When the thickness of paper changes, two major changes have occurred:
First, the pressure between the blanket cylinder and the impression cylinder has changed and the original pressure state cannot be maintained. As the paper becomes thicker and thinner, the pressure between the blanket cylinder and the impression cylinder increases; when the paper becomes thinner, the pressure between the blanket cylinder and the impression cylinder decreases.
Second, the size of the image (print length) printed on the substrate by the printing plate changes. When the thickness of the paper is increased from thin to thin, the length of the image obtained by the impression cylinder is relatively elongated; when the thickness of the paper is reduced, the length of the graphic image obtained by the impression cylinder is relatively reduced.
In order to have a more accurate adjustment of the printing machine when the paper thickness changes, let us first understand the adjustment of the printing pressure and the relationship between the radius of the two rollers and the size of the graphic and text.
Roller pressure
Lithographic offset printing process is to transfer the printing ink on the printing plate to the blanket, and then transferred to the substrate by the rubber. In this transfer process, the surface of the plate cylinder must be sufficiently contacted with the blanket cylinder, the blanket cylinder and the impression cylinder (substrate), that is, there must be a certain contact pressure between the contact surfaces, so that the ink can be fully transferred therebetween. This contact pressure is the printing pressure. Roller pressure: The force of interaction between embossed bodies during printing.
Printing pressure is the primary condition for the completion of lithographic offset printing. The ideal pressure between the rollers is the key to guarantee the printing quality. The ideal printing pressure refers to the pressure on the basis of ensuring that the ink is well transferred (that is, to ensure that the printed material is strong in the print, full of dots but not overlaid, and clear and rich in tone levels), and to minimize the printing pressure between the rollers. Whether or not pressure can be achieved (ie, the use of ideal pressure) is one of the important indicators for measuring the level of technology, and is also an important condition for ensuring high quality and high output.
Too much or too little printing pressure has a great influence on product quality. Excessive pressure can easily cause dot distortion, stencil, dark ink, or even image distortion. Excessive pressure can also accelerate plate wear and machine wear. Too little pressure can result in insufficient or unstable ink transfer resulting in imprint emptiness. , pale ink, unclear image contours, inconsistent ink shades, etc.
How to adjust printing pressure in actual production:
1Adjust the printing pressure by adjusting the center distance of the roller;
2 Adjust the printing pressure by adjusting the thickness of the lining (plate cylinder, blanket cylinder lining).
Note: The size of the center distance is related to the meshing condition of the roller gears. It should be adjusted according to the actual needs of the printing process. It is not suitable for regular adjustment, and must be adjusted within the range specified by the machine operation and must ensure that the pressure on both sides is consistent.
2. The relation between the three-roller radius and the graphic size
General three-cylinder offset press drive: gear transmission with the same number of teeth, modulus, and helix angle. Therefore, regardless of whether the radii of the two rollers are equal, the angular velocities of the three rollers are completely equal.
Ideally, the radius of the drum after lining should be equal to the linear speed of the drum surface in the printing process, and the rolling of the drum should be kept without any slippage. This ensures that the quality points of the printing surface are exactly coincident after each operation. Deformation or ghosting. If the three rollers are rigid bodies, as long as the three rollers have the same radius, the line speed of the three rollers can be guaranteed to be equal.
However, the material of the three-roller coating is different, and the size of the three-roller radius after the packing is different. In order to maintain a pure rolling and no slipping state, the radius of the blanket cylinder after lining should be made smaller than the radius of the plate cylinder and the impression cylinder, and in principle it should be equal to the radius of the pitch circle.
In order to facilitate the study of the relationship between the three-roller radius and the text size, we use the three-roller as a rigid body. Assume that after the installed version, the graphic area accounts for 180o. Now we will study the following situations:
1If the running diameters of the plate cylinder and the impression cylinder are equal and the running diameter of the blanket cylinder is relatively small (or larger), because the angular velocities are equal, when the plate rotates through l80o, the other two cylinders also rotate through 180o, That is, the area of ​​the graphics and graphics occupies a 180° position on the impression cylinder. Since the running diameters of the blanket cylinder and the impression cylinder are equal, the lengths of the imprints obtained on the surface of the printing plate cylinder and the impression cylinder are equal. ).
Conclusion: When neglecting the graphic deformation caused by the elastic deformation of the blanket cylinder, the size of the running diameter of the blanket cylinder does not affect the length of the graphic print, but it will affect the printing quality.
2 If the diameter of the impression cylinder is smaller than the diameter of the plate cylinder, when the plate cylinder rotates through 180o, the other two cylinders also turn through l80o, ie, the area of ​​the graphic image occupies 180o on the impression cylinder, due to the pressure. The operating diameter of the printing cylinder is smaller than the diameter of the printing cylinder, and the length of the graphic image actually obtained by the impression cylinder is relatively reduced (the graphic is shortened).
Conclusion: When the diameter of the impression cylinder is smaller than the diameter of the plate cylinder, the picture is shortened.
