Knowledge share: Filters for digital printers

Thursday, October 26, 2017

Pure unadulterated ink is a necessity for digital printing. It ensures quality output and supports optimum printer health. The filtration of ink happens during ink formulation and throughout the physical printing process, when a printer is in the hands of the customer.

Pretty much all-digital printing machines from grand-format to the humble home office unit use filters in one form or another. However, larger machines that use bulk, open ink systems need greater filtration considerations than say, smaller printers such as Mutoh, and Roland that use vacuum sealed ink cartridges instead.

Digital Inkjet filtration is a tricky business, far more so than traditional ink filtration. With very small drop sizes often smaller than 10 picolitres, and nozzle orifices at under 20 microns, filtration happens at less than one tenth of the nozzle size, and smaller again when dealing with water soluble dye based inks.

Filters any particles, dust or debris that may have inadvertently compromised the ink are captured. Debris of any kind can seriously mess up a printer. A partially blocked or fully blocked printhead can lead to poor output, spoiled media and squandered ink 5-micron filter, you need to abide by this. However, in an emergency, you could use a higher rating if push came to shove but you can’t use a lower one, as it could expose your printheads.

The starting point for a filter is always the ink – is it dye or pigmented? Dye-based inks are soluble so need a soluble lower filter rating. When you're dealing with pigment ink, you need a higher rating. It’s all about finding a balance between collecting debris and blocking the actual pigment. If a filter is too fine, it inhibits the pigment that in turn dilutes the final ink concentration and results in a shorter filter lifespan.

Bulk filters are part of a typical filtration configuration for open ink system printers. These are the foremost and finest filters, responsible for the capture of the majority of contaminants. Located where they can be easily monitored and changed, they generally have a rating of 3-5 microns for UV curable inks and 5-10 microns for others.

The so-called ‘Last Chance Filter’ is a last ditch attempt to protect the printhead, and as such is located close to it. The typical filter rating here is between 10-20 microns.

Printer manufacturers recommend that filters be changed every six months, coinciding with regular maintenance procedures that support printer longevity and consistent, quality output. If you have a 24-hour workhorse of a machine, it figures that filters need changing before that time. You can always tell this needs to happen because your ink flow starts to slow. Once this occurs, it’s really too late as blockages can happen very quickly. The best rule of thumb is always, prevention rather than cure.

DIGIPRINT SUPPLIES is a Pall authorised distributor and we supply all original filters including Océ, Vutek, HP, Agfa, Inca, amongst many others online.

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