MULTICOLOR IMAGE FORMING METHOD AND APPARATUS

Abstract: Multicolor toner images are formed on a receiving surface by superimposing single color toner images formed electrostatically on an image member (1), in registration on a receiving surface. The receiving surface is rotated by a transfer member, for example, a transfer drum (5; 60; 211), repeatedly through transfer relation with the image member. To improve transfer and avoid wear of the image member, the transfer member is driven by the image member during transfer. To improve registration the transfer member is separated from the image member between transfers and reindexed. A stepper motor (17; 117) is used for such reindexing. The invention is particularly usable in transfers using heat, especially to a heat softenable outer surface of a receiving sheet.

Description: MULTICOLOR IMAGE FORMING METHOD AND APPARATUS TECHNICAL FIELD This invention relates to formation of a multicolor toner image, and more specifically to a method and apparatus for precisely transferring a series of different color toner images in registration to a receiving surface. BACKGROUND ART In conventional color electrophotography a series of electrostatic images are created on an image member. They are toned with different color toners and then transferred in registration to a receiving surface to create a multicolor toner image. Typically, the receiving surface is a receiving sheet of paper or similar material which has been secured around the periphery of a transfer drum or roller (used interchangeably herein) . The transfer drum is rotated in contact or near contact with the image member to repeatedly bring the receiving sheet into transfer relation with the consecutive images to overlay them in registration. As color toners get finer, higher resolutions become possible. A limiting aspect of the process is the image registration provided by the transfer process. Some transfer processes bring the receiving sheet into light contact or just out of contact with the image member, and transfer is accomplished by an electrostatic field. Using encoders and separate motors on both the image member and the transfer drum, accurate registration from image to image is possible with such systems. See, for example, U.S. Patents 4,796,054 and 4,872,037. However, fine toners do not generally transfer well electrostatically. Transfer systems using substantial amounts of pressure, sometimes in the presence of sufficient heat to soften or sinter the toner have been more successful in transferring fine toners. Further, for highest quality work, a receiving sheet with a heat-softenable thermoplastic outer layer can be used to receive the toner in the presence of sufficient heat to soften the outer layer and soften or sinter the toner. These transfer processes require more pressure than is common with ordinary electrostatic transfer. Unfortunately, if both the transfer drum and the image member are independently driven at more than light pressures, excessive wear will destroy a normal image member, for example, a photoconductive member, quite rapidly. If the transfer drum is driven by the image member, such wear does not occur but registration is difficult to maintain. Even with precisely machined devices, there is a drift from image to image and also over a number of images. Even slight misregistrations which would not be objectionable in an ordinary color copier, are objectionable in a higher quality print. Also of significance, as the registration drifts, the image may be transferred out of registration with the means for holding the receiving sheet to the periphery of the transfer roller, for example, vacuum holes or gripping fingers. DISCLOSURE OF THE INVENTION It is an object of the invention to provide an apparatus for forming multicolor images in which a series of electrostatic images is formed on an image member, different colored toners are applied to the electrostatic images to form a series of different colored toner images and the toner images are transferred in registration to a receiving surface to form the multicolor image, in which the receiving surface is driven by the image member during transfer but in which the misregistration problems mentioned above are reduced or eliminated. This and other objects are accomplished by an apparatus in which the surface that receives the transfer of the toner images is supported by or defined by a transfer member, for example, a drum. The transfer member is movable through a path which includes a portion in which the transfer member is driven by the image member and a portion in which the drum is out of driven engagement with the image member. The apparatus includes means for moving the transfer member at least a portion of the time it is out of such