Keywords: Overprint accuracy, plate liner blanket
1. Overprint and overprint error concept
The monochromatic printing plates with different screen angles prepared after color separation of the original are overlapped in sequence according to the printing color sequence, and finally the printed products with the same level and color tone as the original are called printing overprint.
Accurate overprinting is the most basic requirement for printing, but due to various subjective and objective factors, overprinting inevitably has a certain deviation (generally this kind of deviation is called "overprinting error"). Therefore, what we often say is "accurate overprinting" actually means that the error of overprinting is controlled within a certain range.
2. Evaluation of overprint accuracy
The permissible tolerance value of overprinting is generally formulated on the basis of human visual observation. Relevant research proves that under normal observation conditions, the human eye's ability to distinguish fine lines can reach 0.10mm. Therefore, the tolerance allowed for overprinting is set to 0.10mm, that is, 0.10mm is used as a reference for overprinting accuracy (but at this time the human eye can also recognize very subtle tones). Practice has confirmed that, for the wet stacking monochromatic overprinting, the tolerance of overprinting can be set to 0.10mm to meet the requirements; but for the wet stacking wet multicolor overprinting, the tolerance of overprinting must be equal to or less than 0.03mm. Under normal circumstances, the overprint standard is usually set in the range of 0.05 to 0.10mm, and for the prints with low quality requirements, the overprint standard can even be relaxed to 0.15 to 0.20mm.
The rule of overprint error is neither uniform nor disorderly, and it is in a normal distribution in mathematical statistics. Further research confirmed that in practice, 68.26% of the overprint errors actually measured are within the single standard error (1s), 95.45% are within the double standard error (2s), and 99.73% are within the triple standard error ( 3s). This provides an important basis for the evaluation of overprint accuracy.
In order to determine the accuracy of overprinting, the "positioning mark method" is usually used to detect changes during overprinting at any time, so that printing operators can take timely measures to improve the accuracy of overprinting. In the offset printing process, the “cross-mark method†is generally used for positioning. This method is marked on the four corners and the middle part of the printing plate during plate making, and is cut off with the paper edge after printing.
In multi-color printing, you can check the overprint of the printed product in the up, down, left and right, and diagonal directions from the degree of misalignment or skew of the line marks of various colors. These marking lines are commonly known as rule lines, sometimes also simply referred to as "rule lines". The rule line is mainly divided into three types: cross line, angle line and cutting line. Since the rule line plays an important role in positioning, you must be very careful when making it. You must ensure that the rule line itself has high dimensional accuracy and geometric accuracy, and its lines Be small, straight, and clear.
Generally, overprinting inaccuracy mostly occurs at the two corners of the tail of the paper, and images in the mouth area are less likely to occur. Moreover, the side where the overprint is not allowed to be more serious must be the part opposite to the side rule.
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3. Influencing factors of overprinting
The following focuses on the analysis of some major factors that affect the accuracy of overprinting in the printing process.
1. Machinery
The key to accurate overprinting is that the paper conveyed by the paper feeder is accurately conveyed to the impression cylinder in accordance with the up and down (that is, the printing direction, along the radial direction of the cylinder) and the left and right directions (that is, the vertical direction of the printing direction, along the cylinder axis) on.
The overprint adjustment structure composed of paper blocking gauges, swinging teeth, gripping teeth, etc., only fixes the paper in the correct position and performs precise movements, and the movements of various components must be very coordinated, and no errors can be registered.
Overprint errors caused by mechanical factors are mainly due to improper adjustment and use of various parts of the machine, resulting in misalignment of parts or mechanical wear. Generally it mainly occurs in the following four processes:
(1) Paper feeding and conveying process of the paper conveyor;
⑵Pre-positioning process;
(3) Side positioning process;
â‘·Handover process of paper transfer (namely, handover between the front rule and transfer gripper and transfer gripper and roller gripper).
