In some examples, an apparatus can include a pawl including an anti-rotation surface and a spring, and a valve housing including a wedge, where the spring biases the pawl to a first position in response to the valve housing being at a closed position such that the wedge of the valve housing contacts the anti-rotation surface of the pawl to prevent rotation of the valve housing at the first position of the pawl.

An example method of performing a null cycle in a liquid electrographic printer is described. The method involves collecting, at a photo imaging plate cleaning station, imaging oil deposited on a photo imaging plate during a print cycle. During a null cycle, the photo imaging plate cleaning station is controlled to apply the collected imaging oil to the photo imaging plate.

An example printing system for printing liquid ink has a developer device to transfer ink to an image plate of the printing system. The developer device has an ink inlet to receive ink from an ink reservoir of the printing system; a developer roller to transfer ink to the image plate, a motor to rotate the developer roller, and an ink sensor to detect arrival of ink at the developer device. The printing system is configured to start the motor in response to the ink sensor detecting arrival of ink at the developer device.

In one example in accordance with the present disclosure, a fluid application device is described. The device includes an application roller to deposit a fluid containing metallic particles on a surface. A squeegee roller of the device forms a first nip with the roller and a cleaner roller forms a second nip with the application roller. A first resistive coating is disposed on a surface of the application roller.

In an example, a print agent application assembly includes a print agent transfer roller to receive print agent and transfer a portion of the print agent to a photoconductive surface and a print agent regulator roller to regulate a film thickness of print agent on the print agent transfer roller. The print agent regulator roller may include a nip forming region and a first mounting region. The print agent application assembly may further include a first resilient component which spans a diametrical width of the print agent regulator roller and acts on the print agent regulator roller outside the nip forming region to impart a lateral force to the first mounting region, the lateral force urging the print agent regulator roller towards the print agent transfer roller.

An example printer comprises an impression cylinder arranged to receive an impression medium thereon and an intermediate transfer member arranged to transfer printing fluid onto a substrate received on the impression medium. The printer further comprises an image sensor arranged to sense an accumulation of printing fluid on the impression medium as sensor data and a controller. The controller is configured to determine, based on the sensor data, an accumulation level of the printing fluid on the impression medium and determine if the accumulation level meets a threshold.

In one example of the disclosure, a first measurement of actual quantity of ink in a target cartridge is taken utilizing a scale. An incremental dosage quantity be deposited into the target cartridge in a dosing pass is determined based upon a desired quantity, the actual quantity, and an undershoot safety factor. A valve is opened to enable a pressure deposit of the incremental dosage quantity of ink to the target cartridge, and then closed. A residue cutter is utilized to scrape the valve and thereby make a scraper deposit of ink to the target cartridge. A second measurement of actual quantity of ink in the target cartridge is taken utilizing the scale. An additional dosage pass is performed if the second measurement is not within an accepted variance of the desired quantity. The making of dosing passes is discontinued if the second measurement is within the accepted variance.

Measures for controlling an engagement force between a photo imaging plate and a developer roller in a printing device are described. A motor is operated to generate rotational motion. The rotational motion is translated into linear motion. The linear motion causes an adjustment to the engagement force between the developer roller and the photo imaging plate. A characteristic of the motor is monitored. The motor is controlled on the basis of the monitored characteristic in order to maintain a desired engagement force between the developer roller and the photo imaging plate.

In one example, a developer roller for liquid electrophotographic printing includes a cylindrical metal inner core, a rigid conductive plastic outer core surrounding the inner core, and a compliant exterior surrounding the outer core.

Ink-based digital printing systems useful for ink printing include a photoreceptor layer configured to receive a layer of liquid immersion fluid. The liquid immersion fluid includes dampening fluid, dispersed gas particles, and charge directors that impart charge to the solid particles. The photoreceptor surface is charged to a uniform potential, and selectively discharged using an ROS according to image data to form an electrostatic latent image. The charged liquid immersion fluid adheres to portions of the photoreceptor surface according to the electrostatic latent image to form a fountain solution image. The fluid portion of the fountain solution image can be partially transferred to an imaging member and/or transfer member to form a dampening fluid image, either or both of which may be electrically biased. The dampening fluid image is inked on the transfer member, and the resulting ink image transferred to a print substrate.