Test of printability test (bottom)

Third, sample test point selection and test content

The selection of sample test points is determined according to the needs of the evaluation. The color and monochrome images of Figures 1 to 8 are visually evaluated according to the characteristics of the natural images. The specific items include the evaluation of the main features, levels, screens, visual effects and others. In several items, the resolution icons and gradient color bars are still based on visual evaluation in composite images, and parameters can be given as quantitatively as possible. Items include: resolution, overprint, moiré, blur, smoothness, gray balance, gradient effects, etc.

The chromatogram synthesis image is the focus of the instrument measurement, that is, all the measurement parameters are from the chromatogram synthesis image, and the test items include the density, dot gain value, trap rate, strike density, dot reproduction, chroma value, K value, etc. ( The specific points are more complicated than one.) The evaluation of texts, spot color screens, geometric dimensions, appearance, and other items were attributed to other projects, using rulers or visual inspections. Since there were 120 lpi, 150 lpi, and 175 lpi proof sheets in all five papers tested, we decided to use the best 150 lpi proof sheets as the test proofs. All data were from these proof sheets.

In short, the comprehensive assessment of proofs is our concluding observations based on the results of various tests. We insist on seeking truth from facts, and strive to objectively, fairly and accurately reflect the actual situation, for your reference.

Analysis of test results:

Papermakers often control many aspects of performance for book printing paper. To meet printing requirements, it is necessary to control the print adaptation performance of a series of papers. In addition, it is also necessary to control specifications, such as quantification and size errors; if it is to be adapted to writing performance, it is also necessary to control the water-absorbing performance; in order to enable printed books and periodicals to be read without breaking, it is also necessary to control Various strength indexes, such as folding resistance, tensile strength, and tearing degree, etc.; In order to make the printed product clean and beautiful, it is also necessary to control the dust index, whiteness, color, and other optical performance indicators. However, as the printing paper, the most important thing is the printing adaptability index.

The so-called "printability" of paper is a general term for ensuring that the printing paper can smoothly pass through the various paper properties where the printing machine does not suffer from printing failures, and the performance of various papers that guarantee a good print quality. For non-coated offset book paper, the performance to ensure smooth passage of the press is mainly the thickness uniformity (usually expressed as the banner quantitative difference), dry and wet printing surface strength, paper loose material, moisture content and scalability indicators. The properties of achieving good print quality include paper smoothness (roughness), ink absorption and opacity. In addition, the tightness also indirectly affects the quality of printed products, because in the case of the smoothness of the paper surface, the smaller the sheet tightness, the better the compressibility of the paper surface, the printing plate and the paper under a certain printing pressure during printing. The better the contact of the surface, the better the quality of the print.

Analyze and compare 5 samples of light non-coated offset printed papers No. 1 to No. 5 of this comparative test, which can basically meet the actual printing requirements in guaranteeing the performance of the smooth printing press. 2.4%, experiments show that no more than 5% can meet the printing requirements. In terms of dry printing surface strength, experiments have shown that when the determination result of medium-viscosity napped oil reaches 0.9 m/s, it can meet the requirement of 0, 000 sheets/hour offset printing at the offset printing speed. The results of the measurement of each sample were only slightly lower on the 2nd and 4th, but if the result of the measurement by Huatai Company prevailed, the measured value could still reach about 1.0 m/s. Therefore, in the actual printing of each sample at a printing speed of 08,000 sheets/hour, the problem of lint falling due to the low strength of the dry printing surface should not occur. As for the wet printing surface strength, ie, the wet-shearing index, the measurement results of the two units are quite different. Generally speaking, wet-greasing of the offset paper is greater than 0, which may cause off-gassing off-grinding (IGT measurement method). The measurement results are all less than 0. The measurement results of Huatai Company have 3 samples greater than 0, and No. 1 is even as high as 11.6%. Only the actual printing effect can be seen. If the problem of lint falling powder occurred in the actual printing No. 1, it indicates that Huatai's measurement results are more sensitive and reliable. Of course, the cause of the dirty version of the printing is the problem of loose paper. If the surface of the paper is subjected to static electricity during the production process of calendering, slitting, and rewinding, dust paper dust in the air is adsorbed, so-called "paper loose material", even if the wet and dry surface strength is high, it will be due to The dust confetti was transferred to the blanket causing a printing failure. The paper surface loose material can be tested by IGT method. If the measurement result reaches level 2, it may cause the printing dirty plate. If it is greater than 2 level, it will definitely cause the printing failure. This time, neither of the two units measured this index. If none of the samples had any problem with the grime plate during printing, this means that there is no problem with paper loose material in each sample.

