Keywords: overprint accuracy, plate liner, blanket, overprint and overprint error concept. Monochrome plates of different network line angles obtained by color separation of the manuscripts are stacked in sequence according to the printing color sequence, and the final printing is obtained at the original document level. The same color prints are called printed overprints.
Accurate printing is the most basic requirement for printing, but due to the influence of various subjective and objective factors, there is inevitably a certain deviation in overprinting (generally this deviation is called "overprinting error"). Therefore, we often say that "accurate overprinting" actually means that the overprint error is controlled within a certain range.
Second, the overprint accuracy evaluation Overprint tolerances, generally based on people's visual observation force based on the formulation. Relevant studies have proved that the ability of the human eye to discern subtle lines can reach 0.10 mm under normal observation conditions. Therefore, the allowable tolerance for overprint is set at 0.10mm, which means that 0.10mm is used as the reference for overprint accuracy (but at this time, the human eye can also recognize the nuance of very slight difference). It has been confirmed by practice that for a wet-dry type monochromatic overprinting, a tolerance of 0.10 mm for overprinting can be satisfied; however, for a wet-dry type multi-color overprinting, the allowed overprinting error must be equal to or less than 0.03 mm. Under normal circumstances, the overprinting standard is usually set in the range of 0.05 to 0.10 mm. For the printing products with low quality requirements, the overprinting standard may even be relaxed to 0.15 to 0.20 mm.
The law of overprinting errors is neither uniform nor disorganized, and it conforms to normal distribution in mathematical statistics. Further research confirms that 68.26% of the overprint errors obtained in practice are actually within the single standard error (1s) range, 95.45% within the double standard error (2s), and 99.73% at the triple standard error ( 3s) within range. This provides an important basis for the evaluation of overlay accuracy.
In order to determine the accuracy of overprinting, the “positioning mark method†is usually used to detect changes that occur at the time of overprinting, so that the printing operator can take timely measures to improve the accuracy of overprinting. In the offset printing process, the “cross-marking method†is generally used for positioning. This method is calibrated at the four corners and the middle portion of the printing plate during the plate making process, and is cut off together with the paper edge after printing.
In multi-color printing, it is possible to check overprinting in the direction of up-down, left-right, and diagonal lines from the misalignment or skewness of line markers of various colors. These marking lines are commonly referred to as rule lines and are sometimes referred to simply as "regular lines." The gauge line is mainly divided into three types: cross line, angle line, and cutting line. Since the rule line plays an important role in positioning, it must be very careful when making, and must ensure that the gauge line itself has high dimensional accuracy and geometric accuracy, and its lines Be small, straight, and clear.
In general, most of the overprinting occurs at the two corners of the paper, and the image at the mouth is less inaccurate. And the side where overprinting is not allowed to be severe is necessarily the part opposite the sideline.
Third, the influencing factors of overprinting The following focuses on the printing process influencing the accuracy of overprinting some of the major factors.
1. The key to the accuracy of mechanical overprinting lies in the fact that the paper conveyed by the paper feeder is accurately conveyed up and down (that is, in the printing direction, along the drum radial direction) and in the left and right direction (that is, in the vertical direction of the printing direction, along the axial direction of the drum). On the impression cylinder.
The overprint adjustment structure consisting of stop gauges, oscillating teeth, grippers, etc., only allows the paper to be fixed in the correct position for precise movement, and the movements of the various components must be coordinated, without any slight errors, before they can be registered.
The overlay error caused by mechanical factors is mainly due to improper adjustment and improper use of various parts of the machine, resulting in misalignment or mechanical wear between the parts. It generally occurs in the following four processes:
(1) Feeding and conveying process of the feeder;
(2) Pre-regulation process;
(3) Side-specific process;
(4) The transfer process of paper transfer (that is, the transfer between the front gauge and the transfer paper, and the transfer between the transfer tooth and the roller).
The paper is sent from the paper feed table to the front of the sheet, until the end of the imprint, the cooperation of the various components in the paper, the process must meet the following principles:
(1) In the process of separating, conveying, imprinting, etc., the paper must always have organic parts strictly and correctly controlled;
(2) The duty cycle, trajectory, etc. of each part of the feeder must be closely matched with each other.
