Overprint faults for offset printing
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Overprint failure
Overprinting is not one of the most typical failures in offset printing. Whether it is new or old, domestic or imported, monochrome or multi-color offset presses, these failures have occurred to varying degrees. In view of this, the causes and exclusions of overprinting failures will be discussed in detail below.
Accurate overprinting is a common quality requirement for printed products. It is usually said that the overprinting is accurate. Under different printing conditions, different types of printed products, the overprinting error does not exceed the specified overprint tolerance, but as an offset press operator. It is necessary to minimize the overprint tolerance as much as possible. Theory and practice have proved that the human eye can recognize the fine lines up to 0.1mm in the normal viewing distance, and the tone of the printed image is generally reproduced by the dot. The diameter of each dot is actually less than 0.1mm. Therefore, the standard of China The overprint tolerance is 0.1mm, and the general high standard overprint tolerance is limited to 0.05mm.
For printing products, the overprinting accuracy is the first. If the overprinting is not accurate, the work of the offset printing machine is meaningless. Overprinting errors are common in the offset printing process, and no offset printing machine does not suffer from overprinting errors.
Overprinting faults are generally classified into two categories: "chronic" and "sudden". Chronic failures are manifested in rising scrap rates, declining product quality, etc. Sudden failures are manifested by sudden overprinting. Whether it is chronic or sudden, it can be solved by a layered approach.
There are many factors that affect overprinting, for about a hundred reasons, but the factors that really affect overprinting are the precision and adjustment control technology of the offset press.
Analysis and measures for misregistration caused by improper adjustment of offset printing machine parts
(1) The cardan shaft or the paper machine and the main unit coupling cone pin are broken and worn. When the paper feeder feeds paper, the paper feed is unstable, and the paper feed is unstable.
(2) The paper tongue, paper feed nozzle, and paper feed roller are not coordinated. The relationship between the three should be: when the paper feed nozzle just sucks the paper, the paper stop tongue begins to have a tendency to lean back, the paper feed nozzle stops sucking, and the paper take-up roller also presses the delivered paper against the paper feed roller. on. If this is not the case, the rules before the arrival of the paper are irregular and the paper feeding speed is different. The relationship between the three is based on the transfer nozzle, and the control cams of the paper stop and the paper take-up roller are respectively adjusted.
(3) The pressure of the transfer roller is not adjusted well. If the pressure adjustment of the two transfer rollers is inconsistent, the paper will be skewed. If the lifting time is inconsistent, check if the clearance of the adjusting screw is the same.
(4) The paper roller (ball) on the paperboard is improperly adjusted, the paper is unstable, and the overprint is affected. It should be adjusted according to the weight of the paper.
(5) The tension of the belt is inconsistent, the paper is skewed, which affects the paper feeding, resulting in inaccurate overprinting. Re-tighten the strap.
(6) The front gauge lifts up (backward) too early and too late. Generally, the paper will be out of control and the paper and the front gauge will grab the paper. This situation often occurs after the front of the offset printing machine is worn or the machine is overhauled. At this time, adjust according to the standard position specified in the manual.
(7) The gap between the upper paper jam and the paper table is not suitable. If the paper is not in place or under the head, it should be adjusted according to the relevant regulations of the operating instructions.
(8) The radius error of the front gauge cam is too large. If there is an error in the radius surface of the cam that controls the swing of the front gauge, the front gauge will have a slight movement when the paper is stabilized on the tooth stage. As a result, the overprinting is not accurate. This is usually the case after the machine has been used for a long time or overhauled.
(9) The transfer paper is too early or too late to close the teeth. The paper-feeding teeth are prematurely closed at the tooth table, and the paper grabbing phenomenon occurs. If the teeth are too late, the paper is out of control, and the closing time of the paper-feeding teeth must be performed at the "zero" speed of the paper feeding teeth, that is, It is carried out while the transfer paper is in a static state, otherwise it is impossible to register. For the upper oscillating type of transfer paper, the paper dent is at the lowest point of the cam, and the two teeth are closed after 2/3 of the time.
