1. Effect on ink transfer rate
The offset printing process is essentially the process in which the ink splits and transfers between the blanket and the substrate. The ink transfer rate is determined by the fact that the split position of the ink layer is closer to the paper and still the blanket, and the split position is closer to the blanket and transferred to The more ink on the paper, the higher the ink transfer rate. The division of the ink layer begins with the perforation of the ink film, and the sparseness becomes larger and larger. The ink is slowly pulled out and the ink is split into two parts here. The sparse formation is first formed near the paper exit area. While moving to the blanket while growing up, the faster the printing speed, the shorter the time the ink is in the embossing area. The sparse movement does not move long enough into the blanket direction. The clutch position is closer to the paper. If the thickness of the ink layer between the paper and the blanket is constant, then the thickness of the ink layer that is transferred to the paper is not thin enough, the ink transfer rate is reduced, and the ink becomes lighter. This kind of color change, for the printing of a large number of dark inks, the human eye can be infected with superficial emotions, and it is necessary to take appropriate measures to remedy the situation.
There are two ways to remedy the situation: one is to increase the ink transfer rate by increasing the printing pressure, but excessive printing pressure will increase the dot, and the variation of the substrate will become more severe. Moreover, after the pressure reaches a certain level, it will increase again. Large pressure forces have half the impact on growth ink transfer rates. Because of this, it is not possible to rely on increasing printing pressure to maintain color stability; another approach is to increase the roll angle of the ink box roll by adding intermittent rolling rolls to make blankets and The thickening of ink between the sheets of paper requires that the thickness of the ink layer transferred to the paper be maintained under the condition that the ink transfer rate is reduced, thereby maintaining the stability of the color of the medium. Until now, the ratio of the drive frequency of the vast majority of printer inking units to the contact arc length and the printing cycle was fixed. The change in ink color caused by the change in printing speed must be reimbursed manually, that is, the ink box roll angle should be increased. If a print speed tracking and reimbursement system is provided on the inking unit of the printing press, the amount of ink can be adjusted semi-automatically in response to changes in printing speed.
2. Effect on the overprint rate
For wet overprinting, the most important factor affecting the overprinting rate is the level of adhesion of the ink layer between the first ink pad and the second ink pad after the overprinting. The first ink pad has a larger adhesion than the latter, and the overprinting rate is high and the overprinting effect is good. The adhesion of the Indian ink layer before the overprinting moment is not only determined by the adhesion of the ink pad ink itself, but also affected by the dryness of the ink layer.
For a specially-designated multi-color printing press, the interval of the paper feeding path between the units is fixed. With the increase of the printing speed, the printing distance is reduced, and the pressure of the ink pad is reduced at the time of the imprinting pressure. The time of free penetration and oxygenation and drying of the paper before the post-imprint superimposition is also reduced. Due to this, the degree of dryness of the first ink ink layer is relatively low, the adhesion is less, the superimposition rate is reduced, and the superimposition effect is reduced. Deterioration affects the balance of gray, accurate reproduction of the color of the trapping mud, and the gamut of printed matter.
If the adhesiveness of two-color printing inks printed successively differs greatly, such as black and yellow, then the variation of the overprinting distance time has half the effect on the overprinting rate. However, as the bright red ink and the blue ink, the difference between the two types of ink itself is half the difference, the change in the printing speed will affect the superimposition rate.
3. The effect of blotting on the back of the blot
The drying of offset printing inks depends on the increased pressure of the embossing flashes to seep out, free penetration and exudation after the embossing is completed, and oxygenation polymerization drying. The printing speed is reduced, the imprinting time is increased, and the pressure permeation and exudation amount are reduced. At the same time, before the second sheet falls on the first sheet, it also has a longer time of free permeation and exudation and contact with oxygen. To help dry the ink, it is not easy to reveal the back of the dirt.
