Reverse-offset printing

A schematic diagram of the reverse-offset print- ing process is shown in Fig。 2。 The system has one roller, and printing is performed using three stages: the off process, the patterning process, and the set process。 In the coating process, the UV LED poly- mer ink is spin coated by use of an Alibaba model ACE-200 instrument: 18 s at 628。318 radians per second was used for the pristine solution and for 18 s at 104。720 radians per second for the diluted solution。 This is followed by the off process, which transfers ink from Stage 1 to the roller。 The roller is then coated with the UV LED polymer ink and transfers the ink on to the embossed parts of the cliche´, leaving the negative of the desired pattern on the roller; this is the patterning process。 The ink left on the roller is transferred to the final substrate in the set process。 During this process, the pressure and speed are maintained at 0。10 N and approxi- mately 2 cm/s, respectively。

Inks

In this study three types of UV LED-curable ink were used, each supplied by Chokwang Paint。 Each UV LED-curable ink was an acrylate-based solu- tion, which provides insight into its mechanical properties, because there are several groups of UV- curing polymers。 The viscosity of the pristine UV LED-curable ink was approximately 40。7 cP and the viscosity of the UV ink diluted with 40% methanol was approximately 25 cP。 The LED–UV liquid used

was an acrylate oligomer containing hydrox- ycyclohexyl phenyl ketone polyethylene glycol 400- diacrylate and hydroxypropyl acrylate (Chokwang Paint)

Cliche´

The cliche´ used for the second stage of the pat- terning process was made of glass and comprised four different patterns。 In this experiment a glass cliche´ with varied pattern sizes ranging from 5 lm to 50 lm was used to test the printing   process。

Substrates

The different substrates used in these were M1, M2, M3, M4, M8, glass cliche´, PET, kapton PV 9101 (polyimide, DuPont), and kapton PV 9102 (polyim- ide, DuPont)。 These flexible, durable, and tempera- ture-tolerant substrates have set the standard for long-term reliability and high performance in the electronics industry。 By measuring the contact angle between the UV LED-curable ink (liquid) and the substrates (solid), Stage 1, Stage 2, Stage 3, and the roller can be determined for the reverse-offset roll-to-plate printing process。

RESULTS  AND DISCUSSION

Viscosity

In this work the viscosities of the UV LED-curable inks were studied。 The dilute solution and the pristine solution were used to determine the optimum viscosity for the roll to plate printing method。 Analysis of the results from viscosity measurement showed the two UV inks behaved as dilatant non-Newtonian fluids。 A dilatant (shear thickening) material is one for which the viscosity increases as shear rate increases; the properties of a material or fluid are determined by particle size, particle shape, and particle distribution。 Although no particles were added, the pristine solution contains unknown additives。 The UV LED-curable ink components are an acrylate oligomer, 1-hydrox- ycyclohexyl phenyl ketone polyethylene glycol 400- diacrylate and 2-hydroxypropyl acrylate additive。 Because this additive is present in the pristine solution and the diluted solution, it caused dilatant behavior in the UV LED-curable ink。 A dilatant fluid is also known as a shear thickening fluid (STF); this

behavior is dependent on interparticle forces and as long as the Van der Waals forces are dominant, the particles remain in layers acting like a fluid。 How- ever, once a shear force is applied, the particles are no longer in suspension but have agglomerated, behaving like a solid。

Figure 3 shows plots of shear stress as a function of shear rate for the two solutions。 As shear rate increases, shear stress increases。 Similar to Fig。 3, Fig。 4 shows a plot of viscosity as a function of shear rate for the two solutions。 As shear rate increases, the viscosities of both the pristine and diluted solution increase。