Patent Publication Number: US-6671486-B1

Title: Common polarity toner duplexing electrostatographic reproduction machine

Description:
BACKGROUND OF THE DISCLOSURE 
     This disclosure relates generally to electrostatographic reproduction systems, and more specifically, it is directed to a common polarity toner image duplexing electrostatographic reproduction machine. 
     The basic process of monocolor electrostatographic reproduction (e.g. black image placed on a white background) comprises exposing a charged photoconductive member. The irradiated areas of the photoconductive surface are discharged to record thereon an electrostatic latent image corresponding to the original document. 
     In electrostatographic reproduction includes cases where an electrostatic charge is deposited image-wise on a dielectric photoconductive member as well as electrophotographic reproduction in which an overall electrostatically charged photoconductive dielectric photoconductive member is image-wise exposed to conductivity increasing radiation producing thereby a “direct” or “reversal” toner-developable charge pattern on the photoconductive member. “Direct” development involves positive-positive development between charge and toner, and is particularly useful for reproducing pictures and text. “Reversal” development is of interest when from a negative original a positive reproduction has to be made or vice-versa, or when the exposure derives from an image in digital electrical signal form, wherein the electrical signals modulate a laser beam or the light output of light-emitting diodes (LEDs). It is advantageous with respect to a reduced load of the electric signal modulated light source (laser or LEDs) to record graphic information (e.g. printed text) in such a way that the light information corresponds with the graphic characters so that by “reversal” development in the exposed area of a photoconductive recording layer, toner can be deposited to produce a positive reproduction of an electronic original. 
     A development system, thereupon, moves a developer mix of carrier granules and toner particles into contact with the photoconductive surface. The toner particles are attracted electrostatically from the carrier granules to the latent image forming a toner powder image thereon. Thereafter, the toner powder image is transferred to a sheet of support material. Following the toner image transfer to the sheet of support material, the support material sheet advances to a fuser which permanently affixes the toner powder image thereto. 
     Essentially, multicolor electrostatographic copying and reproduction (e.g. several colors placed on a white background) repeats the process of monocolor reproduction by repeating a plurality of cycles, each cycle being for a different color. Development stations for each of the different colors apply a specific color toner complimentary in color to the color of a filter utilized to produce the irradiated areas of the photoconductive member. The different color toners are generally, cyan, magenta, and yellow (and sometimes black if a true black is desired), which in one combination or another can be used to generate the full spectrum of visible colors. 
     Through the application of the different colored toners at the respective stations, a plurality of color toner powder images are formed for transfer directly to a sheet of support material or to an intermediate belt for subsequent transfer to a sheet of support material. In either case the images are transferred in superimposed registration with one another. After a plurality of different color toner powder images have been transferred to the sheet of support material in superimposed registration with one another, the multicolor toner powder image is permanently affixed thereto. 
     In recent years, there have been demands for machines, for example duplex machines, providing high productivity, high quality images. Such a machine is disclosed for example in EP0629924 (assigned to Xeikon) and comprises an electrostatographic single-pass duplexing multiple station multi-color reproduction machine. In it a toner image is formed on a photoconductive member of an imaging modules and is then transferred to a paper receiving sheet such as a continuous web whereon the toner image is treated with a pair of opposed corona generating corotrons or “duets” and is then fused. Thereafter, the web is usually then cut into sheets containing the desired image frames. 
     The opposed corona generating corotrons or “duet” arrangement in such a machine is disadvantageous in that it requires use of many corotrons. For example, two corotrons (one on top of the paper and another on the bottom of the paper opposing the top corotron) are needed as a duet for every imaging modules. In a seven imaging modules duplexing machine for example, this translates to 14 corotrons for the duet function. 
     Duets are used in the Xeikon configuration mainly to “correct” the toner charge prior to each color imaging module. In practice “correction” means charging “the toner on the side of the paper that will face the next imaging module&#39;s drum” toward the same polarity that the toner has on the next imaging drum. The alternative would be a more expensive use of different polarity toners. 
