We are a big printing company in Shenzhen China . We offer all book publications, hardcover book printing, papercover book printing, hardcover notebook, sprial book printing, saddle stiching book printing, booklet printing,packaging box, calendars, all kinds of PVC, product brochures, notes, Children's book, stickers, all kinds of special paper color printing products, game cardand so on.
For more information please visit
http://www.joyful-printing.com. ENG only
http://www.joyful-printing.net
http://www.joyful-printing.org
email: info@joyful-printing.net
At present, many small and medium-sized printing manufacturers use monochrome offset printing machines to overprint color work. Simple printing products are printed in two colors. Generally speaking, there are many single-sided and four-color overprinting, but some printing companies have a total of eight colors. This requires the high quality and high precision of the printing machine, as well as the more skilled technology of the printing operator, and pay attention to the overprinting process:
First, paper
Paper is one of the six elements in the printing process. The quality of paper directly affects the speed of printing and the quality of printed matter. First, the paper should be flat and the cutting size should be consistent, and the straight angle and straightness errors are relatively small. For sheet paper used for printing books, the size of the web (width × length) does not exceed ± 1 mm. For printed covers and finer albums, the size of the web (width × length) should not exceed ± 0.5 mm and be marked. The edge, that is, the edge of the rule, the smoothness of the paper refers to the smoothness of the surface of the paper. The smoothness determines the tightness of the contact between the paper and the blanket. It is obviously printed with the paper of low smoothness under the same printing pressure. The image clarity is poor. For example, under the same printing conditions, the use of coated paper is better than the use of coated paper; the paper moisture content is required to be between 6% and 8%, and the content is not too high. The surface tension and surface strength of the paper are lowered, the plasticity is enhanced, and the cured ink film is delayed. If the water content is too low, the paper tends to generate static electricity and tends to be brittle and hard, which is not conducive to printing. Uneven water content will result in unequal deformation of tight edges, ruffles, curls or irregularities, and defects such as pleats may occur after printing. This requires the proper temperature and humidity in the printing shop. Generally, the temperature is controlled between 18 ° C and 25 ° C, and the relative humidity is maintained at 50% - 60%, which is beneficial to the preservation and printing of the paper. In addition, the fiber type and pH of the paper itself will have a greater impact on printing.
Second, the ink
The viscosity and fluidity of the ink play an important role in print quality. The fluidity and viscosity of the ink are closely related. The viscosity of the ink increases the fluidity while reducing the fluidity. On the contrary, the fluidity is improved. The ink with a large consistency is not equal to the viscosity, and the ink with a large viscosity is not necessarily thick. Only the viscosity is the same. Ink, the greater the consistency, the worse the fluidity. If the fluidity is too small, the transfer of ink during the printing process is not smooth and uneven, resulting in shallow and deep ink and other ink failures on the same printing surface; the fluidity is too large, the imprint cannot be accurately transferred, and the graphic level is not clear. The ink is not full, the color is not bright, and at the same time, the excessive fluidity of the ink can also lead to the occurrence of frequent dirty plates and ink sticks. In the printing process, the viscosity and fluidity of the ink can be adjusted by using ink adjustment, but it is forbidden to add corn flour to the ink in order to make the print dry quickly. It can also be achieved by using fast drying inks. Therefore, the viscosity and fluidity of the ink should be adjusted during the printing process. The preservation of ink has always been a headache for printers. Because the ink is placed in the air for a long time, it is easily oxidized by air to form a thick layer of ink. When printing the next ink, it must be The ink is scraped off, which inevitably wastes a part of the ink. The general measure taken by the printing house is to add a certain amount of water to the ink cartridge to prevent the ink from coming into contact with the air. However, in fact, the oxygen contained in the water will also oxidize the ink, and the contact time between the water and the ink is easy to emulsify the ink. When printing on the machine, there is a phenomenon that no ink is applied, although the amount of ink is large, but uniform The ink roller (mainly the hard roller) is still not inked. It is recommended to use wax paper to isolate the ink from contact with air during ink storage, or use plastic paper to seal it. Apply wax paper or plastic paper to the surface of the ink layer, and remove the wax paper or plastic paper before the next printing, but it is best. Do not use plain paper to seal it, and avoid the trouble of tearing the paper when uncovering the paper.
Third, dampening solution
The function of the fountain solution is to wet the blank part of the printing plate and form a metal oxide or hydrophilic salt layer. By using the principle of ink repelling, a barrier is established between the blank portion and the ink to make the ink non-sticky to the printing plate blank. section. Generally, the dampening solution must be a weak acid medium, and the pH of the dampening solution should be adjusted between 5.0 and 6.0. If the PH value is too low, the acidity of the dampening solution will cause severe corrosion of the plate base, and the ink will dry slowly; if it is too high, the diazo compound in the photo-decomposition PS plate will be dissolved, resulting in incomplete image plate. Causes the emulsification of the ink, which seriously degrades the quality of the printed product. At present, the wetting method adopted by the small offset printing machine is mainly the water velvet sleeve wetting device, the Dagren wetting mechanism of the ink and the Kunming direct water supply device, and the different dampening methods according to the printing process. In addition to controlling the amount of water, no matter which type of water supply is used, if the amount of water is too large, water droplets appear on the printed matter, small dots are easily washed away by water, and the printing paper absorbs water and expands and deforms, causing trouble for overprinting. If the amount of water is too small, it will obviously be dirty, but care should be taken to avoid the phenomenon of large water and large ink. The amount of water can be determined according to the area and distribution of the graphic part, the type of paper used, the speed of the machine and the surrounding environment. Offset ink balance is only dynamic relative balance, there is no ideal absolute balance. Mastering the balance of ink and wash is also an important aspect. During the printing process, operators must constantly observe the effect of printing to ensure print quality.
