At the beginning of this century, new laser technologies emerged. YAG and diode lasers started to become common in the plate making process, at first only in mask ablation, but later also for direct laser engraving (DLE).
Several engraving system manufacturers picked up on these technologies and designed their systems around them. Direct laser engraving technology for flexo eliminates the costly outlay for multiple processing stages and equipment commonly associated with more conventional photopolymer sleeve/plate making technologies.
Flexographic engraving systems use single or multiple high-powered CO2 lasers or fiber lasers to directly engrave continuous sleeves or plates in a single step. The laser selectively ablates the non-print area of an elastomer plate or sleeve to produce a print-ready surface without the use of exposures or chemicals.
The Compounds
Charts courtesy of Carey Color Inc.
Rubber has always played an important part in the flexo industry. Going back to the last century, elastomer was mostly used in sleeve form to print background colors in flexible packaging as well as printing on tissue, wall paper and other materials for the decorative market.
There are several reasons elastomer did not really make its way into the classic flexible packaging and label markets until recently. Previously, engraved elastomer plates and sleeves were only suitable for line work and relatively coarse linescreens up to 85 lpi. This had to do with the limited elastomer compounds and the rudimentary laser technology of the day.
With the parallel development of high performance elastomers and more advanced laser capabilities, new levels of quality are possible. Current engraving technologies allow for resolutions of 4,000 dpi and higher (i.e. 5,080 dpi in printed electronics applications). Screen values of 200 lpi are used in everyday production throughout the elastomer plate and sleeve producers around the world.
Material Properties
Today’s high-performance elastomers have more to offer than just improved engraving capabilities. Most current elastomer printing forms are made from a compound called EPDM (ethylene propylene diene monomer). EPDM is a non-polar synthetic rubber compound that is extremely resistant to UV light and ozone. Due to its polarity properties, it also works very well with ketones, esters and alcohols, which are polar substances, making it a perfect fit for flexographic printing, not only with solvent-based, but also with UV and water-based ink systems.
A direct laser engraving workflow involves fewer stages, reducing the possibility for error. By eliminating steps and variables, we are eliminating failure points, thus making the process more consistent.
The following fact can be used as a rule of thumb: Matching polarity between ink and printing plate causes the former to migrate into the latter, causing problems with swelling, premature wear and cracking. Therefore, EPDM does not work with oil-based inks, gasoline and aromatic hydro carbons (i.e. benzene); these substances are non-polar. For printing applications using non-polar inks, such as offset printing, the market uses NBR printing blankets (nitrile butadiene rubber), which is an elastomer with polar characteristics.
Since EPDM-based printing forms do not swell with current flexo ink systems and are extremely abrasion resistant, it makes them a good choice in difficult printing environments, such as plastic cup, tube and aluminum can printing, corrugated or nonwoven applications. Surface properties, such as roughness and surface tension, but also compressibility, are designed to neutralize the feeding characteristics, which a lot of times are the cause of halo effects or mottling. The compounds are also optimized to control ink pickup and release, minimizing buildup.
Elastomer Sleeves vs. Polymer Sleeves
Cost, perceived supply chain problems and limited inventories have held back the use of elastomer sleeves in America, but new advances in imaging and manufacturing are bringing their inherent advantages to the attention of printers.
Elastomer sleeves are thicker and more durable, and can print two to four times longer than polymer sleeves. This eliminates the need to buy multiple plates or polymer sleeves for long runs and repeated, abusive short runs that require many cleanings and press cylinder mountings, and this leads to both cost savings and cost predictability in the long run. A typical comment from printers who have switched from polymer to elastomer sleeves is, “Elastomer usually costs less than polymer, and we’re only buying one set instead of two—so the savings are great, and our print results are more consistent.”
Further cost savings are realized because elastomer sleeves eliminate the need to purchase expensive cushion adapters and replace them in the future.