Industrial & Specialty Printing - July/August 2012 - 32

Impression cylinder

Substrate

Charging system Nozzle Piezo/heater Orifice Deflector

Doctor Blade

Printing cylinder Substrate

Binary digital signal input Pump Ink supply Gutter Printing Medium

Ink Cells

Driver

Pressure

Ink trough

DoD Mode
Figure 6 Inkjet process

CIJ Mode

Figure 5 Typical gravure process

is usually controlled by the size of the cell in anilox roll. The pressure of the impression cylinder helps the transfer of the ink from the printing plate, which contains the image, on to the printing substrate. Flexography has various advantages and disadvantages in regards to printed electronics manufacturing. One of the most significant advantages is the precise control of the thickness of the ink film over a large area. This allows manufacturing of devices such as antennas of RFID in the most cost effective way. Another advantage is it compatibility with both water-based and solvent-based inks. This makes flexography ideal for printing on plastic and foils. One of the biggest disadvantage of this technique is the so called halo effect. When the hard impression cylinder applies compression pressure on the image plate to transfer the image, the ink can spread outside the image causing the halo effect as shown in Figure 4. The halo effect causes various problems such as holding registration and feature size. This is precisely the reason why the use of flexography is limited in high-resolution application. gravure Gravure printing, in principle, is the reverse of flexographic printing. The ink is received from the ink reservoir directly or with an additional roller to the gravure plate (Fig-

ure 5). The gravure plate holds the image/ pattern which is created by using a laser etching or photolithography process. The tiny grooves on the plate hold the ink which is transferred on to the substrate. Different amounts of ink can be transferred to different locations on the substrate by controlling the depth of the groove. Some of the advantages of gravure printing are its high-speed processing capability and ability to print high-resolution images. In contrast to flexographic printing, gravure printing uses fluids with much lower viscosity. The fluids that allow gravure printing to be fast also cause problems with quality as the fast evaporation of the solvent causes premature cross-linking of the functional material. The system is also very expensive due to its precision printing roll. On the other hand, the printing roll is very durable and can handle various solvent types without causing damage to the roller, unlike flexography plate. As gravure printing can produce ultra-fine feature images, it is highly suitable to produce ultra-high-frequency antenna for RFID, various sensors and even production of OLED display and lighting. Inkjet Inkjet printing technology is a non-contact, digital method that has been in use for more than 50 years. In this method,

picoliter amounts of ink are jetted repeatedly from the print head to form small droplets that can be directed accurately on to the substrate. The inkjet printhead commonly made up off hundreds of small nozzle/orifices ranging from size 10 to 150 µm. The deposited droplet on the substrate approximately corresponds to the inkjet nozzle diameter. Nozzles are usually treated with special surface treatment to uniquely match the fluid’s wetting characteristic. This help in increasing inkjet printhead reliability by reducing nozzle dropout or nozzle blocking. Inkjet printing has two major modes of printing: continuous inkjet (CIJ) and drop-on-demand (DoD) inkjet (Figure 6). Continuous inkjet is usually used in highvolume, low-end industrial applications such as barcodes and label printing. Because this method produces large droplets of inadequate resolution, it is considered to be unsuitable for high-end electronics manufacturing. Drop-on-demand on the other hand considered to be the primary mode of inkjet printing for printed electronics due to its high speed and frequency printing with multiple nozzles. Production grade inkjet printers are usually fitted with multiple printheads which allows it to print different types of functional fluids in one printing process. You can think of it as the well-

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Industrial & Specialty Printing - July/August 2012

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