Printed corners and "one line" view
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The "one line" viewpoint is the author's many years of mechanical maintenance work, after a lot of practice and theoretical research. The "one line" view emphasizes that in each transmission and transfer link, the front edge should always be in a line state, without deformation, to ensure the accuracy of printing overprint.
Corner concept
The horns are also called the paper tail shape, which generally shows the following two phenomena.
(1) The printed paper tail sleeve is not allowed, the first color is large in the paper feeding direction, and the latter colors are gradually reduced. This overprinting problem starts from the position of the paper crepe and becomes more serious as it goes to the end of the paper.
(2) Observed with the naked eye, the ruled line is printed accurately, but the printed matter is discolored, the color of a batch of printed products is inconsistent, the color change of the paper tail is serious, the layer of the wire is not layered, the imaginary, the color of the single print is light and heavy, front The text on the tail of several colors (especially the first color) is not clear. When inspected with a 20-fold magnifying glass, it can be found that the rule line on the tail of the paper has obvious jump and the dot is deformed.
The corner problem is a typical form of overprinting failure. Most overprinting faults are caused by corners or accompanied by corners. There are both mechanical and operational reasons as well as paper problems. Strictly speaking, all multi-color printing equipment has corner problems. High-quality new presses have a lighter corner problem, and lighter corners can only occur when printing thinner or ruffled paper. Some corner problems are inherent to certain models. In the 1970s and 1980s, the Heidelberg multi-color offset press had a problem of cornering at the factory due to limitations in equipment design and manufacturing, but it was not serious. After using for a period of time, due to various factors, the problem of the corner of the machine gradually makes the operator feel a headache. Through maintenance, the problem of corners can be minimized to the extent that it is not visible to the naked eye. The domestic multi-color offset printing machine generally has a serious problem of cornering when leaving the factory, and the provisions on the overprinting accuracy in the national standard are too loose, and are not suitable for printing high-quality printed products.
One of the reasons for the corner: the front edge of the paper is wavy when it is transferred to the first group of impression cylinders. When printing, under the common rolling of the impression cylinder and the blanket cylinder, the wave is flattened, but formed on the paper. Micro wrinkles, which are partially or completely flattened during the subsequent imprinting of individual units. Therefore, in the paper feed direction, the first color print of the print is the largest, and the print of the rear unit is gradually reduced. The change in the rule line starts from the mouth of the paper, and the more it changes to the end of the paper. This is the most typical corner problem.
The second reason is that the paper is deformed under the peeling force of the blanket during the printing process, so that the front edge of the paper is not in an ideal straight line state. Under the printing pressure of the rear unit, the wave deformation is flattened and A slight wrinkle is formed on the paper, causing a horn.
The third reason: the vertical section of the paper can not maintain the "one line" state during printing, or the surface of the impression cylinder and the longitudinal surface of the blanket surface in the pressing area cannot be kept "one line" after the blanket cylinder and the plate cylinder are pressed together. "The state, the image on the plate can not be accurately transmitted to the paper, causing the printed matter to be inaccurate and corner.
The fourth reason: the paper is finished in the first color or a certain color after the printing process, due to unreasonable handover time, etc., the paper is deformed at the front edge when the paper is handed over to the back impression cylinder. It is flattened and forms a wrinkle on the paper, causing a horn.
The premise of ensuring the accurate printing of printed products is to minimize the deformation of the paper during each transfer and embossing process, especially the deformation of the front edge of the paper. Although the Heidelberg multi-color offset press has a compensator, this compensation has no practical significance for the large angle of the corner and the deformation of the dot at the end of the paper. There are many reasons for the problem of the corner of the print, and most of the faults are difficult to compensate for the compensator.
1. Paper "one line" problem on the first color impression cylinder
For example, in the case of one of the causes of the corner, due to the difference in the transfer state of each sheet of paper, the rule lines of the first few colors (especially the first color) are blurred, along with the jump of the line, while the rule lines are not completely coincident. The dot is deformed, and the color of the printed product is light and heavy.
To determine if the press has a corner, first check that the paper delivered to the first color impression cylinder is flat. If the paper mouth is wavy, that is, the front edge of the paper is not "one line", the machine must have a corner problem. To ensure the "one line" status of the paper on the first color impression cylinder, the following conditions must be met.
(1) "One line" status when the paper is positioned at the front gauge
The paper at the front gauge is in the last step before all the crepe papers. Ideally, each paper is in the same state and remains absolutely flat. If the front edge is not in the "one line" state, after the paper passes the transfer paper to the impression cylinder row, the front edge of the paper is still not in a straight line. Under the printing pressure, the paper is deformed and converted into a printed corner. In order to keep the paper in the "one line" state of the pre-processing, the following conditions must be met.