The paper is sent to the pre-positioning position of the paper from the paper conveying table until the end of the imprinting. The cooperation of the paper between the components must meet the following principles in technology:
(1) During the process of separation, conveyance and imprinting of paper, organic parts must always be strictly and correctly controlled;
⑵The working cycle and trajectory of each part of the paper conveyor must be closely coordinated with each other accurately;
(3) During paper transport, the paper spacing must be accurate;
â‘· Make full use of the paper positioning efficiency as much as possible to avoid the positioning time change caused by various factors;
⑸ Correctly adjust the paper feeding time of the paper feeder to coincide with the main cylinder cycle;
⑹The time of the front rule, side rule and paper delivery tooth must be accurate when the paper is positioned and transferred;
⑺Pre-set position, the paper should not be bite more, less bite and oblique bite, the upper and lower limits should not exceed 3 ~ 5mm;
â‘»When handing over the transfer of paper teeth and roller teeth, the time must be accurate, and absolutely no leading or lagging phenomenon;
⑼When handing over and controlling paper, the machine must be very stable, and it is not affected by other external forces to change the accuracy of the work;
⑽ The bite of the roller bite the paper and the delivery tooth should be even and consistent, and it should be greater than the pushing force of the roller against the paper;
⑾Each roller gear should be meshed at the tangent point of the pitch circle, and the distance between the two sides of the roller should be equal and parallel;
â‘¿Paper table, side gauges, teeth and rollers can not have axial series movement;
â’€The pinch wheel, brush wheel, brush, etc. that control the accurate positioning of the paper on the paper feeding table, the distance between the trailing edge of the paper, the position of the pressure paper, and the size of the pressure must be strictly controlled and controlled.
The paper handover relationship is very critical. The content of the handover relationship includes two aspects: one is the handover position, that is, the relative position of the two handover parts when handing over the paper, where one part is used as a reference, and the position of the other part is adjusted to be relative to the reference part In coordination, there is a sequence of adjustments between the various parts. The second is the handover time, which refers to the time when the two handover devices control the paper at the handover position (near) at the time of handing over the paper, that is, when the handover device also controls the paper and the paper receiving device starts to control the paper After a period of time, the paper delivery mechanism will stop. There is no order requirement for handover time.
It is worth mentioning that in order to prevent uneven gripping force of gripper teeth and cause overprinting inaccuracy, the following points should be achieved:
â‘´ The adjustment of the gripper should be carried out during the trial printing, and the gripping force should not be changed during the process of overprinting of various colors. For each color unit of the multi-color machine, the gripping force of the gripper teeth on the impression cylinder must be consistent, and the gripper tooth pads must be flat to ensure uniform gripping force.
⑵For front and back self-printing products, the gripping force of the gripper of the impression cylinder must be uniform.
(3) The supporting spring of the gripper shaft must have good elasticity to prevent insufficient gripping force and inaccurate paper positioning.
â‘· Tighten the grip ring of the gripper to prevent axial movement of the gripper. Use high-precision gripper shafts or sleeves to keep grippers in a stable state during gripping, and the gripping distance is always maintained at a constant value.
2. Plate
The commonly used printing plate material is a thin and light metal plate material, which will greatly affect the accuracy of overprinting due to changes in shape or whatever.
â‘´ Tensile deformation
When the two ends of the printing plate are subjected to the tensile force, due to the ductility of the metal plate, an irreversible radial elongation deformation will occur, and the graphics on the surface of the printing plate will inevitably extend, and the elongation value is generated by the graphics Overprint error.
Under the action of tensile force, the elongation value â–³ L of the metal printing plate is proportional to the tensile force F and the length L, and inversely proportional to its cross-sectional area A and elastic modulus E. The formula is expressed as:
△ L = F · L (E · A)
The above formula can be deduced, and the conclusions obtained by practice are:
â‘ The tensile deformation â–³ L of the printing plate is generally caused by operations such as plate installation and plate pulling.
â‘¡ The greater the tension F of the tight plate, the greater the deformation of the printing plate. Therefore, do not apply excessive force when installing or pulling the plate.
â‘¢ The greater the thickness of the printing plate (that is, the cross-sectional area A), the smaller the deformation of the printing plate. Therefore, from this perspective, the use of thicker materials as printing plates can reduce overprinting errors.
â‘£ The longer the length L of the printing plate, the greater the stretching value, which is more serious on machines with larger printing area.
⑤ The greater the elastic modulus E of the printing plate, the smaller the deformation of the printing plate.
In addition, in actual operation, it is also easy to know that the machine with a small wrap angle has a smaller plate stretching value and a more even distribution of the stretching value. By pulling and vibrating when pulling the plate, it can eliminate the pulling due to friction Printing plate; the machine with large wrap angle, the printing plate is not easy to be pulled, and the axial moving parts must be prepared to make the plate clamp produce axial displacement and eliminate the back friction.
