1. Field of the Art
Manipulating, drying, conditioning or shaping continuous or cut sheet surfaces and surfaces of irregularly shaped objects. Examples include drying, curing, treating, plating, coating, etching, polishing and chemical polishing operations. Though specifically applicable to inkjet printing, the techniques are applicable in almost any surface drying, conditioning, manipulating and shaping situation of various materials that benefits from any of: high efficiency, uniformity, low cost, non-contact manipulation and, or conditioning, and controlled and uniform thicknesses. The techniques are especially useful in increasing the rates of diffusion limited processes at surfaces.
2. Description of the Related Art
Processes today are often limited by the speed at which they condition a surface or medium, which processes often require that the conditioned side not be touched.
In one example application, plating baths currently require close, uniform electrode spacing, high diffusion rates of reactants to the surfaces, and good temperature control. Typical existing plating baths incorporate large tanks which must maintain adequate stirring to maintain uniformity of reactants, but chemicals are wasted because very little of the reactants are actually adjacent the substrates, and energy is wasted due to large electrode spacing and bath heating requirements. In another application, in low cost inkjet printers, the printer mechanism waits for ink to dry or cure sufficiently on a previously printed sheet before adding the next sheet to the output stack to avoid smearing of the ink on the previously printed sheet. Typically low cost printers just wait until the ink dries on the previously printed sheet, even though the print mechanism is capable of printing much faster. Printers which print both sides of the page typically wait even longer for the first printed side to dry before the paper is put through a reverser so the second side can be printed since the reverser mechanism tends to smear the ink on the first side if the ink is not dry. Thus printers that print on both sides of the paper print far slower than printers that only print on one side of the paper. Drying and solidification of inks is limited by the slow diffusion rates of solvents away from the media, and also by slow rate of diffusion of the required heat of vaporization from room temperature air to the evaporatively cooled media.
Higher cost/price printers add various heating mechanisms, including heated platens (sometimes with vacuum hold downs to increase heat transfer rates) and radiant heating means with very little of the radiant heat actually being absorbed by the ink. To date, these methods have been costly, bulky, and inefficient, prohibiting their use in small, low cost applications, such as small office and home printing.
Many prior art drying/fixing/conditioning methods include, individually or in combination, one or more of the following:
However, all prior art methods have one or more of the following limitations:
Low cost printers are capable of depositing ink completely covering a page at about a 30 page per minute rate. However, they never actually print at that rate because the ink takes at least 10 seconds to dry adequately before a successive page can be stacked upon the previously printed page. Inkjet printer manufacturers have been unable to make inks that do all of the following:
Typical inks are made with a water carrier, which is environmentally safe, and whose chemistry with respect to pigments and dyes is well understood. The inks further contain surfactants to help the ink penetrate into the paper, humectants to keep the ink moist in the printhead, dyes or pigments for color, and pigments for black. There are often deliberate chemical interactions between the inks to keep one ink from bleeding into another on the paper. A worst case blacked out page at 600 dots per inch of 5 picoliter black dots has about 0.16 cc of ink on the page. The water in the ink sinks into the paper in about 5 seconds, and begins to swell the paper fibers about 1 percent, causing the paper to bow toward the side with ink on it, causing what is known as wet cockle. If the ink is deposited in a swath of a width W, surrounded by dry paper, the paper buckles in a bubble shape about diameter W, and height of about 0.1 W, after, typically, 20 seconds. As the water further penetrates the page, the backside of the paper also begins to swell, tending to flatten, then reverse the direction of the bubbles as the front side dries somewhat, and the back side is being penetrated by water. Eventually the paper is uniformly swelled within the wet swath, and buckles alternating positively and negatively along the swath length, i.e., the width of the paper. As the water in the paper becomes uniformly distributed, and then evaporates, in a minute or more, the fibers tend to return to their former length, but the paper fiber bonds have yielded, and the paper does not return to a completely flat shape leaving residual dry cockle.
Wet cockle can cause a head crash, where the paper buckles enough to hit the scanning printhead, often located about 60 mils above the paper surface. Limiting the size of the swath, and the amount of ink put on the page, can minimize the height of wet cockle, but smaller swaths result in lower print speeds, and less ink results in less dark blacks or less vibrant colors.
Dry cockle is evident in unsightly wrinkled pages and is to be avoided.
Generally the black ink pigment is intended to stay on top of the paper to produce the darkest blacks, with optical densities of 1.3 to 1.4, comparable to offset printed inks. Black pigment inks cannot contain surfactants to the extent that the pigment wicks across fibers, since that would result in jagged edges on letters which is highly undesirable. Thus the black ink pigment is susceptible to smearing, since it is on the paper surface and mechanically in contact with the next sheet of paper which will be stacked on top of it. Though pigments tend to coalesce and solidify when the water carrier is drawn into the paper (after at least 5 seconds), the pigment is often comprised of block copolymers similar to latex paint, thus pigments do not become permanent for days.
Color inks are typically dyes in water solutions with surfactants which help the water penetrate the paper more rapidly. Color inks take less water to cover a region than black inks because the surfactant spread the ink, and because the color inks do not have to be as dense as black is for text. Since the human eye is not as sensitive to color, jagged edges on color droplets are less objectionable. However, color inks would be more vibrant if they were on the surface. One approach would be to use color pigments but pigments typically are ground up solids with particle sizes over 0.1 micron and therefore scatter all colors to some extent making them somewhat duller than dyes that are confined to the surface.
Thus both black pigments and color dyes benefit by being dried rapidly before they can penetrate the surface of the paper. And, problems of paper cockle would also be relieved if paper could be dried substantially in less than 5 seconds (less than 2 seconds for a 30 page per minute printer).
This problem of drying at greater than 30 pages per minute has been continually studied, and to date has not been effectively solved in a way that is suitable for small (less than 1 cubic foot), low cost (less than $100), printers or even printers that are 5 times as large, and 5 times as costly, and ⅕ the speed.
Some ineffective solutions in the prior art include:
U.S. Pat. No. 6,305,796 by Szlucha et al., which discloses an enclosure wherein paper is heated with radiant heat from infra-red bulbs within a reflective enclosure. The enclosure itself is a substantial part of a cubic foot in dimension, the heat required is substantially more than 180 watts due to bulb and absorption inefficiencies, and the paper drying time is longer than the 2 seconds required at a printing rate of 30 pages per minute.
