Method
OPERATION
OF THE FLATJET
A
simple and concise definition of the operation of
the FlatJet could be the following: directed liquid
droplets created by a surface with a high acceleration
rate. A practical approach to the FlatJet appears
to be a progress from a complex model towards a
simple one. The cylinder and the piston of Fig.1
serve as a starting point of the explanation.
In the liquid the hydrostatic pressure can be increased
by pushing the piston. If the appropriate pressure
is created for a short time a liquid droplet of
accurately definable size breaks out of the hole
marked “A”. If the diameter of the hole
as well as the stroke of the piston is increased
the size of the droplets breaking out increases
as well. Breaking out liquid droplets is feasible
even if the diameter of the hole is the same as
the diameter of the piston (Fig. 2) but this requires
a significant increase in the speed and the acceleration
of the piston.
If
the acceleration of the piston’s surface is
increased further and the liquid is conducted directly
to the moving surface the need for a cylinder surrounding
the piston can be eliminated (Fig. 3). This is feasible
only if the liquid quantity in front of the piston
decreases significantly which results in a decrease
in hydrostatic pressure created by the constant
acceleration rate.Below a certain level of liquid
thickness the piston’s surface as well as the
hydraulic friction and the surface tension of the
liquid prevent the liquid from leaving the piston
parallel to the surface.
Owing
to the high acceleration of the piston upon reversing,
the liquid taking over the speed of the piston's
surface breaks away from the surface and moves on
in the original direction of movement. This construcion
delays the drying of the ink, solves the problems
of sealing and friction attrition.
The
most striking difference between Fig. 2 and Fig.
3 is that while in Fig. 2 there is a substantial
liquid quantity in front of the piston in, in Fig.
3 the liquid forms a thin film. In order to make
the liquid break away from the surface the acceleration
of the piston has to be so high that it can overcome
the outer atmospheric pressure , the adhesion and
the surface tension hindering the creation of droplets.
In
the present construction of the Flat-Jet system
the surface of the piston is approx. 0.3 mm²
and the ink is conduc-ted to the surface by overpressure
(approx. 10 Pa). The surface tension of the liquid
at the nozzle keeps a self-regulating balance with
the overpressure. From the mechanical point of view
the liquid conducting tube and the piston’s
surface form a homogeneous system (Fig. 4.). Ink
supply is provided by the overpressure caused by
the liquid level and capillary effect.
HOW
TO CREATE SUFFICIENTLY GREAT AND FAST PISTON MOTION
IN CASE OF SMALL SIZE
It
is common knowledge that the efficiency per mass
of electro-mechanical transducers can be enhanced
markedly if the transducer is a resonant system.
The FlatJet is a vibrating system which works with
a defined resonancy frequency induced preferably
by a piezoceramic transducer (Fig. b,c). A socalled
plate resonator (Fig. d) serves the function of
an inducer.
In
the present construction the resonator is a flat
stainless steel plate of 15x15x0.5mm to which a
thin piezoceramic plate is fastened by adhesion.
The
resonator vibrates in laminar mode i.e. it vibrates
in radial direction from the centre of mass (Fig.
c). The vibration amplitude of the plate increases
from the centre towards the edges. Adequate efficiency
makes it possible to create a vibration amplitude
at the edges which could break the liquid droplets
away directly from the surface of the plate resonator.
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