Reproducing highlights is a key problem in flexography, as there is often huge dot gain in this area (typically 2% will print as 10-15%). Therefore having good colour depth and pattern free halftones in this area is critical.
The standard SGSS hybrid screening offers the desired colour depth, but even though it is stochastic, the FM part isn’t entirely pattern free. This isn’t a problem in offset, where the hybrid screening protects only the highlights under 2-3%, and the slight pattern of the SGSS FM part is practically invisible. However, with the huge dot gain of flexo, it may become visible and offensive.
Therefore SGSS offers a hybrid screening optimized for flexo, increasing the quality of the FM part at the expense of slightly decreased quality in the AM part. Using a principle that allows the centres of the halftone dots to slowly drift away when the percentage falls below a given level (typically 30-35%), reaching an entirely stochastic structure at 3-5%. Removing dots from this stochastic grid will become practically imperceptible, thus visible patterns will disappear almost entirely.
Offset hybrid screening
Flexo hybrid screening
