Patent Application
20090244180
This invention relates to the field of inkjet printing heads and other microfluidic devices. The invention particularly relates to continuous inkjet printheads with integrally formed structures for print drop selection and guttering of non-print drops.
U.S. Pat. No. 6,079,821 issued to Chwalek et al. discloses a continuous inkjet printhead in which deflection of selected droplets is accomplished by asymmetric heating of the jet exiting the orifice.
U.S. Pat. No. 6,554,410 by Jeanmaire et al. teaches an improved method of deflecting the selected droplets. This method involves breaking up each jet into small and large drops and creating an air or gas crossflow relative to the direction of the flight of the drops that causes the small drops to deflect into a gutter or ink catcher while the large ones bypass it and land on the medium to write the desired image or the reverse, that is, the large drops are caught by the gutter and the small ones reach the medium.
U.S. Pat. No. 6,450,619 to Anagnostopoulos et al. discloses a method of fabricating nozzle plates, using CMOS and MEMS technologies which can be used in the above printhead. Further, in U.S. Pat. No. 6,663,221, issued to Anagnostopoulos et al., methods are disclosed of fabricating page wide nozzle plates, whereby page wide means nozzle plates that are about 4 inches long and longer. A nozzle plate, as defined here, consists of an array of nozzles and each nozzle has an exit orifice around which, and in close proximity, is a heater. Logic circuits addressing each heater and drivers to provide current to the heater may be located on the same substrate as the heater or may be external to it.
For a complete continuous inkjet printhead, besides the nozzle plate and its associated electronics, a means to deflect the selected droplets is required, an ink gutter or catcher to collect the unselected droplets, an ink recirculation or disposal system, various air and ink filters, ink and air supply means and other mounting and aligning hardware are needed.
In these continuous inkjet printheads the nozzles in the nozzle plates are arranged in a straight line, their pitch is between about 150 and 2400 per inch and, depending on the exit orifice diameter, they can produce droplets as large as about 100 pico liters and as small as 0.1 pico liter.
As already mentioned, in all continuous inkjet printheads, including those that depend on electrostatic deflection of the selected droplets (see for example U.S. Pat. No. 5,475,409 issued to Simon et al.), an ink gutter or catcher is needed to collect the unselected droplets. Such a gutter has to be carefully aligned relative to the nozzle array since the angular separation between the selected and unselected droplets is, typically, only a few degrees. The alignment process is typically very laborious if done manually and requires precision-machined components for an automatic kinematic alignment, which results in a substantial increase in the cost of print production labor and cost of the print head. Also, the overall print engine cost is increased because each gutter must be aligned to its corresponding nozzle plate individually with separate kinematic alignment components.
The gutter or catcher may contain a knife-edge or some other type of edge or surface to collect the unselected droplets and that edge or surface has to be straight to within a few tens of microns from one end to the other. Gutters are typically made of materials that are different from the nozzle plate and as such they have different thermal coefficients of expansion. Therefore, changes in ambient temperature can produce sufficient misalignment of gutter and nozzle array to cause the printhead to fail. Since the gutter is typically attached to some frame using alignment screws, the alignment can be lost if the printhead assembly is subjected to shocks and vibration as can happen during shipment or operation.
The U.S. publication 2006/0197810 A1-Anagnostopoulos et al. discloses an integral printhead member containing a row of inkjet orifices.
Earlier coassigned filed application Ser. No. 11/748,663, filed May 15, 2007 titled “An Integral Micromachined Gutter for Inkjet Printhead” and application Ser. No. 11/748,620 filed May 15, 2007 titled “Monolithic Printhead with Multiple Row of Inkjet Orifices” are related to this application and disclose formation of silicon printheads with integral gutters and air channels.
The inkjet printhead is an example of a microfluidic device. Microfluidic devices are devices having a network of channels or conduits or flow paths, or otherwise defined regions of fluid flow, wherein at least one dimension is of order 1 mm or less, and in which fluid must travel for intended operation of the devices. The present invention is also relevant to any microfluidic device in which controlled flow of gases or liquids is required and the flow regimes are such that turbulence causes adverse effects on flow uniformity or control. There is a need to decrease fluid turbulence in microfluidic devices.
