Screen printing is a common method used by professional printers all over the world. But if you don't know what you are doing, the limitations can seem difficult to overcome. Because screen printing prints a single color at a time, and doesn't have any control over how much ink is laid down, mixing colors is nearly impossible. With only cyan, magenta, yellow, and black to work with in a typical CMYK printer, the inability to create different shades reduces the possible outputs. Thankfully, printers have solved this problem many years ago using a technique called halftoning. With halftone images, you be able to print photorealistic images even with the limitations of screen printing.
Ordinarily, to print a gradient that goes from black to white with shades of grey in between, you'd use a printer that can apply different amounts of black ink. By doing so, the printer is able to produce a number of different shades of grey, or any other color. When this method isn't possible, or is limited, halftone screening is used instead.
With halftone screening, the same intensity of ink is used throughout the printing process, but it is placed on the surface using differing sized dots. Solid black would be black dots that overlap each other. White would be no dots. The various shades of grey would go from very large dots that are close together on the black end to tiny dots that are far apart on the white end.
When these dots are large, the effect can be very artistic. For a more convincing effect that gives the illusion of continuous color, the dots can be made smaller so they are harder to see from a distance with the naked eye.
Halftone screen printing can be used on any color to create various shades of that color without needing a printer capable of laying down different shades of ink.
To get the best results when screen printing a halftone image, there are a few specifications and best practices to keep in mind. First off is the resolution of your image. Firstly, you don't want to measure your resolution in pixels. Instead, you show go by DPI (dots per inch). To know what DPI to use, you should know what LPI (lines per inch) your printer is set to and set your DPI to twice that value. Remember, it's easier to downscale an image than to upscale one, and a DPI that's too high will not degrade quality but a DPI that's too low will. So, err on the side of caution when designing the image.
A common problem with halftone printing is dot gain. This is when the dots absorb into the material when printing and appear larger (and therefore darker) than they should. The problem is most pronounced in the midtones. This doesn't mean you need to avoid those tones, just that you need to be mindful that they may appear darker than you intended and design around that possibility.
As you can imagine, creating a halftone image from even a simple image by hand would be a lot of work. The real power of halftone printing is that it allows you to print more photorealistic images with printers that would otherwise be incapable of doing so. Those images would be nearly impossible to halftone manually without a lot of time and even more talent.
Thankfully, halftone screen printing is an extremely common technique, and most professional grade software will have the option to let the computer do it for you. In the CoreDRAW Graphics Suite line of products, for example, you can ready an image for halftone screen printing by going to the Color tab of the Print dialog and enabling Color Separations. You'll find plenty of advanced settings to play around with, but keep in mind that halftoning is a sensitive process, and it's easy to mess up the effect, so be careful when changing the settings.
If you are looking to provide the best quality in your screen printed materials without having to rely only on simple shapes that would work well with no color mixing and only a four-color palette, halftoning will open up a whole new world for you.
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