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Choosing Halftone Line Counts For Textile Applications

(October 2002) posted on Thu Nov 14, 2002

Find out how to improve image quality and durability by limiting the halftone line counts you print on garments.


By Mark A. Coudray

Screen printers are a funny lot, especially when it comes to printing process or simulated-process color. This month, I would like to explore an area that gives textile printers needless grief: halftone line count. It amazes me to listen to garment printers brag about the line counts they use, as if the higher the count, the better they are. I call this macho dotism. It is really a pointless exercise, and it leads to confusion and unnecessary problems on press as well as problems for the end user. Let me make it perfectly clear from the outset that we do not need to print a fine line count on textiles to achieve a great-looking shirt. The ideal line count is quite the contrary. Have a good reason When we choose a line count, we must consider many factors. Regardless of which line ruling we decide upon, all of our decisions should be based on a simple concept--visual reality is reality. This simply means that what we see is what we get. If it looks great at 35 lines/in., then this is acceptable. We tend to get hung up on the technical aspects of the printing process. Remember that the consumer has no idea of how the process works, or for that matter, what a halftone dot is. They only care about how great the image looks. I have never heard of a mother sending her child back to the mall to exchange a T-shirt because she took issue with the halftone line count at which the design was printed. What are our objectives? Halftone printing involves considering two entirely different sets of criteria. The first group is what I'll call aesthetic or visual factors: * Does it look good? This is an immediate, gut-level reaction to the image. * Is the tonal range full and complete? * Is the contrast believable? Does the image transition smoothly from white to black? * Are the colors bright and full? * Are there subtle or pastel colors in the image? * Is the detail clearly visible at normal viewing distances? * Is the image too detailed (too busy)? * Does the image read well from 5 ft? 10 ft? 20 ft? * Are all key visual elements of the image easily identified? * Does the image look flat, dark, and muddy? The second set of criteria deals with physical aspects of the print that are of more concern to the printer than the consumer, although a few do apply to the end buyer. These criteria include the following: * Is there enough ink on the garment to prevent fibrillation? * Does a continuous ink film develop in the solid areas? * Does the printed ink film lack penetration into the garment? * At the chosen line count will each and every print maintain the same level of consistency (will the images appear identical)? * Does the printed color shift during the course of the run? * Does platen-height variation affect the image quality? * How often do we need to wipe down the screens during the run? * Is moiré a constant problem, appearing randomly in an unpredictable manner? * Does the image on the garment deteriorate quickly after just a few washings? * Does the image look like it has been washed before it is washed? To put it simply, our objective is to pick a line count that will give us a great-looking print that will last. It does us no good to deliver an excellent visual piece, only to have it wash out when the consumer launders it. On a physical viewing basis, the selection of the halftone line count needs to be partly based on the viewing distance of the image. It is one thing to be looking at a halftone when we read a magazine that's only 12-18 in. from our face. It is quite another to look at the image when an individual is wearing it--most people would probably be displeased to find someone peering at their chest from a foot away. Realistically, normal viewing distances for printed apparel range from 4-20 ft or more. We can use this as the basis for determining an appropriate line count. We do so by applying the rule of 240. This rule simply states that you take 240 and divide it by the halftone line count. What you get is the viewing distance in feet, where the halftone dot pattern is no longer discernable to the human eye. I am not sure where this simple rule originated, but I have heard it referenced frequently over the years, and it works. For textile applications, choosing halftones between 40-65 lines/in. is generally appropriate. This would apply to a viewing range of roughly 3.5-4 ft based on the rule of 240. Using line counts that apply to viewing ranges closer than this becomes an academic decision, one in which you must balance the higher resolution against how the print will perform.Mark Coudray Print contrast has been identified as one of the key factors that influence whether or not an image "looks good" to the viewer. Put simply, contrast is the difference between white and black in the image. More specifically, it has to do with how much detail is clearly visible between the 50% tonal values and the solid (100%) values. It is largely a matter of 3/4-tone detail being distinguishable from the solid areas. Print contrast is most greatly affected by dot gain. The higher the dot gain, the less contrast the image will have. Lower contrast leads to dark, muddy, flat-looking images with detail that is difficult to distinguish. Choosing a lower line count can greatly reduce the amount of dot gain you have. More importantly, it will make dot-gain management more realistic and achievable, because it will increase the space between each dot. Line count influences dot gain by dictating how many halftone dots are in a given area. The higher the line count, the more dots within a given area, and the harder it is to manage the gain that occurs. Below is a simple table that shows how many dots are in a given square inch of space. This value is obtained simply by squaring the line count. The results are enlightening.

