Analysis of Key Factors Affecting Offset Printing Quality

High-quality print is the ultimate goal of the pursuit of printing. The printing quality mainly refers to the comprehensive effect of various characteristics of the printed matter, namely, the absolute quality. On the other hand, it must be requested by the customer. The printed matter is consistent with the customer's sample. This faithfulness to the original copying is called relative quality. There are many factors that affect the quality of prints, this article only from paper

Three key factors affecting print quality, such as printing suitability, prepress technology and reproduction technology, are analyzed to elaborate the internal relationship between them to improve the printing quality.

First, the printability of paper and ink

1. The effect of paper properties on prints

The quality of prints is highly related to the type of paper. Specifically, it relates to five aspects such as surface smoothness, ink absorption, strength, and pH of the paper.

1 The smoothness of the paper affects the quality of the print.

The level of smoothness of paper actually represents the size of the gap on the surface of the paper. Therefore, the level of smoothness has a great influence on the quality of the printed matter. The smoothness of the paper is high, and the layout images on the prints can be faithfully reproduced, especially the small outlets can be accurately restored. For paper with low smoothness, due to the depressions on the surface of the paper, it is easy to make the printing of the dots unclear, and the level of the print layout is not clear. The smoothness and thickness uniformity of the paper will have a direct effect on whether the printing pressure and the ink color can remain uniform. However, to ensure the smoothness of the paper, the method of filling and calendering is mainly adopted. As a result, the tightness of the paper is increased, and the elasticity and ink absorption are relatively decreased, so that the back side of the printed product is prone to smudging. In addition, when the printing pressure is too high, it is easy to spread the ink layer and enlarge the deformation of the print. Based on these circumstances, the unevenness of the smoothness of the paper and the significant difference in the degree of rough light should be classified separately, so that when the paper with poor smoothness is printed, a clear print can be obtained by appropriately increasing the printing pressure and the amount of ink, thereby improving printing. quality.

2 The effect of ink absorption on the quality of printed matter.

The transfer of ink is accomplished by capillary action consisting of pressure and the interweaving gap of the paper fibers. That is, when the paper is pressed, the gaps of the interwoven fibers are compressed, and after the pressure disappears, the surface of the paper is restored to the original state due to the effect of elasticity, and at the same time, the ink is adsorbed, which is the process of ink layer formation. Due to the different elasticity of the fibrous tissue, various papers have different ink absorption properties. The paper with good ink absorption needs a light printing pressure, and the pressure resistance of the printing plate is high. If the paper is too loose, the fiber gap is too large, then when the viscosity of the ink is large, it will cause problems in drawing paper; conversely, when the ink is less viscous, it can easily make the link material penetrate quickly, causing cross-printing phenomenon. This should be done by increasing the viscosity of the ink to suit the printability of the paper. Therefore, paper with tight paper and less ink absorbance is used for color printing. This can prevent pigments from peeling off after the penetration of the link material in the ink, causing the luster of the ink layer to affect the printing quality. However, it is also prone to slow drying and stickiness. Dirty, crystallization and other undesirable phenomena.

3 The effect of the pH of the paper surface on the quality of the printed matter.

The acidic paper and the polyvalent metal salt in the desiccant act to produce a product that is insoluble in oil and grease, prevents the oxidation of the ink from conjunctiva, and inhibits the drying of the ink. Therefore, the desiccant must be properly added during acid paper printing. Neutral or alkaline paper is beneficial to the drying of the ink. The pH of the paper surface is high (alkaline), and the ink dries quickly on the paper surface. For offset printing, the pH of the printing paper also affects the pH of the fountain solution. This affects the drying speed of the printing ink, causing the printed matter to appear poorly dried and the backside sticking and other undesirable phenomena. At the same time, the strong acidity of the fountain solution will inevitably accelerate the corrosion rate of the printing plate, and affect the reproduction of the printed media and the level. When the pH value of the printing paper is too high, ie, the alkalinity is too strong, the alkaline substances in the paper will be continuously dissolved and transferred to the fountain solution during the printing process, resulting in a neutralization reaction with the weakly acidic fountain solution. Change the pH of the fountain solution. In addition, these alkaline substances will also reduce the interfacial tension at the ink interface of the printing plate, promoting the emulsification of the ink, resulting in the non-graphic part of the dirty, so that the gloss of the printed product is reduced, the ink color is dim and so on. The stronger the acidity of the fountain solution, the greater the effect.

4 paper formation on the print quality.