(3) When the paper is transported by paper, the distance between the paper must be accurate;
(4) Use paper positioning performance as much as possible to avoid changes in positioning time caused by various factors;
(5) The correct adjustment of the feeding time of the feeder coincides with the drum cycle of the main body;
(6) When the front gauge, side gauge, and paper handing paper are positioned and transferred in the paper, the time must be accurate;
(7) Before the prescribed position, the paper should not be bite, bite and bite, the upper and lower limits must not exceed 3 ~ 5mm;
(8) When the transfer teeth and the roller teeth transfer the transfer, the time must be accurate, absolutely can not produce lead or lag;
(9) The mechanical parts must be very stable when they are handed over and the paper is controlled. The accuracy of work is not changed by other external forces.
(10) The bite of the roller bite paper and delivery paper should be uniform and greater than the pushing force of the roller on the paper;
(11) Each roller gear should be meshed at the tangent point of the pitch circle, and the distances on both sides of the roller should be equal and parallel;
(12) The feed platen, side gauge, teether and drum must not have axial movement;
(13) Control of the paper-feeding platen The precise positioning of the paper, the distance between the brush roller, the brush, and the trailing edge of the paper, the position and the pressure of the paper, must be strictly controlled and controlled.
The paper transfer relationship is quite critical. The content of the transfer relationship includes two aspects: First, the position of the transfer, that is, the relative position of the two transfer machines when the transfer of paper, in which a machine is used as a reference, and the position of another machine is adjusted to match the reference machine. With coordination, there is a sequence of adjustments among the various parts. The second is the transfer time, that is, the time for the two transfer machines to control the paper at the transfer position (nearby) at the time of the transfer of the paper, that is, the paper is also controlled from the transfer machine, and the take-up machine starts to control the paper. , After a period of time, the paper machine will only stop when the paper is released. There is no order requirement for handover time everywhere.
It is worth mentioning that in order to prevent the uneven gripping force of the grippers from causing misprints, the following should be done:
(1) The adjustment of the gripper should be performed during the trial printing, and the gripping force must not be changed during the process of color overlay. Multi-color machine of the various units, the impression roller on the gripper of the gripper force to be consistent, gripper teeth must be flat to ensure the uniformity of the force.
(2) Positive and negative self-printing products, the pressure of the impression cylinder must be uniform.
(3) The holding spring of the gripper shaft must have good elasticity to prevent the lack of grip force and the positioning of the paper.
(4) Tighten the tight ring of the gripper to prevent axial movement of the gripper. Using a gripper shaft or bushing with a high manufacturing precision, the gripper keeps the card stable during the gripping process and the gripper distance always maintains a constant value.
2. Plates The commonly used printing plate materials are thin and light metal plates, which can greatly affect the accuracy of overprinting due to changes in shape or shape.
(1) Tensile deformation When the two ends of the printing plate are subjected to a pulling force, since the metal plate material has a ductility, an irreversible radial elongation deformation is generated, and the graphic on the printing plate surface is inevitably elongated, and the elongation value thereof is Overprint error generated for graphics.
Under the action of the tensile force, the elongation value ΔL of the metal plate is proportional to the tensile force F and length L, and is inversely proportional to its cross-sectional area A and elastic modulus E. Formulated as:
△L=F·L(E·A)
From the above formula can be introduced, and the conclusion is proved by practice:
1 The tensile deformation ΔL of a plate is generally caused by operations such as plate loading and stencil printing.
2 The larger the tension F of the tight plate, the greater the deformation of the plate. Therefore, do not use excessive force when loading plates or pulling plates.
The greater the thickness of the three plates (that is, the cross-sectional area A), the smaller the deformation of the plate. Therefore, from this point of view, the use of thicker materials for printing plates can reduce overprint errors.
The longer the length L of the printing plate 4, the larger the drawing value, which is more serious in a machine with a larger printing area.
