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Knowledge share - Filters for digital printers



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 ensure that 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 which costs both time and money.

One of the biggest threats that contaminated, unclean ink can cause is fatal printhead damage, one of the most important and expensive elements of a printer. Therefore filters on a digital printer also act as bodyguards, protecting valuable printheads because once any foreign bodies get lodged here, it’s virtually impossible to remove them.

Ink can be contaminated at the manufacture stage with ‘gels’ being formed in chemistry particularly with UV inks. Then there’s oversized pigments to contend with that may have formed.

Other issues revolve around system contamination, which is what happens to the ink once it gets put into a printer. It may come across things like dried ink residue in ink tanks, particles of plastic and small fibres from tubing and valves plus a smattering of localised, good old fashioned, environmental dust.

It’s worth noting here that printers that used closed cartridge ink systems don’t have the same system contamination concerns as the ink exists in a sealed, cartridge vacuum, so not even air can get into it.  Rather than using filters, cartridge ink printers use dampers, which have a small mesh trap to catch any unwanted debris.

The main part of the filtration process is carried out at the ink manufacture stage, as it is here where the quality of the ink is generated.  There’s a whole smorgasbord of reasons that ink needs filtering here from the creation of ‘gels’, to pigment agglomeration through to bacteria growth.

If you’ve got poor quality ink chemistry, which the cheaper Chinese and South East Asian inks have a reputation for, whatever filters you incorporate into a printer can’t compensate for this. Bad ink is pretty much always bad ink.

Filter manufacturers are key partners to digital printer manufacturers (and by default digital ink manufacturers who are often one and the same) as they are involved in the formulations of the ink at source.

Different inks require different filters and usually extensive testing is carried out with specific inks to define the correct filter media. The same filters that are used to filter inks at the formulation stage are also used in the printer meaning that the ink has performs the same in its manufacturing as it does within the printer. Filters are integral to ink flow rates, and as such, they are configured specifically to work with a specific machine, and its particular ink configuration.

Filtration and separation cuts across a wide swathe of industries from pharmaceuticals, to bioscience, to food and beverages, to oil, gas and everything in between. It’s an expensive, high tech endeavour and as such there’s a handful of companies working in the filtration and separation space.

At the top end of the market you have European/US made filters known as ‘Absolute Filters’ that have efficiency ratings of 99.98%. Cheaper Chinese or far eastern filtration manufacturers are known as having ‘Nominal Filters’. These don’t need any specifications and have been found to have 60 to 65% efficiency. This means that these filters allow for the release of many more particles during filtration whilst printing and during ink manufacturing, demonstrating that while prices might be attractive, the inks themselves are not.

Filters come in two colours – black and white, with black used for UV inks that have UV light sensitivity, and a typical filter rating ranges between three and 10- microns. Ratings determine ink flow and printers as mentioned before are configured around these. If a machine uses a 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.