WO 2013/132418 A2
Digital printing process
Landa Corporation Limited
This patent application describes the complete process and makes the basic claims for the overall process. The first drawing is a schematic and is not to scale. Multiple array printheads 300 print an image on to a heated intermediate substrate belt 130 which rotates clockwise around rollers 104 and 106. The image dries partly due to the heated belt but with additional heating from above (not shown). The printheads can be moved backwards from the belt to maintenance and capping stations.
Sheets are fed from a conventional offset press sheet feeder 506 and are transported through the machine by a series of gripper drums. Drums 502 and 504 bring the substrate into contact with the image on the belt, transfer being enabled by pressure from back-up rollers 140 and 142. After the first side of the sheet has been printed at the first impression, it is passed by a further gripper drum to the perfecting cylinder 524. The perfecting cylinder has a circumference twice that of the other drums. The grippers are timed so that as the trailing edge passes drum 526 the grippers grab the rear edge. At the second impression stage the back side of the sheet is then printed. In this way the single imaging engine can be used to print duplex. To do this the images for
each side of the sheet are formed alternately and mirror-imaged so that after transfer they are correctly orientated.
After transfer to the sheet, the ink is dry enough to be handled by the gripper drums and the output stacker 508. No further curing or drying of the image is required.
Other configurations are possible, and in fact a simplex web press was shown at Drupa 2012. There is just one impression cylinder and pressure roller pair 502 and 140. To ensure none of the web substrate is wasted, it’s fed intermittently past the impression cylinder to close up any gaps between the images on the transfer belt. As the web drive has considerable momentum, the intermediate feed is achieved by using dancer rollers 554 and 556.
Going back to the duplex sheet-fed press, the figure below shows the intermediate substrate or blanket support system with the blanket and front support removed. The roller 104 serves to pull the belt past the printheads at a constant speed, while roller 106 is also driven but is adjusted to keep the belt in a state of constant tension across the top run. Roller 104 is mounted in pillow blocks so that it can be moved to allow for belt stretch. Pressure rollers 140 and 142 are mounted inside the support, press against the blanket at the transfer points to the substrate, and can be raised and lowered.
The blanket runs in contact with thermally conductive support plates 130. The junctions between these plates are deliberately staggered to prevent the creation of a line along the belt. Heater elements 132 under the plates supply the heat to the belt along the upper run.
Not shown is a fan system to create a negative air pressure within the blanket support box. This helps keep the blanket in contact with the heated plates, and also keeps the bottom run of the blanket from contacting the impression cylinders when the pressure rollers are withdrawn.
Each of the sides of the blanket support has a track 180 which engages in features fixed to the sides of the blanket. This serves two purposes. Firstly it enables the blanket to be threaded through the machine by entering it into the guides. Although the blanket could be made as an endless belt, it is more economical to make it as a sheet and join the ends within the machine. Secondly it steers the belt past the printheads and maintains tension across the belt.
To the left is a top view of belt 210. Alongside each side are features 270. These can be one half of a zip fastener that has been sewn to the edges of the blanket. The projections 270 are contained within the channel of track 280. Roller bearings 282 can be used to reduce the friction. As the belt is heated the components must be capable of withstanding up to 250C. Suitable materials for the projections include a polyamide reinforced with molybdenum disulphide. The guides are used to stabilise and tension the belt as it passes the printheads to ensure accurate image formation, but can be omitted through the drying station and in other places where the belt is slack.
The blanket is formed into a continuous belt using a zip or hooks and loops fastener. This may be supplemented by adhesive or tape. It is important that the seam is the same thickness as the belt itself, otherwise there will be speed variations when the seam passes over the driven rollers.
Details of the ink can be found in Directions September/October 2013 page 17, and materials and structure of the blanket in the media reviews in this issue of Direction on page 22. The surface of the blanket has a thin upper release layer that is hydrophobic, such as a silicone material. This is formed on a reinforcement layer, such as a fabric.
A pre-treatment liquid can be applied to the blanket by a roller mounted opposite tension roller 106. This coats the blanket with a thin dilute film of charged polymer to modify the surface properties. By the time the blanket reaches the printheads the film is completely dry.
The purpose of this chemical treatment is to counteract the surface tension of the aqueous ink when it contacts the hydrophobic release layer of the blanket. Without the pre-treatment the drops would flatten on the blanket surface on impact, and then surface tension forces would restore the ink into drops or beads. With the pre-treatment the negatively charged polymer particles in the ink drop adjacent to the surface are attracted by the positively charged pre-treatment film. This “freezes” the drop after impact so that beading does not occur. The effect is very short-lived but long enough for the water to be dried out of the image without the drops coalescing, beading or otherwise disrupting the image.
The blanket is heated from below to around 150C. In addition, after the print station external air blowers pass hot air over the image to further aid drying. The polymers in the ink remain tacky at this temperature, aiding the transfer process.
There are many claimed advantages to this process. First of all, it uses aqueous-based inks which are environmentally friendly, and compared to UV-curable inks use relatively low-cost materials.
The freezing of the ink drops on the blanket enables very thin image layers to be produced, typically 500-800 nm. This means the image takes up the surface features of the substrate, so that an image on a glossy surface will be glossy, and matt on a matt surface, matching the non-image areas.
When the drops are flattened on impact with the blanket and then frozen they retain the shape formed on impact, hopefully circular. There is no irregular bleed that is sometimes encountered with direct ink jet printing. In addition inter-colour bleed is also reduced.
The transferred thin film is less likely to split compared to thicker films, reducing the chances of ink being retained on the blanket and thus requiring cleaning.