Reasons for overprinting and solutions (I)

When evaluating the quality of prints, usually one of the two most common standards we use is color, which is overprinting. It can be seen how important it is to achieve good and accurate overprinting for good products. Overprinting generally refers to the fact that colored graphics are transferred to the specified geometric positions of the substrates. The accuracy of the overprinting of the various graphics and texts conforms to the basic requirements of industry standards. For some prints (eg, precision borders, navigation maps, etc.), basic overprint not only includes the above requirements, but also requires that the graphic must conform to the original image size. The general requirement for overprinting accuracy is not limited to the different colors on the same print, but the positive and negative sides of the same print, and the printed sheets of the same batch of products should reach a certain standard. There are many reasons why overprinting is not easy in practical work: From the point of view of workflow, almost every link (material preparation, plate making, and printing) will become the source of overprinting inaccuracies. The reasons for this are various. . Below we do some simple analysis of these reasons and propose some solutions for reference.

First, plate making:

1. Imposition (since the use of CTFilm is becoming more and more widespread, the problem of overprinting caused by imposition has become less and less, but it is still common in manual imposition). The problems that usually arise can only be re-engraved and printed. Careful operation can generally be avoided.

2. The film is deformed during storage.

3. Shaiban led to

Usually rare in a print, often seen in two or more prints. It is suggested that if you need to do multiple printings, you should first set up a countertop paperboard. A qualified company can better ensure the printing accuracy if it uses a continuous dryer.

Baking version

When baking the plate, if the PS plate baking time is too long or the temperature is not uniform, it will easily lead to the plate material becoming soft, the tensile strength is reduced, and it will be easily deformed after being applied to the printing press by the tension force. Lead to overprint problems.

Second, the material:

The material mentioned here is mainly paper.

The main components of the paper are cellulose and hemicellulose, containing a large number of OH groups, plus the porous structure of the paper, water is easily bonded through chemical bonds, adsorption, structure into the paper, so the paper has a strong ability to absorb water, and Because of the rapid swelling of the fibers after absorbing water, the shape and dimensions of the paper are determined to be unstable.

1. Offset printing is different from other printing methods. Water is used in printing. If the paper has bad sizing, or if the straw pulp contains a large amount of hemicellulose, the hydroxyl content of hemicellulose is higher than Paper with high cellulose content has poor shape stability and can be severely deformed during printing, resulting in misregistration (from the first color to the last color, the image portion becomes smaller).

The most fundamental way to meet this kind of paper in printing is to replace the paper.

· In addition, when the printing quality is not very good, we sometimes use the paper to print the water before the formal printing, artificially increase the moisture content of the paper (we commonly known as "running water"), so that the paper has a pre-deformation The process, so that when the formal printing paper deformation will be much smaller.

2. Paper has tight edges or flounces

Wet the paper so that the water content of the edge of the paper is consistent with that of the middle, and try to make the water content of the paper slightly higher than the relative humidity of the workshop (we recommend using the forced humidity adjustment method).

3. The paper is charged with static electricity when the paper is rubbed against each other during transportation, or after the volume changes. The consequences of energizing the paper often result in difficulties in feeding the paper, and overprinting cannot be guaranteed. The solution is to properly increase the relative humidity in the workshop (usually the humidity should be kept at 55%-65% at 20°C). Trying to ensure that the temperature and humidity of the workshop is constant is also an effective means to prevent static electricity.

Third, mechanical reasons:

The reasons for overprinting caused by mechanical reasons are many, but generally do not fall into two categories:
One is the limitation of the overlay accuracy that can be achieved by the machine itself. China's limit on the overprinting error of imported printing presses must reach 0.01L, and the limit on domestic printing presses is 0.02mm. Once the overprint error exceeds 0.07L, it indicates that the performance of the machine itself is extremely poor, and it is impossible to print an overprinted product. In the face of overprinting accuracy, the operator is generally powerless, so the size of the overprint error of the machine is the premise of printing accurate prints.

The other type is improper operation in the production process, improper debugging of the machine, or the problem of overprinting caused by the wear of the machine. Next, we analyze these issues in accordance with the running process of the paper in the machine with more time. And give some solutions:

(I) Problems that may occur during the feeding process of the feeder:
The smooth and correct feeding is the first step to ensure the accuracy of the overprinting of the printing products, especially for the single-color printing machine printing more multi-color printing.
A Usually, the phenomenon that we have not reached the front defined bit line when the side gauges start positioning is called “slow paper”.

There may be many reasons for the slow down of paper. Here we analyze some of the most common failures in actual production.
1 The suction force of the paper suction nozzle and the feed nozzle is insufficient, resulting in the paper being unable to be properly conveyed by the feeder.
Suggestion: Properly increase inspiratory volume (or reduce air pressure)
Regularly clean the air circuit (especially the suction chamber and the trachea that are inhaled are most likely to be blocked) to ensure the airway is open.
Regular maintenance air pump.

