Assessing the control of textile outlets in the printing process

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.

Correct measurement

When the off-page distance reaches its maximum (high/low, low/high), some problems will also arise. The most obvious is the jumps and voids that the ink penetrates into the clothes. Points that are not aligned with the clothing fibers will stay behind the template. If there are more than two low-pressure printing plates forming a column, the ink will continue to accumulate behind the graphic template.

If this template is placed in a medium or high position in the next printing, the extra ink behind it will be superposed twice or even three times, resulting in a certain dot gain. In the next high-position printing (minimum interval sequence), the ink is removed and left on the clothes. The color of the printed pattern immediately after it looks very shallow, because there is no ink stored behind the template.
This entire process is called "dumping." We created a deep/light cycle in the printing process. Among them, samples with poor permeation and incomplete permeation are most typical. Such samples are printed after printing inks with excessive permeation and producing dot gains (50% or more). I recently conducted a trial in a customer's factory and the results showed that the dot-dip value on the platen where the "dumping" phenomenon occurred was higher than 70%.

When we wanted to achieve the best (maximum) printhead/platen distance by increasing the squeegee pressure, even problems with production increased. At this time, extreme pressure changes will occur between the printheads/plates with the smallest gaps, causing "dumping", infiltration and dot gain.

If printing is done on the same press, the ink behind the stencil (not printed on the fibers) will be statistically balanced through all stencils. Dot gains will then gradually reach balance and become stable and predictable, allowing us to manage it.

To get the correct statistics, we need to determine what the real change is. This means measuring the distance between the print head and the platen on each device. Once the true amount of change is determined, we can correct the relationship between dot gain and distance. This can be used to predict the outcome in advance.

First, all printheads and platens must be numbered sequentially. This allows us to assess the relationship between the two by measuring and analyzing the distance between each printhead and the platen. The most important thing is to print the dot enlargement target on the unmarked but marked printer. In order to measure the dot gain value associated with each print head/platen combination. Next, we can measure the distance between the print head and platen in each combination. Then we can adjust the press and platen to a balanced state.

When this process is completed, the distance between all printheads and platens will be the same, and each platen and each printhead will also be on the same level. We may not be able to adjust a printhead and platen combination to a horizontal or parallel state, but we can use the amount of dot gain in the initial printing to determine our expectations for the dot gain of this printing machine.

We need a specific method that can achieve the best performance. First, the platen must be horizontal and parallel to each printhead. The acceptable range of variation depends on the structure of each press and the control of the squeegee and floodbar pressure. In addition, I think the best starting point for distance from the plate should be 0.06 inches.

Second, we should print enough proofs to reach a balance point. Regardless of the dot expansion, the value between the print head and the platen should be consistent, with no more than 1-2% error. If the press is not calibrated, the difference between each printhead/embossing disc combination may be higher than 40%, which is entirely due to pressure and distance variations of the same halftone pattern. In daily printing, except for dot gain, the absolute value we expect is 35-38%.

For now, the choice of 280 or 305 thread/inch mesh made of 34 micron diameter wire is not a problem. They have the same fiber height and the same diameter. We do not know the actual number of meshes after stretching, nor do I know the percentage of blanks after tension. This is mainly because there is 15% of fiber in each inch of the cable in each direction, so the 280 lines/inch mesh will be stretched longer than the 305 line/inch mesh and when it is tight Its blank part will also be larger than the 305 lines/inch mesh.

Since the number of dots per square inch has grown very fast, we also need to consider the number of lines we print. For example, 50 lines/inch correspond to 2400 points/square inch, 55 lines/inch to 3035 points/square inch, and 60 lines/inch to 3600 points/square inch, when the number of printed lines increases to 65 lines/inch At that time, the number of dots per square inch surged to 4,225. With the increase in density per square inch network point, we face more and more problems in the control of network expansion, and in addition to the expansion of mechanical network points, we need to solve more problems of optical network point expansion.

Loss of outlet expansion

We can do at least three things to reduce the phenomenon of mechanical and optical dot enlargement for any number of lines. The first is to use wet white highlights or clear white stencils at the end of printing. This transparent/translucent white can be mixed with wet color to create a talk color. This can effectively reduce or eliminate optical and mechanical dot enlargements in the highlights, while also making the tone transition more natural, reducing the dots that we see on the print. This is a big advantage for the printing of facial images and charts containing meat tones. Second, we can add some high density substrates (3% higher than the weight) to the ink. This does not make the ink look thick, it can also effectively reduce dot gain. When we use a 34-micrometer-diameter screen to make more 280 lines per inch of blank mesh, we can see better tone structure and dot gain reduction.

Third, we can mix the number of screen lines in the color separation chart. For example, if we have a lot of problems in the black control, then we can just reduce the black version of the line (down from 65 lines/inch to 55 lines/inch), which also reduces the density of the dots (Decrease from 4,225 points/in2 to 3025 points/in2). With a lower number of screens, outlets are more dispersed, reducing the number of outlets in the same area. This is an attractive option. Since black ink is often designed in dark areas, the effect of thick dots is not as obvious as in bright areas, such as in areas where flesh or pale colors appear.

Many variables affect the degree of dot expansion in the printing process. but