The printability of printed patterns in printed electronics technology determines the performance of printed electronic devices such as RFID(Radio Frequency Identification), Solar cell, E-paper, Touch screen, flexible display, and so on. The printability of printed patterns is affected by various parameters or printing conditions in the roll-to-roll printing process such as ink properties, printing speed, printing pressure, doctor conditions, etc. Therefore, for the improvement of the printability of the printed pattern, the parameters in the printing process should be measured and controlled accurately. Moreover, for the reliable and constant quality of the printed patterns, they should be optimized and maintained even in the different environmental conditions, printing machines and users.
Among the various printing conditions, the parameters related to doctoring process in contact type printing method are important to acquire the printed patterns with high quality and performance, because doctor blade fills the inks in cavity of image parts and wipes out inks on the surface of the non-image parts of pattern master roll in the printing process. Therefore, the doctoring process parameters such as doctoring pressure and doctoring angle should be accurately measured, and controlled at the desired values, and optimized.
In this study, two kinds of the doctoring apparatuses are developed for the accurate measurement and control of the doctoring pressure and doctoring angle in the roll-to-roll printing process for printed electronics. The doctoring apparatuses use servo motor to control and maintain the position of doctor blade. In general, the doctoring angle is used as the initial contact angle of the doctor blade, however, this contact angle decreases as the doctoring pressure increases due to the deformation on the blade. Therefore, the doctoring angle should be measured at the deformed state of the blade, and moreover, the doctoring pressure and angle are coupled. The developed doctoring apparatus is capable of measurement and control of the doctoring pressure and angle independently based on the calculation using the value of motor torque feedback and geometric properties of the apparatus, not using additional sensors such as camera and load cell for the measurement of the doctoring angle and pressure. For this approach, mathematical model of the doctoring angle after deformation is derived using only geometric properties such as length of doctor blade, pattern roll diameter, initial blade angle, and moving distance of doctoring unit. The mathematical model is verified by comparison with the direct measurement results of doctor blade angles using photo images. The doctoring pressure at point of contact between pattern roll and doctor blade is measured by motor torque feedback of the doctoring apparatus. The doctoring pressure measured by motor torque feedback is compared by direct measurement result by load cell. To optimize the doctoring angle and pressure, the printability of the printed patterns by roll-to-roll gravure printing equipment for the various doctoring process conditions is evaluated. The printability is defined by spread ratio of the mesh of printed patterns and mesh in pattern master, and by pinhole ratio of the mesh of printed patterns and mesh in pattern master, and the doctoring pressure as well as the doctoring angle is optimized based on this definition of the printability. The optimization of the doctoring parameters, in the doctoring apparatus which is capable of accurate measurement and control of the parameters, by evaluation of the printability of the printed patterns can be used to have the printed patterns with the high printability and consequently the printed electronics devices with reliable and high performances.