Design for Print - Print processes - Flexographic

Flexographic

Preprint refers to jobs that require "preprinting" prior to bag conversion. The following are preprint criteria:

• Greater than four colors/prints (maximum of six colors)
• Tight registration between colors
• Fine detail, screen or halftone printing (screens or halftones greater than 65 line screen)
• Greater than 10% ink coverage on claycoated paper
• Any embossed paper order (to ensure maximum emboss depth/texture)
• Any lamination order
• Any UV (ultraviolet) varnish (applied during the preprinting process)

Flexography Tailprint refers to jobs that can be printed during conversion/bag making. The following are tailprint criteria:

• Four colors/print stations or less
• Loose registration between colors (3/32")
• Medium-to-coarse detail, fine screens or halftones at no more than 65 line screen
• Less than 10% ink coverage on claycoated paper
• 100% ink/varnish on uncoated papers

Flexography (Flexo) printing uses a raised-image printing plate. Photopolymer, or rubber, plates are used to transfer the print image. Conventional liquid or sheet photopolymer plates are used for line art. Laser-imaged, direct-to-plates (DTP) are used for all four-color process, all duotone, and most halftone images. As the plate cylinder rotates, it comes in contact with an anilox, or ink roller, which inks the raised portion of the printing plate. The plate then transfers the ink to the web, or paper. The anilox roll is laser-engraved with thousands of recessed cells that fill up with ink. The doctor blade controls the ink transfer to the plate. The doctor blade wipes off all ink except what is in the cells of the anilox roll. Compared to offset printing, this method offers significantly more ink transfer. (Refer to diagram below)

There is "gain," or growth, of Flexo images. For example, if the image area is a process/halftone with a 2% dot in the art/plate (typically the minimum for Flexo), it will print between 9-12% dot on coated papers and 12-15% dot on uncoated papers. This gain occurs for several reasons: As mentioned, the amount of ink transferred increases the image dot size. The contact pressure to transfer from anilox to plate to paper also increases the image dot size.

The substrates needed to ensure efficient machineability of roll paper into bags are not as smooth as those in offset printing. A rougher sheet can increase the size of the image dot because it requires more plate pressure to print. In addition, more ink is absorbed in a rougher paper. There also is more inherent machine movement in Flexo than in offset printing, so registration is not as tight and larger traps between colors are required.

Flexo can maintain resolution levels of 65-110 line on uncoated papers (such as Kraft and white) and 110-133 line on coated papers (clay coat).