What is flexo printing?
What is flexo printing and what can you do to optimize the quality and productivity of your flexo printing process? Read on to find out!
Applications of flexo printing
Flexo printing is a direct relief printing process in which the printing surfaces are higher than the non-printing surfaces. Flexo printing is used in the packaging industry and in corrugated board printing. Especially due to the safety of the mostly water-based inks, flexo is ideal for food packaging. In addition, flexo ink produces beautiful results on absorbent and porous substrates.
Although digital printing is also increasingly being used in the packaging industry, flexo is the most economical printing method, especially for large print runs.
New developments in flexo printing allow for enormous flexibility in color use, detailing, and graphic quality. Combined with safe UV-cured varnishes, flexo printing enables results that were previously only achieved with offset printed liners.
How does the flexo printing process work?
In the flexo printing process, the anilox roll is covered with a defined amount of ink. Then the anilox transfers it to the raised parts of the printing plate, which transfers it as an image to the substrate to be printed.
Construction of the anilox roller
The anilox roller consists of a cylinder of steel or aluminum, with an industrial ceramic coating. This ceramic layer is engraved with very fine holes, also called anilox roll cells. The number, size, shape, and depth of these cells determine ink volume and distribution on the printing plate. The specification of this is shown in transfer volume or in line grid L/cm (lines per cm). Bear in mind that the transfer volume will decrease through wear of the anilox roll (as the cells become shallower) but also by contamination of the cells.
The line screen determines the ink release on the printing plate. For example, line screens of 100-140 L/cm are used with a heavy ink layer, such as for solid color areas or block lettering.
A higher line screen of e.g. 200-240 L/cm gives finer details and is suitable for four-color printing. You can think of photos and detailed graphics here.
It is therefore not surprising that different line screens are required for each color within the same printing job. To meet the increasing demands on the graphic quality of packaging, the anilox technology has not stood still and each application now has its own anilox roller, within which various line screens are possible.
After applying the ink, a doctor blade scrapes the excess ink from the anilox roller. This leaves only the precisely determined amount of ink in the cells.
Then the anilox roller makes contact with the flexo printing plate which transfers it to the substrate as the printed image.
The quality of the doctor blades contributes to a good printing result. A combination of flexibility and sturdiness ensures consistent color distribution and prevents premature wear of doctor blades and therefore less accurate ink distribution.
The flexo ink is pumped from the ink bucket to the ink chamber. An ingenious system consisting of end seals and doctor blades ensures exact ink distribution on the anilox roller. Furthermore, it prevents ink loss from the ink chamber.
The design of the ink chamber is essential for productivity, safety, and graphic quality. Modern chamber systems are lightweight and guarantee easy cleaning and safe doctor blade changes. They also prevent ink loss and premature wear. In addition, there are systems that make changing ink between printing orders easier and more economical than ever.
Flexo printing plates, also called ‘relief plates’ or stereos, are usually made of photopolymer (elastomer or rubber are seen less and less often). The principle of the relief plate is that the image is higher than the area that contains no image.
The photopolymer printing plate contains a mirror image in relief of the desired printing image. After placing the printing plate on the cylinder, the anilox roller transfers the ink to the printing plate. Which transfers the ink to the printable substrate.
In order to maintain printing quality, it is essential to clean printing plates thoroughly immediately after each use. This prevents damage or drying of the ink. Careful storage is also necessary to prevent deformation of the printing plates and ensures that print quality is maintained.
Flexo printing inks
From the anilox chamber, a layer of ink is applied to the anilox roll. This flexo ink must maintain the correct viscosity, pH and temperature throughout the printing process. This is necessary to prevent smearing and insufficient drying.
Flexo printing – substrates
Flexo printing is suitable for printing on a variety of substrates, including:
- Plastic foil
- Corrugated cardboard
- Non-woven fabric
- Metal surfaces
This wide choice of materials makes flexo printing suitable for many products, such as packaging, displays, self-adhesive labels, plastic bags, wallpaper.
The safety of the inks makes Flexo very popular for corrugated board food packaging. Keep in mind that for consistently high print quality, the substrate to be printed needs to be clean, i.e. free from dust, fibers, and other contaminants. This is to prevent contamination of the anilox and ink.