Fine-mesh screen printing analysis (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