Screen printing, also known as screen printing or screen printing, belongs to the perforated printing. Screen printing plate is a porous screen stencil, directly photopolymer or film made of graphics placed on the screen, printing with a rubber squeegee so that the ink through the screen graphic openwork part of the leakage of printing to the substrate under the net. Screen stencil is silk, nylon, polyester or stainless steel wire, copper wire, nickel wire taut on the screen frame, so that its tension is fixed and made.
1.2.1 screen printing plate preparation
Screen printing begins with the preparation of a screen printing plate. Screen printing pre-printing plate has experienced three stages: manual plate making, photographic plate making and computerized direct plate making. Manual plate, photographic plate for analog plate, computer direct plate for digital plate. The application of digital plate to screen printing provides a good condition to improve the quality of printing, printing high-precision products. Screen printing plate production method has a direct method, indirect method and computer direct plate method three methods. Direct method is in the screen directly coated screen printing photopolymer plate, indirect method is in the screen paste screen printing photopolymer film plate, digital computer direct plate should also be coated in the screen screen printing photopolymer. The difference is that the direct method and indirect method are first made of photographic film with images and text of the negative, and then exposure, development made of printing plates, and digital computer direct plate does not require photographic film, only the first image and text information into the computer, and then the use of computers have been coated with screen-printed photopolymer on the screen plate. The same is the three methods of plate making to use screen printing photopolymer.
(1) Screen printing photopolymer
Screen printing plate with four types of photopolymer: dichromate photopolymer, alcohol soluble nylon photopolymer, diazo photopolymer and styrene-based pyridine salt photopolymer.
① Dichromate photopolymer. Is the earliest use of photopolymer, the use of dichromate photosensitive, and some organic colloids (gelatin, fish glue, gum arabic, polyvinyl alcohol, etc.) mixed to make screen printing photopolymer. However, it can only be used now, the use of short life, dark reaction, low resolution, especially the chromium salt toxicity, not only damage to human health, but also serious pollution of the environment, so it is no longer in use.
② alcohol-soluble nylon photopolymer. More benzoin and its derivatives for photoinitiators, dark reaction, and this type of photopolymer plate can not be removed from the film, the screen can not be reused, and is no longer in use.
③ diazo photopolymer. Polyvinyl alcohol and non-water-soluble polymer emulsion (polyvinyl acetate emulsion, ethylene-propylene emulsion, etc.), the sensitizer for the diazo compounds, commonly used double diazo salt, diazo resin or composite diazo resin, etc.. Due to the high resolution, good resolution, fast sensitivity and abrasion resistance, water resistance and solvent resistance are excellent, and water development, as well as easy to remove the film, etc., has become the most widely used screen printing photopolymer.
④ Styrylpyridine photopolymer. Also called SBQ photopolymer, is a single liquid type screen printing photopolymer. It is the use of styryl pyridine salt photodimerization reaction by ultraviolet irradiation, resulting in a four-membered ring structure insoluble in water dimer, water development to stay in the screen after the formation of graphics.SBQ photopolymer thermal stability, high sensitivity, higher than the heavy nitrogen photopolymer 4 to 5 times, the light-sensitive spectra in the range of 315 ~ 430nm, the largest absorption peak in 370nm; high resolution, image clarity, high quality of the prints; Water development, good development performance; plate film and screen adhesion performance is good, high rate of print resistance; waste plate film is easy to remove the film, is conducive to the recycling and reuse of the screen plate. The main disadvantage is that the price is more expensive, affecting the popularization of the use.
Screen printing photopolymer to make screen printing plate process:
Stretching → coating photopolymer → drying → exposure → development → drying → plate
Will select a certain number of mesh screen (nylon, polyester, stainless steel, copper, nickel wire mesh, etc.) in the wooden frame or aluminum frame taut, with taut mesh adhesive on the frame, so that it has a certain tension. And then coated with photopolymer, in (40 ± 5) ℃ oven drying, generally coated 2 to 3 times. Put on the negative has a graphic, vacuum suction, because the screen printing photopolymer absorbs ultraviolet light about 400nm, the use of plate-making fluorescent lamps or metal halide lamp exposure, water development, drying, plate repair, made of printing plates. After the screen printing plate is used up, it can also be stripped and recycled, and the screen can be reused after the photopolymer is removed with the stripping solution.
