Shaiban, proofing process and standardized quality management (4)

The main factors affecting the quality of printing are as follows.

1. The influence of the quality of the original outlets

The quality of the original outlets has two important indicators:

1 Dot density should be high. Positive prints should achieve effective exposure and do not allow light to pass through the dots. Therefore, the dots must have sufficient density. The density of a single dot should reach 3.0 or more, and the density in the field should be reached. 4.5 or more. The problems that often occur in actual production are: First, the dot density is insufficient, especially 1% to 2% of black spots are gray; second, improper revision and corrosion causes the dots to gray and yellow, which affects the quality of the printing plate.

The 2 outlets must be clean and free from imaginary haze. Each mesh requires that its center density is consistent with the edge density, so that the exposure time tolerance can be extended to achieve a faithful transfer.

Problems that often occur in daily production: First, some original point centers have high density, low density around the periphery, and imaginary edges. As a result, small changes in the exposure amount of the printing plate will cause large changes in the dots on the printing plate. Second, the quality of the text is poor, the middle density of the text is large, and the density of pens and pens, etc., is low, which results in the shrinking of printing plate fonts, lack of writing and breaking, and affecting the quality of writing. Therefore, before printing, you should check the quality of the original website and text, and find that the problem is solved in time.

2. Effect of plate quality

The quality of the media mainly includes the following content.

1 version base processing. PS version of the basic version to clean, smooth distribution of grain, thickness, depth and moderate. At present, the PS plate produced by the Institute of Printing Science and Technology of China, the plateau grain is a continuous roughening process, with the advantages of even grain size and moderate thickness. RZ (average depth of sand)=between 5 and 7.5 μm, and Ra (average roughness)=about 0.6 μm.

2 coating of the photosensitive layer. Sensitive coating should be uniform, moderate thickness, not only to completely cover the peak of the sand, to prevent dirt on the blank part, but not too thick, affecting the resolution of the printing plate, the thickness of the coating layer of the PS plate in China and India is 1. 8g/m2, ideal.

The quality issues of the plates in daily production have the greatest impact. The first is the production of different types of PS plates in China and the quality is unstable. If there is a rough plate material, there are many versions of the dirty, dirty, "horseshoe" India and other ills. The second is that the currently used proofing materials are all recycled versions. The quality problem is even more serious, which intensifies the conflict between proofing and printing. Therefore, attention should be paid to the choice of plates.

3. Impact of exposure

The appropriate exposure is an important condition for the correct transmission of the size of the entire tone. The exposure mainly has the following two indicators:

1 light source. Shaiban light source is very important, choose the right and reasonable, not only can shorten the exposure time, but also can improve the resolution of the plate material, reduce dot change. The ideal is a metal halide lamp, which has a strong ultraviolet spectrum, the spectrum between 350 ~ 360um, has the advantages of clean, safe, stable, high efficiency of light efficiency, and the PS version of the spectral sensitivity match. However, it should be noted that the lamp will be aging for a long time, so that the spectrum of radiation will change, the uniformity of the lighting will be affected, it is best to measure the working time of the lamp (preferably automatic timer switch), in the longest specified work Replace the time before the time expires.

2 exposure time. The exposure time of the printing plate is related to the depth of the printing plate. The printing plate of the positive type PS plate has less time for the printing plate to be relatively deep and the plate surface is easy to be dirty. On the contrary, the exposure time is much longer, the printing plates are smaller, the plates are shallower, and the shades of high, medium and dark tone are different. Therefore, when the printing conditions have been selected and fixed, it is necessary to find a suitable exposure. The method is to use a test strip to perform a step-by-step exposure. After development, according to the geometric position correspondence principle, 1% and 99% or 2% and 98% of the black and white dots can correspond to the sun, which is the appropriate exposure.

In daily production, the exposure time required varies depending on the type of PS plate. Therefore, it should be done for each purchase of a batch of PS version, exposure testing should be conducted to obtain the best exposure time.

4. The influence of the contact state between the original plate and the photosensitive plate of the PS plate during printing

When printing, the quality of the original and PS photosensitive layer is an important factor affecting the quality of the printing plate. The contact state is different, and the dot change rule is also different. Good contact is the prerequisite for adjusting the controlled transfer of the dot. The operation should be done with tight suction, high vacuum, and the original plate is in intimate contact with the plate. In daily production, there are many problems in this area, as described below.

1 The printing machine itself has poor vacuum contact. The glass and rubber of the printing plate are not even. Some new plate printing machines are not even. In the middle, it is necessary to put a piece of woolen cloth in the middle to draw flat clothes.

2 The pumping volume of the vacuum pump is too low.

3 Some of the original film overlaps 1 to 3 small pieces and sticks the tape on the sides of the figure, resulting in a lack of realism.

4 There are dirt on the original film and the plate, which causes local exhaustion to be insufficient and the network points to be empty. Check before making a detailed inspection.

5 The temperature and humidity conditions of the studio are poor, and the relative humidity is too low, which will intensify the generation of dust and static electricity and cause poor contact between the film and the photosensitive layer of the PS plate. Humidity fluctuations will affect the developer activity, room temperature 20 °C ± 2 °C, relative humidity (55 ± 5)%.

5. Effect of developing conditions

The correct exposure should be matched with the correct development. The development conditions include the chemical composition of the developer, temperature, development time, and mechanical stirring. Before developing with an automatic developing machine, the developing machine should be circulated and agitated so that the liquid medicine can be maintained uniformly and at a constant temperature. The developer temperature is generally 20 ~ 25 °C, the temperature is not appropriate, it will cause significant or incomplete display.

1 Accurately control developer concentration. Should be based on different types of plates with different developer solution, most of them are now using inexpensive sodium hydroxide and other alkalis as the main agent, and a larger proportion, strong drug, general formula 10000mL water, with 50g. Although this kind of developing solution is fast, but both the oxide layer and the photosensitive layer on the surface of the plate base are etched and dissolved, and the hydrophilicity of the surface of the plate base is also damaged. At present, many domestic and foreign companies use sodium silicate and other weak alkalis as the main agent, although the price is higher, but the development speed, long service life, and can increase the layout of the hydrophilic and printing plate printing rate.

The standard for accurately controlling the concentration of the developer should be tested with a test strip, exposed and developed at normal times, the hollow point after development is relatively clear, and the corresponding small black point is relatively firm, demonstrating that the developer concentration is standard, such as small black points Excessive loss means that the concentration is too high, such as the hollow paste, then the concentration is too low. Should be based on this increase or decrease the developer content of the main agent.

If the correct development effect is visually recognized, there should be no change in the green color of the graphic text on the layout, and the green color of the blank part will completely disappear and the base will be light gray. If the blank part still contains blue-green after development, the developer's drug power is weak or invalid, and the exposure is insufficient.

The problem that arises in daily production is that since the photosensitive layer of the PS plate is not properly matched with the exposure time and the developer density, and the blank part still has yellow-green color after development, two developments are taken so that the dots are lost too much.

2 Measure and replace new fluids. The degree of deterioration of the developer depends on the number of developed plates and the storage condition of the developer. The automatic developing machine generally displays an average of 60 to 70 plates per day. The replacement of new fluid and the reduction of labor in the afternoon can slightly slow down the development speed. Manual development, developer placed in the tray, to be added to avoid contact with air oxidation, should be measured to supplement and replace the new liquid.