Waterless Offset CTP Plate Making
Waterless offset printing plates, including waterless offset CTP plates, consist 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, while the undercoat enhances adhesion between the base and the photosensitive layer to prevent peeling. After exposure, a chemical reaction occurs, weakening the bond between the photosensitive layer and the silica gel. During development, the weakly bonded silica gel is removed, exposing the photosensitive area that will attract ink during printing. The remaining silica gel acts as a non-image area, repelling ink just like fountain solution in traditional offset printing. The surface film protects the plate from scratches during transport and handling on the platesetter.
The process of making a waterless offset CTP plate involves exposure, development, washing, applying a protective coating, and drying. Exposure takes place directly in a standard CTP platesetter, with careful control of exposure time. A protective film is kept on the plate during exposure to avoid damage, but it must be fully removed before development. If not, incomplete development or film residue may block the water jet pump. Once developed, the plate is processed in a specialized waterless automatic processor. This includes pre-treatment, washing, post-treatment, and re-washing. The pre-treatment liquid helps break the interface between the photosensitive and silica gel layers. During washing, a brush removes the exposed silica gel, and any residual chemicals are rinsed off. The image areas are often colored for better visual inspection. Finally, the plate is washed again to remove excess dye. Unlike traditional offset plates, waterless CTP plates cannot be dried with heat; they must be air-dried at room temperature.
Waterless Offset CTP Printing
Waterless offset CTP inks are similar in pigment composition to traditional offset inks, but their binders and solvents differ. Key differences include: 1) Waterless inks require higher cohesion and viscosity to ensure proper adhesion to the photosensitive areas, as there is no fountain solution to repel ink. 2) They also need good transferability, low viscosity, and high thixotropy for smooth ink flow. 3) Since waterless plates are more prone to scratching, the pigment particles should be fine and the ink should have high gloss. 4) As no water is used, these inks don’t need to be water-resistant.
Paper used for waterless offset printing must have high surface strength to maintain print quality. Throughout the printing process, attention should be given to roller cleaning, blanket maintenance, machine adjustments, blanket selection, and environmental control.
Additionally, waterless offset printing requires strict environmental conditions. The printing workshop must maintain a temperature of 23±2°C and humidity between 50% and 60%. Paper temperature should also be controlled according to seasonal changes.
Storage of Waterless Offset CTP Plates
When removing the plate after printing, care must be taken to avoid scratches. When stacking plates, paper should be placed between them. Before unloading, the ink can be removed using the inking roller, making the plate easier to clean. Used waterless CTP plates can be stored for future use with the help of an automatic retainer. However, unlike traditional plates, waterless CTP plates must be coated with a cleaning agent to prevent aging. After cleaning, they should not be sealed with glue but instead air-dried. Soft brushes should be used when handling the plates to avoid damaging the layout. Finally, waterless CTP plates should be stored at a suitable temperature, away from high heat.
This content is selected from Issue 5 of “Printing Technology · Digital Graphic Arts†in 2012. For more journal content, please follow the journal channel.
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