For paper, the part that directly absorbs ink is the coating layer, but when the uniformity of the paper is poor, it will cause the non-uniformity of the adhesive in the coating to penetrate into the original paper layer. This will naturally cause the paint layer to absorb ink. Uniformity. In addition, the poor uniformity of the base paper will also cause unevenness of the coating during the coating process. In particular, the uniformity of the coating layer during the blade coating has a great correlation with the uniformity of the base paper, so that the base paper is not uniform. Sexuality often causes the coating layer to absorb ink non-uniformity.

The surface strength of the paper is not high, and when it is printed, hair loss and powder fall-off may occur. These drop-offs not only paste the printing plate, but also leave white spots on the paper surface where there should be ink. Imprinted ink film will be uneven, even dull, seriously affecting the quality of prints.

2. The effect of ink performance on print quality

1 The effect of ink color performance on print quality.

The color performance of the ink is directly related to the color saturation and hue accuracy of the print color reproduction. Measuring ink color reproduction mainly has the following four parameters:

● The color intensity of an ink, which is also known as the color density, is a representation of the ink's color rendering ability. It affects the neutral gray balance and the accurate reproduction of inter and secondary colors.

● The hue deviation rate is the percentage of the complementary absorption of the basic complementary hue of the basic hue of the ink.

● Grayscale is the ratio of the total absorption of the ink to the three light zones to the light that should be totally reflected, which is the percentage of the maximum reflection density occupied by the equal absorption zone of the ink.

● The color rendering efficiency is represented by the percentage of the actual reflected light of the ink that occupies the ideal reflected light, and is a parameter that comprehensively reflects the size of the ink selected for absorbing capacity.

The above four parameters are closely related to the hue, tinctorial strength, density, fineness, activity, solubility, oil absorption, and hiding power of the selected pigment.

2 The effect of the rheological properties of the ink on the quality of the printed matter.

The rheological properties of inks 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 inhomogeneity of the ink transfer, if the ink is too viscous, the surface of the paper is easily peeled off by the ink film at the time of printing, that is, the hair is pulled. In cases where the viscosity of the ink is too high, it is generally adjusted by adding a dilute solvent or mineral oil.

● When the viscosity is too high, the entire ink system is in an oversaturated state. The pigments are dispersed unevenly and clumped together to accumulate together, resulting in poor ink transfer and fluidity. As a result, the pigments cannot enter or leave the mesh smoothly or even 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, which makes it impossible to form a smooth mask layer upon drying, resulting in dull hue and lack of gloss, that is, color saturation decreases.

● Ink appears imprinted after imprinting, graininess, dull gloss, and low printing ink resistance. The main reason for this phenomenon is that the fineness of the ink is not suitable. The fineness of the ink affects both the quality of the printed product and the imprint resistance of the printing plate during the printing process. The higher the fineness of the ink, the clearer and fuller the dots of the printed product, the higher the ink concentration, the stronger the coloring power, and the higher the quality of the printed product. On the contrary, the fineness of the ink is low, the printing coefficient of friction is large, the ink is not evenly distributed, the version is easy to paste, and the printing plate resistance rate is reduced.

● The adhesiveness of the ink is too large and it is prone to pull-ups. In the high-speed printing process, part of the paper fibers is often pulled up, that is, the printing paper is pulled. The formation of paper-pulling phenomenon is mainly related to the viscosity of the ink and the tightness of the paper.

3 The effect of the emulsifying properties of the ink on the quality of the printed matter.

The emulsifying properties of inks mainly affect the ink balance in printing. The characteristics of offset printing determine that the ink must have a certain ability to contain water. When the proportion of ink changes, the emulsification rate of the ink will also change. When the amount of water supply is small, the ink will absorb all of the fountain solution, and the emulsification rate will rise rapidly. For actual production, it is hoped that the supply ratio of the amount of ink can meet the requirement of maintaining the stability of the emulsification rate and facilitate the quality control in the printing process. Otherwise, the emulsification of the ink will cause changes in ink delivery and distribution, affecting production. The balance of ink and ink is just right, and the ink can be transferred in an ideal way to achieve smooth, neat, clear images, rich layers, and colorful printing effects. It can also ensure the accuracy of overprinting, promote the drying of printed materials, and at the same time ensure the blankness. Partially clean and tidy. If the layout of the water supply and ink imbalance will cause the blank part of the paper is sticky, blotting dull, a wide range of ink emulsification and other issues, seriously affecting the quality of printed matter.

4 ink drying performance of the print quality.

Insufficient ink drying will cause sticky faults. In the printing process, due to the slow drying of the ink, the ink is too soft, too thin, or the amount of ink to be supplied is too large, which may cause the paper graphics to be dirty or the back of the printer to be dirty. This phenomenon occurs mostly in the field where the volume of printed ink is large or in the dense (ie dark) portion of the gradation. It is usually caused by rubbing and soiling and drying in dry late drying. According to the time of generation, the sticky dirt can be generally divided into: the second side is imprinted and dirty (that is, when the back side is printed immediately after one side of the printing is finished, the result is that the other side of the back side of the image is displayed) and all the printing is in contact with the binding. The ink on the surface is contaminated with each other.

