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TONER PRINT SYSTEM WITH HEATED and the speed of contact, among other variables. When one
INTERMEDIATE TRANSFER MEMBER or more of these variables is selected based on independent
considerations, it may prove difficult to then achieve a
BACKGROUND OF THE INVENTION suitable transfer speed or efficiency with the selected vari
The present invention relates to toner imaging systems of 5 aD*e'
the type wherein a latent charge image is developed with a K would therefore be desirable to achieve a printing
pigmented toner, and the developed image is transferred to system in which transfer of a toned image is quickly and
a receiving member to make a printed image. There exist efficiently effected.
many technologies for forming a latent charge image, It would also be desirable to achieve such a printing
including optical image projection onto a charged photo- 10 system wherein multiple toned images are successively
conductive belt or drum; charging a dielectric member with transferred to form a multicolor image, an electrostatic pin array or electron beam; and charge
projection from a so-called ionographic print cartridge or SUMMARY OF THE DSfVENTION
from a plasma generator. Once a latent image is formed, the These and other desirable properties are obtained in a
latent image may be transferred to an intermediate member 15 printing system wherein a first endless imaging member,
before development, or may be developed on the same such as a belt or drum, moves past an imaging station where
member as that on which it is formed, with different system it receives a dry toner image, and contacts a second endless
architectures having evolved to address different process imaging member at a nip to transfer the toner image to the
priorities, such as cost, speed, preferred type of toning second member. The first and the second imaging members
system or intended receiving substrate. The toner may be of 20 are each operated isothermally with at least the second
a liquid-carried or a dry powder type; the former pose member at a temperature T2 higher than the softening
environmental concerns of solvent or carrier management, temperature of the toner which, in turn, is preferably above
especially when printing on so-called plain, or bond, papers, the temperature Tl of the first member,
while the latter developers raise concerns of dust control, ^ &st member has a low surface energV) wMch is
especially as the toner particle size becomes finer. In either preferably under about 20 dynes per centimeter, and has a
case, one must generally also address problems related to hard abrasion-resistant and smooth surface, while the second
erasing or cleaning intermediate image carriers, and fixing member is both softer and has a generauy higher surface
the final image. energy. When the toned image enters the nip formed by the
In general, the toned images, once transferred to a receiv- 3Q first and second member, essentially complete image transing member require heating to dry or fix the final image, but fer to the second member occurs. The second member cannot endure heat at an earlier stage, when the toner is further has a surface energy below that of the ultimate applied, as a dust or liquid suspension, to the latent charge imaging substrate, e.g., below that of paper, while its thickimage. Furthermore, at an even earlier stage, heating is ness and compressibility are selected to allow it to conform generally also to be avoided on or near any photoconductive 35 to this substrate. The second member forms a second nip at elements. Even for charge deposition systems in which an a pressure roller, where the image it received is transferred electric charge is applied to a dielectric rather than photo- from the member to the substrate by contact conductive member, heat may impair the dielectric proper- a preferred embodiment of the printing system, a ties of some common image-holding materials. charge-deposition cartridge deposits an array of charge dots
Thus, a complete imaging system often benefits from, and 40 0n the first member to form the latent charge image, and the
may require, having the image transferred one or more times first member includes a dielectric surface layer which
before the final printing step in order to isolate the charges at each dot to a voltage level which is effective to
chemicals, temperatures or other environment of one imag- attract and hold toner particles. For example, a five
ing process station from those of another. micrometer thick surface layer of Teflon PEA simulta
Another factor which has assumed prominence in imaging 45 neously provides a suitable capacitance and low surface
systems of the foregoing type is the heat transfer, or energy. After the latent image is deposited, the first member
transfusing, of a toner image onto a final receiving substrate. is then developed with a toner, such as a monocomponent
In various prior art constructions, the toned image is simul- magnetic toner, which has preferably been formulated free
taneously transferred to and fixed on the final member in a of waxes or oils, with hard particulate toner particles formed
melted or fused state. It may further be necessary to control 50 of a polymer having a softening temperature somewhat
the precise temperature to vary the relative tackiness or the below the operating temperature T2 of the second imaging
self-adherence of the heated toner, for example, in order to member. A suitable toner is a Coates RP 1442 toner, having
achieve optimal transfer of the image between rollers, or, a particle size of 12-15 micrometers and a tack temperature
when transferring to a final recording sheet, in order to of 90°-110° C. The second imaging member may be a belt
optimize image reflectance properties. U.S. Pat No. 3,554, 55 having a woven Nomex carcass and a 0.5-2.0 mm overlay
836 of Steindorf describes a general approach useful in such of the silicone rubber elastomer, operating at a temperature
multi-transfer systems. According to that patent, intermedi- of 105°-120° C. The silicone rubber has a Shore A hardness
ate rollers may be formed of a silicone elastomer, and of approximately 50 to 80 durometer, and is resistant to
transfer is efficiently arranged between two successive degradation or changes in its physical state so long as it is
image-carrying members by controlling the temperature of 60 operated below about 200° C.
