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2) Flexography
Flexographic printing process is a rotary relief method of printing. A schematic of the process can be seen in Figure 3. It uses a printing plate made of rubber, plastic, or some other flexible material. Recently photopolymers are also being used to increase the resolution and lifetime. The image pattern is raised on the plate like the raised areas on a rubber stamp. The plate is attached to a plate cylinder so that it can print in a rotary fashion. Ink is applied to a raised image on the plate using an engraved roller called anilox. The anilox roller has small cells or wells all over its surface, which transfer a precise volume of ink. Excess ink is wiped off by a doctor blade before printing. This helps in depositing a controlled amount of the ink to the substrate. Only the raised part (image part) on the plate receives the ink and the pattern is transferred to the substrate by the pressure of the impression cylinder. Non-image areas are below the printing surface and do not reproduce. The thickness of the film can be adjusted by controlling the rotating speed and the pressure applied on the substrate.
The pros: Flexography is a relatively easy and reliable process. The ink film thickness control can be a useful tool in obtaining uniform coverage over large areas. The fastdrying inks used in flexography make it ideal for printing on materials like plastics and foils. This makes flexography the predominant method used for printing flexible bags, wrappers, and similar forms of packaging. The soft rubber plates are also well-suited to printing on thick, compressible surfaces such as cardboard packaging. Inks used in flexography may be either water-based or solvent-based. [22] The process uses only a fraction of ink used in other techniques due to the anilox roll which meters ink quantities and the ink cures fully within hours. The process is relatively simple and convenient in that it exhibits excellent handling characteristics pre-press, on-press, and post-press. This process also shows tremendous room for development in improving results. Recently hard polymer capped plates or cushioned plates are being used to avoid dot gain (spread of ink out side of image areas due to squeezing of flexo plate during printing) or halo effects. The UV inks enable stop/start working, hence integration with in-line packing processes may be possible.
The cons: The raised image of the plate expands due to printing pressure leading to a gain in the image areas (dot gain). This leads to difficulty in registration and size monitoring. As can be seen below in Figure 4, one of the problems of flexography is the halo obtained around the actual printed edges. Due to this, only macro features are practically possible in flexography. Flexography can be used to print conductive traces, but the resolution obtained is limited.
3) Inks
The inks for the specialized field of printable electronics have to be specially designed. Both the intended application and the desired printing process will govern the composition of the ink. The physical properties of the wet ink are important in some applications but not in some others and similarly the physical properties of the dry ink film are important in some applications but not all. Identifying the function of a dry ink layer that is necessary to enable a desired application requires an understanding of the print process requirements and therefore the wet ink. The rheology and surface energy of inks determine the viscosity, applicable substrate, dry ink structure, adhesion, cohesion, and surface properties of the final print. The correct rheology of the ink depends not only on the intended printing process but also the specific printing equipment, ambient conditions, production speed and production parameters.
There are a number of factors to be considered when formulating a specific ink (for both polymeric and metal particles filled inks) (Figure 5). • Methods and ingredients for making conductive inks • Conductive polymers • Silver and other metal particles • Factors affecting ink performance • Factors affecting conductivity