⑵Bending deformation
After the printing plate is installed on the printing plate cylinder, it will change from a regular sheet to a cylinder, and bending deformation will inevitably occur. The formula is as follows:
△ L = L · b / （2R + b）
Among them, b refers to the thickness of the printing plate, R refers to the radius of the printing plate cylinder, and L refers to the original length of the graphic.
It can also be inferred from the above formula:
â‘ When the thickness of the printing plate is constant, the bending deformation value of the printing plate decreases as the radius of the printing plate cylinder increases. When the radius of the plate cylinder is fixed, the bending deformation of the plate surface increases with the increase of the plate thickness.
â‘¡The surface coating of the multi-layer metal plate should not be too thick, otherwise it will be excessively deformed due to the bending of the plate surface, which will cause the plating layer to partially crack and form densely distributed moire, causing the blank part of the printing plate to become dirty. In addition, flexible printing plates such as photosensitive resin plates have thicker bases and larger deformations. Therefore, in the layout design, we must focus on the problem of deformation.
â‘¢The multi-color printing machine should use the printing plates of the same thickness to make the bending deformation of each printing plate close to each other, so as to ensure accurate overprinting.
â‘£High-speed rotary printing machine, the printing speed is higher, the radius of the plate cylinder is smaller, the surface utilization coefficient is larger, so the bending deformation of the printing plate is large. In order to control the bending deformation of the printing plate, it is necessary to use a thinner printing plate.
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⑶ How to adjust the printing plate
It is particularly worth mentioning that under normal circumstances, we find that overprinting is not accurate when printing, and most of them are adjusted by stretching the printing plate. The greater the pulling force, the greater the tensile deformation of the layout, and the uneven distribution of the tensile deformation (the stress is greatest on the gripping edge and trailing edge of the layout). From the above analysis, it is known that this is detrimental to the accuracy of overprinting.
In the operation of the printing machine, the standard action for adjusting the printing plate should be: When the printing plate needs to be moved axially, the intimate friend first cancels the circumferential tension on the printing plate, and only then can the friction of the pad against the printing plate The force disappears or is reduced to a sufficient level; when the printing plate needs to be moved in the circumferential direction, the tension at one end of the moving plate must be removed, so that the plate can be loosened, and you must pay attention to not use excessive force. Otherwise, the amount of tensile deformation of the printing plate will increase greatly. Therefore, when adjusting the position of the printing plate, the axial movement and the circumferential movement of the printing plate must be coordinated. Do not pull hard or knock hard.
In addition, if the printing plate is loose during printing, the position of the print will also jump during the printing process, which will also affect the accuracy of overprinting.
In addition, the multi-color machine's platemaker uses the same platemaking conditions to reduce or eliminate various disadvantages.
3. Paper
In the process of multi-color overprinting, graphic overprinting errors are often caused by paper deformation. It is also a major factor affecting the accuracy of overprinting.
The paper deformation is summarized, there are natural deformation and pressure deformation. The former refers to paper without external force, due to changes in its own water content, water absorption or dehydration, resulting in dimensional elongation or shortened deformation. This deformation is closely related to the temperature and humidity of the surrounding environment. The latter refers to the plastic deformation of the linear size and area caused by the displacement of the fibers when the paper is pressed by the roller.
Overprint errors caused by these two deformations of paper are almost inevitable. However, effective control of the paper's moisture content, reasonable adjustment of the ambient temperature and humidity and the pressure of the roller can ensure the stability of the paper's linear size and area, reduce the deformation caused by the paper, and thus reduce the overprint error.
Both theory and actual production confirm that during the process of overprinting, the moisture content of the paper should not change more than ± 1%, otherwise it will affect the accuracy of the overprinting. Therefore, the temperature of the workshop must be strict, and the general change is not more than ± 3 ℃.
The thread of paper also has a great influence on overprinting. In printing, the longitudinal thread paper is mostly used. Due to changes in temperature and humidity, the diameter of a dried fiber after full wetting may increase by 30%, while the length direction only increases by 1% to 2%. Therefore, the stretch rate of the horizontal thread lock is much greater than that of the longitudinal thread lock, which directly affects the accuracy of the overprint. The expansion and contraction of the warp can be compensated by adjusting the lining and other methods.
