Understand the cost savings and higher profitability that comes from knowing how to print halftones well.
One of the barriers to producing good halftones is assuming that the so long as the software and printer appear to be functioning properly, the dots will be of a high quality. The truth is that many companies had better-quality positives years ago using imagesetters (or process cameras before that) than they currently do. Modern positives are often created through inkjet heads that spray an image onto a coated film or directly onto the screen. This means that the bigger dots we need for the screen-printing process are made up of jetted, smaller dots.
It’s important to understand the true image resolution of computer-to-screen machines and inkjet printers. Sometimes, the numbers in manuals are confusing or based on a software manipulation that doesn’t give the accurate number of dots per inch. The critical information to understand is what resolution is actually imaged onto the positive, not what the manufacturer claims is an apparent or simulated resolution. You should ask, “What is the size of the inkjet dots and how many of them are used to create my halftone dots?” The truth about the quality of a printer’s output can always be determined by viewing the printed dots under magnification.
The film may look perfect to the naked eye, but with even a small amount of magnification (10x or 20x), the dots can tell a different story. Fixing dots that aren’t printing well can be a challenge, but it’s important to know you have an issue in the first place. Sometimes the software isn’t working well with the RIP and the dots will have pixilated edges. Other times, the print head will be nearing the end of its life and the spray will be scattered, ruining the halftone edge quality. With film and vellum, the surface of the material may cause the dot edges to bleed or make the ink too transparent.
In order to create a halftone dot with great edge quality, an inkjet print head must be the right distance from the media and it must deliver a sufficient ink volume to produce an opaque image. The media must have the right absorption characteristics, and the software must be calibrated for the printer’s output. If you notice that your system isn’t doing a great job under minimal magnification, then your final halftone printing will likely exhibit some or all of these flaws:
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