Positive Pattern PS Platemaking Process (3)

(2) Change the exposure time and print.

Under the condition that the illumination intensity of the light source is constant, the exposure amount of the printing plate is proportional to the exposure time, and the exposure amount can be adjusted by changing the exposure time. In the actual production, the exposure time is commonly used to express the exposure for convenience.

The gray ladder ruler and the dot ladder ruler (optionally printable ruler) can be overlaid on the same PS plate for exposure and developed. This type of operation is performed separately for the printing plates of various types of printing presses, and the standard exposure time is determined using the dried printing plates.

(3) Determine the standard exposure time.

The dried printing plate is mounted on a printing press for printing, and the printed product is compared with proofs printed with the same ash ladder or screen ladder (or printing ruler) to obtain the same reproduction on the printed matter. The steps of the step ladder, the exposure time of this step is the standard exposure time. For example, when the grey step on the printing plate is stenciled to a level of 6 blanks, the reproducibility of the printed sheets on the Internet is the same as that of the proofs, and the exposure time of the printing of the 6-level blank is the standard exposure time. The so-called "tank level 6 blank" means that the photosensitive layer of the corresponding portion of the plate on the plate and the gray ladder ruler is completely decomposed by light, and after development, it is completely removed from the plate and no ink is adhered. Therefore, " Level 6 Blank is also called "Level 6 Whitening."

The method of determining the standard exposure time by using a printing degree ruler is relatively simple, and the standard exposure time is the exposure time of the specified dot step and the width of the yin-yang line of the printing plate. At present, most of the production practice uses this method to determine the standard exposure of a PS plate.

When printing, if the distance between the light source and the printing plate changes, the exposure time should be adjusted accordingly according to the rule that the exposure time is proportional to the square of the distance from the light source to the layout. If the illuminance of the light source is non-uniform, or the original plate is poorly adhered to the plate, the printing conditions deteriorate, and the balance between the standard exposure time and the reproducibility of the image tone on the plate is destroyed. When this happens, the exposure time must be adjusted using the print height scale and resolution test strip (resolution check).

The above method for determining the standard exposure time is based on the premise that the printing process is in a stable state. The reproducibility of the printing plate corresponding to the original printing plate is also considered based on the standard dot enlargement value of the stable printing. If the printing conditions are not stable, This method loses its meaning.

Second, the development

The PS plate is a plate coated with a lipophilic photosensitive layer on the surface of a hydrophilic aluminum substrate. Therefore, it is necessary to leave a photosensitive layer of the image portion after printing on the plate, and remove the photosensitive layer of the non-image portion to expose the hydrophilic plate. The process of removing the photosensitive layer of the non-image portion by the developer is called development.

1. The principle of development The purpose of developing the positive PS plate is to remove the photo-degraded photosensitive layer on the plate surface. Commonly used 215, 214, and 124 photosensitive resins, see light decomposition reaction, generate tannic acid compounds dissolved in dilute alkaline solution. Therefore, using a dilute alkaline solution as a developer, the PS plate was subjected to a development process, leaving on the plate surface a lipophilic, slightly convex embossed image formed without a photosensitive layer.

The photosensitive layer of the non-image portion, which can be completely removed from the plate after development, first depends on the solubility of the photosensitive layer to dilute lye. If the photosensitive layer is fully exposed during the printing process, the photon-absorbing energy of the photosensitive material enables a certain amount of molecules involved in the photo-decomposition reaction, and better development effect can be obtained.

Positive type

The development of the PS plate is essentially the dissolution process of the photosensitive layer of the light-receiving portion. The higher the temperature, the faster the developing speed, and the more the photosensitive layer is dissolved away. It is generally believed that the development speed increases by 1 time for every 12°C increase in temperature. When the temperature exceeds 30°C, the binding force between the photosensitive material (photosensitive resin such as 215, 214, and 124) and the film forming material (phenol resin, etc.) in the photosensitive layer is weakened, and the anti-alkalinity is reduced, and no phototext is found. Some of the sensitivity will be dissolved, so the temperature will increase further, the percentage of the area of ​​the layout will be reduced (Figure 6), and the reproducibility of the printing plate will be destroyed. In order to stabilize the developing quality, the temperature of the developer should be controlled at 255°C.

Fig. 6 Development Conditions and Dot Reproducibility

If the development time is too long (developing speed is too slow), fine dots on the plate will be easily lost. If the development time is too short (developing speed is too fast), the photosensitive layer of the non-image portion cannot be completely dissolved, causing the layout to be dirty. In particular, the PS plate coated with a centrifugal coater has a large difference in thickness between the central portion and the peripheral portion of the photosensitive layer, and the quality problem caused by improper development time is more prominent. It is generally believed that the development time should be controlled at 30s to 2 min, the better PS plate should have a development latitude of 2 min, and the composition and concentration of the developer should also match the above development time.

(one two three four five six seven eight)