Reasons and Control of Offset Printing

In order to truly reproduce the colors of the original and improve the printing quality, it is necessary to understand the changes in the dots. The dot is the basic unit of ink adhesion, and plays a role in transferring the tone and organizing the colors. In offset printing, the dots are the basic elements that make up an image. Changes in dots will inevitably affect the tone and color of the image, and thus affect the sharpness of the image. Therefore, the key to the quality control of printed matter is to control the printing quality of printed matter on the Internet.

There are two kinds of mechanical and optical conditions for dot gain.

1. Mechanical dot enlargement: The ink attached to the metal plate dot is deformed and expanded by the extrusion pressure.

2. Optical dot enlargement: Due to the light reflection effect, when the light reaches the surface of the dot, surface reflection occurs, and the ink penetrates into the edge of the dot in the paper, forming a diffusion vignette. This kind of reflection vignette forms a tone. The effect is the same as the mechanical dot enlargement during printing.

These two kinds of network expansion have influence on the quality of printed matter to different extents and destroy the balance of the picture. In order to make a true reduction of the outlets on the printed matter, it is necessary to control the delivery process of the outlets (that is, a printing plate, a blanket, and a substrate).

Dot enlargement refers to the fact that the effective area of ​​the web site of the substrate is greater than the effective area of ​​the web access point of the screen. This is a normal process phenomenon, and both the prepress production and the printing process are inevitable. However, it must be controlled within a certain range. Once it exceeds a certain range, the quality of the printed matter will be severely degraded (level loss, density, color distortion, etc.), and this network point increase is abnormal. GB7705-7 stipulates that for 50% of the outlets, the value of the outlets for fine products should be no more than 12%, and that for general products should not exceed 18%. In the prepress production and printing process, it is generally possible to compensate for the normal dot gains (such as the double reflection effect of the laser photolithography, the paper, and the dot gain generated under reasonable printing pressure), but abnormal dots There is often no regularity to increase, and reasonable compensation cannot be made in the prepress process, and can only be controlled during the printing process.

First, the factors that affect the expansion of outlets

(a) Material factors

The poor printing plate has poor photosensitivity. PS version of some of the coarse mesh, some poor photosensitivity, easy to form a halo around the exposure after the outlet. After the PS version was inked, the virtual halo part was also inked. The actual effect was to increase the dot area and cause the dot gain to increase. It is necessary to replace the PS plate with better quality.

During the printing process, the causes of dot gain are:

1 The exposure is too small or the exposure time is too short, making the exposure incomplete.

2 developer developing fatigue or developer pH is too low. Especially in manual development, it is most likely to occur because the developer is exposed to air and reacts easily with carbon dioxide in the air.

3 The development time is short (the unexposed areas on the plate are not completely rinsed), or the development temperature is too low, making the development process slow. Practice has proved that every 5 degrees lower development temperature, the development speed will be reduced by 1 to 2 times.

A reasonable amount of exposure, development temperature, and development time during the printing process can effectively control the dot gain. In addition, when the development temperature is too low, it is appropriate to lengthen the development time. It is recommended that after each batch of PS plates enter the factory, they must be printed with a grey ladder, in order to test the ideal exposure and exposure time, and to facilitate the standardized management of printing plates.

2. Ink factor

1 The cohesion of the ink is too low

When the ink cohesion (viscosity) is too low, the ink will rebound and it will be weak, causing the phenomenon of the printed dot network to increase and the ink spreading. In the ink transfer process, when the ink film is pulled into a wire and broken, and the tensile stress disappears, it mainly relies on the stickiness and elasticity to return to the state before stretching. In the viscous recovery process, because the film has no internal stress, it can only rely on gravity. As a result, the film layer is uneven, the edges are spread, the dots are not smooth, and there are dot gains. This is a very undesirable phenomenon. Recovery status. The recovery of elasticity depends on the recovery of the internal energy of the film. Because the internal stress of the film builds up strong internal stress, the ink is pulled back by the elastic force and shrinks.

When the cohesion of the ink is too low, the elastic recovery ability will be weakened, and can only rely more on viscous recovery, which will cause the dot gain. Therefore, various additives should be used as little as possible during printing to avoid reducing the cohesion of the ink. If the ink itself is a problem, some ink improver or No. 0 varnish can be added as appropriate.

2 ink viscosity and high fluidity

The hydrophilicity of the ink is too sensitive, the thixotropy and yield values ​​are too small, the viscosity is too low, the ink filament is too long, and the fluidity is too strong, and more fountain solution is required to counteract it during printing. The emulsified ink is easy to spread on the PS plate, which easily causes dot enlargement. The transfer to the substrate is not conducive to the solidification of the ink, and no clear halftone image can be obtained.

