What advantages does a closed-cell engraving have over an open-cell engraving?
BATISTATOS: A strength of closed-cell engravings—less than 2.5 bcm and more than 12 bcm—is that they are most repeatable. Our approach to customers is to be upfront and honest. Instead of claiming there’s only a few different engravings that will solve everyone’s problems, we promote different technologies for different situations. In my opinion, the ideal set of anilox rollers will contain different technologies from 60-degree hex cell to Z channel to Ultra X.
HARVEY: The closed cell needs to be qualified and understood. So, let’s understand some basics. If we consider and anilox with 300 lpi as an engraving, 1 inch equals 25,400 microns and 25,400/300 = 85 microns per cell, with single wall and cell walls of approximately 5 microns. Similarly, 300 lpi x 3.2 bcm equals 16 microns deep (ratio 5 opening 1 depth), or 300 lpi x 9.0 bcm = 45 microns deep (ratio 1.7 opening 1 depth). Therefore, it is not sensible to make a generalization statement.
What we can say is that closed cells have a stronger, more stable structure and formation. This is proven to be more resistant against score lines and adopted by some press manufacturers. It is important to understand that the laser beam itself is a circle and not a hexagon.
The best and most consistent mathematical pattern through nature is the 60-degree honeycomb. This pattern is created by pushing the cell walls consistently to create the hexagon shape, allowing the surface to be polished flat to create a smooth surface in the hexagon shape for the doctor blade to glide over the top.
As a buyer, you are looking for the following from the performance of your anilox roll: consistent and repeatable quality, consistent and repeatable transfer, long life and minimal cleaning costs.
A closed cell with optimum volume is better on those stated points, compared to open cells.
WOODARD: Closed-cell engravings offer the printer more control over ink transfer and greater wall support in screen and process applications. However, depth-to-opening ratio is very critical when it comes to engraving quality and transfer. Some printers are apprehensive to use open cells. They achieve success with standard 60-degree hexagonal cells for certain applications and 70-degree or 75-degree extended or elongated cells for applications requiring greater release.
POULSON: There is a more controlled transfer of inks and coatings. This increased control will improve graphics applications. The enclosed 60-degree hex is proof of this, as that angle helps ink transfer at a higher percentage rate and the hexagonal geometry allows for thinner walls, given that it has 15 percent more cell cavity than the 45-degree quad and other geometries.
MIDDLETON: The closed-cell engraving, particularly the 60-degree hex cell configuration, has been the industry standard in flexographic printing due to its uniform distribution of ink. Depending on the type of application, the closed-cell design does provide a higher quality of print.
RASTETTER: Pamarco’s true closed-cell engravings are the ThermaFlo 60-degree and EFlo 75-degree. A closed-cell engraving, like the ThermaFlo 60-degree, enables the ultimate degree of ink control at varying volumes, so it can be used for the highest plate screen counts while maintaining uniform ink delivery across the surface of the web. The construction of an extended cell engraving, such as EFlo 75-degree, provides a cell that delivers a higher ink transfer for applications where high solid ink density is required.
BEESON: When people talk about a closed-cell geometry, they instantly think of a 60-degree engraving. In reality, there are so many different types of closed-cell engravings.
Closed-cell technologies like the HVP (High Volume Process) and the GMX by Sandon Global have outstanding applications. HVP is a fantastic product for increasing the print latitude of one anilox specification. It is utilized in printing Pantone colors, vignettes and white inks that need a high volume. Often, print definition helps create smooth ink delivery. The extended cell of the HVP reduces micro foaming, giving a highly improved transfer to the plate, which reduces pinholing on difficult substrates. When dealing with white ink and large areas of Pantone colors, this is vital, as it can be the difference between a customer rejection or acceptance.
Using elongated cells (HVP), ink collection time is greatly improved. It’s not all about the delivery to the plate—The improved collection is key.
GMX is a proprietary closed-cell technology to Sandon Global, designed for large particle applications such as haptic, glitter or pearlescent. Its unique geometry stops cell blocking of large particles, allowing easier cleaning.
Closed cells will always have a place in flexography as much as open cells do. It’s all a matter of application and what the customer needs to achieve.
What type of engraving works best for high-speed solvent process and why?
BATISTATOS: To accommodate higher speeds, we need the same coverage at the lowest possible bcm engraving. Normally, where you would run a 2.5 bcm, run a 1.9 bcm or 2 bcm with an ink that is designed for stronger and better drying capabilities.
HARVEY: The anilox roll and the engraving is a fixed solid cylinder, or sleeve, that rotates around a fixed central point. The ink, however, is a liquid—a fluid substance that is affected by the forces applied through the physics laws of rotation. Liquid ink is driven backward within the engraving, the greater the velocity/speed of the rotation of the anilox. Therefore, the reality is that all cell-based anilox rolls have back walls that the ink will drive toward and create a lift. Long cells have more ink to drive back that will create an increased ink lift.
High-speed presses suffer from two issues:
- Misting, which is created by the ink raised from the anilox surface being shaved off by the doctor blade. Eliminate the ink lift and you eliminate the misting. From an anilox engraving, all closed cell-based options will risk the misting defect
- Pinholes/low density or opacity, which are created by excessive air entering into the ink. The faster the rotation, the less time the air within the cells has to get removed from the cells and refilled with ink within the doctor blade chamber. Eliminate the aeration of the ink and eliminate the pinholes
One technical answer is a GTT open wave that has the optimum depth-to-opening dynamic. Here, the curve and the angle from the GTT is adapted to allow the ink to flow effectively enough to be replenished in the doctor blade chamber and eliminate the pinholing; while, at the same time, not restricting the ink flow with back walls that create the lift from the surface, resulting in misting.
WOODARD: We find that a conventional 60-degree hexagonal engraving, produced with the finest fiber optic, multi-hit technology, produces the most-consistent engraving with the cleanest, sharpest cell walls. We achieve this with Fibretronics engraving technology from Applied Laser Engineering.
POULSON: At the moment, 60-degree hexagon seems to be used the most, but our new XCAT geometry will support the speeds needed with open-cell geometry. The open channel allows ink to travel without any resistance from added cell walls that will restrict ink flow.
MIDDLETON: The 60-degree hexagonal pattern is recommended for high-speed solvent process and expanded gamut (EG) printing because of the efficient cell design. There is also a demand for the stepped, long hexagon pattern, which offers a more consistent laydown and can be engraved up to 1,200 lpi. In some circumstances, we have seen a decrease in doctor blade wear, ease of cleaning and a better release with this design.
RASTETTER: The industry standard is a 60-degree engraving like the ThermaFlo. But advancements mean that there are many ways of creating a 60-degree engraving. Modern laser technology now allows for production of a cell with the essential flat bottom, uniform wall structure. If the manufacturer maintains the 25 percent to 30 percent depth-to-opening ratio, the engraving will provide a high degree of consistent ink transfer for the longest time period by reducing wear and decreasing the cell’s propensity to plug.
BEESON: The Xpro by Sandon Global is a 61-degree elliptical engraving with a semi-channeled, open-cell geometry. The elliptical engraving is designed to work at high speeds to stop ink starvation, retain optical density and preserve a clean highlight dot. We have seen 2,600 fpm with customers and no ink starvation or loss of quality. With a semi-channeled wall, the ink allows the plate to be lubricated and stops the starvation that can occur with incorrectly specified standard 60-degree engravings. Robust walls with micro finishing support the doctor blade to give a clean wipe, resulting in more constant and accurate results every time.