Flexography Printing

Flexography printing is a process, which adopts direct rotary printing using resilient flexible relief plates typically made from rubber or a photopolymer. The process is an updated, more versatile version of letter press that can be printed onto almost any type of material including plastics, metallic films, cardboard, cellophane and paper. Because of the range of materials flexography can be used on, it is a process that is most commonly used in food packaging, where it has replaced rotogravure and offset lithography printing techniques in some cases, mostly due to its cost effective nature compared with those of the other two printing methods over budget printing runs.

Plate production:

Analogue:

The typical flexographic printing plates are created with a flat polyester base that is coated in a photopolymer layer. This layer is sensitive to ultraviolet light (UV) and when exposed hardens. The steps involved with creating plates are first; creating a ‘floor’ that is set by exposing the back of the flexography plate to UV light, this creates the base of the relief. Next a film mask is placed over the top of the photopolymer layer, and pressed down using a vacuum. The photopolymer layer is then flood exposed to UV light, resulting in a hardening or technically named ‘ cross-linking’ of the two areas of the photopolymer layer not covered by the mask. Finally the mask is removed and the plate is placed in solvents to remove the un-exposed areas of the photopolymer layer, which were covered by the mask, which results in the forming of a printing image. The plate is then left to dry which can take up to several hours. The final result is the raised textured rubber relief plate, ready for printing.

Digitally produced:

Following a similar process as the analogue process, however the plate has a UV- opaque mask layer coated over the photopolymer layer. A digital imager selectively applies this mask layer with a high power laser imaging head t from an image mask that is opaque to UV light in non-treated areas of the plate. Once the mask is made the processing of the plate follows that for conventional flexography plates, except that there is no need to use a vacuum to ensure a perfect contact between the mask and photopolymer layer since the mask layer is combined with the photopolymer layer. Solvents are then finally added to remove any unexposed areas of the plate.

*Close ups of the relief plates

Printing process:

The created flexography plate is then mounted on a flexographic press cylinder using a sticky adhesive layer such as double sided tape or foam. The relief image plate must be mounted in a precise registration on the cylinder to assure perfect alignment when printing.

On the printing press the impression cylinder presses the printing stock (being paper, plastic, card etc) into contact with the flexography relief plate that is attached to the press cylinder. The impression cylinder sets a constant printing pressure that ensures contact between the relief plate and the printed material.

The ink is controlled in the flexographic printing process by the inking unit. The inking unit can be either a “Fountain Roll system” or “Doctor Blade System”. The Fountain roll system is a simple old system yet if there is too much or too little ink this system would likely not control in a good way. The doctor blade inside the Anilox roller uses cell geometry and distribution. These blades insure that the cells are filled with enough ink.

Due to the rectangular printing plate creating a gap or “seam” when wrapped around the cylinder, the rotations create a vibration on the impression cylinder, which is called a “press bounce” or “cylinder bounce”. This vibration is a disadvantage as it restricts the printing speed, if un restricted the registration and other printing errors would occur, however there are a number of techniques used by the technicians to ‘stagger’ the seam so faster printing can be used.

*Image above shows a technician electronically aligning the plates on the cylinders for precise registration. The offset of plates can also be seen in this image, this is used to reduce the 'bounce' during the rotations of the cylinder during printing.

The ink roller called a “ anilox roller” applies ink to the raised parts of the flexography plate which is then transferred to the printing material. The anilox roller has ‘cells’ that carry a specific amount of ink to the plate. The number of cells can vary according to the type of print job and quality print wanted.

Press Types

Flexographic printing is a process done on a rotary web presses. The presses can be divided into three main categories: stack press, in-line press, and common impression cylinder press.

Stack Press: The stack press has separate printing units stacked vertically and each printing unit has its own impression cylinder. This was the first type of press developed for flexography. It is difficult to hold registration between multiple colours printed on stretchable materials such as cellophane using the stack press, even with web tension controls. It is best suited for heavier gauge substances, such as thick paper products, that will not stretch or for print products that do not require good registration.

In-Line Press: Like the stack press, the in-line press has separate printing units for each colour and each unit has its own impression cylinder, but they are arranged in a horizontal configuration just like a rotary offset press. Because of the distance between the printing units, problems with print registration can occur. Tension control equipment are used on the press to hold proper registration between multiple colours. The wider in-line presses are used for printing large items such as corrugated cartons and smaller width webs are used for printing labels at high speeds.

Common Impression Cylinder Press (CIC) : Instead of the printing units being independent from the others, they are all grouped around a common impression cylinder. Substrates do not stretch as they move around the impression cylinder so the common impression cylinder press is a good choice for printing on substrates such as thin plastics that would normally stretch when used on other types of presses. This type of press provides for better registration of multiple colours. Some CIC presses have impression cylinders that are as much as 8 feet in diameter which allows for as many as 8 stations to be placed around the cylinder. The only drawback of the CIC press is that they can only print on one side of the stock or printing material.

Flexography printing inks:

Due to the demand and nature of the flexographic printing process, the printing jobs require and determine different inks fulfil different jobs.

The commonly used types of inks in flexography are:

Water based: Water based flexo inks dry through evaporation and absorption on paper. This evaporation requires a greater amount of energy to dry the ink. Coated papers may be used to control the absorption through the paper. Due to the speed of the presses and volume of inks consumed daily a pollution control system maybe be necessary, especially if the printer is using solvent based inks. A problem with using water-based inks are that the products cause problems when being recycled due to these inks.

Solvent based: These inks contain colorants, which may e pigments and soluble dyes along a binder and various solvents. Both Solvent based and water based inks commonly contain various types alcohol as the primary solvent or drier. Alcohol rapidly dries through evaporation. The inks may also contain glycol ether and/or ammonia, which accelerate drying.

U.V. Curable: UV flexo inks are commonly used for topcoats and lacquers and are responsible for many improvements in image quality of flexographic printing. The uses of UV curable coloured inks are rising within the flexographic printing industry. Water based or UV curable inks may not be an option for some printers due to the substrate being printed or design of the product.

Flexography has an advantage over lithography because it can use a wider range of inks, water based rather than oil based, and can print on a variety of different materials like plastic, foil, acetate film, brown paper, card board, cellophane and paper stocks. So this process is more commonly used for creating corrugated bags, flexible packaging like shopping bags, cartons, adhesive labels, envelopes, wallpaper etc. Because of the flexing nature of the plates used, a wide range of un even surfaces can be printed onto, unlike with Lithography where only smooth flat surfaces can be printed. Flexography inks , unlike those in lithography, have a low viscosity. This allows for faster drying and faster production, which results in lower costs.

The disadvantages to using Flexography are that the plates do not lend themselves to being printed with halftones in a design. The inks used also have a disadvantage, as they do not have an opaque nature. This is because a deposit of ink onto the surface of the material from the rubber printing plate. This creates a resulting ink shadow around areas when printed, especially evident with printed text.