Offset printing is a common printing technique that uses printing plates to transfer an image to a rubber blanket, which in turn transfers the image to paper or other materials. This printing method allows for high-quality printing results and is suitable for mass production. There are many types of printing plates used in offset printing, including the positive PS plate (Presensitized Plate, pre-coated photopolymer plate). It is coated with a photosensitive material that undergoes a chemical reaction when exposed to UV light or other light sources. This reaction causes the exposed area to become insoluble in the subsequent developing solution, thus forming an image. Positive PS plates for offset printing are widely used in commercial printing, magazines, books, packaging and other areas where high quality printing is required.

The process of making a positive PS plate, one of the most widely used printing plates, is as follows:

1. Original Preparation: Firstly, you need to prepare the text or graphics to be printed, which is usually done by computerized typesetting software, and then output as positive or negative.

2. Exposure: The positive film is placed in close contact with the PS plate in the exposure machine, and then irradiated by ultraviolet light, so that the photographic layer on the PS plate undergoes a chemical reaction. In the case of a positive PS plate, the exposed area (i.e., the area corresponding to the transparent portion of the positive) hardens, while the unexposed area remains soluble.

3. Developing: The exposed PS plate needs to be treated with a specific developing solution to remove the unexposed portion and expose the metal substrate. In this way, the image portion becomes ink-friendly while the non-image portion remains hydrophilic.

4. Rinsing and drying: After development, the PS plate needs to be washed with water to remove residual developer and other impurities, followed by drying.

5. Protective Coating: In order to improve the print resistance, a protective coating is sometimes applied to the surface of the PS plate.

6. Installation and calibration: Finally, the finished PS plate is installed on the printing machine and necessary calibration work is carried out to ensure the printing quality.

The characteristics of positive PS plate for offset printing are that it is simple to make, relatively low cost, adaptable and suitable for all types of offset printing tasks. In addition, due to its good print resistance and stable printing performance, the positive PS plate has become one of the top choices in the commercial printing field.