Overprint failure
Overprinting is not one of the most typical faults in offset printing. Whether it is new or old, domestic or imported, monochrome or multicolor offset printing machines, these failures have occurred in varying degrees. In view of this, the causes and elimination of overprinting faults will be discussed in detail below.
Accurate overprinting is a common quality requirement for printed products. It is commonly said that overprinting is accurate, but under different printing conditions and different types of printed products, the overprinting error does not exceed the specified overprinting tolerance, but as an offset press operator It is necessary to make the overprint tolerance as small as possible. Theory and practice have proved that the human eye can recognize fine lines at a normal observation distance of up to 0.1mm. The tone of the printed image is generally reproduced with dots. The diameter of each dot is actually less than 0.1mm. Therefore, our standard The tolerance of overprint is 0.1mm. Generally, the tolerance of high standard overprint is limited to 0.05mm.
For printed products, overprinting is the first priority. If overprinting is not accurate, the work of the offset press is meaningless. Overprint errors are common in the offset printing process, and no offset press does not suffer from overprint errors.
Overprint faults are generally classified into two types: "chronic" and "sudden". Chronic failures are manifested in an increase in scrap rate and a decline in product quality. Sudden failures are manifested as sudden overprinting. Whether it is chronic or sudden, it can be solved by layering method.
There are many factors that affect overprinting, about a hundred kinds of reasons, but the factors that really affect overprinting are the accuracy of the offset press and the adjustment and control technology.
Analysis and Measures of Overprint Inaccuracy Caused by Improper Adjustment of Offset Press Components
(1) The cone pin of the universal shaft or the coupling between the paper feeder and the main machine is broken or worn. When the paper feeder is feeding paper, the paper feeds quickly and slowly to make the paper unstable.
(2) The movement of the paper blocking tongue, paper feeding nozzle and paper receiving roller is not coordinated. The relationship between these three should be: when the paper suction nozzle just sucks the paper, the paper blocking tongue starts to lean back, the paper suction nozzle stops sucking the wind, and the paper pick-up roller just presses the delivered paper against the paper pick-up roller on. If this is not the case, the rules are irregular before the paper arrives, and the speed of the paper varies. The relationship between these three is based on the paper suction nozzle, and adjust the control cams of the paper blocking tongue and the paper receiving roller respectively.
(3) The pressure of the delivery roller is not adjusted well. If the pressure adjustment of the two delivery rollers is inconsistent, the paper is skewed. If the lifting time is inconsistent, check whether the clearance of the adjustment screw is consistent.
(4) Improper adjustment of the paper pressure roller (ball) on the paperboard, unstable paper, affecting overprinting. Should be adjusted according to the paper weight.
(5) The tension of the belt is inconsistent, the paper is skewed, which affects the paper feeding and causes overprinting. Re-tighten the string.
(6) The front rule lifts (lifts back) too early or too late. Generally, it will cause two kinds of phenomena: out-of-control paper and paper-feeding teeth and pre-registration to grab paper. This situation often occurs after the front gauge of the offset printing press is worn or the machine is overhauled. In this case, adjust according to the standard position specified in the manual.
(7) The gap between the paper retaining tongue on the front gauge and the paper table is not suitable. The paper is not in the lower position, or the phenomenon of over the head, at this time, it should be adjusted according to the relevant provisions of the operating instructions.
(8) The radius error of the front cam is too large. Errors in the cam and other radius surfaces that control the swinging of the front gauge will cause the front gauge to move slightly when the paper is stabilized on the tooth table, resulting in inaccurate overprinting. This usually occurs after the machine is used for too long or after overhaul.
(9) Too close the teeth at the tooth table too early or too late. The paper delivery teeth are closed too early at the tooth table, and there is paper grabbing. If the teeth are closed too late, the paper will be out of control. The closing time of the paper delivery teeth must be performed at the "zero" speed of the paper delivery teeth, that is, It is carried out when the gripper is in the static state, otherwise it is impossible to register. For the upper-oscillating delivery teeth, the stationary state of the delivery gripper is that the delivery roller swings the roller at the lowest point of the cam, and also closes the teeth after 2/3 of the time the two sides rest on the brakes.
