Troubleshooting of the shallow screen drawing in t

2022-07-27
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Troubleshooting of shallow wire drawing in gravure printing

wire drawing is one of the problems often encountered in the automatic test of the new controller in gravure printing. It involves many factors such as gravure printing machine, plate cylinder, doctor blade, printing environment, ink and so on. Improper handling will affect production and quality

wiredrawing is also called pulling and starting. When the ink performance and ink color rendering degree are different, the performance degree is also different. Generally, the wiredrawing phenomenon of black, blue and magenta is obvious, and the common detection personnel only use it on a large area of shallow

common wire drawing is mainly divided into three types: shallow position wire drawing, on-site color block wire drawing at the starting position of the doctor blade, and irregular wire drawing at the printing blank

I. wire drawing at shallow position

1 Eliminate wire drawing from the source of design and plate making

fully consider the printability in the original design and plate making. We must consider the printing direction and try to avoid printing from shallow to deep; The black and magenta versions are most prone to shallow upper drawing, especially the black version. Because the carbon particles in the ink are thick, when the black is very light, there is almost no carbon particle in one point, so it is more likely to be brushed. Therefore, a large area of black and light should be set as a special gray version as far as possible to reduce the printing difficulty and scrap rate

2. Use the doctor blade correctly to avoid wire drawing

(1) selection of doctor blade. The surface hardness of the chrome plate cylinder is HV850 ~ 1100. The hardness of the doctor blade should be lower than that of the printing plate, about hv600. If the hardness is too low and the ink cannot be scraped clean, too high hardness will accelerate the wear of the printing plate, produce knife marks and affect the print quality

(2) flatness of doctor blade. When installing the doctor blade, the blade should be flattened first, and the screws should be tightened from the middle to both sides in turn. It should not be done in one step. It needs to be repeated twoorthree times to ensure that the force on the blade is uniform

(3) sharpness of doctor blade. The doctor blade can be polished with 600 × water abrasive paper or 2 × metallographic abrasive paper, or the back of the doctor blade can be ground into a bevel of about 25 with a special grinder, even if it is new

(4) doctor blade angle. The angle of the doctor blade shall be smaller than that in normal printing, and the distance between the contact point and the embossing point of the doctor blade and the printing plate shall be shortened

3. Reasonably select and match the ink to prevent wire drawing (1) when there is a large area of shallow or layer transition on the layout, the ink with good quality should be selected to prevent wire drawing caused by too coarse ink pigment particles. Generally, the fineness of the ink is 15 m. too rough ink particles will cause great damage to the doctor blade and printing plate

(2) when mixing ink, on the premise that it has no effect on other patterns on the same printing plate, consider using slow drying solvent, and do not add too many auxiliary solvents such as ethyl acetate, so as to prevent the ink from changing and normal printing

(3) ensure that the ink viscosity is moderate and stable. The production of wire drawing is directly related to the viscosity of the ink. Generally speaking, the higher the viscosity of the ink, the easier it is to produce knife lines in printing. In addition, because there are too many parts of the ink testing machine of different brands, and the amount of solvent added is also different, the specific data provided by the ink manufacturer can be referred to when mixing the ink, and the specific operation can be carried out in combination with their own printing conditions and the printed products

wire drawing at the starting position of the field color block the wire drawing at the starting position of the field color block shows that the lifting of the doctor blade on the edge of the field color block causes a dotted line on the edge of the printing color block, which is generally a few millimeters of short wire. The main reason is that the ink viscosity is too large and there are too large pigment particle crystals in the ink. The solution is as follows

1. Filter the ink and add a certain amount of new ink during use

2. Adjust the angle of the doctor blade smaller and shorten the distance between the contact point between the doctor blade and the printing plate and the impression point

II. Irregular wire drawing at the printing blank

the irregular wire drawing at the printing blank shows irregular ink channels that are sometimes irregular, long and short, and are usually thick. The causes and solutions are as follows

1. Foreign matters are stuck on the blade or under the doctor blade. The doctor blade can be moved back and forth, up and down during the printing process, and the adhered foreign matters can be removed or directly removed on the blade of the doctor blade with a bamboo stick (it must be completed by a skilled operator to avoid unnecessary losses due to improper operation). If necessary, it is necessary to stop the machine to thoroughly clean the doctor blade

2. Insufficient ink circulation. At the dead corner of some inks in the ink tank, due to poor circulation, the resin precipitates, forming crystals and lumps of different sizes, or the surface forms ink flakes due to excessive drying (ink flakes are easy to occur in summer, and there are many ink crystals and lumps due to the influence of temperature in spring and Autumn). With the circulation of inks, some ink crystals or ink flakes are brought to the blade by the rotating plate roller, The doctor blade breaks it to form irregular drawing. Therefore, we must improve the ink circulation and filtration

(1) cycle improvement. Method 1: tilt the ink tank to the left and right to ensure that the ink can be injected from the higher end and flow out from the lower end to form a good circulation; Method 2: place the ink tank horizontally, but the ink must be injected from both ends at the same time and refluxed from the middle, which can also form a good circulation. It is not allowed to inject ink from one side

(2) filtration improvement. The ink must be filtered for the first time before use. Even the new ink must be filtered. Generally, 140 mesh/inch filtering is used. The filtering work of ink circulation in the ink tank is more important. It can well filter out the resin crystals and external impurities formed during the use of ink. If the samples between the two clamps can maintain the same elongation and assume that they are 500% impurities, the purpose of selection should be finer. The method is to put a silk stocking under the ink outlet bucket to form a filter bucket. After filtering for a period of time, if the filtering is blocked and the ink flow is not smooth, Small holes can be inserted in the upper part of the sleeve bucket with a bamboo stick to increase the ink guiding effect. The lower part of the sleeve bucket can also play a good role in depositing impurities, as shown in Figure 3. Some large printing plants in Shanghai and Guangdong have used this method to control the problem of intermittent ink drawing

3. Ink fatigue. This situation usually occurs when the inking area of the printing plate is relatively small. During the printing process, the resin in the ink is continuously consumed. If the solvent is continuously added without adding new ink, the resin cannot be supplemented, which changes the proportion of each component of the ink, resulting in ink fatigue and affecting normal use. Adding new ink properly can solve the problem

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