This blog was written and submitted to us by Torben Wendland
The back exposure of a flexo photopolymer printing plate is creating the base (or floor) of the plate. The height difference between plate surface and floor defines the relief depth.
Which factors are influencing the relief target?
The required relief depth is depending on several factors. All of them are leading to a target relief depth:
- Print application and substrate: Typically, print applications which require less impression (e.g. narrow web) can use a minor relief depth compared to applications with higher impression or uneven substrates (e.g. corrugated post-print).
- Plate type and thickness: On thicker plates, relief depths of several millimetres are common. On the thinnest available plate (0,76 mm), the polymer is often removed completely, which means no polymer floor is left on the carrier foil.
- Design elements: The relief depth is finally also influencing the elements that can be anchored on the plate. At a lower relief depth, smaller elements like finer dots or lines can be stabilised.
How is the back exposure length determined?
The length of the back exposure needs to be adjusted so that the right floor thickness is achieved. Typically, this exposure time is determined by a step test. Various exposure steps at rising exposure times are exposed on the same plate. With each step the exposure length is increased by a certain increment (usually 3-5 seconds for thin plates, 10 or more seconds for thick plates). After complete processing of the plate, a „staircase“ of different floor heights can be found on the plate. These height of the different steps are measured with a thickness gauge. The required back exposure length can then be derived from the results.
How is the back exposure done in practice?
Light intensity of the UV-A tubes is influenced by the temperature. Before starting the back exposure or doing a back exposure test, it is recommended to preheat the tubes. Otherwise, the relief depth might differ from the previously tested results.
Depending on the exposure equipment, the back exposure of flexo printing plate procedure slightly differs. If the light source is located above (in the lid of the machine), the plate needs to be flipped over before performing the back exposure. For glass bed exposure units, back exposure is done from below – no flipping is needed. In this case, the back exposure is usually done after imaging, in combination with the main exposure.