(10) The size of the brake force on both sides is inconsistent. The brake is the guarantee that the paper is stationary on the tooth table. If the size of the brake force is inconsistent, the twisting of the paper shaft will occur, which may cause the printing to be accurate and the side is not allowed. Of course, there are many reasons for the inaccuracy of overprinting, and the inconsistency of the force on both sides is one of the important reasons.
In addition, the eccentric sleeves of the two transfer paper shafts are not concentric, so that the brake screws on both sides are not pressed at the same time, and the eccentric sleeve should be adjusted to be concentric.
(11) The paper teeth stay on the table for too long or too short. If the paper teeth stay on the tooth table for too long, it means that there are too many brakes on both sides. On the contrary, the stay time is too short, that is, the two sides are too little on the brakes. For the J2205 machine, there are too many jacks. It not only affects the paper-feeding dents on the toothpick, but also greatly increases the impact noise of the brakes, causing the overprinting to be inaccurate. The time of the brakes is often adjusted in accordance with the gap between the control swaying cam and the swinging arm roller. of.
(12) The transfer time between the transfer paper and the embossing (transfer) roller is too long or too short. Here is the time between the transfer paper and the embossing (paper transfer) roller squeezing to control the paper. The two teeth must be synchronized at the time of handover, otherwise the handover will be inaccurate. Generally, if the handover time is too long, the cam synchronization The time is not easy to guarantee, so the handover time should not be too long, just enough, the specific adjustment as mentioned above.
(13) The transfer paper rubs against the front edge of the impression cylinder. If the tip of the paper tooth touches the front edge, you can hear the sound of bumping during operation. The edge of the paper is inaccurate. Usually, the paper swinging cam is adjusted clockwise. If there is less bump, you can also put the tooth. The film moves back a bit.
(14) The paper teeth swing cam roller radial runout. The radial runout of the roller is generally a problem of the roller itself, and also the sum of the runout errors of the ball bearing, so it should be distinguished according to the specific situation.
(15) The swing arm pin or sleeve is worn. After the pin and sleeve are worn, the creping is caused and the overprinting is not allowed. In this case, the replacement pin or sleeve should be repaired in time.
(16) The side pull gauge spring force is too large or too small. The overprint caused by the spring force is not correct, and the compression spring should be replaced according to the thickness of the paper.
(17) If the clearance between the side gauge plate and the paper table is not suitable, the paper may be pulled too far or pulled out. Adjust according to the operating instructions.
(18) Side pull gauge roller radial runout. If the roller jumps radially, the pressure on the paper is large or small, resulting in inaccurate overprinting.
(19) The amount of axial movement of the drum is too large.
(20) The screws that fasten the drum gear are loose, or the pins of the gland and the hub are loose. In this case, the relative working of the drum body and the drum gear changes, causing irregular printing. Check one by one from the transfer roller.
(21) The gripper slips. Reduce the force of the paper, resulting in inaccurate overprinting. Replace the pads or teeth.
(22) The height of the gripper teeth is inconsistent. This situation makes the paper edge uneven, which directly affects the overprint accuracy. Replace the dental pad.
The above is the main reason for the misregistration caused by improper adjustment of the printing machine components.
The influence of the force and the transfer relationship of the second paper on the overprint
In the printing process, the gripper not only transfers the paper, but also fixes the paper at a certain position, so that the graphic on the printing plate or the blanket is transferred to the designated position. However, when the embossing is insufficient or uneven, the position of the paper changes, which may cause the overprinting to be inaccurate.
1. Paper teeth force and overprint
The gripper mentioned here mainly refers to the transfer paper and the roller gripper. The unevenness of the impression cylinder of the embossing cylinder can cause four deformations of the paper, and the printed matter will inevitably cause partial overprinting.
In order to prevent inaccurate overprinting of the crepe paper, the following points should be achieved.
(1) The adjustment of the gripper force of the gripper should be carried out in the trial printing. The force of the multi-color machine should not be changed during the process of overprinting. The force of the gripper on the impression cylinder should be consistent. The gripper teeth must be flat to ensure uniform force.
(2) For the products printed on the front and back sides, the gripper force of the impression cylinder must be uniform.
