In color printing, the printing process plays a role in linking up and down, and the quality of printing quality directly affects the quality of printed products. As far as the current level of domestic printing is concerned, the vast majority of printing production still adopts traditional printing methods. In this process, the faithful reproduction of outlets directly determines the quality of the printed matter. Therefore, the quality of outlets on the PS version plays a decisive role in the process of print delivery.
The following describes a method for controlling the dot transfer during the printing process—using the FOGRA-PMS precision test strip.
FOGRA-PMS Precision Test Strip K Section
The K segment of FOGRA-PMS is divided into line blocks, gap blocks and field blocks. Figure 1 is an enlarged K segment of FOGRA-PMS.
The line blocks and the gap blocks are divided into nine segments marked with numbers from 4 to 40. These figures represent the line width and gap width in microns. Each line has the same width as the adjacent gap.
The solid blocks act as a relief, so that it is easy to visually observe the reproduction effect of fine gap blocks. These individual gaps are not clearly visible to the naked eye, or they cannot be distinguished with the naked eye. If you can see the obvious difference in the adjustment between the solid block and the gap block, it shows that this gap block can be copied out. If the situation is reversed, the same value is used for the real block and the gap block, which means that the gap block cannot be duplicated.
From the transmission status of the line block and the gap block, it is possible to evaluate the light receiving effect on the back side and derive the measurement of the dot transmission. Since then, these measurements can be included in the positive and negative chart printing forms. When the finest lines and the finest gaps of the same step or the same section start to be transmitted, the resolution measured by the K section can be obtained at a certain exposure amount. As the exposure time increases, the backside light effects increase at the same time, so that the lines that get wider and wider will also be less likely to dry out.
The FOGRA-PMS-K print test strip is designed to control the printing process. In addition to the K section, it consists of 5 circular dots with 60 lines/cm (1% and 99%, 2% and 98%, 3% and 97%, 5% and 95%, and 7%. 93%) of the two-block (Yin-Yang dot) measurement block, and one field block with a coverage of 100%. The two-segment dot measurement block can be used to evaluate the minimum and maximum dot gains that can be released under certain exposure conditions. If the printed image has very low and high dot values, it is very important to use this test strip.
When designing the FOGRA-PMS-K printing test strip, it is necessary not only to take into account the need for positive printing, but also to apply it to negative printing.
Using K-section of FOGRA-PMS Test Strip to Determine Pass-threshold Transfer
Light source, vacuum, development and other factors have an important effect on printing. To test the correct exposure time with the FOGRA-PMS-K, before testing, ensure that the light source in the frame is very uniform and that the film is in good contact with the plate. In addition, the development conditions should also meet the fixed numerical requirements recommended by the manufacturer or measured by themselves. Assume that the test is conducted under optimal conditions and tested from the beginning of the exposure. In the test of exposure time must also be taken into account, in some cases, should allow the use of a part of the diffuser exposure, in this case, each selected exposure time should be divided into halftone exposure parts and exposure with a diffuser section. For example, the first half of the exposure time is the time for diffusion exposure with a diffusion sheet.
Positive print
In order to make the test exposure consistent with the actual conditions as much as possible, the PMS-K test strip should be put on a piece of imposition. When multiple exposures are performed, the PMS-K test strip and the imposition patch each time go to the location of the plate test strip. At this time, use a light-proof material (such as a special thin sheet with a thickness of no more than 0.2 mm) to cover the exposed area of ​​the plate and the area to be exposed. In this way, the light of each exposure can only be irradiated with PMS-K. Test strips.
After the film and plate are in good contact, the first exposure is performed. The first exposure is generally too short for some printing plates, and the next exposure time will need to double. For example, exposing with 1/2, 1, 2, and 4 minutes, respectively, and then the results from this group of exposures will give us the information we need. In practical tests, of course, it is not possible to obtain results from a set of tests. Only through exposure tests of the second and third groups can the desired results be obtained with an appropriate average exposure.
Sometimes this also happens: After a series of exposure times, immediately after the exposure to a larger area of ​​the imposition sheet, despite using the same exposure time, the dots on the exposed PS plate are more than those at the time of the experiment. The outlets should be small. The reason for this is that in the case of large-size imposition pieces, the contact between the film and the plate is worse than the exposure test. In this case, you need to adjust the exposure time slightly. In actual operation, the positive printing plate must be carried out at a level 3 to 4 higher than the resolution in order to reach the standard range.
In exceptional circumstances, making full use of the stencil table can help us to accurately correct the transmission of the gradation tone value during printing. While trying hard to reach the printing plate outlets that are less exposed than the standard range of outlets, we must pay attention to the risk of network outlets becoming imaginary (this is related to the contact conditions). With the increase of exposure time, there is a chance of occurrence. Obvious increase. In addition, with the increase of exposure time, the coverage of the network points is in the range of 1% to 5%, and the smallest outlets do not come out. When the quality of the outlets is good, only 2% of the outlets (60 lines/cm) can generally be printed on the printing plate without any loss. It must be noted that dots with very low tone values ​​sometimes do not have enough darkness on some copying screening films.
2. Negative print
The method used in the negative chart printing was the same as that in the positive printing plate. The exposure chart was made using the PMS-K test strip suitable for negative printing, and the exposure time was again used for each exposure time. Observations show that the negative chart version has the same resolution and exposure characteristics as the positive chart example we listed earlier, except that the reversal phenomenon occurs. When the test film was sunk onto the plate, the gap became a line and the line became a gap.
When printing on a negative image, the plate will burn more and more deeply as the exposure time increases. Therefore, through the K section of the PMS test strip, it can be seen that the backside light affects the line width on the test strip, so after the contrast is reversed, the gap is no longer visible on the printing plate, and only a solid area is seen at the middle and lower levels. If you start with the starting point of resolving images as you do in the positive chart, you can say that when a negative chart is printed, the line blocks on the K section of the plate are more likely to become real, that is, the printing plate is too tanned. From this it is also possible to deduce the measure of the transmission of the tone values ​​of the dots. Since, in general, negative-pattern printing plates are formed by the exposed photosensitive layer, in individual cases, it is necessary to ensure that the printing plate obtains a sufficient amount of light, otherwise the firmness or print-resistance of the printing plate will be affected. When the printing degree exceeds the resolution of 1 to 2 levels, the negative printing plate can accurately reach the standard range.
With the increase of the exposure time, when the loss of the lowest part of the positive printing plate occurs, the underside of the low profile part of the negative printing plate receives light. Due to the contrast reversal, loss will occur in the high-profile part. It can be judged from the dot test block of the PMS-K test strip.
The halftone values ​​of the negative and positive prints should be the same, and the same printing effect can be obtained. High-resolved plates should be used for both solar and negative plates, because high-resolution plates can accurately transfer images from 2% to 95% of the total network tone value.
Over the years, due to the lack of data-oriented and standardized management in printing companies in China, the quality of printing plates has become difficult to control, which has directly affected the quality of printed products. Therefore, in order to obtain high-quality prints, it is necessary to control the quality of the PS plates with objective methods.
The printing quality control of the PS version is a systematic project and involves a very wide range. Each printing company should use FOGRA-PMS and other precision test strips according to specific conditions to summarize and improve a set of process technology data and standards suitable for itself. , Standardize the operation, so that the quality of the printing plate can be stabilized and improved.
Source: "Printing Technology"