1 Definition of lightly coated paper
Light-coated paper is a coated rough-processed paper with a lower specified amount. It is also called low-quantity coated paper, and its English name is LWC paper (Light Weight Coated Paper). It is a thin layer of coating on the front and back sides of the base paper. The coating is more uniform and consistent, with excellent rheology and water retention, and then super calendered to improve its physical properties and printability. Sex. The basis weight of the base paper is low, about 30 ～ 60g / ?, the coating amount is usually 5 ～ 10g /? On each side, so the basis weight of the light-coated paper is 40 ～ 80g / ?, which is the sum of the basis weight of the base paper and the coating amount on both sides . Widely used in color printing of general public products such as books, magazines, catalogs, advertisements, trademarks, etc. The printing effect is comparable to coated art paper to a certain extent, but the material cost is relatively low, and now well-known domestic paper manufacturers can also Mass production and put into the market, some characteristics of light coated paper are gradually understood and mastered by people.
2 Characteristics of lightly coated paper
1) The printing quality is better. In terms of paper performance, light-coated paper is between coated paper and offset paper, and has better opacity and smoothness, as well as paper gloss and printing gloss. It is printed on color products with low basis weight paper On the other hand, its printing effect is comparable to coated paper, which is its main characteristic that is different from offset paper. For general color prints, such as color inserts for books and periodicals, and color editions for newspapers and periodicals, all light coated papers with a basis weight of as low as 50 ～ 70g /? Can be used, which can achieve the usual printing effect of coated paper of 70 ～ 90g /? Accepted, favored by print and publishers.
2) Lower production cost. Because light-coated paper is a low-finish paper, the production is mixed with relatively cheap mechanical wood pulp and fillers, so the investment in raw material cost is not high, and the selling price is only about 5-10% more expensive than ordinary offset printing paper. Similar coated paper is about 10% cheaper.
3) The durability is poor, and the scope of adaptation is narrow. The base paper in the manufacture of light-coated paper contains a large amount of mechanical wood pulp, and the residual lignin and other compounds in the mechanical wood pulp will make the paper discolor, brittle and damaged, so it is more suitable for printing products that do not need to be stored and used for a long time. Books and periodicals such as cultural fast food, commercial promotion advertisements, etc. have more applications, and reference books and dictionaries are not suitable for printing on light-coated paper.
4) Lightly coated paper is not suitable for printing high-quality products. Affected by too much filler and too thin coating layer, it is easy to lose paper powder and poor surface gloss when printing (compared to coated paper), which is difficult to print high-end products. At the same time, because of its thin paper, more The accuracy of color overprinting is generally about ± 0.05 ～ 0.1mm. The thin lines and text of multi-color registration will inevitably have deviations, which are in a ghost state, which affects the quality of the product.
5) Lightly coated paper cannot be used as writing paper. Due to the limitation of its paper structure, when writing with ordinary ink, the handwriting is difficult to dry in time and has poor adhesion performance, which is one of its defects. [next]
3 Comprehensive consideration of pre-press platemaking is the key
As the saying goes, "Seven-point plate-making, three-point printing", the quality of the product has been roughly finalized in the plate-making, from the aspects of product quality requirements, printing machine production characteristics, light-coated paper printing suitability, production process application, etc. Comprehensive consideration, and strive to print high-level products, so pre-press plate making is the key.
1) Scan calibration. In the highlights of the printed image, generally 1 to 2% of the highlight area is almost impossible to print (or the dots are not full), that is to say, the highlight part of the printed matter is basically formed by the color of the paper. If the whiteness of the paper is different, it will affect the color brightness and saturation of the main bright part of the screen, which will affect the color contrast of the screen. The surface of light-coated paper has certain smoothness, whiteness and light reflection ability. Therefore, for light-coated paper, the black field calibration is roughly C = 85-955%, M = Y = 80-90%, K = 75- 80%. If there is no white field point on the image, you can use paper color as the reference point for adjusting the white field. It can be adjusted to less than 3% of the CMYK color dots. When there is output CTP, the calibration value can be appropriately expanded and widened.
