Common problems in the printing workshop are well known. There are many factors that affect the quality in the printing process. In addition to the machine itself, there are also substrate, ink, and external factors such as air humidity, workshop temperature, etc. However, when printing operators encounter practical problems in production, it is often not easy to quickly find the root cause of things and effectively solve the problem. Now, we will help you sort out and answer the common questions in the printing process.
1、 One of the common problems in the printing production process is that the edge of the printing handwriting is inverted and the printing is double shadow:
1. Adjust the pressure: the pressure here is divided into upper pressure and lower pressure. The upper pressure refers to the gap between the anilox roller and the printing wheel; Pressing down refers to the gap between the printing wheel and the printing pressing wheel. Large pressure leads to small clearance, and small pressure leads to large clearance. Of course, the effect of overpressuring is different from that of overpressuring. How to judge whether the pressure is too high or too low when the double image problem occurs? It is necessary to carefully observe the printing effect. If the double image appears around the handwriting stroke, it is caused by too much upward pressure. Consider reducing the upward pressure appropriately, that is, enlarging the gap between the anilox roller and the printing wheel, depending on the paper feeding direction. If the double image appears below the paper feeding direction, it is caused by too much downward pressure. Consider reducing the downforce appropriately, that is, increasing the gap between the printing wheels, and the effect will be improved.
2. Layout design: In the actual production process, we often encounter the situation of the field version and small print layout. In the printing process, if you want the field version to be flat and full, the pressure should be appropriately higher, but at the same time, the small print will have a double effect. In addition to the above factors, there are other factors, such as poor transfer of ink itself, uneven cardboard, and uneven compressive strength. When printing, to achieve a bright color, full and good coverage, the first consideration is to adjust the equipment parameters, including pressure.
3. Check the printing plate: The handwriting of the printing plate has been drawn here, and there are many reasons for the double shadow, but the most direct reason is small. The handwriting and stroke surface is not smooth and uniform, or the printing is caused by pressure. In order to achieve a satisfactory printing effect due to factors such as poor ink transfer of the printing plate, the technology, equipment, printing plate, cardboard, ink wash and auxiliary materials of the staff are required to be properly matched.
2、 Ink bar problem
In flexographic printing, the most common fault of the printing machine with gear drive (not shaftless drive) is the ink bar, that is, there are ink tracks in the axial direction of the printed matter. Most ink bars are caused by mechanical reasons. In flexographic printing, ink bars can be divided into two types: one is a comb shaped ink bar with regular axial distribution, and its spacing is a tooth pitch (3.175mm); the other is an irregular one or several ink bars with irregular axial distribution, and the spacing is irregular, which may be a thick one or several thin ones; Or there are half strips on the body or outside. The ink bar is caused by the transmission gear, which can be solved by changing the gear or the color group (color sequence). The second type of ink bar is mainly caused by improper feeler gauge and pressure adjustment. We all know that the pressure adjustment of flexography is the key to ensure the printing quality, and we must not be careless. The pressure adjustment of the three rollers can not only achieve the equal pressure between the three rollers, but also achieve the equal pressure between the two ends of the three rollers. Especially for a new product printing, the pressure between the three rollers must be adjusted.
The most important adjustment is in two aspects: the pressure adjustment between the ceramic anilox roller and the plate roller; The adjustment of the pressure between the plate cylinder and the impression cylinder.
First, adjust the spacing between the three rollers. In the closing state, the spacing between the anilox roller and the plate roller, the spacing between the plate roller and the impression roller is the same: 0.38+1.70=2.08mm, (0.38 mm is the thickness of double-sided adhesive, 1.70 mm is the thickness of the plate). Two 2.08 mm thick standard feelers can be selected and placed at the two ends (no plate or substrate) between the two rollers, and then manually adjust the distance between the anilox roller and the plate roller, and between the plate roller and the impression roller, so that the resistance of the standard feelers at their respective ends is the same. At this time, the spacing is the ideal pressure value in printing, but it is slightly different from the actual printing pressure, which needs to be adjusted by the operator in production.
In the actual production process, the serious imbalance of the three rollers may also cause a kind of ink bar. Therefore, in order to obtain good printing quality, the first thing is to lay a feeler gauge to ensure the balance of the three rolls.
3、 Overprint fault
There are many factors that affect overprint. At present, the tension control system of the unit flexo press is that the operator sets the appropriate tension value according to the conditions of the printing materials to ensure that the alignment line does not drift when registering. The system adopts closed loop automatic control to keep the tension of printing materials constant from beginning to end. The registration control is divided into two directions, with the paper direction as the reference. The horizontal registration refers to moving the version cylinder laterally, and rotating the handle on the version cylinder shaft clockwise or counterclockwise. Generally, in the initial stage of trial printing, the horizontal registration shall be guaranteed first, and then the vertical registration shall be adjusted. First, press the same color until the last unit of printing, stop the machine for coarse adjustment, turn the plate cylinder to make the registration error within one tooth, then start the machine, and use the registration control button to manually fine adjust. The longitudinal registration system uses a stepping motor with high transmission ratio, which drives the plate cylinder to rotate through the imprint cylinder gear, in fact changing the longitudinal relative position between the plate cylinder and the printing material.
