Limiting Ink Density

In past articles, we cautioned that you should limit ink densities to no greater than 250% to 275%. One reason is that the volume of ink that you print on a substrate has a direct effect on the dimensional stability of the film that you are printing on. The more ink that you lay down, the more contraction of the vinyl you can expect as the ink dries. When this happens, the applied graphic can peel at the edges. Even before the film is applied, the vinyl can start to curl at its edges on the release liner. Failure to limit ink density can also lead to other problems, which we will expound upon shortly.

What we would first like to do is define the term "ink density." Let's start with your four primary ink channels; cyan, magenta, yellow and black (CMYK). If you print the maximum amount of each ink, you get 100% cyan + 100% magenta + 100% yellow + 100% black. Add all of these percentages together and you get 400% ink density. That's a lot of ink. About 90% of which is solvent. That's why, when you print a heavy concentration of ink (over 250% to 275%), the solvents can chemically attack the vinyl facestock and can actually penetrate the film and compromise the bond of the adhesive to the substrate. Some films are more prone to shrinkage and edge curl than others. To overcome or compensate for these problems, never print to the edge of the media. Instead, provide 1/4" (6mm) outline around the printed image.

Heavy ink concentration can cause other problems. When you print more ink that the media can handle, the individual dots start to spread and blend together with adjoining dots. The ink can also bleed or puddle into ink islands. When this happens, colors become muddy. As the ink spreads, you also lose detail and crisp edges.

Other issues include banding and excessive drying time. By the virtue of the fact that you have added more solvent onto the surface of media, it naturally takes longer for that solvent to dry. Even when you maintain the total ink limit within the recommended range, it can take at least 24 hours for the print to adequately dry before you can laminate the print.

To minimize bleeding, slow your printing speed. The slower speed also reduces the likelihood of vinyl contraction because the ink has extra time to start drying before another ink layer is printed. The downside is, of course, less printing efficiency.

To accelerate the drying of the inks you may be tempted to increase the dryer temperatures. Easy does it! During heating, the expanding film can tunnel on the release liner. Tunnels between the film and release liner can form as the heated film under the dryer expands and pushes against a cooler, rigid film mass.

When printing higher resolutions, in addition to adjusting the density curve in your profile to control the amount of ink that you print, reduce your print speed. You'll experience less bleeding, especially when printing onto glossy films. You also give the ink more time to dry before the print heads make another pass. Generally, you get better results with matte finished films as opposed to glossy ones.

If you are printing on a dark film, such as RTape's slate grey ChalkTalk® chalkboard film, you should print a white mask first, then print your process colors. Remember that ecosolvent inks are transparent. Without white, the slate grey of the ChalkTalk® film will show through. With a white background, you will be able to print lower percentages of CMY to attain a more desirable color. When printing on ChalkTalk®, limit your ink saturation to 250%

Test, Don't Guess: To minimize problems, test and evaluate your media choice with your printer and inks prior to production. Each ink is unique and will adhere to the various types of print media differently. Ecosolvent ink, which are alcohol based, are milder than the strong solvent inks and do not bite into the media so aggresivily. The traditional solvent inks utilize a strong solvent, such as MEK, and adhere better to a broader range of substrates.

In addition to ink types, also be aware that each printer works a little differently and consequently requires different adjustments with respect to ink limits, temperature settings and print speeds. Settings will vary from one media to another. Cast vinyls require different setting than calendared films and a polymeric vinyl will print differently than a monomeric film.

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