3 If the working diameter of the impression cylinder is larger than the diameter of the plate cylinder, the area of ​​the graphic image occupies the l80o position on the impression cylinder. Since the diameter of the impression cylinder is larger than the diameter of the plate cylinder, the length of the graphic image actually obtained by the impression cylinder is relatively Elongation (text is elongated).
Conclusion: When the diameter of the impression cylinder is larger than the diameter of the plate cylinder, the graphic is elongated.
3. Press regulation when the paper thickness changes
Take paper thickness from thin to thick as an example
1 If the paper changes little from thin to thick, the text does not extend beyond the acceptable range.
Adjustment method: The center distance between the blanket cylinder and the impression cylinder can be directly pulled. The amount of pull-off can be obtained by measuring the change in paper thickness.
(2) If the paper changes greatly from thin to thick, the elongation of the graphic image has exceeded the range of qualified products, but the effect of adjusting the center distance on the meshing condition of the roller gear is still within an acceptable range.
Adjustment method: correspondingly increase the lining amount of the plate cylinder (the increase range can be obtained according to the change of the paper thickness), and simultaneously pull the center distance between the plate cylinder and the blanket cylinder, the blanket cylinder and the impression cylinder, and Re-adjust the ink roller pressure.
3 If the thickness of the paper varies greatly from thin to thin, not only the elongation of the graphic image has exceeded the range of qualified products, but also the adjustment of the center distance has affected the meshing of the roller gear.
Adjustment method: correspondingly reduce the lining amount of the impression cylinder, increase the lining amount of the plate cylinder, and readjust the pressure of the ink roller.
Note: The first adjustment method is most widely used and the adjustment is the simplest; the second adjustment method is more reasonable, and the image size can be kept basically unchanged; the third adjustment method can make the pressure in the normal range, and can also make the figure The size of the paper remains basically unchanged, but due to this adjustment method, the running diameter of the blanket cylinder is different from the running diameter of the plate cylinder and the impression cylinder, so that a slip phenomenon occurs between the rollers, which affects the printing quality.
4. Inspection of printing pressure
The most common and intuitive method for checking the printing pressure is the pressure bar method. The specific operation is as follows: lift the water roller, drop the ink roller, run the machine at a low speed and pressurize, wait until the surface of the three rollers is evenly inked, and then stop. Each cylinder has an ink stick at the parking place. For example, the width of the ink bar is within the value range given by the machine and uniform, and the width of the ink bar between the impression cylinder and the blanket cylinder is greater than the width of the ink bars of the plate cylinder and the blanket cylinder. , it shows that the pressure adjustment is even and suitable.
If the width of the press bar is wide and narrow, the maximum distance between the two sides of the roller may be inconsistent.
First, the pressure between the blanket cylinder and the impression cylinder has changed and the original pressure state cannot be maintained. As the paper becomes thicker and thinner, the pressure between the blanket cylinder and the impression cylinder increases; when the paper becomes thinner, the pressure between the blanket cylinder and the impression cylinder decreases.
Second, the size of the image (print length) printed on the substrate by the printing plate changes. When the thickness of the paper is increased from thin to thin, the length of the image obtained by the impression cylinder is relatively elongated; when the thickness of the paper is reduced, the length of the graphic image obtained by the impression cylinder is relatively reduced.
In order to have a more accurate adjustment of the printing machine when the paper thickness changes, let us first understand the adjustment of the printing pressure and the relationship between the radius of the two rollers and the size of the graphic and text.
Roller pressure
Lithographic offset printing process is to transfer the printing ink on the printing plate to the blanket, and then transferred to the substrate by the rubber. In this transfer process, the surface of the plate cylinder must be sufficiently contacted with the blanket cylinder, the blanket cylinder and the impression cylinder (substrate), that is, there must be a certain contact pressure between the contact surfaces, so that the ink can be fully transferred therebetween. This contact pressure is the printing pressure. Roller pressure: The force of interaction between embossed bodies during printing.
Printing pressure is the primary condition for the completion of lithographic offset printing. The ideal pressure between the rollers is the key to guarantee the printing quality. The ideal printing pressure refers to the pressure on the basis of ensuring that the ink is well transferred (that is, to ensure that the printed material is strong in the print, full of dots but not overlaid, and clear and rich in tone levels), and to minimize the printing pressure between the rollers. Whether or not pressure can be achieved (ie, the use of ideal pressure) is one of the important indicators for measuring the level of technology, and is also an important condition for ensuring high quality and high output.
Too much or too little printing pressure has a great influence on product quality. Excessive pressure can easily cause dot distortion, stencil, dark ink, or even image distortion. Excessive pressure can also accelerate plate wear and machine wear. Too little pressure can result in insufficient or unstable ink transfer resulting in imprint emptiness. , pale ink, unclear image contours, inconsistent ink shades, etc.
How to adjust printing pressure in actual production:
1Adjust the printing pressure by adjusting the center distance of the roller;
2 Adjust the printing pressure by adjusting the thickness of the lining (plate cylinder, blanket cylinder lining).
Note: The size of the center distance is related to the meshing condition of the roller gears. It should be adjusted according to the actual needs of the printing process. It is not suitable for regular adjustment, and must be adjusted within the range specified by the machine operation and must ensure that the pressure on both sides is consistent.
