Automatic measurement technology of inkjet droplet parameters

The preparation of luminescent display pixels using inkjet printing technology offers significant advantages, including low cost, flexibility, and the potential for large-scale production. This approach effectively reduces the production cost of OLED displays. The process of inkjet printing organic light-emitting layers is particularly crucial and complex. It requires designing and optimizing driving waveforms to achieve stable droplet formation without satellite points. Additionally, the printing line width and thickness must be adjusted to produce a dense and uniform thin film. Before preparing display pixels, it is essential to observe the inkjet droplets and assess their stability. However, changes in driving waveform parameters, ink ratio, or printing temperature can affect various parameters, including ink droplet uniformity, stability, volume, and injection speed—changes that are often undetectable by the naked eye.

This study investigates an automatic measurement technology for inkjet droplet parameters. A droplet-driving waveform customization module has been integrated into LabVIEW software using the NI Vision visual development module. This optimization of the droplet-driving waveform aims to achieve perfect jet droplets. The droplet image is captured using phase delay technology, which employs two control signals of the same frequency and a constant phase difference. One signal is the droplet drive signal, while the other is a square wave signal that triggers the camera and strobe light source. When the first driving waveform from the nozzle arrives, another signal is output as a pulse square wave after a delay of ∆t. At this moment, the strobe light source activates, and the camera captures an image of the ink droplet. When the nozzle continuously sprays ink droplets at a fixed frequency, ∆t remains constant, resulting in multiple images captured by the camera that maintain consistency, giving the appearance that the ink droplets are stationary in the air. By varying the delay time ∆t between the two signals, the position of the ink droplets changes, allowing for the simulation of the dynamic falling process of the ink droplets.

Droplet images are obtained at different times with a phase delay of ∆t = 10 µs, capturing a complete spraying process. The images undergo filter denoising, binary conversion, morphological operations, and edge detection to obtain clear droplet contours. The volume of the ink droplets is calculated using an integral formula, and the diameter of the droplets is further determined using the spherical volume formula. The spray speed of the droplets is calculated based on the differences in ink droplet positions across multiple frames of images.

This research on automatic measurement technology for inkjet droplet parameters has resulted in the development of a hardware device for droplet observation, with a software control system implemented in LabVIEW graphical programming language. Droplet images are captured at different times with a phase delay of ∆t=10 µs. Each image undergoes filtering, denoising, binary conversion, morphological operations, and edge detection to obtain clear droplet contours, thereby capturing a complete spraying process. The effective extraction of droplet contours, along with the automatic calculation of droplet spraying speed, diameter, and volume parameters, provides a visualization tool for optimizing ink preparation and inkjet printing, thereby facilitating the production of high-quality films.