Stencil quality control in optical offset printing plate

In the offset printing of the optical disc, the printing plate is an important link between the plate making and the printing, and is an important factor influencing the printing quality. The so-called printing version refers to the process of faithfully transferring the dots on the photographic original (film) to the printing plate (negative pattern PS version) through actinic action. Due to the constraints of processes, materials, equipment and other factors, it is often difficult to achieve the purpose of faithful reproduction. Therefore, in order to ensure the quality of printing, we must control the quality of the printing plate, that is, the transfer of control points. Below, we will analyze how to control the correct transfer of outlets.

First, the factors that affect the PS version network transfer:

Through the analysis of the whole printing process, we can know that the main influencing factors are the following aspects: the quality of the original (film), the reproducibility of the plate, the printing machine and the light source.

1, the original (film) quality

The ideal original quality should be a high dot density (D>3), a very solid idea, and a clean, hard spot. Such outlets have a large degree of exposure and development during printing and are easy to faithfully reproduce. The dot center density and the edge density must be the same, so as to expand the latitude of the exposure time, and completely transfer the dot. The original should be clean and free from dirt, otherwise it will be dirty.

2. Reproducibility of halftone dots

The reproducibility of the printing plate dots refers to the effect that the printing plate can reproduce the original dot reproducibility to the maximum extent during the printing process. The effect is mainly reflected in the resolution of the plate material and the film thickness. As the PS plate is a pre-coated photographic plate, its substrate is an aluminum plate, and a uniform photosensitive film layer is coated on the substrate. Therefore, the treatment process of aluminum base directly determines the quality of the plate. This part is often invisible to the users of the plates. Therefore, it is particularly important to ensure the quality of the plates through the purchase channels. If the plate is rough and the quality is poor, it will lead to a decrease in resolution, which will seriously affect the printing quality. Some inferior plates increase the thickness of the photo-resist film layer in order to mask the grain on the aluminum plate base. Doing so will result in an increase in the diffraction phenomenon at the time of exposure, which will result in decreased resolution and affect the print quality.
The following are the effects of the quality of the media on the reproduction of outlets and their requirements:

(l) Local floral plates and pasting plates The basics should be clean, uniform in thickness, uniform in distribution of grains, moderate in thickness, moderate in depth and depth. The photosensitive liquid is evenly coated with no bubbles or scratches. It is generally required that the average depth of the sand (Rz) is 5 to 7.5 μm, and the average roughness (Ra) is about 0.6 μm. This has affected the quality of the PS version, resulting in local rapid wear and tear, "flower pattern", paste version, and then appear off version and dirty.

(2) PS plate surface roughness is not standard Abundant sand, so that the water is insufficient, easy to dirty: The sand is too fine, so that the photosensitive film in the microporous is not easy to clean, the base is easy to pollution, easy to dirty when printing.

(3) Local word drop, off-grid coarsening of the electroplated base plate after removal of the gray matter is not net, the partial photosensitive layer is not firmly adsorbed; before the coating of the photosensitive liquid, the plate surface is not washed cleanly, leaving traces of acid flow when hanging, will have Cause partial out of print.

(4) difficult to ink PS version has exceeded the storage period or caused by improper storage, such as the preservation of the ambient temperature is too high (more than 30 °C), relative humidity more than 65%, too strong light, etc.: There is photosensitive adhesive viscosity is too low, photosensitive coating The layer is too thin and so on, causing the PS plate to print on the machine is not easy to ink or not at all.

3, printing machine and light source

When printing, it is required that the suction vacuum degree of the printing machine should be as high as possible to ensure good contact between the original plate and the printing plate, which is beneficial to the transfer of the dot; if the suction is not good, the spacing between the original plate and the printing plate increases. Large, it will lead to diffraction is very serious.

The choice of printing plate light source is also very important. In the printing process, it is expected that the ideal light source should emit absolute parallel light, but the light source used in actual production can hardly meet this requirement. Usually in actual production, the light source is required to maintain a distance of about 1 meter from the printing plate so that the light emitted by the point light source can be as close as possible to the parallel light.

Since the exposure is a photochemical process that subjects the photosensitive film to light and undergoes a chemical reaction, the process is affected by two factors, namely the illumination E on the image plane and the exposure time T. The energy obtained per unit area of ​​the photosensitive film, ie the exposure (H) equals the product of the illumination E and the time T:

H=E × T
Therefore, to determine the size of the exposure, it is necessary to consider both the illuminance E (light intensity) and the exposure time T.

The light source of the printing machine is generally a metal halide lamp. Such a light source decays after being used for a long time, resulting in a decrease in the illuminance E during exposure. Therefore, in the actual production, after the metal halide lamp is used for a period of time, it is necessary to increase the exposure time T according to the actual situation of the light source, so as to ensure that the exposure amount during the exposure during exposure is maintained.

