Relationship between offset paper and print quality
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In offset printing presses, there are three important rolls: a PS plate roll, a blanket roll and a pressure roll, which are mounted in a fixed position on the press and the gap between the roll and the roll is fixed. Therefore, it is necessary to fill the gap between the drum and the drum by the bottom paper, thereby precisely adjusting the pressure between the PS plate roller and the blanket cylinder, or the blanket cylinder and the pressure roller. The bottom paper will be placed on the underside of the PS plate or on the underside of the tape. In some machines, it can also be mounted on the platen roller.
A prerequisite for ensuring consistent print quality is the need for stable, stable backing paper, which is why, for many years, the printing industry has used McMurray as a professional backing material: through a special production process – “High Pressure Calibration” "The density, surface and thickness of the inherent heterogeneity of the paper are homogenized, and it becomes a base paper with a standard density, thickness and flatness, and a calibrated thickness function.
Since ordinary paper is not processed by the “high pressure calibration” process, the paper's inhomogeneous properties will undoubtedly affect the print quality, with undesirable results: streaks or foggy spots, unstable dots or dot enlargement, dot deformation, Ghosting, dot misplacement, etc.
1. Minimal manufacturing tolerances
The production process of Meishi's gun base paper is in full compliance with the ISO standard. With its unique technology and expertise, the thickness tolerance value per square meter of the product is controlled below ±0.01mm. One of the most important factors in using an air cushion blanket is that it must have a "hard" gun base paper fit. Because the base tape or the general paper is too soft, it is not suitable for use as a backing material, otherwise it will have a variety of adverse consequences: the mesh is too large, displaced or ghosted, and the blanket will stretch and buckle due to the virtual position, resulting in high temperature. The ink is unbalanced, the quality of the product is unstable, and the printing machine may be damaged in the long run. Only the bottom paper of the process of compression and "high pressure calibration" achieves the required hardness requirements.
2. Image transmission
In general, the tolerance range of the printing process is a few hundredths of a millimeter, and the base paper must have a variety of thicknesses (the Meissis bottom paper is available in 14 thicknesses ranging from 0.015 to 0.5 mm) for fine adjustment. Printing pressure for the best print quality. Especially in multi-color printing, it does not affect the printing quality.
Even when printing at a lower pressure, the printed area (about 2 mm, the contact point between the PS plate roll and the blanket roll) was subjected to a radial pressure of about 100 N/cm 2 and a tangential direction of 250 N/cm 2 .
Due to the rotation of the drum, on the one hand, these forces are transmitted along the tangential direction on the surface of the drum. On the other hand, through the tape/PS plate, the tangential force is transmitted to the axis of the squeegee to reduce the thickness of the blanket, so-called " sink". In general, the blanket sinking ratio can be queried by the manufacturer.
The tape will sink under the whole life cycle. Therefore, it is necessary to use the different thickness of the squeegee paper to compensate the sinking thickness of the tape, and the bottom tape or the bottom glue can not get the effect of the squeegee. If the bottom paper is not used as the backing, the non-burst paper is continuously thinned under the aforementioned pressure and the thickness is continuously reduced. Then, the "controllable" blanket sinks, and the "uncontrollable" non-burst paper sinks, making the original pre-calculated sinking amount impossible to grasp effectively, resulting in an error due to the effective circumference of the drum. problem.
2. Thickness rather than weight
During the manufacture of "high pressure calibration", the base paper has been previously pressed by high pressure to their minimum thickness. Take the 0.30mm thick floor paper of the Meister's base paper as an example: it is made of raw paperboard with a thickness of about 0.45mm, and then is calibrated by high pressure to a thickness of only 0.30mm. Since the pressure generated during the "high pressure calibration" manufacturing process is much greater than the pressure generated during printing, the bottom paper cannot be sunk again during the printing process.
Contrary to ordinary production paper, Max's cannon is made according to the thickness as the priority standard, not the weight (weight per unit area). If you use non-burst paper, the printer only knows the grams of paper. Instead of thickness, it is only possible to estimate the thickness of the underlay, so that errors caused by the use of non-standard effective circumferential length printing of the drum are unavoidable.
