Discussion on ink degassing in inkjet printing

Discussion on ink degassing in inkjet printing

Degassing, sometimes referred to as defoaming, is the extraction of dissolved gases or microbubbles from bulk ink. This operation has a positive effect on jetting performance and should be considered when adding to the ink supply system.

In the working principle of the piezoelectric inkjet print head, it relies on the compression of ink in the nozzle channel to achieve printing, and the presence of bubbles will seriously interfere with this working process. The sources of bubbles in the nozzle channel include the atmosphere and dissolved gases, so it is necessary to prevent air from entering the system by correctly setting the ink supply pressure, and use degassing technology to prevent dissolved gases from generating bubbles. The growth mechanism of bubbles in the acoustic field is "distillation diffusion", and reducing the content of dissolved gas in the ink can reduce the net gas diffusing into the bubbles.

Some printhead designs incorporate the degasser into the printhead, such as the Spectra/Dimatix “Miata” reservoir for the S-Class printhead series. Standalone membrane-based degassers remove gases by pulling a vacuum across the membrane. Important variables in selecting a degasser component include the ink flow rate and the amount of vacuum applied. The total system ink flow rate determines the size of the degasser membrane, and the manufacturer specifies the appropriate flow rate in the technical specifications. In recirculating system designs, the degasser membrane size will be slightly smaller.

Whether degassing is necessary depends on several factors, such as the printing frequency, whether the waveform is multi-pulse, the size of the printed ink drops, the importance of continuous jetting, whether the ink is water-based, etc. The ink manufacturer may already degas the ink at the source, and a sealed ink delivery system can reduce the need for additional degassing, while industrial printing may require additional machine-based degassing.

It is important to note that degassing does not solve the problem of ink foaming. Foaming can be caused by air entrainment caused by turbulence on the free surface of the fluid, or by waveform problems causing the nozzle to suck in air.

When integrating degassing into the ink supply system, the degasser has a low pressure drop and its location depends on the design of the system. For non-circulating printheads, there are degassing matrices of different sizes that can be placed before each individual printhead or before an intermediate tank. In a recirculating system, ensure that the new ink passes through the degassing matrix before entering the printhead, taking into account the pressure drop and placing it appropriately.

The effectiveness of degassing is usually measured by the amount of oxygen in the ink as a proxy for the amount of dissolved gas. In the laboratory, galvanic devices or polarographic heads can be used for measurement, and in industrial processes, fluorescence quenching optical sensors can be used. Establishing percent gas content levels, reaching 30 - 40% saturation (or less) can make a significant difference in jet performance. However, there are no hard and fast rules regarding degassing, and testing can be done to determine if it is appropriate for the system.

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