How to make a flexible printing plate

2022-09-20
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How to make a flexible printing plate

printing plate is an indispensable and important factor of printing technology at present. In the printing process, on the one hand, the receiving ink roller should adjust the length of the ejector rod and fix it; (2) The ink transmitted by the improper installation position of the reversing switch, on the other hand, transmits the graphic ink to the surface of the substrate or other intermediate printing carriers. The quality of the printing plate will directly determine the quality of the printed matter. The printing plate used in flexographic printing is higher than the non graphic part and in the same plane, which belongs to the category of letterpress. Therefore, the quality of the printing plate should not only meet the quality requirements of the metal relief plate, such as the top and end faces of the image and text should be flat and consistent, and the edges and sides of the image and text should have a certain slope and relief depth; Moreover, due to the low printing pressure, in order to achieve the full tightness between the printing plate and the substrate and make the ink transfer well, the printing plate should also have appropriate softness, so these factors must be taken into account when making the printing plate. So, how to make a flexible printing plate? I think we should do the following work to make a good flexographic plate

I. quality control of plate making negatives

the negatives of current flexo plate making, like those of offset lithography, are made through the color desktop publishing system. However, flexo has its own characteristics, and the negative must meet these characteristics before it can be used in plate making

1. requirements for film density

the plate making negative of the flexo is a positive negative image, and the density value of the opaque non graphic part is higher, which should generally not be less than

4.0. This is because the main exposure time of the flexo is longer, and if the density value of the non graphic part is less than 4.0, it will cause light leakage and make the image relief depth lighter. The transparent image and text part should be very transparent, and the haze value should not be greater than 0.05, otherwise the light will be blocked during exposure, so that the end face of the image and text will be uneven due to insufficient light and insufficient crosslinking and hardening of the photosensitive resin

2. Requirements for negative pictures and texts

① the text and text should avoid using small words and thin lines, otherwise the foundation is narrow due to insufficient light during exposure, so that the printing plate cannot withstand the brushing of the plate brush during the development and corrosion process, resulting in broken pens, missing strokes or lines, and incomplete lines

② there should be no bars or field patterns parallel to the axial direction on the negative film, otherwise it will cause uneven ground in the circumferential direction, and the machine will jump due to the circumferential height when running

and cause stripe defects on the product

③ large area of field and small characters and dots should not be designed in the same page. Because the ink layer is required to be thick on the spot during printing, the printing pressure must be increased to achieve. In order to prevent small characters and dots from expanding and deforming, or even pasting, a small printing pressure is used. If the field is designed in the same layout with small characters and dots, one will often lose the other when printing. Taking care of the field, we must make small characters and dots expand, deform or paste up; Taking care of the small characters and dots, the field is under insufficient pressure, and it is empty and blooming

3. Requirements for film grain level

① grain level requirements

the printing plate characteristics of flexographic printing belong to relief printing, so the image grain level it copies is not as rich as that of lithographic printing process with the increase of functions, because the image grain level of flexographic printing plate cannot be avoided in the high-profile part, and it must be supported by small dots. The dot area should not be less than 5% to make the dot stand in the printing. Secondly, the flexographic version is relatively soft and will deform and expand after being pressed, so the dot area of the darkened part should not be greater than 85%, and it should not be on the ground, otherwise the darkened part will be pasted in printing. It can be seen that the grain level of flexographic printing is limited by the characteristics of the printing plate, and can only be limited to the range of 5% - 85%. For example, if the graphic level of the original is very rich, and the gradation level is 0-95%, high temperature resistant and anti-aging materials are considered to be the most promising technology. They must be properly compressed before they can be applied to plate making during prepress color separation. At the same time, the hardness of the printing plate should be controlled at 70 ° shore

② number of lines

flexographic printing uses metal roller to transfer ink to the printing plate. Because the size and depth of the inking hole of the metal roller are always

unchanged, it can ensure that the ink transfer to the printing plate is quantitative and uniform. In order to ensure that ink can be obtained at every point on the flexographic plate, the relationship between the number of lines on the plate and the number of lines on the metal roller must be considered. Practice has proved that when the number of lines on the printing plate is 1/3 of the number of lines on the metal roller, it can ensure that every point on the printing plate can get ink. In order to better get ink at all points of the plate pattern, it is recommended to use the number of plate lines: the number of metal lines is 1:4

③ striation angle

the striation angle of the metal striation roller of the flexible version is generally 45 ° (60 ° is also used). In order to avoid the moire caused by the overlap of the groove angle of the metal groove

roller and the groove angle of the plate pattern, the groove angle of the plate making negative should avoid the 45 ° angle. The pattern angle of the negative film can be yellow 90 °, magenta 15 °, green 60 ° and black 30 °. Or use yellow 82.5 °, magenta 67.5 °, cyan 97.5 ° and black 37.5 °

④ the shape of points and the number of lines added

at present, most printing processes use printing pressure to transfer the graphic ink on the printing plate to the surface of the substrate

because of pressure, point expansion is inevitable. As long as the expansion value of the point is within the effective control range, it can be considered as normal printing. The enlargement value of a point is often related to the shape of the point and the number of lines added:

the enlargement value of a point is directly proportional to the perimeter of the edge of the point. Because the dot expands to all sides during printing, under the same condition

, the edge perimeter of the square dot is the largest, so the dot expansion value is also the largest; The perimeter of the edge of a circular point is the smallest, so its point expansion value is also the smallest. Flexible printing plate is relatively soft, and the printing time point expansion value is large. Using circular dots is the most ideal choice