It can also be recorded in the process file after the waste simulation test. At the same time, it is necessary to ensure that the plate pasting is accurate, do not stretch, eliminate the bubbles between the plate tapes, and compact. In order to prevent the printing plate from deforming, it can be removed together with the tape when removing the plate. In overprint, although our accuracy can reach 0.05mm, due to the paper deformation, printing plate deformation and other factors, we need to further overprint after the alignment line is registered. When registering, we should first understand the graphics thoroughly. There are major parts and minor parts of the graphics, and the major part is the key to the image. Therefore, we can consider the main part and reject the secondary part, and register the major part as much as possible on the basis that the minor part is acceptable.
4、 Ink, white spots, milky white ribbons, etc. appear on the real part of the print
In flexographic printing production, there are often many white spots on the real part of the printed matter, and the white spots still exist after the plate is wiped. In this case: first, check whether the electrostatic eliminator and the dedusting part have faults and need to be handled immediately; Next, check whether the viscosity of the ink (UV) is too high, and then measure the temperature of the ink. If the viscosity is high and the temperature is below 30 ℃, we should add some adhesive remover and stir more to enhance the fluidity of the ink. The printing suitability of the ink is better when the temperature is 33 ℃ in production. Another situation is that the white spots in the field are concentrated on the circumference of the fixed part, especially on both sides. At this time, it is necessary to readjust the pressure. Adjusting the three roll balance can solve this problem. Another is to increase the humidity of the workshop, which can reduce the dust in the air and reduce the probability of white spots.
When there are ink filaments in the circumferential direction on the printed matter, first check whether there are ink filaments on the surface of the anilox roller after scraping. If there are ink filaments on the surface of the anilox roller, it may be that there is a problem with the ink scraper (such as a gap). After changing the scraper, if there are still ink filaments, check the scraper. If the scraper is intact, check the viscosity of the ink. If the viscosity is too high, add the adhesive remover.
Milky white ribbons appear on the field of printed materials. We use domestic UV inks and imported UV inks in production. When using domestic inks, several kinds of inks such as transparent white or covered white are added to ordinary four-color inks to prepare spot color inks, which will produce milky white ribbons on the printed products. It is preliminarily believed that this is due to the poor mutual solubility and stability of various connecting materials, solvents, and free bodies. A good method is to let the manufacturer directly supply the prepared spot color inks, with good results. Or add more mixing or high speed driving after preparation during production, or prepare the ink two or three days in advance, which can effectively alleviate the problem. When matching inks, the spot colors of inks from different manufacturers must not be mixed with each other without experiments. In particular, the mixing of domestic inks and imported inks is a big taboo in printing. In this way, the depth will be different, and the inks will quickly deteriorate and glue.
5、 Fault on die cutter
There are mainly two kinds of die-cutting knives in production: semi cutting and full cutting.
In half through die cutting, if it is placed within the printing tension range, it will affect the nearest printing color deck registration, so it is better to place it outside the printing tension range. The pressure adjustment is very important in the die-cutting process, especially for the knives that are approaching the service life, the pressure should be increased appropriately when increasing the speed, and the pressure should be reduced slightly after the paper is received, so as to prevent paper breaking or waste paper breaking. After shutdown, keep away from the press knife, do not hit or touch hard objects, and pay attention to cleaning the gear, shoulder iron and bottom roll.
In full cutting, the knife is outside the range of printing tension, so pay attention to pasting elastic sponge on the knife. Different types of medium and hard sponges can be selected according to different live parts, which can ensure the die cutting accuracy and normal output of finished products. If the area of the die cutting finished product is large, paste the entire sponge on the knife plate, which will cause the die cutting finished product to jam with the rotation of the knife roller. Therefore, the sponge can be cut into strips and pasted along the circumference, but at the same time, increase the number of electrostatic eliminator groups, especially near the knife roller.