5 ink lightfastness.

Under the effect of light, printed products will gradually fade away from the color of the printed products. Usually, light-colored inks are more obvious, such as yellow, because the molecular structure of the ink contains more unsaturated bonds, especially the molecular structure of the pigments has changed. Caused. Therefore, when selecting the ink, the lightfastness of the ink should be taken into account, otherwise the fade of the printed 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 copying colors, which are often expressed through pictures. Image scanning is to convert the color information on a picture to a storable and editable digital signal through a scanning device, and then process these digital signals through application software such as image processing software and typesetting software. Therefore, whether or not the image can be accurately reproduced with high quality, ie, the information such as tone, gradation, contrast, and saturation on the original as much as possible, is a basic standard for measuring the quality of image scanning.

Image scan quality evaluation:

1 Accurate calibration of the original is the basis for the true restoration of the original.

The second tone is the hierarchical structure of the whole picture, and the complete tone can make the picture complete. This is because it is easy to lose the level in the darkest place and the brightest place.

3 When the original has a rich gradation, strong contrast, and high saturation, it is sufficient to restore the gradation, contrast, and saturation of the original as accurately as possible during scanning.

2. Influence of graphic edition on quality of printed matter

Graphic group edition is to combine the scanned image and the entered text according to the requirements of the customer. If the production requirements of the subsequent processes are not fully taken into account in the process of group printing, all the printed products may be scrapped. If you do not follow this standard at the time of group edition, you may increase the error rate of post-processes such as cutting, beer boxes, etc., resulting in unnecessary waste. The group edition is a rather complicated operation process, and attention must be highly concentrated during the group edition. Otherwise, a small mistake may lead to large-scale rework.

3. Effect of development on print quality products

1 developer concentration.

The concentration of developer is too large and the development is too fast, which can easily cause excessive development of the printing plate. The lines of the images and lines are thin, the fine dots are missing or the images are thin and unclear, affecting the printing quality; the concentration of the developer is too small, and the concentration of the developer is too small. Difficult to clean, easy to dirty when printing on the machine.

2 development time.

If the development time is too long, the film surface where the printing plate is not exposed to light will be easily dissolved, and the printing plate image will become light and thin, resulting in false printing and unclear printing; the development time is too short, and the photolysis film is too short. It is not easy to completely remove the surface and it is easy to get dirty when printing. The proper development time is when the plate is washed after development, and the photolytic film surface just flushes out. If the concentration of the liquid is relatively large, the development time should be shortened accordingly; otherwise, the development time should be extended accordingly.

4. Effect of output on print quality

The layout produced in the computer needs to be output as a four-color film by a laser imagesetter or output by a computer-to-plate (CTP) printer or a digital proofer. Whatever the output, the error rate for this process may be the highest. Several aspects that are prone to errors in the output process:

1 text, pictures.

Text problems in the output process are the most common, often appear text garbled, text walk, text leakage and font errors, these are mainly caused by the font, output software, output parameter settings and output devices. The problems that appear in the output during the output process are mainly wrong and missing graphs.

2 output device.

Output devices are also often the main cause of errors. For digital proofers, if the color of the proofs shows more obvious deviations, it may indicate that the proofs were previously color managed.

The profile generated at this time is inconsistent with the current situation. Therefore, before outputting the file, you must ensure that the output device is in the best condition and that the material used is the same as the characteristic curve or characteristic file used by the software.

5. Effect of Proofing on Print Quality

Proofing plays a role that is linked to the past and is crucial for printing. It can check the previous image scanning, text entry, layout, etc. It is also a standard for subsequent printing. Since the proofing is analog printing (regardless of traditional proofing or digital proofing), proofing proofs must be different from the final printed matter. However, one of the principles that must be followed is that the proofs should be able to catch up with proofs. Therefore, the quality of the proofs and the size of the difference from 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 proofing is matched with the printed color space by the color management system (CMS) to simulate the printed color. of. However, several digital proofers currently in widespread use cannot implement spot color proofing. This cannot but be said to be a major flaw.

