Analysis of key factors affecting offset quality
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High-quality prints are the ultimate goal of print work. Printing quality mainly refers to the comprehensive effect of various characteristics of printed matter, that is, absolute quality. On the other hand, it must be in accordance with customer requirements, and the printed matter is consistent with the customer sample. This faithfulness to the original copy is called relative quality. There are many factors affecting the quality of printed matter. This paper analyzes the three key factors affecting the quality of printed matter, such as the printability of paper and ink, prepress technology and reproduction technology, and expounds the intrinsic relationship between them to improve the printing quality.
First, the printing suitability of paper and ink
1. The effect of paper performance on printed matter
The quality of the print is highly correlated with the type of paper. Specifically, it is related to five aspects such as surface smoothness, ink absorption, strength, and pH of paper.
1.1 The influence of the smoothness of the paper on the quality of the print.
The smoothness of the paper actually appears as the size of the gap on the surface of the paper. Therefore, the smoothness of the paper has a great influence on the quality of the printed matter. The smoothness of the paper is high, and the layout of the printed matter can be faithfully reproduced, especially the small dots can be accurately restored. The paper with low smoothness has a concave shape on the paper surface, which makes the dot printing unclear and the layout of the printed product is not clear. Paper smoothness and thickness uniformity have a direct impact on whether print pressure and ink color remain uniform. However, to ensure the smoothness of the paper, the method of filling and calendering is mainly used, so that the tightness of the paper is improved, and the elasticity and the ink absorbing property are relatively lowered, so that the back surface of the printed product is liable to be smeared. In addition, when the printing pressure is too large, the ink layer is easily spread and the print is expanded and deformed. Based on these conditions, the unevenness of paper smoothness and the difference in coarse light should be classified separately, so that when printing smooth paper with poor smoothness, a clear imprint can be obtained by appropriately increasing the printing pressure and the amount of ink, thereby improving printing. quality.
1.2 The influence of the ink absorbency of the paper on the quality of the print.
The transfer of the ink is accomplished by capillary action consisting of pressure and interwoven spaces of the paper fibers. That is, when the paper is pressed, the interlaced fiber gap is compressed, and after the pressure disappears, the surface of the paper is restored to its original state due to the elasticity, and the ink is adsorbed at the same time, which is the process of forming the ink layer. Various types of paper have different ink absorbing properties due to the tightness of the fiber structure. Paper with good ink absorption has a light printing pressure and a high pressure resistance of the printing plate. If the paper is too slack, the fiber gap is too large, then when the viscosity of the ink is large, there will be a problem of pulling the paper; otherwise, when the viscosity of the ink is small, the connecting material is easily infiltrated, causing a phenomenon of offset. This should be done by increasing the viscosity of the ink to suit the printability of the paper. Therefore, paper with tight paper and low ink absorption is mostly used for color printing, which can avoid the pigment falling off after the infiltration of the binder in the ink, and the loss of gloss of the ink layer affects the printing quality, but at the same time, it is easy to produce slow drying and easy sticking. Dirty phenomena such as dirt and crystallization.
1.3 The effect of the PH value of the paper surface on the quality of the print.
The acidic paper reacts with the polyvalent metal salt in the desiccant to form a product that is insoluble in the oil and fat, prevents the oxidative conjunctiva of the ink, and inhibits the drying of the ink, so the desiccant should be appropriately added during the printing of the acid paper. Neutral or alkaline paper facilitates ink drying. The pH of the paper surface is high (alkaline) and the ink dries quickly on the paper. For offset printing, the acidity and alkalinity of the printed paper also affects the pH of the fountain solution. Therefore, the drying speed of the printing ink is affected, and the printed matter is poor in drying and the back is dirty. At the same time, the acidity of the fountain solution is too strong, which will inevitably accelerate the corrosion rate of the printing plate and affect the reproduction of the tone and level of the print. When the pH of the printing paper is too high, that is, the alkalinity is too strong, during the printing process, the alkaline substance in the paper is continuously dissolved and transferred to the dampening solution, and neutralized with the weakly acidic dampening solution. The pH of the dampening solution is changed. In addition, these alkaline substances also reduce the interfacial tension at the ink interface of the printing plate, promote the emulsification of the ink, cause the non-image parts to be dirty, the gloss of the printed products is lowered, and the color of the ink is dim. The stronger the acidity of the fountain solution, the greater the effect.
1.4 The influence of paper formation on the quality of printed matter.
For paper, the part that directly absorbs the ink is the coating layer, but when the uniformity of the paper is poor, it will cause the unevenness of the penetration of the adhesive in the coating into the original paper layer, thus naturally causing the coating layer to absorb the ink. Uniformity. In addition, the poor uniformity of the base paper may also cause unevenness of the coating during the coating process, especially the uniformity of the coating during the coating of the blade has a great correlation with the uniformity of the base paper, so the unevenness of the base paper Sexuality often causes the coating layer to absorb ink unevenness.
1.5 The influence of paper surface strength on the quality of printed matter.
The surface strength of the paper is not high, and there will be hair loss and powder falling during printing. These detached materials not only paste the plate, but also leave white spots on the paper where there should be ink. The imprinted ink film will be uneven, even dull, seriously affecting the quality of the printed matter.
2. The effect of ink performance on print quality
2.1 The effect of the color performance of the ink on the quality of the print.
The color performance of the ink is directly related to the color saturation of the printed color and the accuracy of the hue. There are four main parameters for measuring ink color reproduction:
● The color intensity of the ink, which can be called the color density, is a representation of the ink coloring ability, which affects the neutral gray balance and the accurate reproduction of the intermediate and complex colors.
● Hue deviation rate is the percentage of the corresponding amount of complementary absorption of the ink that interferes with the basic hue.
Grayscale is the ratio of the total amount of absorption of the ink to the three zones of light that is supposed to be completely reflected, that is, the percentage of the maximum reflection density of the ink to the equivalent absorption zone of the spectrum.
●The color rendering efficiency is expressed as the percentage of the actual reflected light of the ink to the ideal reflected light, which is a parameter that comprehensively reflects the size of the ink to absorb.
The above four parameters are closely related to the hue, tinting strength, density, fineness, activity, solubility, oil absorption and hiding power of the selected pigment.
2.2 The influence of the rheological properties of the ink on the quality of the print.
The rheological properties of the ink mainly include viscosity, viscosity, fineness and adhesion.
● The greater the viscosity of the ink, the greater the force required to peel the ink layer. Due to the unevenness of the ink transfer, if the ink is too viscous, the surface of the paper is easily peeled off by the ink film during printing, that is, the hair is pulled. In the case where the viscosity of the ink is too large, it is usually adjusted by adding a diluent solvent or mineral oil.
● When the viscosity is too large, the whole ink system is in a super-saturated state, the pigment dispersion is uneven, and agglomeration and accumulation are caused, resulting in poor ink transfer effect and fluidity, so that the pigment cannot smoothly enter and exit the mesh, or even not transfer at all. . When the viscosity is too low, it indicates that the organic solvent content in the ink is large, and the components such as the resin and the pigment are relatively small, so that it cannot form a smooth mask layer upon drying, resulting in a dull hue, a lack of luster, that is, a decrease in color saturation.
● After the ink is printed, the imprint is not real, there are granular, the gloss is dim, and the printing plate has a low printing durability. The reason for this phenomenon is mainly that the fineness of the ink is not suitable. The fineness of the ink affects both the quality of the printed product and the printing durability of the printing plate during the printing process. The higher the fineness of the ink, the clearer and fuller the printed dots, the higher the concentration of the ink, the stronger the tinting strength, and the higher the quality of the printed product. On the contrary, the ink has low fineness, large printing friction coefficient, uneven ink dispersion, easy paste, and reduced printing durability of the printing plate.
● The adhesion of the ink is too large, and it is prone to hair pulling. In the high-speed printing process, it is often the case that some of the paper fibers are pulled up, that is, the printed paper is pulled. The formation of the paper pulling phenomenon is mainly related to the viscosity of the ink and the tightness of the paper.
2.3 The effect of the emulsification performance of the ink on the quality of the print.
The emulsification performance of the ink mainly affects the ink balance in printing. The characteristics of offset printing determine that the ink must have a certain water content. When the proportion of ink changes, the emulsification rate of the ink will also change. When the amount of water supplied is small, the fountain solution into which the ink is added is completely absorbed, and the emulsification rate rises rapidly. For the actual production, it is desirable that the supply ratio of the amount of ink can reach the requirement of maintaining the emulsification rate stable, and is convenient for quality control in the printing process. Otherwise, emulsification of the ink can cause changes in ink delivery and distribution, affecting production. The ink balance is just right, the ink can be transferred ideally, so that the dots are smooth, tidy, the image is clear, the layer is rich, the color is bright, and the printing accuracy is ensured, the print is dried, and the blank is ensured. Partially clean and tidy. If the water supply and the amount of ink in the layout are out of balance, the blank part of the paper will be dirty, the print will be dull, and a wide range of ink emulsification will seriously affect the quality of the print.
2.4 The effect of ink drying performance on the quality of printed matter.
If the ink is not dry enough, it will cause a dirty malfunction. In the printing process, due to the slow drying of the ink, the ink is too soft, too thin or the amount of ink is too large, which may cause the paper image to be dirty or the back surface to be dirty. Most of this phenomenon occurs in the field where the amount of printed ink is large or the density of the layer is high (ie, dark). It is usually caused by friction and smear and in the late drying of dryness. According to the time of generation, the sticky dirt can be generally divided into: the second side is imprinted and sticky (that is, when the back side is printed immediately after one side is printed, the result is the other side of the image on the back side) and all the printing is in contact at the time of binding. The ink on the surface is contaminated with each other.
2.5 ink light resistance.
Under the action of light, the color of the printed product will gradually recede. Generally, the light-colored ink is more obvious, such as yellow. The reason is that the molecular structure of the ink contains more unsaturated bonds, especially the molecular structure of the pigment changes. Caused. Therefore, the ink should be considered when it is selected. Otherwise, the fading of the product will affect the quality of the product.
Second, prepress technology
1. The effect of image scanning on print quality
The most basic job of printing is the reproduction of colors, which are often expressed through pictures. Image scanning is to convert the color information on the image into a storable and editable digital signal through the scanning device, and then process the digital signals through application software such as image processing software and typesetting software. Therefore, whether or not the image can be accurately and accurately reproduced, that is, the information such as tone, gradation, contrast, and saturation on the original is retained as much as possible, which is a basic standard for measuring the quality of image scanning.
Evaluation of image scanning quality:
1.1 Accurate calibration of the original is the basis for the true restoration of the original.
1.2 the hierarchical structure of the whole picture. The full scale can make the picture complete, which is because it is easy to lose the level in the darkest and brightest parts.
1.3 When the original is rich in layers, contrast is strong, and saturation is high, it is sufficient to restore the gradation, contrast, and saturation of the original as accurately as possible during scanning.
2. The influence of graphic group on the quality of printed matter
The graphic version is a combination of the scanned image and the entered text according to the customer's request. If the production requirements of the subsequent processes are not fully considered in the process of the assembly, the printed products may be completely scrapped. If you do not follow this standard in the group version, it may increase the error rate of the cutting process, the beer box and other post-processes, resulting in unnecessary waste. The group version is a rather complicated operation process. Attention should be highly concentrated during the grouping process, otherwise a small mistake may lead to large-scale rework.
3. The effect of development on printing quality
3.1 developer concentration.
If the developer concentration is too large and the development is too fast, it will easily cause excessive development of the printing plate, the lines of the graphic will become thinner, the small dots will be lost or the graphics will be faint, which will affect the printing quality. If the developer concentration is too small, the film surface of the photolysis will be seen. It is not easy to wash, and it is easy to get dirty when printing on the machine.
3.2 development time.
If the development time is too long, the film surface of the printing plate that is not visible will be easily dissolved, and the printing plate will become lighter and thinner, resulting in imprinting and unclear printing. The development time is too short, see the photolysis film. The surface is not easy to remove completely, and it is easy to get dirty when printing. A suitable development time is when the printing plate is developed and washed, and the film surface of the photodecomposition is just washed out. If the concentration of the liquid is relatively large, the development time should be shortened accordingly; on the contrary, the development time should be extended accordingly.
4. The impact of output on print quality
The finished layout in the computer needs to be output into a four-color film by a laser imagesetter or output into a printing plate by a computer direct plate-making machine (CTP), or a digital proofing sample can be output through a digital proofing machine. Regardless of the output, the error rate for this process is perhaps the highest. Several aspects that are prone to errors in the output process:
4.1 text, picture.
Problems in the output process are the most common. Text garbled characters, text moves, white text and font errors often occur. These are mainly caused by fonts, output software, output parameter settings, and output devices. The problems that appear in the picture during the output process are mainly the wrong picture and the missing picture.
4.2 output devices.
Output devices are also often the main cause of errors. For the digital proofer, if the color of the sample shows a significant deviation, it may indicate that the profile generated during the previous color management is not consistent with the current situation. Therefore, it is important to ensure that the output device is in an optimal state before outputting the file, ensuring that the materials used are consistent with the characteristic curves or characteristic files applied by the software.
5. The impact of proofing on the quality of printed matter
Proofing plays a role that is crucial to printing. It can check the previous image scanning, text entry, layout, etc., and is also a standard for post-printing. Since the proofing is analog printing (whether traditional proofing or digital proofing), the proofing sheet will definitely be different from the final print, but one of the principles that must be followed is that the print can be caught up. Therefore, the quality of the proof sheet and the difference between the finished product and the printed product will have a great impact on printing.
For digital proofing, since the ink used for proofing is different from the printing ink, in analog printing, the color space of the proof is matched with the printed color space by the color management system (CMS) to simulate the printed color. of. However, several digital proofers that are currently in common use cannot achieve spot color proofing, which is a big flaw.