A microfluidic device comprising a monolithic superstructure, wherein the superstructure contains fluid channels, and in at least one of the fluid channels, in an area where the channel changes direction or intersects another channel, the channel is greater in cross-section than in other areas of said channel.
In another embodiment of the invention, a microfluidic device comprising fluid channels where said channels comprise projections into at least part of the channel to aid in a uniform laminar flow of fluid over the entire length of the printhead.
b is a schematic partial cross-section of a printhead with steps in the area of greater cross-section.
The invention has numerous advantages over the prior practices. The invention inkjet printhead has improved flow of ink droplets from the inkjet printhead because the flow of gases is less turbulent. Further, the airflow is improved even though the superstructure containing the channels for airflow is formed of a silicon wafer structure wherein etching is not able to produce rounded corners in channels. As is known etching silicon wafers produces only Manhattan skyline type structures with raised and lowered portions joining each other at right angles. These and other advantages of the invention will be apparent from the Figures and the description below.
The invention provides a method to achieve improved airflow in silicon monolithic micromachined structures forming a fluidic network. More generally, a microfluidic device comprising a monolithic superstructure, 3-dimensional network of fluid channels, and means to improve laminar fluid flow are embodied in this invention. The phrases such as “monolithic silicon superstructure” and “monolithic micromachined structures” refer to structures for inkjet printheads or other microfluidic devices that are formed by bonding together wafers of machined material to form a unitary monolithic structure containing channels needed for an inkjet head or other microfluidic device. In a preferred form the monolithic structures or superstructures are formed of silicon wafers that are machined by a process such as Deep Reactive Ion Etch (DRIE) and then bonded together. The improved airflow is an advantage in the utilization of these structures for printhead superstructures.
In the present art, the interaction between the moving ink drops and the surrounding gas flow is crucial in determining the ink drop trajectory and hence the print quality. For example, airflow aligned to the motion of the ink drops, called collinear airflow, helps to reduce adverse drag effects and consequent slowing of ink drops as they move towards the media. In cases where air or gas flow is used for drop separation, gas flow is also employed across the motion of the ink drops. This cross flow air is called deflection air. Instabilities or disturbances in the collinear or deflection airflows result in errors in ink drop position. Typically, this is avoided by using moderate airflows and aerodynamically shaped gas conduits and by maintaining the flow in the laminar flow regime. In addition, other features such as screens and compression zones are used upstream of the printhead to pre-condition air to suppress turbulence arising from instabilities or disturbances in the source pressure and air flow.
In silicon micromachined integrated continuous inkjet printheads, “Manhattan” like geometries are most common. The “Manhattan” geometry is characterized by rectangular features having steep sidewalls. In order to maintain feature widths, highly anisotropic etching processes are commonly used. The Deep Reactive Ion Etch (DRIE) processes used to machine silicon wafers that create the integrated print head are highly anisotropic etch processes that create “Manhattan” like geometries. Although there are other processes that can etch angled walls in silicon, DRIE is a preferred process for etching silicon as it is more suitable for high volume manufacturing and is available commercially from most silicon foundries. The present invention overcomes this limitation of silicon fabrication by adding design features to minimize unsteady gas flow experienced by ink drops. First, it is important to minimize turbulence (random, three-dimensional motion of fluid particles in addition to mean motion) by maintaining the gas flow in a laminar regime, where the gas flow is characterized by smooth motion in laminates or layers. The turbulent and laminar regimes are determined by Reynolds number (Re), which is a dimensionless parameter defined as the ratio of inertial force to viscous force in a given flow field. Re is calculated as Re=ρUD/μ, where ρ is the fluid density, U is the velocity magnitude, D is the characteristic dimension defining the flow geometry, and μ is the fluid dynamic viscosity. In general the flow is laminar at lower values of Re and turns to turbulent at high values of Re. For internal pipe flows, the critical value of Re above which the flow transitions to turbulent is around 2000-2300. Therefore, for a given gas, dimension D is designed such that flow is in the laminar range for the required gas velocity U. However, there are many sources of flow disturbance in the integrated silicon printhead due to its Manhattan geometry. These include sudden expansion zones, flow mixing zones, sharp turns, and sharp edges in the flow path. The device described in this invention addresses this issue. The invention provides an increase in the cross-section of at least one channel in the printhead where the channel bends or where the channel crosses or intersects with another channel. The channel would have an enlarged cross-section area at bends. The channel increase may be in the area, or mouth, at the intersection with another channel. Another advantage of the current device is the integrated micromachined flow straighteners and screens (i.e. flow conditioning aids) that reduce the flow disturbances coming from the source and the tortuous path in the device. These flow conditioning aids are preferably located just before the gas enters the main channel, where it interacts with the ink drops, thus reducing the adverse effect on drop motion. These and other advantages of the invention will be apparent from the Figures and the description below.
