The reasons for the expansion of offset printing dots and their control

The reasons for the expansion of offset printing dots and their control

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To print the original color of the original and improve the printing quality, it is necessary to understand the change of the dot. The dot is the basic unit of ink adhesion, which plays the role of conveying the tone and organizing the color. In offset printing, the dot is the basic element of the image. The change of the dot will inevitably affect the tone and color of the image, which will affect the sharpness of the image. Therefore, the key to print quality control is to control the print quality of print outlets.

The expansion of dot gain is both mechanical and optical.

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

2. Optical dot enlargement: caused by light reflection, 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 to form a diffuse vignette, which forms a hue. The effect is the same as the mechanical dot enlargement at the time of printing.

The expansion of these two outlets affects the quality of printed matter to varying degrees, destroying the balance of the picture. In order to make the dots actually restore on the printed matter, it is necessary to control the transfer process of the dots (ie, a printing plate, a blanket, and a substrate).

The expansion of outlets means that the effective area of the substrate's Internet access point is larger than the effective area of the screened netting point. This is a normal process phenomenon, whether it is prepress production or printing process is inevitable. However, it must be controlled within a certain range. Once it exceeds a certain range, the quality of printed matter will be seriously degraded (gradation loss, density increase, color distortion, etc.), and such dot gain is abnormal. GB7705-7 stipulates that for 50% of the outlets, the dot gain of fine products should be no more than 12%, and the dot gain of general products should be no more than 18%. In the pre-press production and printing process, it is generally possible to compensate for the normal dot gain (such as laser photorangement, double reflection effect of paper and reasonable dot gain generated under printing pressure), but abnormal dot The increase often has no rules to follow, and reasonable compensation cannot be made during the prepress process, which can only be controlled during the printing process.

First,  Analysis of factors affecting the expansion of outlets

(1) Material factors

1. Plate factor

Poor prints have poorer photosensitivity. Some of the PS versions have thicker meshes, and some have poorer photosensitivity. It is easy to form a faint around the dots after exposure. After the PS version is inked, the faint part is also inked. The actual effect is to increase the area of the dot, causing the dot to increase, and it is necessary to replace the PS plate with better quality.

During the printing process, the reasons for the increase in the dot are:

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

1.2 The developer is fatigued during development or the pH of the developer is too low. Especially in manual development, it is most likely to occur, because the developer is exposed to the air, it is easy to react with carbon dioxide in the air.

1.3 The development time is short (the unexposed portion of the plate is not completely rinsed), or the development temperature is too low, so that the development process is slow. Practice has shown that for every 5 degrees of development temperature reduction, the development speed will be reduced by 1 to 2 times.

Appropriate exposure, development temperature, and development time can be used during the printing process, and the dot gain can be effectively controlled. In addition, when the development temperature is too low, the development time is appropriately extended. It is recommended that each batch of PS plate should be printed with a gray ladder after entering the factory to test the ideal exposure and exposure time, which is convenient for standardized management of the printing plate.

2. Ink factor

2.1 ink cohesion is too low

When the cohesion (viscosity) of the ink is too low, the ink rebound will be weak, causing the dot of the printed matter to increase and the ink to spread. In the process of ink transfer, when the ink film is broken into filaments and broken, after the tensile stress disappears, it mainly relies on viscosity and elasticity to return to the state before stretching. In the process of viscosity recovery, since the film has no internal stress, it can only rely on gravity. The result is uneven film layer, edge diffusion, unclean dots, and dot gain. This is a very undesirable phenomenon. Restore status. The elastic recovery is the recovery caused by the inner energy of the film layer. The ink is pulled back by the elastic force to shrink due to the accumulation of strong internal stress inside the film layer.

When the cohesion of the ink is too low, the elastic recovery ability is weakened, and only more dependent on the viscous recovery, which leads to an increase in the dot size. Therefore, various additives should be used as little as possible in the printing to avoid reducing the cohesion of the ink. If there is a problem with the ink itself, some ink improver or No. 0 varnish may be added as appropriate.

2.2 ink viscosity is small and the fluidity is too high

The hydrophilicity of the ink is too sensitive, the thixotropic and yield values are too small, the viscosity is too low, the ink is too long, the fluidity is too strong, and more dampening solution is required to compete with it during printing. The emulsified ink is easy to spread on the PS plate, which tends to cause the dot to increase. Transferring to the substrate is not conducive to the solidification of the ink, and a clear mesh image cannot be obtained.