     There is therefore a need for an economical common polarity toner duplexing electrostatographic reproduction machine. 
     In accordance with the present disclosure, there is provided a common polarity duplexing electrostatographic reproduction machine that includes (i) a first plurality of toner image producing electrostatographic modules that each include a first image forming surface, image forming devices, and charged toner particles having a first polarity, (ii) a second plurality of toner image producing electrostatographic modules that each have a second image forming surface, image forming devices, including charged toner particles having a polarity common with the first polarity, (iii) a charged toner polarity reversing device mounted against each module of the second plurality of toner image producing electrostatographic modules for reversing a polarity of toner particles forming the second set of toner images from the first polarity to a second an opposite polarity; and (iv) a transfer device for transferring the second set of toner images having the second polarity onto a second side of the web of recording media. 
    
    
     In the detailed description of the disclosure presented below, reference is made to the drawings, in which: 
     FIG. 1 shows a section of a common polarity toner image duplexing electrostatographic reproduction machine including plural imaging modules according to the present disclosure; 
     FIG. 2 represents a diagrammatic cross-sectional view of an imaging module of the imaging modules of the machine of FIG. 1; 
     FIGS. 3-8 are each an enlarged schematic of part of a conventional (prior art) toner image duplexing machine including a use of “duets” or of a pair of opposed corotron devices; and 
     FIG. 9 is a schematic of the machine of FIG. 1 showing the pre-transfer transfer toner polarity reversing devices of the present disclosure. 
    
    
     While the present disclosure will be described in connection with a preferred embodiment thereof, it will be understood that it is not intended to limit the disclosure to that embodiment. On the contrary, it is intended to cover all alternatives, modifications, and equivalents as may be included within the spirit and scope of the disclosure as defined by the appended claims. 
     Referring now to FIG. 1, there is illustrated a common polarity duplexing electrostatographic reproduction machine  10  in accordance with the present disclosure. As shown, the electrostatographic reproduction machine  10  includes a supply station  13  in which a roll  14  of web material  12  is housed, for example, in sufficient quantity to print, say, up to 5,000 images. The web  12  is conveyed into a tower-like printer housing  44  in which support columns  46 ,  46 ′ are provided, housing a number of similar imaging modules A to D, A′ to D′. In addition, each column includes a further module E, E′ in order to optionally enable printing an additional color, for example a specially customized color, for example white. The imaging modules A to E, A′ to E′ are mounted in a substantially vertical configuration resulting in a reduced footprint of the machine  10 , and additionally making servicing easier. The columns  46 ,  46 ′ may be mounted against vibrations by means of a platform  48  resting on springs  50 ,  51 . 
     Thus, as shown in FIG. 1, each column  46 ,  46 ′ of the electrostatographic reproduction machine  10  comprises 4 imaging modules A, B, C, D, E as well as A′, B′, C′, D′ and E′ which are arranged for printing for example yellow, magenta, cyan, black and an optional color toner images respectively on the respective sides  12 L,  12 R of the web  12 . The imaging modules (i.e., image-producing stations) A, B, C, D, E and A′, B′, C′, D′, E′ are arranged in a substantially vertical configuration, although it is of course possible to arrange the stations in a horizontal or other configuration. A web of paper  12  unwound from a supply roller  14  is conveyed in an upwards direction past the imaging modules in turn. 
     After leaving the final imaging modules E, E′, the composite duplex image on the web  12  is fixed or fused by means of image-fixing stations  16  and  18 , and are then fed to a cutting station  20  (schematically represented) and to a cut web/sheet stacker  52  if desired. As discussed above, the web  12  is conveyed through the machine  10  by the two drive rollers  22   a ,  22   b  which are shown one positioned between the supply station  13  and the first imaging modules A, A′, and the second positioned between the image-fixing stations  16 ,  18  and the cutting station  20 . The drive rollers  22   a ,  22   b  are driven by controllable motors,  23   a ,  23   b.    
