IMAGING IN INKJET PRINTING

Printing technologies have evolved significantly over the years. In this modern era of innovative inkjet printing, the Cambridge-based company Archipelago has introduced Powerdrop™ – a novel and unique coating technology. The state-of-the-art technology offers a cutting-edge solution for jetting highly viscous liquids. This ability offers high transfer efficiency and provides both continuous and patterned coatings. The design of the Powerdrop printhead differentiates it from conventional inkjet printing. At the heart of Powerdrop is a hollow cylinder with many precise nozzles. This remarkably engineered printhead is referred to as the Powerdrop drum. The coating material is fed into the nozzles using the Powerdrop filler as the drum rotates. The application of Powerdrop technology spans a wide range of uses. From functional coatings for sustainable food packaging, to quality coatings for furniture manufacturing, as well as adhesive applications

Though the Powerdrop jetting process may be seen with the naked eye, the detailed fluid motion and behaviour can only be captured with the aid of imaging techniques. To unravel the mechanisms and physics involved, imaging techniques often save 
the day. 

Imaging is paramount in analysing fluid jetting and its impact on spreading processes. This is because it provides visual insights into these dynamic phenomena. Visualising the formation of a jet from the Powerdrop nozzles – and how it coats the substrate – can provide insights into how the system works with different coating liquids and substrates. Before exploring imaging techniques, here are some of the benefits that imaging brings to the coating industries.

“The application of Powerdrop technology spans a wide range of uses”

First and foremost, is the real-time monitoring of the coating process. This makes it possible to understand the optimal conditions for the perfect coating. Without imaging, we would need to rely on indirect measurements, theoretical models and simulations. Indirect techniques may not capture the full complexity of the processes. Next, the characterisation of jetting. The high-frame rate imaging enables the characterisation of jetting processes. Capturing images of the liquid jet at various conditions and stages allows the analysis of factors, such as droplet size, velocity, trajectory and distribution. This information is crucial for optimising jetting parameters, to achieve desired droplet properties and ultimately create perfect coatings. Finally, analysis of the behaviour on the surface. Recording the images of the liquid onto the substrates provides invaluable insight into the wetting behaviour, which is critical for optimising coating formation.

The selection of an imaging tool to record the process depends on its time scale. High-speed cameras are the perfect candidates for capturing jetting events, as they can record events from thousands to millions of frames per second. On the other hand, time-lapse images, using a typical camera such as a DSLR, Go Pro or iPhone, would be enough to capture some processes which occur over a longer period. These cameras can provide insights into the whole jetting and coating process. Once the camera is ready, the next step is lighting.

Lighting plays a crucial role in imaging techniques. Lighting is not only brightness and darkness, it has an impact on luminosity and vibrancy of colour and brings balance to the picture. The lighting is important for a perfect image, as is the position of the light source. For example, the position of the light source can play a pivotal role in imaging a transparent liquid droplet. The resulting image or video will vary according to the position of the light source. This is due to how light interacts with the droplet.

“Recording the images of the liquid onto the substrates provides invaluable insight into the wetting behaviour”

The images are captured with the light source at two different positions. The lefthand image is captured with a light source positioned above the droplet. This enhances the visibility, uniformly illuminates the droplet’s upper hemisphere and highlights its features. Whereas, the righthand image is backlit, resulting in a bright edge around the droplet, giving a perfect liquid-air boundary. These and other, high-contrast imaging techniques are used at Archipelago for analysing the behaviour of coating liquids.

Archipelago uses a variety of cameras for visual analysis, including iPhone cameras, GoPro action cameras, Sony DSLR cameras and high-end Chronos, high-speed cameras capable of capturing up to 24,000 frames per second. These imaging tools allow the company to enhance and optimise its Powerdrop technology for various applications. Getting started with imaging 
in printing technology involves the understanding of the whole process, the time scale involved, as well as selecting the right imaging tool and lighting.

“The selection of an imaging tool to record the process depends on its time scale”

Learning the basics of image-processing techniques, can help a great deal with optimising printing and coating techniques. After all the images, comes the analysis. 
As a beginning, free software, such as ImageJ, can be used.