1 Introduction
The quality of the reproduction of the printed matter is mainly evaluated from the aspects of color reproduction and gradation reproduction of the image. The tone level replication mainly depends on the number of dots in the unit area and the number of dots continuously changing from small to large. The more dots in the unit area, the more dots in the copied area, the clearer the printed matter The higher the degree, the richer the layers and the more realistic the colors; the more the dot changes, the more expressive the printed layers and the more delicate it looks. In the printing process, people change the number of screen lines to control the number of dots per unit area. The higher the number of screen lines, the more the number of dots per unit area, and the smaller the size of the reproduced dots. The number of commonly used coated paper plus screen wire is between 150 ~ 175lpi. But with the continuous improvement of people's appreciation level, this kind of precision printed matter can no longer meet people's visual needs. Therefore, more and more printing companies use high-screen technology to obtain printed products with more layers, higher definition, richer colors, and reproduction effects close to the original, to meet people's needs for high-end picture albums and famous packaging.
As one of the important contents of the Hefei Sino-German Printing Training Center and the German KBA company's printing seminar and KBA equipment exhibition held in Hefei, it is to carry out the printing test of 300 screen printing. With the help of KBA technicians, we completed the printing comparison test of 300lpi printed matter and 150lpi printed matter.
2 Test analysis
As we all know, dot expansion in the printing process (including mechanical dot expansion and optical dot expansion) is an inevitable phenomenon of printing methods that achieve image transfer through pressure. The size of the dot expansion value is usually proportional to the sum of the perimeters of the dots per unit area. For the same dot value, the sum of the perimeter of the dots of 300lpi is greater than the sum of the perimeter of the dots of 150lpi, which means that the expansion rate of the dots of 300lpi is greater than that of 150lpi. In view of this, we need to test and make a suitable output characteristic curve before outputting the printed version, so that it can be called when outputting.
The diameter of the 2% dot of 300lpi is about 11.6μm, which is almost the same as the diameter of the dust particles, so the dust falling on the glass of the printing plate or the film is easily exposed, forming the so-called "pseudo dot", which interferes with printing The normal operation of, so we should strictly ensure that it is dust-free in the process of PS version drying. In addition, the PS plate developer should be used to develop the PS plate, and to ensure that the development process is stable.
The selection of test materials is also relatively strict, we should fully consider the characteristics of the relatively small dots of 300lpi printed matter, and choose the appropriate printing material.
In the printing process, the printing workers are required to have a good professional level and good professionalism, to achieve "three diligence", and to maintain a good water-ink balance during the printing process.
3 Test equipment and test materials
â‘ Printing equipment: Adopt the KBA Rapida 74 four-color five-color plus light sheet-fed offset printing machine just introduced by Sino-German Printing Training Center.
â‘¡Paper: Korean Hansen 157g / m2 coated paper with high smoothness and good absorption capacity is adopted.
â‘¢Printing ink: CMYK ink provided by German Hostmann Steinberg (affiliated with the Amber Group) with strong coloring power, low strength and good rheological properties is used.
â‘£Fountain solution: Ace DINDH2010 fountain solution (pH value controlled between 4.8 ~ 5.2) provided by German Vigra company free of charge.
⑤PS version: The locally produced Jindongyang pattern PS version is used.
â‘¥Blanket: Use the special blanket provided by KBA.
⑦ Bag lining: Use the special polyester film base provided by Gao Company with smooth, smooth surface and uniform thickness.
4 Test process
(1) Original production We used digital manuscripts provided by the German KBA company. These digital manuscripts are stored on the disc in eps format. There are a total of 8 images, and the data size of each image is 83MB. We selected three representative images as test manuscripts, which are skin color image, wine image and furniture image. We didn't do any processing on the originals, we imported them directly into the group version of Pagemaker, and then called the specific output characteristic curve in the Delta workstation, and finally output the film on the Hercules imagesetter. Because the imagesetter can not output 300lpi and 150lpi on the same page at the same time, we may output 300lpi and 150lpi films respectively, and then use the manual imposition method to combine the 300lpi and 150lpi page information on the same page.
(2) Printing plate printing Before printing 300lpi dots, we did a series of tests, including PS plate resolution and testing to find a suitable exposure time. If the resolving power of the PS version cannot reproduce the smallest outlet, obviously it cannot meet the requirements. We use UGRA1982 printing proofing strips and test strips, and use the method of graded exposure to test the Jindongyang pattern PS version. After developing the PS version under normal development conditions, observe the reproduction of the yin and yang round lines used to test the angular force of the PS version on the UGRA1982 printing proofing strip (based on at least 505 for clear reproduction), and judge the PS version accordingly The resolving power is 6 μm. According to calculations, the diameter of 100% of the dots on a 300 lpi printed matter is 83 μm, and the diameter of 2% of the dots is 11.6 μm. It can be said that the PS version fully meets the test requirements.
