Flexography

flexographic print.

A flexo or cliché plate.

The flexography is a high-speed printing technique that uses a flexible plate with relief, that is, the printed areas of the shape are raised compared to the non-printed areas. The plate, called cliché or plate, is generally made of photopolymer (previously it was vulcanized rubber) which, being a very flexible material, is capable of adapting to a number of very varied printing media or substrates. Flexography is the characteristic printing system, for example, of corrugated cardboard and plastic supports. It is a method similar to that of a printer's stamp.

In this printing system, liquid inks characterized by their great speed of drying are used. This high drying speed is what allows printing high volumes at low costs, compared to other printing systems. In any case, for low-absorbent supports, it is necessary to use dryers located in the printer itself (for example, in the case of coated papers or UV varnishes).

History

The origins of flexographic printing date back to the 1920s and 1930s, with a machine known as the aniline press as it used inks based on this chemical. These inks were dissolved in alcohol, so the print was of a very poor quality and not very resistant.

After this, the MossType Corporation carried out a survey to decide the name that would be given to a new printing method, since the use of aniline in packaging for human consumption had been prohibited by the FDA (Food and Drug Agency)., for its acronym in English) for being harmful to health.

Three names were decided in this survey:

• Rotopack
• Permatone
• Flexography

Print process

Scheme of a flexographic printer.

Printers are usually rotary, and the main difference between these and other printing systems is the way the cliché receives the ink. Generally, a rotating roller made of rubber or other materials, such as polyurethane or urethane, picks up the ink and transfers it by contact to another cylinder, called an anilox. The anilox, by means of microscopic-sized alveoli or holes, generally formed by abrasion of a laser beam on a steel roller and with a chrome or ceramic cover, transfers a light layer of regular ink and uniform to the printing, engraving or cliché form. Subsequently, the cliché will transfer the ink to the support to be printed.

The number of inks that can be used ranges from one to ten, including different types of finishes such as varnishes (machine, high gloss or ultraviolet), plastic lamination and film stamping.

In the event that transparent materials are used, the use of white is essential, compared to offset, where white is obtained from the paper support, which is why four-color printing is used. However, in the case of flexography, the use of prepared colors (spot colors or direct colors) or pantone colors is very common. For example, if a product with photography is made, it is printed in eight colors, white, cyan, magenta, yellow, black (for the photo), black (for the barcode and text) and two more colors for make saturated backgrounds, among these may be gold, silver and/or the pantone of the company.

Applications

The labels of many products are usually printed using flexography.

The flexography process is characteristic for printing self-adhesive roll labels, which can be printed on paper, films and plastics. It is also one of the most used printing methods for packaging, from corrugated cardboard boxes, films or plastic films (polyethylene, polypropylene, polyester, etc.), paper and plastic bags, to the printing of napkins, toilet paper, etc. folding cartons, newspapers, etc.

Flexography is one of the most economical printing methods with respect to the final product, because it allows a greater number of reproductions at a lower cost.

Design for flexo

To design a flexographic package, a broad knowledge of printing procedures and their technical specifications is required. Because of this, it is a job that is mainly commissioned to graphic designers and packaging designers. First of all, the customer's needs must be known, what type of product is sold, if it is solid, liquid or frozen, then the substrate that the customer wants and the number of units that the package will contain.

Commonly, graphic designers have to be in charge from the production process of the graphic piece, to the separation of colors; these clear insights help to have an efficient, easy and profitable process. Prior to the production of the plates, different methods can be used to perform color tests, for example: Color Key, Cromalin, Color Check, Cromacheck, Dylux, etc.^{[citation required]} These proofs allow you to get very close to the final printing colors and are done to avoid errors, which can be very costly for the production process.

Technical aspects

The design process for flexo requires knowledge of several technical aspects, such as:

Tolerances and compensation percentages

Tolerances are measurements provided by printer manufacturers, which are used to determine the compensation percentages necessary for the production of the final artwork and color separation.

The compensation percentages, in turn, help to adjust the design at the time of color separation and that the necessary size is obtained when making the printing plates, although the shrinkage of the color must also be taken into account. the same in a percentage of 1.5 to 2%. These numbers do not apply to photopolymer plates.

Trapping

The trapping occurs when expanding a color and mounting it on another so that, at the moment of printing, if the plates move or if they are elongated by the wrapping or by the double-sided tape placed to stick them on the main cylinder, they do not move and thus they can cover the rest of the colors with the excess, forming an intersection of them. If the trapping is not done properly, white lines or gaps can be generated in the area where the colors join. The normal measure goes from 0.3 pt to 1 pt.

This procedure only happens in the case of flexography, since the printing plate is flexible. In the case of offset, trap is not done since the plates do not stretch and the registration is more reliable.

Positives and Negatives

“Positive” and “negative” refer to the relationship between the figures and the background of the design; that is, a positive indicates that the figure is printed and the background is not, while in the negative, only the background is printed. This printing nuance is important when adding fine elements such as lines and thin body text, as ink can change the way these elements appear in the final product.

If lines are used, care must be taken not to make them too thin: in the case of a positive, because the line risks disappearing; in the case of the negative, it is made wider than in the previous one because the ink filling in the interior spaces tends to decrease the thickness. For example, a line of 1.5 points, when printing on the support can be transformed into one of 1pt.^{[citation required]}

Point Size

Dot size is affected by the pressure the printer exerts on the substrate or print medium. When the design is carried out, it must be taken into account that in printing there is an increase of between 10 or 20% in the plots, since the force exerted on the film causes the dot to expand. That is, if a screen of 25% is sent, it increases to 35% or 40% in printing. These variations must be anticipated before making the design, in addition to not using many gradations because they tend to enlarge the point and, as a consequence, increase the thickness of the color or printed object.

Moire

Moiré is a problem of halftone interference, which causes unwanted lines or streaks of color to be seen, as a result of improper handling of the angulations or an erroneous pattern of the rosette.

Taking into account that the angulation of the anilox roller cells is 45º with respect to the material coil, the following values must be considered for printing the different colors:

If it is printed in a single color, it is recommended to use 45º frames, or in its own way, use a 30º difference between each color, the most used angles are:

• Cian: 105o
• Magenta: 45o
• Yellow: 90o
• Black: 75o

These values are the most common, but depending on the arrangement of the anilox, variations can be generated by adding or decreasing 7.5º.