U.S. Pat. No. 6,463,674 by Meyers et al, which discloses an air impingement drying system that also fully encloses the paper, and, because of the large air boundary layer inherent in the geometry, Meyers system is inadequate to meet conditions stated above in the discussion of the Szlucha patent and only slowly dries the paper.
U.S. Pat. No. 6,382,850 by Freund et al., which discloses a large, complex system of heaters and air knives disposed along a 10 inch linear vacuum belt with the paper being held by the back side. In the Freund system the paper is moved at 5 cm per second, therefore drying at only a 12 pages per minute rate.
U.S. Pat. No. 5,510,822 by Vincent et al., which discloses a heated platen which physically contacts the backside of the paper, and the paper is held in close contact to the platen by a vacuum which is only released to allow the paper to move. This has a high enough heat transfer rate, but would require, at a 30 pages per minute printing rate, that the vacuum hold down pressure be released and restored at least 4 times per second (for a 1 inch swath print mechanism), and it has no provision for adequate air movement to dry the ink.
None of the above are suited to simultaneous double sided printing since they all hold one side of the paper in the drying process.
That which is disclosed here enables low cost, compact, non-contacting, low energy usage means and apparatus for drying/conditioning/shaping/manipulating media, including media that is being dried/conditioned on both sides, without the disadvantages outlined above.
The purpose of the present invention is to support a medium, with applied material on or within it at a fixed distance from a platen by action of fluid flow. The region between the platen and the medium is available for reactions that may be carried out in, or with the aid of the fluid. The configuration enables high surface tangential rates of flow, thus decreasing boundary layer thicknesses, and accelerating diffusion limited processes at the media-fluid interface. By confining the fluid to a small region, with high local velocities, much higher reaction rates occur in a much smaller geometry.
One disclosed preferred embodiment includes a platen which is supplied with both positive and negative pressure which propels fluid through orifices which said orifices are configured to hold the media a predetermined height above the platen, by appropriate fluid flows and consequent forces. The platen surface shape can be flat, ruled, or any arbitrary shape. The platen itself may be rigid or flexible. The media may be heated directly with heated fluid which supplies part of the media suspension means, or alternatively, with heaters thermally coupled to the platen or by radiation whose source is incorporated in the platen, or the media may be heated prior to entry into the region adjacent the platen. The media may be exposed to radiation, catalysts, or reactants. The suspension of the media by action of the fluid a small, fixed, distance above the platen eliminates friction, and enables efficient and predictable energy transfer and, or, application of reactive chemicals, in a very thin, and therefore easily controlled reaction region.
a-2e are perspective views of the housing and Platen of
a is a perspective top view of a Platen having an alternating interlaced pattern of the Negative Pressure Orifices and the Positive Pressure Orifices with the Positive Pressure Orifices being substantially larger than the Negative Pressure Orifices;
b is a perspective top view of a Platen that illustrates an alternative pattern of the Negative and Positive Pressure Orifices with the Negative Pressure Orifices surround by a square shaped groove that each includes an orifice (shown in the left most corner of the groove) that is coupled to the top end of a tube of the Manifold (not shown) from the positive pressure plenum;
c is a perspective top view of an alternative Platen that illustrates a pattern of the Positive and Negative Pressure Orifices with individual radiation sources distributed among the orifices;
a is a perspective view of opposing curved upper and lower Platens with a wider initial opening to facilitate receipt of Media to pass between the two platens to treat both the top and bottom surfaces of the Media;
b is similar to
a illustrates a simplified schematic representation of a Media processing apparatus for simultaneously processing both the top and bottom surfaces of the Media at each step of the operation. Said Media processing apparatus is comprised of input and output pinch roller sets (with the Media feed from right to left through an optional Media surface treatment station), a dual print station, and opposing Platens to an output station;
b illustrates an alternative Media processing apparatus for processing both sides of media with most of the processing of the top and bottom of the media performed at different times;
a-c show various ways to supply and recirculate Fluid to the Platen;
a and 10b show alternative power and airflow requirements that enable drying of a sheet of fully printed paper to be dried at 30 pages per minute;
a and 11b show alternative designs of the Manifolds which allow distribution of fluids at high flow rates, while maintaining uniform pressures throughout the Plenums; and
Though the concepts described below are specifically applicable to inkjet printers, they are also applicable to other processes involving drying and/or conditioning and/or manipulating and/or shaping of various media, including those that are more or less flexible than paper. These concepts are especially applicable where one or more processes that are diffusion limited occur at a surface.
One object of the designs of the disclosure is to dry or condition a medium which may have Applied Material distributed throughout, or have Applied Material on one or both sides.
In this document, all units are SI units, unless otherwise designated and temperatures are in degrees Centigrade.
In this document, the following definitions are used:
Negative pressure is provided to the Fluid by negative pressure source 6 which is connected to housing 1 by conduit 8 to draw Fluid through Negative Pressure Orifices Platen 4 from beneath Media 5. Negative pressure source 6 may contain a fan, pump, or blower to exhaust Fluid, for example air, to create a partial vacuum on conduit 8, and may optionally contain collection vessels to capture any materials in the recovered Fluid that enters the Negative Pressure Orifices in Platen 4. The negative pressure supplied by the fan, blower or other pressure apparatus may be constant or pulsed.
Conduit 8 may be as long as convenient, subject to being sized so no substantial pressure drop occurs along its length. Alternatively, negative pressure source 6 can be mounted directly on housing 1, and fluidically connected to it.
Similarly, positive pressurized Fluid is supplied from positive pressure source 7 which may imbue the Fluid with Additional Applied Material, and/or Reactants, or heat the Fluid. Positive pressure source 7 may contain a fan, pump, or blower, and may optionally contain apparatus to introduce Reactants or Additional Applied Materials into the Fluid. Positive pressure source 7 may be a pulsed source to increase heat or material transfer rates and/or may optionally contain a heater to heat the Fluid. Such devices are commonly available in many combinations and configurations. The Reactants and additional Applied Material may be liquids, solids (particles), or gasses, or combinations.
