When evaluating the quality of printed matter, one of the two most commonly used standards is color and the other is overprinting. It can be seen how important it is to achieve good and accurate overprinting for printing good products. Overprinting generally refers to the transfer of graphics and texts of various colors to the specified geometric position of the substrate, and the overprinting accuracy of the contours of graphics and texts of various colors meets the basic requirements of industry standards. For some printed materials (such as: precise national boundaries, navigation maps, etc.), the basic overprint not only includes the above requirements, but also requires that the graphics and text must conform to the original image size. Generally, the requirements for accurate overprinting are not limited to different colors on the same printed matter, but also the front and back sides of the same printed matter, and the printed sheets of the same batch of products should reach a considerable standard. There are many reasons for misregistration in actual work: From the perspective of workflow, almost every link (material preparation, plate making, printing) will become the root cause of misregistration, and the reasons for it are also various . Below we do some simple analysis of these reasons and propose some solutions for reference.
1. Plate making:
1. Imposition (due to the increasingly widespread use of CTFilm, the overprint problems caused by imposition have become less and less, but it is still common in manual imposition). The problems that arise generally can only be re-made and printed. Careful handling can generally be avoided.
2. The film is deformed during storage.
3. Printing results
are generally rare in one printing, but often seen in two or more printings. It is suggested that the table paper should be exposed first when multiple exposures are required. Enterprises with conditions can better ensure the accuracy of the exposure if they use a continuous exposure machine. 4. When baking the plate, if the baking time of the PS plate is too long or the temperature is uneven, it will easily cause the plate to become soft and the tensile strength will decrease
.
It is easily deformed, causing overprinting problems.
Second, the material:
The material mentioned here is mainly paper.
The main components of paper are cellulose and hemicellulose, which contain a large number of OH groups. In addition to the porous structure of the paper, water can easily penetrate into the paper through chemical bonding, adsorption, and structure, so the paper has a strong water absorption capacity. Because the fiber will expand rapidly after absorbing water, the shape and size of the paper is not stable.
1. Offset printing is different from other printing. Water will be used in printing. If the paper with poor sizing, or straw pulp (straw pulp contains a lot of hemicellulose, and the hydroxyl content in hemicellulose is higher than Paper with high cellulose content has poor shape stability, and will be severely deformed during printing, resulting in inaccurate registration (the graphics and text of the first color to the last color become smaller in turn).
·When encountering this kind of paper in printing, the most fundamental way is to replace the paper.
·In addition, when printing paper with poor quality, we sometimes use water printing on the paper before official printing to artificially increase the water content of the paper (commonly known as "water run"), so that the paper has a pre-deformation The process, so that the deformation of the paper will be much smaller when it is officially printed.
2. The paper has tight edges or ruffles
to adjust the humidity of the paper, so that the moisture content of the edge and the middle of the paper is the same, and try to make the moisture content of the paper slightly higher than the relative humidity of the workshop (the forced humidity control method is recommended) .
3. Paper is charged with static electricity
. During transportation, paper rubs against each other or its own volume changes. The consequences of paper electrification usually lead to difficulties in paper feeding and overprinting cannot be guaranteed. The solution is to appropriately increase the relative humidity of the workshop (generally, the humidity should be kept at 55%-65% at 20°C). Trying to keep the temperature and humidity of the workshop constant is also an effective means to prevent static electricity.
3. Mechanical reasons:
There are many reasons for the inaccurate registration caused by mechanical reasons, but generally there are two types:
One is the limitation from the overprint accuracy that the machine itself can achieve. my country's limit on the overprint error of imported printing machines must reach 0.01L, and the limit on domestic printing machines is 0.02mm. Once the overprint error exceeds 0.07L, it means that the performance of the machine itself is extremely poor, and it is impossible to print products with accurate overprint. In the face of a machine whose overprint accuracy cannot meet the requirements, generally speaking, the operator is helpless, so the size of the overprint error of the machine itself is the premise of whether it can print out accurate overprint prints.
The other type is improper operation, improper machine debugging, or misregistration problems caused by machine wear and tear during the production process. Next, we will spend more time analyzing these problems according to the paper’s running process in the machine. , and give some solutions:
(1) Problems that may arise during the paper feeding process of the paper feeder:
stable and correct paper feeding is the first step to ensure accurate overprinting of printed products, especially for single-color and double-color printing machines that print multi-color It is more obvious when printing.
A Usually we call the phenomenon that the paper does not reach the previous positioning line when the side gauge starts to position it as "slow paper loading".
