Interpreting the Operational Skills of the Flat Color Screen Printing Process

The color sequence on the machine is magenta, cyan, yellow and black. The reason why magenta is placed in the first color sequence is because the layout of the magenta version layout is relatively large, which facilitates the overprint of the later colors; the black version is shorter and is suitable for the last. Printing inks generally use magenta, sky blue, medium yellow, and black four-color inks. Like offset inks, if large color shifts occur, red, blue, lemon yellow, or transparent yellow may be used instead.

Network distance is an important factor influencing the screen printing process and quality. The screen distance of plane color screen printing is mostly 5mm. The net distance is too bad to overprint, and it is easy to break the plate; the net distance is too small to stick to the net. Printing pressure is divided into the horizontal direction of the ink shear force and the vertical downward pressure, the latter is to ensure that the screen and the substrate contact, the ink is a good prerequisite for transfer. The mastering of the actual printing pressure is to ensure that the tone is basically complete, and that the key position is faithfully restored to the principle of color.

At present, China's flat color screen printing is still in the exploratory stage. There is no corresponding standard data management system in color reproduction management.

Screen printing is known as "universal printing". As the application of thermal transfer printing and water transfer label printing is becoming more and more widespread, color screen printing has once again become a focus of attention. Compared with general screen printing, flat color screen printing has higher process requirements for materials and equipment, because flat color screen printing products mostly contain halftone images and require multi-color overprinting. Here only the heat transfer decal paper screen printing as an example, specifically discusses the process of overprinting and color reproduction of flat color screen printing.

Screen Printing Basics - Plate Making

1. Selection of plate-making film

To get rich screen printing, we must first make films like offset printing plates, and use 100-133 lines/inch outlets. Too high lines are not conducive to the reproduction of low-key levels. If a circular dot is selected, since the screen printing dot reproduction curve is reduced below 20%, 70% of the dot dots have an increase phenomenon, and the tone adjustment of the image must be performed before the film is taken out. The general screening angle of the screen is: 15° for the magenta version, 75° for the cyan version, 90° for the yellow plate, and 45° for the black plate. Practice has proved that rare overprinting occurs. In addition, in order to ensure the accuracy of the post-printing, even if the linework screen printing, sulfuric acid paper negative film is not used as much as possible. If you do not have a pre-press production department, do not commission advertising banner production companies to make the printing film, because they have a lower number of film lines and layers, which will affect the print definition.

2. The choice of screen

The high-mesh yellow plain-woven polyester mesh is a necessary condition for high-precision screen printing. The yellow color is to reduce the diffuse reflection of the exposure light during the printing; the plain weave mesh can form a thinner screen, which facilitates the reproduction of high-level, low-key levels. The same number of mesh screens are available in a variety of wire diameters, and should not be easily broken. The ratio of the number of screen meshes to the number of film screening lines is 3.5:1, and if the number of screen lines is 120 lines/inch, the 420 mesh screen should be selected.

3. Stretching

Large-format mechanical stretching machine can be used to obtain greater tension, to prevent tension during the printing process, screen deformation; the same set of screen tension should be consistent; oblique stretch net, net cloth latitude and longitude and the network frame was 22.5 ° folder Corners can help to reduce the network defects and make fine text and lines perfect. When stretching the net, the initial tension should be larger. After every 1 hour, pull the net once. After 12 hours, measure the tension of the screen with a tensiometer. The difference between the tension of each part should be no more than 1N/cm (the tension after the sticking is 24~25N after printing. /cm). In addition, an aluminum alloy or steel mesh frame should be selected, and the wooden frame is easily deformed after being immersed in water.

4. Screen production

Take the direct legal system as an example. After the screen has been pretreated, a high resolution sensitizer is used, the squeegee is applied twice, the printed surface is coated once, the printed side is laid flat (should be in a dust-free environment); subsequently, at a suitable temperature ( The drying is generally completed within 35-45°C), the measured tension is not less than 18 N/cm; the thickness of the photosensitive paste layer is controlled at 10-18μm, the thickness of the printing surface emulsion layer is approximately 10% of the screen thickness, and the Rz value (rough surface) Degree) is less than the thickness of the emulsion layer. During the coating process, it is very necessary to control the thickness of the screen. When the screen is too thick, the dark tone cannot be separated and the highlight cannot be printed. If the screen is rough and uneven, the screen will increase abnormally and the tone will change. These data can be controlled through instrument measurements.

The production quality of screen printing is the basis for accurate overprinting, stable reproduction of network points and good reproduction of tone levels. If the customer provides offset image files, offset proofs or inkjet proofs, the color print reproduction of screen printing will depend on the operator's knowledge of screen printing. Before printing, it is necessary to check the attachment of the emulsion surface and the screen printing surface of the positive image film, and through the rotation angle, the film and the screen have no moire, and then the tape is fixed with the tape.

Sophisticated equipment - printing

Sophisticated equipment guarantees us good print quality. To ensure accurate overprinting, high-precision screen printers should be selected, and screen lifts and screen printers should be preferred. The printing format should not be too large. Comprehensive consideration should be given to A3 format. The printing material can be selected from transfer paper or low-stretch transfer film, which is pre-shrinked with hot air before printing. Printed materials that have not been printed in one day are wrapped with plastic film to prevent moisture. The hardness of the squeegee is chosen to be HS80-90. Under the premise of not affecting the color tone, the hard squeegee is used as far as possible to form a thin ink layer, which is conducive to reproduction of the tone level.

Source: China Screen Printing Forum

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