Patent Grant
7997709
The present invention relates to the field of drop on demand inkjet printers, and more particularly to the improvement in ejection frequency and response time of such drop on demand printing systems.
Traditionally, digitally controlled color ink jet printing is accomplished by one of two technologies; “continuous stream” or “drop on demand.” In both, liquid, such as ink, is fed through channels formed in a print head. Each channel includes a nozzle from which droplets are selectively extruded and deposited upon a recording surface. Continuous stream printing uses a pressurized liquid source that produces a stream of droplets that are selectively steered toward a recording surface to imagewise deposit thereon, or are captured to be recycled
On the other hand, drop on demand printing, provides droplets for impact upon a recording surface. Selective activation of an actuator causes the formation and ejection of a flying droplet that strikes the recording surface. The formation of printed images is achieved by controlling the individual formation of droplets. For example, in a bubble jet printer, liquid in a channel of a print head is heated, creating a bubble that increases internal pressure to eject a droplet from a nozzle opening of the print head. Piezoelectric actuators, such as that disclosed in U.S. Pat. No. 5,224,843, issued to VanLintel, on Jul. 6, 1993, have a piezoelectric crystal actuator in a fluid channel that flexes when an electric current flows through it, forcing a droplet out of a nozzle.
Drop on demand inkjet printing systems have traditionally suffered from a problem of limited droplet ejection frequency. Once a single droplet is ejected form the print head, the ink cavity behind the nozzle opening needs to refill with ink before a second droplet can be ejected. Additionally, the system must dampen the perturbation associated with drop ejection and the system returned to steady state conditions before the next drop can be fired. All of this places constraints onto the fire frequency of drop on demand printing systems and reduces the response time of the system.
By increasing the speed capabilities of drop on demand printing system, it becomes possible to exploit the low manufacturing costs of these systems compared to faster and more expensive counterparts. It is an object of the present invention to increase the speed capabilities of a drop on demand print system by creating continuous flow through in an internal cavity of a drop on demand style print head, and to incorporate a flow stagnation point centered at each nozzle opening in the internal flow path.
It is possible to reduce this limitation by having a continuous flow of fluid from behind each orifice. Continuous fluid flow internal to the system decreases the time to refill the fire chamber directly behind the nozzle opening after droplet ejection. This in turn dramatically increases the response time of the system.
Accordingly, it is a feature of the present invention to provide a drop on demand ink jet print head having a chamber with a plurality of liquid passages into and out of said chamber, such that liquid is continuously moved into the chamber to a stagnation point adjacent to the nozzle opening, whereat the fluid comes substantially to rest, and out of the chamber from the stagnation point such that vector sum of liquid flow derived forces within the liquid channels is neutral. An actuator associated with the chamber is adapted to selectively increase the pressure of the liquid at the stagnation point to thereby eject a liquid drop from the nozzle opening.
In the detailed description of the preferred embodiments of the invention presented below, reference is made to the accompanying drawings, in which:
The present description will be directed in particular to elements forming part of, or cooperating more directly with, apparatus in accordance with the present invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art.
Bernoulli's equation states:
P+½ρV2+ρgh=constant,
where p is pressure, ρ is density, V is velocity, h is elevation, and g is gravitational acceleration. When a steady flow impinges on a perpendicular plate, as shown in
Bernoulli's equation along the stagnation streamline gives
pe+½ρVe2=p0+½ρV02,
where the point e is far upstream and point 0 is the stagnation point. Since the velocity at the stagnation point is zero,
pe+½ρVe2=p0.
The stagnation pressure, p0, is the pressure measured at the point where the fluid comes to rest. It is the highest pressure found anywhere in the flowfield, and it occurs at the stagnation point. It is the sum of the static pressure and the dynamic pressure measured far upstream. The dynamic pressure is so named because it arises from the motion of the fluid. The dynamic pressure is not really a pressure at all. It is simply a convenient name for the quantity (half the density times the velocity squared) which represents the decrease in the pressure due to the velocity of the fluid. We can also express the pressure anywhere in the flow in the form of a non-dimensional pressure coefficient Cp, where
At the stagnation point Cp=1, which is its maximum value. In the freestream, far from the plate. Cp=0.
Referring to
An actuator 32, such as a piezoelectric, acoustic, thermal, or electrostatic actuator, inside pressure chamber 18 is operable to force ink from the pressure chamber through passage 22 and out of nozzle opening 24, causing a droplet 34 to be ejected from nozzle opening 24 toward a recording surface (not shown). During operation, one or both of the ink jet apparatus and the recording surface may be moved relative to the other. By selective ejection of droplets from an array of such nozzle openings along the nozzle plate, a desired image is produced on the recording surface.
Fluid enters pressure chamber 18 of print head 20 from passages 16 as shown by directional arrows or flow streamlines 36 and 38. Fluid travels past actuator 32, and turns into passage 22 towards nozzle opening 24 as indicated by directional arrow or stagnation (dividing) streamline 40. Just before passage 22, the flow splits (see directional arrows or flow streamlines 42 and 44) and exits the firing chamber via ink return passages 30. A stagnation point exists on the directional arrow or stagnation (dividing) streamline 40 directly inside nozzle opening 24, preventing air ingestion through the nozzle opening. See
The stagnation point directly inside the nozzle opening allows printing at a higher frequency than the traditional drop on demand devices as a result of the forced refill after droplet ejection. By creating a stagnation point with flow symmetry above the nozzle opening by dual port input and output flow paths, this invention promotes proper jet directionality and improved refill time.
In ink jet print heads, suitable stagnation flow geometries can result from several formats, such as directing ink toward the nozzle opening perpendicular to the plane of the nozzle opening array as illustrated in
The mechanism by which the ejection of the droplet occurs differs upon choice of the energy source. Still referring to
In an alternative embodiment wherein thermal energy sources 46 and 48 are brought to the point of fluid vaporization, the thermal energy serves to decrease the effective area of fluid flow in direction 40, raising the pressure in the cavity just inside nozzle opening 24, and ejecting a droplet.
In yet another alternative embodiment, side wall energy sources 46 and 48 may be piezoelectric (PIT) crystals. In which case, an acoustic energy pulse is sent through the fluid. The pulse is operable to raise the pressure in pressure chamber 18 and creates droplet 34.
The embodiment shown in
Stagnation flow geometry can be achieved between opposing in flows that are parallel to the plane of the array wherein the fluid meets directly adjacent to the nozzle opening and exits the fire chamber in one or more directions, which are different from the input flow paths. Referring to
The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.
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