The Flexographic Printing Process
From its origin as a kind of a rubber-stamp printing method, flexographic printing has evolved into an advanced technology that allows full color printing of numerous products on every conceivable substrate. The flexographic printing process is relatively simple to understand.
As with any printing process flexographic printing too begins with the conception of an idea and a design to be printed. The graphic designer creates a comprehensive layout, which shows what the final product should look like. This layout indicates accurately where all the design and type elements are to be placed and in what color. The paste-up artist then prepares the display type, body copy and illustrations and pastes them onto a stiff based sheet. This is the paste-up or the mechanical. Often the graphic designer and paste-up artist may be the same person.
After the complete paste-up has been proofread and approved, the camera department photographs it with a large process camera to make the film negative. The film negative is used to make the printing plate.
Two kinds of artwork are used in flexographic printing. Line art uses just solid areas, pen and ink drawings or type. Solid black line art is pasted directly onto the paste-up. The photographic negative thus produced contains only black and clear areas – no shades of grey. Continuous-tone art is any artwork that contains tones of light and dark areas rather than solid areas only. Photographs, transparencies, painting and shaded drawing are examples of continuous-tone art. Since flexographic plates print only even layers of ink in the image areas, continuous-tone art must be converted to another form to be printed. A photograph or other continuous-tone illustration must be rephotographed with a graphic arts process camera through a special halftone screen. The screen breaks up the image into different-sized dots. The screening process is called halftoning, and the resulting image is a halftone. When printed, the image will resemble the original photograph, with the smaller dots producing light shades or highlight areas and the larger dots making the dark shades or shadow areas.
To print color photographs or transparencies the original must be rephotographed in a manner that breaks the colors that you see into different halftone negatives. The image can then be reproduced as different-sized dots for printing yellow, magenta, cyan, and sometimes black. This rephotographing is called color separation, and these four colors are called process colors. For artwork that prints in more than one solid color an overlay must be made for each separate color. These overlays represent flat or spot colors. Each overlay is used to make a separate plate making negative, and each plate will be inked with a separate color when put on the press.
After the paste-up is complete and the negatives have been made, the next step is making either rubber or photopolymer flexographic plates. There are three basic steps in rubber plate making.
Step 1: Engraving
In the first step an engraving is made from the negative. The negative is placed over a light sensitized metal sheet and exposed to intense light in a vacuum frame. This metal sheet is then etched with acid in an etching machine. The metal sheet thus becomes an engraving with the image areas high and non-image areas low.
Step 2: Molding
The mold is made by pressing the metal engraving against a heated matrix material that hardens in the molding press. Molding pressure is then generated by hydraulic power to the bottom platen while the top platen stays stationary.
Step 3: The Rubber Plate
The molding press comes into play once again. The relief image is formed on the rubber plate by pressing the matrix against it under the appropriate heat, time and pressure. The plate must be stripped from the mold while still warm.
In case of photopolymer plates there is no need for engraving. The plate is made directly from the negative by placing the negative over a pre-cast sheet of light-sensitive photopolymer and exposing it to ultra-violet light. Liquid photopolymer can also be used to make a plate. In both cases, the exposure to ultra-violet light causes the polymer to harden in the image areas. The exposed plate is then put into a processor, which removes the unhardened polymer from the non-image areas. The plate then goes into the drying unit.
Once the plates are ready, they are mounted on the printing cylinder in a mounting and proofing machine. Once attached to the plate cylinder the plate is inked with a roller and then rolled against a piece of proofing paper that is attached to an impression cylinder. The plate and proof are then inspected for precision. If approved, the plate is now ready to be printed.
Flexographic printing uses thin fluid inks that dry quickly. These inks are mainly water based or alcohol based.
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