The enlargement value of

points is inversely proportional to the size of the point area. In the unit area, the higher the number of lines added, the smaller the area of points,

the more the number of points, the larger the total perimeter of the point edge, and the larger the expansion value of points. On the contrary, if the number of added lines is low, the area of the point is large, and the number of points is small, so the total perimeter of the point edge is also small, and the expansion value of the point is also small. It can be seen that when the number of lines of the flexible version is less than 133 lines/inch, it is more advantageous

II. Requirements for plate making operation

1. exposure control

① determine the exposure

different batches of flexible photosensitive plates. Due to different production dates and storage times, the raw and auxiliary materials used are also different

and the exposure is also different. In order to correctly master the exposure of flexible photosensitive plates, exposure tests should be done before use to obtain the correct exposure parameters. The test can be carried out on a small piece of flexible photosensitive plate, which is equally divided into several pieces, and the exposure time of each piece increases proportionally. After the small sample is exposed, developed and corroded, compare the point level, size and relief depth of the negative and layout on the sample viewing platform to determine the ideal exposure parameters

② control of exposure time of photosensitive plate

a back exposure. The main function of back exposure is to increase the thickness of the plate base, determine the depth of graphic relief, and improve the printing resistance. The exposure time of back

exposure is generally controlled at 1-2 minutes. If the exposure time is too long, the printing plate will be too thick, the relief of graphics and texts is too shallow, and the non graphics and texts are easy to be printed with ink; On the contrary, the exposure time is too short, the relief of graphics and texts is too deep, and the support for words, dots and thin lines is not enough, which is easy to be washed away when the printing plate is corroded, resulting in incomplete graphics and texts

b main exposure, also known as front exposure. Its function is to form pictures and texts in the layout. The main exposure time can be appropriately longer, generally

is controlled at 5-8 minutes. Because the negative film is negative, it is overexposed, resulting in the thickening and enlargement of words, lines and lines, and the decline in the clarity of graphics and texts, or even paste. If the exposure time is too short, the text, lines and dots become smaller and thinner, and the hardening degree of the photosensitive layer is not enough, it is easy to cause the small characters and dots to be lost and the lines to be incomplete and bent during the development corrosion of the printing plate

Exposure after

c. The function of post exposure is to completely crosslink and harden the polymer resin without crosslinking and hardening inside the printing plate, so that the printing plate can reach the due hardness, and improve the corrosion resistance and wear resistance of the plate to the ink solvent. The exposure time of post exposure is generally about 10 minutes. With less exposure time, the printing cannot meet the purpose of thick exposure, and excessive exposure time makes the printing plate hard

③ light source

the plate making light source of flexo printing plate generally adopts 365nm ultraviolet light. Because the new and old lamps of the light source are different, and the luminous intensity

is different, the new lamps that have just been replaced usually have strong luminous intensity, which can reduce the exposure time a little during exposure; After the lamp is used for a period of time, the luminous intensity is weakened, and the exposure time for plate making can be appropriately extended

2. Operation precautions in the plate making process

① the surface of unexposed and sun dried flexo has a certain viscosity. After it is closely connected with smooth negative film, it is difficult to vacuum the air between them to form spacing bubbles, resulting in deformation, thickening or blurring of graphics and texts after plate making. The ideal surface of flexographic plate making negative should be frosted, which is conducive to the export of air after being close to the flexographic plate. This requires that special frosted polyester film base photosensitive materials should be selected to make negatives. If a smooth offset plate making negative is used, a layer of fine light powder can be applied on the surface of the negative to solve the problem of poor air conduction, but the thinner the powder is applied, the better, otherwise the fog of the negative will be increased

② as the unexposed flexo is soft, it's best not to put pressure on the layout before exposure. Even if the vacuum film is to be flattened,

don't press it by hand. Instead, roll it gently and then cure it in the furnace through the pouring process, otherwise it will leave indentation defects

③ when removing the protective film before the exposure of the plate, it should be done in one breath, and do not stop for a while in the middle, otherwise a trace will be left on the

plate, causing the ink amount at this place to be greater than that at other places during printing, resulting in ink bar marks

④ for back exposure of flexographic plates, it is carried out in semi vacuum, and for front exposure, it is in full vacuum

at the same time, it is also necessary to pay attention to whether there are residual bubbles between the negative and the plate, and if so, eliminate them

⑤ when developing corrosion, the pressure of the developing brush, the concentration of the developing solution and the speed of plate washing should be well controlled. Generally, the

pressure of the developing plate brush depends on the distance between it and the plate table. If the distance is small, the plate brush has a high pressure on the printing plate. Although the developing corrosion time can be shortened, due to the high pressure, the small characters, thin lines and dots of the printing plate are easy to be brushed off; Large distance leads to small pressure, long development corrosion time, and the printing plate expands due to the adsorption of too much developer, resulting in the bending and deformation of text lines and the loss of small dots. The concentration of the developer should be controlled. Generally, some new solvents should be added appropriately every time the developer is corroded, so as to ensure that the solvent concentration of the developer is controlled within the effective range. If it is not replenished in time or the replenishment amount is not enough, there will be too many photosensitive resin monomers left in the layout, which will become sticky and the surface smoothness will become poor. The development corrosion should be washed at an appropriate speed, otherwise the development and washing speed is too fast, and there are too many non photosensitive polymer resin monomers left in the layout, which will affect the relief depth of the pictures and texts after post exposure; If the plate washing speed is too slow, it is easy to make the printing plate absorb solvent expansion,

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