Screen printing of fine mesh (5)
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●Ink
Regarding inks, especially monochrome screen printing or three-color, four-color screen printing inks, I think solvent-based inks will soon become obsolete. In short, solvent-based inks are difficult to use for fine mesh printing. So, I will only talk about the UV ink system.
Regarding the physical and chemical properties of the ink, the most important thing is still the thixotropy of the ink (the thixotropy will be described later). Ink with poor thixotropy is easily diffused (resulting in darker images, larger dots, and smaller contrast). The thixotropy ink keeps the dots in shape, but in any case, these are caused by the ink flowing through the screen.
The UV ink is actually not dried on the screen, so the fine dots on the screen can be completely printed. In addition, we have to consider the role of the screen, the film and the resolution of the photoresist. However, it is much less troublesome and wasteful than using solvent-based inks.
On the other hand, the quality of the image is related to the thixotropy of the ink (not directly related to UV itself) in maintaining the shape-related aspects and surface of the negative dot.
We must know that the thickness of the ink layer of the thixotropy UV ink is 2~3μm thicker than the UV ink with poor thixotropy. Although the latter does not contain an organic solvent, the ink diffuses slightly under the mesh due to the capillary action in printing and the momentary gravity of the printing and passing through the UV lamp. Therefore, in order to obtain better image quality and as thin an ink layer as possible, it is required that the ink manufacturer must determine the amount of the thixotropic agent to be added according to the type of screen used for the mesh to be printed. It is important to know that UV ink systems (UV, water-based UV, fully water-based or conventional solvent-based) are important, depending on the following aspects, otherwise problems can arise, especially fine mesh printing.
★ thixotropic - other rheology, etc.;
★ color quality;
★ the concentration of the colorant, which determines the density of the printing color;
★ printing theme or printing target;
★ images to be copied;
★ type of printing material;
★Printer type;
★ printing speed;
★ print use;
★ Relative transparency;
★ impurity;
★ sun protection;
★ resistance to physical and chemical properties (external solvent);
★ color map (cold or hot);
★ If UV ink is not used, the ink uses a type of slow drying agent.
Especially the screen printing, choose the ink system. It is decisive, and it is important to control this variable. But sometimes, due to restrictions on the investment, habits, purpose and type of printing of existing equipment, the ink is often not selected, and the existing ones are often used. Obviously, printing UV screens with UV inks or water-based UV inks is much easier, but it doesn't mean you can't print without them. Before UV ink appeared on the market, as early as 1968, I printed 150 and 175 lines/inch. Of course, it was very difficult because the fine mesh was dried on the screen and some corrosive solvents were applied to the stencil. The sheet and the photoresist layer cause damage.
UV inks are not dried on screens. There are some benefits to using some water-based inks or "self-solvent" solvent-based inks, but regardless of the ink system, the most important variable (or key factor) that the ink can control is the ink. The thixotropy, which is a very important physical property of the ink, enables the semi-fluid ink to completely become in a liquid state when it is agitated, and when the agitation is stopped, it can be maintained in a stable state again. The ink should not flow or spread after passing through the screen. Further, once the ink is printed on the surface of the printing material, during the drying (solvent evaporation), the dots can maintain the original shape, and the hue value does not increase, especially It is the negative dot of the dark area of the image.
The main chemical properties of thixotropic agents, in simple terms, are to cut the long molecular chains of the ink, preventing their expansion, while also preventing light reflection on the dry ink film. Because of this, it is very difficult to use inks with both thixotropic properties and good gloss. Therefore, if the customer asks us to do glossy four-color screen printing, we always use matte ink and then a layer of varnish. Of course, we have to explain to customers the need to do so. If you print a rougher mesh of 75 lines/inch or less, this is not so strict.
Of course, the ink also has other properties (or physical properties) such as viscosity, hair suction (the ability to flow through the mesh at a certain rate), solvent, water or monomer content, solid component (nutrient) particle size, In particular, the content of fluids (dyes), the separation tendency of different additives such as pigments, binders (main components), solvents or water, the rapid ability to evaporate (dry) and polymerize (some solvents or water-based inks, Especially using UV ink).
In order to obtain better results in the screen printing, some controllable parameters of the ink should be controlled. For example, the viscosity of the ink should be controlled according to the speed of printing and drying (hereinafter, the printing machine and the drying equipment).
The capillary adsorption of the ink on the screen should be as low as possible to avoid sticking the ink to the screen, even if it is local.
The content of solvent, water or monomer in the ink should also be controlled. If the ink is too thick, it is difficult to pass through the screen; if it is too thin, it is easy to flow, the edges of the image are not neat, the definition is not good, and the dark area is cloudy. The dot points will be lost, resulting in a lower overall density
The particle size of the solid primary fraction, especially the color of the particles, should also be well controlled. The particles of the toner are too large, and it is possible to block the mesh when the mesh is opened, which changes the color density (lightening) of the printed image. In UV ink printing, especially if the screen used is one-sided (on the side of the substrate) flattened. Because the ink may stay in the mesh.