driven engagement and for timing said movement to register the toner images on the receiving surface. According to a preferred embodiment, the transfer member is a drum and the drum includes timing indicia. The apparatus includes means for sensing the presence or absence of the timing indicia at a predetermined location. The image member also includes image location indicia and the apparatus includes means for sensing the image location indicia. The means for rotating the drum when the drum is not engaging the image member rotates the drum until the timing indicia is located at the predetermined location. It then rotates the drum from said predetermined location in timed relation to the sensing of the image location indicia. Thus, for every revolution of the drum and every image that is transferred, the drum is separately reindexed according to the location of the image on the image member. According to a further preferred embodiment the means for rotating the drum when it is out of engagement with the image member is a stepper motor. With presently available equipment we have found that the drum can be positioned at a stopping point within 0.00025 inches and similar accuracy is obtainable in engaging the transfer member and the image member upon sensing the image location indicia on the image member. This provides quality registration usable for multicolor image reproduction in both the regular photographic and graphic arts industries. BRIEF DESCRIPTION OF THE DRAWINGS In the detailed description of the preferred embodiment of the invention presented below, reference is made to the accompanying drawings, in which: Fig. 1 is a side schematic of a multicolor image forming apparatus. Figs. 2, 3, 4 and 5 are top views illustrating receiving sheets having various size image areas prior to cutting. Fig. 6 is a perspective view of an embodiment of a portion of the apparatus shown in Fig. 1, with many parts eliminated for clarity of illustration. Figs. 7, 8 and 9 are side schematics of the apparatus portion shown in Fig. 6 illustrating the relative movement of a transfer roller with respect to an imaging drum. Fig. 10 is a side schematic of a portion of an embodiment of the apparatus shown in Fig. 1 illustrating another mechanism for moving the transfer roller with respect to the imaging drum. Fig. 11 is a partial cross-section of the transfer roller and imaging drum shown in Fig. 10 illustrating a preferred form of transfer. Figs. 12, 13, 14 and 15 are side schematics of the transfer roller and imaging drum illustrating one approach to coordination of those members. Figs. 16-19 are side schematics similar to Figs. 12-15 illustrating an alternative approach to coordination of those members. Figs. 20-23 are side schematics similar to Figs. 12-15 illustrating another approach to coordination of those members. Figs. 24-27 are side schematics similar to Figs. 12-15 illustrating still another approach to coordination of the transfer roller and imaging drum. Figs. 28 and 29 are top views of two different instances in the revolution of an alternative transfer roller drum embodiment to that shown in Figs. 6-10. Figs. 30 and 31 are side schematics similar to Figs. 13-14 illustrating coordination of the transfer roller and imaging drum shown in Figs. 28 and 29. Fig. 32 is a side schematic section of a multicolor image forming apparatus constructed according to another embodiment of the invention with many portions eliminated for clarity of illustration. Fig. 33 is a top view of the apparatus shown in Fig. 32 also with many elements eliminated for clarity of illustration. Fig. 34 is a perspective view of a transfer roller portion of the apparatus shown in Figs. 32 and 33. BEST MODE OF CARRYING OUT THE INVENTION Fig. 1 illustrates a multicolor image forming apparatus utilizing electrophotography. Most of it is conventional. An image member, for example, a photoconductive drum 1, is rotated by a motor, not shown, past a series of electrophotographic stations, all well-known in the art. A charging station 2 uniformly charges the surface of the drum 1. The uniformly charged surface is exposed at an exposure station, for example, laser exposure station 3, to create a series of electrostatic images, each representing a color separation of a multicolor image to be formed. The series of electrostatic images are toned by different color toner stations 20, 21 and 22, one different color for each image, to create a series of different color toner images. The images are then transferred in registration to a receiving sheet carried on the periphery of a transfer roller 5. The drum 1 is cleaned by cleaning station 6 and reused. The receiving sheet is fed from a receiving sheet supply 23 into a nip 10 between drum 1 and roller 5. As it approaches