opener

Flexo post printing process

Corrugated box makers around the globe are able to exploit new markets by improving the quality of their flexo post-print. BOBST has been at the leading edge of this technology since the introduction over forty years ago of the FLEXO -1575, and today our flexo printing presses set the standard for what can be achieved with the offline flexo post-press process.

We have achieved this by researching and developing new systems, working with industry leading partners, and by constantly listening to the views of our customers.

BOBST flexographic printing machines cover a wide range of different production needs, from straightforward block print to the highest quality process work, along with offering the performance, versatility, and reliability which modern manufacturing demands.

Flexographic printing process

Flexographic printing is a method that uses a flexible relief plate to print onto a substrate, which may be a wood-pulp based, synthetic, or laminated material. The flexo post-printing offline process is used on corrugated and micro-flute boards, including uncoated, semi-coated, and coated substrates. Typical applications of the process are multi-color packaging and in-store displays.

In broad terms, a flexo printing machine of this type is used to print, varnish, dry, and quality check the sheet ready for further processing such as die-cutting and folding & gluing.

Today's flexo printing presses use photopolymer printing plates containing a mirror relief image of the required print. The raised areas on the plate have ink applied to them by an anilox roller and then transfer it onto the substrate.

Flexo printing presses and process description

There are three different architectures for flexographic printing presses, with offline machines normally having the print units laid out horizontally in a line, generally with a loader unit at the feed end and delivery at other.

In a modern flexo printing press, each print unit comprises of:

  • Anilox roller: a cylinder of highly engineered metal and/or ceramic, which is laser engraved with minute cells of a given angle, line screen, and volume to deliver the required fineness of print
  • Chambered doctor blade system: a device that delivers a measured amount of ink to the cells of the anilox roller
  • Rubber rolls: an alternative system to the chambered doctor blade, consisting of two rubber rolls in contact with each other. One roll sits in an ink bath and transfers the ink to the second which is in contact with the anilox roll. A negative blade may be added to the anilox roll to limit ink flow
  • Plate cylinder: which the printing plate is mounted upon
  • Impression cylinder: a sleeve that supports the substrate as the printing plate is pressed against it
  • Inking system: ink holding tank, ink pump with delivery and return ink lines, plus other elements to ensure the maintenance of ink supply and viscosity

Between the print units, driers may be installed so that subsequent colors can be applied to the substrate without merging into those previously printed. These driers may utilize hot air, infra red, or ultra-violet light, depending on the application.

During the printing process, the ink is pumped into the ink chamber of the doctor blade system. Two blades within the ink chamber, the barrier blade and the doctor blade, seal either end and confine the ink to the chamber while it is in contact with the anilox roller. As the anilox roller rotates, the cells in contact with the doctor blade system collect ink, and then any surface excess is removed as the roller passes under the doctor blade. As the anilox rotates, its surface comes into contact with the raised areas of the printing plates mounted on the plate cylinder, transferring the ink. The printing plate then rotates and transfers the image onto the substrate.

Process advantages

The flexo printing press offers users a number of advantages, not least that the most readily available inks are water based and therefore easier to work with and faster to dry. The manufacture of flexo printing plates is relatively straightforward and the process is also easily adaptable to a wide range of substrates.

BOBST expertise and knowledge

In recent years the development of new pre-press systems, doctor blade chambers, inks, inking systems, and anilox rolls has led to a growth in high quality graphics printing, allowing flexographic printing presses to compete against gravure and litho machines when it comes to the manufacture of fiber-board packaging.

BOBST flexo printing machines have been at the forefront of this movement, with the development of high precision feeding, vacuum sheet transport, highly effective inking and drying systems, and intelligent register control systems, allied to high running speeds.