     As shown, after passing the first imaging module A, the web  12  passes successively to imaging modules B, C D and E on the one side where images in other colors are transferred to the web, and modules A′, B′, C′, D′ and E′ on the other side where images in various colors are formed and transferred to the web. The moving web  12  is in face-to-face contact with the drum surfaces  26 ,  26 ′ over a desired wrapping angle at each module as determined by the position of guide rollers  36 . After passing the last imaging modules E and E′, the web  12  then passes over a roller  150  and through an image-fixing station  16 , an optional cooling zone  18 , and thence to a cutting station  20  where the web  12  is cut into sheets for discharge to an output tray  52 . 
     The imaging modules A, B, C, D, E and A′, B′, C′, D′, E′ are each identical except for the inclusion of a pre-transfer toner polarity reversing corotron device  100  (in accordance with the present disclosure, and to be described in detail below) on each of the imaging modules A′, B′, C′, D′, E′, for example. Detailed description of one of the modules, for example the module A′, will thus suffice as a description of each of the other modules, given proper notification of the exception mentioned above. 
     Thus, as shown in FIG. 2, A′ (and hence the rest of the other modules) comprises a cylindrical drum  24 ′ having a photoconductive outer surface  26 ′. circumferencially arranged around the drum  24 ′ there is a main corotron or scorotron charging device  28 ′ capable of uniformly charging the drum surface  26 ′ to a potential having a desired level and polarity. There is also arranged an exposure station  30 ′ which may, for example, be in the form of a scanning laser beam or an LED array, which will image-wise and line-wise expose the photoconductive drum surface  26 ′ causing the charge on the latter to be selectively discharged, thus leaving an image-wise distribution of electric charge or “latent image” on the drum surface  26 ′. 
     This so-called “latent image” is then rendered visible by a developing station  32 ′, which by means known in the art will bring a charged developer into contact with the drum surface  26 ′. The developing station  32 ′ for example may include a developer drum  33 ′ which is adjustably mounted thus enabling it to be moved radially towards or away from the drum  24 ′. According to one embodiment, the developer contains (i) charged toner particles, for example negatively charged toner particles as shown, containing a mixture of a resin, a dye or pigment of the appropriate color and normally a charge-controlling compound giving triboelectric charge to the toner, and (ii) carrier particles charging the toner particles by frictional contact therewith. 
     The carrier particles may be made of a magnetizable material, such as iron or iron oxide. In a typical construction of a developer station  32 ′, the developer drum  33 ′ contains magnets carried within a rotating sleeve causing the mixture of toner and magnetizable material to rotate therewith, to contact the surface  26 ′ of the drum  24 ′ in a brush-like manner. Toner particles charged triboelectrically to an appropriate level and polarity (e.g. negative polarity) are attracted to the “latent image” areas on the drum surface  26 ′ by the electric field between these areas so that the latent image becomes visible. All reference numerals, e.g.  24 ′ used in reference to the modules A′, B′, C′, D′, and E′, are the equivalent for example of  24  for the modules A, B, C, D, and E. 
     In accordance with the present disclosure, after the toner image is developed or made visible as such, each of the imaging modules only on one side of the web  12  (for example A′, B′, C′, D′, E′ on the side  12 R) includes a toner polarity reversing corotron device  100  located on the photoconductive drum  24 ′ thereof, and upstream of the point of toner image transfer to the web  12 , for reversing the polarity (e.g. from negative to positive as shown) of the toner image on the surface  26 ′. The imaging modules only on one side of the web  12  could equally have been the modules A, B, C, D and E. In either case, pre-transfer toner polarity reversal as such advantageously enables the use of the same or a common polarity developer (e.g. negative polarity as shown) and toner in all the imaging modules for both simplex (using just A, B, C, D and E or A′, B′, C′, D′ and E′), and duplex using (A, B, C, D and E as well as A′, B′, C′, D′ and E′) operations of the machine  10 . Pre-transfer toner polarity reversal as such also reduces the number of corotron or corona devices needed in machines of the present disclosure as compared to conventional machines using the “duet” arrangements. 