The tendency of the ink to dissociate should also be well controlled. It also changes the density and chromaticity of the image and causes the ink to adhere poorly to the substrate during the printing process.
Other variable factors that need to be controlled, such as color quality, should be controlled in two ways:
The first is the quality (purity) of the colorant. The other color components in the main color material should be as small as possible, and should be as transparent as possible to reduce the problem of "primary color".
Second, the density of the basic colors is adjusted according to the copied image.
These densities are not predetermined and should be adjusted between 0.95 and 1.35 depending on the image to be copied (if printing some POP/POS light boxes or posters at bus stops, even higher).
Controlling these variables is not just a technical issue, it is more of an artistic perception problem, and it is largely an uncontrollable variable, because the perception of the image cannot be defined in scientific terms.
Also, make sure the ink color is appropriate for the material you want to print. Because the absorbency of the printing material will affect the color sequence of the printing and the color after printing (the material with strong hygroscopicity needs to reduce the density of the ink, especially the first printed color, that is, the main color).
The type of printing press usually determines the printing speed (for example, the speed of the roller printing machine is faster than that of the flat printing machine, and the mesh distance is smaller), which also has a certain influence on the change of the chromaticity.
The use of printed matter is also a factor that requires control of density. It is obvious that the backlit print requires a higher density of the dominant color, and at the same time, the coloration is adjusted according to the "color temperature" of the light source. If there is an intermediate material between the printed material and the light source (usually diffusing the light), then the color of the material and the intermediate material is also printed at a high speed.
Other problems associated with the use of the print, such as magnification (large size), give the impression that the space between the enlarged dots is larger than the space on the original small-sized original, so that the final image appears lighter. This means that it is necessary to increase the density of the film, especially the density of the highlight area, and to change the density of the printed color.
The relative transparency of the basic colors should also be well controlled. If the transparency of one or several basic colors is not sufficient, the printer has to print an opaque color (or opaque ink) on the surface of the substrate. But the disadvantage of doing this is that it has an effect on the main color, which will affect the visual quality of the image.
Regarding noise, except for yellow, no basic color is absolutely pure. A completely pure basic color must be able to fully absorb its complementary color, and when the human eye receives it, it will show its own color. Unfortunately, cyan contains a small amount of yellow and magenta, while magenta contains a small amount of cyan and more (sometimes up to 60%) yellow. It is important to hire a pure magenta, even if it is the best choice, it should be corrected in the color separation stage.
The fastness of color (for ultraviolet radiation in sunlight) is another variable that should be controlled. If prints are required (such as long-term outdoor advertising, etc.) to have this feature, this is related to the problem of choosing the right product. The light resistance of inks is usually divided into 1 (not light) to 8 (good light resistance). If the ink is required to be exposed to sunlight for more than two weeks, the ink with 8 levels of light resistance should be selected.
The sunscreen properties of the ink have nothing to do with the oiliness, wateriness or UV ink of the ink. It is only related to the quality of the ink pigment.
In contrast, the physical (mechanical) and chemical strength (drying or polymerization) of an ink is absolutely related to its chemical composition. Due to the cross-linking polymerization of UV systems, UV inks have good mechanical strength and corrosion resistance, and are resistant to rain, snow, sand, dust, pollution, and chemical agents (solvents or gases). Choosing UV inks is a good way to control this variable.
What kind of slow drying agent should I use if I don't use UV ink?
Most use of indulgent-type inks, and sometimes water-based inks, must overcome the tendency of the ink to dry on the screen. The key factor of control is the speed of solvent or water.
The evaporation rate of diethyl ether was set to grade 1 (evaporation in 1 s). For most solvents, a good slow-drying agent was benzyl alcohol (1800 times slower than ether evaporation).
For water-based inks, the addition of 5% ethylene glycol (or butyl acetate) slows the dissociation between the ink and the screen due to the movement of the doctor blade.
●How to control each variable
The way to control these variables is to print prints and substrates for a variety of uses using only one matt UV ink. For some materials that are difficult to print, a hardener can be added to the ink.
For other materials that are difficult to print, such as glass, the method is to modify the design of the UV reactor (dryer) so that the ink can polymerize above or below the glass and reach deep polymerization, which is relatively firmly attached. On the glass.
Ink on ceramic printing can now be found on the market, some are a mixture of UV ink and enamel, which has the advantages of UV ink, such as not drying on the screen.
Just pay attention to the problem of "overlap", which may cause cracks on the ink layer. With the UCR/GCR system, ink overlap under black and dark adjustments can be limited to eliminate this problem.
●The influence of printing materials on the printing of the mesh
Regardless of whether the mesh is thicker or thinner, in most cases, the influence of the printing materials is relatively large. Unfortunately, we have to face the reality that printing materials are often a factor beyond the control of screen printing workers. .
The choice of substrate is often based on the intended use of the print and the specific needs of the customer. Screen printers often only choose between one or two. Only the printer can explain and make reasonable choices for the choice, the customer is willing to accept the choice of screen printing personnel, for example, our company explains to customers why we use PVC foam printed with a relatively smooth surface, that is, the surface is smooth Ability to print fine meshes.