the colored image layer so that it is in a rubbery state, while With these two members, when the first and second belts
the members have surfaces of silicone elastomer of increas- are each operated isothermally at 60°-65° C. and 115°-120°
ing energy to enable relatively effective transfer of the Q, respectively, complete image transfer is achieved at
heated image material from one roller to the next speeds of 15-30 inches per second. The thickness and soft
Nonetheless, the transfer of a toner image from one 65 durometer of the second member increase the nip width and
member to another remains highly dependent on the mate- allow the second member to deform slightly so it conforms
rials used, as well as on the characteristics of the transfer nip with the entering image, enhancing dwell in the nip and
increasing the area in contact with the toned image. The first member may be maintained at a temperature below the toner aggregation temperature by a simple blower, while a heater in the second member maintains it at the temperature T2. Essentially, only a small quantity of heat enters the first member, through the first nip, so each member operates isothermally and little extraneous heat loss occurs between the members.
A final print or image is produced by transferring the hot toner image carried by the second member onto a sheet or substrate passing through a second nip. The substrate is preferably preheated to a temperature not substantially below the toner softening temperature so that when it passes through the second nip contact with toner is complete and the heated toner wicks onto the substrate, separating from the second imaging member.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other features of the invention will be understood from the description and claims herein, taken together with several illustrative drawings, wherein:
FIG. 1 illustrates steps of a basic embodiment of the method of the present invention;
FIG. 2 is a schematic view in section of single-color printer embodying the invention;
FIG. 3 shows a detail of the embodiment of FIG. 1;
FIG. 4 illustrates temperature within the printer of FIGS. 2 and 3;
FIG. 5 illustrates a multicolor embodiment; and
FIG. 5A illustrates another multicolor embodiment.
FIG. 1 illustrates a printing method 10 in accordance with the present invention, which basically addresses the effective transfer of a dry toner image in an electrographic printer. The method includes a step 2 of forming a toned image on a first member which has a hard, low-energy surface, and a step 4 of heating the toned image during a short dwell time as it passes under pressure through a contact nip formed between the first member and a second member having a higher surface energy and softer bulk hardness property.
The method further includes the step 6 of separating the toned image onto the second member, by rotating the second member through the nip, each of the first and second members being maintained isothermally, with at least the second member being at a temperature above the temperature of the image in the nip. Advantageously, the second member is arranged to form a second nip through which in a second transfer step 8 a receiving substrate is passed in synchrony with passage of the heated image that was received at the first nip. The substrate is preferably preheated to a temperature somewhat below that of the second member, so that as the substrate passes a nip formed between the second member and a pressure member, the toned image undergoes a final step of sticking to the substrate and cooling.
Thus, a second or intermediate member picks up the toned image as it raises the toner temperature, and then releases it to a receiving member as it lowers the toner temperature, each of the two temperature transitions occurring nearinstantaneously during dwell time of the transfer nips, while the second member runs isothermally in the middle between the first and last imaging steps.