nip 10 it is secured to drum 5 by a vacuum means, gripping fingers or other mechanism. For example, the leading end of the sheet can be secured by a row of vacuum holes 28 and the trailing end by a row of vacuum holes 29. After all 3 (or 4) color separation toner images have been transferred to the surface of the receiving sheet, the leading edge of the receiving sheet is stripped from roller 5 by stripping mechanism 18. The receiving sheet is pushed by further rotation of roller 5 onto a sheet transport 24 which carries it to a fixing device 25 and then to a cutter 26. After the sheet has been cut by the cutter 26 the resulting prints are collected in a tray 27 or more sophisticated print collecting device. The input for exposure station 3 begins with a color scanner 40 which includes a color responsive CCD 41 for scanning an original to be printed, for example, 35mm color negative film. The output from CCD 41 is fed to a signal processor 42 which converts the CCD signal into a form suitable for storing in memory. For example, signal processor 42 can use suitable compression algorithms to save on storage, enhance the image in both its color aspects and its resolution including color masking, halftone screening, etc. all processes well known in the art. After such signal processing, the image information is stored in a suitable storage 43. Because this system demands substantial storage, a preferred form of storage is a system using magnetic disks. A logic and control 30 is capable of accessing the storage 43 and also receives inputs from various portions of the machine including encoders (not shown) on drum 1 and roller 5 and various stations to manage the timing of the entire apparatus. One of the inputs to logic and control 30 is a print size designation portion 45 of an operator control panel. As shown in Fig. 1, the operator can press a button beside any of four print sizes ranging from 4x6 through 5x7, 8x10, 11x17 and also 5x7 with borders. The logic and control 30 then receives the input from the print size designation portion 45 and the memory 43 and supplies that information in an appropriate form to raster image processor 46 which lays out the bit map for the ultimate exposure. The output from the raster image processor 46 is fed to an electronic driver 47 for electronic exposure station 3 to control the intensity of a laser, LED printhead, or the like, making up that station. Prior electrophotographic color apparatus capable of providing a variety of sizes of sheets has a sheet supply 23 which can be loaded with different sizes of sheets and a transfer drum 5 which is capable of holding different size sheets. These devices have specific complexities that are undesirable in such apparatus including the flexibility in the sheet supply and other portions of the paper path. However, the most serious problems arise in securing the sheets to transfer roller 5. If both the leading and trailing edge of the sheet are to be held by a vacuum means 28 and 29 as shown in Fig. 1, those vacuum means must be separated by different lengths of the drum periphery for different size sheets. The drum being the same size, small prints will be produced at the same slow rate that larger prints are produced. More significantly, image quality in the large sheets will suffer from the vacuum holes that are necessarily under their image areas. To solve this problem, the apparatus as shown in Fig. 1 is capable of taking a single large size sheet, for example, 12 inches by 18 inches. Only 2 sets of vacuum holes are provided and the apparatus is optimized for productivity for the single size sheet. The sizes are chosen to allow later cutting for the print size produced. Examples of preferred image locations for different sizes of image are shown in Figs. 2-5. According to Fig. 2 a 12 inch by 18 inch receiving sheet 75 can hold nine 4x6 images exposed edge-to-edge with no waste. The other common sizes will produce some waste with a 12x18 inch receiving sheet. Although vacuum holes 28 and 29 will in fact be under the very leading and trailing edges of the receiving sheet 75, they can be limited to the leading and trailing one quarter inch where such defects are least likely to be noticed. Alternatively, a narrow leading and trailing margin can be provided and later trimmed. Figs. 3 and 4 show two approaches to positioning 5x7 images on a 12x18 receiving sheet. Other such arrangements for 5x7's can be designed. The Fig. 4 arrangement is the most efficient, fitting five 5x7 , s on a 12x18 receiving sheet. The Fig. 3 arrangement fits only four 5x7's in the same space, but has several advantages. A primary one is that if the receiving sheet is cut along the dotted lines shown in Fig. 3, a 1/2 inch border is provided for each print. Further, the