2) Properly adjust the contrast, sharpening and halftone ratio of the image. The stronger the absorption of paper, the greater the dot expansion value. Specifically, the absorption of light-coated paper is greater than that of coated paper and lower than that of offset paper. During the plate-making process, it is necessary to compensate for the printability of the impact of the increase in dots. Generally, the high-tone and dark-tone dot increase values ​​of light-coated paper printing are 10 to 15%, and the mid-tone dot increase value is large, as much as 15 to 25%, so the image should be appropriately sharpened and moderated. The tone contrast adjustment makes the image clearer, which must be at the expense of the middle tone level of the image, so that the printed matter has good color and saturation, and the color is more vivid, which can reflect a certain level and contrast. The contrast between the light and dark junctions is increased, making the bright areas brighter and the dark parts darker to achieve the purpose of sharpening? Improve the viewing of pictures, and also facilitate printing and restoration. However, excessive contrast and sharpening will make the image too distorted, blunt and unnatural.
â‘ The printing suitability of light coated paper is between coated paper and offset paper, and the number of hanging net lines is set to about 133 to 175 lines when printing by lithography machine; when using good quality light coating paper, the number of hanging net lines can be up to 200 Line; Rotary machine printing time should be reduced to 133 ~ 150 lines. The products printed with light-coated paper are generally not very high-grade and have high quality requirements. When the screen printing is too high, it will easily lose powder and pile up ink, but it will not achieve the expected effect. At the same time, the printing plate has a low printing resistance. Cost and quality control outweigh the gains.
â‘¡ Set the requirements of the flat screen. The large flat screen set by light coated paper should be between 5 and 65%. The dark tone overprint in the CMYK color version is not easy to design too much, because the ink load on the surface of the thin paper is relatively small and it is thin itself, and the imprint is easy to pass through the paper surface; As a result, the dots at that location are enlarged too much and it is difficult to dry in time.
â‘¢In order to maintain the color brightness and gorgeousness of the image restoration, it is necessary to appropriately adjust the blue and black plates in the medium and dark tones and dark colors. On the premise of meeting the tone of the image, reduce the necessary component ratio, generally Reduce the value by about 5 to 10%, and the same can also moderately increase the value by about 8 to 15% for the medium and dark tones and the yellow and red editions in the dark colors.
3) Total ink setting. Due to the thinness of light-coated paper and the limitation of the production speed of the printing press, it is determined that the total amount of ink in the image cannot be too high. These must be adjusted in place during pre-press production. After actual production comparison, the parameters are adjusted accordingly. : The total ink volume of light-coated paper printing is generally controlled at 270-300%, of which 280-300% is printed by lithography machine and 260-290% by wheel transfer. The thinner the paper is, the smaller the total ink volume is, and it is difficult to remove the ink layer. In addition to being dry, it will also cause dirty, through-print quality accidents. The setting of this value is determined by the production process of each unit and the actual printing situation, and depends on the dot expansion rate during printing. If the dot expansion rate is too large (when the paper quality is ≥ 25%, such as some domestic B-grade light-coated paper), this value needs to be reduced, otherwise, the dark tone part is easy to be dead, and the medium and dark tone layer loss is serious; If the dot expansion is basically normal (such as imported light coated paper ≤ 15%), the value can be increased appropriately to ensure that the dark tone part is full of color, showing the dark tone regional level, which is convenient for the necessary reproduction of the entire tone.
4) Establish the necessary color management system, make good data records, be good at comparative testing, use multi-data analysis, and formulate suitable light-coated paper to print the output characteristic curve, so that it can be used for standard output. [next]
4 Master the characteristics and deal with printing
Paper is the most important substrate for printing, and the printability of the paper will directly affect the quality of the printed matter. The gloss, surface strength, whiteness, absorbency, smoothness and stretchability of the surface of lightly coated paper are slightly worse than that of coated paper. Some domestic lightly coated papers are prone to different degrees of de-powdering due to process defects. The expansion and contraction variation, especially the low-gram weight varieties are prone to curl, paper wrinkle and other defects, which affect the accuracy of overprinting. If improper operation, it will also cause quality defects such as stickiness, chalking, slow drying, and ink color under gloss. In actual operation, the printing characteristics of light-coated paper should be fully grasped, and the printing parameters should be determined according to its performance to improve the quality of the printed matter.