3. Relationship between paper and outlet expansion

The surface of the paper is not completely smooth. There are many pits and pores. When printing, under the pressure of the ink, the ink is first filled with the pits in the pressure direction, and the remaining ink spreads to the outside of the dot. The printability of the paper depends on the surface characteristics of the paper, such as the amount of ink absorbed and accepted. The faster the speed of absorption, the greater the degree of expansion of the printed network. The rate of ink absorption by the paper plays a key role in the extent of dot gain. The higher the paper smoothness, the smaller the dot gain value; the lower the paper smoothness, the larger the dot gain value. In printing paper, coated paper has lower absorptivity than non-coated paper, non-coated paper has high absorption performance, and as a result of high absorbency, the percentage of dot gain is high. Even if the same coated paper, because of the different origin, the smoothness of the paper is also very different. Solving the quality problems brought about by this difference can be achieved only through the adjustment of the printing pressure.

The effect of paper on dot gain is also reflected in the double reflection effect of paper. After the dots are printed on the paper, most of the white light shining on the paper is reflected, but there is still a considerable part of the white light absorbed by the ink layer. The remaining light is reflected by the paper and reflected by the ink layer to become a diffuse reflection light. In the dual role of reflection and diffuse reflection of the paper, a halo circle is formed at the edge of the dot, resulting in the phenomenon of visual dot enlargement. In fact, these dots have not been expanded, but merely the illusion of dot enlargement. If the surface of the paper is high in smoothness, there is much reflected light and diffuse reflection is not serious, and the double reflection effect is small.

4. The relationship between blankets and dot gain

The blanket is used as the medium for the ink transfer of printing plates. It has the characteristics of strong elasticity and good oleophilicity. It deforms when subjected to pressure during printing. Under the effect of printing pressure, the ink will spread around the dots. At the same time, due to the elastic deformation of the blanket, relative slippage between the printing plate and the blanket and between the blanket and the paper is caused. As a result of the above-described expansion and slippage, it is inevitable that a dot expansion phenomenon occurs. This is an important factor that causes dot gain in lithography.

Blanket is divided into air blanket and ordinary blanket. In order to make the expansion value of outlets within the minimum range, make the density of the outlets even, and the periphery smooth, it is necessary to debug the correct printing pressure and use a printing blanket with good printability. The air-cushional blanket has excellent printability on the structure, and it has a large printing capacity and a small amount of deformation. When the same compression amount is produced, the pressure applied by the air-cushion blanket is smaller than that of the ordinary blanket roller, so the dot gain is also small. If a common blanket is used, a rigid lining is used because of the large elastic modulus of the hard lining, the smaller amount of compression deformation and the width of the embossed area, and the better dot reproducibility.

With good printing blankets and correct liners, care must be taken to keep the blanket surface clean. Preventing the aging of the surface of the blanket by the aging of the conjunctiva and the oxidized conjunctiva to smoothen the surface of the blanket and affect the lipophilic and hydrophobic properties of the blanket surface. In the printing process, the surface of the blanket is covered by paper fluff, paper powder, residual particles in the ink, adherence of fountain solution, dusting, and friction during high-speed operation, greatly reducing the spread of the blanket. The performance of ink, resulting in blurring of dots and floating on the ground, directly affects the quality of the printed matter. Therefore, it must be cleaned frequently to keep the surface of the blanket clean and to exert its best effect. The blanket liner is too thick or too soft to increase the pressure between the plate cylinder and the blanket cylinder, the blanket cylinder and the impression cylinder (the transfer blanket blanket cylinder is both the transfer cylinder and the impression cylinder), and the blanket compression deformation amount And the width of the contact area is relatively large, the quality of the dot is not ideal, the dot reproducibility is poor, and the dot gain value is relatively high.

(b) Printing factors and process factors

1. Determination of printing pressure

In the printing process, the transfer of the ink and the transfer of the print require proper printing pressure, and the deformation of the web and the deformation of the blanket are bound to cause deformation and increase of the dots. Therefore, in the process of drying the PS plate, it is necessary to combine different models (different printing presses have different dot gain values) to select an appropriate exposure time so that the dots on the PS plate are slightly smaller than the sample drafts, and the printed matter can be kept consistent with the proofs. . Under the effect of printing pressure, the ink will spread outwards. If the printing pressure is too large, dot gain will occur. Therefore, the printing pressure should be strictly controlled. For papers with high surface smoothness (coating paper), the compression should be controlled between 0.10 and 0.15 mm, while the paper with low surface smoothness (offset paper) should be controlled to have a compression of about 0.2 mm. The principle of control is to use as little printing pressure as possible on the basis of good ink transfer.