cutting itself is far simpler than that in Fig. 4. Although automatic equipment is available that will cut and slit the geometry shown in Fig. 4, equipment to cut that geometry shown in Fig. 3 is far simpler. Fig. 5 illustrates two 8x10 images on a 12x18 receiving sheet. With this geometry, if the sheet is cut exactly in half the 8x10's will have 1/2 inch borders on the top and bottom and l-inch borders on the sides. The 11x17 format is not illustrated. It, of course, would fit on a 12x18 with half-inch borders on top, bottom and both sides. Although snapshots are popular without a border, larger prints often are mounted with a border. The 12x18 size allows production of four 5x7's, two 8x10's and one 11x17 with even borders. It is presently within the skill of the art to provide a selection of colors for borders produced on color copiers. This same technology can be used to permit choice of border color for each print larger than 4x6 in the apparatus shown in Fig. 1, by exposing the border area uniformly for one or more of the color separations. The 12 inch x 18 inch size is particularly useful in the United States with the standard print sizes discussed above. Most of these sizes have been standard in the United States for printing from 35mm film for more than 40 years. However, other size receiving sheets may be optimum for other circumstances. For example, in markets in which 4x6 is not the high volume size, integer multiples of whatever that size is is a preferred starting point in determining the preferred receiving sheet size. Further, if customers prefer snapshots with borders, which are not presently popular, then the receiving sheet would either have to be made larger by the amount of the borders, or the image area reduced in size. Other arrangements could include a combination of different size prints from the same receiving sheet, for example, seven 4x6 prints and one 5x7. All of this is well within the skill of the art within the general framework of this description. The slitting, chopping or cutting arts are extremely well developed. Devices are presently available that can be set to slit or cut any sheet at a variety of locations. However, a medium volume photofinishing operation may install a less expensive automatic cutting device that cuts only a single high volume print, for example, 4x6 snapshots and allows all other sizes to be trimmed by hand. In such a device the cutter 26 would be either disableable or there would be a path around it for sizes larger than the high volume size. The process illustrated in Fig. 1 can be capable of extremely high-quality imaging. The quality of that imaging is dependent on many portions of the process. In particular, it is dependent on the resolution of the exposure device 3, the size of -li¬ the toners used to create the toner images and the registration associated with the exposure and transfer stations. To compete with ordinary photography in making prints, extremely fine toners are necessary. It is presently possible to tone images with toners as small as 3.5 microns and smaller which toners provide extremely high-quality images if correctly registered. Transfer of extremely fine toners is difficult to do electrostatically. Better results are obtained by a combination of heat and pressure. If substantial pressures are used in the transfer process, for example, pressures in excess of forty pounds per square inch, for example, 100 pounds per square inch, and both the transfer drum or roller and the image drum or web are independently driven, excessive wear will result to the surfaces in contact, which wear is especially damaging to a photoconductive surface of the imaging member. To maintain extremely precise registration for full utilization of extremely fine toner particles and high quality exposure, the transfer roller is separated from the image member and reindexed for every revolution of the transfer roller. This particular approach provides extremely precise registration of the transfer roller or drum 5 and is illustrated in Figs. 6-10. According to Fig. 6 transfer roller 5 has a pair of cam disks 7 and 8 of the same diameter as roller 5 but centered about a different axis. They are fixed to roller 5 and are rotated with roller 5. Roller 5 is rotated by engagement with image member 1 during transfer. When transfer is finished, cam disks 7 and 8 acting either directly on drum 1 or on separate disks (not shown) journaled on the same shaft as drum 1, separate roller 5 from engagement with drum 1. After such separation, roller 5 is driven by a stepper motor 17 through a clutch 16. Roller 5 is driven by motor 17 to a home position controlled by an indicia 11 associated with roller 5 and sensed by a sensor 12. From the home position, motor 17 drives roller 5 up to its appropriate speed and rotational position for reengagement as controlled by disks 7 and 8. This process is illustrated in Figs. 7-9. According to Fig. 7, roller 5 is in contact with drum 1 during image transfer. As shown in Fig. 8, after transfer, disk 7 separates transfer roller 5 from drum 1 and stepper motor 17 (Fig. 6) drives roller 5 until indicia 11 is sensed by sensor 12, at which point stepper motor 17 stops. The rotational position of drum 1 is also sensed, for example, by sensing an indicia 14 with a sensor 13 on the periphery of drum 1. Logic and control 30 receives signals from both sensors 12 and 13. In timed relation to receiving the signal from sensor 13 motor 17 is started again and driven until engagement of roller 5 with drum 1 is completed. Motor 17 is a high-quality stepper motor which is capable both of accurately positioning roller 5 at its home position and accurately driving it from that home position so that it is going at the same speed as drum 1 during engagement, which engagement is shown in Fig. 9. After engagement clutch 16 is disengaged and roller 5 is again driven by drum 1. With the quality of stepper motor and sensors presently available, registration of a higher quality can be maintained between images than if the drum 1 is allowed to rotate the roller 5 through the entire cycle. Fig. 10 illustrates this approach in an apparatus similar to that of Fig. l. Note that a transfer roller 60 is driven by engagement with drum 1 except when a cam 70 rotating with roller 60 contacts drum 1 and disengages roller 60 therefrom. Fig. 11 illustrates the type of transfer with which the structure shown in Figs. 6-10 is particularly usable. According to Fig. 11, extremely small toner particles making up a toner image are transferred from drum 1 to a receiving sheet 75, which receiving sheet includes a thermoplastic outer layer 76. Layer 76 is heat softenable as is the toner. The receiving sheet 75 and particularly outer layer 76 is raised to a temperature which softens the thermoplastic layer 76. This is accomplished by heating roller 60 internally, using heating lamp 62 and also may be accomplished by externally heating thermoplastic layer 76, for example, by externally located lamp 80. The heat of the receiving sheet 75 also heats the toner in the nip 10. The edges of the toner sinter or become soft. The layer of toner which first contacts the softened thermoplastic layer 76 becomes embedded in it and the rest of the toner which has also become soft or sintered sticks to the toner that is so embedded. Transfer is thus effected to the thermoplastic layer 76 as shown in Fig. 11. This process works best at relatively high pressures, for example, pressures well in excess of 100 pounds per square inch. Lower pressures are also effective, especially working with larger sized toners with more modest quality images on regular uncoated paper. Good thermal transfer from the interior of the roller 60 while maintaining relatively high pressures is best obtained if both drum 1 and roller 60 are hard rollers and do not contain a nip widening compliant surface. Again, for less high-quality work, especially with plain paper, roller 60 can have a slightly compliant surface as is more customary in the art. It is common in many color copiers using a drum photoconductor to provide a substantial fly wheel driven with the drum to even out the rotation of the drum to avoid image defects. Defects that are repeated in all colors in the same way are less objectionable than a defect that occurs only in one color and not in others. The latter defect may show up as a shift in color and be quite noticeable. For that reason, it is common to have both the drum and transfer roller of the same size and form one image for each revolution of each. Thus, any variations in the periphery of the drum or transfer roller or variations that occur repeatedly in the motion of either will be repeated with each image and not show as a misregistration of colors. As mentioned above, precision in the exposure station is one of the ingredients which provides extremely high quality results that makes an electrophotographic application competitive with regular photography. Referring to either Figs. 1 or 10 it is noted that the transfer station and the exposure station are approximately 180° apart from each other. At the same time that the electronic exposure station 3 is writing an image, the transfer station is going through its cycle. The transfer station itself involves discontinuities in the mechanical interface between drum 1 and roller 60. For example, the registration arrangement illustrated in Figs. 6-10 include an engagement step between the roller 60 and the drum 1 and a disengagement step. In between engagement and