Conduit 9 transports the Fluid from positive pressure source 7 to positive pressure port 3 with conduit 9 being as long as convenient, subject to being sized so no substantial pressure drop occurs along its length. Alternatively, positive pressure source 7 can be mounted directly on housing 1, and fluidically connected to it.
Housing 1, in addition to providing support for Platen 4, encases Fluid paths therewithin coupled to the respective one of the negative and positive pressure ports 2 and 3 and corresponding Positive and Negative Pressure Orifices in Platen 4 as will become clear in the discussion of
The Orifices shown in Platen 4 permit the passage of Fluids, for instance air, and are interspersed so that the Positive Pressure Orifices, supplied with Fluid under positive pressure tends to distance Media 5 from Platen 4 while the Negative Pressure Orifices supplied with negative pressure, tend to attract Media 5 closer to Platen 4. A balance of forces is achieved at a specific distance of Media 5 from Platen 4, with Media 5 remaining at a specific designed distance from Platen 4 (i.e., at an equilibrium point), as determined by the relative size, geometry, and disposition of the Positive and Negative Pressure Orifices, the pressures supplied, and the pressure head losses in the Manifolds (not shown) between the corresponding negative and positive pressure ports 2 and 3 supplying the corresponding Negative and Positive Pressure Orifices.
Since Media 5 is supported above Platen 4 by the Fluid from the various Pressure Orifices, there is little friction between Media 5 and Platen 4, and thus there is no smearing of the Applied Material which may be on the side of Media 5 adjacent Platen 4. The action of the Fluid, with design parameters to be discussed below, holds Media 5 at an equilibrium distance spaced away from Platen 4. The Fluid forces are substantially greater than gravitational forces, therefore allowing alternative configurations of Platen 4, such as with Platen 4 above Media 5, with the Applied Material on Media 5 facing Platen 4, or in a configuration where the Media is fed vertically.
a is an enlarged lengthwise perspective vertical cross-sectioned view of housing 1 and Platen 4 of
b is an enlarge left end view of the perspective vertical cross-sectioned view of housing 1 and Platen 4 of
c is an enlarge right end view of the perspective vertical cross-sectioned view of housing 1 and Platen 4 of
d is a different perspective view of the vertical cross-sectioned view of Platen 4 of
e shows a portion of the view of
The numbers and sizes and spacing of the Orifices, the Manifold components, the Plenums, and other details of the examples are variables which can be changed with the application and desired function of an implementation. The illustrations are not to scale, and the relative sizes or positions of the components are not necessarily as shown in the drawings.
At high Fluid flow rates, the design of the Manifold shown in
An alternative design for the Manifold which minimizes such pressure drops is shown in
b shows a simplified cross section of the same branched Manifold 216 as in
Returning for simplicity of discussion to embodiments of the invention where high Fluid flow rates do not demand a branched Manifold such as shown in
a and 3b are included here to aid in the discussion of the hydraulics between Platen 4 and Media 5 relative to the position of Media 5 with respect to Platen 4. For purposes of this discussion, Media 5 in
When Media 5 is very close to Positive Pressure Orifices 31 and Negative Pressure Orifices 30, Media 5 acts as a valve limiting the flow rate of Fluid through and between Orifices 30 and 31 to a level well below what it would be if Media 5 was further away from them, or not there at all.
With Media 5 in close proximity of Platen 4, between Positive Pressure Orifices 31 and Negative Pressure Orifices 30, there is a graduated pressure distribution, governed by the equations of fluid flow, which acts on Media 5 and Platen 4. If pressure is supplied to Positive Pressure Orifices 31 while Negative Pressure Orifices 30 are held at zero gauge pressure, then a pressure distribution develops between Media 5 and Platen 4. Similarly, if negative gauge pressure (partial vacuum) is applied to Negative Pressure Orifices 30 while Positive Pressure Orifices 31 are at zero gauge pressure, a corresponding pressure distribution develops. If both types of Orifices are pressurized, the resulting pressure distribution is the algebraic sum of the corresponding pressure distributions, and the average pressure over the bottom surface of Media 5 is the sum of the pressure distributions resulting from application of pressure to positive and Negative Pressure Orifices 31 and 30.
The Orifices 30 and 31 and central bore 72 of tubes 32 of Manifold 33 supplying pressure to Positive Pressure Orifices 31 are designed so when Media 5 is close to Platen 4, the spatial averaged pressure from the applied pressure at Orifices 30 and 31 is positive, and pushes Media 5 away from Platen 4.
When Media 5 is distant from Platen 4, i.e., beyond the equilibrium distance, the Fluid flows through Orifices 30 and 31 almost as freely as it would if Media 5 were not there. However, by design, there is a flow induced pressure loss at Positive Pressure Orifices 31 due to a series flow resistance deliberately designed into the size of central bore 72 of tubes 32 of Manifold 33 supplying Fluid to each of Positive Pressure Orifices 31. Thus the net pressure just above Positive Pressure Orifices 31 is significantly reduced to the point that the spatially average pressure applied to Media 5 becomes negative, thereby attracting Media 5 to Platen 4. One method of achieving a greater flow induced pressure drop prior to Fluid entering the Positive Pressure Nozzle is a long, thin Manifold central bore, 72, as shown. Alternatively, the Manifold could be designed somewhat larger in diameter, and have a constriction somewhere in it. Whatever method is chosen, it is an object of the geometry of the Fluid path to fix an equilibrium position, and Fluid flow rate at that position.
Therefore, there is a sharply defined equilibrium position for Media 5 which can be calculated based on standard fluid flow mathematics, and/or found experimentally. Alternative implementations, with different pressures applied from pressure sources 6 and 7, different Orifice sizes and geometries, for the Positive and Negative Pressure Orifices, and different flow induced pressure drops in the Manifold, or elsewhere, will allow somewhat different Media 5 to Platen 4 equilibrium separations. Fluid flow velocities, and Orifice separations can be optimized to enable efficient Drying and Conditioning, Manipulating and Shaping of Media 5, as can be determined by one skilled in the art, and are further described below.
Orifices 30 and 31 are spaced apart so the pressures above each Positive Pressure Orifice 31 and Negative Pressure Orifice 30 produce forces insufficient to bend either Media 5 or Platen 4 between the Orifices substantially compared to the equilibrium Platen 4 to Media 5 spacing. However, Orifice 30 and 31 spacing and pressure distributions can be designed so cumulatively Orifice supplied pressure distributions force Media 5 to conform to the shape of Platen 4, or alternatively, for Platen 4 to conform to the shape of Media 5, if Platen 4 is non-rigid. This capability of gradually deforming Media 5 can be used, for example, to hold an otherwise non-flat Media 5 to a flat shape over a wide area, as might be advantageous in the print zone of an inkjet printer, or to Manipulate, and possibly Condition or Dry, Media 5 in a curved path without friction in a printer. Alternatively, the capability to bend Media 5 can be used to transport Media 5 around corners with little friction.
In
There is an intermediate separation of Media 5 and Platen 4 where the spatially averaged pressure is zero, which is the equilibrium separation distance, tequilibrium. Media 5 is held at this distance by the action of the Fluid in the conditions set by the Orifice sizes, geometry and separation, and the Manifold Fluid flow resistance and the positive and negative applied pressures by sources 7 and 6, respectively. This separation is stiff in the sense that there is a strong restoring force returning Media 5 to the equilibrium separation distance, tequilibrium.
The following simple example illustrates how the equilibrium position is maintained. If the pressure drop across the length of bore 72 of Manifold 33 is W*R, where R is the Fluidic resistance and W is the volume flow rate, the pressure drop, for low Reynolds numbers flow, between Orifices 30 and 31 is of the form F*W/t3, where F is a constant and t is the separation between Media 5 and Platen 4. The spatial average pressure applied to Media 5 is (Bpositive*Ppositive+Bnegative*Pnegative), where Bpositive and Bnegative are geometry dependent constants, and Ppositive is the pressure at Positive Pressure Orifices 31, and Pnegative is the pressure at Negative Pressure Orifices 30. For this illustrative example, we can take Bpositive and Bnegative to be the same constant B, and set Ppositive to be 2 P, and Pnegative to be −P. The spatial average pressure acting on Media 5 is then:
But W=3*P/(R+F/t3),
so that
P
Media
=B*P(1−(R*(3/(R+F/t3))))
As t approaches 0, PMedia=B*P; i.e., positive. As t becomes large, Pmedia=−2*B*P; i.e., negative. There is an equilibrium spacing, tequilibrium, where PMedia is zero, implying no net force on Media 5, and that Media 5 is in a stable position. There actually are, of course, additional gravitational forces, however they are small relative to the Fluid pressures, so that gravitational forces only very slightly alter the equilibrium separation, tequilibrium.
Since PMedia is a strong function of t, the separation is stiff and stable. For turbulent flows, the relation between t and flow is an even stronger function of t, implying that for both laminar and turbulent flows, Media 5 is stiffly supported at an equilibrium distance tequilibrium that depends on the pressure ratios of the positive and negative pressure sources, and on the geometry of the Orifice sizes, Orifice spacings and the inner diameter, length, and shape of the elements of the Manifold, and hence the Manifold flow resistance.
In
In this configuration, heat is used to accomplish Drying or Conditioning of Media 5. As shown in
A heater also may be designed so that said heater heats the air in the positive pressure Plenum 22 by making the heater in the form of a finned heat exchanger, or any other configuration that efficiently transfers heat to the Fluid. The heater may have a large heat capacity compared to the power applied, so it may average the energy required, minimizing peak power demand. This is particularly useful if the Fluid is recirculated, as described below.
In the configuration of
To enhance the Drying/Conditioning process, suitable Reactants may be added to the Fluid to react chemically with the Applied Material, rendering the Applied Material immobile, or otherwise changing its properties, or the properties of Media 5. In the case of inkjet printing, it is important to keep these Reactants away from the Orifices of the inkjet print head—which is easily accomplished because all the Reactants are confined to the thin layer of Fluid between Platen 4 and Media 5 which is located after Media 5 has left the location of the print heads (see
In prior art, relating to inkjet printers, air jets have been used to enhance evaporation rates, but those air jets tend to disturb the flight path of ink droplets and, because the air jet width is much larger than the thermal diffusion layer, are inefficient in delivering warmed or high velocity air to the boundary layer. In the current designs, having all the Fluid movement between Media 5 and Platen 4, and in pairs of Positive and Negative Pressure Orifices 31 and 30, there is little possibility of disturbing Fluid outside the region between Platen 4 and Media 5.
It should be noted that the key is not to any specific arrangement of Positive and Negative Pressure Orifices, rather that the arrangement of Orifices create alternating regions of positive and negative pressure above Platen 4. Thus, the Orifices may be of different sizes, shapes and positioning.
In the example discussed above, with the Positive and Negative Pressure Orifices being the same size, it is necessary to have the supplied positive pressure from source 7 be greater than the absolute value of the negative pressure from source 6. However, this need not be the case if different sizes or configurations of Orifices are used. For example,
For example, if the Positive Pressure Orifice 92 has radius rp and pressure at the exit of the Positive Pressure Orifice 92 is P+, and the Negative Pressure Orifice 91 has radius rn and pressure at the entrance of the Negative Pressure Orifice 91 is P−, with the spacing between the centers of each of the Orifices is b, the average pressure on Media 5 is approximately:
P
Media
=P
+*{1−(rp/b)2)/(2 ln(b/rp)}+P−*{1−(rn/b)2)/(2 ln(b/rn)}
wherein ln is the natural logarithm, and the terms in { }'s (multiplying P+ and P−) are Bpositive and Bnegative respectively, and are no longer equal.
Thus, one can chose the ratios of rp to rn such that if the pressure applied to the bore of 72 is P, not 2*P as above, there will still be a stable equilibrium position tequilibrium. This allows the use of two identical pressure sources for the positive and negative pressure sources 7 and 6 respectively, or, as will be discussed later, the use of a single pressure source, if desired for cost or other reasons.
b offers a different configuration of the Orifices than in previous figures.
It is often desirable to introduce heat or radiation into region between the Platen and the Media, to, for instance, increase reaction rates.
In this configuration, Platen 4c also serves to introduce radiation and/or electric and/or magnetic and/or electromagnetic fields into the reaction region between Media 5 (not shown) and Platen 4c. In recesses 106, there may be placed sources 105 of infra-red, visible, ultraviolet, or other radiation which may serve to enhance reactions or catalyze processes in the space between Platen 4c and Media 5. Similarly, electrodes which supply DC or oscillating fields may be placed in recesses 106. Platen 4c may also be made of a material that is transparent to such radiation or fields, and alternatively the radiation sources may be located in the bulk of Platen 4c or on the back surface of said Platen that is transparent to said radiation or said fields.
The local arrangement of Orifices, pressures, and Manifold sizes, may be different from region to region of a Platen to achieve different objectives of Platen 4 to Media spacing, and/or different local rates of processes. Some Orifices may be connected through valves (not shown) to vary the local rate of processes depending on whether the valves are open, closed, or partially open.
In Drying, often one of the objectives of Drying is to consume minimal power. In most of the alternatives mentioned throughout this patent, and in prior art, heating the Applied Material to increase its reaction rate, or diffusion rate, also requires heating Media that the Applied Material is in contact with, which wastes energy. In one alternative afforded by the Platen 4c configuration, infrared radiation, for example, absorbed selectively by the Applied Material and not Media 5, may heat the Applied Material much faster than heat is conducted by Media 5. Thus all the heat (in this example) will be supplied only to the Applied Material. To be more specific, in the case of an inkjet printer, with Media 5 being paper, paper is transparent to most infra-red radiation, whereas water absorbs the infrared radiation. When pulsed, high intensity infrared radiation is supplied by light emitting infrared diodes in Platen 4c, such that it heats the ink on the paper faster than about 1/10th of a second, the water in the ink will evaporate before any substantial heat is transferred to the paper. For this technique to be effective, there must be sufficient airflow so that the vaporized water is carried off and not re-deposited on the adjacent paper. Similarly, radiation that is used to crosslink polymers, such as ultraviolet radiation, may be efficiently used, since such radiation is absorbed both by Media 5 and the Applied Material, is confined to the region between Platen 4c and the Media 5, and does not crosslink ink in nearby printheads.
The previous discussion centered around the use of a single Platen. However the Platen or Platens may be configured in various ways to Dry or Condition simultaneously, or successively, both sides of Media 5.
In
In the double sided configuration shown in
b shows another double sided configuration with flat housings 120 and 121, that also serves to guide Media 5 into the space between them. Housings 120 and 121, and their incorporated Platens are operated with positive pressure supplied to the Positive Pressure Orifices and negative pressure applied to the Negative Pressure Orifices. Housing 121 and attached Platen 4f are positioned somewhat to the right of housing 120 and its corresponding Platen. Media 5 is shown approaching housing 121 and Platen 4f at a substantially slight angle, and is attracted toward Platen 4f by the pneumatic forces the Positive and Negative Pressure Orifices as described previously in relation to
Referring to
A Platen may alternatively have any surface shape corresponding to an already similarly shaped Media. The Media may approach the Platen roughly perpendicularly to the Platen surface, and the Media may be drawn to the Platen, and held in place at spacing, tequilibrium, from the Platen by the combined forces of the Positive and Negative Pressure Orifices. The Media may be attracted to the Platen from a slightly separated position by the hydraulic forces described in the text discussing
A Platen of various shapes may simultaneously deform, transport, Condition, or Dry the Media.
It is frequently desirable to process both sides of a Media simultaneously.
Traditionally, inkjet printers have not been able to print simultaneously, or nearly simultaneously, on both sides of a sheet of paper because of the difficulty of Drying the paper with total volume of ink on the paper, and the difficulty in keeping ink from smearing where handling mechanisms necessarily would touch the paper to move it. The techniques discussed herein enables such printing of both sides since those techniques Dry the paper much more rapidly than previously possible, and since the paper is not touched through the processing steps other than advancing the paper through the apparatus at positions prior to printing and after the ink is already Dry.
Those steps and operations that are common to the configurations of both
a and 8b each depict, schematically, a cross section of the paper path of an inkjet printer that incorporates the features of the present invention. In
In
Another exemplary design is shown in
Though the Platens 141a,b, 143a,b, 152 and 154 are shown as being flat in the examples of
a,b,c are schematic representations depicting how pressure sources may be connected in various alternative configurations in the invention, with each configuration having distinct advantages.
In
In this configuration, if the Positive and Negative Pressure Orifices are of the same size and disposed as in
In
Frequently it may be desirable to recycle only a portion of the Fluid, and introduce some new Fluid. This would be the case, for example, if the Fluid is cooled somewhat or the Reactants are depleted, or if there are unwanted reaction products.
c shows a similar configuration to that of
Thus, it is possible to use pressure sources of the same pressure (of opposite sign) to drive the positive and negative pressure inputs of the Platens, or even use a single pressure source to drive both ports, as illustrated in
The following is a detailed discussion of a low cost inkjet printer incorporating the features discussed above that includes a Dryer/Conditioner in the paper path.
Inkjet printers and ink jet inks that take advantage of the features disclosed above are able to:
1. Dry ink and paper more rapidly than about 2 seconds on the printed page without the use of expensive, power intensive, and bulky driers;
2. have sharp edge acuity when printed;
3. have dark blacks and vibrant colors; and yet
4. do not dry out, and clog Orifices of the printhead when the printer is not in use.
The design techniques discussed previously achieve the printing requirements of 1-4 above for an exemplary printer paper path by:
A. Confining all the air (Fluid) to a narrow region between the Media and the Platen, thus providing both high heat and mass transfer rates, and nearly unity efficiencies;
B. Using many Orifices to allow high flow rates with small pressure losses suitable for inexpensive fans;
C. Supporting the paper at the target flying height without friction;
D. Optionally recycling unused heat, and, Reactants or additional materials; and
E. Taking advantage of the fact that, at high enough airflow (Fluid flow) rates, a large fraction of the latent heat of evaporation required will be supplied by unheated ambient air.
