Patent Description:
In some liquid electrophotographic printers, a transfer member, such as an intermediate transfer member (ITM) is used to transfer an intermediate image to a print medium. For example, an intermediate image comprising print fluid aligned according to a latent image, may be transferred from the photo imaging plate to a transfer blanket of the intermediate transfer member. From the intermediate transfer member, the intermediate image is transferred to a substrate, which is placed into contact with the transfer blanket, such that a printed image is formed on the substrate.

Various features of the present disclosure will be apparent from the detailed description which follows, taken in conjunction with the accompanying drawings, which together illustrate features of the present disclosure, and wherein:.

An example of a printing apparatus, generally designated <NUM>, is shown schematically in <FIG>. In some examples, the printing apparatus <NUM> is part of a liquid electrophotographic printer.

The printing apparatus <NUM> comprises a photo imaging plate (PIP) <NUM>, an intermediate transfer member <NUM>, and an electrically conductive member <NUM>. The photo imaging plate <NUM> is to be held at a first voltage having a first polarity in use. In some examples the first voltage may comprise a negative voltage of around -900V. The photo imaging plate <NUM> is to hold an intermediate image in use, for example by attracting charged print fluid particles from an image development unit. In some examples the intermediate image is held at a negative voltage in the region of -50V. An intermediate image in some examples herein may comprise an image present at a stage in the printing apparatus <NUM> between a latent image formed on the photo imaging plate <NUM> and a printed image formed on a substrate.

The intermediate transfer member <NUM> is to receive the intermediate image from the photo imaging plate <NUM>, and is to be held at a second voltage having a second polarity opposite to the first polarity. In some examples the second voltage may comprise a positive voltage in the region of +<NUM>-650V. The intermediate transfer member <NUM> is to transfer the received intermediate image to a substrate in use, such that a printed image is formed on the substrate. The printed image in some examples may be a modified version of the intermediate image, for example a version of the intermediate image that has been modified by the application of heat thereto.

The applicant has found that during use of the printing apparatus <NUM> absent the electrically conductive member <NUM>, corona discharge may occur between the photo imaging plate <NUM> and the intermediate transfer member <NUM> in view of the voltage difference between the photo imaging plate <NUM> and the intermediate transfer member <NUM>. Such corona discharge may have an impact on the intermediate transfer member <NUM>, which may lead to increased wear and reduced maintenance interval times. In particular, corona discharge may result in the formation of a high concentration of hydroxyl radicals, which may oxidise an outer surface of the intermediate transfer member <NUM>. This may increase the surface tension of an outer surface of the intermediate transfer member <NUM> with the number of impressions printed. Consequently, print fluid releaseability from the intermediate transfer member <NUM> to a substrate may reduce as the number of impressions printed increases.

By placing the electrically conductive member <NUM> between the photo imaging plate <NUM> and the intermediate transfer member <NUM>, the applicant has found that the effects of corona discharge on the intermediate transfer member <NUM> may be reduced, which may lead to reduced wear, for example reduced oxidation of an outer surface of the intermediate transfer member, and increased maintenance interval times. The electrically conductive member <NUM> defines a leakage path for corona discharge between the photo imaging plate <NUM> and the intermediate transfer member <NUM>. In some examples, such as that of <FIG>, the electrically conductive member <NUM> is disposed between the photo imaging plate <NUM> and the intermediate transfer member <NUM> such that the electrically conductive member <NUM> does not contact the photo imaging plate <NUM>. In some examples, such as that of <FIG>, the electrically conductive member <NUM> is disposed between the photo imaging plate <NUM> and the intermediate transfer member <NUM> such that the electrically conductive member <NUM> does not contact the intermediate transfer member <NUM>.