(10) The force of braking force on both sides is inconsistent. Retaining the brake is the guarantee that the delivery tooth is still holding the paper on the tooth table. If the top force of the resting brake is inconsistent, there will be a situation where the delivery gripper shaft is distorted, which is likely to cause the drawback of overprinting one side and one side. Of course, there are many reasons that affect the accuracy of one side to the other, and the inconsistency of the braking force on both sides is one of the important reasons.
In addition, the eccentric sleeves of the delivery shafts on both sides are not concentric, so that the brake screws on both sides do not press against the brakes at the same time. At this time, the eccentric sleeves should be adjusted to be concentric.
(11) The time that the paper delivery tooth stays on the tooth table is too long or too short. Too long staying time of the delivery tooth on the tooth table is equal to too much jacking on both sides. On the contrary, too short staying time, that is, too little jacking on both sides. For J2205, too much jacking, Not only affects the paper gripper on the tooth table, but also greatly increases the impact noise of the brake, causing overprinting inaccuracy. The lift time of the brake is often adjusted in accordance with the gap between the control cam and the roller of the swing arm. of.
(12) The hand-over time between the delivery tooth and the impression (paper transfer) roller gripper is too long or too short. This refers to the time that the paper delivery tooth and the impression (transfer) roller gripper jointly control the paper. The two teeth must be synchronized during the handover, otherwise the handover will be inaccurate. Generally, if the handover time is too long, the synchronization of the cam The more difficult it is to guarantee the time, so the handover time should not be too long, and it is enough. The specific adjustments are as described above.
(13) The delivery paper knocks against the front edge of the impression cylinder. If the tip of the delivery tooth bumps against the front edge, the knocking noise can be heard during operation. The edge opening is not properly set. Usually, the swing cam of the delivery tooth is adjusted clockwise. The piece moves back a bit.
(14) The roller of the delivery paper tooth swing cam jumps radially. The radial runout of the roller is generally a problem of the roller itself, and there is also a combined sum of the runout errors of the ball bearings, so it should be distinguished according to the specific situation.
(15) The pin or sleeve of the swing arm is worn. After the pin and the sleeve are worn out, there is a movement and the overprint is not accurate. At this time, the pin or sleeve should be repaired and replaced in time.
(16) The side pull gauge spring force is too large and too small. The overprint caused by the spring force is not accurate, and the compression spring should be replaced according to the thickness of the paper.
(17) If the gap between the side pressure plate and the paper table is not suitable, the paper may be pulled over or out of position. Adjust according to operating instructions.
(18) The side pull gauge rollers run out radially. If the roller jumps in the radial direction, the pressure on the paper will vary greatly, resulting in inaccurate overprinting.
(19) The axial series movement of the drum is too large.
(20) Loose screws tightening the drum gear, or loose pins on the gland and hub. This situation changes the relative working positions of the cylinder body and the cylinder gear, causing irregular overregulation. Check one by one from the transfer roller.
(21) The paper teeth slip. Reduce the gripping paper force, resulting in inaccurate overprint. Replace dental pads or dental tablets.
(22) The height of gripper teeth pads is inconsistent. This situation makes the edge of the paper uneven, and directly affects the accuracy of the overprint. Replace the dental pad.
The above is the main reason for overprinting inaccuracy caused by improper adjustment of printing machine components.
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Influence of gripping force and handover relationship of two grippers on overprint In the printing process, the gripper not only transfers the paper, but also fixes the paper at a certain position, so that the graphics on the printing plate or blanket are transferred to the designated location. However, during imprinting, if the gripping force of the gripper is insufficient or uneven, and the position of the paper changes, overprinting will be inaccurate.
1. Paper gripping force and overprinting
The grippers mentioned here mainly refer to transfer paper and roller grippers. The uneven gripping force of the teeth of the impression cylinder can cause four kinds of deformation of the paper, and the printed products will inevitably produce partial overprinting.
Figure paper deformation caused by uneven gripping force
In order to prevent inaccurate overprinting of uneven gripping force of paper teeth, the following points should be achieved.
(1) The gripping force adjustment of gripping teeth should be carried out during trial printing. The gripping force must not be changed during the process of overprinting of various colors. The gripping force of gripping teeth on the printing cylinders of the multi-color machine's various color units and impression cylinders must be consistent The gripper pads must be flat to ensure even gripping force.
(2) For the products overprinted on the front and back, the gripping force of the gripper of the impression cylinder must be uniform.
(3) The support spring of the gripper shaft must have good elasticity to prevent insufficient gripping force and inaccurate paper positioning.
(4) Tighten the clamping ring of the gripper to prevent the axial movement of the gripper. Use a gripper shaft or bushing with high manufacturing precision to keep the gripper in a stable state during gripping and keep the gripping distance at all times Keep a constant value.
2. Handover relationship and overprint
During the entire transfer process of paper feeding, regular parts, paper gripper, and paper transfer roller, there are problems with the transfer relationship. As mentioned above, the transfer relationship refers to the transfer position and transfer time, and the transfer relationship is essentially guaranteed The paper is conveyed without error, and the phenomenon of runaway paper is never allowed. The handover position refers to the handover of the paper at a certain point, and the handover time is the time for the handover mechanism to complete the handover of the paper. This time is required to be adjusted in strict accordance with the regulations, neither short nor long. For example, The handover time of the impression cylinder gripper, if the handover time is short, the overprint must be inaccurate, if the handover time is long, the edge of the mouth will be torn. Therefore, the adjustment of the handover relationship is a very cautious technical work, and must not be careless.
3. The relationship between printing pressure, paper peeling force and overprint inaccuracy
During the printing process, the paper is deformed by printing pressure and peeling tension. For paper with large plastic deformation, after leaving the embossing area, the amount of deformation cannot be recovered, causing a large overprint error.
1. Printing pressure and paper deformation
Paper is a material with plastic deformation, and different papers have different plastic deformation characteristics. Under the action of large printing pressure, paper with large plastic deformation will expand slightly in width and length of the two corners of the trailing edge, resulting in inaccurate overprinting. Especially in the trailing part of the paper is more serious.
The greater the printing pressure, the more obvious the deformation of the paper, so 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 situation, for example, the printing pressure of multi-color offset presses Often the random group decreases.
The deformation of the paper is closely related to the thread of the paper. Considering the general offset press, the paper produces an overprint error caused by the deformation of the paper under the action of printing pressure. It is difficult to adjust in the axial direction than in the radial direction, so when printing, the single sheet The paper should be as parallel as possible to the longitudinal direction of the paper and the axis of the cylinder, and the same batch of products, the direction of the silk of the printing paper must be the same, in order to ensure the accuracy of overprinting, the same batch of products are never allowed to use different silk paper for printing
Due to the fan-shaped deformation of the paper under the pressure of printing, the images that have high requirements for overprint accuracy or the parts that have an important role in the theme should be arranged near the mouth during pre-press processing and plate making. Because when printing, the deformation of the paper near the mouth is small, so it is easy to register accurately.
2. Paper peeling tension and paper deformation
The paper after printing in the embossing area is held by the roller gripper, forcing it to detach from the blanket, and the ink on the printing plate is used as a medium to adhere the printed sheet to the surface of the blanket with a strong adhesive force. At this time, the fibrous structure on the printed sheet is affected by the force in two directions, one is the gripping force of the gripper to the paper downward, and the other is the upward adhesion of the ink to the paper. The directions of these two forces are opposite. When the printed sheet is peeled off from the blanket, the trailing edge of the paper is easily curled and does not fit with the impression cylinder, forming a folding angle, called the peeling angle. If the adhesion of the ink is large, the longer the printed sheet is peeled down by the gripper, the greater the peeling angle, the greater the peeling tension, the greater the peeling deformation of the paper, the paper is elongated in the radial direction of the cylinder, and overprint The accuracy drops.
During the printing process, if the gripping teeth of the impression cylinder have enough gripping force to ensure that the paper does not move, the size of the peeling tension becomes the main factor affecting the peeling 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 and stretching deformation, and vice versa.