There may be many reasons for the slow loading of paper. Below we will analyze some of the most common faults in actual production.
① The suction force of the paper separation nozzle and the paper feeding nozzle is insufficient, so that the paper cannot be sent out by the feeder reasonably.
Suggestion: Properly increase the inhalation volume (or reduce the air pressure) and
regularly clean the air circuit (especially the air distribution chamber and trachea for inhalation are most likely to be blocked) to ensure the smooth flow of the air circuit.
Regularly maintain the air pump.
② The rubber ring of the paper separation nozzle of the feeder is too small or too thin.
③ The presser foot and the paper separation nozzle quarrel (the presser foot steps on the paper that has been separated), this is because the paper pressure foot steps on this depression. It is equivalent to lowering the position of the paper separation nozzle from the paper pile, and it is impossible to level the paper tail, which will cause the presser foot to step on the paper tail when it falls.
Suggestion: first use the method of inserting wedges or laying paper tape to level the paper, and then adjust the height of the feeder.
·The relative height between the presser foot and the paper separation nozzle is too low.
Suggestion: Properly increase the height of the paper separation nozzle, or lower the height of the presser foot.
④ The paper pile is too lower than the front paper stopper
. The outgoing paper is blocked, resulting in slow loading.
Suggestion: Do not let the paper pile be lower than the axis of the front paper stopper. The general position requirement is lower than the front paper stopper. 5-10L at the top of the paper blocking block is sufficient. If this happens, the feeder can be properly raised (actually, the altitude detection device is raised).
⑤ The falling time of the front splicing pendulum roll is late or the wear is serious
. Make the pendulum roll just fall to press the paper when the paper feeding nozzle is about to stop sucking the paper, and the pressure is enough to feel a certain pulling force when the machine stops. (In fact, it is more reasonable to adjust according to the angular position of the machine, because the transmission time of the feeder should be adjusted based on the swinging roller, not the other way around. We will discuss it in detail later; In addition, pay attention to the accurate adjustment of each part of the swing and roll, because different machine mechanisms are different, no more complaints).
·If the wear is serious, it should be replaced in time.
⑥ The paper feeding time of the feeder is too late (compared to the paper splicing roll)
. On the premise that the swing time is correct, adjust the feeder transmission time according to the falling time of the swing roll (BEIREN’s PZ4880-01 and Heidelberg There is a sprocket in the transmission box of the feeder that can adjust the relative position) The specific timing relationship is shown in ④.
⑦ a The tension of the paper tape is small, or the pressure of the pressure roller is too light. Solution: Tension the transfer belt, or properly increase the pressure of the pressure roller.
b. The machine using the vacuum conveyor belt shows high air pressure, so the pressure of the pressure roller should be increased appropriately.
⑧ a The paper delivery pad is too low, or the height of the up-swing front gauge is too low, or the cover gauge on the down-swing front gauge is too close to the paper delivery table.
·Adjust according to the correct position (the distance between the cover gauge on the up-swing type paper transfer dog pad, the up-swing type front gauge and the down-swing type front gauge from the paper feeding table should be: thin paper thickness*3 or thick paper Paper thickness +0.2mm).
b The lower-swing paper delivery teeth are higher than the paper feeding table.
·Adjust according to the correct position (the tooth pad should be equal to or slightly lower than the height of the cardboard table). a, b The reason for the inaccurate registration is that the improper adjustment of the machine parts
prevents the accurate positioning of the paper
Adjust the height of the cardboard platform according to the standard, and then adjust the height of the front gauge according to the height of the cardboard platform.
⑨ The paper feeding time of the paper feeder and the falling time of the front gauge are improperly matched
. When the front gauge just reaches the positioning position, the distance between the paper on the paper feeding board and the front regulation line is 3-5mm.
⑩ The paper is bounced back when the front gauge is positioned
·It may be caused by the wrong position of the brush and the brush wheel on the paper feeding board (it should be tangent to the back edge of the paper).
·In addition, when printing thick paper, especially when paper is jammed, you can consider using a paper pressure ball.
If the height of the paper teeth at the front gauge is too low, the paper that has been positioned will be pushed back
. It is recommended that the height of the paper teeth should not be lower than 20° (it can be adjusted by adjusting the height of the teeth opening protrusions)
⑾ Slightly larger and thicker model. Improper coordination between the paper feeding time of the paper feeder and the falling time of the previous regulation makes the paper reach the previous positioning line ahead of the previous positioning time.