The superstructure 14 of the printhead is formed of silicon wafers 46, 48, 50, 52, and 54 joined to form a monolithic assembly. The superstructure 14 is joined to the wafer 56 comprising the inkjet nozzle plate 12. Shown in
The invention relates to control of the flow in the channels of the superstructure 14. The airflow channels are provided with notches 62, 64, 66, 68, 72, and 74 where the channels change direction (in this case, make a 90-degree turns) or intersect with other channels. These notches remove the sharp edges that cause adverse flows in the ink stream path and create intentional flow separation and recirculation zones 73. These recirculation zones act as an artificial wall for the main gas flow and help to make a gradual transition in the flow direction. The arrows in
It is understood that the notches each of 62, 64, 66, 68, 72, and 74 can be divided into notches formed of small steps 60 as shown in
In
Polymer materials suitable for passing through the superstructure to form the rounded filling in the notches may be any suitable material. Typical examples of such materials are thick photoresist SU-8 and polyimide. The preferred material is the polyimide because this material effectively gathers in the notches and it may be placed in the channels by spin-coating.
In
The flow conditioning aids consist of vertical parallel ribs placed in the channels 23 and 25 for the incoming air for the ink stream channel 33 and the cross direction air in channel 29. These projections, 82, 84 and 86 are etched in silicon wafers 46 and 48 and can have a variety of geometrical configurations, including vertical ribs or posts of rectangular, square, or other cross-sectional shape.
Instead of ribs, posts aligned in the direction of airflow could be utilized in the channel to aid in creating a laminar flow of the air. As illustrated in
While the posts have been described as extending from one side of the channel or as being placed across the channel it is possible that the posts or ribs could extend up from the bottom of the channel and downwardly from the top. Such ribs or lines of posts could either meet or be interlocking, with the upper ribs or posts located between the lower ribs or posts. Combinations of ribs and posts also would be suitable. The rib and posts could also extend in from the sides of the channel rather than the top and bottom as illustrated.
Screens can also be formed integrally by using many small through holes in wafer 56 instead of large channels 16, 17, and 18. Theses holes can range from 10 to 100 micrometers in diameter. These screens will act in the same way as the ribs 92 or posts 93 projections at 82, 84 or 86 to further precondition the incoming air to remove turbulence and have uniform flow across the entire length of the printhead.
The integral gutter device of the invention may be formed by any of the known techniques for shaping silicon articles. These include CMOS circuit fabrication techniques, microelectromechanical structure fabrication techniques (MEMS) and others. The preferred technique has been found to be the deep reactive ion etch (DRIE) process, because, in comparison with other silicon formation techniques, the DRIE process enables more efficient fabrication of high aspect ratio structures with large etch depths (>10 micrometers) required for this device.
The techniques for creation of silicon materials involving etching several silicon wafers, which are then united in an extremely accurate manner, are particularly desirable for formation of printheads, as the distance between the nozzles of the printheads must be accurately controlled. Further, there is need to put channels for ink and air handling into the silicon structure in an accurate manner.
The methods and apparatus for formation of stacked chip materials are well known. In
In
While illustrated with particular inkjet printheads, the invention could be utilized in other embodiments. For example, the invention could be utilized in a printhead that printed with small drops and recycled the large drops. Furthermore, the invention could be utilized for improved fluid flow (e.g., flow of gas, liquid, or supercritical fluid) in microfluidic devices that process fluids at flow rates that, without the present invention, are sufficient to produce adverse turbulent effects. The invention could be used in microfluidic devices such as lab-on-chip devices, on-chip chemical synthesis, and microfluidic chips for biomedical applications.