     Thus, on the one hand (i.e. for each of modules A′, B′, C′, D′ and E′ on the side  12 R of the web  12  as an example), after toner image development with negative or common polarity charged toner, the polarity of the toner image adhering to the drum surface  26 ′ is reversed by a positive charge generating corotron device  100 . After such reversal, the toner image (now positive) is then transferred as a positive toner image to the side  12 R of the moving web  12  with the aid of a negative transfer corona device  34   a ′. The negative charge sprayed by the transfer corona device  34   a ′, being on the opposite side of the web  12  relative to the drum  24 ′ of the module A′, and having a polarity (negative) opposite in sign to that (positive) of the charge now on the toner image, operates electrostatically to attract the toner image away from the drum surface  26 ′ and onto the side  12 R of the web  12 . The transfer corona device  34   a ′ serves to generate a strong adherent force between the web  12  and the drum surface  26 ′ in addition to urging the toner particles into firm contact with the side  12 R of the web  12 . 
     After image transfer from the surface  26 ′ to the side  12 R of web  12  as shown, the drum surface  26 ′ is pre-charged to a suitable level by a pre-charging corotron or scorotron device  40 ′ thus making the final charging by the corona  28 ′ easier. Following such pre-charging, any residual toner remaining on the drum surface  26 ′ is then easily removed by a cleaning device  42 ′. The cleaning unit  42 ′ for example may include an adjustably mounted cleaning brush  43 ′, the position of which can be adjusted towards or away from the drum surface  26 ′ to ensure optimum cleaning. After such cleaning, the drum surface is ready for another recording cycle starting with charging by the corona device  28 ′. 
     On the other hand, for each of modules A, B, C, D and E on the side  12 L of the web  12 , (as also shown in FIG.  4 ), after toner image development with negative or the common polarity toner, such toner image adhering to the drum surface  26  (of each of modules A, B, C, D and E), is transferred as a negative toner image (without reversal) to the moving web  12  with the aid of a positive transfer corona device  34 . The positive charge sprayed by the transfer corona device  34 , being on the opposite side of the web  12  relative to the drum  24 , and having a polarity (positive) opposite in sign to that (negative) of the charge on the toner image, operates electrostatically to attract the toner image away from the drum surface  26  and onto the side  12 L of the web  12 . The transfer corona device  34  serves to generate a strong adherent force between the web  12  and the drum surface  26 , in addition to urging the toner particles into firm contact with the side  12 L of the web  12 . 
     Referring to FIG.  2  and applying it to the modules A, B, C, D and E, after image transfer from the surface  26  to the side  12 L of web  12  as shown, the drum surface  26  is pre-charged to a suitable level by a pre-charging corotron or scorotron device  40  thus making the final charging by the corona  28  easier. Following such pre-charging, any residual toner remaining on the drum surface  26  is then easily removed by a cleaning device  42 . The cleaning unit  42  for example may include an adjustably mounted cleaning brush  43 , the position of which can be adjusted towards or away from the drum surface  26  to ensure optimum cleaning. After such cleaning, the drum surface is ready for another recording cycle starting with charging by the corona device  28 . 
     FIGS. 3-8 are each an illustration of part of a conventional “duet arrangement type duplexing machine showing use of a duet  58 L and  58 R after each set of opposite modules, for example, A and A′. As shown in FIGS. 2 and 8, at each of the modules, for example A, A′ and B, a developer unit  35 ,  35 ′ deposits negative toner (for example) on the surface  26 ,  26 ′ of the drum  24   a ,  24   b ,  24   a ′. As shown in FIGS. 2 and 7, at the module A, a positive corona device  34   a  assists in transferring the negative toner image from the surface  26  onto the side  12 L of the web  12 , but also changes the toner image on side  12 L to positive. At the next module A′ as shown in FIGS. 2 and 6, a positive corona device  34   a ′ also assists in transferring the negative toner image from the surface  26 ′ onto the side  12 R of the web  12  but also changes the toner image on side  12 L to positive. Importantly as shown In FIGS. 2 and 5, in order for the positive toner image now on the side  12 L not to transfer back onto a negative drum surface  26 , this arrangement employs a negative corona device  58 L for reversing the polarity of the toner image on the side  12 L from positive back to negative. For reasons to be explained below, it is also necessary to use the second and opposed corona device  58 R. At the next module B, as shown in FIGS. 2 and 4, a negative toner image can then be formed on the surface  26  of drum  24   b , and transferred to side  12 L with the help of a positive corona device  34   b.    
