Fine mesh screen printing (1)

●Ink

Regarding inks, especially monochromatic halftone printing or three-color and four-color halftone printing inks, I think that solvent-based inks will soon become obsolete. In short, solvent-based inks are difficult to use for fine mesh printing. So, I only talk about the UV ink system.

With regard to the physical and chemical properties of the ink, the most important one is still the thixotropy of the ink (thixotropy will be described later). Thixotropic inks tend to diffuse (resulting in darker images, larger dots, and less contrast). Thixotropic inks can keep dots in their shape, but in any case, these are caused by the ink flowing through the screen.

UV inks actually do not dry on the screen, so the fine dots on the screen can be printed completely. In addition, the factors that we must consider are the effect of the screen, the resolution of the film and the photosensitive adhesive. However, it is much less troublesome and wasteful than using solvent inks.

On the other hand, the quality of the image is related to the thixotropy of the ink (which is not directly related to the UV itself) in terms of maintaining the aspect and the surface of the negative dot pattern.

We must know that the ink thickness of the thixotropic UV ink is 2~3μm thicker than the thixotropic UV ink. Although the latter does not contain organic solvents, due to capillary action in printing and a momentary gravitational effect under printing and passing through UV lamps, the ink spreads slightly under the mesh. Therefore, in order to obtain better image quality and the ink layer as thin as possible, it is required that the ink manufacturer must decide the amount of thixotropic agent to be added according to the screen type to be used and the screen type to be used. It is important to know that choosing a UV ink system (UV, water-based UV, fully water-based or traditional solvent-based) according to the following aspects may cause problems, especially fine-mesh printing.

★ Thixotropy - other rheological properties;
★ Color quality
★ The concentration of pigment, which determines the density of the printing color;
★ printing theme or printing target;
★ images to be copied;
★ printing material type;
★ type of printing machine;
★ printing speed;
★ print use;
★ Relative transparency;
★ impurities;
★ Sunscreen;
★ resistance to physical and chemical properties (external solvents);
★ Color map (cold or hot);
★ If you do not use UV ink, the type of slow drying agent is used for the ink.

Especially for screen printing, select the ink system. It is decisive that it is important to control this variable factor. But sometimes, due to limitations on the investment, habits, purpose, and type of printing of existing equipment, inks are often not chosen, and the most frequently used ones are the so-called “best use”. Obviously, using UV inks or water-based UV inks to print halftones is much easier, but it does not mean that you can't print without them. Before the appearance of UV inks in the market, I had printed 150 and 175 lines/inch as early as in 1968. Of course, it was very difficult because the fine dots were fixed on the screen, and some corrosive solvents were added to the stencil (membrane). The film and the photosensitive layer cause damage.

UV inks do not dry on the screen, and there are some advantages to using certain water-based inks or "auto-solubilized" solvent-based inks, but regardless of the ink system, the most important variable (or key factor) that the ink can control is the ink. Thixotropy, which is a very important physical property of the ink, enables the semi-fluid ink to completely become liquid when it is agitated, and when the stirring is stopped, it can be maintained in a stable state again. To make the ink flow through the screen, neither flow nor diffusion, and then, once the ink is printed on the surface of the printing material, during the dry solid (solvent evaporation), the network can maintain its original shape, the tone value does not increase, especially It is the negative dot of the image dark area.

The main chemical properties of thixotropic agents, simply put, is to cut off long molecular chains of the ink, prevent their expansion, and at the same time prevent light reflection on the dry ink film. Because of this, it is very difficult to use both thixotropy and good gloss of the ink. Therefore, if the customer asks us to do a glossy four-color screen printing, we always use matte ink and then a layer of varnish. Of course, we must explain to the customer the necessity of doing so. If you print a rougher mesh tone below 75 lines/inch, this is less strict.

Of course, the ink also has other properties (or physical properties), such as viscosity, hair-absorption (ability to flow through the mesh at a certain speed), solvent, water or monomer content, solid component (nutrient) particle size, In particular, the content of fluids (dye), the separation tendency of different additives such as pigments, binders (main points), solvents or water, evaporation (drying) and the rapidity of polymerization (some solvents or water-based inks, Especially using UV inks).