disengagement the drive for drum 1 drives both drum 1 and roller 60. While the two members are disengaged the drive for drum 1 need drive only drum 1. These discontinuities in the engagement between drum 1 and roller 60 can cause a variation in the motion of drum 1 which will affect the quality of exposure provided by electronic exposure station 3. In systems in which the drum 1 and roller 60 are not disengaged, the edges of the receiving sheet also provide a discontinuity, not nearly as large as. hat illustrated in Figs. 6-9 but still capable of affecting imaging. This later variation of the problem will be discussed with reference to Figs. 28-31. The affect of such discontinuities can be totally eliminated by not writing with exposure station 3 during the discontinuities. Such an arrangement is illustrated in Figs. 12-15 in a system in which one image is created for each revolution of image member 1. According to Fig. 12, one-half or less of the circumference of drum 1 is written on as drum 1 passes exposure station 3. An image is written through an arc ABC as shown in Fig. 12 as drum l rotates in a clockwise direction past exposure station 3. The arrow inside the drum 1 illustrates the progressive direction of scan as seen by the drum 1, which is in the opposite direction to the rotation of drum 1 past exposure station 3. According to Fig. 13, roller 60 and drum l are engaged at the edge of transfer sheet 75. At substantially the same instant of engagement, the exposure station 3 is turned off having finished its scan. According to Fig. 14, transfer takes place through half a revolution of the drum 1 and roller 60. As transfer finishes with point C on drum 1 reaching the trailing edge of the receiving sheet 75, laser 3 begins its scan at point A and roller 60 is separated from drum 1 as shown in Fig. 15. From the process described in Figs. 12-15, it can be seen that both discontinuities at engagement and disengagement can be outside the scanning time of exposure station 3 if slightly less than one-half of drum 1 is used for that exposure. In fact, with the preciseness available with this system, very close to one-half of drum 1 can be used for such exposure. This system thus has a duty cycle of approximately 50%. Figs. 16-27 illustrate three approaches to increasing that duty cycle utilizing the imaging scheme described with regard to Figs. 1-5. According to Figs. 16-19, exposure station 3 is 90° from the transfer nip and three-fourths of the circumference of drum 1 is utilized in creating a large image made up of nine smaller images as diagrammed in Fig. 2. Laser exposure station 3 exposes the large image over an arc ABCD. The cross-track interfaces between small images occur at points B and C. This is true whether the long dimension or the short dimension is in the cross-track dimension because both dimensions are divided in thirds according to the Fig. 2 scheme. As shown in Fig. 16, roller 16 is being brought toward engagement with drum 1 as the scan of exposure station 3 approaches its end at point D. As shown in Fig. 17, engagement between roller 60 and drum 1 at point A occurs just as the point D is reached in the scan and the exposure station is turned off. According to Fig. 18 disengagement occurs as point D reaches the point of contact with roller 60. This occurs as point C is being scanned by exposure station 3. Point C is the boundary between the second and third sets of small images shown in Fig. 2. Any fluctuation in the motion of drum 1 would show up as an image artifact at or close to that boundary. Considering the preciseness of the system in general, it is likely that the artifact will occur at a place eliminated in the cutting operation. However, even if it is slightly displaced from that position, it occurs at the least objectionable portion of the image, that is, next to that boundary. The condition shown in Fig. 17 of engagement involves both the impact of roller 60 moving into drum 1 and also the increased load of driving roller 60 by the drive system for drum 1 and is therefore the greater discontinuity. This system locates that defect essentially outside of any of the image areas with impact occurring as the laser quits writing. The effect of the separation of roller 60 which occurs at the position shown in Fig. 18 primarily is affected only by the immediate decrease in load on the drive system for drum 1. This discontinuity has less effect on image writing than does the condition in Fig. 17 of engagement. Its positioning with the writer at the point C is a preferred approach to reducing discontinuities in the writing overall. The Figs. 16-19 location of the exposure station 3 forces both the cleaning station and the charging station into the quadrant between the transfer nip and that exposure