FIG. 2 is a schematic sectional view of a complete printer 100 for performing single color printing of an image in accordance with the method of FIG. 1.
In this embodiment, a first imaging member 102, shown as a belt, receives an image and carries it to a nip 110 formed between member 102, and second member 104 which is also a belt. At nip 110, the developed toner image is transferred
5 to the second member 104, which then carries it around to a second nip 120 formed between the member 104 and a pressure roll 105. There the image is transferred a second time, from the intermediate belt 104 to a recording substrate 107, such as a sheet of paper. Drive rolls 108, 109, move
10 synchronously and define a precise nip where the respective belts 102,104 contact. Similarly, pressure roll 105 may be driven synchronously with roll 109. It will be understood that one or more of the rolls may be an idler roll driven by contact with the opposing sheet, belt or drum.
15 In the embodiment of FIG. 2, the first imaging member 102 is a thin hard belt with a very low surface energy, and with at least a thin surface portion formed of dielectric material to receive and hold a latent charge image. Suitable belt constructions for forming such an imaging member are
20 shown in commonly-owned U.S. Pat Nos. 5,103,263 and 5,012,291, which are both incorporated herein by reference. The belt travels counterclockwise past a biased corona rod 112 which establishes a uniform or null level of charge on the belt surface, and continues past a charge deposition print
25 cartridge 114. The charge disposition print cartridge is a controlled array of electrodes configured to generate localized silent glow discharge and to direct charged particles from point-like regions of the array as shown, for example in U.S. Pat Nos. 4,160.257 and 4.992,807 and others, to the
30 imaging belt 102. An imaging module 116 provides electronic control signals to electrodes of the print cartridge 114 in an appropriate order to deposit the desired latent image of text, graphics or the like.
Once the latent image is deposited on the first imaging
35 member 102, it travels past a toner applicator 124 in which a magnetic brush 122 brings a thin layer of monocomponent magnetic toner into proximity with the surface of the belt causing the toner to selectively adhere to the charged areas of the latent image. The toner is formulated of hard polymer
40 particles, free of waxes, and the member 102 is both smooth and hard, so adherence of the toner is due essentially to attraction by the latent image charge. In this manner the latent image is toned. While not illustrated, a temperaturecontrolled switch preferably actuates a blower to maintain a
45 flow of room air over the inner surface of the belt or over a drive roller contacting the belt to maintain its temperature below a certain limit, e.g., about 65°-75° C.
In accordance with a principal aspect of the present invention, both belts 102 and 104 are operated isothermally,
50 that is, each is at a constant temperature, and substantially complete transfer of the toned image is achieved due to the surface properties of the belts. To achieve this, the first member 102 has a non-elastomeric hard coating of Teflon PEA having a hardness of 65-70 Shore D, with a surface
55 energy below about 20 dynes/cm and approximately 5 micrometers thick, resulting in a surface capacitance of about 400 pf/cm2. This material allows the charge deposition cartridge to charge surface dots to a 50-250 volt potential, and maintains excellent charge dot resolution. Applicant has
60 also successfully used a hard non-conductive silicone rubber of somewhat higher surface energy. Suitable charge deposition cartridges for latent image formation are sold, for example, by Delphax Systems, of Mississauga, Ontario, Canada. The underlying belt may be a Kapton conductive
65 polyimide film, a stainless steel belt, or other thin continuous sheet or surface having a conductive backplane. The imagereceiving belt 104, by contrast, is much thicker and has
entirely different surface properties. One representative belt 104 is built with a woven Nomex body, and coated with a Vt-2 mm thick layer of a silicone or fluorosilicone rubber, having a surface energy of about 22-35 dynes/cm, and a hardness in the range of approximately 50-80 Shore A. The 5 materials are also selected so that operation at temperatures up to 200° C. will have little effect on their elastomeric, mechanical and release properties, and so that they have sufficient strength to sustain the level of strain energies occurring at the two transfer nips 110, 120. 10
In general, where the ultimate print is to be on plain paper, the thickness of the silicone or fluorosilicone rubber layer and its hardness are selected to assure a high degree of conformability to the receiving substrate, as described in the aforesaid U.S. Pat. Nos. 5,012,291 and 5,103,263, while 15 thinner and/or less compressible formulations may be used for printing on or transferring to smoother substrates. The thickness also affects the effective dwell time in nip 110. Nips 110 and 120 are formed with only a moderate nip pressure, about twenty-five pounds per linear inch, and 20 dweE times at paper feed speeds of 15-30 inches per second are in the range of 2-20 milliseconds or more. Since the thermal time constant of a ten micron toner particle is quite short, these are effective to fully heat the toner image at nip
FIG. 3 shows an enlarged detail of the toned image 200 passing through the nip 110 between belts 102 and 104. As shown, belt 104 has an in extensible and strong support 104a coated with the silicone elastomer surface layer 104£>, while
belt 102 has a much thinner dielectric surface coating 102fc on a support formed of conductive Kapton film 102a, both of which are quite thin and hard.