An exemplary inkjet printer Dryer/Conditioner is in the configuration of
The evaporative process requires a gradient of water concentration, between the Applied Material surface (in this case, water in the ink) and the Fluid environment (in this case air), which supports a diffusion process. The water vapor concentration at the surface of the paper is determined by the maximum saturated water possible in air at the surface temperature, as is known from psychometric charts. The higher the temperature, the higher the water vapor saturation concentration, the higher the gradient, and hence the higher the evaporation rate. However, when water is evaporated from a surface, the evaporated water absorbs and takes with it a corresponding latent heat of vaporization.
That heat must be replaced from one of three sources:
A. the bulk of the substrate (i.e., paper) on which the water resides;
B. the heat from the (forced convection) ambient air which is at a temperature above the evaporatively cooled temperature of the water surface; and
C. heat of the water itself from temperatures above the temperature of the evaporatively cooled equilibrium temperature (generally a small contribution to the total heat needed).
The water vapor concentration gradient, ΔCwater vapor/Δx, is also partly determined by Δx, the distance over which the gradient occurs. In natural convection over a piece of paper, the separation distance between the surface of the paper and ambient water saturation (boundary layer thickness) is a significant fraction of the size of the paper. However, in forced convection, as in this invention, the boundary layer can be made very small—thus increasing dramatically the evaporation rate.
Since the water gradient is influenced by the water vapor pressure in the channel between the Platen and the paper, the input air (Fluid) must have enough capacity for vapor generated so that the gradient persists as water is evaporated.
There is another gradient that is important—that of temperature from the air between the Platen and the paper. That gradient determines the rate of heat flow from the air to the water surface on the paper, providing heat mentioned in subparagraph B above. Again, making the space between the paper and the Platen much smaller than in the thermal diffusion length (thermal boundary layer) in natural convection, by forced internal convection allows a much greater heat transference rate, in a smaller space, than would otherwise have been possible.
In normal unforced thermal convection, the heat transfer rate between two planes is:
Q=(k*A*ΔT)/tseparation
where Q is the rate of heat transfer, tseparation is the spacing between source and sink, k is the thermal conductivity of air, A is the surface area of the Platen and Δt is the temperature difference between the air and the Platen.
In forced convection, the local heat transfer rate is
Q=(Nu*k*A*ΔT)/Dh
where Ph is the hydraulic diameter, and in this case it corresponds to 2*tequilibrium. Nu is the dimensionless Nusselt number, reflecting the geometry, velocity, and viscosity of the Fluid.
If the airflow is low enough that the air passing over the page is saturated with water vapor at ambient temperature, there will be no evaporation, and no evaporative cooling. Hence, one goal of the invention as applied to inkjet printing is to provide adequate airflow, which provides both a destination for the water vapor from the ink, and heat of vaporization to allow the water in the ink to evaporate.
There are multiple approaches to drying which are differentiated by the source of heat. They include providing heat by preheating the Media, by heating the Media from ambient air as it passes between the Platen and Media, by heating the Media with preheated air as it passes between the Media and the Platen, and by using heaters located within the Platen itself.
If the airflow between the Media and the Platen is fast enough to not become saturated with water vapor, the surface ink temperature will drop to the wet bulb temperature corresponding to the water content of the ambient air—which for 20° C. ambient air with 20% relative humidity is −5° C. Then the surface ink will remain at the wet bulb temperature, and there will be a 25° C. difference between the ambient temperature of the air and the surface ink. Heat will flow to vaporize the ink from: the paper; from the remaining initially room temperature ink; and from the air that is being driven through the Orifices. The temperature difference between the ambient and the wet bulb temperature times the thermal capacity of the paper and the ink will provide 122 out of the 368 joules required to evaporate ink from the page. The air driven through the Orifices, if ambient air at 20° C., will have a 25° C. temperature difference to supply the remaining heat. If the Fluid (air, in this case) is heated above ambient temperature, the wet bulb temperature of the paper corresponding to the Dry bulb temperature of the heated air will provide a temperature gradient that is the difference between the wet bulb, and the Dry bulb temperature for the air.
The relevant equations for combined mass transfer and heat transfer in the case of evaporation of water in ink on a surface are:
A Mass Transfer (by diffusion/forced convection) Equation:
And a Heat Transfer (by diffusion/forced convection) Equation:
where
In this case, Sh and Nu are both the same function of velocity of the airflow, and geometry, and viscosity of the Fluid. In the above the brackets [ ] mean “as a function of”.
Combining these equations leads to a single differential equation for ink temperature Tink.
Thus if Tink starts at an ambient temperature, and there is air flowing, the Nusselt and Sherwood numbers will be non-zero, and the ink will cool since the first term on the right hand side will be much larger than the second term, and ink will evaporate rapidly. As the ink evaporates and cools, Cs[Tink] decreases dramatically, since saturation concentrations are exponential functions of 1/temperature, to the point where the two terms on the right hand side are equal, and further cooling stops. This temperature is defined as the dew point, Tdew. When the ink and the paper it is sitting on are no longer cooling, by the mass transfer equation above, the water vapor in the ink is still evaporating.
from the heat transfer equation,
then
and therefore,
As the ink is transitioning from its initial temperature to the dew point, a certain amount water in the ink is evaporating. That amount is just that amount that can be vaporized by the latent heat of vaporization supplied by the transition of the paper from its initial temperature to the dew point.
Thus, after a short delay when a fraction of the ink is evaporated as the ink and Media moves to the dew point (essentially, flash evaporization), heat transfer from the Fluid (air) supplies the heat to evaporate the remaining water in the ink. When the water in the ink has vaporized, the Media and ink temperature begins to rise from the dew point towards the temperature of the supplied air.
Thus a model for the time required to evaporate the ink on a page is simply the time for a heat exchanger formed by the paper and the Fluid (air) flow to supply the heat of vaporization for the water remaining after the initial cooling phase where the ink temperature lowers to the dew point.
In current typical printers, about ⅓rd of the water initially on the page can be flash vaporized by the transition of the paper temperature from its initial (usually, room) temperature to the dew point. The remaining water must have heat transferred to it through either heating the paper prior to printing, supplying heat from ambient air, or supplying heat from heated ambient air.
Even in the case of the Media thermal mass being adequate to supply all the heat necessary to flash evaporate the water in the ink, there still needs to be an airflow to maintain a water vapor gradient. However, the amount and velocity of that air can be considerably less than in the case where externally supplied heat via heated air is required.
