How the quality of the printing plate is determined by the exposure of the platelet first. Excessive exposure is too shallow plate image, a small dot loss, dot reduction, a serious loss of some of the highlights, and even affect the ability to resist India; underexposure is too deep plate image, dot expansion, dark tone easy to paste version. In order for the printing plate to faithfully reproduce the original dot layer, it is necessary to determine the appropriate exposure amount for the printing plate.
The PS version of the sensitizer is substantially completely decomposed into a substance that is soluble in the developer after being irradiated with a certain amount of ultraviolet rays, so that the PS plate releases grayish white sand. This amount of light is the critical exposure of the PS plate. In actual operation, as long as the original version is closely attached to the PS plate, the plate-making machine has a good vacuum effect, and the exposure of the printing plate tends to exceed the critical exposure, though it is a little. This can make the blank part of the layout fully exposed to the original fingerprints, tape marks and other stains are sunburned, reducing the amount of revision.
However, it is not allowed to extend the printing time without limit, otherwise the small outlets will be lost due to the diffusion of light.
According to the national professional standard of the positive PS version, it is known that the step size is printed on the corresponding step, and the value of the step density is between 0.65 and 0.80. The drying area should have the ink performance. In this way, the second level 0.35 area with a second-level third-level density value must be white. At this time, the exposure of the exposed children is the standard exposure of the fish, which is the best exposure. However, the original ladder scale is expensive and generally not available for printing manufacturers. Through the following derivation, the relationship between the critical exposure amount of the optimal exposure sentence can be obtained, which provides convenience for manufacturers without a ladder.
As shown in the figure below, the optimal exposure level for the third stage sunburn is E. When the E passes through the step scale, the third stage having a density of 0.35 is partially blocked and becomes E3, and then reaches the PS plate photosensitive layer, so that the third stage of the PS plate is not ink-absorbed. Here E3 is the critical exposure of this PS version. The critical exposure of each PS plate can be measured experimentally, that is, the amount of exposure just whitening when there is no shield on the PS plate.
We know that E = TIt, where T is the transmittance, T0 = 100%, I is the light intensity.
Because E=E0=T0I0t0,E3=T3I3t3
Simultaneous printing I=I0,t0= t3
So E/E3=T0I0t0/T3I3t3=T0/T3
When D3=0.35, T3=10-0.35=0.45
E=(T3I3t3)E3=2.2E3
After clarifying the relationship between E and E3, the best exposure E can be determined by measuring E3.
After two different speeds of PS plates Samples A and B were exposed on a platemaking machine, they were developed by hand for 40 minutes with 40 ‰ sodium hydroxide and 3% sodium metasilicate mixed developer (25°C). During printing, the PS plate specimen was pressed under a glass plate and exposed directly without an obstructer. A sample was taken every 10 seconds. Observe the layout after developing and record the time of full whitening. At the same time, use the gray step scale to print, and record the drying time required for the three levels of whitening and density values ​​between 0.65 and 0.80. Comparison of the list for comparison, from the data in the table shows that basically meet the E = 2.2E3 this relationship.
When drying text or original lines, you can step into a white four-step white, so that the actual best printing time can be longer. The same reasoning can be derived for E=3.1E4.
In short, the best exposure is 2 to 3 times the critical exposure. The lower limit of 2 times can be selected when printing fine mesh originals, while the original text line can be up to 3 times the upper limit.