 

 
Halftone
line count
(lines/in.)
Dot count
(dots/sq in.)
45 2025
55 3025
65 4225
75 5625
85 7225

As this table shows, if we use halftones that are 65 lines/in., we will have 1200 more dots to manage in the same space as we would with halftones at 55 lines/in. and more than twice as many as at 45 lines/in. If we multiply these differences by the total area of our image, we may find that we are managing hundreds of thousands more dots every time we increase line count. When I came to this realization, lower line counts became very attractive. Another influence on halftone line-count selection is the surface that we are printing on. Unlike paper or vinyl film, T-shirts and sweatshirts are not solid surfaces. At best, we are going to hit a fabric fiber perhaps 65% of the time. The coarser the surface or knit of the garment, the less chance we have of hitting a thread. When a thread opening is missed, a microscopic dot of ink is left on the backside of the screen. On the next pass, if that droplet gets printed, it has the same effect as a double hit, causing dots to expand beyond their desired size. This is why sweatshirts are so hard to print with halftones. This is also the reason why screens need to be wiped down during the run. The more ink that is left behind, the faster dot gain reaches unacceptable levels. If the line count we choose is too high, we greatly increase the chances that the dot will not hit a thread. Figure the dot population into the equation, and we will see that a dot-gain nightmare looms on the horizon. If we know that we are going to be printing sweatshirts, we can give ourselves a better chance at success by dropping the line count by a minimum of 10 lines/in. from what we would normally choose. At the very least, this will reduce the visual impact of the inevitable gain that occurs. Print quality vs. image durability Besides the technical issue of dot gain, there is a more fundamental factor to consider when working with a particular halftone line count: Are you printing a heavy enough ink film to withstand fibrillation? Fibrillation is a term used to describe the very fine strands of garment fiber that pop up from the printed surface after the shirt has been washed. Every garment contains millions of these very, very small strands (they are only a few microns in diameter--much finer than a human hair). The strands are so small that the plastisol cannot physically bond to them. The ink is only secured by mechanical adhesion. Because the surface area on these fibers is so small, it doesn't take much for them to pop up from the ink film during laundering. The visual effect this causes is that the image appears washed out. The occurrence of fibrillation really doesn't have to do with ink that is improperly cured and washes off the garment, although this can contribute as well. The solution to fibrillation is to make sure that we are printing enough ink to form a fully fused film. It is also necessary for the ink to penetrate the garment so that it has a surface to which it can bond. However, in satisfying these conditions, our results are radically influenced by dot gain. When there is a high degree of dot gain on press, it is customary to decrease print pressure to help reduce the gain. In doing so, the amount of ink reaching the surface is also reduced, as is the ink's penetration into the garment. While the overall visual appearance of the image may improve, the improvement comes at the expense of image durability. The key to solving fibrillation is to reduce the influence of dot gain without reducing the amount of ink being deposited on the shirt. This is best done by dropping the line count of the halftone.now it should be very clear where I am headed with this discussion. Halftones in the 45- to 65-line/in. range offer significant improvements in visual quality, contrast, tone, dot gain, image consistency, and durability. There are so many positive reasons to drop the line count that they outweigh the limited benefits of increased detail and improved saturation offered by higher counts. The best proof is to let the customer decide which count gives the best results. I will almost guarantee that the image of choice will be the one printed at the lower count. It's a case where economic and marketing aspects of a job overshadow the technical capability we can provide. About the author Mark Coudray is president of Coudray Graphic Technologies, San Luis Obispo, CA. He has served as a director of the Screenprinting and Graphic Imaging Association Int'l (SGIA) and as chairman of the Academy of Screenprinting Technology. Coudray has authored more than 70 papers and articles over the last 20 years, and he received the SGIA's Swormstedt Award in 1992 and 1994. He covers electronic prepress issues monthly in Screen Printing magazine. He can be reached via e-mail at drdot@aol.com.


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