Thus the forced convection described in this invention is suitable for both situations, though the design parameters are different.
The choice of a design for an inkjet printer Dryer includes:
In general, a number of simultaneous constraints must be met by design parameters including:
The mass of air moving through the heat exchanger, multiplied by an efficiency factor must supply the necessary heat of vaporization not already supplied by the thermal mass of the paper.
M
air
*C
air*(Tin−Tdew)*ηheat exchange+Mpaper*Cpaper*(Tpaper initial−Tdew)>=H*mink
Mair=Mass of air required to process one page
Cair=the specific heat of air
ηheat exchange=the efficiency of the heat exchanger
Mpaper=the Mass of a sheet of paper
Cpaper=the specific heat of paper
Tpaper initial=the paper temperature entering the plating region
The efficiency of the heat exchanger is consistent with Fluid flow rate, and the geometry of the heat exchanger. For a parallel plate heat exchanger, that relationship is described by:
−ln [1−ηheat exchange]=k*A*Nu/(2t*Mair*Cair)
k=the air thermal conductivity
M=the air mass flow rate
We want the efficiency to be greater than required, so the equation above is written as a constraint:
−ln [1−ηheat exchange]<k*A*Nu/(2t*M*Cair)
The air moving over the paper must be able to absorb all the moisture in the ink. That air can absorb at most the difference in its current moisture content, and its saturated air content. Thus, analogous to the Heated air mass per page equation above, there are equations for minimum amount of air that must be passed over the paper, and for the efficiency of that absorption process.
M
air(Csat[Tin]−Cin[Tin])>=Mink
Csat[Tin]=the saturation concentration of water at air temperature Tin
Cin[Tin]=the actual concentration of water at Tin
Analogous to heat exchangers, there is an efficiency in transferring mass, described by a similar relationship:
−ln [1−ηmass transfer]=ρair*D*A*Sh/(2t*M)
ηmass transfer=the mass transfer efficiency
D=the air diffusivity
ρair=the density of air (or Fluid)
Analogous to the heat flow efficiency equation above, the requirement to achieve a given efficiency is therefore expressed as
−ln[1−ηmass transfer]<ρair*D*A*Sh/(2t*M)
Since, when air is the Fluid, ρair*D is almost exactly equal to k/Cair, mass and thermal transfer efficiencies are approximately equal. However, since the ambient air has typically much more capacity for absorbing the water in the ink than the warm air has energy to supply heat, the required mass transfer is lower, and hence the mass transfer efficiency does not provide a design limitation.
However, in the case where the Media has sufficient heat capacity to vaporize all the water, the Heated Air Mass per Page, and the Heat Transfer Efficiency-airflow inequalities would no longer be a constraints, but the Dry Air Mass per Page equation, and the Mass Transfer Efficiency-airflow equation would still be constraints.
The flow rate of air per page per second is consistent with the number of pages per minute to be printed.
M=(Mair*ppm/60)
Where:
The wet bulb temperature (also called the dew point temperature) is determined by the ambient temperature of the input air, and its water content. Tdew can be found by solving the following Clausius-Clapeyron equation:
T
in
−T
dew=2.07*103*((1.34*106exp(−5295/(273+Tdew))−Cin)
The pressure from the blowers just equals the flow resistances in the Fluid path. Thus the pressure drops from the Manifolds, Orifices and the region of Fluid between the paper and the Platen must be just equal to the assumed driving pressures from the positive and negative blowers, at the required flow rates. Pressure drops may have terms both linear in flow velocity, and quadratic in flow velocity. Equations for the flow losses due to linear terms are derived from Hagen-Poiseuille equation for various geometries.
The pressure-flow relationship is:
+Positive pressure from the positive source
−pressure drop in positive Manifold [=a term linear in velocity—Loss pl]
−pressure drop in positive Manifold [=a term quadratic in velocity—Loss pq]
−pressure drop between Orifices [=Loss pmemb]
−pressure drop in Negative Orifice [=a term linear in velocity—Loss nl]
−pressure drop in Negative Orifice [=a term quadratic in velocity—Loss nq]
−Negative pressure from the negative source=0
Each of the pressure terms in the pressure-flow equation just above can be expressed in the terms of geometrical and other design parameters. Substituting geometrical and other design parameters in the simplified equation just above, the pressure-flow relationship is expressed in an equation with 7 corresponding terms on the left hand side (below):
Where
The sum of all the forces caused by fluid pressure must, at the equilibrium height, tequilibrium, be zero.
Pressure at Positive Pressure Orifice*geometry factor+Pressure at Negative Pressure Orifice*geometry factor+Pressure from Change in momentum of Fluid=0
That is mathematically stated as:
(Ppos−Losspl−Losspq)*(1−(rp/b)2)/(2 ln(b/rp))+(Pneg+Lossnl+Lossnq)*(1−(rn/b)2)/(2 ln(b/rn))+Change in momentum of Fluid=0
In many cases the “change of momentum contribution” of the Fluid can be made small, and is neglected here. Depending on the actual geometry, one may include a correction term for the Fluid momentum contribution, or use a 3-d Fluid flow model to compute the forces more accurately.
In addition to balancing the forces at the equilibrium position, it is important that at t much larger than the desired equilibrium position, tequilibrium, there is a strong restoring force returning the Media toward the equilibrium position. This means that one would like the average pressure at “large paper to Platen spacing” to be comparable to the pressure of the negative supply. A simple equation for this is:
(Ppos−Loss pl−Loss pq)=0 when the paper to platen spacing is 2 times the equilibrium spacing, tequilibrium.
Per the reference, “Heat and Mass Transfer”, by Baehr, 2nd Edition, p 354, equation 3.258, the average Nusselt number is geometry dependent, and can be approximated by:
Nu=No*tan h(2.432Pr1/6X1/6) where X=(L/d Pe), and No=3.65.
L is a characteristic length, and d is a characteristic thickness (Respectively b and t in this patent), and Pe is the Peclet number. Pr is the Prantl number, approximately 1; tan h is the hyperbolic tangent.