2. The relationship between printing pressure and dot gain

The reduction of outlets needs to use the effect of printing pressure. Printing pressure directly affects the transfer of ink and it is very important to properly adjust the working pressure of the printing press. When the printing pressure is too small, the printing surfaces cannot fully contact each other. The molecular force between the ink and the paper is very small. Only a small amount of ink may be transferred to the surface of the paper. The ink's conversion rate is very low, and the printed product is light in ink. , And there is a "emptiness phenomenon", and even graphic incomplete. If the printing pressure is too high, the ink will be squeezed into the blank space outside the graphic. On the one hand, the dots will enlarge and the image will be blurred; on the other hand, the ink transfer will also show a downward trend, causing the prints to appear shaded. Clear, the parts of the field and outlets cannot reproduce the colors of the originals.

The printing pressure is not stable, the ink transfer is excessive and sometimes insufficient. The tone reproduction and color reproduction of the print cannot meet the desired requirements, and high-quality prints can be obtained only within an appropriate printing pressure range. Choosing the right printing pressure becomes a very important part of the printing process. The determination of printing pressure, in addition to the structural performance of the machine itself, is also related to the ink, paper, dampening fluid, blanket, ink roller, printing speed and other factors used in the printing process, because these small changes in printing conditions will be Hue affects. Therefore, in the printing process, it must be adjusted according to the actual printing conditions to get the ideal printing pressure.

3. Relationship between printing speed and dot gain

The change in printing speed directly affects the quality of the product. When the printing pressure is set at certain data, the printing speed is slowed down, the contact time between the printing surfaces becomes longer, the contact of the printing surface is sufficient, the transfer rate of the ink is high, the ink absorption of the dots is full, and the ink of the image is bright. When the printing speed increases, the contact time between the printing surfaces becomes shorter, the printing surface is not sufficiently contacted, the ink transfer rate is low, the ink absorption of the dots is insufficient, and the image is white. If the printing speed is not stable in a batch of printed products, it will cause inconsistencies in the ink before and after.

4. Ink transfer affects outlet expansion

The transfer of the ink is done by the ink roller. The ink is transferred from the ink fountain through the ink transfer roller to the uniform ink roller. Under the shearing action of the uniform ink roller, the thixotropic phenomenon occurs, the structure is fully destroyed, and the film is extended into a uniform film, which is transferred to the surface of the printing plate through the ink roller. The distribution of the ink film on the ink roller has strict rules, and only the ink system ensures its high-precision ink supply, so that the ink can be continuously and evenly transferred to the printing plate. The ink is squeezed between the printing plate and the blanket, and then squeezed between the blanket and the paper. The ink layer formed after two extrusions is usually very thin. If the ink layer is thicker, the problem of dot gain will occur. At the same time, the hardness of the rubber roller must meet the requirements, the surface of the rubber roller must not have the vitrification phenomenon, and it must have good ink transmission performance, so that the dots of the printing plate have enough ink absorption.

For the ink to be smoothly transferred from the ink roller system to the plate, the ink is also required to have good fluidity. When the offset ink is stirred or under mechanical action, the ink will become soft and the fluidity will increase. Stop stirring, rest for a period of time, and return to the original solid state, this phenomenon is called thixotropic ink. Therefore, to ensure that the ink can be transmitted well, we must constantly stir the ink fountain. Use the thixotropy of offset printing inks to increase the flow of the ink and ensure that the dots can achieve the desired ideal color.

5. Ink transfer device is poorly adjusted

The relative position between the ink rollers changes, and the shear force between the roller and the roller also changes, and the ink transmission relationship is destroyed. Relying on the pressure of the printing roller, the amount of extrusion deformation is large, the diameter of the rubber roller becomes fine, and the line speed of the plate roller and the platen roller are inconsistent, and the dot is stretched in the circumferential direction of the roller, resulting in tensile deformation. The stencil pressure on the printing roller is too large, which causes the ink to spread on the printing plate when it is transferred from the ink roller to the printing plate. Therefore, it is necessary to readjust the pressing pressure of the printing roller. Equipment wear causes dot gains, such as dot gain caused by gear wear, which generally manifests as ink bars and ghost images. Solution: Generally, the tooth spacing can be properly reduced without tipping.

6. Ink is too large

Under ideal conditions, the thickness of the transferred ink layer should be controlled within 25 μm. As the amount of ink is increased, the thickness of the ink film increases, and the separation force increases rapidly. The excess ink cannot be instantly transferred by the substrate. The ink on the surface of the printing plate and the blanket is more, and the excess ink spreads out to the outlet. Caused by the increase of outlets, and even a paste version, it is necessary to strictly control the amount of ink to be printed.