However, in addition to these choices, there are many variable parameters that have a significant impact on the printing effect:
(1) The surface condition of the printing material (thicker or thinner);
(2) physical properties of paper and plastic materials, including textiles;
(3) Its flexibility or rigidity determines the choice of printing machine type (platform or roller);
(4) size (size);
(5) chemical properties that determine the choice of ink;
(6) Stability of dimensions.
Controllable variables of the substrate - surface condition is an important parameter related to the shape and quality of the printed dots.
Printing materials with micropores on the surface, such as coated paper, optical discs, etc., will print better than the printing materials of the coarse holes. Like rougher paper, non-woven fabrics and some stretchable textiles and plastics, the first printed ink will “eat” some of the aesthetic fibers and some openings, so that this color will become ours. Said the main color. In other words, it will stand out in the entire print pattern.
To control this problem means to reduce the density of the first color, or the density of the color itself, or both, by correcting the template, on the other hand, reselecting the printing order of the color. The first printed color is selected based on its importance in the final printed image, for example, to print a bright mesh on a more hygroscopic paper, preferably first magenta or pink. But if it is a natural scenery, it is better to print black first.
It should be emphasized that the flatness, stability and thickness of the substrate are also important.
Screen printing workers should stand on the customer's side to decide whether a certain material can be used or not and tell the customer as much as possible.
The smoothness of the material and the flatness of the surface are inconsistent. How can we ensure that the quality of the printed image of the mesh is always consistent?
Unfortunately, production tolerances such as cardboard, hard corrugated board, or thick plastic materials are often close to ±7 (especially for some products that are easy to stretch, the actual stretch makes the thickness difficult to control). In contrast, screen printing workers cannot fully control these variables, which is an almost uncontrollable variable. However, when customers require four-color screen printing on these substrates, you must pay special attention to ensure that the required print quality is provided to the other party. This is absolutely necessary.
This problem is not so simple, because, especially in fine mesh printing, the screen and template must be adapted to the material to be printed, which means that many factors are also considered, such as those commonly used in screen printing. Materials such as cotton paper, various types of plastics, stretchable products, and textiles.
Therefore, printing a general image on any material uses a direct photosensitive plate.
As mentioned earlier, in most cases the substrate is fixed or specified by the customer, not by the screen printer. In this case, the screen printing worker can only present his opinions, suggestions or inform the customers who provide the materials that the materials are harmful to the quality of the screen printing.
Screen printing workers should explain to customers why this material is more suitable than the other, so we say that choosing a certain substrate for screen printing is a half controllable factor.
In most cases, involving a myriad of physical and chemical variables, you should explain to the customer as clearly as possible, for example, why choose polystyrene instead of PVC, why is the pH neutral paper better than Paper with a higher pH, or why a cotton material is better than another (textile), etc., because it can send and improve print quality, thus delivering the final image quality.
Of course, the surface of the rougher porous material can also get good print quality, but the screen printer must adjust the screen exposure intensity, the choice of screen making technology, the density and viscosity of the ink, and the printing parameters, such as the scraper. Factors such as pressure and printing speed are also adjusted, which are naturally related to the physical properties of the substrate - surface properties, including the flexibility or rigidity of the material. For example, with regard to the choice of printing press, if the substrate has sufficient flexibility, it can be printed on a cylinder press, which is better for screen printing. The 0.7 mm thick material can be printed on a small roller press and the thickness of 1.5 mm can be printed on a large cylinder press. For harder printed materials, a platform press should be chosen.
In short, the use of automatic platform printing presses, the thickness of the material must be absolutely considered, according to the maximum opening of the paper cutter. In other words, the nature of the surface of the substrate is also adjusted depending on whether a platform printer or a cylinder printer is used.
The size of the substrate is also a physical variable that affects the type of press used, the screen, the plate making method, and the choice of ink. For example, to print a customized bus stop sign (120cm × 176cm), or a large-format map (120cm × 160cm), I usually use a screening line of 110~150 lines/inch (44~60 lines/cm). In order to get the quality that customers expect.
This means that when printing large-format screens, it must be done like other small-sized screens.
Especially when using solvent inks or aqueous inks, it is necessary to accurately select other important variables related to the chemistry of the substrate.
This simple fact indicates that some plastic materials cannot be printed using water-based inks or solvent inks. To carefully and accurately select the type of ink, it is necessary to select a chemically compatible ink for the substrate while at the same time Consider solutions for other parameters such as thixotropic.
As far as I am concerned, UV inks are often used to solve this problem. I can use a UV ink for printing various substrates. This means that the variables that need to be controlled are greatly reduced.
The dimensional stability of the substrate should also be tested and well controlled. In addition, in four-color screen printing, accurate positioning is important regardless of whether it is fine-tuned or not.
If it is possible to choose between two substrates with similar physical and chemical properties, the most stable one should be chosen.