The toner particles, which may be formed of iron oxide, lamp black and thermoplastic resin or fusible polymer, have 35 a mean particle size ten to fifteen micrometers in diameter, and are compounded without waxes or low-temperature binders. Belts 102 and 104 are each maintained at fixed temperature, with at least belt 104 being above the toner melting temperature. By way of example, the Coates RP 4Q 1442 toner becomes tacky at 90°-110° C. and fuses at about 105° C. Belt 102 may be maintained at a relatively low temperature, below about 65° C, while running belt 104 at 120° C. allows the toner image 200 to reach an equilibrium temperature above its softening state. 45
As further shown in FIG. 3, in this state the toner particles do not wet the first imaging belt 102 and they present a relatively small contact area to that belt, whereas the side of the toner image contacting the hotter and softer intermediate or transfer belt 104 both wets that belt and presses into and 50 conforms with the belt surface over a relatively larger area. Since the forces of adhesion between the toner image and a belt will generally be proportional to the area of contact as well as surface energy, the heating contact preferentially causes the toner image to adhere to the second belt 104, and 55 image transfer is effected with essentially 100% efficiency. In related experiments conducted with another toner at room temperature, transfer efficiency was only about 80% without the benefit of the nip-softening of powdered toner. As further shown in FIG. 3, the toner particles 201 a, 201&, 201c m forming the toned image 200 coalesce with neighboring particles under the influence of temperature and pressure in the nip. This renders the transferred image quite stable.
Continuing now with a description of FIGS. 2 and 3, thereafter, the belt 104 carries the received and heated toner 65 image to the second nip 120, where it is "transfused", or simultaneously transferred to and fused on a receiving sheet
as described in the aforesaid U.S. Pat. Nos. 5,012,291 and 5,103,263. The surface energy of belt 104 is less than that of sheet 107 (FIG. 2), and this, together with the "wicking" of the thermoplastic toner into the sheet promotes the complete transfer of the toned image from belt 104 onto the final recording sheet.
As noted above, belt 104 operates above the softening and melt temperatures of the toner. Sheet 107, which may for example be a sheet of twenty pound paper stock, therefore is contacted by flowable toner at the nip. In order to assure that the contact and wicking is relatively complete and is not disrupted by excessively fast cooling, sheet 107 is preferably preheated to a temperature slightly below the toner softening point, e.g., to about 85° C. for the described toner, so that its surface immediately attains a temperature in the nip which allows the toner 200 to flow or wick into the textured surface even as the toner itself undergoes a drop in temperature due to contacting the paper. In general, the surface energy of sheet 107 is above forty dynes/cm, so the toner image is preferentially held by the receiving substrate, and it will release from belt 104 and transfer to the receiving sheet as it moves through the nip.
