing, sometimes referred to as liquid electrostatic printing, uses liquid toner to form images on paper or other print media. LEP printing is often used for large scale commercial printing. Basic LEP printing process involves placing a uniform electrostatic charge on a photoconductor, the photoconductive surface on a rotating drum for example, and exposing the photoconductor to light in the pattern of the desired printed image to discharge the areas of the photoconductor exposed to the light. The resulting latent electrostatic image on the photoconductor is developed by applying a thin layer of liquid toner to the photoconductor. Liquid toner generally consists of charged toner particles dispersed in a carrier liquid. The charged toner particles adhere to the discharged areas on the photoconductor (discharged area development—DAD) or to the charged areas (charged area development—CAD), depending on the charge of the toner particles, to form the desired toner image on the photoconductor. The toner image is transferred from the photoconductor to an intermediate transfer member and then from the intermediate transfer member to the paper or other print medium.

In some LEP printers, the photoconductive element includes a replaceable film of photoconductive material wrapped around a rotating drum. This drum is commonly referred to as the PIP (Photo Imaging Plate) and the thin film of conductive material as the PIP foil. The PIP foil is replaced periodically, once or twice a work shift for example depending on the printing volume, to maintain the good print quality. A new PIP foil is accurately aligned to the PIP drum during installation to help ensure good print quality and to minimize the risk of damaging the PIP foil during installation and printing.

FIG. 1schematically illustrates main parts of an LED printer100according to an example. Printer100may include three drums (the drums in this drawing are not presented to scale): PIP drum102, ITM104and impression drum106. PIP foil200, in accordance with an example, is wrapped around PIP drum102. Magenta114, yellow116, cyan118and black120developers are arranged in series adjacent PIP drum102. Charge roller110is designed to roll over PIP foil200and charge it with electrostatic charges. Writing head112is designed to irradiate PIP foil200with a laser beam113, in a predetermined pattern forming an image which is to be printed. Areas on the PIP foil200which are irradiated by the laser beam113from writing head112are discharged while other areas on the PIP foil remain charged. Charged ink from the color developers (Magenta114, yellow116, cyan118and black120—each in turn) is then applied on the PIP foil, being attracted to the laser irradiated areas while being repelled from other areas of the PIP foil200. In another example, and with oppositely charged ink, ink would be repelled from areas of the PIP foil that were irradiated by the laser and attracted to other areas of the PIP foil. The formed image is then transferred in the “first transfer” to the blanket (ITM104) and from there, in the “second transfer”, to the paper101(or other printable medium) which is passed between and impressed by ITM104and impression drum106.

A designated cleaning station107may be provided, which is designed to apply a coolant (e.g. cooling oil) from a coolant orifice105to cool the heated PIP drum102and foil200. Cleaning station107may further include wiper blade108(e.g., a rubber bar) to wipe dust and dirt particles which remain on the PIP foil200after the image has been transferred to the blanket (ITM104), and the PIP drum102has completed a rotation, and to maintain a thin layer of imaging oil on the surface of the PIP foil200. Additionally, a sponge roller (not shown in this figure) may be used.

Wiper blade108may be firmly pressed against PIP foil200(e.g. employing a force of about 50 N/m), defining a contact line111across the PIP foil200, where the wiper blade108maintains effective contact with PIP foil, to ensure a smooth thin layer of imaging oil on the surface of the PIP foil200for high printing quality during the next printing cycle.

However, dust and dirt particles may be trapped at the tip of the wiper blade108. As a result a significantly thicker layer of imaging oil may thus be applied on the corresponding place on the PIP foil200, which might locally damage the functionality of the PIP foil. Due to the continuous rotation of PIP drum102, the inferior localized wiping ability of wiper blade108may evolve into a sharp brutal scratch throughout the vertical axis of the actual print.

Even in the best case scenario, in which the operator notices this in real time and stops the printer to manually clean the wiper blade, there is a significant impact on both the customer's experience (TCE) and the press utilization aspects. Unfortunately, it is not rare for operators to not act immediately in real time (e.g. due to a non-sensitive job). In such cases, excessive quantities of imaging oil may form “rings” on the charge roller110and even on the developer roller of the binary ink developer (BID, e.g.114,116,118,129), which are harder to spot and the replacement of which can be costly. Additionally, if the trapped particle is not removed within a few cycles it may eventually induce irreversible damage to the surface of the PIP foil200, e.g. in the form of a mechanical scratch or a localized change in chemical characteristics.

Thus a “passive” particle removal mechanism is introduced, according to an example, to relieve particles trapped at the tip of the wiper blade108without any intervention by the operator. It was suggested in the past to roughen the Mylar (PET) layer on the trailing edge of the PIP foil so as to assist in relieving trapped particles, however if notches are made throughout the entire width of the PIP foil, the foil may tear upon impact with the wiper blade.