Third, reproduction technology

1. Effect of image color shift on the quality of printed matter during printing

When processing the image on the monitor, if the monitor is well calibrated, the image color may be relatively normal, but this does not mean that the image is printed with the same color, because the monitor uses the color light additive method to achieve the color, and Printing uses the subtractive color method to achieve color, so the reproduced color will be different. In fact, the ink to be considered for printing will not suffer from the problem of color shift, that is, how the gray balance of the used ink is grasped. Briefly, gray balance refers to a combination of colors that can produce a gray color. The same amount of yellow, product, and blue do not produce neutral gray. They produce a lighter, more turbid, brownish gray than true grey. The reason for this is due to the imperfect absorption of colored light by the above-described inks, that is, due to 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 tone, so the dot gain will make the whole picture darker. Moreover, the dot size of the dots with different sizes increases to a nonlinear relationship, and the dots formed by the characteristics of paper and ink generally increase in an exponential manner. The other thing to note is that in the process of plate-making graphic output and printing, due to the material and equipment factors, the dot gain will also occur.

The dot gain is an unavoidable phenomenon due to certain factors in the printing process. In order to be able to faithfully reproduce the color and gradation of the original, it is necessary to compensate for the influence of dot gain in the prepress processing. This compensation process can be applied directly during the graphics processing process, or it can be added to the file first during processing, and then compensated during printing and photocopying.

3. Effect of Total Ink Control on Print Quality

In theory, the three primary colors of CMY pigments can restore the desired color. However, in practical printing, K is added indispensably. However, when printing with CMYK ink, a lot of ink will accumulate in the dark tone area. When an ultra-black appearance effect is to be achieved, for example, the covers of books, magazines and pictorials, only K is Far less than black, often need to use C50M50Y50K50, or even C100M100Y100K100, this is because the amount of ink accumulation must reach at least 200% or even 400%. Since the ability of different substrates to maintain ink is different, if no measures are taken to control the amount of ink applied, the dark areas will be pasted and the dark levels will be lost. Therefore, the total amount of ink needs to be set in the prepress image processing.

4. Impact of print registration on print quality

Register refers to the accurate alignment of two or more print images with each other. The registration problem is an unavoidable error caused by the mechanical factors of the printing equipment, and is an important aspect in printability. The registration problem refers to the problem that arises when the four separation layouts are printed while the overlapping position is inaccurate. During 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, problems such as color undercolor and discoloration of the color pattern will occur, and even the pattern will not be generated. clear. Especially in the production of high-speed on-line printing, this phenomenon will be more subtle and complicated, and the quality of the product will be seriously affected.

5. The effect of the color arrangement of the offset press on the quality of the printed matter

The color sequence of multi-color offset printing machines mostly uses the inverted color sequence, namely K, C, M, Y. Multi-color printers are wet and wet, meaning that the ink is superimposed on another ink in a very short time without drying the oxidized conjunctiva. The color sequence of a multicolor offset printing press is arranged according to the viscosity of the ink, and the ink with high viscosity is printed on the front side. In this case, the overprint fastness will be better, and the “reverse overprint” will not be caused due to the size of the viscosity. As a result, the printed product has a blurred picture, dark colors, and poor gloss. However, the effect of ink overprinting cannot be attributed entirely to the ink viscosity, but it should be said that the ink viscosity and the amount of ink play a role. Therefore, in order to achieve a good overprinting effect, it is required that the ink should have the following points: The ink must have a proper viscosity and concentration, and should have a moderate ink supply in wet and wet printing, too much and too little. , The ink requires positive hue, high transparency, good gloss, delicate ink particles, acid and alkali, good chemical stability.

6. The effect of printing pressure on print quality

Printing pressure refers to the interaction between the plate cylinder and the blanket cylinder, the blanket cylinder and the impression cylinder, and it is a necessary condition for ink transfer. Printing pressure is too small, ink is not easy to transmit, ink is light, ink will not transfer well, and printing pressure is too high. The amount of ink spreads to the non-graphical part, making the dot gain serious, image distortion, and cracks. Easily make the printing plate and blanket resistant to printing force drop. Therefore, the mechanical printing pressure must be properly adjusted. Different quantitative papers require different printing pressures, and only proper printing pressure can ensure uniform ink and uniform shades.

7. Effect of ink balance on print quality

The ink-and-wash balance is a relative balance, not an absolute balance. In the process of high-speed operation of the equipment, both the graphic part and the non-graphic part of the printing plate are inked and watered, and they infiltrate each other. Therefore, ink emulsification will inevitably occur on the printing plate. If the amount of water and ink is not well controlled, ink emulsification is bound to deepen, resulting in stencil and dirty prints. Printing methods are usually used to reduce the amount of water, the appropriate increase in the amount of ink, but some will reduce the amount of ink. In addition, the ink emulsification will vary with changes in the environment and conditions. In actual production, the viscosity of the ink and the pH of the fountain solution should be properly controlled to control the amount of ink and the amount of water. Paste, not dirty version prevail.