When Kodak started to work with Maxtone, the focus was on the individual halftone dot. If a media representative determined the minimum dot value to be 5%, it was understood that a 48-micron opening in the mask was required to expose a dot that would develop to full relief height and withstand impression on a printing press. However, testing showed that even though the minimum AM dot was clearly 48 microns in size, in many cases, Maxtone at 48 microns would not properly develop.
Maxtone at confirmed minimum dot size fails to develop
The reason for this is that you cannot consider exposures only at the level of an individual halftone dot. What matters is the overall energy that reaches a square area of the flexo plate. A minimum dot in a full AM grid is part of a minimum overall level of energy in a given area required to activate the floor and neighboring dots. When Maxtone is applied, it makes each individual dot at the minimum dot size, but it also removes neighboring dots from the grid. Consider a situation where Maxtone has removed half of the dots to create a tone area that is half of the minimum dots to properly render tone values in lighter areas.
Full AM 5% and Maxtone 1%
Note: In the Maxtone area, only 20% of the UV energy reaches the plate, compared to the full AM area.
What is occurring in the Maxtone area is that much less of the UV energy reaches the plate surface, compared with the full AM area. There is insufficient energy to activate the floor and the individual Maxtone dots. What happens is that the flexo plates require a threshold energy limit to be reached before plate material starts to polymerize. By removing dots in a Maxtone grid, you are removing energy, and a point will be reached where the exposure falls below a critical energy threshold. In the Maxtone at confirmed minimum dot size fails to develop image above (see first image in this topic), you can see that the only place Maxtone is developing is up against the outside rule. In this case, Maxtone is benefiting from the extra energy that spills over from the exposure of the outside rule. However, the dots in the interior of the area are isolated, and have insufficient energy to develop properly.
HyperFlex was developed by Kodak to replace this missing energy, in order to activate the plate and bring it up to the energy threshold that allows Maxtone to develop.
Maxtone at minimum dot size, with added HyperFlex, reaches energy threshold and develops
Maxtone without HyperFlex (left) and Maxtone with HyperFlex added (right)
HyperFlex replaces the missing UV energy caused by removing Maxtone dots from the grid
The image above shows a simulation of how extra energy is directed to the floor by HyperFlex, to maintain the energy activation threshold, as dots are progressively removed from the grid into the highlights.
HyperFlex is a light valve technology. Small openings are made in the mask to allow extra UV light to hit the floor between the halftone dot exposures, but they are kept small enough that the light valves do not themselves cause dots to form on the floor of the plate. The key is to make light valves large enough to allow for the maximum amount of extra activation energy, but small enough that they do not cause any visible features on the plate.
When Maxtone size has been established, you must make test plates to fingerprint for the maximum size of HyperFlex that can be applied. For example, you might determine that a 3-pixel HyperFlex causes no visible artifact on plate, but a 4-pixel HyperFlex causes a tiny artifact on the plate. In this case, you would choose to use HyperFlex at 3 pixels.
Left: floor is clear with 3 pixels of HyperFlex; right: 4 pixels of HyperFlex is causing an artifact on the plate floor