Whether it is four-color printing, simulation printing or index separation, textile printers have to face a large number of dot expansion problems, especially compared with image printers, this problem is more prominent. There are many uncertainties faced by garment printing factories. For example: 65-90% of the surface of the substrate they print is solid, which means that they have a 10-35% chance of losing the printable surface. This surface is also absorbent and compressible. It is almost impossible to make a gasket with a stencil (stencil) to maintain the integrity of the dots. Most importantly, this interwoven fiber structure is very irregular.
The hot-wetting-wet-on-wet processing characteristics are also one of the reasons for the dot gain, as dot and color are constantly shifting and distorting. Flashing between colors is slow, so this is not a real solution. In addition, the excess heat generated by the flash exposure also destroys the structure of the ink, resulting in a change in the viscosity of the ink, which causes dot gain, ink hardening, and unnecessary imprinting.
The stencils involved in image printing and the control of the surface spacing are very limited, unless the image printing plant uses a multicolor inline printer, otherwise the dot-spreading situation can still be controlled. However, textile printers usually manage 8 to 20 platens and 6 to 18 printheads. This also caused a large number of surface distance variations and combinations.
The author hopes to find an effective way to maintain the stability of the printing press during the printing process of the garment and to be able to correct any printing press before printing. The most difficult of these is the relationship between the parallel and off-print distances of the platen and the stencil in the printing.
Print head and platen
With the increase in changes between the print head and the platen, dot gains have become more severe. The dot-difference between stamping plates can vary by up to 50% (this is beyond the range we often see). The author once saw a severely deformed and uneven platen. Some imprinting discs will no longer work, while other imprinting discs will produce dot gains of 80% or more.
To change this situation, we must find a way to determine the distance between the platen and the printhead. By numbering the printhead and platen, we can see the different effects produced by different printheads or platen combinations.
Ink and template
The ink itself also needs inspection. It must be suitable for halftone printing. A general-purpose hybrid system may not be suitable because of the high content of dye fill in this system. For example, the black ink we use may be too thick for halftone printing. When the filler is too heavy, black looks darker than the desired hue. This will also cause the phenomenon of "optical network expansion". If we want to print a 50% dot, even if we control the entire printing process well, the true printing effect of this dot is even darker than 50%. This means that, on the one hand, we need to print a smaller proportion of dots in order to achieve a suitable optical value. On the other hand, we must also control the ink to a certain level to achieve the correct optical value. The effect of the dye filling can only be measured by a reflection densitometer. Without reflection densitometers, we will make many detours and make many mistakes.
To evaluate dot gains, we need to make appropriate measurements of color intensity before printing. Of course, any change in the mesh height (diameter of the wire) or the template (emulsion on the mesh, EOM) will have a detrimental effect on the effect.
The use of a relatively thin mesh, such as a 280 or 305 thread/inch mesh made of a 34 micron diameter wire, can help us to control the dot gain caused by the large ink volume. The thicker the mesh and the template, the thicker the ink film.
Similarly, the thinner EOM and total stencil height will also result in a thinner ink film. This thinner film does not appear to be so dark (less thick). In this way, since the amount of ink used is reduced, the mechanical dot gain generated during the printing process is reduced. This means that the amount of ink needed is also reduced when printing wet and wet.
(to be continued)
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