What are the effects of paper, ink and plates on offset quality?
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First, the paper factor
Physical and chemical properties and technical indicators of paper, including physical properties, optical properties, printing performance indicators, chemical performance indicators. Among them, closely related to the suitability of offset printing are: tightness, tensile strength, stretchability, whiteness, absorbency, smoothness, gloss, pH and the like.
The tightness of the paper and the value of the tensile strength. When the pressure is separated during the printing process, the viscosity of the ink and the adhesive force generated by the surface moisture of the blanket may cause hair loss and powder drop, resulting in burr-like spots or solid spots on the printed matter. Whitening failure.
The stretchability of the paper is related to the direction in which the fibers are arranged, and the expansion ratio of the filaments in the direction of the filaments is smaller than the stretch ratio in the direction of the two sides of the fiber array, so that the offset printing of the filaments in the direction parallel to the axis of the drum is performed to minimize the printing. The image is overprinted due to wet elongation.
The whiteness and smoothness of the paper are related to the color vividness and saturation of the printed image. After the color white ink is printed on the paper with high whiteness, the light projected onto the paper is less absorbed by the paper and has more reflection. Better to present the original color of the ink color. The paper with poor whiteness has a partner, absorbs more projection light, less reflected light, and the printed image is gray. The paper with poor smoothness has rough surface, strong oil absorption, dullness after blotting, and the projection light is absorbed more, and the reflected light is diffusely reflected. The visual effect of the printed matter is poor, and the color is gray and not bright.
The oil-absorbing paper absorbs the binder in the imprinting ink in a large amount before the imprinting has not dried the conjunctiva, resulting in a rough conjunctiva on the surface of the imprint and a flawless product. In severe cases, there is also a blotting failure.
Generally, the pH of the paper is slightly acidic at 5.5 or higher. When the pH drops below 5.5, the acidity of the paper increases. This paper can destroy the dryness of the ink and cause a failure of the print.
Second, the ink factor
The ink used to express the image and color of the image is a plastic fluid, which is transported by an ink roller, coated on the graphic on the printing plate, coated with a blanket, and finally transferred to the paper surface, and dried and conjunctiva. It becomes solid afterwards. Offset printing requires the main indicators of ink printing suitability: fixability, dryness, coloration, fineness, viscosity, fluidity, and resistance to acid, alkali, water, light and light.
With the increase in the speed of offset presses and the emergence of multicolor machines, the fixation of fast-fixing inks is becoming more and more important. The printing speed of the multi-color machine is mostly above 8000 sheets/hour, and each printing sheet takes about 0.5 seconds (s) to be transferred to the second color printing. If the fixing speed of the ink is slow, the front color imprint will be more retained on the back color blanket and gradually reversed, so that the back color will cause color mixing defects. The drying of the ink blot is generally more than four hours, and the early drying ink is crusted on the rubber roller, causing trouble for normal printing and cleaning work. If the print printed on the paper is too dry, crystallization (vitrification) occurs, so that the back color is not printed. On the contrary, if the print is not dry, the back color cannot be printed, which reduces the production efficiency.
The higher the coloring rate of the ink, the better, it is required to reach 100%, but some ink coloring rate can only reach 80-90%. In order to achieve the color saturation degree, the offset printing operator has to take the method of improving the thickness of the imprinted ink layer to remedy. However, the ink layer is printed too thick, which tends to cause the back of the product to be dirty and the dots in the dark area to be blurred.
Generally, the diameter of the ink particles is about 7.5 μm. If it is calculated according to the line area of 1613 μm 2 of 200 lines, it can accommodate 68 pigment particles, and the printing suitability requirement is more than 15 pieces, which is more than enough. However, in actual printing, the domestic ink has poor resistance to polymerization. Because the ink rollers rub against each other during the operation of the machine, the viscosity of the ink increases, and the powder on the paper is mixed with the paper wool, which makes the particles coarser and thicker, affecting the printed matter. The clarity and level of expressiveness.
Third, the printing factor
Offset printing plates have now entered the PS version stage, but there are still a large number of factories using protein plates, PVA plates, multi-layer metal plates, and in recent years, the Zinc Oxide paper base plates have been developed. These plates have different plate making processes. As a printing process, the PS plate is the simplest, and the PVA plate and multi-layer metal plate process are more complicated.
The development, corrosion and painting of the PVA version are all controlled by hand. Therefore, the printing of the printing plate is difficult to control, and the color reproduction of the printed matter often fails to meet the original requirements. Moreover, the thickness of the zinc-coated version of the matte is controlled by experience. Quality is not guaranteed. Sometimes the number of grinding of the zinc skin is much higher, the plate material is reduced from the original 0.55mm to 0.45mm, and the machine operator also relies on the experience to lining the printing plate, often the padding is too much or too little, which is equivalent to changing the plate cylinder. The radius also changes the size of the inner image, making the inner image overprinted.
The multi-layer metal version has a printing durability of several million sheets, and is suitable for printing invoices, tax bills, thin books, books, etc. with uniformity and large printing volume. However, in the printing process, if the sand particles falling from the paper are rubbed, the chrome layer of the plate is often crushed and exposed to copper, which causes the viscous viscous ills. It is very troublesome to eliminate the chrome plating and crushing the smear. It is necessary to use potassium citrate first. It is more time-consuming to remove the dirt and then use special equipment to chrome.
The zinc oxide paper base plate has low cost, and the manufacturing process is simple, but the environmental temperature and humidity, the current voltage of the electrostatic plate making machine, the water content of the basic body of the paper plate, the ink sensibility, and the electrostatic conductivity of the toner are required to be high. Often, some links are slightly biased, and the printed version of the text is white or full of dirty, and there are many bad versions.