Reasons for the inaccurate printing of printed materials and solutions
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When evaluating the quality of printed matter, usually the two standards we use most often are one color and one overprint. It can be seen how important it is to achieve good and accurate overprinting for printing good products.
Overprinting generally means that each color graphic is transferred to the geometric position specified by the substrate, and the contouring accuracy of each color graphic conforms to the basic requirements of the industry standard. For some printed materials (eg, precise borders, navigation maps, etc.), basic overprinting not only includes the above requirements, but also requires that the graphic must conform to the original size.
Generally, the exact requirements for overprinting are not limited to different colors on the same printed matter, and the front and back sides of the same printed product should meet the equivalent standards between the printed sheets of the same batch of products.
There are many reasons why the overprinting is not allowed in actual work:
From the perspective of the workflow, almost every link (material preparation, plate making, printing) will become the root cause of inaccurate overprinting, and the reasons for it are also diverse.
Below we make some simple analysis of these reasons and propose some solutions for reference.
First, 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). Problems that arise can only be re-imposed and printed. Careful handling can generally be avoided.
2. The film is deformed during storage.
3. The printing plate leads to
Generally not seen in a single print, often seen in two or more prints. It is recommended that the paper should be used first when it is required to be printed many times. If the company has the condition of using the continuous drying machine, it can better ensure the accuracy of the printing.
4. Bake version
In the baking version, if the PS version of the baking plate is too long or the temperature is not uniform, it will easily lead to the softening of the plate material, and the tensile strength will decrease. When it is applied to the printing machine, it will be easily deformed by the tension. Lead to 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 amount of OH groups. With the porous structure of paper, water can easily enter into paper through chemical bond bonding, adsorption and structure penetration, so paper has strong water absorption capacity. Because of the rapid expansion of the fiber after it absorbs water, it determines the shape and size of the paper and is unstable.
1. Offset printing is different from other printing. Water is used in printing, if paper with poor sizing degree, or straw pulp (grass pulp contains a lot of hemicellulose, and the content of hydroxyl in hemicellulose is higher than Paper with a high content of cellulose has a poor shape stability and will be severely deformed during printing, resulting in inaccurate overprinting (from the first color to the final color of the image portion).
· The most fundamental way to encounter this kind of paper in printing is to replace the paper.
· In addition, when printing paper of poor quality, we sometimes use the paper to print the water before the official printing, artificially increase the moisture content of the paper (we commonly called "running water"), so that the paper has a pre-deformation. The process is such that the paper is much less deformed during official printing.
2. Paper has tight edges or ruffles
The paper is conditioned to make the water content at the edge and the middle of the paper consistent, and the water content of the paper is slightly higher than the relative humidity of the workshop (forced humidity control is recommended).
3. Paper with static electricity
Paper is easily rubbed against each other during transportation or after its own volume changes. The consequences of paper charging often result in paper feeding difficulties and overprinting is not guaranteed. The solution is to increase the relative humidity of the workshop (usually the humidity should be kept at 55%-65% at 20 °C). Try to ensure the constant temperature and humidity in the workshop is also an effective means to prevent static electricity.
Third, the mechanical reasons:
There are many reasons for the inaccuracy of overprinting caused by mechanical reasons, but there are generally two categories:
One type is the limit of overprint accuracy that can be achieved by the machine itself. China's restrictions on the overprinting error of imported printing presses must reach 0.01mm, and the limit for domestic printing presses is 0.02mm. Once the overprint error exceeds 0.07mm, the performance of the machine itself is extremely poor, and it is impossible to print an overprinted product. In the face of a machine that does not meet the requirements for overprinting accuracy, the operator is generally powerless, so the size of the overprint error of the machine itself is a prerequisite for printing an accurate printed matter.