In some examples, such as that of <FIG>, the electrically conductive member <NUM> is grounded, which may provide a safe path for corona discharge that may occur between the photo imaging plate <NUM> and the intermediate transfer member <NUM> in use. In some examples the electrically conductive member <NUM> may be held at a third voltage. This may, for example, attract corona discharge to the electrically conductive member <NUM>. The third voltage may be sufficiently large to attract corona discharge to the electrically conductive member <NUM> without being so large as to cause further corona discharge from the electrically conductive member <NUM> to the intermediate transfer member <NUM>. In some examples, the third voltage is within <NUM>% of the value of the second voltage, for example within <NUM>% of the value of the second voltage. The electrically conductive member <NUM> may be held at a third voltage in the region of <NUM> to 1000V, in the region of <NUM> to 900V, or in the region of <NUM>-800V. In some examples the third voltage may be similar to the second voltage at which the intermediate transfer member <NUM> is held, for example a voltage of around <NUM>-650V.

In the example shown schematically in <FIG>, the photo imaging plate <NUM> comprises a cylindrical drum rotatable in a first direction indicated by an arrow <NUM> about a rotational axis. Similarly, in the example of <FIG>, the intermediate transfer member <NUM> comprises a cylindrical drum rotatable in a second direction indicated by an arrow <NUM> about a rotational axis. The second direction is opposite to the first direction, as depicted in <FIG>. In some examples, the electrically conductive member <NUM> extends along an axis parallel to the rotational axis of the intermediate transfer member <NUM>, and extend along an axis parallel to the rotational axis of the photo imaging plate <NUM>, with such an example shown schematically in <FIG>. In the example of <FIG>, the electrically conductive member <NUM> extends along the entire length of the intermediate transfer member <NUM>. This may provide protection from corona discharge along substantially the entire length of the intermediate transfer member <NUM>.

In some examples, such as the example of <FIG>, the electrically conductive member <NUM> comprises a length and a cross-sectional width, the length more than <NUM> times the cross-sectional width. In such an example the electrically conductive member <NUM> comprises a member that is considered elongate, and may have the form of a wire. In some examples, the electrically conductive member <NUM> may comprise an edge, for example with the electrically conductive member <NUM> comprising a tear-drop shaped cross-sectional shape. In examples herein, it will be appreciated that a cross-sectional area of the electrically conductive member <NUM> may be significantly smaller than a cross-sectional area of either the photo imaging plate <NUM> or the intermediate transfer member <NUM>, and hence that the relative size of the electrically conductive member has been enlarged in the figures to aid understanding.

In some examples, the cylindrical drum of the photo imaging plate <NUM> is in contact with the cylindrical drum of the intermediate transfer member <NUM> in a contact region, with the electrically conductive member <NUM> located adjacent the contact region. One such example is the example shown schematically in <FIG>, where the electrically conductive member <NUM> is located adjacent the contact region <NUM>. The contact region <NUM> is rather small, for example in the region of <NUM>-<NUM> microns along a circumferential direction of the drums when viewed in a direction orthogonal to a rotations axis of the drums, and so it will be appreciated that the scale of <FIG> has been exaggerated for ease of understanding.

Transfer of the intermediate image from the photo imaging plate to the intermediate transfer member occurs in the contact region <NUM> in the example of <FIG>, and hence may also be referred to as an image transfer region. For example, the intermediate image held on the photo imaging plate <NUM> comprises a first electrical charge in view of the first voltage, and the intermediate transfer member <NUM> comprises a second electrical charge in view of the second voltage. Transfer of the intermediate image from the photo imaging plate <NUM> to the intermediate transfer member <NUM> thereby takes place via both an electromagnetic attraction mechanism and physical contact in the contact region <NUM>.

In some examples, as shown in <FIG>, the electrically conductive member <NUM> is held adjacent the contact region <NUM> by a support structure, the support structure comprising first <NUM> and second <NUM> support members spaced apart along the length of the electrically conductive member <NUM>. Utilising spaced apart first <NUM> and second <NUM> support members may allow the electrically conductive member <NUM> to be adequately supported along its length without requiring an intrusive support structure extending across the full length of the electrically conductive member <NUM>. In the example of <FIG>, the first <NUM> and second <NUM> support structures are tapered, which may allow the electrically conductive member <NUM> to be located closer to the contact region <NUM> than, for example, a non-tapered arrangement, with the tapered regions able to penetrate further into the airgap between the photo imaging plate <NUM> and the intermediate transfer member <NUM>. The first <NUM> and second <NUM> support structures hold the electrically conductive member <NUM> in any appropriate manner, provided that the structures are capable of holding the electrically conductive member <NUM> in position.