·The correct mating time should be that when the front gauge falls to the positioning position, the position of the paper on the paper feeding board should be 5~8mm away from the previous positioning line. (J2205 can adjust the matching time of relative motion by adjusting the flange plate under the transmission surface of the paper feeder; Heidelberg SM series can also adjust the matching time by changing the gear meshing relationship in the transmission box of the paper feeder) B Usually we put the
paper Before the front regulation arrives at the positioning position, it has reached the previous specified position line or has exceeded the previous specified position line, which is called paper-loading fast.
① The paper feed stack is too high, even higher than the top of the front paper stopper.
The height of the paper pile can be reduced by adjusting the height of the feeder or inserting a wedge. See (1) for the adjustment requirements.
② Excessive blowing volume of loose paper
·The paper is unstable on the paper table, easy to drift, or the paper blown too high is dragged out when a paper is fed by the paper feed nozzle.
③ Paper feeding mouthpiece is too low
and improperly adjusted.
·Specifically, if the 'damping hole' on the feed nozzle of the SZ206 paper feeder in the domestic printing machine is blocked, it will cause the suction nozzle to fall and fail to lift.
④ The paper splicing pendulum falls too early
. Adjust according to the coordination relationship mentioned in (1) A ④ and ⑤.
⑤ The pressure wheel and brush wheel on the paper feeding board are too tight and loosen properly
⑥ For vacuum feeding belt machines, the air pressure of the feeding belt is too low. The paper feeding time of the air pressure paper feeder is improperly coordinated with the falling time of the previous regulation, so that the paper reaches the previous positioning line ahead of the previous positioning time.
See (1) for the adjustment method.
In addition, there is also the paper skew caused by the fast or slow loading of the paper on the body surface and the transmission surface. Generally, we can directly observe this situation on the paper feeding board (between the edges of the running paper. jagged).
The most important point to prevent the paper from skewing is to pay attention to the position of each part of the paper feeding, and the adjustment should be symmetrical along the center line of the machine. For example: the suction force of the left and right suction nozzles of the feeder must be the same, the tension of the paper feeding belt must be the same, and the falling time of the paper receiving balance wheel must be consistent. During the paper feeding process
until the end of printing, any group of symmetrical parts should be adjusted asymmetrically. All may lead to overprinting inaccurate.
Note: In the production operation, we should pay attention to the sequence of adjusting the machine:
the first set of impression cylinders (feeding cylinders) - swinging paper delivery teeth - paper feeding table - front gauge - paper arrival time - flying Paper feeding time (opposite to the running direction of the paper)
the first set of impression cylinders - transfer cylinders - the second set of impressions... (opposite to the running direction of the paper)
we can see that the first set of impression cylinders (transfer rollers) Paper roller) is the adjustment benchmark of the whole machine, adjusting in this order can ensure the accurate relationship between the various parts of the whole machine.
In addition, there are a few points worth noting in the adjustment:
① The transmission cam of the swing shaft of the paper receiving balance wheel on the paper delivery table is static at the low point, and there should be about 0.15 between the roller on the swing rod and the cam mm clearance to ensure smooth movement when picking up paper.
② The positioning of the front gauge is also a static stroke, and corresponding means must also be used during adjustment (many machines adjust the limit nut) to ensure the stability of the positioning.
③ When adjusting the feeder, it is recommended to adjust the height of the feeder and other parts on the premise of ensuring that the paper pile surface is flat (by padding paper tape and inserting wedges), so as to ensure stable paper separation and feeding.
④ In the case of loose paper, pile less paper each time on the paper feeding table, so as to avoid the excessive concave amount of the paper tail under the pressure of the paper pressure foot, which will cause paper feeding failure and cause overprinting problems.
(2) Overprinting problems caused by inaccurate positioning of the front and side gauges:
Whether the paper can be accurately positioned and passed out by the paper delivery teeth is the key to ensure overprinting. Let's explore some of the more common failures we encounter in production.
① The top-swing front gauge is too high, or the cover gauge on the bottom-swing front gauge is too high, which makes the front edge of the paper easily warped during positioning.
· See (1) A for the correct position. ⑦
② The falling time of the front gauge is not correct, and the correct positioning cannot be performed.
· Re-adjust the falling time (change the position of the
cam )
Use the position of the hole to change the angular position of the cam. After the adjustment, you must re-drill the positioning pin hole and drive in the positioning pin to prevent the position from changing in the future operation. For sheet detection, it is easy to hinder the positioning of the paper because the reed is too hard (this kind of reed generally used in domestic printing machines is not a standard part, and it is easy to have uneven thickness. In this case, you can use sandpaper to thin it out) ④ wheel
or Foreign matter (such as paper hair or ink dots) on the bezel will cause the paper to not reach the positioning position smoothly.
