The development of modern CI flexo
Modern CI flexo enables the converter to: increase machine speed, reduce both startup and process wastage, reduce solvent loss, substantially reduce the total volume of solvent evaporated, reduce energy in drying, simplify the web path, reduce changeover time, and make small jobs viable in addition to long run jobs.
Central impression (CI) flexo differs from stack or inline flexo in that it has a single large impression cylinder (called a CI drum) accommodating all the print stations (typically 6 to 10) around its circumference. The stations are arranged symmetrically on the left and the right of the central vertical axis of the CI drum.
The CI drum supports the web and reduces the threadle through path to 15-20m. A CI flexo press is, indeed, very compact.
The introduction of the chambered doctor blade
A development that has revolutionised modern CI flexo is the chambered doctor blade. Earlier, flexo stations would have an ink pan from which a fountain roller would pick up ink and transfer to the anilox, which would transfer a metered quantity of ink to the printing plate. The chambered doctor blade has eliminated the ink pan and fountain roller.
The compact, fully enclosed chambered doctor blade takes in the ink and transfers it directly to the anilox roller. This has made the ink stations compact, which enables the entire machine, including the CI drum itself, to be made more compact as the print stations can now be positioned closer to each other.
Making the CI drum smaller has reduced the mass of the CI drum and lowered the motive power required to rotate and control the CI drum. This has facilitated an increase in machine speed and a reduction in motive power consumption. The CI drum is now controlled by advanced direct drive systems.
The ink drums are now positioned fairly close to the ink stations and the ink circulation path is much reduced, which has cut power consumption for ink circulation. Solvent evaporation drops significantly, both reducing solvent cost and making viscosity control simpler.
Benefits in ink
Flexo inks average 8% more solid content (33%) than gravure inks so less solvent needs to be evaporated. The chambered doctor blade design combined with high solid inks means flexo has a drastically shorter drying energy load than gravure.
Flexo inks can be diluted by a single alcohol solvent, simplifying auto viscosity control. The alcohol chamber is kept centrally on top and helps easy viscosity control at all stations. Auto viscosity control helps in shade maintenance during run with steady and high machine and results in lower process wastage.
The chambered doctor blade, the short ink paths and the ease of circulation of solvent enables automatic wash up after job completion. This has significantly reduced solvent consumption in wash up and drastically reduced changeover times.
Gearless operation
Modern CI flexo is gearless and allows a vast range of repeats with any plate cylinder circumference from 400mm to 800mm. Servo control of anilox and plate cylinders on multiple axes allows precise pressure settings and alignment control in the print stations with inter-station interlocking of plate positions in pre-register mode.
Longitudinal and side-lay register controls are done with high precision and very short response time. In fact, response is almost instantaneous. Pre-register ensures the job is in register in barely 20 to 50 metres.
Improved measurement and management
Latest generation plate topography measurement, recording and co-ordination systems (measurement and recording off line and on line) have made perfect register possible from the second rotation of the plate cylinder. Wastage due to register setting and control is practically down to zero.
New colour management systems based on spectrometric measurements during proofing and matching of L-a-b values on line within Delta E (rather than relying on ink densitometry only) and advanced video camera systems combined with 100% defect detection systems now enable shade match within 25 rotations of the plate cylinder.
A job can now be set for register and shade within 20m. Even factoring in climatic issues, ink issues, anilox issues and plate issues, a job can be set in under 100m.