Waterless Offset CTP Plate Making
Waterless offset printing plates, including waterless offset CTP plates, are made up of several layers: an aluminum base layer, an undercoat layer, a photosensitive layer, a silica gel layer, and a protective surface film. The aluminum base provides structural support for the plate. The undercoat layer enhances adhesion between the aluminum base and the photosensitive layer to prevent peeling. After exposure, a chemical reaction occurs, reducing the bond strength between the photosensitive layer and the silica gel. During development, the weakly bonded silica gel is removed, exposing the photosensitive area. This exposed part attracts ink during printing to form the image area. Meanwhile, the silica gel acts like a fountain solution, repelling ink to create the non-image areas. The surface film protects the plate from scratches during transport and processing in the platesetter.
The process of making a waterless offset CTP plate involves exposure, development, water washing, applying a protective coating, and drying. Exposure is done directly on a standard CTP platesetter, with careful control over exposure time. To avoid damage, the protective film should remain on the plate during exposure but must be fully removed before development. If not removed completely, residual film can cause incomplete development or even block the water jet pump. Once the plate is ready, it is processed in a specialized waterless automatic processor. This includes pre-treatment, water washing, post-treatment, and re-washing. In pre-treatment, a special liquid breaks down the interface between the photosensitive and silica gel layers. During washing, a brush is used to remove the silica gel from the image area, and any remaining treatment liquid is rinsed away. The image area is then colored to improve visibility, and finally, the plate is washed again to remove excess dye. Unlike traditional water-based plates, waterless offset CTP plates cannot be dried using heat; they must be air-dried at low temperatures.
Waterless Offset CTP Printing
The ink used in waterless offset CTP printing is similar in pigment to traditional offset ink, but differs in binder and solvent composition. First, because the non-image areas rely on the silica gel to repel ink, waterless offset ink needs higher cohesion and greater viscosity than regular offset ink. It also requires good transferability and low viscosity with high thixotropy. Second, since waterless plates are more prone to scratching, the pigment particles in the ink should be fine to ensure a high gloss finish. Lastly, as no fountain solution is used, waterless ink does not need to be water-resistant.
For optimal results, the paper used in waterless offset printing must have strong surface strength to maintain print quality. Throughout the entire printing process, attention should be paid to cleaning the ink rollers and blankets, adjusting the press, selecting the right blanket, and controlling temperature and humidity.
Additionally, waterless offset printing requires strict environmental control. The printing area must maintain a temperature of 23±2°C and a humidity level between 50% and 60%. As seasons change, the paper's temperature should also be carefully managed.
Storage of Waterless Offset CTP Plates
After printing, care must be taken to avoid scratching the waterless offset CTP plate. When stacking plates, sheets of paper should be placed between them to prevent damage. Before unloading, a few sheets can be pressed together with the inking roller raised to remove excess ink, making the plate easier to clean.
Used waterless offset CTP plates can be stored for future use. An automatic storage system can help preserve them. However, unlike traditional offset plates, waterless plates require a cleaning agent to prevent aging. They should not be coated with glue after cleaning but can be air-dried at low temperatures. Due to their sensitivity, soft brushes should be used when handling the plates to avoid scratches. Finally, waterless offset plates should be stored at the proper temperature and kept away from high heat.
This content is selected from the 5th issue of “Printing Technology · Digital Graphic Arts†in 2012. For more journal content, please follow the journal channel .
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