Clichés

The flexography plates were improved along the time, resulting in larger productivity, durability and quality of the printed paper. Among the technologies used or in use stand out the plates of manual carve, vulcanized rubber, photootopolimer and liquid photootopolimer.

Manual carve

This process is accomplished manually, beginning by tracing the image on a vegetable paper to a specific plate for that end. Then, the non image areas are removed with a blade or bistoury. This cliché doesn't resist to high volume presses, being of low resistance

In the cliché of manual carve, the art is traced / drawn and carved with bistoury or similar.

Commonly it has 2 layers, being a canvas and an rubber covering, but there is also the cliché of base machinable, that has a third layer previous to the canvas, used to rectify the cliché. The rectifying process guarantees that the thickness of the cliché is uniform after the incision.

 

Vulcanized rubber

Vulcanization is the hardening process applied to the rubber of this cliché type. The making process of  that plate runs trough some different stages.

For obtaining of the secondary original, it is accomplished through the sensitization of a foil of zinc with a photographic copy of the original. In that foil a chemical engraving is accomplished, turning image areas into high relief.

Just after that a negative plate is created, beginning from the polymerization under high temperature and pressure, of a plate of baquelite powder, generating a mold in low relief.

The cliché of raw rubber is then vulcanized on the baquelite plate and, later, rectified so his thickness is uniformized. That process generates a cliché of low durability, with poor capacity of details reproduction.

 

Photopolimer

The photopolímer cliché is essentially formed by 4 different layers: the polyester base; the photopolimer itself; the anti-adherence layer; and the protection polyester.

Layers of the photopolímer cliché used in the flexography. The layers are not proportionally represented in this illustration.

The base polyester has as purpose to sustain the photopolimer and to guarantee the dimensional stability of it. His thickness and uniformity are highly controlled, in order to guarantee a perfect deposition of the photopolímer.

The photopolímer, as the own name says, it is the light sensitive material (photosensitive), responsible for the formation of the image and non-image areas. During the exposure to the UV rays, the polymerization of the monomers of the photopolímer happens (hardening the cliché material), leaving those areas resistant to the chemical during the revelation process. The non exposed areas are removed during that process (more details bellow).

The anti-adherence layer (slipfilm) is put on the photopolímer to prevent his adherence to the protection polyester and/or photolith. In case that layer is acidentally removed when of the retreat of the protection polyester, the plate should be wasted.

Finally, the protection polyester is the last layer of the cliché, which has the function of protecting the photopolímer against risks, dirtiness or any other damages that can happen during the plate transportation. That layer is  removed just at the moment of the plate exposure. You can find the protection polyester easily, because unlike the base polyester (shiny), that is opaque and it loosens himself with easiness.

Photopolímer cliché used in the flexografia. Source: Magazine Conversión.

 

Liquid Photopolímer

It is a relatively recent technology that uses specifics materials and equipments. For its production, a viscous resin is used, allowing to vary the thickness of the cliché easily.

The process consists of putting the photolith on the plate of glass of the recorder, with the layer upward. On him a protecting film of polyester is put, avoiding contact with the resin. The resin is "spilled" on those components until completing certain height, when then the base polyester is applied.

Notice that in that process the cliché is set up in the inverse sequence to the conventional one. However, the exposure of the photopolímer is accomplished on both sides simultaneously. The main exposure makes the polymerization of the image areas, while the exposure by the verse creates the height of the non-image areas.

After that stage, the plate follow to the wash of the cliché with detergent solution to remove the non-polimerized resin and the wash with water to remove the solution detergent. A subsequent drying with hot air and the post-exposure completes the cycle and allow a perfect polymerization of the cliché.

Bibliography

ABTG. Flexografia: um carimbo muito caro?
ROTOFLEXO & CONVERSÃO. N32 Julho/agosto/setembro 2005.
ROTOFLEXO & CONVERSÃO. N34 Janeiro/fevereiro/março 2006.
SENAI. Tecnologia de Flexografia, 2002.
TOLEDO, Osvaldo de. Impressão em flexografia: Manual de treinamento, 2006.