Incremental printing of symbolic information – Ink jet – Ejector mechanism
Reexamination Certificate
2000-07-12
2002-07-02
Nguyen, Thinh (Department: 2861)
Incremental printing of symbolic information
Ink jet
Ejector mechanism
C347S011000, C347S014000
Reexamination Certificate
active
06412925
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The invention relates to an ink jet apparatus.
2. Description of Related Art
Among non-impact printers that have expanded their market by supplanting existing impact printers, ink jet printers are simplest in principle and easily realize color printing as well as printing in multiple gradations. Particularly, drop-on-demand type ink jet printers, which eject ink droplets for printing, are rapidly becoming widespread because of their excellent ejection efficiency and low running costs.
Typical drop-on-demand type ink jet printers include a Kyser type disclosed in U.S. Pat. No. 3,946,398 and a thermal jet type disclosed in U.S. Pat. No. 4,330,787. However, the Kyser type is difficult to miniaturize, while the thermal jet type requires heat-resistant ink because intense heat is applied thereto.
To simultaneously overcome the above-mentioned problems, U.S. Pat. No. 4,879,568 proposes, as a new system, a shear mode type printer utilizing piezoelectric ceramics.
FIGS. 12 and 13
show an exemplary sectional view of a shear mode type ink jet head. The printhead
600
includes an actuator substrate
601
and a cover plate
602
. Formed in the actuator substrate
601
are a plurality of ink channels
613
shaped like a narrow groove and extending perpendicularly to the sheet as shown in
FIG. 12
, and a plurality of dummy channels
615
carrying no ink. The ink channels
613
and the dummy channels
615
are isolated by sidewalls
617
. A sidewall
617
is interposed between each ink channel
613
and each dummy channel
615
. The sidewalls
617
are composed of upper walls
609
and lower walls
611
, which are polarized in directions P
1
and P
2
, respectively. The directions P
1
and P
2
are opposite to each other and parallel to the height direction of the side walls
617
.
A nozzle
618
is provided at one lengthwise end of each of the ink channels
613
. Provided on the other end is a manifold for supplying ink. The dummy channels
615
are closed at the manifold-side ends to block the entry of ink and do not have a nozzle at the other end. Electrodes
619
,
621
are provided, as a metal layer, on opposite side surfaces of each of the sidewalls
617
. More specifically, two adjacent sidewalls
617
,
617
are separated by an ink channel
613
, and electrodes
619
,
619
,
621
,
621
are provided on opposite side surfaces of the two adjacent sidewalls
617
,
617
to constitute one set of actuators. Each electrode
619
provided on the internal surface of the sidewalls
617
,
617
of each of the ink channels
613
is grounded. Electrodes
621
,
621
, each provided on the side surface facing an associated dummy channel
615
, are connected to an associated output circuit
34
(
FIG. 4
) that generates drive signals.
Upon application of a voltage to two adjacent electrodes
621
,
621
on sidewalls
617
separated by an ink channel
613
, the upper and lower walls
609
,
611
of the two adjacent sidewalls
617
,
617
deform, by a piezoelectric shearing effect, in such directions that the volumetric capacity of each of the ink channels
613
increases. More specifically, as shown in
FIG. 13
, when an ink channel
613
b
is driven, a voltage of E [V] is applied to two adjacent electrodes
621
c
,
621
d
, which are separated by the ink channel
613
b,
while the electrodes
619
of ink channel
613
b
are grounded. Consequently, electric fields are generated on sidewalls
617
c
,
617
d
in the directions E, and the upper and lower walls of the side walls
617
c
,
671
d
deform, by a piezoelectric shearing effect, in such directions that the volumetric capacity of the ink channel
613
b
increases. At this time, the pressure within the ink channel
613
b
, including in the vicinity of the nozzle
618
b
decreases. By maintaining such a state for a period of time required for a pressure wave to propagate, one way, along the ink channel
613
b,
ink is supplied from the manifold (not shown) for that period of time T.
The one-way propagation time T represents a time required for a pressure wave in the ink channel
613
b
to propagate longitudinally along the ink channel
613
b
, and is given by an expression T=L/c, where L is a length (perpendicular to the sheet of
FIG. 13
) of the ink channel
613
b,
and
c
is a speed of sound in the ink within the ink channel
613
b.
Based on the theory of propagation of a pressure wave, upon expiration of the time T after the application of a voltage of E [V], the pressure in the ink channel
613
b
is reversed to a positive pressure. Concurrently with the reversing of the pressure, the voltage applied to the electrodes
621
c
,
621
d
are reset to 0 [V].
Then, the sidewalls
617
c
,
617
d
return to their original states, as shown in
FIG. 12
, and pressurize the ink. The pressure reversed to a positive pressure in addition to the pressure generated upon returning of the sidewalls
617
c
,
617
d
generates a high pressure in the vicinity of the nozzle
618
b
of the ink channel
613
b.
As a result, an ink droplet is ejected from the nozzle
618
b.
If a time period between application and resetting of the voltage of E[V] does not agree with the one-way propagation time T, energy efficiency for ink ejection decreases. Particularly, when the time period between application and resetting of the voltage is even multiplies of the one-way propagation time, no ink is ejected. Normally, when the time period between application and resetting of the voltage agrees with the one-way propagation time, energy efficiency reaches its peak, and so does the ink droplet ejection velocity. Thus, the time period between application and resetting of the voltage is preferably odd multiplies of the one-way propagation time.
Recently, demands for higher printing resolutions have increased in order to improve print quality. To respond to such demands, it is preferable to reduce the ink droplet volume. The ink droplet volume is usually reduced by reducing the nozzle diameter or by reducing the drive voltage, that is, the ink droplet ejection velocity.
In the printhead
600
, when a nozzle
618
is exposed to air in a non-ejection state for a while, the ink solvent in the vicinity of the nozzle
618
evaporates, and the viscosity of ink around the nozzles
618
increases. Consequently, the ink droplet ejection velocity and the ink droplet volume decrease, and the ink trajectory is curved by a sidewind generated when the printhead
600
travels. As a result, ink droplet striking positions are displaced. Ink droplets as tiny as 20 pl (picoliters) or less in volume, are especially susceptible to such a problem. As one of the conventional methods to solve the above-described problem, when the nozzles have been exposed to air in a non-ejection state for a predetermined time, a higher drive voltage than usual is applied to increase the ink droplet ejection velocity. However, changing the drive voltage for each print command increases the cost of a power source. Further, changing the drive voltage requires extra time and disables high-speed printing.
SUMMARY OF THE INVENTION
In view of the foregoing problems, an object of the invention is to provide an ink jet apparatus capable of obtaining excellent print quality, at low cost, without changing the drive voltage.
To achieve the above object, an application time of an ejection pulse is elongated in response to a print command, for at least an initial dot, issued after a nozzle has been kept in a non-ejection state. More specifically, a period of time during which an ejection pulse is applied to an actuator is elongated by widening the pulse width of an ejection pulse or by increasing the number of ejection pulses. By doing so, the volume of an ejected ink droplet is increased, and thus, the ink droplet trajectory becomes unlikely to curve under the influence of the sidewind. Consequently, even when the nozzle has been exposed to air in a non-ejection state for a while, excellent print quality can be obtained without displ
Brother Kogyo Kabushiki Kaisha
Nguyen Thinh
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