Inkjet offers new CTP approach

A new system allows the black mask layer in flexo CTP to be applied by inkjet. Andy Thomas reports

Black mask ablation has become the dominant way of imaging digital flexographic and letterpress plates. The plate is pre-coated with a black layer which is burned away (ablated) in the image areas by a laser, followed by conventional plate exposure and processing. 

But at Labelexpo Europe a start-up company called Digiflex demonstrated a new approach which uses a high definition inkjet to print the black mask directly onto a standard flexo, letterpress or analog dry-offset plate.

There are three main elements to consider when using inkjet ink as a mask to prevent UV exposure light from reaching the plate:

• Optical density of the ink in the UV range (over 4.5)
• Smooth operation of the print-head, eliminating any nozzle clogging or misdirection of the droplet
• High print quality on top of any non-porous flexo plate, eliminating any ink clustering or bleeding

The desired optical density is achieved by adding UV absorbent elements to the ink in order to ensure high UV light blocking.  Since no solid particles are used in the ink formulation, there is no source for nozzle clogging, and Digiflex says smooth jetting ‘is ensured throughout the system’s lifetime’.

The main obstacle preventing earlier pioneers from reaching the high print quality required to print a negative mask on top of the photo-polymeric plate was that inkjet ink is very dilute and non-viscous. Jetting it on top of a non-porous substrate often caused print quality failures.

DigiFlex overcame this difficulty by developing its own unique bi-component reactive ink approach.

The flexo plate is coated with a special primer layer which contains a reactant 'A'. The ink, jetted on top of the plate contains a reactant 'B'. When the ink droplet hits the plate surface, chemical reactants 'A' and 'B' both react instantaneously to cause the gelation of the ink droplet.  The resulting highly viscose ink droplet causes no print quality deterioration and behaves almost like a conventional offset ink. This mechanism ensures that the tiny ink droplet forms a minimal dot on top of the flexo plate, eliminating any ink spreading.

The DigiFlex system uses an advanced inkjet unit which jets ink droplets of 3.5 pL at 2880 dpi resolution.  Combined with a very accurate plate feeding table, this system achieves 180 lpi quality on the plate.

So how does it work? A photopolymeric plate is coated with a special thin coating via a lamination process. This process transfers the thin coating layer from a substrate onto the plate. The plate is then introduced into the system, and the RIPped separations are printed on top of the plate. The ink is dried in-line and the plate comes out of the machine. Printing speeds are from one m2/hour up to a possible two m2/hour. Once the plate mask is ready, the standard processes of exposure, wash-out, dry-out and development are performed.

The benefits

According to Dr Moshe Frenkel, founder and CTO, DigiFlex, there are many benefits to the process. 

Most importantly, no oxygen reaches the plate during the curing process. Exposing the plate to UV light in the presence of oxygen at the top of the plate causes a round-top dot to form, and this shape can introduce stability problems during printing. A flat-topped dot shape ensures instantaneous set-up since the pressure on the plate has almost no effect on dot gain.  ‘Using the DigiFlex inkjet flexo CtP with its bi-component ink technology, a flexo plate with flat-topped dots is produced directly, enabling fast set-up time, with no dot gain,’ says Dr Frenkel.

Any analog plate can be used, including water washable, solvent washable, flexographic, letterpress or dry offset.

Pictured: A DigiFlex Flexojet system

This article was published in L&L issue 2, 2012