(2) Screen printing photographic film
Photographic film, also known as photographic film or film, screen printing photographic adhesive evenly coated in polyester sheet base, after drying and winding storage, that is, open and ready to use, time-saving and fast. The use of screen printing photopolymerized film plate can ensure that the screen printing surface is smooth, and can easily control the thickness of the screen, precision and durability.
Screen printing photographic film plate making process:
Stretching → Pasting photographic film → Drying → Exposure → Developing → Drying → Printing plate
Screen printing computer direct imaging plate (CTS) is with the computer direct plate technology (CTP) in the field of offset printing and flexographic printing success, screen printing has also had a far-reaching impact and the emergence of the CTS is by the computer to the input image and text output directly to the screen printing screen plate, the development process to form a screen printing plate. Currently there are two ways to realize CTS: inkjet direct plate (jet screen) and direct exposure plate (direct light processing, DLP).
① Inkjet direct plate making. The screen will be coated with photopolymer and dry, or paste film, through the inkjet system to block the black ink or wax sprayed on the photopolymer layer, with ultraviolet light exposure, not sprayed ink or wax place to see the light curing, sprayed ink or wax part of the development was removed, the formation of the plate with the text.
② Direct Exposure Platemaking. The same need to coat the screen plate photopolymer or paste film, through the computer-controlled UV light source directly on the screen plate exposure, see the light part of the photopolymer cross-linking curing occurs, not see the light of the part of the development of the development of the liquid is removed, the formation of the plate with the text.
Now CTS is further developed for screen printing plate without coating photopolymer, that is, through the computer directly spraying a kind of coating, after the formation of images on the stencil, after the plate development can be obtained by the screen plate technology. Plate dot angle, spray material thickness, density, exposure time, etc. are set by the computer, according to the original image scanning imaging, direct scanning plate making, and then water to wash out the unreacted chemical materials and made of printing plates. This direct imaging on the stencil plate method is the future direction of screen printing.
In addition, there is a direct imaging plate making method, is in a special metal alloy film directly on the computer laser scanning etching imaging, this plate making method has no deformation of the stencil, alignment quasi-precision, high printing accuracy, its application makes screen printing high quality prints.
Direct digital imaging plate (CTS) has many advantages, more technologically advanced, a step can be made into a printing plate, completely without the use of silver salt film, which avoids the environmental pollution caused by flushing chemicals. Computer-aided design data can be transmitted directly to the template, no intermediate photographic steps, so that the clarity of the image is better, and easier to control. The use of computer direct plate, with plate simple, fast, prevent pollution, improve the quality of plate and printing features, is today’s photopolymer screen printing plate of the latest plate method, its appearance and application to the traditional screen printing plate process has brought about fundamental changes, marking the screen plate technology has entered a new stage of digitalization.
1.2.2 The characteristics of screen printing
The main features of screen printing are as follows.
① Thick ink layer. General offset, letterpress, flexographic ink thickness of only a few microns, gravure only 9 ~ 20μm, while the screen printing ink thickness of up to 30 ~ 100μm, so the ink covering ability is particularly strong.
② can use a variety of ink printing. Screen printing can be used solvent-based, water-based, heat-curing, light-curing, electron-beam curing and other types of inks, printing ink drying, including volatile drying, penetration drying, oxidation polymerization drying, heat-curing drying, light-curing drying, electron-beam drying and so on.
③ The plate is soft, printing pressure is small, can be printed on a variety of substrates. Screen printing plate is soft and flexible, so not only can be printed on paper, film, textiles and other soft materials, but also in the metal, hard plastics, ceramics and other hard materials such as direct printing, because of screen printing printing pressure is very small, so it can also be printed on fragile materials such as glass.
④ There is no limitation on the shape and size of the substrate. Screen printing can be printed on special shaped molded objects (bottles, cups, industrial parts, etc.) and various kinds of flat objects, and can print on oversized advertisements, backdrops, flags and other materials, as well as on ultra-small, ultra-high-precision materials such as thick-film integrated circuits.