(3) The spring of the crepe shaft should have good elasticity to prevent the lack of force and the paper is not positioned properly.
(4) Tighten the tight loop of the gripper to prevent the axial movement of the gripper, and use the gripper shaft or sleeve with high manufacturing precision to make the gripper in a stable state during the picking process. Maintain a constant value.
2. Handover relationship and overprinting
In the whole process of paper feeding, rule parts, transfer paper squeezing, and paper transfer roller, there is a problem of handover relationship. As mentioned above, the handover relationship refers to the handover position and handover time, and the handover relationship is essentially guaranteed. Accurate and accurate delivery of paper must not allow paper to run out of control. The transfer position refers to the transfer of paper at a certain point, and the transfer time is the time for the transfer of the parts to complete the transfer of the paper. This time is required to be adjusted strictly according to the regulations, neither short nor long, for example, the transfer paper and If the handover time is short, the overprinting will be inaccurate. If the handover time is long, the crevice will be torn. Therefore, the adjustment of the handover relationship is a very prudent technical work, and it must not be careless.
The relationship between printing pressure, paper peeling force and overprinting inaccuracy
During the printing process, the paper is deformed by the printing pressure and the peeling tension. After the paper with large plastic deformation, after leaving the embossing area, the deformation amount cannot be recovered, resulting in a large overprinting error.
1. Printing pressure and paper deformation
Paper is a material with plastic deformation, and the plastic deformation characteristics of different papers are also different. Under the action of large printing pressure, the width and the length of the two corners of the paper may be slightly expanded, resulting in inaccurate overprinting. Especially in the tip of the paper is more serious.
The greater the printing pressure, the more obvious the deformation of the paper. Therefore, when printing the same batch of products, the pressure of each color plate should be as consistent as possible, but sometimes the pressure should be adjusted according to the specific conditions, for example, the printing pressure of the multi-color offset printing machine. It is often a random group decrement.
The deformation of the paper is closely related to the silk thread of the paper. Considering the general offset printing machine, the overprinting error caused by the deformation of the paper under the printing pressure is difficult to adjust in the axial direction than in the radial direction, so when printing, the sheet is single. The paper should make the longitudinal direction of the paper parallel to the axial direction of the drum as much as possible, and the same batch of products, the direction of the silk paper must be the same in order to ensure the accuracy of overprinting, and never allow the same batch of products to be printed on different silk paper.
Under the action of printing pressure, due to the phenomenon of fan-shaped deformation, the image that has high requirements for overprinting accuracy or the part that plays an important role in the theme should be arranged near the cornice during pre-press processing and plate making. Because of the small deformation of the paper near the mouth when printing, it is easy to overprint accurately.
2. Paper peeling tension and paper deformation
The paper after printing through the embossed area is caught by the roll gripper, forcing it to detach from the blanket, and the ink on the plate acts as a medium to adhere the sheet to the surface of the blanket with a strong adhesive force. At this time, the fibrous structure on the printed sheet is subjected to the force in two directions, one is the force of the gripper against the paper, and the other is the adhesion of the ink to the paper. The two forces are opposite in direction. When the printed sheet is peeled off from the blanket, the trailing edge of the paper is easily curled and does not fit against the impression cylinder to form a folding angle, which is called a peeling angle. If the adhesion of the ink is large, the longer the sheet is peeled off by the gripper, the larger the peel angle, the greater the peeling tension, the more the peeling deformation of the paper is increased, and the paper is elongated along the radial direction of the drum, overprinting The accuracy is reduced.
During the printing process, if the gripper of the impression cylinder has sufficient force to ensure that the paper does not shift, the amount of peeling tension becomes the main factor affecting the deformation of the paper. The amount of deformation depends on the following aspect:
(1) The nature of the paper. Paper with large plastic deformation has large peeling deformation, and vice versa.
(2) Printing pressure and time when the rollers are mutually imprinted. The greater the printing pressure, the longer the embossing time, the longer the paper is in contact with the ink on the blanket, and the greater the peeling deformation.