     Thus, in advance of the third image-producing module B, and also between each subsequent pair of opposite image-producing modules (not shown), an opposed pair of corona discharge devices  58 L and  58 R are positioned one on each side of the web  12 . The polarity of the corona discharge devices  58 L and  58 R are chosen to reverse the charge carried on the toner particles carried on the adjacent face  12 R and  12 L respectively of the web  12 . As shown, between the modules A′ and B, the positively charged toner particles on the face  12 L of the web  12  are reversed to carry a negative charge as they pass the negative corona device  58 L, while the negatively charged toner particles on the face  12 R of the web  12  are reversed to carry a positive charge as they pass the negative corona device  58 R. As can be seen the toner particles of the first color on the face  12 L are now negatively charged as they reach the negatively charged drum  24   b  and they are therefore repelled by the charge on the drum preventing their removal from the web, assisted by the positive charges from the transfer corona  34   b . The web therefore continues to the next module in the electrostatographic reproduction machine carrying toner particles of both the first and second colors on the face  12 L in the desired amounts according to the image to be produced. 
     The “Duets” ( 58 L,  58 R) are needed in order to avoid severe toner image retransfer that would otherwise occur when the same polarity toner is to be transferred for image on image simplex and duplex operation of such a machine. Thus in order to avoid such severe retransfer without resorting to using different polarity toners in the simplex and duplex development systems, “duets” as such have to be employed. 
     Thus, “duets” are used in conventional such machines mainly for “correcting” the toner charge prior to each subsequent color imaging module. In practice “correcting” means charging “the toner on the side of the web that will face the next imaging module&#39;s drum” toward the same polarity that the toner has on that next imaging drum. For example, with negative polarity toner used in the imaging drum modules, the duet is arranged to spray negative charge toward the web on the side of the web that will face the next imaging drum. Thus a “loner charge correction” is needed because, in prior imaging module transfer zones, the polarity of the toner on that side of the web will get reversed compared to the polarity of toner on the drum (due to the charge deposited toward the web by the transfer corotron at the previous transfer station and also by the charge deposited by the previous duet. If the toner charge on that side of the web is not “corrected” to be the same polarity as the toner on the next imaging module drum, then the toner on that side of the web will transfer back to such next drum when the transfer corotron is adjusted to try to make the right signed toner on the drum transfer to the web. 
     Unfortunately however, this necessary toner charge “correction” is done on the web prior to the transfer zone while the web is relatively far from any reference grounded conductors. As a negative consequence, the capacitance between the web and nearby conductors thus is very, very small. As such, a “duet” must be used because use of a single corona device to attempt correct the toner charge on the web (even in very small amounts of charge deposited onto the web) will cause the potential on the web to head toward “infinity” (very, very high). Such very high potentials will in effect operate instead to prevent significant charging by such a single corotron device in such an arrangement. 
     Thus, “duets” are necessary because the corona device (e.g.  58 L) used to try to correct the toner charge on one side ( 12 L) of the web  12  “in free space” must have an additional corona device ( 58 R) on the opposite side ( 12 R) of the web for depositing a reversal polarity charge on such opposite side. This thus prevents the very, very high potentials and thereby allows sufficient charge deposition for correcting the toner charge. In a qualitative sense, the additional “duet” corotron on the opposite side of the web acts like a “pseudo ground” for the “toner charge correcting corotron”. At any rate, a penalty is that two corotrons instead of one are needed for the simple function. 