Third, the reproduction technology
1. The effect of color shift of image on the quality of printed matter during printing
When processing images on the display, if the display is well calibrated, the color of the image may be normal, but this does not mean that the color is the same after the image is printed, because the display uses color and color to achieve color, and Printing uses color subtraction to achieve color, so the reproduction color will be different. In fact, the ink to be used in printing should have the problem of color shift, that is, how to grasp the gray balance of the ink used. Briefly, gray balance refers to a combination of colors that produce a gray color. The same amount of yellow, yellow, and green does not produce neutral gray. They produce a lighter, turbid, brownish gray rather than a real gray. The reason for this is due to the undesirable absorption of the color light by the ink described above, that is, by the impureness of the ink used.
2. The effect of dot gain on print quality
The dot size is directly related to the tone and hue, so the dot gain will make the entire image darker. Moreover, the dots of different sizes increase to a non-linear relationship, and the dot gain formed by the characteristics of paper and ink generally exhibits an exponential expansion law. Other things to note are that in the process of graphic output and printing, the dot size increases due to material and equipment factors.
The dot gain is an inevitable phenomenon due to a specific factor in the printing process. In order to truly reproduce the color and level of the original, it is necessary to compensate for the influence of the dot gain in the prepress process. This compensation process can be applied directly during the graphics processing, or the compensation function can be added to the file first during the processing and then compensated during the printing and photographing process.
3. Influence of total ink control on print quality
In theory, the three primary colors of CMY pigments can restore the desired color. However, in the actual printing, K is indispensable, but when printing with CMYK ink, a lot of ink will accumulate in the dark area. To achieve the super-black appearance, such as the cover of books and magazines, only K is Far from being black, it is often necessary to use C50M50Y50K50 or even C100M100Y100K100, because the ink accumulation is at least 200% or even 400%. Since different substrates maintain different ink abilities, if no measures are taken to control the amount of ink applied, the dark areas will be pasted, thus losing the dark level. Therefore, it is necessary to set the total amount of ink in prepress image processing.
4. The influence of printing registration on the quality of printed matter
Registration refers to the accurate alignment of two or more printed images with each other. The registration problem is an inevitable error caused by the mechanical factors of the printing equipment, and is also an important aspect in the printing suitability. The registration problem refers to the problem caused by the error of the overprint position when the four color separation layouts are respectively printed. In the printing process, the printing pattern of the latter group is accurately placed on the printing pattern of the previous group. If the registration accuracy is greater than a certain absolute value, the color pattern may be under-colored, discolored, etc., or even the pattern may not be caused. clear. Especially in the production conditions of high-speed online printing, the phenomenon will be more concealed and complicated, and the product quality will be seriously affected.
5. Influence of color sequence arrangement of offset printing machine on print quality
Most of the color sequence arrangement of multi-color offset printing machines adopts inverted color order, namely K, C, M, Y. The printing method of the multi-color printing machine is wet and wet, that is to say, the ink is overprinted by another ink in a very short time without drying of the oxidized conjunct film. The color sequence of the multi-color offset printing machine is arranged according to the viscosity of the ink. The high viscosity ink is placed on the front, so that the printing fastness will be better, and the "reverse printing" will not be caused by the viscosity problem. As a result, the printed image is blurred, the color is turbid, and the gloss is poor. However, the effect of ink-stacking is not entirely attributed to the viscosity of the ink, but it should be said that the viscosity of the ink and the amount of ink play a role. Therefore, in order to achieve a good overprinting effect, the ink should have the following points: the ink should have a proper viscosity and concentration, and when wet-wet printing, it should have a moderate ink supply amount, too much and too little is not good. The ink requires positive hue, high transparency, good gloss, fine ink particles, acid and alkali resistance, and good chemical stability.
6. Influence of printing pressure on print quality
The printing pressure refers to the interaction force between the plate cylinder and the blanket cylinder, and between the blanket cylinder and the impression cylinder, which is a necessary condition for ink transfer. The printing pressure is too small, the ink is not easy to convey, the ink is light and faint, and the ink transfer is not good; the printing pressure is too large, and the ink quantity expands to the non-image part, causing the dot to increase seriously, the image is distorted, and cracks are generated, and also It is easy to reduce the printing durability of printing plates and blankets. Therefore, the mechanical printing pressure must be properly adjusted. Different amounts of paper require different printing pressures, and only the proper printing pressure can ensure uniform ink color and uniformity.
7. Influence of ink balance on print quality
Ink balance is a relative balance, not an absolute balance. During the high-speed operation of the equipment, the printed and printed parts of the printing plate are both inked, watered and infiltrated, so ink emulsification will inevitably occur on the printing plate. If the amount of water and ink is not well controlled, it is bound to deepen the emulsification of the ink, resulting in paste and dirty plates in printing. Printing is usually done by reducing the amount of water and increasing the amount of ink, but some will also reduce the amount of ink. In addition, ink emulsification will vary with the environment and conditions. In actual production, the viscosity of the ink and the pH of the fountain solution should be well controlled according to the actual situation, and the amount of ink and water should be controlled. Paste plate, not dirty version shall prevail.