1 Introduction
The quality of print reproduction is mainly evaluated from two aspects of image color reproduction and tone reproduction. Tone-level copying mainly depends on the number of dots in a unit area and the number of consecutive changes in the dot network from small to large. The more number of dots in a unit area, the more dots in the copy area, the more clearly printed copy. The higher the degree, the richer the level, and the more realistic the color; the more dot progressions, the stronger the expressive level of the printed matter, and the more delicate it looks. In the printing production process, people control the number of dots in a unit area by changing the number of screen lines. The higher the number of screen lines, the greater the number of dots in a unit area and the smaller the size of the reproduced dots. At present, the commonly used coated paper lines are between 150 and 175 lpi. However, with the continuous improvement of the level of people's appreciation, this kind of precision print can no longer meet people's visual needs. Therefore, more and more printing companies use high-line technology to obtain prints with more layers, higher definition, richer colors, and copy effects close to the original to meet people's needs for high-end albums and famous packaging.
As one of the important contents of the printing seminar held by Hefei Sino-German Printing Training Center and the German KBA company held in Hefei and the Gaobao Equipment Exhibition, the printing test of 300 network cable printing products was conducted. With the help of the technical staff of KBA, we completed a printed comparison test of 300 lpi prints and 150 lpi prints.
2 test analysis
As we all know, the dot enlargement in the printing process (including the mechanical dot enlargement and the optical dot enlargement) is an inevitable phenomenon in the printing mode that realizes image transfer through pressure. The size of the dot gain value is usually proportional to the sum of the perimeters of the dots within the unit area. For the same network point value, the sum of 300 lpi network perimeters is larger than the sum of 150 lpi network perimeters, which means that the 300 lpi network expansion rate is larger than the 150 lpi network expansion rate. For this reason, we need to test and make a suitable output characteristic curve before the output of the printing plate, so that it can be called when it is output.
The 2% dot diameter of 300 lpi is approximately 11.6 μm, which is almost equal to the diameter of the dust particles. Therefore, the dust that falls on the glass or film of the printing plate can easily be exposed to sunlight, forming a so-called “pseudo dotâ€, which interferes with printing. The normal conduct of the process, so we should strictly in the PS printing process to ensure clean. In addition, PS plates should be used to develop the PS plate, and to ensure that the development process is stable.
The selection of test materials is also more stringent. We should take full account of the relatively small features of the 300 lpi print outlets and select the appropriate printing materials.
In the printing process, the printing workers are required to have a very good professional level and good professionalism, to achieve "three services", and to maintain a good balance between the ink and the printing process.
3 test equipment and test materials
1Printing equipment: The Chinese-German printing training center has just introduced the KBA Rapida 74 four-color five-color plus light sheetfed offset press.
2 paper: Korean Hansen 157g/m2 coated paper with high smoothness and good absorption capacity was used.
3 Printing inks: CMYK inks supplied by Hostmann teinberg (subordinate to Amber Group) in Germany are provided for their strong coloring strength, low strength and good rheological properties.
4 fountain solution: using Ace from Vega, Germany
DINDH2010 fountain solution (pH is controlled between 4.8 and 5.2).
5PS version: The locally produced Jin Dongyang graph PS version.
6 Blanket: Special blankets provided by KBA.
7 Lining: Special polyester bases with smooth, flat and even thickness are provided by the company.
4 Test process
(1) Original production
　　 We used digital manuscripts provided by the German KBA. These digital manuscripts were stored on the disc in eps format, for a total of 8 images, each with a data volume of 83 MB. We selected three representative images as test originals, namely skin color images, wineware images, and furniture images. We did not do any processing on the originals. We imported them directly into the composition of the Pagemaker. Then we called the specific output characteristic curve on the Delta workstation. Finally, we output the film on the Hercules photo-composing machine. Because the imagesetter can not achieve 300lpi and 150lpi output on the same page at the same time, so we may output 300lpi and 150lpi film, and then use the manual imposition method to combine 300lpi and 150lpi page information on the same layout.
(2) printing plate drying
We conducted a series of tests before drying 300 lpi dot prints, including the resolution of the PS plate and the test to find the right exposure time. If the resolution of the PS plate cannot reproduce the minimum dot, it obviously cannot meet the requirement. We used the UGRA1982 proofing proofing control strip and applied the graded exposure method to test the Jindongyang pattern PS version. After developing the PS plate under normal developing conditions, observe the reproduction of the yin and yang circular lines used to test the PS plate angulation force on the UGRA1982 proofing test strip (according to at least 505 clear reproduction), judge the PS plate accordingly The resolution is 6 μm. According to calculations, the diameter of a 100% dot on a 300 lpi print is 83 μm and the diameter of a 2% dot is 11.6 μm. It can be said that the PS plate satisfies the test requirements completely.
