There’s a world of difference between the direct laser engraving (DLE) of just a decade ago and the pinnacle of this application today. As the expression goes, “This isn’t your grandfather’s rubber printing plate.”
Laser engraving technology has continued to change in pursuit of the perfect printrun. However, the optimal printrun today isn’t just the application of newer laser technology. It comes from a combination of elements—from file handling to dot manipulation, from engraving materials to fingerprint testing.
The Beginning of ITR Laser Engraving
Why is in-the-round (ITR) laser engraving important to the flexo industry? ITR laser engraving was initially developed to solve several inherent problems with plate mounted images. Plates are difficult and often take a skilled craftsman time to mount accurately.
Plates have additional drawbacks:
- Seams between mounted plates produce “bounce” that can vibrate the entire press
- Plates can lift off the mounting surface during high speeds
- Even with current image modifying software, there can still be print distortion caused by bending a flat engraving over a round press cylinder
Early DLE required the production of an image negative, scanning of the negative, then laser cutting the plate. This was time consuming. Costs ran high and quality was suspect.
DLE sleeves don’t have seams. The engraving process takes place directly on the curved surface of the sleeve (ITR), eliminating distortion from bending the flat plate. Sleeves typically have a much longer press life. And the time to go from digital graphic files to ready-to-run sleeves has been dramatically reduced.
All that is great, but the real excitement comes from what the newest DLE machines can do in achieving the perfect printrun.
3D Dot Control
DLE uses a precise laser to create an image directly on the round rubber surface—No post-processing steps are required. With the latest lasers, DLE operators have the ability to shape dot shoulder angles, relief depths and below-surface heights. This is essentially controlling all the critical dimensions of every dot of any image.
With the new DLE technology, the shoulder angle can be just about anything desired. The average shoulder for DLE is 75 degrees to 80 degrees in the highlight and 90 degrees in the shadow. Sharper shoulders produce sharp print. The step provides a precise dot surface to accept the ink so it won’t pool against the substrate.One of the most significant advances in DLE technology is the ability to control image dot height. This is critical to applying just the right amount of ink under pressure on the press. Old TIF (Tagged Image File) images had the same shoulders, whether positive or negative dots. New lasers can undercut the highlight dots below the surface. Otherwise, the engraved dots would have to be at one uniform surface and cut-backs would need to be applied to the dots. The right reduced height can produce clean highlights while the same pressure generates clean solids.
Testing & Fingerprint
A good DLE operator knows the ideal engraving needs to control every parameter of every dot on the sleeve. While operator experience makes a huge difference, running a test sleeve on the customer’s press is the sure way to identify the specific dot structure best suited to that press, ink, press pressure and intended substrate.
Customers with a lot of tones and blended colors in their art often opt for a test sleeve first. A test sleeve is produced to fit their press and coverage requirements. With a range of various “cuts” on the sleeve, including relief heights, a quick test on press determines the ideal engraving profile.
This data becomes a sort of fingerprint for that particular press and substrate—and the parameters for the ideal engraving of all subsequent sleeves ordered for that press. The laser can then be dialed in to get the best solids and halftones. The final engraved sleeves will have the right dot shoulders and support of positives (line) that won’t fold, roll over or wear out prematurely.