1 The paperboard should have strict flatness
The paperboard is not flat, and the front edge of the paper cannot achieve the "one line" state. When the equipment is overhauled, the flatness of the paperboard should be corrected to within 0.05mm.
2 The positioning plane of all front gauges must be on “one line”
This is especially important when printing thin paper. The positional deviation of a front gauge will cause the paper to partially deviate from the straight line formed by the pre-positioning plane, which will destroy the "one line" state of the paper.
3 Paper can be correctly positioned at the front gauge
If the tongue of the individual front gauge is low, the paper will not smoothly enter the positioning plane of the front gauge, which will cause the straightness of the front edge of the paper to decrease. Therefore, the operator should adjust the feeder's paper-cutting time and the position and pressure of the platen roller and brush wheel on the paperboard and the tightness of the paper-feeding tape according to the specifications to ensure the accuracy of the pre-defined position.
4 The paper is smoothly attached to the cardboard
Different models are equipped with different devices to ensure the flatness of the paper when it is in the front position. In the old Heidelberg 102 series offset press, a suction device is arranged below the paperboard and a flat spring leaf is placed on top. In the new machine, the front paperboard is equipped with a blowing device, and the negative pressure formed by the blowing process is used to attach the paper to the paperboard. Therefore, the operator should carefully adjust the suction or blowing device on the cardboard, the gap between the flat spring and the paper. On the paperboard, the paper must not have the problem of warp angle, otherwise there will be more serious corner failure.
For thinner papers, no matter what kind of equipment, the flatness of the paper cannot be completely solved. Therefore, the paper should be carefully knocked before printing to ensure the stiffness of the paper. The author suggests that when designing the printing, a device can be installed at the pre-specified position to allow the paper to withstand the pulling force in the paper-feeding direction at the front gauge, and to flatten the paper.
(2) The “one line” state of the paper at the instant of paper picking on the paperboard
When repairing or manufacturing equipment, it should be ensured that the paper feeder is in a "line" state when the paper is picked up on the paperboard. To this end, the following conditions must be met.
1 The occlusal plane of the transfer pad is a standard plane
After long-term use, the transfer paper pad is deformed, and the sprayed layer at part of the occlusal plane is detached, so that the occlusal plane of the transfer paper pad is not standard, and the front edge of the transfer paper is uneven after the paper is jammed. Therefore, the transfer paper must be repaired or replaced. Dental pad.
2 The ideal gap between the transfer pad and the paperboard is the thickness of the printed paper, so that the paper has the least deformation when the paper is taken.
(3) "One line" status after the paper is fed
1 hand paper picking paper time accurate
The ideal paper take-up position for the transfer paper is the vertical position of the transfer paper pad and the paperboard, and the angle reading on the dial is 11.5 degrees. Due to the wear of the transfer paper, the tooth pad and the adjustment, the paper pick-up time of each tooth on the transfer paper is not consistent. When the transfer paper is taken at the front gauge, the transfer paper pad is not absolutely stationary, but there is a slight rotation, which is a problem that the rotary transfer paper cannot solve. The paper pick-up time is inconsistent, resulting in inconsistent paper conditions when the individual sheets are being picked up, and the front edge of the paper is deformed. Some teeth only pick up the paper after the transfer paper starts to rotate, causing a serious deformation of the front edge of the paper. Some teeth have too early closing time, and the paper is cut at the moment of the paper smashing, so that the paper edge or the paper edge of the tooth portion is backward. After the other teeth are creped, the front edge of the paper is deformed.
Due to the wear of the transfer shaft, the dental shaft bearing, the copper sleeve of the socket, and the open ball, the paper take-up time of the transfer paper will change each time the paper is taken, which makes the paper pick-up time of the transfer paper inaccurate. The reasonableness of the paper take-up time and the consistency of the straightening time of the teeth cannot be guaranteed, resulting in deformation of the paper.
2 Correctly adjust the paper transfer position
During printing, the operator should be accustomed to adjusting the paper transport position according to the position of the front gauge, and should not center the paper without thinking. When the paper is transported, if the edge of the paper is close to the side of the pull gauge (the front gauge of the paper edge), the front gauge will hinder the pulling action of the pull gauge when the paper is pulled by the pull gauge, so that the paper edge Deformation. In this state, the crepe paper state of the transfer paper is not ideal, and the front edge of the paper is not in a "one line" state.
(4) "One line" problem at the front edge of the paper during the movement of the paper
Sometimes, the teeth are removed due to the individual pieces of paper; after the teeth are worn, they are not adjusted according to the standard; there are dead teeth on the transfer paper, which affects the force and causes the paper to loosen after the paper is fed. These will cause the "one line" state of the front edge of the paper to change.
