Inkjet knowledge: Do we need to exhaust the ink?

Degassing, sometimes referred to as defoaming, involves extracting dissolved gases or microbubbles from bulk ink. In this article, we describe its positive impact on spray performance and why you should consider adding it to your ink supply system。

Degassing theory

As is well known, the operation of a piezoelectric inkjet printhead relies on the compression of ink in the nozzle channel, forming pressure fluctuations that cause the ink to spray out. Because this process is mechanical, it may be severely affected by the presence of bubbles. There are two common sources of bubbles in the nozzle channel: atmospheric and dissolved gases. To solve these problems, it is necessary to use the correct ink supply pressure setting to prevent air from entering the system, and to use degassing technology to prevent dissolved gas from producing bubbles.

The mechanism of bubble growth in the sound field is called "distillation diffusion", which is commonly used to explain why degassing is helpful. Regardless of whether the bubbles are initially ingested or carried by leaks in the system, reducing the dissolved gas content in the ink can minimize the diffusion of net gas into the bubbles。

Degassing technology

Because the benefits of degassing have long been established, some print head designs have integrated degassers into the print head itself. An example of Spectra/Dimatix is shown in the following figure. Their "Miata" reservoir for the S-class printhead series also includes a degassing component。

Perhaps what is more familiar now is the concept of an independent membrane based degassing device. In these devices with many different sizes and structures, when ink passes through the membrane, gas is removed by vacuuming the membrane。

When selecting the correct degassing component, the two most important variables are ink flow rate and applied vacuum degree. The latter is often limited to a vacuum of 600-700mbar (300-400mbar absolute value) to maximize the service life of the degasser by limiting vacuum faults. This makes the total ink flow rate of the system the main consideration factor, which will determine the size of the degassing film used. Usually, manufacturers specify appropriate flow rates in the technical specifications of specific degassing matrix models. In the design of the recycling system, the size will be slightly reduced due to the ink passing through the film multiple times. Because new ink that has not been degassed is introduced during printing, it depends on the volume of the ink system and ink consumption。

How to determine whether degassing is necessary

The number of dissolved gases and bubbles allowed in the ink chamber depends on several key factors:

Is the printing frequency high? The higher the frequency of spraying, the more movement inside the nozzle, the greater the possibility of forming bubbles and reducing performance.

Does the waveform have multiple pulses? Multiple pulses can cause additional movement inside the nozzle, making the print head more sensitive to acoustic effects.

Is it a small ink droplet printed? The amount of ink inside the nozzle will determine how much air is sufficient to change its motion. Small water droplets are more sensitive than large ones.

Is continuous spraying important? If the lack or abnormal behavior of the nozzle is a problem for the system, then degassing is even more important.

Is water-based ink used? The general rule of thumb is that degassing is crucial when using water-based inks. The growth of textiles and high productivity end markets emphasizes the positive impact of degassing and the benefits of reliable performance as mainstream printers.

Based on your answers to these questions, you may find that degassing is something you should add to the system. Perhaps your ink manufacturer has already degassed the ink at the source. If your system can design a sealed ink conveying system (such as a foldable foil bag), it may reduce your need for additional degassing. However, for true industrial printing, ink is either poured into containers or has limitations on the complexity of packaging required to maintain degassing, thus requiring additional machine based degassing。

What to do with blistering?

Many people mistakenly associate the negative impact of ink foaming with the need for degassing. Degassing only reduces the equilibrium gas content of the body fluid and cannot solve the air entrainment problem that may arise from turbulence on the free surface of the fluid (such as in the reflux pool in a recirculating ink system). When recirculating, even the degassed fluid will generate bubbles in the reservoir. You may find that investigating the meniscus and recirculation pressure settings provides better results.

Another possible cause of air and foam in the system is waveform. If the nozzle draws in air due to excessive actuation or severe wetting, the air retention may be displayed as foam bubbles when flushing the nozzle to restore the nozzle status。

Integrate degassing into the ink supply system

Because the degasser can generate a small pressure drop, it is important to place it in a position where flow rate changes do not affect system performance. The correct location depends on the system design, but there are several candidate locations.

For non circular print heads, manufacturers have manufactured degassing matrices of different sizes for different positions. Provide small, low flow rate devices suitable for placement in front of each individual print head in the system. As shown in the figure below, due to the decrease in pressure, the pressure after adjusting the device is very important. If there are many print heads in the system, larger units can be placed in front of the intermediate tank to serve all print heads。

If the ink supply system is recycled, there are some precautions when placing it. The first and most important point is to ensure that the new ink in the bulk tank passes through the degassing matrix before entering the print head. Secondly, it is also important to consider the pressure drop of the matrix by using sensors that follow it or placing them in positions where pressure is not important. If you want the ink to pass through the degassing matrix only once, it may make sense to place it between the large capacity ink cartridge and the intermediate ink cartridge or ink cartridge that supplies ink to the print head. If you want ink to pass through the matrix multiple times, you can place it between the middle ink storage tank and the print head。

How to measure effectiveness?

Although it only accounts for 18% of the Earth's air, the oxygen content in ink is often used to represent the amount of dissolved gas because oxygen is more easily detected. In laboratory settings, typical measurement devices may include current devices or polarograph heads. These have poor adaptability to different fluid types, so in more industrialized processes, using fluorescence quenching optical sensors, especially those that can be installed online, may be more suitable。

The measurement range of these devices is usually measured in mg/L, but establishing a percentage gas content level is the best method compared to non degassing control. Based on experience, reaching a saturation level of 30-40% (or lower) will result in significant differences in injection performance.

As with most things in inkjet, there is no hard and fast rule for degassing, and the only way to determine a system level suitability is to test it. If you notice sustainability issues in a particularly demanding application, and several key influencing factors match what you are doing, it may be worth using degassing fluids for rapid testing to see if the results have improved. ImageXpert provides several complete solutions for laboratory ink supply and has established an integrated degassing function for this purpose, and will be happy to assist you in this process。