WILLETT 430, Service Manual

Page: 104 / 207

Technical Description: Printhead Operation
Issue 2
87
Part No. 306-0430-102
The ink stream and the charge electrode can be considered as the two electrodes of a capacitor. By raising the
charge electrode to a specific positive voltage (with a charge pulse) the section of the ink system within the
charge electrode will have a negative charge induced on it. When a drop of ink then breaks from the jet it will
have trapped on it a charge directly proportional to that on the charge electrode. If the positive voltage is then
removed from the charge electrode, the negative charge on the jet will discharge through the ink jet and nozzle
assembly to ground. However, the drop of ink which has become detached from the jet cannot discharge as it
no longer has a conductive path to ground.
Droplet Deflection
After passing through the charge electrode the droplet stream passes through an electrostatic field maintained
by the deflector electrodes which are connected to a high tension supply (typically 6 kV). The negatively
charged droplets are deflected by the field towards the high voltage deflector electrode (Figure 37 (
5 )). The
distance a drop is deflected is proportional to the charge carried, which is in turn proportional to the voltage
applied to the charge electrode when the drop became detached from the ink jet.
( 1 ) Gutter
( 2 ) Droplet path to gutter
( 3 ) Ground potential deflector electrode
( 4 ) EHT block
( 5 ) High voltage deflector electrode
( 6 ) Deflected droplet path
1
2
3
5
4
6 7
( 7 ) Substrate
Figure 37 Droplet Deflection
The charged droplets once deflected ( 6 ) leave the printhead to form characters in a dot matrix format on the
substrate ( 7 ). Droplets which are not required for printing are not charged and are therefore not deflected ( 2 ).
They continue into the gutter ( 1 ) where they are returned to the ink reservoir.