Whether it’s four-color process, simulated process, or index separations, textile printers are faced with addressing dot gain on a massive scale, especially compared to graphics printers. Garment shops are up against a number of variables that compound themselves. For instance, the surface on which they print is 65-90% solid, which means there is a 10-35% chance of missing a printable surface. The surface also is absorbent and highly compressible. It is almost impossible to form a good gasket with the stencil in order to maintain dot integrity. On top of this, the twisted yarn structure is highly irregular.
The wet-on-wet nature of plastisol printing also contributes to dot gain because dots and color continuously move and distort. Flash drying between colors is slow and, therefore, not really a solution. Additionally, the excess heat generated during the flashing process is damaging to the ink and contributes to viscosity change in the ink, which leads to more dot gain and stiff, objectionable prints.
Graphics printing involves managing a much more limited number of screens and surface distances. Unless a graphics shop uses a multicolor inline press, the situation is much more controllable. But textile printers are typically faced with managing eight to 20 platens and six to 18 printing heads. This creates a huge number of distance variations and combinations.
This month, I would like to look at what it takes to create a stable situation on press and establish a workable prepress correction for any press inconsistencies in garment printing. The single most problematic area is the parallelism and off-contact relationship between the platens and the screens in the printheads.
Heads and platens
When variation between heads and platens increases, dot gain goes wild (Figure 1). The variation in gain between platens alone can be almost 50% (this is in addition to what we would normally expect to see). I have seen cases where platens were seriously out of level, where some platens did not print, and where others had 80% or higher gain.
In order to remedy the situation, we must have a way to determine platen-to-head distance relationships. Numbering the heads and platens gives us a way to see what is happening between any given head or platen combination.
As Table 1 shows, platen and head heights can combine in multiple ways that result in three general levels of dot-gain predictability. Those shown in green have a relatively constant relationship to each other, the yellow relationships are marginal, and the reds will yield the biggest variation. How much additional distance qualifies as a step up or step down to a different level is up to us to determine based on the line count we’re printing and the tolerance for color variation within the run. A good starting point is to set the minimum off-contact to 0.06 in. An increase of 0.03 in. would constitute a step up to the next level. The highest level would be 0.12 in. or more. This may be too much, but it is a starting point. I use 0.04, 0.06, and 0.08 in. on my own presses.
|Table 1 Printhead and Platen Relationships|
Blue areas of this table indicate relationships that will lead to consistent gain, gray areas represent configurations in which dot gain will be marginally predictable, and white areas show conditions that will lead to unpredictable gain.
Ink and stencil issues
The ink itself also needs to be examined. It must be suitable for use in printing halftones. A general-purpose mixing system may not be acceptable because the pigment load may be too high. As an example, the black we create may be too dense for halftone printing. When the load is too heavy, the black appears darker than it needs to be. This leads to an effect called optical dot gain. If we print a 50% dot, the actual visual appearance is darker than 50%, even if we completely control the printing. This means we either need to print a lighter percentage dot to create the proper optical value, or modify the ink to the proper level to achieve the correct value. The effect of pigment load can only be measured with a reflection densitometer. Without one, we are in for a lot of trial and error.
To assess dot-gain profiles, we need to have the proper color strength measured before we even print the dot. Of course, any change in total mesh height (thread diameter) or stencil profile (emulsion over mesh, EOM) will adversely affect the results.
Using a relatively thin mesh, such as a 280 or 305 thread/in. plain-weave fabric with 34-micron thread diameter, helps us to control the physical growth of the dot that results from high ink volume. The thicker the mesh and stencil profile, the thicker the ink film. As the ink-film thickness increases, it provides physical volume that can smash and move around.
Likewise, thinner EOM and total stencil height will deliver a thinner ink film. This thinner film will appear less dark (not as dense). Since there is less volume of ink, there is less physical dot gain. This means there is less ink to squash out when other colors are printed wet-on-wet on top.
A couple of problems arise when the off-contact distance is greatest (high/low, low/high combinations). The most obvious are skips and voids with bad ink penetration into the garment (Figure 2). Any dot that doesn’t hit a garment fiber is left on the backside of the screen. If there are more than two low platens in a row, the ink continues to build on the back side of the screen in the image area.
When that screen hits a medium or high position on the next print, the extra ink on the backside acts like a double
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