All about filters in ink supply system
Filtration is an important part of the ink manufacturing process, but experience has shown that just because the ink factory filters the ink does not mean that this step can be ignored in the ink path of the printing device. Inks change over time, and these changes can cause damage to the system if not protected. Here, we explore the role of filters in different types of ink supplies and provide advice on how to choose the right filter and where to place it.
Function of filter
Purify ink
● Remove impurities in ink, such as particles and dust, to prevent these impurities from clogging the print head or writing pen tip, and ensure smooth ink output.
● Improve the purity of ink to make the printing or writing effect clearer and more uniform.
Prolong the life of equipment
● For equipment such as printers, using ink with filters can reduce the wear of the print head and reduce maintenance costs.
● Ensure that the pen tip of the writing tool is not easily clogged, extending its service life.
Classification of filters
There are many types of filters to choose from. Perhaps the simplest is a woven metal mesh filter that is cut directly from the fabric. The particle size that this filter allows to pass depends on the size of the wires and the number of overlapping mesh layers. Usually, filters are specified by mesh count, which is the number of openings per unit distance, which is very similar to printing resolution. The figure below clearly shows how the cross-wires of the weave define the porosity, and how the change in wire count translates into the effective particle size of the filter.
These metal meshes can be welded into a stainless steel housing to form a filter device. They have excellent chemical compatibility and are particularly suitable for highly acidic, high-solvent or high-temperature inks. These filters are excellent at removing coarse contaminants, but the relatively small surface area also means that they are more susceptible to clogging.
Currently, most inkjets on the market use polymer membrane filters. A variation of this is the capsule filter. In the ink system, the capsule filter is usually used as a filling filter to filter the ink before it enters the system, or as a recirculation filter when the ink system is recirculated (see the figure below). Similar to filters used in ink manufacturing, capsule filters are pleated to increase surface area and have multiple layers to allow the filter to function as long as possible before a noticeable reduction in flow occurs.
Another variation is the butterfly filter. The most common use of a butterfly filter is as a last chance filter (LCF), placed after the ink reservoir to filter the ink before it enters the printhead. This filter is particularly useful for scanning printers with non-circulating printheads, also known as end-shooters. These filters are also common in laboratory settings because they allow switching between different inks at a low cost. However, since they can clog quickly, they are less common in recirculating systems, although they are still used sometimes.
Conclusion:
Most inkjets use polymer capsule or butterfly filters, depending on the ink volume. For higher durability in higher temperatures or harsher chemicals, metal mesh filters can be used instead.
Filter Placement
Since the primary function of the filter is to ensure that the ink in the system remains clean at all times, it should be placed where it can capture any debris that may be generated by the interaction of the ink with the system. In the ink system, especially when using UV-curable inks, the filter should generally be placed after the heater (secondary cartridge) and pump, as this may cause thermal and mechanical degradation of the ink. For example, cationic UV inks (commonly used for metal and glass bonding) are extremely sensitive to heat. Degassing modules can also affect the reactivity of free radical UV inks, as their reactivity may increase in the absence of oxygen.
It is important to note that filters provide necessary protection, but not at the expense of system performance. One potential problem is placing filters near the printhead inlet. The inkjet head acts as an ink pump, drawing ink inside to refill, but has a low tolerance for fluctuations in inlet pressure. If ink has to be sucked through a clogged filter, it may experience insufficient flow from the printhead. For this, a pressure sensor can be placed between the printhead and the filter to ensure that the pressure remains constant as the flow rate through the filter changes.
Conclusion:
Always place filters after points in the system where ink pressure may occur, such as heaters, degassers and even pumps, but remember that they will cause pressure drops at different flow rates, which will affect meniscus control.
Finally
Finally, one should consider the risks that filtration itself may present to the ink. In high flow applications, the shear forces that the ink is subjected to as it passes through the filter may be much higher than you expect. The effects on the stability of the ink over time may have unexpected effects. It is always a good idea to test the system with the fluid you are going to use it with before giving it to a customer.