     Referring now to FIGS. 1 and 9, on the one hand, each of the modules A, B, C, D, and E on the one side  12 L of the web  12  has a drum  24  with surface  26  and negative polarity developer development station  35  for forming negative polarity toner images on the surface  26 . Each thus can form a negative toner image that is transferred as such onto the side  12 L of web  12  with the help of a positive transfer-assist corona device  34   a ,  34   b ,  34   c ,  34   d , and  34   e . On the other hand, each of the modules A′, B′, C′, D′, and E′ on the other side  12 R of the web  12  has a drum  24 ′ with surface  26 ′ and negative polarity developer development station  35 ′ for forming negative polarity toner images on the surface  26 ′. Each module thus can form a negative toner image on the surface  26 ′. In addition, each such module A′, B′, C′, D′, and E′ includes a toner polarity reversing corotron or corona device  100  for reversing the polarity of the formed toner image from negative to positive. In accordance with the present disclosure, the polarity of the toner image on each of the drums  24 ′ is thus reversed from negative to positive before such toner image is then transferred as positive onto the side  12 R of web  12  with the help of a negative transfer-assist corona device  34   a ′,  34   b ′,  34   c ′,  34   d ′, and  34   e ′ as shown. 
     The corotron device  100  is thus used at a pre-transfer location for conditioning the toner image on the photoconductor or drum  24 ′ on one side ( 12 R) of the web by reversing the polarity of the toner right on the drum  24 ′. This thus enables the use of the same or common polarity developer and toner packages on both sides of the machine for simplex and duplex operations. Use of the corotron device  100  as such also reduces the number of such corona devices that are needed for such duplexing operations as compared to the conventional “duet approach. 
     There are significant advantages from using common polarity toners as above, and then reversing such polarity on the drum  24 ,  24 ′ on one side (for example  12 R) of the web, before transfer of the reversed polarity image to the web  12 . For one thing, in accordance with the present disclosure, only  5  corona devices  100  (as opposed to  10  in a duet arrangement) would be needed. Such a reduction in the number of corona devices of course saves cost (less parts, power supplies, etc.), improves reliability (less parts to go wrong) and reduces service cost and/or customer annoyance (reduced number of corotron cleaning actions. 
     Note that if common polarity toners are not used in the immediate duplex configuration, then a different polarity and developer formulation for each of Y,M,C,K color toners would have to be used on one side of the web versus the other side of the web  12 . This is undesirable because ordinarily, it is frequently a major challenge to develop one good set of color developer formulations for a product. Needing to double the set of compatible color developer formulations Y,M,C,K for a machine. Developing more than one set of course would obviously be a major challenge because in order to have acceptable image quality, the toners in both sets must be “identical” relative to colorants, for example. 
     To maintain commonality in the imaging systems used for both sides of the print, the two different toner formulations would need to have compatible fixing, transfer, cleaning, and development, performance for examples. It is generally unlikely that any machine developers would even consider taking on such challenges. Even if different formulations for the two sides were achieved, there are other disadvantages. For example, now the Y,M,C,K color developers for one side of the print must be packaged stored separately, for example, from the other Y,M,C,K color developers of the other side, and a system must be in place to make sure “one side&#39;s developer does not get put into the wrong side imaging system”. As can be seen, there has been provided a common polarity duplexing electrostatographic reproduction machine that includes (i) a first plurality of toner image producing electrostatographic modules that each include a first image forming surface, image forming devices, and charged toner particles having a first polarity, (ii) a second plurality of toner image producing electrostatographic modules that each have a second image forming surface, image forming devices, including charged toner particles having a polarity common with the first polarity, (iii) a charged toner polarity reversing device mounted against each module of the second plurality of toner image producing electrostatographic modules for reversing a polarity of toner particles forming the second set of toner images from the first polarity to a second an opposite polarity; and (iv) a transfer device for transferring the second set of toner images having the second polarity onto a second side of the web of recording media. 
     While the embodiment of the present disclosure disclosed herein is preferred, it will be appreciated from this teaching that various alternative, modifications, variations or improvements therein may be made by those skilled in the art, which are intended to be encompassed by the following claims: