Third, the printing material factors
Offset 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.
1. Paper factor
The physical and chemical properties and technical indexes of paper, including physical performance indexes, optical performance indexes, printing performance indexes, chemical performance indexes, etc. Among them are closely related to the suitability of offset printing: tightness, tensile strength, stretchability, whiteness, absorbency, smoothness, gloss, pH, etc.
The paper's tightness and anti-brush strength value, when the pressure is separated during the printing process, the viscosity of the ink and the adhesive force generated by the moisture on the surface of the blanket will cause hair loss and powder loss, causing burr-like spots or solid spots Failure to spend money.
The stretchability of the paper is related to the direction of the fiber arrangement. The stretch rate in the filament (sin) direction is less than the stretch rate in both directions of the fiber arrangement. Therefore, offset printing is usually printed on paper with the filament direction parallel to the cylinder axis to minimize Image overprint caused by wet elongation of paper is not accurate.
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 color ink, the light projected on the paper is absorbed less by the paper and reflected more, so it can Better present the original appearance of ink color. The paper with poor whiteness has a member, which absorbs more projection light and reflects less light, and the printed image is gray. The surface of the paper with poor smoothness is rough, strong in oil absorption, dull after the print is dried, 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.
Before the blotting has dried the conjunctiva, the oil-absorbent paper absorbs a large amount of the binding material in the blotting ink, resulting in a rough conjunctiva on the surface of the blot, and the product is not tarnished. When it is serious, it will also produce a pulverization failure.
Generally, the pH value of paper is slightly acidic above 5.5. When the pH value drops below 5.5, the acidity of the paper increases. This paper can destroy the dryness of the ink and cause the printing to dry.
2. Ink factor
The ink used to express the image and color of the image is a kind of plastic fluidity, which is transmitted through the ink roller, coated on the graphics on the printing plate, used by the blanket, and finally transferred to the paper surface, after drying the conjunctiva Later it becomes solid. The main indicators of ink printing suitability for offset printing are: fixability, dryness, coloring rate, fineness, viscosity, fluidity, and acid resistance, alkali resistance, water resistance, light resistance and other resistance.
With the increase in the speed of offset presses and the advent of multi-color 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 it takes about 0.5 seconds (s) to print the second color for each printing. If the fixation speed of the ink is slow, the front color imprint will stay more on the back color blanket, and gradually reverse the transfer, so that the back color will produce a color mixing disadvantage. The drying of ink blots usually takes more than four hours. The early drying of the ink will cause skinning on the rubber roller, which will cause trouble for normal printing and cleaning work. If the blots printed on the paper are over-dried, crystallization (vitrification) will occur, and the back color will not be printed. On the contrary, if the blots are 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 achieve 100%, but some ink coloring rate can only reach 80 ~ 90%, in order to achieve color saturation, offset operators have to take the method of increasing the thickness of the printed ink layer to remedy , But the ink layer is printed too thick, it is easy to cause the back of the product to be dirty and the dots of the dark tone area are blurred.
In general, the diameter of ink particles is about 7.5μm. If calculated according to the area of ​​200 lines and a dot area of ​​1613μm2, 68 pigment particles can be accommodated. According to the printability requirements, more than 15 particles are more than enough. However, in actual printing, the resistance of domestic inks is poor. Due to the friction of the ink rollers 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, affecting the printed matter. The clarity and level of expressiveness.
3. Plate factors
Offset printing plates have now entered the PS version stage, but a considerable number of factories are still using protein plates, PVA plates, and multi-layer metal plates. In recent years, zinc oxide paper base plates have been developed for printing printed copies. The plate-making processes of these printing plates are different. As the plate-making process, the PS plate-making process is the simplest, and the PVA version and the multi-layer metal plate process are more complicated.
The operation of developing, corroding, and applying base paint on the PVA plate are all manually controlled. Therefore, the coating of the printing plate is difficult to control, so that the color reproduction of the printed matter often fails to meet the original requirements. Moreover, the thickness of the zinc plate frosting is controlled by experience. Quality is not guaranteed. Sometimes the number of grinding times of zinc skin is increased, the plate material is reduced from the original 0.55mm to 0.45mm, and the machine operator also pads the printing plate based on experience. Often, too much or too little padding is equivalent to changing the plate cylinder The radius also changes the size of the inner picture, making the overprint of the inner picture inaccurate.
The multi-layer metal plate can have a printing durability of up to several million sheets, which is suitable for the printing of invoices, tax bills, thin books, account books, etc. with a uniform and large printing volume. However, in the printing process, if the paper is rubbed by sand particles, the chromium layer of the plate is often crushed and exposed to copper, resulting in fat-sensing and ill-effects. To eliminate the chromium-plating layer is very troublesome, and potassium flavinate must be used first Decontamination, and then use special equipment to pass chrome plating, which takes more time.
The cost of zinc oxide paper base plate is low. Although the manufacturing process is simple, it has high requirements on the environment temperature and humidity, the current and voltage of the electrostatic plate making machine, the water content of the basic body of the paper plate, the ink sensitivity, and the electrostatic conductivity of the toner. Often there are some deviations in some links, and the produced text is white or dirty, and there are many bad versions.
4. Operating environment factors
At present, there are many new offset printing machines and new offset printing workshops in China. However, due to the insufficient understanding of the offset printing process by managers, the temperature and humidity of the operating environment cannot be strictly controlled. Problems such as inaccurate overprinting caused by paper bowing and wrinkling, and the impact of static electricity on paper have long been difficult to solve completely. They cannot produce properly and exert their due benefits. The operating environment here refers to area, height, level, Paper drying area, doors and windows and ground, workshop orientation and lighting, process arrangement, logistics direction, and air pump and pool and safety, dustproof, shockproof, fire prevention, heating, ventilation, lighting and other contents. The operating environment has an important influence on the printing quality. A good operating environment can reduce the occurrence of failures.
V. Maintenance factors
Practice has proved that in addition to the design and manufacturing factors of the machine itself, many mechanical failures are not well maintained, and improper operation is also an important cause of failures. Similarly, if a piece of equipment is properly maintained, it can be used for about ten years without major repairs without accidents. On the contrary, it may take as little as three years or more than five years to print fine products. Although it may delay some time and affect some output during maintenance, it is worth it in the long run. That kind of task requires little or no maintenance when busy, and maintenance when not busy is actually an irresponsible attitude; some even take the way of driving fast and building equipment, which is even more wrong.
Maintenance is nothing more than according to the requirements in the instruction manual, according to the parts are divided into daily maintenance, weekly maintenance, monthly maintenance, seasonal maintenance and half-year maintenance. Its content includes regular oiling, oil changing, cleaning parts, cleaning various parts of the filter; maintenance of various motors and electrical appliances, adjustment and replacement of carbon brushes; regular inspection, verification and inspection of various data of water (ink) roller and roller pressure Whether each part is loose or worn; and the cleaning and sanitation of the machine are well done, so that the equipment is always in good operation. In order to prolong the service life of the machine and reduce various failures caused by poor maintenance.
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