The above characteristics of screen printing make it possible to print with screen printing in cases where offset printing, letterpress printing, flexographic printing and gravure printing are difficult. Screen printing can be used for paper, cardboard, plastic, metal, wood, glass, ceramics, fiber, thick film and other materials, regardless of its shape, thickness and size, both manually operated and automated machine printing, so it is widely used in the field of packaging, decorating, advertising, printed circuits, electronic components and so on.
Usually the properties of screen printing ink can be expressed as follows.
① Viscosity. Is to stop the liquid flow of a property, if the liquid flow resistance viscosity is too large, the ink is not easy to pass through the screen, resulting in prints printed ink; viscosity is too low, it will cause the print to expand, affecting the clarity of prints and resolution.
② plasticity. Refers to the deformation of the ink by external forces, but still maintains its deformation before the nature of the ink plasticity is conducive to improving printing accuracy.
③ Thixotropy. Is the ink sol and gel interchangeable phenomenon, manifested in the ink after a certain period of time after thickening, viscosity, the ink stirred after thinning, viscosity and become smaller, but also conducive to improving the printing accuracy.
④ Fluidity (leveling). Refers to the degree to which the ink spreads in all directions under the action of external forces. Mobility is the inverse of viscosity, mobility and ink plasticity and thixotropy have a relationship, plasticity and thixotropy, mobility is small, and vice versa. Mobility is easy to expand the imprint, mobility is easy to appear in the imprint of small knots. The knots of the intertwined nodes of the ink phenomenon is also known as mesh. The mobility of screen printing ink is generally 30-50mm.
⑤ Visco-elasticity. Refers to the ink in the scraper scraping process is sheared off after the rapid rebound performance. Requirements for ink deformation speed, ink rebound quickly in favor of printing.
⑥ dryness. Requirements for ink drying on the screen plate, the slower the better, the ink transferred to the substrate, the faster the better.
⑦ Fineness. Pigment and solid material particle size in 15 ~ 45μm, suitable for printing ink fineness should be 1/4 ~ 1/3 of the mesh opening.
⑧ Drawing. Ink shovel to pick up the ink, silk ink stretched and not the degree of fracture is called pulling the ink silk long, ink in the printing surface appears a lot of filaments, so that the substrate and the printing plate stained (dot expansion, dot clogging hair), resulting in the inability to print. Pulling phenomenon is due to the ink in the linker molecular weight is too large and the viscosity of the ink, sometimes due to the expiration of the ink and produce more pulling phenomenon.
⑨ Color. Color has three attributes, namely, hue, brightness, purity. Hue is the color inherent color phase appearance, a certain wavelength of light waves represent a certain fixed hue, hue is different, the wavelength of its light waves are different, hue is the main difference between color and color. Brightness, also known as brightness, for a series of colors with the same hue, they are different due to the brightness, looks deep and light, the closer to white, the greater the brightness. Purity, also known as saturation, that is, the degree of color close to the standard color, the closer to the standard color, the higher the purity, and vice versa, the lower the purity.
Ink transparency and hiding power. They are a pair of contradictory relationships, the better the covering power of the ink, the worse the transparency, and vice versa, the transparency of the ink is good, the covering power is poor.
Weatherability of ink. Ink printed on the substrate, its color, fastness, etc. in the natural conditions of exposure to the outdoors to remain unchanged nature is called weatherability. The weatherability of the ink mainly depends on the pigment, UV ink due to the presence of photoinitiators, will also affect the weatherability of the ink.
1.2.3 Preparation of UV Screen Printing Inks
The viscosity of screen printing ink is large (1-10 Pa-s), the printing speed is low (5-30 m/min), and the ink layer is thick (10-30 μm), so the formulation should be designed according to the above performance characteristics when preparing UV screen printing ink.
(1) Selection of Oligomer
UV Screen Printing Ink commonly used oligomer for epoxy acrylic resin, fast light-curing rate, good overall performance, inexpensive, but also appropriate to join the polyurethane acrylic resin, to improve the brittleness, improve flexibility and adhesion. Can also use part of the polyester acrylic resin and amino acrylic resin.
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