As seen in the example of <FIG>, the electrically conductive member <NUM> is located between the photo imaging plate <NUM> and the intermediate transfer member <NUM> in a region downstream of the contact region <NUM>. A region downstream of the contact region <NUM> is, as shown in <FIG>, a region in which the photo imaging plate <NUM> and the intermediate transfer member <NUM> are rotating away from the contact region <NUM>. This is in contrast to a region upstream of the contact region <NUM>, which is a region in which the photo imaging plate <NUM> and the intermediate transfer member <NUM> are rotating toward the contact region <NUM>. A region downstream of the contact region <NUM> comprises an airgap between the photo imaging plate <NUM> and the intermediate transfer member <NUM> in some examples, such as those shown in <FIG>, <FIG>. For example, pigment particles and any carrier fluid may be compressed in the contact region <NUM>, such that the region downstream of the contact region <NUM> is relatively dry compared to a region upstream of the contact region <NUM>, which may comprise carrier fluid and pigment particles. Given the relatively high dielectric constant of air, for example compared to a relatively lower dielectric constant of a carrier fluid for pigment particles present in a region upstream of the contact region, corona discharge may be more likely to occur in such an airgap. Thus, the electrically conductive member <NUM> may be usefully placed in a region downstream of the contact region <NUM> to mitigate for the effects of corona discharge in use. In some examples the electrically conductive member is located no more than <NUM>%, or no more than <NUM>%, of a value of a circumference of the photo imaging plate <NUM> away from the contact region <NUM>. In some examples, the electrically conductive member <NUM> has a diameter of less than <NUM>, for example a diameter in the region of <NUM>. Given the relatively small distance from the contact region <NUM> and the relatively small cross-sectional area of the electrically conductive member, the electrically conductive member <NUM> may be thought of as being between opposing outer faces of the photo imaging plate <NUM> and the intermediate transfer member <NUM>.

It will be appreciated that the electrically conductive member <NUM> may also find utility in a region upstream of the contact region <NUM> if corona discharge is found to occur in such an upstream region, for example if a carrier fluid is used that has a relatively high dielectric constant. In some examples there may be electrically conductive members located both upstream and downstream of the contact region <NUM>. In some examples there may be a plurality of electrically conductive members located in either or both of regions downstream and upstream of the contact region <NUM>.

In some examples, the printing apparatus <NUM> comprises an image development unit to deposit print fluid onto the photo imaging plate <NUM> to form the intermediate image, the print fluid comprising pigment particles suspended in a carrier fluid. The print fluid in some examples, such as those discussed herein, comprises ink, and the intermediate image comprises an inked image. The carrier fluid may comprise an imaging oil. An example print fluid is HP ElectroInk™. In this case, pigment particles are incorporated into a resin that is suspended in a carrier fluid, such as Isopar™. The pigment particles may be electrically charged such that they move when subjected to an electric field. The pigment particles may be negatively charged and are therefore repelled from negatively charged portions of the photo imaging plate <NUM>, and are attracted to discharged portions of the photo imaging plate <NUM>. The carrier fluid may comprise a dielectric constant of <NUM> or less, which may be sufficient to inhibit corona discharge in a region upstream of the contact region <NUM>.

In some examples, the print fluid comprises an epoxy-based print fluid. The applicant has found that the impact of corona discharge on an intermediate transfer member may be exacerbated when epoxy-based print fluids are used. In particular, corona discharge may cause oxidation of an outer surface of an intermediate transfer member, and the risk of print fluid reaction with the outer surface of the intermediate transfer member may increase with an increased oxidation level since epoxy moieties tend to react with oxidized species (such as peroxides, hydroxyls and carboxyls) at elevated temperatures. Use of the electrically conductive member <NUM> as disclosed in examples herein may reduce oxidation of an outer surface of the intermediate transfer member <NUM>, which may facilitate use of epoxy-based print fluids.

In some examples, the intermediate transfer member <NUM> comprises an outer surface formed of polydimethylsiloxane. The applicant has found that an outer surface formed of polydimethylsiloxane may be vulnerable to oxidation as a result of corona discharge, and use of an electrically conductive member <NUM> as disclosed in examples herein may reduce the risk of oxidation of an outer surface of an intermediate transfer member <NUM> formed of polydimethylsiloxane.

As previously mentioned, in some examples, the printing apparatus <NUM> is part of a liquid electrophotographic printer. An example of a liquid electrophotographic printer (LEP) <NUM> is shown schematically in <FIG>, with the operation of the LEP <NUM> described below.

The LEP <NUM> comprises a photo imaging plate <NUM>, a charging element <NUM>, an imaging unit <NUM>, an image development unit <NUM>, an intermediate transfer member <NUM>, an impression cylinder <NUM>, and an electrically conductive member <NUM>.

In the example of <FIG>, the photo imaging plate <NUM> comprises an imaging cylinder rotatable about a central axis in a direction indicated in <FIG> by an arrow <NUM>. A latent image is formed on the photo imaging plate <NUM> by rotating a clean segment of the photo imaging plate <NUM> under the charging element <NUM>. The charging element <NUM> may include a charging device, such as corona wire, a charge roller, scorotron, or any other charging device. A uniform static charge is deposited on the photo imaging plate <NUM> by the charging element <NUM>. Thus, it may be considered that the photo imaging plate is held at a first voltage. In some examples the first voltage is in the region of -900V.

As the photo imaging plate <NUM> continues to rotate, it passes the imaging unit <NUM> where one or more lasers dissipate localized charge in selected portions of the photo imaging plate <NUM> to leave an invisible electrostatic charge pattern, having a significantly lower voltage of around -50V, that corresponds to the image to be printed, i.e. a latent image. Print fluid is then transferred onto the photo imaging plate <NUM> by the image development unit <NUM>. Although shown in <FIG> as having one image development unit <NUM>, it will be appreciated that in practice there may be multiple image development units <NUM> present, for example one for each colour. An image development unit <NUM> may also be referred to as a Binary Ink Developer (BID) unit. During printing, the appropriate image development unit <NUM> is engaged with the photo imaging plate <NUM>. The engaged image development unit <NUM> presents a uniform film of print fluid to the photo imaging plate <NUM>. The print fluid contains electrically charged pigment particles which are attracted to the image areas of the photo imaging plate <NUM>. The photo imaging plate <NUM> then has a single colour print fluid image on its surface, i.e. an intermediate image. In some examples the intermediate image is held at -50V.

The print fluid comprises pigment particles suspended in a carrier fluid. An example print fluid is HP ElectroInk™. In this case, pigment particles are incorporated into a resin that is suspended in a carrier fluid, such as Isopar™.

The photo imaging plate <NUM> continues its rotation to transfer the intermediate image to the intermediate transfer member <NUM>. In the example of <FIG>, the intermediate transfer member <NUM> comprises a cylindrical drum <NUM> and a transfer blanket <NUM> disposed on the cylindrical drum <NUM>. The transfer blanket <NUM> is removable and replaceable upon the cylindrical drum <NUM>. In the example of <FIG>, the transfer blanket <NUM> comprises a release layer made of silicone rubber, for example, polydimethylsiloxane (PDMS). The transfer blanket <NUM> is held at a second voltage, which in some examples is in the region of <NUM>-650V. The transfer blanket <NUM> is in contact with the photo imaging plate <NUM> in a contact region <NUM>, which may sometimes be referred to as a nip. Transfer of the intermediate image from the photo imaging plate <NUM> to the transfer blanket <NUM> takes place in the contact region <NUM> via electrostatic attraction and physical contact. The transfer of the intermediate image from the photo imaging plate <NUM> to the intermediate transfer member <NUM> may be deemed a "first transfer", and so the contact region <NUM> may be referred to as the T1 nip.

The transfer blanket <NUM> in the example of <FIG> is heated, which causes pigment particles carried on the transfer blanket <NUM> to partially melt and blend together into a film. The combination of heating and pressure at the contact region <NUM> may at least partially evaporate the carrier fluid, such that substantially no carrier fluid is located in a region downstream of the T1 nip, i.e. downstream of the contact region <NUM>. In some examples the pigment particles may be alternatively or additionally heated by an external heat source rather than directly heating the transfer blanket <NUM>.

Once the pigment particles are heated and merged into a film, the film is transferred to a substrate <NUM> at a contact region <NUM> between the transfer blanket <NUM> and the impression cylinder <NUM> to form a printed image on the substrate <NUM>. The impression cylinder <NUM> both mechanically compresses the substrate <NUM> into contact with the transfer blanket <NUM> and also helps feed the substrate <NUM>. The transfer of the film from the transfer blanket <NUM> to the substrate <NUM> may be deemed a "second transfer", and so the contact region <NUM> may be referred to as the T2 nip.

In a similar manner to that discussed above in relation to the example of <FIG>, due to the large voltage bias between the photo imaging plate <NUM>, for example the intermediate image held on the photo imaging plate <NUM>, and the transfer blanket <NUM> (for example around <NUM>-700V), corona discharge can occur in the region downstream of the T1 nip, i.e. downstream of the contact region <NUM>, between the photo imaging plate <NUM> and the transfer blanket <NUM>, where an airgap exists due to a reduced presence of carrier fluid at this location. Such corona discharge does not occur upstream of the T1 nip due to a relatively larger presence of the carrier fluid as part of the intermediate image on the photo imaging plate <NUM>, with the carrier fluid in the example of <FIG> being Isopar™, which may have a relatively low dielectric constant of around <NUM>.

As mentioned above, in the example of <FIG> the transfer blanket <NUM> comprises a release layer made of silicone rubber, for example, polydimethylsiloxane (PDMS). The applicant has found that an outer surface of the transfer blanket <NUM> formed of polydimethylsiloxane may be vulnerable to oxidation as a result of corona discharge, and that may increase the surface tension of an outer surface of the transfer blanket <NUM> with the number of impressions printed. Consequently, print fluid releaseability from the transfer blanket <NUM> to the substrate <NUM> may reduce until replacement of the transfer blanket <NUM>.

Furthermore, the applicant has found that the impact of corona discharge on the transfer blanket <NUM> may be exacerbated when epoxy-based print fluids are used. In particular, corona discharge may cause oxidation of an outer surface of the transfer blanket <NUM>, and the risk of print fluid reaction with the outer surface of the transfer blanket <NUM> may increase with an increased oxidation level since epoxy moieties tend to react with oxidized species (such as peroxides, hydroxyls and carboxyls) at elevated temperatures. This may cause print fluid to stick to the transfer blanket <NUM>.

To mitigate the impact of corona discharge on the transfer blanket <NUM>, the electrically conductive member <NUM> in the example of <FIG> is placed in the region downstream of the T1 nip, for example in the region immediately adjacent the contact region <NUM>. The electrically conductive member <NUM> in the example of <FIG> takes the form of a grounded electrically conductive wire, which may intercept corona discharge in the region immediately adjacent the contact region <NUM>, and may inhibit oxidation of the transfer blanket <NUM>.

In some examples the electrically conductive member <NUM> may be held at a third voltage. This may, for example, attract corona discharge to the electrically conductive member <NUM>. The voltage may be sufficiently large to attract corona discharge to the electrically conductive member <NUM> without being so large as to cause further corona discharge from the electrically conductive member <NUM> to the transfer blanket <NUM>. In some examples, the third voltage is within <NUM>% of the value of the second voltage, for example within <NUM>% of the second voltage. The electrically conductive member <NUM> may be held at a third voltage in the region of <NUM> to 1000V, in the region of <NUM> to 900V, or in the region of <NUM>-800V. In some examples the third voltage may be similar to the second voltage at which the transfer blanket is held, for example a voltage of around <NUM>-650V.