(2) The printing pressure and the time for the cylinder to imprint each other. The greater the printing pressure, the longer the embossing time, the longer the contact time between the paper and the ink on the blanket, the greater the peeling and deformation.
(3) Ink receiving area on the paper surface. The larger the inked area, the thicker the ink layer, the greater the peeling and extension deformation.
(4) Viscosity and adhesion of ink. The greater the viscosity and adhesion of the ink, the greater the peeling and stretching deformation, and vice versa.
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Four examples of typical overprint faults
1. Constant speed and invariable speed.
Isokinetic registration refers to the same-speed registration of several colors printing, but not registration of different speeds means that the registration of different speeds is not accurate. Speed ​​does not necessarily register.
cause of issue:
(1) The hand-over between the delivery tooth and the impression (paper transfer) roller is not at the cut point.
(2) The gripping force of the paper transfer tooth and the impression (paper transfer) roller is inconsistent.
(3) The transfer time is too long or too short.
(4) The rule is too early or too late before the paper arrives.
(5) The paper has poor stability before the paper arrives.
(6) Paper expansion and contraction.
2. One side of the pull gauge is accurate and the other side is not accurate. It is simply called the operating side and the transmission side is not.
cause of issue:
(1) The braking force on both sides is inconsistent.
(2) The eccentricity of the delivery eccentric is asymmetric.
(3) The gap between the upper gauge of the front gauge and the two sides of the feed iron table are inconsistent.
(4) Paper feeding failures include:
a The tape is not tight.
b The pressure of the pressure roller is asymmetrical.
c Brush pressure is inconsistent.
d The pressure of the pickup roller is inconsistent.
(5) The front gauge and the side gauge are not at right angles.
(6) The gripping force of the paper gripper and roller gripper is too different.
3. The two sides of the different speed overprint are not parallel. This means that under two speed conditions, the error of the cross line printed at low speed and the cross line printed at high speed is equal, and its manifestation: one is that the high speed is not in place. The other is the high speed, which is very regular and has equal distances.
cause of issue:
(1) The time before the paper arrives is late.
(2) It is late to hand the paper tooth and close the tooth.
(3) Inconsistent tightness of the tooth pads.
(4) The time before the paper arrives is early.
(5) Closing teeth early after handing paper teeth.
4. The current rules are not allowed to be combined. It is correct when the overprint is not used alone and the side rules are not used. However, it is compatible with the side pull gauge, that is, when the front gauge and the side gauge are used at the same time, overprinting is not allowed. The printing operator calls this type of failure to be incorrect.
cause of issue:
(1) The side pull gauge is bad.
(2) The gap between the side pull gauge plate and the iron table is too small.
(3) When handing the paper to grip the teeth and close the teeth, the side pull gauge lifts up late.
(4) The paper is late.
(5) The paper residence time is short.
(6) The front and side gauges are not at right angles.
5. Reasons for faults are:
(1) The printing pressure is large.
(2) Inconsistent lining of two printings.
(3) The printing plate is huge.
(4) The blanket is loose.
(5) The paper stretches.
6. Both sides can register, the middle set is not allowed
(1) The roller of the impression cylinder slips.
(2) Middle gripper pine.
(3) The blanket is uneven.
(4) The printing pressure is too large.
(5) There is a problem with the printing plate.
7. The middle can register, the left and right sets are not allowed
(1) The previous rules are not in line.
(2) Individual grippers 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) Throw angle.
8. The overprinting of individual prints in the print is not correct, that is, more than 90% of the prints in a stack can be registered. Only the individual prints are not overprinted. This phenomenon is mainly a problem of paper feeding, followed by:
(1) There is a groove in the front board.
(2) The mixing of horizontal and vertical strands of paper.
9. Constant speed overprinting is not accurate. This kind of fault is the most difficult to solve. No matter how it is debugged, the rule line is always a curved line.
cause of issue:
(1) The impression cylinder of the impression cylinder is bent.
(2) The transfer of paper gripper and impression roller gripper is not allowed to be synchronized.
(3) The clearance of the drum gear is too large.
(4) The opening and closing tooth cam (block) is worn.
(5) The machine has been used for a long time.
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