Editor's note: Although CTP has been used for a long time and has begun to spread in the country, some printing companies that are not very strong still use PS plates. PS plate is still the printing plate with the largest proportion of domestic use at this stage. In the following article, the author discusses two important conditions that determine the quality of PS plates: the quality of the original plate and the quality of the plate.
In offset printing, plate making is the guarantee of printing quality. Only by guaranteeing the quality of PS plate making can the quality of printed matter be ensured. Well, good quality printing plates need to have those points, let us explain in detail from the original plate quality and plate quality.
1. Original quality
The original version refers to the film or sulfuric acid paper output from the pre-press. Its quality directly affects the plate-making effect of the PS version. There are two important indicators of original quality:
(1) Dot density
When printing a positive image, the dots must have sufficient density. The density of a single dot is above 3.0, the solid density is above 3.5, and the transmission density of the blank area of ​​the film should be less than 0.05.
(2) Bright and clean dots
The density of the dot center and the density of the edge are the same, so that the tolerance of the exposure time can be expanded. The original version is clean and free of dirt. It is best to use a complete film without a knife edge and collage tape to ensure that the film is tightly combined with the PS version to avoid virtual dots caused by light scattering.
2. Plate quality
(1) Edition base
The base should be clean, uniform in thickness, evenly distributed in sand, moderate in thickness and moderate in depth. The photosensitive liquid is coated evenly, without bubbles and scratches. Generally, the average depth of sand mesh (RZ) is 5 to 7.5 μm, and the average roughness (Ra) is about 0.6 μm.
(2) Photosensitive adhesive layer
The photosensitive coating is uniform and the thickness is moderate, which completely covers the peaks of the sand mesh, but not too thick, which affects the resolution of the printing plate. Generally, the thickness of the coating layer of the PS plate is about 1.8 g / m2.
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