2 Feida's paper suction nozzle rubber band is too small or too thin

3 The paper presser foot and the splitter mouth dispute each other (pressing the foot to the already-separated paper). • The position of A in the figure shows a large amount of dishing. This is most likely to happen because the paper is pressed into the depression. On the other hand, it means that the position of the sub-sheet from the paper pile is reduced, and it is impossible to level the paper tail. This will cause the presser foot to fall to the end of the paper.
Suggestion: Use a wedge or paper strip to level the paper first, then adjust the height of the feeder.
• The relative height of the presser foot and the separator nozzle is too low.
Suggestion: Increase the height of the paper mouth properly or reduce the height of the presser foot.

4 The paper stack is too far below the front trim stop. • The paper being ejected is blocked, resulting in slow paper feeding. Suggestion: Make sure the stack is not lower than the axis of the front stop paper swing axis. The general position requirement is lower than before. Block the top of the paper 5 ~ 10L can be. This situation can be appropriately raised Feida (actually height is height detection device).

5 The front-feeding pendulum roll falls too late or is worn badly. • When the pendulum rolls on the feed nozzle to send the paper to stop the air suction, the paper just drops down when the suction has not stopped, and the pressure can be felt when the machine stops. (Actually, a more reasonable approach is to adjust the machine according to the angular position, because the feeder's transmission time should be adjusted based on the swing roll, and should not be reversed. We will discuss in more detail later; Also pay attention to accurately adjust the various parts of the roll, because different machine organizations are different and no longer tired.
・If it is seriously worn, it should be replaced in time.

6 Feeder's paper feeding time is too late (as opposed to the paper feed roller)
·When the rollover time is correct, the feeder's transmission time is adjusted according to the falling time of the pendulum roll (BEIREN's PZ4880-01 and Heidelberg's SM have a sprocket with adjustable relative position in the feeder's transmission box. ) See 4 for the specific time relationship.

7 a The tape tension is low or the pressure on the pressure roller is too light. Solution: Tighten the transfer tape, or increase the pressure on the roller.
b The machine adopting the vacuum conveyor belt is characterized by large air pressure, and the pressure of the pressure roller must be appropriately increased.

8a The paper feed pad is too low, or the upper front height of the swing type is too low, or the cover gauge on the front of the hem type is too close to the feed table.
• Adjust to the correct position (The distance between the upper swing-type pad, the upper swing-type front rule, and the hem-type front gauge should be from the platen: thin paper thickness * 3 or thick Paper thickness +0.2mm).
b The hem pendulum is higher than the transfer board.
• Adjust to the correct position (the pad should be level with or slightly lower than the height of the transfer board).
a, b The reason for overprinting is due to the improper adjustment of the machine to prevent the accurate positioning of the paper.
* In addition, it should be noted that whether in the swing-type or hem-type adjustment in the adjustment should be based on the height of the transfer paper to adjust the level of the transfer board, and then to the height of the transfer board to adjust the height of the front rule .

9 The paper feeder's paper feeding time is not properly matched with the drop time of the front gauge. • When the gauge just arrived at the positioning position, the paper on the paper feeding board should be 3~5mm away from the previous specified bit line.

10 The paper is bounced back when it is positioned at the front gauge. This may be due to the wrong position of the brush and the brush wheel on the paper feeder (it should be tangent to the trailing edge of the paper).
• In addition, when printing thick paper, especially paper jams, consider using a ball.
If the height of the prosthesis at the front teeth is too low, the already positioned paper will be topped and the height of the recommended teeth should not be lower than 20° (adjustable by adjusting the height of the open teeth).

(11) Slightly larger and thicker models. The misfeeding time of the paper feeder and the time of falling of the front gauge were improperly matched to make the paper ahead of the previous specified time to reach the predetermined bit line.
• The correct time for cooperation should be when the current gauge falls to the positioning position. The position of the paper on the paper feeding board should be 5 to 8 mm from the previous specified bit line. (The J2205 can adjust the relative movement timing by adjusting the flange below the conveying surface of the conveying board; the Heidelberg SM series can also adjust the matching time by changing the gear meshing relationship in the transmission box.)
B Usually, we have already reached the front defined bit line before the front gauge has reached the positioning position, or we have already called the bottom paper faster than the front specified bit line.
1 The paper stack is too high, even higher than the top of the front stop.
·You can reduce the height of the paper stack by adjusting the feeder height or inserting the wedge. See (1) 3 for adjustment requirements.
2 Too much air blown paper • The paper is not stable on the paper table, it is easy to drift, or the paper that is blown too high is pulled out when the paper feed nozzle feeds a piece of paper.
3 Paper feed nozzle too low • Improper adjustment.
· What is more special is that if the 'difficult hole' on the feed nozzle of the SZ206 type feeder in the domestic printing press is blocked, it will cause the nozzle to fall and not lift.
4 The falling of the paper-feeding pendulum is too early. It is adjusted according to the coordination relationship in (A) A 4 and 5.
5 The roller and brush wheel on the paper roll are too tight and properly relaxed. 6 The vacuum conveyor belt machine has a low belt pressure. The paper feeding time of the pneumatic paper feeding machine is not properly matched with the falling time of the front gauge, so that the paper advances before the prescribed bit time before reaching the predetermined bit line.