A method <NUM> that utilises an electrically conductive member as discussed herein is shown schematically in the flow diagram of <FIG>. The method <NUM> comprises providing <NUM> a photo imaging plate held at a first voltage having a first polarity; providing <NUM> an intermediate transfer member to receive an intermediate image from the photo imaging plate, the intermediate transfer member held at a second voltage having a second polarity opposite to the first polarity; transferring <NUM> an intermediate image from the photo imaging plate to the intermediate transfer member via an electromagnetic attraction mechanism; and providing <NUM> an electrically conductive structure located between the photo imaging plate and the intermediate transfer member, whereby the electrically conductive member is to intercept corona discharge between the photo imaging plate and the intermediate transfer member.

An experimental set-up used to verify the utility of an electrically conductive member as discussed herein will now be discussed with reference to the set-up schematically shown in <FIG>. As seen in <FIG>, an electrically conductive mesh <NUM> was placed above a 10cmx12cm rectangular piece of transfer blanket material <NUM>, which in the present example comprises polydimethylsiloxane (PDMS). A manual corona generator was used to apply a manual corona discharge in the region of the electrically conductive mesh, with an application time of <NUM>. A similar application of manual corona discharge was applied to a 10cmx12cm rectangular piece of transfer blanket material absent the electrically conductive mesh <NUM>. The properties of the treated pieces of transfer blanket material were compared by: i) measuring a water droplet on transfer blanket material contact angle; ii) measuring a di-iodomethane droplet on transfer blanket material contact angle; and iii) measuring a surface energy of the transfer blanket material. The results of the measurements can be seen in Table <NUM> below for the transfer blanket material absent the electrically conductive mesh, and in Table <NUM> below for the transfer blanket material <NUM> with the electrically conductive mesh <NUM>.

As can be seen from a comparison of Table <NUM> with Table <NUM>, utilizing an electrically conductive member in the form of the electrically conductive mesh <NUM> where a manual corona discharge is applied for <NUM> provides results substantially similar to the case where no corona discharge is applied. This is in contrast to the case where no electrically conductive mesh is utilised and a manual corona discharge is applied for <NUM>.

The impact of the corona discharge on transfer blanket material when utilised with epoxy-based print fluids can be seen from <FIG>. In particular, after the manual corona discharge was applied, <NUM> of <NUM>% NVS yellow print fluid was placed on the treated transfer blanket pieces and heated at <NUM> until total drying of the print fluid. Attempts were then made to remove the print fluid from the transfer blanket material.

As can be seen from <FIG>, where no corona discharge was applied, the print fluid was successfully removed from the transfer blanket material. As can be seen from <FIG>, where corona discharge was applied in the absence of the electrically conductive mesh <NUM> for a time duration of <NUM>, substantially all of the print fluid remained adhered to the transfer blanket material after removal was attempted. As can be seen from <FIG>, where corona discharge was applied in the presence of the electrically conductive mesh <NUM> for a time duration of <NUM>, substantially all of the print fluid was removed from the transfer blanket material in the region of the transfer blanket material that was covered by the electrically conductive mesh <NUM> after removal was attempted.

From the discussion above, it can be seen that use of an electrically conductive member as disclosed herein may mitigate the effects of corona discharge on a transfer blanket of an intermediate transfer member of an LEP printer.

Claim 1:
A printing apparatus comprising:
a photo imaging plate (<NUM>, <NUM>) to be held at a first voltage having a first polarity;
an intermediate transfer member (<NUM>, <NUM>) to receive an intermediate image from the photo imaging plate (<NUM>, <NUM>), the intermediate transfer member (<NUM>, <NUM>) to be held at a second voltage having a second polarity opposite to the first polarity; and
an electrically conductive member (<NUM>, <NUM>) disposed between the photo imaging plate (<NUM>, <NUM>) and the intermediate transfer member (<NUM>, <NUM>);
characterized in that:
the electrically conductive member (<NUM>, <NUM>) is configured to act as a leakage path in case of corona discharge between the photo imaging plate (<NUM>, <NUM>) and the intermediate transfer member (<NUM>, <NUM>).