Other methods of high-volume/long-run printing are Flexography and Rotogravure. Both are similar to Offset Lithography in that they are CMYK (plus PMS Spot colour) based. The predominant difference is how the inked image transfers to the substrate. GSM takes a look…
Flexography
Flexographic printing (‘Flexo’) is a form of high-speed, high-volume, web-fed relief printing, meaning the image area is raised.
Like offset lithography, flexography also uses printing plates. However, flexo plates are rubber or plastic and feature a raised image area. During printing, the raised area on the plate is inked and directly contacts the passing substrate. There is no offset cylinder or transfer blanket.
Flexo is particularly effective for printing on non-porous surfaces such as plastic film and metal foil. For this reason, it is mainly the packaging industry that uses Flexo to produce confectionery wrappers, convenience foods and plastic bags of all types. We can also use Flexo to print onto corrugated boxes and produce wallpaper.
Rotogravure
Rotogravure (‘Gravure’) is a form of high-speed, high-volume, web-fed intaglio printing, meaning the image area is recessed.
The image is engraved directly on a metal impression cylinder instead of a plate. As the impression cylinder spins, it is inked and then scraped clean by a blade so the ink sits in the recessed image area only. The ink is transferred directly onto the passing substrate; there is no offset cylinder or transfer blanket.
Gravure can print onto paper and plastics and is an alternative to Offset Lithography and Flexography.
The advantage over the other two processes is that the metal impression cylinder is significantly more durable than plates. This means it can produce hundreds of thousands of high-quality impressions before any visible degradation occurs. Due to this high durability, Gravure printing is used instead of Web-fed Offset printing for very high volume, high-end magazines (often print runs in excess of 100,000). And, instead of Flexography for very high volume, high-quality packaging where print runs can go into the millions. The disadvantage is the initial set up cost, as the impression cylinders are significantly more expensive than printing plates to produce.
Flexography & Rotagravure Ink Station
Flexographic or Rotogravure printing presses look very similar to a web-fed lithographic press. Indeed the main differences between these processes are in the details—more specifically, of what happens during the ink transfer process. The image below shows a simplified version of an ink station on a Flexographic press. An ink station on a Rotogravure press is not dissimilar—but with some notable differences (see point 5 below):
- The substrate, which could be paper, plastic or foil, is fed from a continuous web at high speed. The photo (above right) shows the magenta ink station on a small scale flexo-press—the substrate is plastic.
- The substrate is guided by an Impression cylinder.
- Ink is fed into a reservoir tray. An Anilox ink cylinder, which is partly submerged in the ink reservoir, spins, picking up ink—you can see the ink tray in the photo (above right).
- The inked Anilox cylinder contacts a Doctoring Blade which helps to even the ink spread consistently across the roller surface.
- Looking at the cross-section Flexographic close-up, the image area on the rubber or plastic printing plate (A) is in raised relief. You can also see this clearly in the close-up photo of a flexo plate (above left). The plate is wrapped around the Plate Cylinder (B), which spins at very high speed. As the plate cylinder spins, the printing plate contacts the Anilox roller, picking up the ink. The Ink will only pass onto the relief area (C) on the plate. By comparison, looking at the Rotogravure cross-section close-up; Rotogravure does away with the plate—instead the image is directly etched into a metal Impression Cylinder (D) which replaces the Plate Cylinder. In Rotogravure, an Anilox Cylinder is not required—instead, the Impression Cylinder is partly submerged in the ink reservoir. A doctoring blade will remove the ink from the surface of the Impression Cylinder as it exits the reservoir, leaving ink in the recessed (Intaglio) area (E) only.
- As the Plate or Impression cylinder spins, the inked image transfers directly onto the passing substrate.
- A second Doctoring Blade cleans the Anilox, or Impression Cylinder, as it circles back into the ink reservoir.