For this patent, this reduces to, as a function of geometric parameters:
Nu=0.136[M4b7Pr2/ρ4v4A4t3)]1/6
Where v=the kinematic viscosity.
It is desirable that the deflection of the Media not be large enough to contact the Platen.
Deflection of a Media can be approximated by plate clamped on all 4 edges, which equivalent to the boundary conditions in the middle of a large Media. From well-known shell deflection theory:
Where
We can require that
φ<0.2t
The above equations and inequalities above may be used as constraints in a general purpose non-linear optimizer, such as the Nminimize function of Mathematica to minimize variables of interest—typically Platen area and power input—for a desired paper throughput. Thus, the optimizer is given the equations as constraints, and, say, Platen area to be optimized, and then chooses all the other design variables (within ranges) to optimize the Platen area. It should be recognized that all equations above are 1-dimensional equation approximations of 3 dimensional geometries, and as such, can be made more accurate by fluid flow simulations or experiment.
In
In the
From the table of
In some designs, such as design 15 of
The above specific solutions for inkjet printers are illustrative of relevant equations for other applications. For use of the concepts and designs presented in this invention in other applications, there will always be a pressure-flow equation, and a force balance equation, but other equations relating the diffusion related phenomena or other phenomena will depend on other process objectives.
In an alternative design, if the heat contained in the paper between the Fluid wet bulb temperature and the ambient is greater than the heat of vaporization of Fluid on a blacked out page (because the printer uses smaller drop volumes per pixel, or the paper is heavier weight, or the Fluid has a lower heat of vaporization), Drying would be limited by only mass diffusion, which is significantly faster than heat diffusion. The required Fluid flow rate would be limited by that required to keep the Fluid from being saturated with water vapor, and maintaining the necessary vapor gradients. The simulation equations above would be the same equations, but the mass transfer efficiency air flow equation would be the limiter, not the heat transfer efficiency-air flow equation. One could design using the same procedure outlined above a high efficiency mass flow transfer apparatus with efficiencies of over 80%. Then, if a page had 0.05 cc of Fluid on it (roughly ⅓ of that assumed in the simulations above, and ⅓ the current practice), one would only require that the air passing through the Platen could absorb 0.05 cc/0.8=0.0625 cc Fluid without saturating. For 30% humidity air, the amount of additional Fluid that can be added to air before saturation is 0.012 grams of Fluid per gram of air. Thus removing the Fluid from a page with 0.05 cc ink would require 0.0625/0.012=5 grams of air per page. At thirty pages per minute, this is 2.5 grams of air per second, or about 2.3 liters per second—considerably lower than the 18 liters per second required when ambient air is used to supply heat to the paper (see
Thus there is considerable incentive to reduce the amount of ink on the page.
One method of reducing the amount of ink on a page is described by U.S. Pat. No. 6,155,670, by this inventor and others, and assigned to Hewlett Packard Co.
Another way to reduce the heat of vaporization required is to use organic solvents, which have lower heat of vaporization. However, the use of volatile organics have potential negative environmental problems, and may result in rapid clogging of a printhead.
Alternatively, Media that allow a certain amount of ink Fluid to remain on the page without cockle or smear, such as coated Media, reduce Drying requirements and make possible smaller Platen sizes, lower airflow rates, and decrease or eliminate the need for additional heat. However, coated Media is not favorably received by the public for everyday use because it is expensive and not widely available, so it cannot be viewed as a generally useful solution to the Drying problem.
Returning to the conditions where the printer ejects 5 picoliter drops on a 600 dpi pitch, and 4 mil thick paper, in design 25, the Fluid (air) is supplied at 27 degrees C. (i.e., slightly above room temperature) and about 14 liters per second; the resulting power consumption is about 121 watts. However, typical pages have less than 10% density ink coverage. Thus the warmed air can be recycled so the heater has to supply only fraction of the peak heat requirement on average. With little ink on the paper, power is still used to heat the paper from 20 to 27 degrees C. (with 85% efficiency), consuming about 45 joules of what would have been over 242 joules had the paper been covered in ink. However, the remaining energy can be recycled, resulting in about 70% energy saving. Thus, the average power required to Dry paper with on average 10% printing density using 27 degrees C. heated air would be about 36 watts if the Fluid (air) is recycled.
a shows schematically a non-recycled supply scheme with Platen 160 fed by positive Plenum 167, and exhausted by negative Plenum 161 by blowers 164 and 163, with arrows showing direction of Fluid flow. Pump 164 receives Fluid from reservoir (which, in the case of an inkjet printer, could be the environment) 169 via conduit 168. Pump 162 exhausts the Plenum via 161 through conduit 166 to a reservoir, or sump (in the case of an inkjet printer, the environment).
A recycling scheme is indicated schematically in
In
The fraction of air (more generally, Fluid) that is recycled may be held constant, or varied by a controller, as determined by the amount of ink on previous pages, and the thermal mass and other characteristics of the heater, or by measurements of such parameters as the exiting Fluid temperature, or concentration of Reactants in the exiting Fluid. The thermal mass of the heater (such as 80 in
Since Media 5 is supported by the current designs without contact with a surface Media 5 can be Dried or Conditioned simultaneously on both sides—enabling simultaneous double sided printing—heretofore not possible in low cost inkjet printers.
Compared to prior inkjet art, printers incorporating the current designs enable much faster printing, enable double sided print, and result in more vibrant and sharp print, and more permanent print, at a very small increase in cost over the current state of the art printers.
Similar corresponding benefits are available in other diffusion limited processes involving surfaces, such as the plating example mentioned above.
An other advantage of the current designs is that the apparatus can force Media 5 to conform to a flat shaped Platen 4 thus maintaining an arbitrarily large flat zone. This in turn, allows the use of print heads larger than the current state of the art, ⅚th inch swath can be used, thus increasing the print rate correspondingly.
While the features disclosed herein have been described with respect to various designs and focused more on inkjet printing, there are other designs which could be implemented to utilize those disclosed features and many other applications for other types of Media whether they be fabric, sheet materials such as plastic and rubber and metals such as aluminum and steel, and even semiconductor materials to name just a few. Clearly one skilled in various arts could foresee many different applications to many different materials that are similar and equivalent to what has been discussed here.