6、 Faults in waste blowing and waste collection
In the production of stickers, the connection die cutting, slitting, waste blowing, waste collection and rewinding are completed at one time. The common fault in waste blowing is that there are bad edges on the paper edges, which makes it difficult to collect and blow waste. Therefore, when the paper is rolled on the machine, carefully check whether there are bad edges on the paper edges. If there are bad edges, remove the paper. In addition, when printing full width, pay special attention to that the paper edge will slightly rub against the gear when just moving the paper or adjusting the edge, or check whether there is a large rubber block at the edge of each paper guide roller, screen roller and rubber roller, which will also cause damage to the paper edge. During waste blowing, the paper edge pull down direction is perpendicular to the paper feeding direction. Waste collection, especially the collection of stickers, is difficult because of the thin face paper. The key to waste collection is typesetting. When typesetting according to the paper direction, pay special attention to avoid sharp edge effect. If there are sharp edges on the printed matter, place the concave part in the front and the convex part in the back. Pay attention to the spacing between labels, which is related to materials and production speed. The spacing in the middle can be 3~4mm less than the width at both ends. If the arrangement is most likely to break, that is, because of the irregular label arrangement, the sharp edge effect and the tension when the intermediate interval is scrapped increase sharply here, it should be considered carefully before typesetting. During waste collection, the waste collection roll and the paper pressing roll sometimes make the waste collection roll into an oval shape, which makes the waste collection tension unstable when the vehicle speed is high, and is easy to cause waste collection broken line. Therefore, it can be corrected by padding paper strips at the low recesses of the waste collection roll.
7、 Ink bar
The axial (horizontal) light or dark streaks on the image surface of printed matter are called starting bars.
There are many reasons for the occurrence of horizontal bar. In addition to the improper adjustment of the equipment, the screening method is also a reason. In addition, the spray powder or paper powder accumulated on the rubber, the poor or damaged printing plate or rubber, the supply of ink or fountain solution and even the quality of ink can cause problems. The state of the machine may also lead to a rising bar, such as poor arrangement and adjustment of the ink roller, improper cylinder liner (too much pressure), etc. The transversely movable ink roller against the printing plate often causes similar horizontal stripes in the printing plate.
Solution:
• Use proper cleaning paste to treat and restore the surface state of the rubber roller every week, and remove the surface sediment;
• Correctly adjust the pressure of the ink roller and regularly check and maintain it;
• Replace the rubber rollers that have become hardened and the surface has been vitrified and worn;
• Carefully adjust the backup ink roller;
• Keep the flushing roll clean and hydrophilic;
• The roller bearing shall be kept in good working condition;
• Properly lubricate the roller bearing;
• Try to lighten the pressure of the plate leaning roller on the plate to reduce the impact;
• Shoulder iron shall be kept clean and free of oil;
• The roller liner paper must be clean, with proper size cutting and correct installation;
• The pressure between the cylinder and the rubber cylinder shall be set between 0.1mm and 0.13mm at most;
• Print with as little printing pressure as possible;
8、 Ghost Shadow
The Yin/Yang pattern of the print itself forms its own Yang/Yin shadow in the circumferential direction, commonly known as ghost, which is a phenomenon that the layout image forms a similar image vacancy or ink volume accumulation on the layout through multiple backup ink rollers. The ink roller becomes hard or is adjusted incorrectly, and the use of low concentration ink may aggravate ghost. On the contrary, a good ink and wash balance and minimizing water consumption can reduce the impact of ghosts.
resolvent:
• When maintaining the ink roll, use a suitable lotion, and use cleaning paste to remove the deposits on the surface of the ink roll every week to restore the ink roll characteristics
• Adjust the ink roller pressure according to the operating specifications and check it regularly
• The top roller surface will become hard and shiny due to dry ink, and wear will be accelerated, so the old ink roller should be replaced in time
• Ink roller string momentum is set at the maximum position
• Make the ink roller of the backup plate can move horizontally
• Adjust the starting time of series operation
• Reduce the use of fountain solution
• Use high concentration ink
• Open the water roller differential ink removal device
• Try to turn around the printing plate (or turn around before printing)
9、 Printed hair flower
The so-called printing patterns and uneven printing patterns mean that the surface of the printed matter is not smooth like clouds. This is due to uneven ink absorption caused by uneven paper surface. If the paper surface has spots, or the uneven coating forms a cloud like surface, then the ink can only be partially absorbed. In four-color printing, this situation will be exposed on various colored rubbers. To determine whether it is a paper problem, light the back of the paper or UV light the front of the paper to observe and judge whether the pattern on the subsequent rubber is consistent with the paper surface. In addition, the ink absorption characteristics of the paper surface can also be tested through the special "scratch test ink".
Solution: If it is determined that the problem is paper, it is better to use another paper with a flat surface as a substitute.
10、 Ink emulsification
When the ink and fountain solution are mixed, the fountain solution is evenly dispersed in the ink with small particles to form emulsion. When the water content in the ink reaches about 20%, it will form a "stable" emulsion state that is most suitable for printing. If the water content is too high or the pH value of fountain solution is too low, it will lead to problems such as decreased ink dispersion ability, reduced liquidity, unstable emulsification, and ink accumulation. Different types of inks will produce different emulsions. Compared with ordinary inks, synthetic inks, metallic inks and fluorescent inks are more sensitive to water content and pH value. Emulsified ink will delay drying time and increase the risk of back printing.
resolvent: