Offset printing materials mainly include paper, ink, printing plates, blankets and cushions. Various materials must meet the printability requirements of offset printing characteristics, otherwise various printing failures will occur.
0 1. Paper factors
The physical and chemical properties and technical indicators of paper, including physical performance indicators, optical performance indicators, printing performance indicators, chemical performance indicators, etc.
Among them are closely related to the suitability of offset printing: tightness, tensile strength, stretchability, whiteness, absorbency, smoothness, gloss, pH value and so on.
The tightness and tensile strength value of the paper, the ink viscosity and the adhesive force generated by the moisture on the surface of the blanket when the pressure is separated during the printing process, will cause lint and powder loss, resulting in burr-like spots or solid spots on the print Gray failure.
The stretchability of the paper is related to the direction of the fiber arrangement. The stretch rate in the filament direction is smaller than the stretch rate on both sides of the fiber arrangement. Therefore, offset printing usually uses paper whose filament direction is parallel to the axis of the cylinder for printing to minimize Inaccurate image registration caused by wet elongation of the paper.
The whiteness and smoothness of the paper are related to the vividness and saturation of the printed image. After the paper with high whiteness is printed with colored ink, the light projected on the paper is less absorbed by the paper and reflected more, so it can The original appearance of the ink color is better presented.
Paper with poor whiteness has ash content, absorbs more projected light, less reflected light, and the printed image is dark. The paper with poor smoothness has a rough surface and strong oil absorption. After the imprint is dry, it is dull, the projected light is absorbed more, and the reflected light is diffusely reflected. The visual effect of the printed matter is poor, and the color is dark and not bright.
The oil-absorbent paper absorbs a large amount of the binder in the printing ink before the conjunctiva is dried, which makes the conjunctiva of the imprinting surface rough, and the product is all luster. In severe cases, the fault of imprinting and chalking may also occur.
Generally, the pH value of paper is above 5, which is slightly acidic. When the pH value drops below 5, the acidity of the paper increases. This kind of paper can destroy the dryness of the ink and cause the problem of non-drying.
0 2. Ink factors
The ink used to express the image and color is a plastic fluid substance. It is transported by the ink roller and coated on the graphic on the printing plate. It is transferred to the paper with a blanket and finally transferred to the paper. The conjunctiva becomes solid after drying.
Offset printing requires the main indicators of ink printability:
Fixation, dryness, coloring rate, fineness, viscosity, fluidity, and resistance to acid, alkali, water, and light.
With the increase in the speed of offset presses and the emergence of multi-color presses, the fixability of fast fix inks is becoming more and more important. The printing speed of the multi-color machine is mostly above 8000 sheets/hour, and it takes about 0.5 seconds to transfer to the second color printing for each sheet. If the fixing speed of the ink is slow, the front color imprints will stay on the back color blanket and gradually reverse transfer, causing the back color to produce color mixing.
The drying of the ink imprint generally takes more than four hours. The early drying ink will crust on the rubber roller, which will bring trouble to the normal printing and cleaning work. If the print printed on the paper is too dry, crystallization (vitrification) will occur, and the subsequent color will not be printed. On the contrary, if the print is not dry, the subsequent color cannot be printed, reducing the production efficiency.
The higher the coloring rate of the ink, the better, it is required to reach 100%, but some inks can only achieve 80-90% coloring rate. In order to achieve color saturation, offset printing operators have to take measures to increase the thickness of the ink layer to remedy the problem. , But if the ink layer is printed too thick, it is easy to cause smearing on the back of the product and blurry dots in the dark tone area.
Generally, the diameter of ink particles is about 7.5μm. If calculated according to the area of 200 lines per dot area of 1613μm 2, it can hold 68 pigment particles, which is more than 15 according to the requirements of printability.
However, in actual printing, some inks have poor agglomeration resistance. Because the ink rollers rub against each other when the machine is running, the viscosity of the ink increases, and the powder on the paper is mixed with the paper wool, which can make the particles thicker and thicker. The clarity and level of expressiveness of the printed matter.