7. Control of fountain solution usage

The small amount of water in the printing plate prevents the water film in the blank portion from effectively resisting the spread of the ink layer on the web. Solution: Adjust the pressure of the water roller, so that it can transfer water evenly and stably, or increase the amount of fountain solution or fountain solution. However, if the amount of the fountain solution is too large or too small, the dot gain may be increased. Therefore, the amount of the fountain solution must be reasonably controlled in the printing process, and the pH of the fountain solution should be within a reasonable range. Between 4.8 and 5.3. In the actual printing, it is necessary to properly adjust the actual conditions, such as the use of desiccant in the ink, can properly reduce the pH of the fountain solution. However, it is necessary to control the over-emulsification of the ink to prevent the ink film on the dots from being able to maintain a relatively clear boundary with the blank portion of the paper, resulting in an increase in the dot size. In this case, the amount of fountain solution or wetting powder can be appropriately reduced.

8. The deformation of the blanket

Due to the elastic deformation of the blanket, relative slip occurs between the printing plate and the blanket, between the blanket and the substrate, causing dot gain. Therefore, when adjusting the printing pressure and changing the center distance of the roller, the surface line speed of the roller should be as uniform as possible. But complete agreement is impossible because the surface speed of the blanket cylinder itself is not uniform due to the compression of the blanket. The tightness of the blanket is not enough or the compressibility is not good. After the new blanket had been printed on the machine for the first time for five or six thousand sheets, the blanket was once stretched. After printing one or twenty thousand sheets, the blanket was again stretched.

Second, the measurement of outlet value

There are two methods for measuring the dot gain value: one is the reflectance densitometer measurement method, and the other is the Bruner printing and measurement stripe measurement method.

1. The reflection densitometer measurement method uses a density meter to measure the density, and then uses the Murray-Davies formula to calculate the dot gain value, which can be used for quantitative analysis and data printing and standardization. The dot enlargement value is equal to the dot area on the printed matter minus the dot area of ​​the corresponding part on the original (film for printing).

2. Bruner printing control bar measurement method. The Bruner measure has a fine line segment of 60 lines/cm and a coarse line segment of 10 lines/cm. The fine mesh and coarse mesh have the same 50% dot area rate. The ratio of fine lines to coarse lines is 6:1. That is, the ratio of the edge length of the thin network to the length of the coarse edge is 6:1. Since the amount of dot expansion is proportional to the length of the edge of the dot, the fine dot gain is large and the density is large under the same conditions. When the calculation is based on the coarse network segment and the difference between the density of the fine network segment and the coarse network segment, the 50% dot area expansion value can be calculated. Its calculation formula is:

= 50% fine segment density - 50% coarse segment density

According to Brunel's research, the formula for calculating the dot gain value of about 75% is:

= (75% fine segment density - 75% coarse segment density) ÷ 2

About 25% of dot gain values ​​are calculated as:

= (25% fine segment density - 25% coarse segment density) × 2

Third, the control of offset printing outlets

Although the increase in outlets is inevitable, it can be controlled. Under normal circumstances, the fine print dot gain of coated paper is less than 12%, the general print is less than 18%, and the print control is within 35% (generally 50%). In the production process, effective control can be made from the following aspects.

1. Select high quality media, reasonably control exposure time and developer concentration

The clear printing plate of the dried printing plate is clear and the outlets are strong, so as to ensure the transfer effect of the outlets and ensure that the printing plate has a higher resistance to printing forces. Under normal circumstances to ensure that 5% of the network is not lost, 95% of the network is not blurred.

2. Strictly control the ink and water balance in printing

In the printing process, if the water supply is too large, the ink supply on the layout will increase, the ink layer will become thicker, and the ink emulsification will be severe, resulting in dull and dull prints. In the production process, in order to reduce the start-up waste, the water level is usually increased to 150% of the normal value when the machine is turned on. After normal printing, the amount of water should be gradually reduced to the normal value to ensure clear graphics and saturation.

3. Control various printing pressures within a reasonable range

The pressure between the plate cylinder and the blanket cylinder is controlled between 0.15 and 0.20mm, and two printing rollers are parallel to the printing plate roller. The printing roller width between the plate roller and the plate roller with a diameter of 100mm or less is 3 ~4mm, the printing roller width of printing plate roller is 5~6mm.

4. Correctly control the concentration of fountain solution

The control of the fountain solution concentration plays a crucial role in the balance between ink and ink and the quality of the printed product. A suitable concentration of fountain solution can quickly flush away the "dirt" around the dot. If the concentration of the fountain solution is too low, the ink around the dots cannot be removed, and if the dots are light, the dots will increase, and when the weight is too high, the stencil will be caused. If the concentration of the fountain solution is too high, the resistance of the PS plate will be reduced and the fountain solution will be reduced. A lot of waste. In general, the pH of the fountain solution is controlled between 4.0 and 6.0, and the conductivity is controlled at 1400±200 μs.

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