Fluorescent lamps and light-emitting diodes (LED) are different, but the same plate chemistry is expected to work flawlessly. UV LED exposure systems deliver energy to the photopolymer differently. LEDs emit UV radiation at a very different level of intensity (15 to 20 times greater) and with a narrower emission spectrum. Figure 4 illustrates the differences.
Another major dissimilarity is that flexographic plate making UV LED exposure systems generally scan over the plate during main exposure, instead of applying a constant exposure on fluorescent lamp systems (see Figure 5).
Figure 6 compares two samples of the same type of flexographic plate, exposed on an LED system and a bank system. It shows how much direct exposure one specific image element on a plate receives during each of these processes. LED systems typically scan, and thus, the image element gets exposed several times during the process, however at a significantly higher energy level than on conventional systems. Consequently, the exposure gets interrupted several times and the image element is in “the dark” between passes.
Photopolymer printing plates which are currently used may be exposed using these two parameters:
- Bank exposure: 20 mW/cm2, 420 seconds (constant) = 8,400 mJ/cm2
- LED: Output 300 mW/cm2, five passes, 20 seconds (per pass), cycle time 26 minutes = 30,000 mJ/cm2
The photopolymer undergoes two totally different processes of crosslinking. This is why so much effort is put on developing an optimal “recipe” (set of exposure parameters) to let LED-exposed plates match or exceed the characteristics of the same plate exposed with a bank system.
In this example, the LED exposure delivers about 3.5 times the amount of energy (30,000 mJ/cm2 vs. 8,400 mJ/cm2) to the plate. However, due to several passes and long gaps between the exposure cycles, it resembles the bank-exposed plate’s performance. For non-optimized photopolymers, using this unique exposure process can require compromises in productivity and quality.
While one can achieve very short exposure times using the maximum UV output, tradeoffs in quality may occur. Often, the combination of high output and short exposure time leads to larger minimum dots, pronounced shoulders of printing elements and cupping, which all can contribute to less latitude on press and lower print quality. This challenge often requires LED exposure times to be longer in order to prevent the quality issues mentioned above.
Referring to our real-life example of how to cook a steak, LED resembles a blowtorch, while bank exposure may be compared to slow cooking. The challenge would be to achieve a crisp surface, but also a sufficient cooking of the center—in flexographic terms, forming fine elements and surface structures while photopolymerizing the bulk of the material sufficiently for stable shoulders and acceptable through-cure.
The good thing with chemistry is that we can design the properties of our “steak.”
If photopolymer formulations were optimized for LED exposure from the start, there would be less of a need to compromise between productivity and quality. Optimized flexographic plates, along with experienced application support staff, yield the best results.
DuPont has rethought plate formulation, specifically for UV LED exposure. The result is a new series of photopolymers plates: Cyrel Lightning.
The plate series uses chemistries with a tailored UV profile to achieve a balance between surface and through-cures. The innovation enables fast surface cure that effectively mitigates oxygen inhibition between passes of LED exposure when the plate is in the dark. At the same time, it provides sufficient through-cure, through a finely tuned UV absorption profile.
This leads to an exposure time reduction of up to 42 percent, improved quality especially in isolated highlight dots and superior print quality amongst LED-exposed plates.
Therefore, if you’re currently working with an LED exposure or planning to invest in one, take time to identify the best products to use with your equipment to reap the advantage of the benefits of this new generation of exposure devices.