⑤ The side gauge and the cover gauge are too high to pull it over, or it is too low to pull it out of position.
· The correct height should be thin paper*3 or thick paper+0.2.
⑥ The spring is too tight to pull over or too loose to pull it out of position
. Make corresponding adjustments according to the thickness of the paper
**③④⑤The mechanical side gauge mentioned is prone to overprint failure**
Pneumatic side gauge It is easy to be blocked by paper wool, etc. It is recommended to clean it frequently.
⑦ The side gauges fall too early, causing the paper to be pressed by the side gauges before being correctly positioned by the front gauges.
·Adjust the side gauge to the exact falling time according to the angular position of the machine, and wait until the paper is correctly positioned by the front gauge before dropping and pressing the paper (or starting to suck paper on the vacuum side gauge).
When adjusting the side gauges, pay attention: the side gauges should only be lifted when the paper delivery tooth bites the paper, so as to avoid "three nosing" (that is, the paper is in a free state on the paper feeding board) and drift.
⑧ The side gauge is lifted too late
. When the paper delivery teeth have started to hold the paper and leave the paper feeding board, if the side gauge has not been lifted, there will be a phenomenon of "scrambling for mouth", sometimes causing the paper to slide backwards, resulting in overprinting not allowed.
<The lifting time of the side gauge should be after the paper delivery jaw grips the paper and before it leaves the paper feeding board> (
3) Regarding the possible reasons for misregistration during the paper feeding process.
① If the swinging teeth are too high at the transfer position of the paper, it may cause uneven jaw size (and overprint problems)
. The correct position is thin paper thickness*3 or thick paper thickness+0.2.
② All paper delivery teeth and roller teeth If the paper bite force is too small, it is easy to cause problems in paper transfer, resulting in inaccurate registration.
Re-adjust correctly (the size of the bite force should be based on the ability to pull out the bitten paper strip tightly, do not blindly increase the bite force of the teeth, which will cause serious wear and tear on the relevant parts) <Sometimes when the bump is
closed The teeth, and cam-controlled high-point closed teeth machines may be caused by the wear of the tooth rollers >
③ The running track of the pendulum type is not stable
. The running track of the pendulum type is an inverted drop-shaped curve as shown in the figure , it is the running track of the swinging teeth, it should be a smooth curve as shown in Figure a, but if it is found that there is a zigzag part as shown in Figure b during inspection, it will cause overprinting due to unstable paper transfer error. This trajectory can be drawn with a pencil tied to the side of the tooth row. (Generally, it is recommended that the machine should be overhauled in this situation, because once the cam
of the machine has such a failure, it proves that many parts of the machine have been severely worn)
Back slip.
It is recommended to replace it (if it is to be replaced, try to replace it in the whole row, and it is still not guaranteed to prevent uneven overprinting
of teeth )
·Adjust each row of dental pads to the same level.
⑥ The distance between the teeth of the cylinder and the tooth pads of the swinging teeth is not suitable (multi-color machines will also occur in the relative position of the tooth pads passed between the cylinders) and local misregistration is prone to occur.
·The height of the tooth cushion of the paper transfer tooth is adjusted according to the teeth of the first paper transfer roller or the teeth of the impression roller (multi-color machine is adjusted backwards according to the direction of paper feeding by the teeth of the first paper transfer roller) It is two sheets of paper thick, plus 0.2mm for thick paper.
<When adjusting, you can use an elastic steel bar whose length is the width of the machine, about 10mm wide, and about 0.2mm thick. After the previous positioning, let the paper delivery teeth bite, click to the handover position, release the paper delivery teeth, and then Judging and adjusting the height of the transfer pad according to the bending of the steel sheet>
⑦ Uneven teeth force on the impression cylinder can easily lead to misregistration
. It may be caused by machine wear or improper adjustment.
<Sometimes it is caused by the uneven strength of the large tooth springs on both sides of the tooth shaft>
Also, the wear of many parts during the paper transfer process may cause overprinting problems. During operation, we must pay attention to lubrication maintenance and timely repair.
(4) Overprint problems caused by the operation of the rollers:
① Axial movement of the three rollers during axial operation
Sometimes it may be due to the loosening of the jacking screws at the end of the shaft, and some may be due to the thrust bearings on both sides of the axial direction caused by wear and tear.