2. The relation between the three-roller radius and the graphic size
General three-cylinder offset press drive: gear transmission with the same number of teeth, modulus, and helix angle. Therefore, regardless of whether the radii of the two rollers are equal, the angular velocities of the three rollers are completely equal.
Ideally, the radius of the drum after lining should be equal to the linear speed of the drum surface in the printing process, and the rolling of the drum should be kept without any slippage. This ensures that the quality points of the printing surface are exactly coincident after each operation. Deformation or ghosting. If the three rollers are rigid bodies, as long as the three rollers have the same radius, the line speed of the three rollers can be guaranteed to be equal.
However, the material of the three-roller coating is different, and the size of the three-roller radius after the packing is different. In order to maintain a pure rolling and no slipping state, the radius of the blanket cylinder after lining should be made smaller than the radius of the plate cylinder and the impression cylinder, and in principle it should be equal to the radius of the pitch circle.
In order to facilitate the study of the relationship between the three-roller radius and the text size, we use the three-roller as a rigid body. Assume that after the installed version, the graphic area accounts for 180o. Now we will study the following situations:
1If the running diameters of the plate cylinder and the impression cylinder are equal and the running diameter of the blanket cylinder is relatively small (or larger), because the angular velocities are equal, when the plate rotates through l80o, the other two cylinders also rotate through 180o, That is, the area of ​​the graphics and graphics occupies a 180° position on the impression cylinder. Since the running diameters of the blanket cylinder and the impression cylinder are equal, the lengths of the imprints obtained on the surface of the printing plate cylinder and the impression cylinder are equal. ).
Conclusion: When neglecting the graphic deformation caused by the elastic deformation of the blanket cylinder, the size of the running diameter of the blanket cylinder does not affect the length of the graphic print, but it will affect the printing quality.
2 If the diameter of the impression cylinder is smaller than the diameter of the plate cylinder, when the plate cylinder rotates through 180o, the other two cylinders also turn through l80o, ie, the area of ​​the graphic image occupies 180o on the impression cylinder, due to the pressure. The operating diameter of the printing cylinder is smaller than the diameter of the printing cylinder, and the length of the graphic image actually obtained by the impression cylinder is relatively reduced (the graphic is shortened).
Conclusion: When the diameter of the impression cylinder is smaller than the diameter of the plate cylinder, the picture is shortened.
3 If the working diameter of the impression cylinder is larger than the diameter of the plate cylinder, the area of ​​the graphic image occupies the l80o position on the impression cylinder. Since the diameter of the impression cylinder is larger than the diameter of the plate cylinder, the length of the graphic image actually obtained by the impression cylinder is relatively Elongation (text is elongated).
Conclusion: When the diameter of the impression cylinder is larger than the diameter of the plate cylinder, the graphic is elongated.
3. Press regulation when the paper thickness changes
Take paper thickness from thin to thick as an example
1 If the paper changes little from thin to thick, the text does not extend beyond the acceptable range.
Adjustment method: The center distance between the blanket cylinder and the impression cylinder can be directly pulled. The amount of pull-off can be obtained by measuring the change in paper thickness.
(2) If the paper changes greatly from thin to thick, the elongation of the graphic image has exceeded the range of qualified products, but the effect of adjusting the center distance on the meshing condition of the roller gear is still within an acceptable range.
Adjustment method: correspondingly increase the lining amount of the plate cylinder (the increase range can be obtained according to the change of the paper thickness), and simultaneously pull the center distance between the plate cylinder and the blanket cylinder, the blanket cylinder and the impression cylinder, and Re-adjust the ink roller pressure.
3 If the thickness of the paper varies greatly from thin to thin, not only the elongation of the graphic image has exceeded the range of qualified products, but also the adjustment of the center distance has affected the meshing of the roller gear.
Adjustment method: correspondingly reduce the lining amount of the impression cylinder, increase the lining amount of the plate cylinder, and readjust the pressure of the ink roller.
Note: The first adjustment method is most widely used and the adjustment is the simplest; the second adjustment method is more reasonable, and the image size can be kept basically unchanged; the third adjustment method can make the pressure in the normal range, and can also make the figure The size of the paper remains basically unchanged, but due to this adjustment method, the running diameter of the blanket cylinder is different from the running diameter of the plate cylinder and the impression cylinder, so that a slip phenomenon occurs between the rollers, which affects the printing quality.
4. Inspection of printing pressure
The most common and intuitive method for checking the printing pressure is the pressure bar method. The specific operation is as follows: lift the water roller, drop the ink roller, run the machine at a low speed and pressurize, wait until the surface of the three rollers is evenly inked, and then stop. Each cylinder has an ink stick at the parking place. For example, the width of the ink bar is within the value range given by the machine and uniform, and the width of the ink bar between the impression cylinder and the blanket cylinder is greater than the width of the ink bars of the plate cylinder and the blanket cylinder. , it shows that the pressure adjustment is even and suitable.
If the width of the press bar is wide and narrow, the maximum distance between the two sides of the roller may be inconsistent.
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