The greater the elastic modulus E of the 5 printing plates, the smaller the deformation of the printing plate.
In addition, in practical operation, it is also easy to know that a machine with a smaller wrap angle has a smaller plate stretching value and a more uniform distribution of the drawing value. When the plate is pulled, the tumble vibration can eliminate frictional force. Plates; Machines with large angles, the plate is not easily pulled, and must be prepared axial movement of the parts, so that the plate clamp to produce axial displacement and elimination of the back of the friction after the serial printing plate.
(2) After the bending deformation plate is mounted on the printing plate cylinder, it is changed into a cylindrical shape from a regular plate shape, and bending deformation is inevitable. Formulated as follows:
△L=L·b/(2R+b)
Where b is the thickness of the printing plate, R is the radius of the plate cylinder, and L is the original length of the graphic.
From the above formula can also be inferred:
1 When the plate thickness is constant, the plate bending deformation value decreases with the increase of the plate cylinder radius. When the radius of the plate cylinder is constant, the bending deformation of the plate increases with the increase of the plate thickness.
2 The surface plating layer of the multilayer metal plate should not be too thick, otherwise it will be excessively deformed due to bending of the plate, resulting in partial cracking of the plated layer, forming a dense pattern of moiré, causing the blank part of the printing plate to become sticky. In addition, photosensitive plates such as photosensitive resin plate, plate base thicker, deformation is also greater. Therefore, it is necessary to consider the amount of deformation in the layout design.
3 Multi-color printing presses should use printing plates of the same thickness so that the bending deformation produced by each printing plate is close to ensure accurate overprinting.
4 High-speed rotary presses have a higher printing speed, a smaller radius of the plate cylinder, and a larger coefficient of surface utilization, which results in a greater amount of bending deformation of the printing plate. In order to control the bending deformation of the printing plate, it is necessary to use a thinner printing plate.
(3) The method of adjusting the printing plate is particularly worth mentioning is that under normal circumstances, we found overprinting inaccurate when printing, most of them adjusted by stretching the printing plate. The greater the tensile force, the greater the tensile deformation of the layout, and the uneven distribution of the tensile deformation (the maximum force is applied to the crease and drag edges of the plate). From the above analysis we can see that this is unfavorable for overprinting accuracy.
In the operation of the printing press, the standard action of adjusting the printing plate should be: When the printing plate needs to move axially, the intimate friend first cancels the circumferential tension of the printing plate, and only then can the pad rub against the printing plate. The force disappears or reduces to a sufficient degree; when the plate needs to be moved in the circumferential direction, the tension at the other end must be removed from the end of the plate, so that the plate can be loosened, and absolutely no attention should be paid to it. Otherwise, the amount of tensile deformation of the printing plate greatly increases. Therefore, when adjusting the position of the printing plate, the axial movement and the circumferential movement of the printing plate must be coordinated, and do not hard-knock the hard knock.
In addition, if the printing plate is loosened during printing, the position of the imprinting will also jump during the printing process, which will also affect the overlay accuracy.
In addition, the printers of multicolor machines are manufactured under the same plate making conditions to reduce or eliminate various unfavorable factors.
3. In the process of multi-color overprinting of paper, the overprinting error of the image is often caused by the deformation of the paper. It is also a major factor in the accuracy of overprinting.
The deformation of the paper summed up, there is a natural deformation and compression deformation. The former refers to the paper in the absence of external forces under the conditions of its own water content changes, absorb water or dehydration resulting in size extension or shorten the deformation. This deformation has a close relationship with the temperature and humidity of the surrounding environment. The latter refers to the plastic deformation of the straight line size and area caused by the fiber misalignment when the paper is subjected to a pressing force during roller rolling.
The overlay error caused by the two deformations of paper is almost inevitable. However, effectively controlling the moisture content of the paper and reasonably adjusting the ambient temperature and humidity and the pressure of the roller can ensure that the straight-line size and area of ​​the paper are stable and reduce the deformation caused by the paper, thereby reducing the overlay error.
