The precondition for the printing pressure to ensure the full transfer of ink is that the lining of the stamp body undergoes compression deformation. In actual production, the control pressure of the roller pillow printing machine is mainly by adjusting the thickness of the liner, selecting the appropriate liner material (such as soft and hard properties, Poisson's ratio, yield limit), etc., using the maximum compression or pressure of the liner The printing width indirectly indicates the printing pressure. To master the adjustment of the lining, we first need to have a full understanding of its series of properties.
Compression characteristics of viscoelastic materials
The lining is a viscoelastic material and has unique deformation characteristics under the action of printing pressure. The compression deformation of viscoelastic materials consists of sensitive elastic deformation, hysterelastic deformation and plastic deformation. Sensitive elastic deformation means that the external force deforms immediately, and the external force disappears and immediately returns to its original state; hysteresis elastic deformation means that the external force will slowly deform, and when the external force disappears, it will slowly recover and eventually return to its original state; The disappearance of external force will never return to its original state.
Packing compression deformation
The compression deformation of the liner in the printing process only considers the sensitive elastic deformation, and does not need to consider the hysteretic elastic deformation and plastic deformation. This is because the time for the hysterelastic deformation to fully recover is much longer than the time interval between two printings; the plastic deformation will become less and less during each printing load and unload process, and finally reach a stable state. Among them, the elastic modulus E value is a measure of the size of the sensitive elasticity, the material with a large E value is hard, and the material is soft. Keyin
The selection of the maximum compression amount λ of the lining has the following rules. Soft-lined lambda: offset press is about 0.2mm, letterpress printing machine is about 0.4mm; hard-lined lambda: offset press is about 0.1mm, letterpress printing machine is about 0.15mm; neutral-lined lambda is at Between the soft liner and the hard liner. In actual production, the size of λ should be comprehensively considered according to factors such as the accuracy of the printing press, the fineness of the printed matter, the printing speed, and the quality of the paper.
The thickness of the lining refers to the design thickness of the lining of the printing press. The thickness of the lining of the offset printing machine is the size of the cylinder diameter reduction. The cylinder diameter reduction of different types of offset presses varies greatly and varies between 2 and 4 mm. The thickness of the lining has little effect on the soft and hard properties of the lining, which is mainly affected by the elastic modulus E value of the lining. The liner of the offset printing machine with a diameter reduction of about 2mm can only be composed of a blanket and a few pieces of liner paper, and only a small λ can be obtained, so it is a rigid liner; the diameter reduction is 3.0 to 3.5mm The liner within the range is wrapped with a 1.85mm thick rubber cloth on the rubber roller, and a rubber liner and an appropriate thickness of lining paper are allowed on the back, or a layer of felt and an appropriate thickness of lining paper are added , The obtained λ is larger, which is a neutral liner; the liner with a diameter reduction of more than 4mm can be wrapped with a rubber pad, a felt, or two rubber pads on the back of the blanket. Λ is larger, so it is a soft liner.
The relationship between printing pressure, compression amount and embossing width
The relationship between the printing pressure and the amount of compression and the relationship between the printing pressure and the width of the imprint are not fixed. The relationship between the soft and hard properties of the lining is different. But the general trend of the relationship curve is consistent. Among them, the shape of the relationship curve between printing pressure and compression amount approximates an exponential function, and the shape of the relationship curve between printing pressure and imprint width approximates a normal function.
For the relationship curve between printing pressure and compression amount, the harder the lining, the greater the slope of the curve, and the softer the lining, the smaller the slope of the curve, indicating that the same compression changes, the hard lining printing pressure changes greatly. In actual production, if you use a hard liner, you must pay great attention to the huge changes in printing pressure caused by small changes in the amount of liner compression. Moreover, whether it is a soft liner or a hard liner, the amount of compression is large enough. At the value, the change of printing pressure is very sensitive and rapid, indicating that under the same printing pressure, the compression of the soft pack lining is greater than that of the hard pack lining.
For the relationship between printing pressure and embossing width, under the same printing pressure, the harder the lining, the smaller the embossing width, and the higher the maximum printing pressure.
For equal-diameter printing cylinders, the relationship between compression and embossing width is:
(Where, λ is the maximum compression of the liner, b is the imprint width, and R is the radius of the cylinder)
It can be seen that the changes of λ and b are consistent, so that these two curves have a certain relationship. Through the analysis of these two curves, it can be concluded that when using a soft liner, since the compression amount and the imprint width of the soft liner are larger than the rigid liner, the dots on the printing plate, blanket, and paper are easy to increase Large, can only be used to print less demanding prints, but the thickness of the ink layer is larger than that of the hard liner, the ink layer is thick, suitable for printing lines, solid ground. On the contrary, the dots using the hard liner have good reproducibility, the dots increase less, the dots are clear, and the reproducibility is good. The ink on the paper is less than the soft liner, which exceeds the allowable thickness. Print fine prints. The printing machine using the rigid liner is affected by the precision of the printing machine. With the improvement of the precision, the rigid liner and the neutral liner are generally used.
Stability of lining deformation
After thousands of compressive printings, the lagging elastic deformation of the lining is negligible, the plastic deformation has reached a stable state, and the sensitive elastic deformation has also reached a stable state. Printing is to rely on stable and sensitive elastic deformation to ensure the quality of printing, to make the ink of each printed product uniform and the color reproducibility consistent. After printing a large number of prints on the same liner, its sensitive elastic deformation will be less and less, and eventually disappear. Keyin
If there is no deformation, normal printing cannot be performed. In order to compensate for this situation, a new lining can be replaced or an additional lining can be added to compensate for less and less sensitive elastic deformation. Usually the printing pressure is reduced due to the decrease of the sensitive elasticity. The soft lining can reach 20% to 30% of the maximum pressure, and the hard lining can reach 10% to 15% of the maximum pressure. To achieve a stable amount of sensitive elastic deformation, it takes about 4,000 sheets when the blanket is softly lined with felt, and about 1,000 sheets when the blanket is hardly lined with paper. Therefore, the stability of the liner deformation can only be maintained at a certain stage in the printing process. At this stage, the printing pressure is stable, the compressive deformation of the liner is stable, and the quality of the printed matter is stable. Once a large amount of printing is exceeded, the liner loses its elasticity and cannot be printed.
Change the situation of roller lining
(1) When the thickness of the printed sheet changes, the adjustment of the roller lining. The print mark increases with the thickness of the printing paper. The specific adjustment method is to make the print footprint larger, the thickness of the printing plate liner needs to be reduced; to make the print footprint smaller, the thickness of the printing plate liner needs to be increased. The increase in the thickness of the paper increases the footprint, and the thickness of the lining of the printing plate can be appropriately increased to limit the growth of the footprint. For printing paper of 0.15 ～ 0.5mm, the ratio of the thickness of the printing paper and the thickness of the printing plate lining increases at the same time is 2: 1. For example, if the paper increases by 0.2mm, the printing plate thickness should increase by 0.1mm.
(2) The increase in paper humidity causes the paper to grow, and the adjustment of the roller lining. During printing, the paper is swelled and increased due to the infiltration of the wetting fluid. In order to compensate for this growth, if it is a four-color printing, the thickness of the one-, two-, three-, and four-color printing plate lining needs to be gradually reduced. This gradually increases the width of the print. Generally, the change in the width of the print is 4 times the change in the thickness of the printing plate liner.
(3) Adjustment of the roller lining when correcting the imprint width.
â‘ To reduce the picture and text, it is necessary to increase the plate cylinder lining; to increase the picture and text, it is necessary to reduce the plate cylinder lining.
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