In the described embodiment, both of the toner carrying members 102,104 are shown as belts, but in other embodiments one or both members may be a drum or even a flat plate. A belt is preferred for the first member 102, because the contact region with the hot nip 110 may thus be more conveniently positioned away from both the cartridge 114 and the toner reservoir 124. Furthermore, a dielectric imaging belt may be made quite thin, limiting the amount of heat energy that it takes from the second transfer member, and allowing it to reach a low thermal state without using any cooling other than a small fan, as it travels around its path between imaging stations.
In general, the dwell time in the nip will depend on the belt speed, which in the prototype machines is 15-30 inches per second, on the thickness and compression of the elastomeric layer, and on the diameters and spacing of the drums or pressure rolls which define each transfer nip. These dweE times may be quite smaE, despite the fact that image fusing is being carried out, because the transfer processes are dependent entirely on surface to surface properties and the temperature of the thin toner layer. The characteristic thermal relaxation time for the toner particles or image is under one miUisecond, so in the designs described above the toner quickly attains a temperature or changes state in the nip. In general at the process speeds described herein, the toner is beEeved to attain a substantiaUy uniform temperature in the nip which is intermediate between the temperatures of the transferring member and the receiving member or sheet Thus, very precise control over the actual temperature of the toned image in each transfer nip is obtained by simply adjusting the temperature of one donor or receiving member.
FIG. 4 iEustrates, in non-dimensional units the temperatures of the various elements of the printing apparatus of FIGS. 2 and 3.
Temperature is plotted on a short time Ene corresponding to passage of a toned image portion through the nip, with the segment between vertical dashed Enes indicating the period of nip contact. As shown by temperature curve 102', member 102 Ees at a substantiaEy uniform temperature tx which rises sEghtly in the nip region and quickly returns to equilibrium as the belt rotates further out of the nip. The member 104 resides at a higher temperature 104' which is generaEy at temperature 12, above the toner fusing temperature. This temperature drops sEghtly in the first nip, but remains above
the toner softening temperature range ts. The toner image that transfer their toned images onto a common transfer belt
200 has a temperature shown by line 200'. This image is at Typically the four single colors may comprise three primary
the temperature of the belt on which it resides, and in the nip colors and black, with the black imaging station preferably
quickly rises to a temperature (t^t^ which as noted above being the last one to transfer its images—the bottom station
lies above the toner softening temperature, so that under the 5 in the illustrated system with a clockwise-moving transfer
pressure of the nip, the toner coalesces as well as adhering belt. In this embodiment a pair of positioning rollers, each
to the receiving member 104. At the second transfer nip, the pair simply denoted (Ra, Rb) supports the transfer belt 504
receiving substrate 107 is fed in at a third temperature t3, so that it runs tangentiaUy around the circumference of the
which, as illustrated, is lower than temperature t2. In general, imaging belt or drum at its contact nip with each of the
the image-receiving substrate will be substantially thicker image-carrying members. This provides an enhanced dwell
than the toner image layer 200, i.e., will be 0.1 to 0.3 mm time in the nip, proportional to the length of the circumfer
thick, and unlike the toner image will be a substantially ential contact path, and allows the transfer belt 504 to
continuous uniform sheet Its temperature change will there- operate at a lower temperature without reducing its transport
fore be much slower, and only its contact surface may be speed.