The other type is the improper operation in the production process, improper machine debugging, or the problem of overprinting caused by machine wear. Next, we use more time to analyze these problems according to the running process of paper in the machine. And give a certain solution:
Problems that may occur during the paper feeder's paper feeding process:
Smooth and correct paper feeding is the first step to ensure accurate printing overprinting, especially for single-color printing presses.
A Usually, the phenomenon that we have not reached the pre-defined bit line when the paper starts to be positioned at the side gauge is called “slow paper down”.
There are probably many reasons why the paper is slow. Let's do some analysis on some of the most common faults in actual production.
1 .The paper feed nozzle and the paper feed nozzle have insufficient suction, so that the paper cannot be sent out reasonably.
Recommendation: Properly increase the inspiratory volume (or reduce the air pressure)
Regularly clean the air circuit (especially the inspiratory air separation chamber and the air tube are most likely to be blocked) to ensure smooth air flow.
Regular maintenance of the air pump.
2 .The rubber band of the feeder paper nozzle is too small or too thin
3. Pressing the paper foot and the paper separation nozzle (the pressure foot is stepped on the paper that has already been separated). The position of A in the illustration is large, and this is the most likely situation. This is because the paper foot is stepped on this depression. If it is above, it is equivalent to the position of the paper separation nozzle from the paper pile, and it is impossible to flatten the paper tail, which will cause the pressure foot to step on the paper tail when it falls.
Suggestion: First use the wedge or pad to flatten the paper, then adjust the height of the feeder.
• The relative height of the platen foot and the separation nozzle is too low.
Recommendation: Properly increase the height of the separation nozzle or lower the height of the presser foot.
4. The paper stack is too low below the front paper stop
·The paper that is delivered is blocked, causing the paper to be slow
Recommendation: Do not let the paper stack be lower than the axis of the swinging axis of the front paper block. The general position requirement is 5~10mm below the top of the front paper block. When this happens, the feeder can be raised appropriately (actually, the height detecting device is raised).
5. The front paper roll is falling late or wears badly
·When the roll is sent to the paper feed nozzle, the paper will stop sucking. When the suction is not stopped, the paper will just fall and the pressure will be felt when the pressure stops. (In fact, it is more reasonable to adjust according to the angular position of the machine, because the transmission time of Feida paper feeding should be adjusted based on the swinging roller, and should not be reversed. We will discuss it in more detail later; In addition, it is necessary to pay attention to the accurate adjustment of the various parts of the roll, because the different machine mechanisms are different and no longer complain.
· If the wear is serious, it should be replaced in time.
6 .Feida's paper feeding time is too late (relative to the paper roll)
·Adjust the flying time of the feeder according to the time when the rolling time is correct (BEIREN's PZ4880-01 and Heidelberg's SM have a sprocket that can adjust the relative position in the feeder's gearbox. See the specific time coordination relationship.
7. a The tape tension is small or the pressure of the pressure roller is too light. Solution: Tighten the paper tape or increase the pressure on the pressure roller.
b The machine using the vacuum paper feeding belt is characterized by a large air pressure, and the pressure of the pressure roller should be appropriately increased.
8.
a. The transfer pad is too low, or the height of the front gauge is too low, or the cover gauge on the hem of the hem is too close to the transfer table.
·Adjust the correct position (the distance between the upper pendulum type pad, the pendulum type front gauge and the hem type of the front gauge on the front plate should be: thin paper thickness *3 or thick Paper thickness +0.2mm).
b .The hem type of transfer paper is higher than the transfer board.
• Adjust in the correct position (the pad should be level or slightly lower than the height of the pallet).
a, b The reason for the misregistration is that the improper adjustment of the machine prevents the accurate positioning of the paper.
* In addition, it should be noted that both the swing-up type and the hem type should adjust the height of the paperboard table according to the height of the paper-feeding pad, and then adjust the height of the front gauge according to the height of the paperboard table. .
9. The paper feeder's paper feeding time and the front gauge falling time are not properly matched.
· When the previous step is reached, the paper on the cardboard should be 3 to 5 mm from the front specified line.