Both theory and actual production have confirmed that the moisture content of the paper should not change by more than ±1% during the overprinting process, otherwise the overprinting accuracy will be affected. Therefore, the workshop temperature must be strict, and the general changes are not excessive.
Accurate printing is the most basic requirement for printing, but due to the influence of various subjective and objective factors, there is inevitably a certain deviation in overprinting (generally this deviation is called "overprinting error"). Therefore, we often say that "accurate overprinting" actually means that the overprint error is controlled within a certain range.
Second, the overprint accuracy evaluation Overprint tolerances, generally based on people's visual observation force based on the formulation. Relevant studies have proved that the ability of the human eye to discern subtle lines can reach 0.10 mm under normal observation conditions. Therefore, the allowable tolerance for overprint is set at 0.10mm, which means that 0.10mm is used as the reference for overprint accuracy (but at this time, the human eye can also recognize the nuance of very slight difference). It has been confirmed by practice that for a wet-dry type monochromatic overprinting, a tolerance of 0.10 mm for overprinting can be satisfied; however, for a wet-dry type multi-color overprinting, the allowed overprinting error must be equal to or less than 0.03 mm. Under normal circumstances, the overprinting standard is usually set in the range of 0.05 to 0.10 mm. For the printing products with low quality requirements, the overprinting standard may even be relaxed to 0.15 to 0.20 mm.
The law of overprinting errors is neither uniform nor disorganized, and it conforms to normal distribution in mathematical statistics. Further research confirms that 68.26% of the overprint errors obtained in practice are actually within the single standard error (1s) range, 95.45% within the double standard error (2s), and 99.73% at the triple standard error ( 3s) within range. This provides an important basis for the evaluation of overlay accuracy.
In order to determine the accuracy of overprinting, the “positioning mark method†is usually used to detect changes that occur at the time of overprinting, so that the printing operator can take timely measures to improve the accuracy of overprinting. In the offset printing process, the “cross-marking method†is generally used for positioning. This method is calibrated at the four corners and the middle portion of the printing plate during the plate making process, and is cut off together with the paper edge after printing.
In multi-color printing, it is possible to check overprinting in the direction of up-down, left-right, and diagonal lines from the misalignment or skewness of line markers of various colors. These marking lines are commonly referred to as rule lines and are sometimes referred to simply as "regular lines." The gauge line is mainly divided into three types: cross line, angle line, and cutting line. Since the rule line plays an important role in positioning, it must be very careful when making, and must ensure that the gauge line itself has high dimensional accuracy and geometric accuracy, and its lines Be small, straight, and clear.
In general, most of the overprinting occurs at the two corners of the paper, and the image at the mouth is less inaccurate. And the side where overprinting is not allowed to be severe is necessarily the part opposite the sideline.
Third, the influencing factors of overprinting The following focuses on the printing process influencing the accuracy of overprinting some of the major factors.
1. The key to the accuracy of mechanical overprinting lies in the fact that the paper conveyed by the paper feeder is accurately conveyed up and down (that is, in the printing direction, along the drum radial direction) and in the left and right direction (that is, in the vertical direction of the printing direction, along the axial direction of the drum). On the impression cylinder.
The overprint adjustment structure consisting of stop gauges, oscillating teeth, grippers, etc., only allows the paper to be fixed in the correct position for precise movement, and the movements of the various components must be coordinated, without any slight errors, before they can be registered.
The overlay error caused by mechanical factors is mainly due to improper adjustment and improper use of various parts of the machine, resulting in misalignment or mechanical wear between the parts. It generally occurs in the following four processes:
(1) Feeding and conveying process of the feeder;
(2) Pre-regulation process;
(3) Side-specific process;
(4) The transfer process of paper transfer (that is, the transfer between the front gauge and the transfer paper, and the transfer between the transfer tooth and the roller).
The paper is sent from the paper feed table to the front of the sheet, until the end of the imprint, the cooperation of the various components in the paper, the process must meet the following principles:
(1) In the process of separating, conveying, imprinting, etc., the paper must always have organic parts strictly and correctly controlled;
(2) The duty cycle, trajectory, etc. of each part of the feeder must be closely matched with each other.
