When evaluating the quality of printing, the two most commonly used standards are color and overprinting, which shows how important it is to achieve good and accurate overprinting for printing good products.
Overprint generally refers to the transfer of various colors and graphics to the geometric position specified by the substrate. The accuracy of the contour overprint of each color and text conforms to the basic requirements of industry standards. For some printed materials (such as precise national borders, navigation maps, etc.), basic overprinting not only includes the above requirements, but also requires that the graphics and text must meet the original size.
Generally, the requirement for accurate overprinting is not only limited to different colors on the same printed matter, but also on the front and back sides of the same printed matter, the printed sheets of the same batch of products should reach a considerable standard.
In practical work, it is easy to cause misregistration for many reasons:
From the perspective of workflow, almost every link (material preparation, plate making, printing) will become the root cause of inaccurate overprinting, and the reasons for its generation are also varied.
Below we do some simple analysis of these reasons and propose some solutions for reference.
1. Plate making:
1. Imposition (due to the increasing use of CTFilm, the problem of overprinting caused by imposition has become less and less, but it is still very common in manual imposition). The problems that can arise can only be re-set and printed. Careful operation can generally be avoided.
2. The film deformed during storage.
3. Exposure is generally rare in one exposure, often seen in two or more exposures. It is recommended that the table paper should be printed first when multiple printing is required. If the conditional enterprises use the continuous printing machine, they can better guarantee the printing accuracy.
4. When baking the plate, if the PS plate baking time is too long or the temperature is not uniform, it is easy to cause the plate material to become soft and the tensile strength will decrease. It is easily deformed, causing overprinting problems.
2. Materials:
The material mentioned here is mainly paper.
The main components of paper are cellulose and hemicellulose, which contain a large number of OH groups. With the porous structure of paper, water is easily incorporated into the paper through chemical bonding, adsorption, and structure penetration, so the paper has strong water absorption capacity, and Because the fiber expands rapidly after absorbing water, the paper's shape and size are not stable.
1. Offset printing is different from other printing. Water will be used in printing. If paper with poor sizing degree or straw pulp contains a large amount of hemicellulose, the hydroxyl content in hemicellulose is higher than Paper with a high content of cellulose> has poor shape stability, and will be severely deformed during printing, resulting in inaccurate overprint (from the first color to the last color, the graphics and text will become smaller in turn).
· The most basic way to meet this kind of paper in printing is to replace the paper.
· In addition, when printing paper with poor quality, we sometimes use the paper to print the water before the official printing, artificially increase the water content of the paper (we commonly call it "running water"), so that the paper has a pre-deformation Process, so that the deformation of the paper during official printing will be much smaller.
2. The paper has a tight edge or a lotus leaf edge to adjust the humidity of the paper, so that the moisture content of the edge and middle of the paper is consistent, and try to make the moisture content of the paper slightly higher than the relative humidity of the workshop (recommended to adopt the mandatory humidity adjustment method) .
3. The paper is charged with static electricity. The friction between papers during transportation, or the change of their own volume, is easy to be charged. The consequences of electrification of paper usually make it difficult to feed paper, 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 way to prevent static electricity.
3. Mechanical reasons:
The reasons for the inaccurate overprint caused by mechanical reasons are many, but generally there are only two types:
One is from the limitations of the overprinting accuracy that the machine itself can achieve. China's limit on overprinting errors of imported printing presses must reach 0.01 ?, and the limit on domestic printing presses is 0.02mm. Once the overprint error exceeds 0.07 ?, 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 machines where the accuracy of overprinting does not meet the requirements, in general, the operator is helpless, so the size of the overprinting error of the machine itself is the prerequisite for printing accurate overprinted products.
The other type is the problems of improper operation, improper machine debugging, or inaccurate overprinting caused by machine wear during the production process. Next, we spend more time to analyze these problems according to the running process of paper in the machine. And give a certain solution:
(1) Problems that may occur during the paper feeding of the paper machine:
Smooth and correct paper feeding is the first step to ensure the accuracy of overprinting, especially for single-dual-color printing machines when printing multi-color prints.
A Normally, we call the phenomenon that the paper does not reach the pre-specified bit line when the side gauge starts to be positioned as "slow paper release".
â‘ The suction power of the paper separation nozzle and paper feed nozzle is insufficient, so that the paper cannot be fed out by the feeder reasonably.
Suggestion: Properly increase the inspiratory volume (or reduce the air pressure)
Regularly clean the air path (especially the inhaled air distribution chamber and trachea are most likely to be blocked) to ensure that the air path is unobstructed.
Regularly maintain the air pump.
â‘¡ The rubber band of the feeder nozzle of Feida is too small or too thin
③ The presser foot competes with the separation nozzle (the presser foot has stepped on the paper that has been separated) · This kind of situation is most likely to occur when the amount of depression in the position of A in the figure is large, because the presser foot has stepped into this depression If it is on, it means that the position of the separation nozzle from the stack is lowered, and the paper tail cannot be flattened, which will cause the presser foot to step on the paper tail when it falls.
Suggestion: First insert the wedge or paper pad to flatten the paper, and then adjust the height of the feeder.
· The relative height of the paper pressing foot and the paper separation nozzle is too low.
Suggestion: Properly increase the height of the separation nozzle, or reduce the height of the presser foot.
â‘£ The stack of paper is excessively lower than the front paper stopper. The paper being fed is blocked, resulting in slow paper loading. Suggestion: Do not let the stack of paper fall below the axis of the swing axis of the front stopper. The general position requirement is lower than the front The top of the block can be 5 ~ 10? When this happens, the feeder can be properly raised (in fact, the height detection device is raised).
⑤ The swing time of the front paper swing roll is late or the wear is serious. Make the swing roll drop and press the paper when the paper is sent out by the feed nozzle. The pressure just falls and presses the paper. The pressure can be felt when the machine stops. (In fact, a more reasonable approach is to adjust according to the angular position of the machine, because the transmission time of the feeder feed should be adjusted on the basis of the swing roller, and should not be reversed. We will do a more detailed discussion later; In addition, we must pay attention to accurately adjust the various parts of the roll, because different machine mechanisms are different and no longer complain).
· If the wear is serious, it should be replaced in time.
â‘¥ The feed time of Feida is too late (relative to the paper roll)
⑦ a The tension of the paper tape is low, or the pressure of the pressure roller is too light. Solution: Tension the paper belt, or increase the pressure of the pressure roller appropriately.
b The machine adopting vacuum paper conveying belt behaves as atmospheric pressure, and it is necessary to increase the pressure of the pressure roller appropriately.
⑧ a Tooth-feeding pad is too low, or the height of the front swing-over front gauge is too low, or the cover gauge on the front swing-down front gauge is too close to the paper table.
· Adjust according to the correct position (the distance between the upper swing-type paper transfer tooth pad, the upper swing-type front rule and the lower swing-type front rule from the paper table should be: thin paper thickness * 3 or thick Paper thickness + 0.2mm).
b The bottom-feeding paper delivery teeth are higher than the paper feeding table.
· Adjust according to the correct position (the tooth pad should be level with the cardboard table or slightly lower).
a, b The reason for the overprinting is due to the improper adjustment of the mechanism, which prevents the accurate positioning of the paper
* In addition, it should be noted that whether it is an upper swing type or a lower swing type, the height of the paper conveying table should be adjusted according to the height of the paper delivery pad, and then the height of the front regulation should be adjusted according to the height of the paper conveying table. .
⑨ Improper coordination between the paper feeding time of the paper feeder and the falling time of the front gauge
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