1) Influence of PH value
The PH value of domestic light coated paper is generally acidic. The printing process requires the pH of the paper to be neutral. However, due to differences in sizing materials, filler properties, and color composition during paper and pulping, the paper may be either acidic or alkaline. For example: when there is a certain amount of residual chlorine and organic acid in the sizing slurry used in the papermaking process, the paper will be acidic; during the papermaking process, excessive caustic soda, alkaline fillers and colorants, and pulp paper are used Will be alkaline. Most of the domestic light-coated papers we often come into contact with are acidic, which will inhibit the drying of the printed ink conjunctiva. The stronger the acidity, the slower the ink drying speed.
The acidity of the paper also affects the translucent color of the printed product and is generally dark. In addition, the acidic paper will accelerate the emulsification of the ink and cause the phenomenon of smudging, which seriously affects the quality of the product. When printing gold, silver, and bronzing (silver), when the paper is acid or alkaline, it will chemically react with the metal powder, discoloring the printed and bronzing electrochemical aluminum foil, and losing its original luster and color.
2) Influence of ink
From the perspective of printing technology, a large number of color books, newspapers, and periodicals papers tend to be printed with light coated paper. It is much higher quality than offset paper, but due to cost considerations, it is at a low basis weight (50 ~ 70g / ?) When printing ordinary color products on paper, it is inevitable to choose light coated paper instead of coated paper. In the mass printing of books and periodicals, most of them are completed by semi-commercial and commercial rotary presses. Ordinary books and periodicals are not competent because of the lack of heating and quick drying devices. Due to the slow drying of the ink layer, which affects the production schedule, at present, few domestic expensive UV inks are used to print light-coated paper, and more special inks are used to print.
â‘ When printing light-coated paper with semi-commercial and commercial rotary machines, high-speed thermosetting ink should be used instead of cold-setting ordinary ink.
â‘¡When printing light-coated paper on a lithographic four-color offset press, you can't use ordinary bright quick-drying ink at will, and you need to use special light-coated paper quick-drying ink.
Listed example 2: When I printed light-coated paper on a lithographic four-color machine for the first time, due to insufficient understanding of its characteristics, after printing on coated paper, it was not easy to change the special light-coated paper quick-drying ink, which caused frequent occurrences in the printing process. Dirty, especially the thick graphic part of the ink volume, although increasing the powder spraying, adding dry oil, and lowering the PH concentration of the fountain solution can not effectively control the quality of the product, the more terrible thing is after printing off the machine The semi-finished products were not completely dried after being baked and dehumidified for 3 to 4 days in the copy room, which seriously affected the post-printing production cycle and delayed the delivery. In the end, I had to borrow a coating glazing machine and irradiate the ultraviolet rays one by one quickly Drying, so as not to be able to dry the ink layer, the product can be transferred to the post-press department.
3) The influence of ink and ink balance
The standard of water-ink balance is: on the premise of ensuring the printing quality, the minimum water consumption is used uniformly, and a certain amount of anti-dirt water is left. The criteria used to identify the balance of water and ink with printed materials are: well-printed dots are restored, dots are not blurred, dots are not spent, and colors are restored well.
Since the special ink used for printing light coated paper requires good fluidity, there are certain strict requirements for the control of fountain solution. Take the common Heidelberg CD102 lithographic four-color machine as an example:
â‘ Properly increase the alcohol content of the fountain solution, increase the control force of the hydrophilic layer on the plate surface, which can prevent paste. The amount of alcohol added should be controlled within 15%. General operators will think that the larger the amount of alcohol added, the less dirty the plate will be. Therefore, the amount of alcohol is always too large, but it is not true. When the amount of alcohol is too high, it will make the surface of the paper dissolve more, the printed paper is prone to increase the paper powder and the ink is easily over-emulsified, and the printed product will have flower spots and color saturation Phenomenon that affects print quality such as decline.
â‘¡The axial blowing amount of dampening mechanism should be increased appropriately to prevent the emulsification of ink caused by the increase of alcohol, and the product quality is difficult to control.
③If the amount of fountain solution is too large, the ink is easily emulsified, the ink color is not easy to control, and the phenomenon of flicking and flying ink will occur, polluting the water bucket and the water roller, the ink roller does not apply ink, and the ink is easy to accumulate on the blanket . When the two values ​​of PH value and conductivity value are within the range of 5.3 to 5.7, the printing is most stable, the printing plate has a high printing durability, the best printing quality, and the ink and ink balance are most easily controlled. The PH value is not only determined by the amount of fountain solution added. It is also affected by the acidity and alkalinity of the paper. The conductivity should generally be controlled at 1400 ± 200μs. The conductivity value is susceptible to the hardness of water, the amount of alcohol, dust and ink inclusions, so the conductivity value of 800 ~ 2000μs is allowed, but when the fountain solution seems to have been obviously When contaminated, it must be reconstituted.
