Introduction
The minimum dot is a topic of interest and mystery for many in flexography. It can bring the production floor to a screeching halt while people begin troubleshooting or cleaning plates if the characteristic hard edge in the print can’t be recognized. A flexographic printing plate that doesn’t hold a minimum dot is easy to identify once you know what to look for. It is a phenomenon that has always been an issue, but warrants explanation for those new to the flexographic process.

Spots on the plate where dots are completely missing.

Mechanics
The minimum dot phenomenon originates from the fact that light energy must pass through a negative or mask to harden the photopolymer enough to form a dot. The opening in the mask must be large enough to allow enough light to react with the plate polymer for a given exposure time. If the opening is too small, the flexo dot will not fully form, which people tend to refer to as “drop off.”

The black dots in the sky drop off leaving a noticeable hard edge.

Importance
Why is minimum dot such a big concern for flexographic printing? Two hand-in-hand reasons: tone break and dot gain. To explain further, the opening in the negative might, for example, need a minimum opening size of 20 microns to allow enough light to pass through and sufficiently react with the photopolymer. Let’s say this size opening corresponds to a 3 percent dot in the electronic file. Therefore, the minimum dot would be a 3 percent file dot. Any image element smaller than 3 percent in the electronic image (like highlights in a person’s face or features in a sky) will disappear altogether because the plate won’t hold it. Therefore, a sudden tone break is noticeable in the printed image that is objectionable to the viewer.

Distinct drop of the cyan dot in the red ball leaves a noticeable edge.		Notice the significant jump in density from white substrate to smallest black dot.

Example
Dot gain now comes into effect and compounds the issue of tone break. Here’s how it works using a simple one color on white substrate as an example. The substrate is the brightest highlight (0 percent) that can be achieved. The next highlight tone after the substrate is the minimum dot that can be held on the printing plate. This minimum plate dot might correspond to a 3 percent file dot. This minimum dot then grows as result of dot gain to something like a 13 percent printed dot. The eye detects a harsh transition from 0 percent to 13 percent and the viewer interprets this as a defect in the image. To compound this, ink drying can occur at this minimum dot edge and cause a build up of ink that appears as an even darker edge in the print.

Vignettes or Gradients require special care since they inherently fade to 0 percent or substrate and show the hard edge.

Forms of Minimum Dot
This brings up a very real issue that can get overlooked in handling a minimum dot. There are actually two forms of minimum dot a plate provider must be aware of. First is what we have been discussing all along—the physical minimum dot that can be held on the printing plate. Second is the production minimum dot. This dot is the on the plate that will dynamically hold up during a production press run. Most commonly, this can relate to the drying speed of the ink. If the press crew prints with a fast-drying ink, the 3 percent file dot could realistically dry up, meaning it may not be the best choice as a minimum dot. A press characterization will not always recognize this because the press run is too short. That is why running control targets on all jobs is important. They give long-run, “live” print data.

One of many ways to set a minimum dot is with curves in Photoshop. Notice an input value of 0 percent is forced to an output value of 1 percent.

Digital Bump
With the growing popularity of digital flexo plates, the minimum dot is still a concern that must be addressed. The term used in digital plates is “bump.” Digital dots shrink (aka sharpen) during UV exposure. Because of this, the mask opening is “bumped” open wider so a minimum physical plate dot is held. Usually RIP output is manipulated so a 1 percent file dot is bumped up to, as an example, a 7 percent corresponding dot opening in the mask. That 7 percent mask opening will sharpen back down to a very small (sometimes less than 1 percent) dot on the plate. This takes care of the physical plate; however, the electronic file dot must still be handled to ensure there is no dot less than 1 percent in the image. This is why the separator or plate maker must always take care to set a minimum electronic dot in the artwork even when using digital plates. It is also important to mention that an ICC profile does not recognize a minimum dot and this must still be manually set after conversion from RGB to CMYK.

The minimum dot is a topic that has multiple discussion elements we can go into; however, the fundamental mechanics have been related in this article.

Clemson University’s Department of Graphic Communications hosts a variety of training programs for the printing and packaging industry through its Print/Con Center. For more information please visit www.clemson.edu/printcon or call 864-646-5749.

Copyright 2008 Salmon Creek Publishing