(3) When the paper is transported by paper, the distance between the paper must be accurate;
(4) Use paper positioning performance as much as possible to avoid changes in positioning time caused by various factors;
(5) The correct adjustment of the feeding time of the feeder coincides with the drum cycle of the main body;
(6) When the front gauge, side gauge, and paper handing paper are positioned and transferred in the paper, the time must be accurate;
(7) Before the prescribed position, the paper should not be bite, bite and bite, the upper and lower limits must not exceed 3 ~ 5mm;
(8) When the transfer teeth and the roller teeth transfer the transfer, the time must be accurate, absolutely can not produce lead or lag;
(9) The mechanical parts must be very stable when they are handed over and the paper is controlled. The accuracy of work is not changed by other external forces.
(10) The bite of the roller bite paper and delivery paper should be uniform and greater than the pushing force of the roller on the paper;
(11) Each roller gear should be meshed at the tangent point of the pitch circle, and the distances on both sides of the roller should be equal and parallel;
(12) The feed platen, side gauge, teether and drum must not have axial movement;
(13) Control of the paper-feeding platen The precise positioning of the paper, the distance between the brush roller, the brush, and the trailing edge of the paper, the position and the pressure of the paper, must be strictly controlled and controlled.
The paper transfer relationship is quite critical. The content of the transfer relationship includes two aspects: First, the position of the transfer, that is, the relative position of the two transfer machines when the transfer of paper, in which a machine is used as a reference, and the position of another machine is adjusted to match the reference machine. With coordination, there is a sequence of adjustments among the various parts. The second is the transfer time, that is, the time for the two transfer machines to control the paper at the transfer position (nearby) at the time of the transfer of the paper, that is, the paper is also controlled from the transfer machine, and the take-up machine starts to control the paper. , After a period of time, the paper machine will only stop when the paper is released. There is no order requirement for handover time everywhere.
It is worth mentioning that in order to prevent the uneven gripping force of the grippers from causing misprints, the following should be done:
(1) The adjustment of the gripper should be performed during the trial printing, and the gripping force must not be changed during the process of color overlay. Multi-color machine of the various units, the impression roller on the gripper of the gripper force to be consistent, gripper teeth must be flat to ensure the uniformity of the force.
(2) Positive and negative self-printing products, the pressure of the impression cylinder must be uniform.
(3) The holding spring of the gripper shaft must have good elasticity to prevent the lack of grip force and the positioning of the paper.
(4) Tighten the tight ring of the gripper to prevent axial movement of the gripper. Using a gripper shaft or bushing with a high manufacturing precision, the gripper keeps the card stable during the gripping process and the gripper distance always maintains a constant value.
2. Plates The commonly used printing plate materials are thin and light metal plates, which can greatly affect the accuracy of overprinting due to changes in shape or shape.
(1) Tensile deformation When the two ends of the printing plate are subjected to a pulling force, since the metal plate material has a ductility, an irreversible radial elongation deformation is generated, and the graphic on the printing plate surface is inevitably elongated, and the elongation value thereof is Overprint error generated for graphics.
Under the action of the tensile force, the elongation value ΔL of the metal plate is proportional to the tensile force F and length L, and is inversely proportional to its cross-sectional area A and elastic modulus E. Formulated as:
△L=F·L(E·A)
From the above formula can be introduced, and the conclusion is proved by practice:
1 The tensile deformation ΔL of a plate is generally caused by operations such as plate loading and stencil printing.
2 The larger the tension F of the tight plate, the greater the deformation of the plate. Therefore, do not use excessive force when loading plates or pulling plates.
The greater the thickness of the three plates (that is, the cross-sectional area A), the smaller the deformation of the plate. Therefore, from this point of view, the use of thicker materials for printing plates can reduce overprint errors.
The longer the length L of the printing plate 4, the larger the drawing value, which is more serious in a machine with a larger printing area.
The greater the elastic modulus E of the 5 printing plates, the smaller the deformation of the printing plate.