Due to the small number of teeth of the rotary transfer paper, a part of the paper is out of control, and the external force cannot be withstood during the transfer of the paper by the transfer paper. Therefore, the time for the side gauge to stop pulling the paper must be reasonable (standard time is 10.5 degrees), otherwise the paper will grab the paper with the side gauge, so that the front edge of the paper is deformed; the pressure of the paper brush wheel on the paperboard puts pressure on the paper. It must be appropriate, otherwise a part of the front edge of the paper will be pulled backwards.
(5) "One line" problem of paper when the transfer paper and the impression cylinder are handed over
1. The delivery time of the transfer paper and the impression cylinder is accurate
The ideal intersection of the transfer paper and the impression cylinder to the paper should be at the tangent point of the two rollers. In order to ensure the stability of the transfer process, there must be a common transfer time of 1.0 to 1.6 degrees between the impression cylinder and the transfer paper. The midpoint of this handover time is the tangent point of the two rollers. In this intersection, the distance between the transfer pad and the impression pad is the closest, and the change is minimal, which can minimize the deformation of the paper during the transfer. The opening time of the transfer paper is (204.9+0.3) degrees, and the closing time of the impression cylinder is (203.9-0.3) degrees. If the handover time is too long or the junction point is not based on the tangent point of the roller, the gap between the pads will increase when the transfer is made, and the front edge of the paper will be deformed.
Factors affecting the handover time include: wear of related parts in the transfer paper row; inaccurate opening time of the transfer paper; inaccurate adjustment of the closing time of the impression cylinder row; in the impression cylinder, the dental shaft and the dental shaft There is a “dead tooth” in the copper sleeve, the open ball wear or the tooth row, which changes the tooth closing time of the impression cylinder or the closing time of some cavities, and the junction point is uncertain.
2.The gap between the paper pad and the impression pad is reasonable.
The most reasonable gap is the thickness of the paper to minimize paper distortion. When adjusting the interstitial space, pay attention to the adjustment standard of the height of the dental pad. When printing thin paper, it is recommended to adjust the gap between the transfer pad and the impression pad to 0.15 to 0.20 mm.
There are two factors that affect the gap between the transfer pad and the impression pad. The first is the impression cylinder pad. The plane formed by the upper surface of a row of dental pads is the reference plane during the transfer of the printed products. Therefore, the height of the dental pads should be carefully measured before installing the dental pads, and the height error of each row of dental pads should be controlled within 0.02 mm. The second is the transfer paper pad. After the transfer pad is bent, it is difficult to ensure the flatness of the occlusal plane by the adjustment of the operator alone. If the sprayed layer of the individual bite points falls, the gap between the transfer pad and the impression pad is increased. When replacing the transfer pad, the position of the pad must be re-corrected, corrected with the impression roller pad or impression cylinder body or with a special tool.
2. "One line" problem at the front edge of the paper during imprinting
Due to the effects of roller dent, roller body, blanket, printing pressure and dampening solution, the paper undergoes various deformations during the embossing process.
As shown in Figure 1, in the Heidelberg 102 series offset press, the impression cylinder has only 14 cavities. When printing positively, there are only 10 crepe papers, the distance between the two fangs is large, and the paper has 45 at each of the two front corners. The out of control part of mm.
The deformed part of the paper always starts from both sides of the print mouth and the fangs, so that the image position of the printed product and the printing plate cannot completely coincide or cannot change according to the same law. There is a difference in the location and extent of deformation between printing units due to differences in printing conditions. Due to the wet-wet printing, the deformation of the paper is finally converted into the ghosting and corner failure of the printed product, and the following factors may cause the corner failure of the printed product during the printing process.
(1) The two front corners of the paper are dragged backward by the blanket peeling force, and the paper is not in the "one line" state when the impression cylinder is handed over to the first small transfer cylinder.
(2)Due to the peeling force, the paper is deformed between the two-phase molars, not a line state. Especially when printing longitudinal filament paper, the front edge of the paper is more severely deformed due to the expansion of the paper fibers.
(3)There is a "dead tooth" on the tooth row, which can not produce enough force on the paper. Under the action of the peeling force, the paper is pulled backwards for a distance in this part, so that the front edge cannot maintain the "one line" state.
(4)Use teeth and pads that do not meet the requirements. The contact between the tooth piece and the tooth pad is in the form of a line contact. Since there is not enough occlusal area, the paper cannot overcome the peeling force between the paper and the blanket cylinder at the nip, so that the paper is deformed at the cornice.
(5) If the paper pull is not good, the adhesive is too sticky or the printing pressure is too high, and the paper will also be deformed at the mouth.
3. "One line" problem in the embossed area
If the impression cylinder is deformed, the quality of the blanket is not good, or the blanket is not flat, the longitudinal section of the contact area between the impression cylinder and the blanket is not a straight line. When the impression cylinder body is deformed greatly, the printed product will wrinkle, causing partial overprinting or cornering.
4. "One line" problem during paper transfer
(1) The intersection of the tooth row and the common handover time
The ideal intersection point for the transfer of the rollers should be at the tangent point of the two rollers. In order to ensure the stability of the transfer process, there must be a common transfer time of 1.0 to 1.6 degrees between the rows of the rollers (in the single-sided printing state, the common transfer time of the reverse roller to the impression cylinder is 1.0 to 1.8 degrees, and the roller is turned over to the storage roller. The common handover time is 1.0 to 2.0 degrees). The midpoint of the intersection time between all the rows of the rollers is the tangent point of the two rollers.
When the outer surface of the roller or the open ball is worn, the transfer time between the rows of the rollers becomes small or there is no handover time, and should be readjusted at this time. If the wear exceeds a certain limit, the joint time of the roller row will not be adjusted. At this time, the tooth piece and the tooth ball must be replaced. When the tooth pad is worn, the paper is wavy on the tooth row. At this time, the roller pad must be replaced. When the roller open ball has obvious gap, the dental shaft and the socket copper sleeve are worn, and the dental shaft bearing is damaged, the opening and closing time of the roller tooth row or a certain tooth decay is uncertain and cannot be adjusted normally. Wear parts in the row.
For large paper rollers and storage rollers, the opening and closing time of the two rows of cavities should be strictly consistent. Compared with other models, the Heidelberg 102 series offset press has a smaller roller diameter, and the common transfer time between the rows of teeth is not too long (the transfer length is 2.5 to 3.5 mm). Since the roller row has a slope of 2.5 degrees, the deformation of the paper on the junction is increased, which limits the extension of the time between the rows of the rollers. Therefore, the standard time data must be strictly observed when adjusting the handover time. In particular, when adjusting the opening and closing time of the turning roller, careful operation is required.
(2) Dental pad clearance
The most reasonable gap between the roller pads is the thickness of the paper. Pay attention to the adjustment level of the height of the dental pad. When printing thin paper, it is recommended to adjust the gap between the rollers to 0.15 to 0.20 mm.
For small transfer rollers, the operator can change the height of the pads to meet the printing needs of different thickness papers. When the equipment is fully maintained and overhauled, the height of the small paper roller pad should be carefully corrected. For the turning roller, the relative position of the molar fixed piece and the turning roller should be carefully adjusted.
When repairing the flipper tooth row, some maintenance personnel can change the front and rear position of the tooth row (without the positioning pin on the tooth row seat) and the positioning position of the gears at both ends of the tooth row in order to increase the force of the flipping tooth row. At the junction, the gap between the pad that flips the roller and the storage roller and the pad of the impression roller deviates significantly from the standard data, and the front edge of the paper will be severely deformed during the handover process.
(3) In the process of the handover of any two rows of rows of teeth, the "one line" problem of the position of the cusps of the teeth is mutually overlapped.
Due to the slope of the surface of the dental pad, the position of the cusps of the mutually overlapping dental teeth is in a straight line, which can reduce the deformation of the paper. Due to manufacturing, installation and other reasons, it is difficult to strictly do this.
(4) "One line" problem of paper during transmission
The paper should be prevented from deforming at the front edge of the paper during paper transport.
When there is a "dead tooth" in the tooth row, the paper will be out of control at this part, and the two front corners of the paper are generally out of control (the part other than the tooth decay). When the paper encounters an external impact, the uncontrolled portion of the paper is dragged backwards, so that the position of the backing paper is inaccurate, causing the paper to deform.
For the first small transfer roller, the tail of the paper is still between the impression cylinder and the blanket cylinder when the tooth is discharged. If a tooth is a "dead tooth", the paper will be deformed at this location.
(5) "One line" problem with the size of the jaws
Under normal circumstances, the size of the crepe paper of each unit's roller should be equal or no significant change. In some machines, the first group of impression cylinders has a size of 6 mm. In the latter unit, the size of the mouth of the impression cylinder is gradually reduced, and the last unit becomes 2 mm, so that the paper is in the mouth. There will be distortion at the place.
This is because the joint transfer time between the rows of the rollers is too long, the teeth of the pickup roller are too early to make the mouth smaller; there is no joint transfer time between the rows of teeth, and the whole paper is out of control (handling) at the time of handover; The pad gap is not reasonable. Solve the problem of corners with the "one line" point of view
In order to accurately print, every step of the paper must be ideal during the transfer. However, most maintenance personnel do not understand the corners of the equipment.