4. Effect on positioning accuracy
The printing speed increases, the speed of paper transport is accelerated, the time for regular positioning is shortened, and the paper easily rebounds or changes in the rules. The safety and stability of paper transport and the precision of positioning will be reduced; the inertia of reciprocating swinging mechanism will be greater. However, the time for permitting seismic elimination has actually been reduced. This has the disadvantage of accurately delivering paper.
With the use of monochrome machines to print multi-colored products, after adjusting for good conditions at low speeds, at a growth rate, the original situation of registering will often be destroyed and the demand will be upright. Because of this, we must try our best to keep the printing speed relatively stable during the printing process. In particular, when the monochrome machine prints the first color, it is necessary to pay attention to the positional deviation that may be caused by the change in the printing speed. For products that are printed on a multi-color machine at one time, the slight deviations in positioning are generally not awakened.
5. Effect on paper variation
The embossed paper, on the one hand, is tamped by the impression cylinder and forced to engage the blanket cylinder. The adhesiveness of the ink layer on the blanket and the residual viscosity of the blanket adhere the print sheet to the appearance of the blanket. The force on paper that is exactly the same as the direction of the paper is called stripping tension. Under the effect of strip tension, the stripping deformation occurs in the leveling direction, which has an unfavorable effect on the accuracy of the overprinting.
The deformation of the sheet that occurs under strip tension is determined by the volume and uniformity (extrinsic) of strip tension, and the volume (internal cause) of sheet tensile strength and ductility. The higher the printing speed is, the greater the acceleration of the paper-ink clutch is, and the greater the stripping and pulling force, the more severe the priming of the paper, and the greater the impact on the accuracy of the overprinting.
6. The effect of paper on the hair
The higher the printing speed, the greater the pull off force when the paper and the blanket are peeled off and the easier it is to pick up the hair. Practice has proved that under certain ink stickiness conditions, the printing speed is the main factor influencing whether the paper will be pulled during the printing process. Under normal circumstances, the faster the printing speed, the more surface the paper pulls. Therefore, for papers with poor surface strength, printing speed should be controlled and printing should be performed at a slower speed.
7. The effect of uniform ink
From the theory of rheology, it can be obtained that ordinary fluids are flashed as fluid when they are sheared at a slow speed, and they tend to be solid when they are fast. The printing speed is fast, and the ink is quickly cut when the ink is transferred between the ink rollers. The ink tends to be in a solid state, which is disadvantageous to even ink. Therefore, the viscosity and adhesiveness of the ink must be lowered in advance.
Due to the existence of the outer surface of the printing roller, when the ink roller, the water roller and the plate roller are rolling, and when the three rollers are rolling, the relative pressureless condition of the roller is rotated to the printing surface. Pressure conditions will certainly start to oscillate, so that the dot pattern of the contact surface and the thickness of the ink layer will be changed to create scratches. The increase in printing speed will increase the degree of shock, and the offset presses with poor precision and contact pressure adjustment will show up.
Due to poor mechanical precision or wear of the machine during operation, the idleness between the drum journal and bushing, unfair bushing and wall plate hole is too large; gear machining precision is not enough, gear wear and backlash Too large; multi-color machine vertical fine-tuning mechanism slack, bearing wear, etc., so that the drum in the process of operation shock. This vibration occurs when the printing speed is changed or the roller separation pressure is applied, and when the roller shifts from the neutral position to the printing position, not only will the above-mentioned so-called streak failure be induced, but the relative two drum bodies will also be in operation. Misplaced position and induced ghosting failure, this ghosting occurred in the printing press. When the printing speed changes, the ghosting position is usually revealed at the printout.
In short, to ensure stable and high-quality printing, it is very important to control the production process variables. In addition to ink thickness, fountain solution supply, and other factors, printing speed is also a process variable that affects print quality. Operators need to understand the relationship between different process variables and the effect of various process variables on the final print result. In practice, on the one hand, we must do our best to prevent more times from speeding up and speeding down and maintain a stable printing speed. On the other hand, when speed is indispensable for debugging, we must take appropriate measures to repay the impact of speed changes. .