Due to irregular operation, the bottom paper is infiltrated by dampening solution or car wash water, and may become damp and swell after a period of use. Therefore, some printers prefer to use plastic sheets as substrate sheets. However, the use of plastic sheets as the backing has the following drawbacks: the film will generate static electricity; the thickness of the plastic sheet has a large error; the plastic does not absorb moisture, and the residual solvent of the car wash water or the water bucket accelerates the corrosion of the drum; after a long time of use, the film It is often difficult to peel off from the roller; it is impossible to increase or decrease the thickness of the plastic sheet; when there is foreign matter sandwiched between the rollers, the plastic sheet is not easily damaged and absorbs the impact force, thus causing the roller to be damaged; the plastic sheet is more than the gun The backing paper is more expensive.
3. Application of waterproof technology
The Mein's gun bottom paper technology has the function of maximizing moisture resistance. The special component of moisture-proof is not only used for the surface treatment of the squeegee paper, but also thoroughly penetrated into the squeegee paper to prevent the solvent such as car wash water from infiltrating from the both ends of the drum to the edge of the squeegee paper and expanding.
This feature makes Max's gun bottom paper have the advantage of being similar to plastic sheets. Because plain paper does not have any moisture protection properties, theoretically using plain paper as the backing requires more downtime to replace the backing material. The backing paper which is only water-repellent on the surface can only prevent water from penetrating into the paper from the surface, and cannot prevent the solvent such as water from the water from penetrating from the both end edges of the drum to the edge of the paper to cause expansion.
4. Rolling error
The use of air-cushion blankets tends to produce excessive printing pressure, and the most common problem with excessive pressure is the accordion folding. The larger the effective circumference error of the drum (the greater the difference in the thickness of the gun bottom paper), the larger the tangential force (force), and the more chance the bottom paper has to be folded. Although the accordion fold is caused by the tangential force, it is ultimately caused by the error of the effective circumference of the drum. However, the quality of the gun's backing paper, especially its surface quality, determines the tolerance range of the effective circumference of the drum. The key is to maximize the tolerance range that can be compensated for the bottom paper, and try to avoid premature sliding movement of the gun.
In addition, the German Meishi gun bottom paper adds an additional process to the production process, successfully integrating the “slip resistance” performance in the gun bottom paper. This process neither increases the surface coating nor roughs the surface of the slab, as these have fatal consequences for the dots. No wax is used because the wax melts and the surface is smoother.
Obviously, we can't explain in detail how to get the technical details of this anti-slip effect. But the fact is that today's new generation of Meister's cannon paper can compensate for the larger drum effective perimeter error without causing the transfer of the bottom paper or the accordion fold.
5. Color series
If you do not use the Meister's floor paper, the printer usually only uses white backing paper regardless of the thickness. Since substrates of different thicknesses are required, the thickness of the entire stack can only be estimated by measuring the individual thickness of each sheet of paper. Due to the lack of a suitable paper thickness gauge, the use of commercially available micrometers does not accurately measure the exact thickness of the bottom paper. In addition to the large amount of time required to combine a substrate thickness, the measurement error is increased. Such thickness errors can be accumulated up to several hundred millimeters, resulting in a decrease in print quality and a technical failure associated with an increase in effective circumferential error. .
This is why the German Max has come up with the "color coding system" decades ago, and the specific thickness of the gun bottom paper is assigned a specific color. This is the only way to combine a padding as quickly and easily as you don't have to rely on inaccurate measurements. This has the advantage that even a low-tech worker in the pressroom can easily set the backing without making mistakes.
6. Other products
The air cushion bottom tape can be used in some old printing machines, because the machine is no longer accurate, and the slight imbalance can be compensated by the compression of the air cushion bottom tape. Air-cushioned backing tapes are not suitable for sheetfed offset printing, but for older rotary presses, such as KIMLON R and Finito R. In the market, air-cushioned backing tapes, plastic sheets or ground papers are often misled to be thickness-corrected and can replace cannon-backed paper. In fact, only Max's gun bottom paper is truly pressure-corrected to meet the needs of long-run printing.