FIG. 2illustrates a PIP foil200, according to an example. PIP foil200includes a main printing area202(e.g. the organic photo conductor—OPC—the functional layers of the PIP) on which the latent image is to be formed. A trailing edge205of PIP foil200, which is the edge of the PIP foil200which follows the main printing area202in the direction of rotation of the PIP drum102, includes a zone of roughening pattern206, confined between two substantially opposite lateral margins204a,204b. The zone of roughening pattern206includes a roughening pattern208, which, for example, as shown in this figure, may include a plurality of notches arranged in a substantially parallel arrangement.

The notches of the roughening pattern208do not stretch across the entire width of the trailing edge205of the PIP foil200, leaving the lateral margins204a,204b, of the PIP foil200intact to act as enforcements to prevent inadvertent tearing of the trailing edge205of the PIP foil200.

If the lateral margins204a,204bare too wide, some portions of the wiper blade108may not encounter the notches. According to some examples the width of the lateral margins204a,204b, are designed such that the wiper blade extends substantially across the zone of roughening pattern206, to preserve the mechanical integrity of the PIP foil200.

Further, if the notches of the roughening pattern208are provided substantially parallel to the horizontal edge215of the PIP foil200, the impact of the wiper blade108may induce undesired stretching of the entire PIP foil200. Consequently, the segment of the PIP foil which is under the laser beam at that instance may slightly shift, causing the laser beam to impinge on the wrong place of the PIP foil200, leaving a thin missing strip on print. The length of the arc over the PIP drum102between the wiper blade108and the writing head112determines the location of that missing strip on print. To prevent such inadvertent stretching the notches of the roughening pattern208are designed to be slightly inclined with respect to the horizontal edge215of the PIP foil200, reducing the abruptness of the impact between the wiper blade and the PIP foil200to prevent stretching. According to some examples, the notches of the roughening pattern208are inclined with respect to the horizontal edge215in an angle θ that, in some examples, is greater than 0 and smaller than 10 degrees (e.g. 0<θ<10), and in some examples angle θ ranges between 2 to 6 degrees (e.g. 2<θ<6). In a more specific example, angle θ is about 5 degrees.

The PIP foil200may be installed such that the trailing edge is adhesively attached to the underlying preceding revolution (close to the leading edge of the foil), thanks to an intermediate wetting layer of imaging oil.

Detachment of the trailing edge—a failure mode also known as PIP buckle—may result in dangerous contact between the wiggling trailing edge and the various PIP satellites, and at the very least requires replacement of the PIP. It is clear that the notching process induced extraneous strains exerted on the Mylar layer of the PIP foil. The inventors have found that if the notches of the roughening pattern are kept shallow enough so that at least some of the underlying layer of the Mylar is left intact (e.g. 40-50 micrometers), then the likelihood of PIP buckling may be significantly reduced (practically to about the same level of likelihood of PIP buckling in non-notched PIP foils).

According to some examples, the zone of roughening pattern is heated (e.g. to a temperature of about 70 to 80 degrees, and in some examples to a temperature of about 75 degrees Celsius, during the notching process. The inventors have found that if the zone of roughening pattern in the trailing edge of a PIP foil is subjected to such heating during the notching process, then the chances of that PIP foil to buckle on the LEP printer are substantially decreased. This may perhaps be attributed to increased ductility of the PIP foil matter, thus enabling quenching the extraneous strains the foil experiences during notching.

The inventors have conducted several experiments in which one half of a trailing edge of a PIP foil was left smooth, whereas a roughening pattern of notches was embedded on the other half. Some manipulation was used to generate excessive sludge and fused ink on the wiper blade of the cleaning station. There was a significant difference in the number of scratches on print between the two halves of the PIP foil (significantly less scratches on the side of the print corresponding to the half of the PIP foil that included the roughening pattern), attesting efficient removal of particles from the tip of the wiper blade in the roughening pattern zone. Specifically it was found that the percentage of pages being rejected due to wiper induced scratches was reduced from 14.5% (historic reference) to 2.9%—a 5-fold reduction.

Correspondingly, the average lifespan of a PIP foil may thus increase by some 30%. The frequency of wiper-related interventions by operators (cleaning/flipping or replacing the wiper) may also decrease significantly.

According to an example, a roughening pattern may include 4-7, and in a more specific example 5, substantially parallel notches which are spaced some 2 mm from each other. In some examples, each of the notches is 15-50 micrometers wide and 15-30 micrometers deep. In a specific example, each of the notches is about 20 micrometers wide and about 20 micrometers deep

Accordingly, a method300for enhancing wiping a PIP foil of a photo imaging plate of an LEP printer, according to an example, is illustrated inFIG. 3A. Method300may include providing on a trailing edge of the PIP foil a zone of a roughening pattern which includes notches that are inclined with respect to a contact line of the wiper blade, the zone of roughening pattern confined between two opposite margins lacking any roughening.

FIG. 3Billustrates a method350for enhancing wiping a PIP foil of a photo imaging plate of an LEP printer, according to an example. Method350may include heating352the zone of roughening pattern of the trailing edge of the PIP foil in a process for providing the roughening pattern on the trailing edge. Method350may also include providing354the roughening pattern on the trailing edge.