The consistency of the moisture content of the printed page and the humidity conditions of the printing shop is another important condition for ensuring the smooth passage of the paper through the printing press. If the humidity of the printing shop is controlled at about 60%, and the moisture content of the paper sheet is controlled at about 6%, the paper sheet will be guaranteed to be flat, no pleats will be printed during printing, and overprinting will not occur. The test item of the paper sample did not measure the moisture content, so it could not be evaluated. However, as long as there was no paper unevenness and printing pleats or overprinting problems in the actual printing, the paper sheet moisture and the humidity in the printing shop were consistent. . As for the scalability index, this requirement for multi-color continuous printing has been less stringent. Although the standard for offset printing paper stipulates that the transverse stretchability is not more than 2.5%, 0 is 2.8%. As long as the moisture content is not too high or too low, the problem of overprinting is not likely to be caused. Tests have shown that the most important reason for the inability of four-color continuous offset printing is not the flexibility of the paper, but the inconsistencies between the moisture content of the paper and the humidity conditions of the printing shop. With respect to the wet repellency performance, since this printing failure does not occur in non-coated offset book paper, it is generally not necessary to perform evaluation control. However, for coated offset printing paper such as coated paper, this is one of the important printing suitability that must be assessed.

In terms of ensuring good print quality, there are large differences in the performance of the five samples, among which the important performance indicators are smoothness, and the order of the five samples is: No. 2> No. 4> No. 3> No. 5> No. 1 The higher the smoothness, the better the printing effect. However, as mentioned above, the tightness of the paper sheet also indirectly affects the printing effect. That is, the smaller the tightness, the better the compressibility of the paper surface, the better the printing plate and the paper surface contact effect under the printing pressure, and the higher the quality of the printing product. high. Therefore, although the smoothness of No. 4 is not the highest, its quality is better than No. 2 because its tightness is significantly lower than No. 2. From 5 copies of printing dot reproducibility and printing tone reproduction effect, the order of goodness and badness is: 4 is better than 2 is better than 3 is better than 1 and 5. Of course, there is another reason why No. 4 printing works well, that is, its ink absorption is the lowest. For offset printing paper, the lower the ink absorbability, the lower the amount of ink permeation to the paper before the ink layer is cured, the higher the density and gloss of the paper ink layer, and the better the color reproducibility, ie, the paper surface. The better the "ink support" is. However, for these non-coated light offset book papers, because the ink absorbency is much higher than the level that can produce a good print gloss (both higher than 40%), even at the 4th, the print gloss is still very low, only About 10%. Therefore, for non-coated printing papers, it is obviously meaningless to assess the print gloss. Only coated printing paper, such as coated paper, coated with light after the press, the ink absorption reduced to about 20%, in order to obtain a good print gloss, such as coated paper printing gloss is generally more than 90%. Of course, for the coated printing paper, it is also necessary to control the ink absorptivity not too low, otherwise there will be a problem of stickiness caused by too slow ink drying speed.

There is another indicator for assessing the quality of printed products, that is, the degree of print penetration, especially for double-sided prints, the problem of print penetration is even more important. The main paper properties that determine the degree of print penetration are opacity, and the higher the opacity, the lower the degree of printability. However, the ink absorption of the paper also has a certain influence on the print penetration of the printed product. The stronger the ink absorbability, the greater the depth of penetration of the printed ink into the paper, and the higher the degree of print penetration of the printed product. The result of this test was No. 1 but the opacity of No. 4 was only 1.0, but the printability of printed samples was significantly higher than that of No. 4. The reason for this may be that the ink absorption of No. 1 was significantly higher than that of No. 4. , thereby increasing the degree of transparency. Comparing the measurement results of each sample, the opacity measured in the laboratory and the print penetration degree obtained by the printing test show a correlation that is fully consistent with the law, that is, the more severe the paper is, the lower the opacity of the paper is. The order in which the five samples were arranged to reflect the severity of strikes was: No. 4 is better than No. No. 2 is better than No. 5 is better than No. 3.

In summary, the printability test of major light papers in the Beijing market, organized by the Publications Association's Publishing Materials Working Committee, passes the measurement results of the two laboratories and the quality of the actual prints printed at Beijing Ruibao Picture-in-Picture Printing Co. Based on the analysis and comparison, the following conclusions can be drawn for reference:

(1) Although the measurement results of the two laboratories have large differences in individual performance indicators, there is good correlation and good and bad consistency between the laboratory test results and actual printing results and print quality.
(2) If 5 samples did not show the problem of scumming and stencil printing in the actual printing, and the print failure of the stencil of the uneven printing of the sheet, it can be said that 5 samples did not cause a printing failure in the smooth printing press. The performance indicators are all in accordance with the printing requirements; a comprehensive evaluation is performed on the performance indicators that guarantee the quality of the prints, ie, the laboratory test results and the quality of printed products such as dot reproducibility, print tone reproduction effect, and print visibility In comparison, No. 4 is the best, No. 2 is the second, No. 3, No. 1 and No. 5 are again. Proof of the sample According to the proofs provided, the preliminary analysis results are as follows: The best sample should be number 4: the dots are solid, uniform, and small in deformation. Followed by No. 2, No. 0, No. 5, No. 1. No. 3 outlets are evenly distributed, but the overall situation is weak. The main reason is that the pressure is insufficient. If the pressure is increased, the effect may be close to No. 4. The above results are for reference only. If the process conditions change, the results may change.



Source: Printing Quality and Standardization