In addition, in practical operation, it is also easy to know that a machine with a smaller wrap angle has a smaller plate stretching value and a more uniform distribution of the drawing value. When the plate is pulled, the tumble vibration can eliminate frictional force. Plates; Machines with large angles, the plate is not easily pulled, and must be prepared axial movement of the parts, so that the plate clamp to produce axial displacement and elimination of the back of the friction after the serial printing plate.
(2) After the bending deformation plate is mounted on the printing plate cylinder, it is changed into a cylindrical shape from a regular plate shape, and bending deformation is inevitable. Formulated as follows:
△L=L·b/(2R+b)
Where b is the thickness of the printing plate, R is the radius of the plate cylinder, and L is the original length of the graphic.
From the above formula can also be inferred:
1 When the plate thickness is constant, the plate bending deformation value decreases with the increase of the plate cylinder radius. When the radius of the plate cylinder is constant, the bending deformation of the plate increases with the increase of the plate thickness.
2 The surface plating layer of the multilayer metal plate should not be too thick, otherwise it will be excessively deformed due to bending of the plate, resulting in partial cracking of the plated layer, forming a dense pattern of moiré, causing the blank part of the printing plate to become sticky. In addition, photosensitive plates such as photosensitive resin plate, plate base thicker, deformation is also greater. Therefore, it is necessary to consider the amount of deformation in the layout design.
3 Multi-color printing presses should use printing plates of the same thickness so that the bending deformation produced by each printing plate is close to ensure accurate overprinting.
4 High-speed rotary presses have a higher printing speed, a smaller radius of the plate cylinder, and a larger coefficient of surface utilization, which results in a greater amount of bending deformation of the printing plate. In order to control the bending deformation of the printing plate, it is necessary to use a thinner printing plate.
(3) The method of adjusting the printing plate is particularly worth mentioning is that under normal circumstances, we found overprinting inaccurate when printing, most of them adjusted by stretching the printing plate. The greater the tensile force, the greater the tensile deformation of the layout, and the uneven distribution of the tensile deformation (the maximum force is applied to the crease and drag edges of the plate). From the above analysis we can see that this is unfavorable for overprinting accuracy.
In the operation of the printing press, the standard action of adjusting the printing plate should be: When the printing plate needs to move axially, the intimate friend first cancels the circumferential tension of the printing plate, and only then can the pad rub against the printing plate. The force disappears or reduces to a sufficient degree; when the plate needs to be moved in the circumferential direction, the tension at the other end must be removed from the end of the plate, so that the plate can be loosened, and absolutely no attention should be paid to it. Otherwise, the amount of tensile deformation of the printing plate greatly increases. Therefore, when adjusting the position of the printing plate, the axial movement and the circumferential movement of the printing plate must be coordinated, and do not hard-knock the hard knock.
In addition, if the printing plate is loosened during printing, the position of the imprinting will also jump during the printing process, which will also affect the overlay accuracy.
In addition, the printers of multicolor machines are manufactured under the same plate making conditions to reduce or eliminate various unfavorable factors.
3. In the process of multi-color overprinting of paper, the overprinting error of the image is often caused by the deformation of the paper. It is also a major factor in the accuracy of overprinting.
The deformation of the paper summed up, there is a natural deformation and compression deformation. The former refers to the paper in the absence of external forces under the conditions of its own water content changes, absorb water or dehydration resulting in size extension or shorten the deformation. This deformation has a close relationship with the temperature and humidity of the surrounding environment. The latter refers to the plastic deformation of the straight line size and area caused by the fiber misalignment when the paper is subjected to a pressing force during roller rolling.
The overlay error caused by the two deformations of paper is almost inevitable. However, effectively controlling the moisture content of the paper and reasonably adjusting the ambient temperature and humidity and the pressure of the roller can ensure that the straight-line size and area of ​​the paper are stable and reduce the deformation caused by the paper, thereby reducing the overlay error.
Both theory and actual production have confirmed that the moisture content of the paper should not change by more than ±1% during the overprinting process, otherwise the overprinting accuracy will be affected. Therefore, the workshop temperature must be strict, and the general changes are not excessive.