② Axial and circumferential movement of the plate cylinder during operation
· In addition to appealing reasons, there are usually plate adjustment mechanisms on both sides of the plate cylinder of modern printing machines, and sometimes it may be caused by circuit problems of the plate adjustment mechanism.
4. Problems that are prone to occur in the printing process:
(1) Printing plate:
① The improper method of pulling the plate causes the deformation of the printing plate
. It is recommended to remove the tension on the printing plate before pulling the plate, and resolutely prevent hard pulling and hard top
② Various colors The thickness or padding between the printing plates is different.
<Taking a 270mm cylinder and printing a 600mm graphic as an example, every 0.1mm increase in the thickness of the printing plate or pad thickness will shorten the imprint by 0.25mm> Sometimes
we will also use this feature of the printing plate to purposefully change The length of the blot.
(2) The amount of water:
Too much water will cause the deformation of the paper
<No matter from which point of view (drying of printed matter, overprinting, ink color, etc.), we should advocate reducing the amount of water as much as possible within a reasonable range.
(3) The thread direction of the paper used in printing:
·We generally advocate the use of longitudinal (consistent with the printing machine's paper-feeding direction) paper printing with the paper-feeding direction perpendicular to the wide edge of the paper.
<The experiment proves that when a fiber absorbs water, its diameter increases by 25% to 30%, while its length only increases by about 1%. In addition, using this kind of paper printing can largely avoid the deformation of the paper caused by the backward tension of the paper during the embossing process and the reduction in the strength of the paper after absorbing water. (4) Temperature
and humidity control in the workshop
· When the ambient temperature changes At five degrees, the moisture content of the paper will change by 0.15%. In multi-color printing, when the moisture content of the paper changes by 0.1%, it will affect overprinting. Under normal circumstances, the ambient temperature of the workshop should not change by more than three degrees.
When the humidity changes by 10% (below 80%), the moisture content of the paper changes by 1%. When the relative humidity of the environment exceeds 80%, the moisture content of the paper changes by 1.5-2.0%, so we must be strict below 80%. Control changes in ambient humidity.
Enterprises with conditions are best to ensure that when the temperature and humidity in the workshop are constant at 20°C, the humidity is kept at 55%--65%. Forgot a
word, whether it is a machine failure or an inappropriate process application, it must be overprinted on the printing plate. It is a bit outrageous to start to analyze whether there are these faults if the printing plate cannot be fitted.
alright! So far, we have briefly discussed some reasons for the inaccurate registration of printed matter from the aspects of plate making, material, printing, etc. In actual production, we may encounter more and more problems, which requires We analyzed specific issues in detail.
Here it is necessary to talk about "trapping", this part mainly refers to the "Help" that comes with PS.
There is a "Trapping" menu under the "Image" menu in Photoshop, which can help you complete the trapping of the edge of the hollowed out image, but if you are not a professional output person, or if you do not have enough understanding of "Trapping", it is recommended that you do not use it . So what happened to trapping?
After converting the image to CMYK, you can adjust the color trapping. Trapping is an overlap that ensures that slight deviations or movements of the plate while printing do not affect the final appearance of the print job. If there are significantly different colors in the image, these may need to be embossed slightly to prevent tiny gaps in the printed image. This technique is called trapping. In most cases, the printer will determine whether trapping is required and tell you the values entered in the Trapping dialog box.
The picture below shows the misalignment without trapping and the misalignment with trapping.
Remember that trapping is used to correct misalignment of solid undertones in CMYK images. Normally, do not
Create traps for continuous-tone images such as photographs. On C, M, and Y plates, too much trapping can produce a marker line effect (or even a crosshair). These issues may not be seen in the composite pass, and only show up when outputting to film. So, I say that if you are not a professional, don't use trapping, because you can't see the actual effect of trapping on the screen. We design people are generally used to what you see is what you get, and don't be confused by unpredictable things Dizzy head melon seeds, making the trap into a trap. So what to do? Don't worry, after talking about trapping, I will introduce to you the specific method of manual trapping (trapping).
The Trapping value determines how far overlapping colors are extended (non-blocking) to compensate for typographical misalignment.
Remember the four rules for using trapping in Photoshop:
■All colors expand under black. Mainly use the tolerance of black to other transparent colors.
■Light colors expand under dark colors.
■Yellow expands under cyan, magenta, and black.
■ Pure Cyan and Pure Magenta expand equally below each other.