Lithographic printing plates, also known as litho plates or lithographic plates, are key components in the lithographic printing process. They are thin, flat sheets typically made of aluminum or, in some cases, polyester or other materials. Lithographic plates play a crucial role in transferring the inked image onto the substrate during the printing process.
The lithographic plate consists of two main regions: the image areas and the non-image areas (also called background or non-printing areas). The image areas contain the desired design or artwork that needs to be printed, while the non-image areas repel ink and remain ink-free. The contrast between these two regions enables the transfer of ink from the plate to the substrate while keeping non-image areas clean.
The process of preparing a lithographic plate involves several steps:
- Plate Coating: The plate is coated with a light-sensitive material, known as a photosensitive or photosensitive emulsion. This coating allows the plate to capture the image during the exposure process.
- Image Exposure: The plate is exposed to a film negative or a digital image file using a specialized process known as plate exposure. This exposure causes the photosensitive emulsion to undergo chemical changes, resulting in the formation of the image areas on the plate.
- Plate Development: After exposure, the plate goes through a development process where the unexposed areas are washed away, leaving behind the image areas that will attract ink.
Once the lithographic plate is prepared, it is mounted onto the plate cylinder of the printing press. During the printing process, the plate cylinder rotates, and ink is applied to the plate using an ink system. The ink adheres to the image areas, which have an affinity for oil-based inks, while the non-image areas repel the ink due to the presence of a thin water film applied by the dampening system.
As the substrate passes between the plate cylinder and the blanket cylinder, the inked image on the plate is transferred to the blanket cylinder. The blanket cylinder, in turn, applies the inked image onto the substrate with the help of pressure from the impression cylinder.
Lithographic plates offer several advantages in printing, including excellent image quality, sharp detail, and the ability to reproduce fine lines, text, and halftones accurately. They are widely used for high-quality commercial printing, such as magazines, brochures, packaging, and other printed materials.
It’s important to note that there are different types of lithographic plates available, including conventional (analog) plates and digital plates. Conventional plates require the use of a film negative and chemical processing, while digital plates allow for direct imaging using computer-to-plate (CTP) technology, eliminating the need for film and enabling more precise and efficient plate production.
In summary, lithographic printing plates serve as the carriers of the image in the lithographic printing process. By utilizing the principles of ink-water balance, these plates allow for the accurate transfer of inked images to the substrate, resulting in high-quality printed materials.