Let's find the right exposure time. Here we mainly consider the drying conditions of 300 lpi cable. If 2% dots and 98% dots of the 300lpi cable can be clearly reproduced, the 2% dots and 98% dots of the 150lpi cable must be clearly reproduced. We also use the UGRA1982 proofing control strip to find the right exposure time by means of a graded exposure method. In this process we walked a detour. We first used glass to clean the glass on the printing plate, wiped 300 lpi strips with alcohol (we used a transmission densitometer to measure the maximum solid density of the signal strips to 5.6) and cleaned the PS plate with a clean gauze, and exposed them to 450, respectively. Beat, 380 exposure beats, 315 exposure beats on the normal exposure of the printing plate (without using the exposure process), after normal development found: in the exposure of 450 exposure exposure of the positive version of the plate more than 90% of the network paste, 10% of the following The outlets have been lost; more than 80% of the dots on the positive version with 380 exposure beats and 315 exposure exposures have been pasted, and 5% or less of the outlets have been lost. Obviously, this type of printing plate can not meet the printing requirements, mainly reflected in the loss of light levels, dark tone in the role of printing pressure will appear unacceptable dot gain phenomenon. Therefore, we must shorten the exposure time of the printing plate and reduce the maximum solid density of the film.
According to a colleague from Germany, as long as the maximum solid density of the original film is above 2.4, it can ensure that the original version of the Internet site is completely transferred to the PS version. Therefore, we changed the output characteristic curve, set the maximum solid density to 3.4, output the 300 lpi signal strip for testing again, and then normal exposure of the printing plate with 250 exposure beats, 300 exposure beats, and 380 exposure beats. No supplemental exposure process was used. After normal development, it was found that 90% of dots on the positive plate with 300 exposure beats, 380 exposure beats, and 250 exposure beats are clearly visible, and only the positive version exposed with 250 exposure beats The top 5% of outlets still remain. Therefore, we set the exposure time to 240 exposure cycles.
(3) Printing process
In order to achieve the desired print reproduction effect, we replaced the new air-cushion blankets and special liners with good transfer performance before printing. The specific print job was completed by the experienced printing technician Hasse of the German KBA and the young backbone of the Sino-German Print Training Center. During the printing process, Mr. Hasse impressed his Chinese counterpart with the uniqueness of the Germans. He carefully sampled and compared the references from the beginning to the end, and drew the best possible print. The highest printing speed reached 10,000 sheets/hour.
5 results analysis
By observing the contrast images after printing, we found that the difference between 300 lpi prints and 150 lpi prints is still very obvious, mainly in the following aspects.
(1) Whether there is rose spot
　　 When we looked at the prints at close quarters, we noticed that the highlights and midtones of the images with 150 lpi screens showed rose spots that were easily recognizable by the human eye, making the prints appear rough and lacking in luster, while the 300 lpi prints Can not see the rose spots. This rose spot is caused by the overprinting of dots of ink on multiple dots, and the diameter is usually 2 to 4 times the dot length. The side length of the 150 lpi mesh is 167 μm (1×2.54×10000/150), which means that the diameter of the rose spot is between 334 μm and 668 μm. When the photopic distance is small, the rose spots will be clearly visible when the photopic distance is very good. When large, the rose spots will be clearly visible. When the distance between the eyesight and the eyes is large, the diameter of the rose spots is within the limit of the visual resolution of the human eye, and people cannot see the phenomenon of rose spots. The side length of 300 lpi is 83 μm, and accordingly the diameter of the rose spot is also reduced to 167 μm to 334 μm. Due to its small diameter, we cannot see the rose spot phenomenon even when the distance is small.
(2) Sharpness of printed matter
The image sharpness of the 300 lpi print is obviously better than that of the 150 lpi print because the dot size of the 300 lpi print is only a quarter of the 150 lpi print dot size, that is, the content originally reproduced by one dot is now composed of four Small dots reproduce, leading to increased sharpness of the image hierarchy boundaries and correspondingly improved image sharpness.
(3) Color softness
The AM screening process reproduces the gradation changes and the shades of color of a manuscript by changing the dot size. In the current color printing field, we often scan originals with a scanning resolution of 300 lpi, and then output the images on an imagesetter with a number of 150 lpi hanging lines. This means that the first dot duplicates the color information of the four ingested pixels. In most cases, the gray values ​​of these four pixels are not the same, but the computer will ignore four pixels when dots are generated. The difference between the gray levels, taking the four pixel gray average as the grey point value of the dot. This is also the main reason why we feel that the low screen printing is not as soft as the original. If we use the 300 lpi hanging line number output, which means that each copy of a pixel's color information, the original gray scale can be copied one by one, the resulting color is also relatively soft, not significantly stiff.
(4) The level of expression of print
The level of expression of the print screen is also the level of subtle level of reproduction, that is, the so-called texture based on the resolution of the subtle level of the original image, but it is also affected by other factors such as the number of screen lines. Because when the imagesetter is outputting at 150 lpi, the laser accuracy is 2400 dpi (150×16, where herbal 50 represents the number of screens, 16 represents the square root of the grayscale 256), and the photosetter is 300 lpi. When the number is output, its laser precision is 4800dpi (300×16). Obviously, the number of 300 lpi dots in the same area is 4 times the number of 150 lpi dots, and the size of a single 300 lpi dot is a quarter of the corresponding 150 lpi dot size, so the level expressiveness of 300 lpi prints is obviously improved.