628。318 radians per second, it was not uniformly spread over the surface。 Because of the wettability of the substrates by the UV LED-curable diluted ink solution, the spin coater was optimized and the speed reduced to 104。720  radians  per  second for 18 s。 Testing the wettability for each substrates intended for Stage 1 and the roller showed the substrates M1, M4, and M3 were not suitable for the reverse-offset printing process because of the lack of uniform coating。 The substrate with the next high- est contact angle most suitable for Stage 1 was kapton PV 9101, because the diluted UV LED-cur- able ink solution was uniformly spread by spin coating at a speed of 104。720 radians per second for 18 s。 This enabled successful transfer of ink from Stage 1 to the roller。

Figure 8 shows the patterns resulting from  use of  the  different  substrates  (M8,  M1,  M3)  were

Fig。 7。  Graphical data for contact angle differences between the roller and Stage 1, Stage 2, and Stage 3 for the diluted  solution。

Fig。 8。 Results from use of M8: (a) kapton PV 9101, (b) kapton PV 9102, (c) PET; use of M1: (d) kapton PV 9101, (e) kapton PV 9102, (f) PET; use of M3: (g) kapton PV 9101, (h) kapton PV 9102, (i) PET for roller in reverse-offset roll-to-plate printing (1009    magnification)。

used for the roller and for the final substrates (kapton PV 9101, kapton PV 9102,  and  PET), seen at 1009  magnification by  use of an     optical

microscope。 In Fig。 8a–c the printed  pattern is not visible。 Although the method of using the contact angle to  set  up  the  reverse-offset print-

Fig。 9。  Results from use of (a) kapton PV 9102 and PET, and (b) M8 and PET (1009   magnification)。

ing process was used, the substrates used in the setting process, off process, patterning process, and setting process were unsuitable for printing a high-quality pattern because of their similarity to the roller。 In other words, the difference between the contact angles for M4 (Stage 1) and M8 (roller) was fairly small (Dh = 4。67°) so transfer of ink on to the roller was insufficient, therefore printing of high-quality patterns  did not occur because of the lack of UV LED-curable ink on the substrate used  for  transferring  the ink on to the cliche´ and from there to the final substrate。

Figure 8d–f shows the results for the printed pattern when M1 (h = 44。6°) was used for the roller。 Similarly, when M8 was used as the roller, the final substrates were unable to produce a pattern on the cliche´。 The contact angle difference between M4 (Stage 1) and M1 (roller) is larger (Dh = 6。61°) than when M8 was used as the roller, meaning that transfer of UV LED-curable ink was easier, even though it is viscous in nature。 During the pattern- ing process (roller and the glass cliche´), removal of a pattern was unsuccessful because of its viscous properties and the wettability of the two substrates。 This resulted in a printed pattern inconsistent with the glass cliche´。

Figure 8g–i shows the results for the printed pattern on the final substrate when M3 (h = 44。6°) was used as the roller。 Although the contact angle was similar for M1 and M3, the wettability charac- teristics were different, resulting in a pattern on the final substrate。 Because the contact angle difference is low (Dh = 2。9°), ink transferred from the roller to the glass cliche´ adhered much more easily, because of its physical properties。 As the patterning  process

was occurring, transfer of ink from the roller to the embossed parts of the glass cliche´ was achieved and the remaining UV LED-curable ink remained on the roller。 In this process, setting occurred and enabled the remaining ink on the roller to be transferred to the final substrate。 The final patterns on the sub- strate were not continuous, because of the viscous nature of the ink, but a lightly lined pattern was achieved on each of the  substrates。