3. The relationship between paper and dot enlargement

The paper surface is not completely smooth. There are many pits and capillary holes. When printing, the ink is first pressed under the pressure of the pit, and the remaining ink is spread to the periphery of the dot. The printability of the paper depends on the surface characteristics of the paper, such as the amount of ink absorbed and received. The faster the absorption, the greater the extent to which the printed dots are enlarged. The rate at which paper absorbs ink plays a key role in the extent of dot gain. The higher the smoothness of the paper, the smaller the dot gain value; the lower the smoothness of the paper, the larger the dot gain value. In printing paper, coated paper has lower absorbency than non-coated paper, non-coated paper has high absorption performance, and high absorbency results in a high percentage of dot enlargement. Even if it is the same as coated paper, the smoothness of the paper varies greatly depending on the place of production. Solving the quality problems caused by this difference can only be achieved by adjusting the printing pressure.

The effect of paper on dot enlargement is also reflected in the double reflection effect of paper. After the dots are printed on the paper, most of the white light on the paper is reflected, but a considerable portion of the white light is absorbed by the ink layer, and the remaining light is reflected by the paper surface and reflected by the ink layer to become diffuse reflection light. Under the dual action of paper reflection and diffuse reflection, a halo circle is formed at the edge of the dot, which causes visual dot enlargement. In fact, these dots do not expand, but the illusion of dot enlargement. If the surface of the paper is smooth, the reflected light is large, the diffuse reflection is not serious, and the double reflection effect is small.

4. The relationship between blanket and dot enlargement

As a medium for ink transfer on the printing plate, the blanket has the characteristics of strong elasticity and good lipophilicity. It deforms when subjected to pressure during printing. Under the action of printing pressure, the ink will expand to the periphery of the dot. At the same time, due to the elastic deformation of the blanket, a relative slippage occurs between the printing plate and the blanket, between the blanket and the paper. Due to the above expansion and slippage, the enlargement of the dots is inevitably caused. This is an important factor in the expansion of outlets in lithography.

The blanket is divided into an air cushion blanket and a general blanket. In order to control the expansion value of the dot to a minimum range, the density of the dots is uniform, and the periphery is smooth, it is necessary to debug the correct printing pressure, and select a blanket with good printability. The air cushion blanket has excellent printability in structure, and has a large printing capacity and a small amount of deformation. When the same amount of compression is produced, the air cushion blanket is used to withstand less pressure than the ordinary blanket cylinder, so the dot gain value is also small. If a plain blanket is used, a rigid lining is used because the elastic lining has a large elastic modulus, a small amount of compression deformation and a nip area, and good dot reproducibility.

With a blanket with good printing properties and the correct lining, always pay attention to keeping the surface of the blanket clean. Prevent the surface of the blanket from aging the conjunctiva and oxidize the conjunctiva, smoothing the surface and affecting the lipophilic hydrophobic properties in the rubber layer on the surface of the blanket. During the printing process, the surface of the blanket is covered by paper wool, paper powder, particles in the ink, adhesion of dampening solution, dusting, etc., and friction during high-speed operation, which greatly reduces the transmission of the blanket. The performance of the ink, causing blurring of the dots, and the phenomenon of floating on the ground, directly affect the quality of the printed matter, so it is necessary to clean frequently to keep the surface of the blanket clean and to exert its best effect. The blanket 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 rotary blanket cylinder is both the transfer roller and the impression cylinder), and the compression deformation of the blanket The width of the contact area is relatively large, the quality of the dot is not ideal, the reproducibility of the dot is poor, and the dot gain is relatively high.

(2) Printing factors and process factors

1. Determination of printing pressure

Both the transfer of the ink and the transfer of the print during the printing process require a suitable printing pressure, and the deformation of the dots and the deformation of the blanket by the roller tend to cause deformation and enlargement of the dots. Therefore, when printing the PS version, it is necessary to combine different models (different printers have different dot gain values) to select the appropriate exposure time, so that the dots on the PS plate are slightly smaller than the proofs, and the prints can be consistent with the proofs. . Under the action of printing pressure, the ink will spread outward. If the printing pressure is too high, the dot gain will occur. Therefore, the printing pressure should be strictly controlled. For paper with high surface smoothness (coated paper), the compression amount should be controlled between 0.10 and 0.15 mm, and 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 ensuring a good transfer of the ink.

