Positive PS version failure analysis - blank part of the printing press

There are many common faults in the use of positive PS version. Analysis of the reasons is not only the quality problems of the PS version, but also improper use in the process of plate making and printing is also an important cause of various failures. According to the actual working conditions of plate making and printing, we will carry out a detailed analysis of the common faults of the PS version of Yang Tu, and suggest ways to eliminate it so that the printing plant can make better use of the PS version.

First, printing blank parts on the dirty:

1, underexposure:

The principle of plate making of the PS plate is that during the exposure process, light passes through the light-transmitting portion (blank portion) of the positive plate to cause the photosensitizer to undergo solution, emits nitrogen gas, rearranges the molecular structure of the photosensitizer, and produces an anthrone compound; this compound Insoluble in water, but soluble in dilute alkali solution, so exposed oxide film is a blank part. However, the part of the image that has not seen light is not changed into the printing plate part of the ink. If the exposure time of the blank portion is too short and the light is too weak, the photosensitizer may not be completely decomposed when the exposure is insufficient, and the photosensitizer may not be completely dissolved during the development, and a residual photosensitizer may cause the blank portion to become dirty. Therefore, you should use the print test strip to test the correct exposure time when printing, so that the density is less than 0.35 and 0.35 under the sun without ink (line). Or in the exposure, development, washing, drying PS blank part of the blank with acetone, ethyl ether or the correct solution to remove the background color in the blank part, visually no obvious traces, or with a reflection densitometer did not remove the background color and remove The difference in the density of the background area is <0.03 (line)>.

2, underdeveloped:

The blank part of the photosensitizer is not fully dissolved, and there is still a residual photosensitizer that causes the blank part to become dirty. The reasons are:

(1) Insufficient developer concentration: The developer with the specified concentration shall be prepared according to the prescribed preparation method, and shall be supplemented according to the degree of attenuation after the developer is developed.

(2) Insufficient development time: Generally, it should be 35±10 seconds. It should not exceed 1 minute at most.

(3) developer temperature is too low: the optimal temperature of the developer should be 22 ± 2 °C. Because the temperature is 10°C, the development speed varies 3-4 times, that is, the temperature is high and the development temperature is low. The development is slow; the concentration is high and the development is fast, the concentration is low and the development is slow; the platemaking can be flexibly controlled according to the specific circumstances.

(4) developer fatigue: Developer due to continuous development or exposed to the air for a long time, due to the absorption of CO2 in the air, the developing ability will gradually reduce the recession, continuous development time is too long and the developer concentration is too low. similar situation. Therefore, the developer should be kept from exposure to the air for as long as possible.

(5) Contamination of the developer: Since the sensitizer of the PS plate contains a small amount of dye, during development, the dye will be dissolved in the developer together with the photosensitive tree finger exposed to photolysis and accumulated. When the developed PS plate reaches a certain amount and the dye accumulation reaches a certain level, the dye in the developer is adsorbed by the micropores of the PS platen to cause self-contamination. Therefore, new developer must be replaced.

3, the original film and printing plate dirty:

(1) The blank part of the original film is exposed to dust and dirt. The density is too high and the transparency is not good. The exposure and development time may be appropriately extended or the smear may be removed by a double exposure method of a scattering film. When using a diffuser film exposure, the main exposure time is generally reduced by a quarter, and then the double exposure of the Mongolian diffuser film is used; when the masking time is one-third to one-half of the main exposure, the film can be contaminated and Film edges and tapes are removed. If the original film ash can only be seriously re-plate, so that the density of blank parts does not exceed 0.05.

(2) Stainer glass should be cleaned regularly to maintain hygiene. When the machine is not in use, the glass should be covered to prevent dust.
4, uneven exposure: due to the light source of the printing plate light source printing spot light source distance from the PS version of the various parts, the exposure of the middle part of the PS version from the light source near the exposure, large amount of light, the four corners away from the light source, the exposure is small, easy Causes the surrounding area to be less exposed than dirty. The distance of the light source can be appropriately increased, the light distance difference of each part of the PS plate can be reduced, and the exposure time can be appropriately increased. When printing, you should take into account the exposure of each part of the PS version. The general lamp distance is 1.2M. The formula for calculating the exposure time after changing the lamp distance is:

t=t. (r/r)2
t. - The original exposure time
r. - The distance between the original lights
r - Change in lamp distance
t - the exposure time after the requested change

5, the normal exposure after the development of washing the layout is still blue:

(1) During the production process of the PS plate, the drying temperature is too high, which causes the aging of the photosensitive layer, and the exposure and development time can be appropriately increased. If the photosensitive layer of the PS plate is aged before exposure, the plate material has to be discarded.

(2) PS plate storage temperature is too high, storage time is too long (more than one year), or exposure time in the air is too long, the photosensitive layer is oxidized by air aging, can be appropriately increased exposure and development time to solve.

6, improper ink on the dirty:

When the ink is removed, the gauze is not clean. There is sand or dried ink. If it is too strong, wipe the plate with dirt. Under normal circumstances, use gasoline to wash it off. Then clean the plate and then re-rub or lift the ink. Take care when cleaning the ink. The method of operation, so that evenly rubbed evenly.

7, wipe the protective rubber and dirty:

(1) The protective glue concentration is too low and the film is too thin to cause oxidation of the PS plate.

(2) Wipe the protective glue to cause uneven oxidation or dirt leakage.

(3) The PS plate grain is too fine, and the protective glue concentration is too low. The surface tension is too small, and it is easy to condense into a small water droplet to make the PS plate oxygen dirty.

8, in addition to dirty method caused by the blank part of the dirty:

In addition to the dirty agent dried on the printing plate is not timely to clean the ink on the dirty, need to promptly wash away the photosensitive agent rinse.

9, improper processing before and after copying cause viscera:

The plate should be completely removed before the plate and the wipe protection liquid should be rubbed. Normally, the amount of a half-split plate is 30ML. After the plate is cut, it should be thoroughly developed. Wash the protective liquid and wipe the protective gel.

10, improper operation of the offset press causes blank parts on the dirty:

(1) The ink used in the offset press is too thin and the oiliness is too strong. The ink should be thickened.

(2) The workshop temperature is too high, it should be generally 20-25 °C.

(3) The dampening syrup is not suitable. Wash the dampening solution matching the PS plate and add a small amount of gum dampening.

(4) The pressure of the offset press is too heavy or uneven, and the pressure should be reduced as much as possible while ensuring the quality.

(5) The offset roller is too heavy and the roller pressure is too light. It should be adjusted properly.

(to be continued)