Correctly grasping the exposure time is very important for the printing process: overexposure will cause some small dots to be sunburned, which will result in the loss of brightness levels; and underexposure will cause the decomposition of the blank (non-textual) part to be incomplete. And easy to stain. Therefore, the following factors need to be taken into account during the exposure process:

(1) Choose a light source that matches the spectral sensitivity of the PS platen—short wavelength light source (UV light source). On the one hand, it can shorten the time of printing, on the other hand, it can also reduce the thermal effect of long wave.
(2) Minimize the effect of natural light on the PS plate. In general, the use of incandescent lighting is relatively safe, even after the exposure is complete, but also to minimize the light, to prevent the graphic part of the re-decomposition.
(3) The exposure time should be coordinated with the development, as stable as possible, which is conducive to print quality.
(4) During the period from the development to the printing, if the time permits, the plate should be dried as much as possible in the cool place to prevent the damage of the photosensitive layer caused by strong light or high temperature.

In the printing process, in order to ensure the accurate reproduction of the network points, we must pay full attention to the tight integration of the original version and the PS version. The unevenness of the printing glass and blanket, the dirt of the pad, the tape on the film, hair and dirt, and the insufficient vacuum between the film and the PS plate will cause the film and the photosensitive layer of the plate. Poor adhesion causes light to diffract when passing through, causing dots on the printing plate to become imaginary or shrinking, or non-ink-absorbing white spots, affecting the reproduction of halftone dots, resulting in reduced reproducibility of image reproduction, and resulting in deterioration of printing quality. Can not be ignored. In addition, the aging of blanket printing blankets will cause a small amount of air leakage at some edge positions, which will also cause the PS plate and the film to stick tightly, making the drying out of the picture.

Since each optical disc company is equipped with a developing machine at random when the offset printing machine is purchased, and each type of developing machine has a similar function and structure to the Japanese Toray development machine, it can be set according to the developing speed recommended by each device manufacturer. However, it should be noted that the pre-treatment solution must be replenished in time according to the number of plates. Otherwise, the concentration of the pre-treatment solution will be insufficient, and the development speed will not be adjusted, resulting in incomplete decomposition of non-graphical components, resulting in blurred plate images and easy Dirty. Water is the Toray PS version of the developer, so ensuring water quality is critical. It is recommended to clean the filter once a week and change the water. The post-treatment solution is mainly used to color the dots so as to observe the reproduction of the dot after development. Because we can't distinguish the transfer of the dots with the naked eye after the print version of the PS plate, we must use the developed image to determine the reproducibility of the dot, so the control of the developing stage is also very important. In particular, the temperature of the pretreatment liquid (usually set at 38°C or more for the developer to start) and the concentration, water quality, etc., as well as the height of the brush and the hardness of the bristles all affect the development.

We can visually compare the developed dots with the dots on the film with a 15x or more magnifying glass. If the size is comparable, it is proven that the printing and development are relatively normal. If they do not match, you must find out the reason and resolve it as soon as possible. Due to the high latitude of the development, it can be detected once a week.

Second, the control of outlets

In order to ensure the accuracy of network transfer, control bars are usually used in actual production to determine the exposure required for printing. The control strips used for printing include continuous ash grading scales, UGRA test strips, and Japanese FUJI test strips.

Here we are going to introduce the use of the continuous adjustment hairline as the exposure control strip in the usual production. Generally speaking, there are 15 grades and 21 grades for the gray ladder, with a grade difference of 0.15. That is, for each additional level, the density increases by 0.15. Indicates that the corresponding exposure will increase by 10%. If we are unable to find a negative-graded grayscale ruler, we can also use the grayscale ruler (which can be found at any printing plant or non-forest export company) to test it temporarily. However, it should be noted that in this case, the dots in the ladder should be viewed in reverse (because the grey scale for paper printing is positive and the opposite is true for the printing of the disc). That is, after labeling 95% of outlets after printing, it is necessary to observe whether the remaining 5% of the outlets can still be seen. If they can be seen, 5% of outlets are not lost. Also note that the 10% dot part can be seen if the remaining 90% of the dot prints are printed. If it can be seen, we can say that 90% of the dot prints have not been smeared. Another thing to note is that the number of lines for offset printing on paper is exactly the same as the offset number on the disc, so this method can only be used as a reference.


Of course, if the conditions permit, you can go to the film output company requirements to produce 175-line negative tone grading step (UGRA test strip, Japan's FUJI test strip is generally a few thousand dollars, and the production of negative-type continuous gray ash ladder ruler Only need one or twenty yuan). In this way, the negative grading scale can be used as the original (film) when printing, and it is closely associated with the PS plate. Determine a fixed light intensity (illuminance), adjust the length of the printing time, respectively, to test the sun. After the PS plate was developed, a half-doctor lens was used to observe the dot shift. If 2% - 3% of the outlets are not lost, more than 95% of the outlets will not be smeared. Proof of exposure is basically suitable. The exposure time and light intensity at this time can be recorded, and the printing can be performed according to this parameter in the subsequent production.

As mentioned earlier, the light source of the printing machine will decay after prolonged use. This requires that we must constantly detect and adjust the exposure of the printing machine, so that the network can be transferred within a controllable range.

Shaiban is an important process link in the offset printing of optical discs. Its quality will directly affect the production quality level of large CD players. Therefore, it must be paid attention to. At the same time, printing is a process that requires practical skills and operating experience. It requires people to understand the basic principles of the process and continuously explore it in practice. Only in this way can the best process quality be achieved in production.

Reprinted from: China CD Forum