In addition, the amount of moisture in the printing process also affects the accuracy of overprinting. In general, the moisture should not be too large, otherwise the paper will stretch too much.
In addition, the cutting accuracy of the printed paper will also affect the overprint error. The cutting accuracy of paper mainly includes dimensional accuracy and verticality.
General requirements for good cutting accuracy:
⑴∠The verticality of BCD is very important, its right angle must be controlled within the allowable accuracy:
⑵The cutting specifications of the paper must be AB = CD, and AD and BC are parallel, and CD and BC are vertical.
4. Pad
Changing the thickness of the liner makes the imprint on the printed product and the size of the printing plate of the cylinder inaccurate, which usually occurs in the radial direction of the cylinder. This change in the size of the picture and text due to the change in the thickness of the roller pad has two cases:
â‘ When the radius of the printing plate cylinder is larger than the radius of the blanket cylinder, the size L of the plate image is smaller than the size L of the paper image. The registration error at this time is: â–³ L = L paper-L version.
â‘¡When the radius of the printing plate cylinder is smaller than the radius of the blanket cylinder, the size of the printing plate is larger than the size of the paper. The registration error at this time is: â–³ L = L version-L paper.
In order to change the error value of the extension or shortening of the picture and text, the method of changing the thickness of the roller pad can be used to balance the error. In actual operation, in order to make the size of the plate image and text extend in the radial direction, it is often used to reduce the thickness of the plate cylinder liner; The thickness of the roller pad is used to maintain the increase and decrease of the pad. On the contrary, in order to shorten the size of the printing plate and text in complex labor, increase the thickness of the plate cylinder liner according to the same reason; or increase the thickness of the plate cylinder liner while reducing the blanket cylinder The thickness of the liner keeps the increase and decrease of the liner equal.
However, when increasing or decreasing the padding operation, it should be noted that the maximum allowable value for the increase or decrease of the padding of the roller can generally only be controlled within the range of ± 0.10mm.
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5. The deformation of the rubber blanket will also cause the size of the picture and text to change and cause overprint errors. It mainly comes from the effect of pressure and tension and the quality of the blanket itself.
Tightening the blanket on the roller not only produces a relative friction on the surface of the roller, but also has a close relationship with the overprint error caused by the change in the graphic size caused by the extrusion deformation and elongation of the blanket. It is known from the law of the deformation of the blanket that its predatory deformation mainly depends on two points: â‘ the direction and magnitude of the friction force; â‘¡ the elongation of the blanket itself. Under the same friction force, if the elongation of the blanket itself is different, the degree of deformation must also be different. At the same time, it also depends on how tight the blanket is on the cylinder, that is, the tighter the stretch, the smaller the elongation during the printing process, and the smaller the graphic size change, but the amount of friction on the layout has increased; Ran. In addition, if the position of the blanket (including the pad below) changes during the printing process, the print often changes, which also affects the registration accuracy.
To analyze the causes of the overprint errors caused by the above-mentioned blankets in the changes of graphics and text, you must pay attention to in daily production:
a. Try to choose a rubber cloth with a smaller elongation to reduce deformation;
b. Maintain the tightness of the blanket, not too loose or too tight, and make it constant;
c. The overprint error caused by the change of the graphic size caused by the tightness of the rubber blanket should not exceed 0.10mm;
d. If the machine is shut down for a long time, it is better to loosen the rubber cloth tightly on the roller.
Finally, it must be stated that among the above factors, factors such as machinery, paper and printing plates are the most important factors.
4. Measures and suggestions for improving the accuracy of printing and overprinting
1. When designing and making printing plates, we must ensure the accuracy of overprinting. In particular, it is necessary to take into account the amount of deformation of the printing plate during the layout design and compensate for this.
2. The operation of adjusting the printing plate must be standardized.
3. Properly control the printing pressure (preferably the minimum pressure), increase or decrease the thickness of the liner according to the actual situation, and reduce the deformation of the printing plate, paper, blanket, etc.
4. Strictly control the temperature and humidity of the printing environment, reduce the natural deformation of the paper, and ensure the cutting accuracy of the paper.
5. Adjust all the machine parts and their handover relations that affect the accuracy of the overprinting, such as the paper transport mechanism, positioning mechanism, and paper delivery mechanism of the printing press, to the best position.
6. Strengthen the sense of responsibility of the operating personnel, do diligent inspection of printed products, find faults, and eliminate them in time.
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