expected to dependably reach a quick thermal equilibrium J5 The invention further contemplates that by providing the
with an opposed member at a transfer nip. The temperature positioning rollers Ra, Rb on movable members so that their
of this region is indicated by curve 107'. As the sheet passes selective positions may be adjusted or varied, it is possible
through the nip this temperature rises to a temperature in the to separately set the dwell or contact time for the transfer
fusing range y of the toner, at approximately (t,+t3)/2. After belt with each of the color image carriers. This also allows
passing the nip. the temperature of the sheet falls, eventually 2Q the transfer belt to be retracted from contact with the
reaching room temperature which is off of the scale of the imaging subsystems of one or more colors, so that the
FIGURE. In this FIGURE, the right hand portion of curve machine may be run in a single- two- or three-color mode
200' corresponds to the toner on member 104 which contacts without loss of image quality or unnecessary wear of the
and is transferred to sheet 107. Thus the right hand portion unused imaging belts. Registration between colors may be
of curve 107 is a continuation of the toner temperature curve 25 effected in a straightforward fashion by detection of the
200', showing the evolution of the image at the second nip. relative positions of each color on belt 504, and a compen
As described above, this second curve may be shifted up or sating adjustment of the riming of electric control signals to
down by varying the temperature of the incoming sheet 107. the imaging cartridges in order to shift the position of a color
This may be accomplished by passing sheet 107 through a image on eacn 0f me donor belts by an appropriate distance,
radiant heater section, by applying additional heat to the 3Q ^ operation, the printers of FIGS. 5 and 5A operate by
sheet via pressure roll 105, or by other means known in the forming the specific color-separation portion of a desired
^ image separately at each of the imaging units 510,520,530,
The image transfer process has been described for illus- and 540^ wi(h phase delays or successive distance offsets
trative purposes by reference to a printer using a single aiong each imaging belt corresponding to the distances
toning operation. However, in other embodiments, the 35 traveled by belt 504 between receiving successive images,
invention includes multicolor or multistage printers of After the first powder toner color image is carried to and
diverse types. transferred to the belt 504 where it remains as a melted
FIG. 5 shows a multicolor printer 500 in accordance with image preferentially adhered to belt 504, the next color this aspect of the invention, in which an intermediate image image is similarly contacted to and adhered to belt 504 on transfer member 504 corresponding in function and physical 40 top of the already-received first image. Note that in the characteristics to member 104 of FIG. 2 is arranged to embodiment of FIG. 5A, the transfer belt moves synchroreceive a single color toned image from each of a plurality nously in contact with the imaging belt at a very low contact of image forming stations 510,520,530,540. In each of the pressure, and the adherence of the subsequent color to the image forming stations an imaging member 512, 522, 532, already-deposited color will in general be at least as great as or 542, respectively, corresponding to the member 102 of 45 to the transfer belt, and greater than to its own low-energy FIG. 2, receives latent charge image which is toned by a imaging belt, so the additional color is released as it contacts single-color toner forming a toned image. The toned images transport 504 at a low contact force and without impairing each travel to a respective transfer position T15 T2, T3, or T4 the first color image. This process continues, with successive where the dry powder image is heated and relinquished to colors and transferred onto the existing images where they the transfer member 504. The image charging, toning and 50 reside at the temperature T2 of the transfer belt. The cornrelease operations of each image forming belt may be bined multicolor image is then transferred to the recording essentially the same as shown for the single color embodi- sheet 107 at a pressure nip with roller 105, as in the single ment of FIG. 2. A cleaner station as indicated in the drawing color embodiment.
may also be included between the image transfer and the it wiu be appreciated that the foregoing description is erase station. Since transfer is effectively total, a simple felt 55 intended as illustrative only and that variations and modiwiper or cloth roller is sufficient to assure belt cleanliness. fications of the invention will occur to those skilled in the
As in the single color embodiment, both the imaging belts art, so that many practical implementations may be further
and the transfer belt 504 operate isothermally and at different varied by additional features not specifically disclosed
temperatures from each other. Belt 504 has a relatively high above. For example, a release agent may be applied to one
thermal mass, and may be heated by heaters (not shown) 60 or more of the imaging belts 102, 512 . . . , or may be
within one or more of its transport rollers, or by radiant incorporated in one or more of the toners, to assure complete
heaters positioned along its path. In the embodiment of FIG. image transfer under varying conditions of speed, tempera
5, the image transfer from each imaging belt to the transfer ture or humidity. Similarly, the printer may be operated to
member occurs at a nip defined between opposed pairs of heat each color toner to a different temperature, for example
rollers. 65 using different temperatures Tl,- for the different color
FIG. 5Ashows another multicolor printer 500', which also imaging belts, to assure that even though the belt 504 runs
utilizes a plurality of separate single color imaging stations isothermally, each color toner is transferred to the transfer