Fourth, the testing machine is good, the overprinting is accurate
Before printing, be sure to check whether the printing machine can be overprinted accurately. After printing 10 to 20 prints, check the heaven and earth and the left and right rules. If the rule is not on the paper (the paper is slightly smaller than the rule line), you can artificially draw a rule on the plate corresponding to the edge of the paper, and apply some ink on the rule line and then print to check the overprint. It is also possible to print the printer again and check if it can be accurately overprinted after two printings.
In the printing process, it is first necessary to ensure that the paper feeding is stable, the paper can reach the front gauge in parallel and flat, and the distance between the paper and the puller side baffle is required to be 3mm-5mm, ensuring that the pull gauge can pull the paper to the desired position, and Do not pull the paper too long if the paper is pulled too long. Then, the pressure of the tension spring should be moderate. When printing thick paper, use a thick compression spring. When printing thin paper, use a fine compression spring, and the spring force of the spring should be adjusted by the spiral on it. Excessive pressure of the compression spring causes the paper to be pulled for too long, and it is easy to deform the paper and affect the movement of the pull gauge with other mechanisms. If the pressure is too small, the paper is not easily pulled to the side baffle, and it is obvious that the left and right overprinting may occur. Only when the distance between the paper and the puller side baffle is correctly set, and the spring pressure is appropriate, can ensure the proper paper pulling force and accurate overprinting. Small offset printing using the push gauge should pay attention to the proper distance between the paper and the gauge baffle, and the other reed has a certain amount of compression when the paper is pushed.
Fifth, the paper position
In the printing process, it is generally said that the small offset printing machine requires the left and right position of the paper, that is, the left and right position requirements of the pull gauge are not very strict, but in some cases, whether the overprinting of the printed product is accurately related to the position of the paper. Because the small offset printing machine has its speciality, unlike the large offset printing machine, the continuous paper feeding method used by the printing machine with large four-open format is usually the direct paper feeding or the indirect paper feeding method, and the small offset printing machine generally uses the paper feeding method. In the paper feeding, the paper continues to be fed to the impression cylinder by the friction between the upper and lower paper rollers, and the paper is positioned once at the front gauge to perform secondary positioning at the impression cylinder. Due to the poor position of the paper, there are two cases of overprinting: one is that after the paper reaches the front gauge, the front end of the paper just touches the side end surface of a front gauge hook during the drawing process. In this way, the paper is an unstable movement during the process of being pulled, or the paper is not pulled into position due to hitting the front side end face, resulting in inaccurate overprinting. In another case, the paper does not touch the front end face at the front gauge, but in the secondary positioning, the edge of the paper just touches the small gauge end face on the impression cylinder, and in this case also Inaccurate overprinting due to poor paper position may occur. Therefore, in the overprinting of small offset printing machines, attention should also be paid to the influence of the left and right positions of the paper on the overprinting.
Six, reasonable arrangement of printing color sequence
Generally, the reasonable arrangement of the four-color machine printing color sequence is: black, blue, red, yellow, and also printed in inverted yellow, red, blue, and black. When printing in a four-color machine, the printing interval between color and color is short, the ink does not have drying time, and it is a superimposed wet printing method, while the monochrome printing machine prints a longer printing interval between colors and colors, and prints a second. When the color is used, the first color is already dry, so it is a dry-stacked overprinting method. It is precisely because of the different printing methods that the color sequence arrangement is different. The color sequence arrangement of the monochrome machine is many, but the specific color sequence can be considered from the following aspects: 1. According to the brightness of the three primary colors, the color sequence should be printed first, then the black color, black and black. Printed, red and yellow printed. 2. According to the transparency and hiding power of the three primary colors, the color sequence is arranged, the blue and black with poor transparency are printed first, and the yellow and red ink with good transparency is printed. 3. Arrange the color sequence according to the size of the graphic area, then print the small graphic first, then print the full or large format graphic. 4. According to the characteristics of the original, the color picture based on warm color should be printed in blue or black first, and the color picture based on cold tone should be printed in red or yellow first. 5. Considering overprinting, it is first to print a color that is difficult to overprint. Generally, the landscape is printed in blue and yellow, while the image of the character is printed in red and yellow. The printing color sequence recommended for printing four colors in a monochrome machine is: blue, red, yellow, black, so that as long as the first two colors are printed accurately and the color tone is good, the latter two colors are not too accurate even if they are not allowed. obvious. The operator should pay attention to the ink change. In order to keep the ink color pure, when the cleaning ink is about to be completed or finished, it is better to add a little ink to be printed and re-clean it again. Especially when the dark ink is changed to light ink, it is best to clean the machine twice.
Seventh, other
In the process of overprinting, if it is not printed on the printed version or printed with more color patches, it is best not to use the dusting device. Although the dusting can make the ink dry quickly and prevent the back of the printed matter from being dirty, the dusting is not good. It directly affects the finish and brightness of the printed matter. Generally, the powder is used in the final color printing, and in the four-color overprinting process, it is better to use the same speed to print, and not to change the printing speed. Because the change in speed affects the overprint accuracy of the print more or less. In addition, if the accuracy of the color printing is very high, the qualified printing factory can also run the water one time before the official printing, that is, the printing plate is folded, the printing ink roller is folded down or the ink supply system is not supplied with ink, and only the water supply is provided. "Printing" without ink supply can reduce the inaccuracy of overprinting caused by paper deformation.
In summary, the monochrome offset printing machine overprints color prints, both for the machine and for the operators, has high requirements. Only by mastering the overprinting technology can fine-quality prints be printed under the conditions of sophisticated equipment and printing consumables.