Let's find the appropriate exposure time. Here we mainly consider the drying situation of 300lpi network cable. If the 2% dots and 98% dots of the 300lpi cable can be clearly reproduced, the 2% dots and 98% dots of the 150lpi cable can also be clearly reproduced. We also use the UGRA1982 printing proofing control strip to find the appropriate exposure time through the grading exposure method. In the process, we walked a detour. We first clean the glass on the plate holder with glass, wipe the 300 lpi signal strip film with alcohol (we measured the maximum solid density of the signal strip with a transmission density meter to be 5.6) and clean the PS plate with a clean gauze, and then expose it with 450 Beats, 380 exposure beats, 315 exposure beats for normal exposure of the printing plate (without using the supplementary exposure process), after normal development, it was found that more than 90% of the dots on the positive plate exposed at 450 exposure beats, and less than 10% Of the outlets have been lost; more than 80% of the outlets on the positive plate exposed at 380 exposure beats and 315 exposure beats are dead, and less than 5% of the outlets are lost. Obviously, such a printing plate can not meet the requirements of printing, mainly manifested in: the loss of gradation in the bright tone, and the unacceptable expansion of the dots under the influence of the printing pressure in the dark tone. Therefore, we must shorten the exposure time of the printing plate and reduce the maximum solid density of the film.
According to a German colleague, as long as the maximum field density of the original film is above 2.4, it can ensure that the original Internet point is completely transferred to the PS version. Therefore, we changed the output characteristic curve, set the maximum field density to 3.4, and output the 300lpi signal strip film for testing again, and then exposed the plate to normal exposure at 250 exposure beats, 300 exposure beats, and 380 exposure beats (same No supplementary exposure process is used), after normal development, it is found that 90% of the dots on the positive plate exposed at 300 exposure beats, 380 exposure beats, 250 exposure beats are clearly visible, and only in the positive plate exposed at 250 exposure beats The top 5% of outlets are still retained. Therefore, we set the printing time to 240 exposure beats.
(3) In the printing process, in order to achieve the ideal printing and copying effect, we replaced a new air cushion rubber cloth and special liner with good transfer performance before printing. The specific printing operation was jointly completed by Mr. Hasse, an experienced printing technician of KBA and the young backbone of the Sino-German Printing Training Center. During the printing process, Mr. Hasse impressed the Chinese counterparts with the German's unique perseverance. He sampled from beginning to end, carefully compared with the reference look, and obtained the most ideal printed matter as much as possible. The highest printing speed reached 10,000 sheets / hour.
5 Results analysis
By observing the comparison image after printing, we found that the difference between the 300lpi printed matter and the 150lpi printed matter is still very obvious, mainly in the following aspects.
(1) The presence or absence of rose spots When we looked closely at the printed matter, we found that the bright and midtone areas of the image with the screen number of 150lpi showed rose spots that are easily distinguishable by the human eye, making the printed matter look rough and color Lack of gloss, and rose spots are not visible on the 300lpi print. This rose spot is caused by the overprinting of multiple dot inks, and the diameter is usually 2 to 4 times the side length of the dot. The dot length of 150lpi is 167μm (1 × 2.54 × 10000/150), that is, the diameter of the rose spot is between 334μm and 668μm. When the clear vision distance is very small, the rose spot will be clearly visible, when the clear vision distance is very When it is large, the rose spot will be clearly visible. When the distance of the bright vision is large, the diameter of the rose spot is within the limit of the visual resolution of the human eye, and people cannot see the rose spot phenomenon. The side length of the dot of 300lpi is 83μm, and the diameter of the rose spot is also reduced to 167μm ~ 334μm. Due to its relatively small diameter, we cannot see the rose spot phenomenon even when the distance is small.
(2) The clarity of the printed matter The image clarity of the 300lpi printed matter is obviously better than that of the 150lpi printed matter, because the dot size of the 300lpi printed matter is only a quarter of the dot size of the 150lpi printed matter, that is to say, the original one dot The reproduced content is now reproduced by four small dots, which leads to an increase in the sharpness of the image gradation boundary and accordingly improves the clarity of the image.
(3) The color softness amplitude modulation screening process is to reproduce the change of the original layer and the change of color depth through the change of the dot size. In the current color printing field, we often scan originals at a scanning resolution of 300 lpi, and then output them on the imagesetter with 150 lpi of the number of hanging lines. That is to say, the first dot replicates the color information of the four ingested pixels. In fact, in most cases, the grayscale values ​​of these four pixels are different, but the computer will ignore the four pixels when the dot is generated. For the difference between gray scales, the gray scale average value of four pixels is taken as the gray value of the dot. This is the main reason why we think that low-line prints are not as soft as the original. If we use the output of 300lpi for the number of hanging lines, that is to say, each dot replicates the color information of one pixel, and the grayscale of the original can be copied one by one correspondingly. The resulting color is also softer and does not appear stiff.
(4) Hierarchical expressiveness of printed matter The hierarchical expressiveness of the printed picture is also the reproduction level of subtle levels, that is, the so-called texture is based on the resolution of subtle levels of the original image, but is also affected by other factors such as the number of screen lines. Because when the imagesetter is output with the number of hanging lines of 150lpi, its laser accuracy is 2400dpi (150 × 16, where the herbal 50 represents the number of screened lines, and 16 represents the arithmetic square root of the gray scale of 256), while the imagesetter has a hanging line of 300lpi In digital output, its laser accuracy is 4800dpi (300 × 16). Obviously, in the same area, the number of 300lpi dots is 4 times the number of 150lpi dots, and the size of a single 300lpi dot is one-quarter of the corresponding size of 150lpi dots, so the hierarchical expressiveness of 300lpi prints is significantly improved.
Kitchen Cleaning Tool,Reusable Household Glove,Disposable Kitchen Cleaning Cloths,Rubber Dishwashing Gloves
vchomy , https://www.vichenivchomy.com