â‘£ Before each printing, the pH value, conductivity value and alcohol concentration of fountain solution should be tested.
⑤ Excessive moisture. In addition to being prone to ink emulsification and false imprinting, it also causes light-coated paper to absorb too much water and become deformed, and the fit is not accurate. This is a special concern when printing on light-coated paper, whether it is necessary for lithographic and rotary printing machines. Control moisture.
4) Influence of equipment
â‘ Equipment maintenance is very important. In addition to daily routine maintenance and maintenance, after printing 50 to 60 orders of light-coated paper (especially domestically produced paper), the pipeline of the dampened water bucket should be cleaned and dredged in time. If it is printed for 3 to 4 working days in a row Coated paper, water bucket fluid should be replaced with new fluid in time to ensure that such higher-end printing presses maintain intact and stable technical performance.
â‘¡ Clean the rubber and ink roller frequently. The structure of light-coated paper contains a certain amount of filler, because the density of the coating layer is far less than that of coated paper? Therefore, the powder dropping phenomenon is more serious (domestic light-coated paper is more obvious), and the rubber and ink roller are also one of the ways to improve the printing quality Necessary measures.
â‘¢ Adjust the feed of Feida. Due to the thin paper, double (multiple) and crooked sheets are susceptible to normal use of the equipment.
5) Do not easily replace commonly used lightly coated paper
Necessary testing should be carried out when the materials are put into the factory and storage, and long-term brands and suppliers should be determined for commonly used materials, and cooperative and mutually beneficial partnerships should be established. Do n’t change frequently for cost, which makes it difficult for operators to adapt, so that quality and efficiency There is no effective guarantee. For each batch of light-coated paper, before being put into use, a series of printing suitability issues such as smoothness, acidity, alkalinity, absorbency, dryness, stretchability and surface bonding strength of the paper should be tested and analyzed, and targeted Corresponding technological measures, such as adjusting the viscosity, fluidity and anti-sticking measures of ink, establishing relevant basic data, and arranging production rationally and scientifically.
Listed example 1: A store entrusted the unit to print its regular weekly color promotional newsletter. Due to the large volume (100 reprints per period), it was printed on a well-known light-coated paper for a long time. In order to save costs, the price was temporarily changed to low (per (The paper can be reduced by about 20 yuan.) The light-coated paper of a factory was printed. Due to paper powder and paper wrinkles, the printing machine failed to adapt. As a result, two additional shifts were used on the imported four-color machine for two consecutive periods. Under printing, the paper consumes more than 6 reams of paper than usual. After comprehensive comparison and calculation, the savings in paper prices are far less than the loss of materials and depreciation. More importantly, due to printing problems, product delivery has been delayed and cooperation has been lost for many years. The trust of our customers really picked up the sesame seeds and threw away the watermelon. [next]
5 Notes on post-press processing
The post-printing requirements of light-coated paper are basically the same as offset paper and coated paper, but there are some requirements that should be noted.
1) Lightly coated paper is brittle and not durable. When folding, the pressure of the folding roller is not easy to be too large, and at the same time, it is necessary to prevent the paper from scratching and folding.
2) Poor drying. When binding a book into a book, bundling and flattening the book block and cutting the finished product, pay attention to whether the book pages are accidentally sticky or dirty, and the inspection strength should be increased. When glazing or laminating is required after printing, more attention should be paid to the drying of the ink layer. Generally, there are methods such as dehumidification and heating and drying after printing.
3) Easy to absorb moisture and deform. After receiving the printed semi-finished products, it is necessary to take moisture-proof measures, place them in a dry place, add a plastic cover, etc., to reduce the phenomenon of ruffles and elongation deformation of the printed products.
4) When gluing books and periodicals, use hot-melt glue packs with strong fluidity, penetration, and viscosity, and increase the depth and spacing of the slot to increase encryption to prevent quality accidents such as glue opening and page dropping.
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