A new way of making digital flexographic printing plates reduces dot-gain problems and delivers crisp detail.

With a flat-top dot structure (top image), flexo plates can print high-quality details, and with minimal modifications to electronic files.

Over the past several years, the word “digital” conjures up an image of a state-of-the-art product.  Yet some flexo printers, especially those in the flexible packaging market, wondered why digital flexo plates were not effective for their processes, why they struggled with color and why plates with vignettes seemed to wear out so quickly. 

Further examination of conventional digital plates, when viewed through a scanning electron microscope, reveal a dot profile that is rounded with a fairly shallow relief depth. It is this profile that can cause problems in a print production environment, as the rounded tip needs to be over-impressed to achieve the print target. But this over-impression will also deliver greater dot-gain in flexible substrates, as well as “dirty” print.

But a “flat top dot”-that is, a dot profile that is completely flat-just about eliminates this type of conventional digital dot profile and represents an accurate delivery system for the ink. The degree of the bevel (28 degrees from the dot tip to the plate floor) enables the dots to flex, much like a shock absorber, which all but eliminates the problems of printing vignettes and dot-gain.

A number of suppliers now offer flat-top dot digital flexo plates. In 2007, PRPflexo brought its solution to market. The added relief depth of the Digital ExSpect plates (up to 30% on average), when compared side-by-side with conventional digital plates, offers another benefit. This important difference is not in the open area where there is no content, but is in the space between the dots, where it offers more responsiveness to the plate, adding significantly to the overall plate life, especially in vignette areas.

Recent tests have proven that Digital ExSpect plates last up to 200% longer than conventional digital plates, and since the latitude for the impression settings is much greater,  fewer “tweaks” during set-up need to be made, leading to greatly reduced set-up times. The best comparison is to a bicycle tire: The one with the thicker tread will always take you further.

In the illustration, you can see the crisp transition from the face to the bevel. Also note that the bevel is engineered to redirect some of the dot compression away from the face to the bevel of the dot, ensuring that the dots maintain integrity under impression. In particular, for flexible packaging printers, the independent compression of the dots does more to reduce dot gain, while allowing for the possibility of using harder mounting tapes to achieve better solids without compromising on halftone printing.

For all flexographic printers, the absence of a conventional “bump” curve means that there is much less compression of the electronic file. A “bump” curve is required for conventional digital plates, so that the minimum dots form sufficiently to maintain form all the way to the print surface of the plate. Typically, a minimum ablation spot-size is an 8% to 10% dot, to generate a minimum dot for conventional digital. This dot is treated as though it were a 1% dot.

For Digital ExSpect plates, the percent-dot ablated from the carbon-and-wax mask is exactly the same size as the finished halftone dot. Consequently, no “bump” curve is required.  This is a true what-you-see-is-what-you-get process, leading to a much truer rendition of the original artwork and less compromise.

In many cases, depending mostly on the volume of ink delivered, no cutback curves are required at all for this “flat-topped” dot technology. Dot gain is minimal, and the only modification to the electronic file required is a minimum dot, typically of 2%, to ensure reliability of the minimum dot structure. Though many printers are obsessed with what kind of minimum dot they can hold in a plate, the minimum printed-dot density is the better target. The minimum 2% dots in these Digital ExSpect plates typically print one to three points less than the minimum dots in conventional digital plates.

Interestingly, the imaging process of this plate technology permits 0.5-point type to be printed cleanly and clearly, while the applications for microtext printing for anti-counterfeiting solutions are boundless. Print trials of 200-line screen and beyond have proven the reliability of the flat-top dot structure, and there are options for even higher imaging resolution. There is no “rounding” or smoothing effect like that of conventional digital plates, so line-work and text has crisp sharp corners, and detail you may have thought impossible from a flexographic plate. 

Chris Green, business development manager at PRPflexo, can be reached at 317-783-3226 x1148 orcrjgreen@prpflexo.com.