2. The relationship between printing pressure and dot gain

The reduction of the dot needs to be through the printing pressure, the printing pressure directly affects the transfer of the 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 be fully contacted, and the molecular force between the ink and the paper is small. Only a small amount of ink may be transferred to the paper surface, the ink conversion rate is very low, and the printed product is light in color. And there is a "empty phenomenon", and even the picture is incomplete. If the printing pressure is too large, the ink will be squeezed into the blank space outside the graphic. On the one hand, the dot will be enlarged and the image will be blurred; on the other hand, the ink transfer will also show a downward trend, making the print appear dark. Clear, the parts of the field and the dots cannot reproduce the color of the original.

The printing pressure is unstable, the ink is transferred excessively, and sometimes it is insufficient. The tone reproduction and color reproduction of the printed matter cannot meet the expected requirements, and high-quality printed matter can be obtained only under the appropriate printing pressure range. Choosing the right printing pressure is a very important part of the printing process. The determination of printing pressure, in addition to the structural properties of the machine itself, is related to the ink, paper, dampening solution, blanket, ink roller, printing speed and other factors used in the printing process, because slight changes in these printing conditions will be Hue has an effect. Therefore, during the printing process, it is necessary to adjust according to the actual printing conditions to obtain the desired printing pressure.

3. The relationship between printing speed and dot enlargement

Changes in printing speed directly affect the quality of the product. When the printing pressure is set to a certain data, the printing speed is slowed down, the contact time between the printing surfaces becomes long, the contact of the printing surface is sufficient, the transfer rate of the ink is high, the ink dots are full, and the ink color is vivid. When the printing speed is increased, the contact time with respect to the printing surface becomes short, the printing surface is not sufficiently contacted, the ink transfer rate is low, the dot is insufficiently sucked, and the image is white. If the printing speed is unstable in a batch of printed products, the ink color will be inconsistent before and after.

4. Ink transfer affects the expansion of outlets

The transfer of ink is done by an ink roller. The ink is transferred from the ink fountain to the leveling roller through the transfer roller. Under the shearing action of the ink roller, the thixotropic phenomenon occurs, the structure is fully destroyed, and the film is extended into a uniform film, which is transmitted to the surface of the printing plate through the ink roller. The distribution of the ink film on the ink roller has a strict law. Only the inking system ensures high-precision ink supply, and the ink can be continuously and uniformly transmitted 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 thick, the problem of dot enlargement occurs. At the same time, the hardness of the rubber roller should meet the requirements. The surface of the rubber roller should not be vitrified. It should have good ink transfer performance, so that the dot of the printing plate has sufficient ink absorption.

In order for the ink to smoothly transfer from the ink roller system to the printing plate, the ink is also required to have good fluidity. Under the action of stirring or mechanical action, the ink will become soft and the fluidity will increase. Stirring is stopped, and after a period of inactivity, the original solid state is restored. This phenomenon is called thixotropy of the ink. Therefore, to ensure that the ink can be transported well, the ink fountain is often stirred. The thixotropy of offset inks is used to increase the fluidity of the inks and to ensure that the dots are able to achieve the desired desired color.

5. Ink transfer device is not well adjusted

The relative position between the ink rollers changes, and the shear force between the rollers and the rollers also changes, causing damage to the ink transfer relationship. The pressure of the plate roller is heavy, the deformation amount is large, the diameter of the rubber roller is thinner, the line speed of the plate roller and the plate roller are inconsistent, and the mesh point is elongated in the circumferential direction of the roller to cause tensile deformation. The pressure of the plate by the plate roller is too large, causing the ink to spread on the plate as it passes from the ink roller to the plate. Therefore, it is necessary to readjust the pressure of the plate by the plate roller. The wear of the equipment causes the dot to increase, such as the increase of the dot caused by the wear of the gear, which is generally represented by the ink stick and the ghost image. Solution: Generally, the tooth spacing can be appropriately reduced without the top teeth.

6. The amount of 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 is increased, and the separation force is also rapidly increased. The excess ink cannot be transferred instantaneously by the substrate, and more ink is accumulated on the surface of the printing plate and the blanket, and the excess ink is diffused outside the dot. The dot is enlarged and even the paste is formed, so the amount of ink to be supplied must be strictly controlled according to the print.

7. Control of the amount of dampening solution

The printing plate has a small amount of water, so that the water film in the blank portion cannot effectively resist the expansion of the ink layer on the dot. Solution: Adjust the pressure of the plate water roller so that it can transfer water evenly and stably, or increase the amount of dampening solution or dampening solution. However, if the amount of dampening solution is too large or too small, the dot size may increase. Therefore, the amount of dampening solution must be properly controlled during the printing process, and the pH of the dampening solution should be within a reasonable range. Between 4.8 and 5.3. In actual printing, it is necessary to appropriately adjust according to actual conditions. For example, if a desiccant is used in the ink, the pH of the dampening solution can be appropriately lowered. However, it is necessary to control the excessive emulsification of the ink to prevent the ink film on the dot from being able to maintain a relatively clear boundary with the blank portion of the paper, resulting in an increase in the dot size, and the amount of the fountain solution or the wetting powder can be appropriately reduced.

