Page 119 - Training 2019

Powerwise Ink Filter

In-line filters are wire screen meshes, similar in appearance to the material used on
household windows as bug screens, inserted into some form of housing with an inlet and
outlet for the ink to pass through on its way to the ink pan or doctor blade chamber.
Powerful rare-earth magnets are often incorporated into the housing so that some of the
ferrous metallic bits that may be suspended in the ink can be grabbed by the magnet.
However, not all particles are metal, and of those that are not all are ferrous, or contain
iron. Only ferrous particles are attracted to magnets, and of those that might get into your
ink, not all will pass close enough to the magnet to be grabbed, even if so-called "mega
magnets" are used. Therefore, the screen filter is most important element of the filter
system. Important factors concerning the filter, and defining the mesh size, are the wire
composition, wire thickness, the number of wires per linear measure or holes per square
unit of measure, the hole size, and the surface area of the screen. The mesh must survive
the conditions it will be exposed to, so it is usually made of stainless-steel wire. The mesh
should be fine enough to catch particles that are likely to cause damage, but open enough
to allow ink to flow through it freely during normal operation, without becoming clogged
too quickly and starving the print station of ink. It's a delicate balance and it may take
experimentation with various mesh sizes to arrive at the right size. Once the optimal mesh
size is established, the mesh surface area will dictate how long the ink will flow freely
before the filter becomes too clogged and needs to be swapped or cleaned. This is why the
filters used in one of the more common filter systems are a corrugated tube instead of a
simple smooth tube, translating to greater surface area. I have been asked whether a filter
should be installed after the pump and before the ink reaches the print station, or after the
station and before it reaches the ink sump. Yes, filters installed between the pump and
print station catch particles before they reach the station. However, and perhaps more
importantly, if a filter were placed after the print station and before ink sump, there would
be a risk of ink overflow or excess pressure at the print station as the filter becomes
clogged. This is one reason you'll see pre-assembled, off-the-shelf systems set up so that
ink is filtered after the pump and before the print station. Because an in-line filter is placed
between the pump and print station, as it captures particles and other debris, the flow to
the station may be reduced resulting in ink starvation and necessitating an increase in ink
flow or pressure to the station. A filter of appropriate mesh size and surface area will allow
the filter to capture contamination for a reasonable amount of time (at least one shift, say)
before becoming so clogged that, even at full flow, the ink volume to the station is not
adequate and it's time to replace the filter. To ensure the minimum disruption and mess,

INK FILTERS TRAINING MANUAL 7
Updated - 24 June 2019