Process

Flexographic printing uses a flexible relief plate to print onto a substrate, which may be a wood-pulp based, synthetic, or laminated material. When carrying out flexo printing and flat bed die-cutting inline, the printing process is directly followed by flat bed die-cutting of the sheet. The technology is predominantly found in the manufacture of packaging made from corrugated and micro-flute board, including uncoated, semi-coated, and coated substrates. Other applications of the process include the manufacture of in-store displays.

Flexo printing and flat bed die-cutting inline is particularly useful where complex die-cutting is required and/or high quality graphics combined with high productivity is needed, although the process is also often used for much less difficult work.

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.

A typical installation may include a loader unit, a feeder, multiple flexo printing units with drying and quality control, a flat bed die-cutting section, a waste stripping unit, and a breaker/palletizer. This configuration may be created by the addition of several flexo units and peripherals to a central flat bed die-cutter, or may involve the linkage of an offline printer to a flat bed die-cutter and the addition of peripheral units.

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.

The exact configuration of a flat bed die-cutter element of the line will vary depending on the application but, in broad terms, will consist of:-

• In-feed: Takes control of each individual sheet and registers it to the cutting tools using mechanical or dynamic registration
• Platen section: This is the heart of the machine, bringing the cutting die and cutting plate together under pressure. Held between them, the sheet of substrate may be flat bed die-cut or creased, depending on the application and substrate
• Stripping section: Internal waste, and sometimes side and rear trim, is removed using an upper stripping tool, central stripping board, and, in some tooling arrangements, lower stripping pins
• Delivery: the delivery creates either a pile of full sheets or a layer of blanks attached to each other by nicks. The front trim of the sheet may be removed at this stage
• Peripheral units: such as breakers and palletizers

By flexo printing and flat bed die-cutting inline users gain a number of advantages, not least that the process requires a single make-ready while ensuring excellent print quality and the ultimate in die-cutting precision in one pass.

Additionally the most readily available inks are water based and therefore easier to work with, faster to dry, and are more sustainable. The manufacture of both the flexo printing plates and the die-cutting tools is relatively straightforward.

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 graphics printing.

BOBST equipment for flexo printing and flat bed die-cutting inline has been at the forefront of this movement with the development of high precision feeding, vacuum sheet transport, highly effective inking and drying systems, on-board register and print quality control systems, and high running speeds, all of which contribute to high printing quality and high performance.