In order to obtain better results in halftone 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 (to be followed by printing presses and drying equipment).

The capillary adsorption of the ink on the screen should be as low as possible to prevent the ink from sticking to the screen even if it is localized.

The content of solvent, water or monomer in the ink should also be controlled. If the ink is too thick, it will be difficult to pass through the screen; if it is too thin, it will flow easily, the edges of the image will be irregular, the resolution will be bad, and the darkness of the shadow area Graph dots are lost, resulting in a lower overall density

The particle size of the solids, especially the particles, should also be well controlled. The particles of the colorant are too large and may clog the mesh when the screen is opened. This changes the color density (lighten) of the printed image. In UV ink printing, especially if the screen used is flat on one side (on the side of the substrate). Because the ink may stay in the mesh.

The tendency of the ink to dissociate should also be well controlled. It will also change the density and color value of the image and cause the ink to adhere poorly to the printing material during the printing process.

Other variables that need to be controlled, such as color quality, should be controlled in two ways:

The first is the quality (purity) of the color material. The main color material contains other noise components as little as possible. It should be as transparent as possible to reduce the problem of “main colors”.
Second, adjust the density of the base color according to the copied image.

These densities are not pre-determined and should be adjusted between 0.95 and 1.35 depending on the image to be copied (if printed on some POP/POS boxes or posters at bus stops, or even higher).

Controlling these variables is not just a technical issue. It is more of a matter of artistic perception. It is largely an uncontrollable variable because the perception of images cannot be defined in scientific terms.

Also, make sure that the ink color is suitable for the material you want to print. Because the absorbability of printing materials will affect the color sequence of the printing and the color after printing (materials with higher hygroscopicity need to reduce the density of the ink, especially the color of the first printing, ie the main color).

The type of press usually determines the printing speed (for example, the speed of the cylinder press is faster than that of the plane printer, and the distance from the web is smaller), which also has a certain influence on the change of the chroma.

The use of prints is also a factor that requires the control of density. Obviously, the back-illuminated prints require a higher density of the main color, and at the same time, the coloring should be 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 the light is diffused), the color of the material and the intermediate material is also printed at high speed.

Other issues related to the use of prints, such as magnification (large size), give the impression that the space between enlarged dots is larger than the original size on small-size originals, making the final image appear 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 printing color.

The relative transparency of the basic colors should also be well controlled. If the transparency of one or several basic colors is not enough, the printer must first print a layer of opaque color (or opaque ink) on the surface of the printing material. However, this has the disadvantage of affecting the dominant color and affecting the visual quality of the image.

Regarding noise, no basic color other than yellow is absolutely pure. The completely pure basic color must be able to completely absorb its complementary color, and when the human eye receives it, it will only 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 as high as 60%) yellow. It is important to hire no pure magenta, even if the best choice is made, it should be corrected during the separation phase.

The color fastness (UV radiation in sunlight) is another variable that should be controlled. If you require prints (such as long-term outdoor advertising, etc.) to have this feature, it is also related to the problem of choosing the right product. The lightfastness of inks is usually classified into 1 (not lightfast) to 8 (good lightfastness). If the ink is required to be exposed to sunlight for more than two weeks, lightfastness grade 8 ink should be selected.

The sunscreen properties of inks are not related to the oily, watery, or UV inks of the ink. It is only related to the quality of the ink colorant.

In contrast, the physical (mechanical) and chemical strength (dry or polymeric) of an ink are absolutely related to its chemical composition. Due to the cross-linked polymerization of the UV system, UV inks have good mechanical strength and corrosion resistance and are resistant to rain, snow, sand, dust, pollution, and chemicals (solvents or gases). Selecting UV ink is a good way to control this variable.

If you do not use UV ink, what kind of slow drying agent should you use?

Most of the use of sunken type inks, and sometimes the use of water-based inks, must overcome the tendency of the ink to dry on screens. The key factor to control is the speed of the solvent or water.

The evaporation rate of ether is rated at level 1 (evaporation within 1 s). For most solvents, a good slow drying agent is benzyl alcohol (1800 times slower than ether evaporation).

For water-based inks, the addition of 5% ethylene glycol (or butyl acetate) can slow the dissociation between the ink and the screen due to the squeegee movement.