Flexographic Design Principles

Graphic Elements and Printing Parameters

The thin photopolymer plates used in flexographic printing deliver very good image quality, but, because this is a direct impression form of printing, the plates have their limitations. Certain types of very fine detail can be difficult to reproduce properly at higher equipment speeds. In many cases, minor design changes or logo modifications can produce artwork that will print very well in flexography.

Here are some graphic elements and design characteristics that often pose printing problems. While these things are not necessarily to be feared or avoided, you should be aware of the them.

• Very fine type.
• Fine lines within or around logos.
• Type with small serifs.
• Ornate or decorative typefaces.
• Reverses of fine type, thin lines, or fine logos.
• Small dots (screens, tints, vignettes, and halftones).

The examples at right show the differences between the original art and the final reproduction when problematic art elements are used. As you can see, thin type and objects (top) widen when printed (bottom). As the plate wears, this fattening will increase. Reverses, on the other hand, will react in the opposite manner. They appear thinner on the plate impression (bottom) and may even disappear as the plate wears.

These elements may be necessary in some graphic designs, so don’t completely avoid them. Just be aware that to make these items print correctly, it may take additional prepress production time. If you are concerned about particular elements in a design, please ask for a review on the artwork and a quote on the prepress work to adjust the art for best results.

Halftones

Reproducing patterns of small dots also present special challenges for flexographic printing. Screened tints, vignettes, graduated fills, and halftones all utilize small dots to give the illusion of varying tints, shadings, and tones of color. Using these elements carefully and thoughtfully can give a product an excellent finished appearance. Understanding the characteristics of flexo printing will help to create these graphic elements in ways that work well in flexography.

The dots used in a screen are rated by a percentage of the area they cover. For example, 100% dot is solid color, and 50% dot is half the area covered which results in a lighter tone of the color. Any type of printing process causes the dots to gain a certain amount on the printed piece. Flexographic printing has a dot gain of approximately 12-15% when used with doctor blade equipment and about 25-30% gain on a standard printing system.

Essential points to understand about art with screens, graduated tones, or photos:

• Screens and tonal graduations should stay between 85% and 5%. Heavier screens will fill in and lighter screens will not reproduce cleanly.
• High-quality, high-resolution image files should be supplied with the artwork for proper preparation of the images for flexo.
• Placed photographic images in the artwork should have a border to hide any jagged appearing edges from the halftoning and provide trapping on multi-color registered jobs.

If the artwork contains large screened areas and/or includes halftones, you should be aware that there are important considerations to make on this type of reproduction and the type of equipment that will be used for the job. The sales representative should discuss the artwork with the customer and with production associates to determine the best way to produce a successful envelope product.

Trapping

The nature of flexographic inks and the tolerances of color registration also require consideration in the graphics and prepress process. In order to print multi-color jobs correctly, the different colors of ink must overlap slightly in areas where colors meet. This is referred to as trapping.

Trapping is critical to compensate for minute misalignments in the printing units which occur as envelopes are run at high speeds. In order to maintain a high quality product, proper trapping is essential.

The customer needs to be aware of this necessity and of the minor effects created by trapping. When translucent flexo inks are printed on top of one another, a third color results. As shown in the example at left, the overlap of colors created by trapping has created a thin line in a darker color.

The consequences of not trapping the colors is demonstrated in the examples at far left – any misregistration of printing shows up as a thin, paper show-through line along one or two sides of the character or object. The trapped image is far less noticeable or objectionable if slightly misaligned.

Water Based Ink Mark Created for Tension Customers

To assist customers in their environmental public relations efforts, a symbol was developed to use on Tension envelopes. The symbol promotes the fact that water based inks were used in the flexographic printing of the envelopes. Water based inks contain much lower levels of volatile organic compounds (VOCs) than solvent or petroleum based inks.

Flexography is a form of direct-impression printing using cylinder-mounted flexible plates and fluid-type inks. Flexographic printing units are incorporated into Tension’s high-speed envelope folding equipment. This modern envelope flexography utilizes thin, photo-sensitive polymer plates for crisp imaging and precise registration control.

Tension was a pioneer in the use of DuPont’s Cyrel® photopolymer plates and a leader in the envelope industry conversion from solvent based inks to today’s low-emission water based inks. Tension has won numerous printing craft awards for high flexographic printing quality, including top recognition in both national and international competitions.

The water based ink symbol above is for the exclusive use of Tension customers. It is another way that companies and organizations can express their concern for the environment to their mail recipients. Talk to your Tension Account Executive.