(3) The ink receiving area of the paper surface. The larger the ink area, the thicker the ink layer and the greater the peeling deformation.
(4) Viscosity and adhesion of the ink. The greater the viscosity and adhesion of the ink, the greater the peeling deformation, and vice versa.
Four typical overprint fault examples
1. Constant speed, the speed is not allowed.
Isometric index refers to the same overprinting speed of several colors printing. Different speeds are not allowed to indicate that different speeds are overprinted. Generally speaking, all-speed can be registered, and the constant speed can be registered, and the constant speed can be registered. Speed may not be able to register.
cause of issue:
(1) The transfer of the transfer paper and the embossing (transfer) roller is not at the cutting point.
(2) The force of the transfer paper and the embossing (paper transfer) drum are inconsistent.
(3) The handover time is too long or too short.
(4) It is too early or too late before the paper arrives.
(5) The stability of the paper before the arrival of the paper is poor.
(6) Paper stretching.
2. The side of the pull gauge is accurate and the other side is not allowed. Simply speaking, it is called the operation side, and the drive side is not allowed.
cause of issue:
(1) The two sides are inconsistent with the brake top force.
(2) The eccentricity of the paper eccentric is asymmetric.
(3) The gap between the front gauge and the paper feeder is inconsistent.
(4) Paper feeding failures include:
a cloth strap is inconsistent.
b The pressure of the platen roller is asymmetrical.
c brush pressure is inconsistent.
d The paper roller pressure is inconsistent.
(5) The front gauge and the side gauge are not at right angles.
(6) The difference in the force of the transfer paper and the roller is too large.
3. All-speed overprinting is not parallel on both sides
This means that at two speeds, the crosshair printed at low speed, and the crosshair printed at high speed, the error is equal, and its expression: one is that the high speed is not in place. The other is high speed over the head, very regular, and the distance is equal.
cause of issue:
(1) The time before the paper arrives is late.
(2) The delivery paper is closed late.
(3) The elastic of the transfer paper pad is inconsistent.
(4) The time before the paper arrives is early.
(5) The paper teeth are closed early.
4. Not allowed together
The current rules for individual overprinting are not required when using side gauges. However, when combined with the side pull gauge, that is, when the front gauge and the side gauge are used at the same time, the overprinting is not allowed, and the printing operator refers to such a fault as being inaccurate.
cause of issue:
(1) Defective side pull gauge.
(2) The clearance between the side pull plate and the iron table is too small.
(3) When the paper is lifted and the teeth are closed, the side puller is lifted up late.
(4) The paper is late.
(5) The paper stays for a short time.
(6) The front gauge and the side gauge are not at right angles.
5. Squatting, not allowed
cause of issue:
(1) The printing pressure is large.
(2) The two printing packages are inconsistent.
(3) The printing plate is large.
(4) The blanket is loose.
(5) The paper stretches.
6. Both sides can be registered, the middle set is not allowed
cause of issue:
(1) The impression cylinder is slippery.
(2) Middle molars.
(3) The blanket is not flat.
(4) The printing pressure is too large.
(5) There is a problem with the printing plate.
7. The intermediate can be registered, and the left and right sets are not allowed.
cause of issue:
(1) The former rules are not in line.
(2) The individual fangs on both sides of the impression cylinder are loose.
(3) The water on both sides of the printing plate is large.
(4) There is a problem with the printing plate.
(5) Corner.
8. The individual printed sheets in the print are not allowed to be printed.
That is to say, in a stencil, more than 90% can be registered, and only a single imprint is not allowed. This phenomenon is mainly a paper feeding problem, followed by:
(1) There is a groove in the front gauge cardboard.
(2) Mixing of horizontal and vertical silk paper.
9. Constant speed overprinting is not allowed. This kind of fault is the most difficult fault to solve. No matter how to debug, the rule line is always bent and bent.
cause of issue:
(1) The impression cylinder is bent by the impression cylinder.
(2) The transfer of the paper to the tooth and the impression of the impression roller are not allowed.
(3) The roller gear gap is too large.
(4) Wear and tear of the opening and closing teeth cam (block).
(5) The machine has been used for a long time.