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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 (claycoat).

Flexographic printing image
Summary
  • Medium resolution up to 133 line screen
  • Image "gain" (2% dot to 9-12% dot for coated papers, 12-15% dot for uncoated papers)
  • Raised printing surface
  • Moderate-grade paper stocks
  • Registration +/- 3/64" for preprint
  • Registration +/- 3/32" for tailprint



Gravure

Gravure refers to jobs that use large rolls of paper fed into a gravure printer to produce printed rolls of paper. It is a preprint application only and is prior to bag conversion. The following are gravure criteria:
  • Up to seven colors on one side
  • Can print inside and outside simultaneously (maximum of five outside and two inside)
  • Very fine detail or screens or halftones as high as 175 line screen
  • Any ink coverage
  • Very tight registration between colors (minimal movement)
  • Claycoated papers only
  • Excellent color strength/density: Dark colors and especially metallics print very well with gravure
  • Inside print does not have to be random. It can be registered with the outside print portion
  • Technology and partnership allow for affordable prepress and engraving costs.

Gravure prints from a "cut-in," or recessed, surface. An engraved metal printing cylinder is used to transfer the printed image to the paper. As the gravure image cylinder rotates in the ink pan, the engraved cells of the image are filled with ink. The doctor blade controls and meters the ink by wiping the excess from cylinder surface, leaving ink only within engraved cells. The gravure cylinder then comes in contact with the paper substrate, transferring the printed image with the help of the rubber impression cylinder, which pulls the ink from the engraved cells. (Refer to the diagram below.)

There is "gain," or growth, in gravure printing. For example, if the image area is a process/halftone with a 3% dot in the art/cylinder (typically the minimum for gravure), it will print between 5-7% dot. Gravure is excellent for printing strong metallic inks.

Gravure prints on rolls of the same claycoat paper as Flexo and therefore has many of the same characteristics. However, gravure offers tighter registration that requires minimal traps and can maintain resolution levels of 150-175 line.

gravure printing

Summary

  • High resolution: 150-175 line
  • Minimal distortion, or "gain"
  • Recessed printing surface
  • Moderate-grade paper stocks
  • Very tight registration
  • Excellent for metallic colors
  • Large orders/long runs
  • Claycoated papers only

Sheet-Fed Offset

Sheet-Fed Offset refers to jobs that use precut sheets fed into an offset printer. It is a preprint application only. The following are offset criteria:
  • Unlimited number of colors (because sheets can be fed through presses/printers more than once)
  • Very fine detail with screens or halftones as high as 150 line screen
  • Any ink coverage
  • Extremely tight registration between colors (very little movement)
  • Dark colors/metallics may require a "double bump" (printing the color twice) to achieve satisfactory color strength/density

Sheet-fed Offset prints from a flat surface. Thin metal, photosensitive plates carry the image. As the plates are developed, the photosensitive surface remains in the image area only. The adage of "oil and water don't mix" truly is the basis for this printing application: The image area attracts ink and repels water, and the non-image area attracts water and repels ink. The term "offset" is derived from the transfer, or offsetting, of the printed image from the plate to the rubber blanket and then to the sheet (paper). (Refer to diagram below.)

Nine ink rollers squeeze together and "meter" the ink before transfer. A very thin film of ink transfers to the plate, blanket, and sheet-thereby offering the highest printing resolution. This precise control of minimal ink transfer allows for high-line screen printing (150-175 line). Offset lithography also ensures minimal distortion, or "gain," of images. For example, if the image area is a process/halftone with a 1% dot in the proof/art, the plate can be compensated to print a 1% dot on the sheet.

In addition to the actual printing process, the substrate to be printed has a significant impact on resolution. Sheet-fed offset is printed on high-grade, extremely smooth paper stock, as it does not require the same characteristics for machineability as roll-fed applications. Offset also offers extremely tight registration requiring minimal traps.

offset printing

Summary

  • Precise, minimal ink transfer
  • High resolution: 150-175 line
  • Minimal distortion, or "gain"
  • Smooth/flat printing surface
  • High-grade paper stocks
  • Extremely tight registration
  • Dark colors/metallics may require a "double bump" (printing the color twice) to achieve satisfactory color strength/density

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