8. The deformation of the blanket

Due to the elastic deformation of the blanket, relative slippage occurs between the printing plate and the blanket, between the blanket and the substrate, causing the dot to increase. Therefore, when adjusting the printing pressure and changing the center distance of the drum, the surface speed of the drum should be as uniform as possible. However, it is impossible to be completely consistent, because the surface speed of the blanket cylinder itself is inconsistent due to the compression of the blanket. The blanket is not tight enough or has poor compressibility. After the new blanket was printed on the machine for five or six thousand sheets for the first time, tighten the blanket once again. After printing one or two thousand sheets, tighten the blanket once more.

Second, the measurement of the expansion value of the outlet

There are two methods for measuring the dot gain value: one is the reflection densitometer measurement method, and the other is the Brunel printing measurement bar measurement method.

1. The reflection densitometer measurement method uses the Murray-Davies formula to calculate the dot gain value after measuring the density by a densitometer, and can perform quantitative analysis to realize the printing data and standardization. The dot gain value is equal to the dot area on the printed matter minus the dot area of the corresponding portion on the original plate (film used for printing).

Murray-Davies calculates the formula as:

Where: the density of the dot density of the part to be tested on the printed matter, the value of the solid density on the printed matter or the measuring strip, the area of the dot of the part to be tested on the printed matter, the original area of the part to be tested on the printed matter, and the expanded value of the dot.

2. Brunel printing measurement and control strip measurement method. The Brunel measure strip has a fine mesh segment of 60 lines/cm and a thick mesh segment of 10 lines/cm, and the 50% dot area ratio of the fine mesh and the coarse mesh is equal. The ratio of the line number of the fine mesh and the coarse mesh is 6:1, that is, the ratio of the edge length of the fine mesh to the edge length of the coarse mesh is 6:1. Since the amount of dot gain is proportional to the edge length of the dot, the density of the fine dot is large and the density is large under the same conditions. When calculating, taking the coarse network segment as the reference and taking the difference between the density of the thin network segment and the coarse network segment, the 50% dot area expansion value can be obtained. Its calculation formula is:

= 50% fine mesh 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 mesh segment density - 75% coarse segment density) ÷ 2

The calculation formula for the dot gain value of about 25% is:

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

Third, the expansion of the offset printing network control

Although the dot gain 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 newspaper printing is controlled within 35% (generally referred to as 50% dot). In the production process, it can be effectively controlled from the following aspects.

1. Select high quality plate, reasonable control of exposure time and developer concentration

The printed plates are clear and the dots are strong, so as to ensure the transfer effect of the dots, and to ensure that the plates have a high printing durability. Usually, 5% of the outlets are not lost, and 95% of the outlets are not blurred.

2. Strictly control the ink and water balance in printing

In the printing process, if the water supply is too large, the amount of ink supplied to the layout will increase, the ink layer will become thicker, and the ink will be emulsified seriously, resulting in a dull and dull impression. In the production process, in order to reduce the start-up waste, the water level is usually increased to 150% of the normal value at the time of starting. When the printing is normal, the water quantity should be gradually reduced to the normal value to ensure that the graphic is clear, saturated and shiny.

3. Control various printing pressures within a reasonable range

The pressure between the plate cylinder and the blanket cylinder is controlled to be 0.15 to 0.20 mm, and it is ensured that the two ink rollers are parallel to the plate cylinder, and the offset roller and the plate cylinder having a diameter of 100 mm or less have an embossing width of 3 ~4mm, the printing roller with a diameter of 100mm or more and the plate cylinder embossing width is 5 ~ 6mm.

4. Correctly control the concentration of dampening solution

The control of the concentration of the fountain solution plays a crucial role in the balance of the ink and the quality of the printed product. A suitable concentration of dampening solution can quickly wash away the “dirt” around the dots. If the concentration of the dampening solution is low, the ink around the dot can not be removed, which will cause the dot to increase, and the paste will cause the paste; if the concentration of the dampening solution is too high, the printing plate of the PS plate will be reduced and the dampening solution will be A lot of waste. Normally, the pH of the dampening solution is controlled at 4.0 to 6.0, and the conductivity is controlled at 1400 ± 200 μs.