These techniques improve the density and opacity of the solid areas (the 100% tones) by adding a texture on the surface of flexo plates. That way the ink holds better on the plate and transfers more uniformly on the substrate.
Without these techniques, it is possible that the 100% areas print with un-inked holes (this is called "pin holing") so that the solid ink density is lower than expected. Because of this, blends going to 100% can show tone reversal: the density of the tints in the shadows (for example 95%) is higher than the density at 100%.
Another and older technique to prevent this tone reversal is by screening solids. This can be done by using curves that adjust the 100% areas to a specific gray level (for example 100% is adjusted to a 95% screen). This can be done by creating a DGC curve with the Maximum Value set to 95%, and the Keep 100% option deselected. When this curve is used to RIP a job, solids or 100% tones will be screened at 95%.
In previous version of Imaging Engine, the effect of the Keep 100% option was different for standard screens and for screens using surface screening.
From Imaging Engine 18.0.1 onwards, the effect is the same for all types of screens.
This change also has an effect on PressSync curves. This is explained in another article, see:
Behavior of the "Keep 100%" option in Imaging Engine 18.0 and older
When using a Standard Screen
When using a screen without surface screening, the Keep 100% option behaves as explained above:
Selecting Keep 100% keeps the solid areas as they are (solid and unscreened).
deselecting Keep 100% screens the 100% tones in the job to the gray level set as Maximum Value.
When using a Screen with Surface Screening
When using a screen with a surface screening technology, the Keep 100% option has no effect: the 100% tones in the job will always be screened with the surface screening pattern, whether Keep 100% is selected or not.
Behavior of the "Keep 100%" option in Imaging Engine 18.0.1 and newer
From Imaging Engine 18.0.1 onwards, the Keep 100% option behaves the same way for all screens (with or without surface screening technology): deselecting Keep 100% screens the 100% tones in the job to the gray level set as Maximum Value.
Consequences of the new behavior
When an adjustment curve with a Maximum Value set below 100%, and Keep 100% switched off is applied to a screen that uses surface screening, the solids in the job will be screened to a gray level equal to the maximum value. In this case the surface screening pattern will no longer be used on the solids.
Appendix: Esko Screens that (can) use Surface Screening
These are static screens which use a line pattern in the solids. They use the dot mnemonic prefix "GVY" (for example screen GVY3).
These are screens that can be customized with the ScreenManager application. They use the dot mnemonic prefix "SCR" (for example screen SCR01). These screens use surface screening if the Surface Screening effect is enabled in ScreenManager.
(Full) HD Flexo Screens
These screens can be created with the HD Flexo application. They use the dot mnemonic prefix "HD" (for example screen HD01).
HD Flexo screens typically use surface screening, and the full name of a particular HD Flexo screen indicates the type of surface screening it uses. For example: the "HD Flexo C25 TPH_SD03 MC16P_FADE65" screen uses the "MC16P" surface screening pattern, that fades into the screen from 100% to 65%.
These screens can be created with the Printing Control Wizard application. They use the dot mnemonic "CRS" (for example screen CRS01). You can see whether a Crystal screen uses surface screening in the screen information displayed in Curve Pilot and the Printing Control Wizard.