station. Some machine configurations may not lend themselves to locating those two stations in such a limited space. Figs. 20-23 show an alternative approach, similar to Figs. 16-19, but which allows more space for the cleaning and charging stations. According to Figs. 20-23, the exposure station is located directly opposite the transfer station as in Figs. 12-15. However, the impact of engagement shown in Fig. 21 occurs when the exposure station 3 is at point C, the boundary between the second and third sets of small images shown in Fig. 2. Disengagement, shown in Fig. 22, then occurs at point B which is the boundary between the first and second set of small images in Fig. 2. In this embodiment, both discontinuities occur within the large image but at a boundary between the small images. Although this is not as desirable from an image defect standpoint as the embodiment shown in Figs. 16-19, it is superior in terms of machine geometry. Figs. 24-27 show a third location for the exposure station 3. With this approach, engagement shown in Fig. 25 occurs when point B is being written while disengagement occurs as point A is being written. This approach, while acceptable, is inferior to the other two embodiments in that the larger discontinuity of engagement (Fig. 25) occurs while writing at one of the boundaries between small images rather than outside or nearly outside of the entire large image area. At the same time, from a machine geometry standpoint all of the toning stations must be crowded between the exposure station and the transfer nip. For some toning stations, this would be impossible. However, if toning stations are cycled into and out of position as in some color apparatus, the Fig. 24 through 27 embodiment could be used. Larger-size prints, for example, those made with the image arrangements shown in Figs. 3-5 will generally end up with the image defect in the images. The larger the overall image, the less serious is a given size of defect. Thus, for the larger sizes the defect may be acceptable. However, if it is not acceptable in very high quality work, the larger images can be done at lower productivity than snapshots, skipping every other frame to avoid the defect entirely. Note that the Fig. 4 arrangement of 5x7's could be used without the two images on the left side and the discontinuities would occur while the lasers were writing in the borders between the other three images. Figs. 28-31 show another variation of the apparatus shown in Figs. 6-10. The cam disks 7 and 8 in Figs. 6-9 and the cam 70 are replaced by a pair of disks 161 and 162 shown in Figs. 28 and 29. According to Fig. 28, internally heated transfer roller 60 is driven by engagement with drum 1 during transfer as in Fig. 10. A stepper motor 117 drives roller 60 through shaft 125 when between transfers. Disks 161 and 162 have a radius slightly less than the combined radius of heated transfer roller 60 and receiving sheet 75 when sheet 75 is compacted by drum 1 and heated roller 60 in the nip 10. Shaft 125 is spring loaded as shown to obtain the desired pressure for transfer. As shown in Fig. 29, at the end of transfer, as the receiving sheet 75 leaves the nip, the loaded shaft 125 moves toward drum l until disks 161 and 162 contact the surface of drum 1 outside the image area. Disks 161 and 162 are mounted on shaft 125 and are free to rotate with respect to it. As shown in Fig. 29, roller 60 is now separated from drum 1 and can be rotated by motor 117 through shaft 125, essentially as described with respect to Fig. 6. That is, stepper motor 117 rotates roller 60 until it reaches a home position as controlled by sensor 12 sensing a mark 111 on the periphery of roller 60. Roller 60 is then rotated from its home position in timed relation to rotation of drum 1. This can be controlled by sensor 13 which senses a mark 112 on drum 1 or by encoder 67, or both. Drum 1 is driven by a motor 118 and may also include a flywheel (not shown) to steady its movement. The thickness of receiving sheet 75 and the separation between drum 1 and disks 161 and 162 have been exaggerated in Figs. 28 and 29 to aid the explanation. Disks 161 and 162 can be made less than .002 inches in radius less than the radius of the compacted receiving sheet and roller. Thus, the movement from the edge of the receiving sheet to the disks is very slight. Nonetheless, even that small an impact can result in a discontinuity of visible proportions at the exposure station. The discontinuities in such a system do not occur exactly as shown in Figs. 12-27, i.e., at the center of the nip. The thickness of the receiving sheet 75 will cause engagement slightly upstream of the nip center and will cause disengagement slightly downstream of the nip center. In systems like that shown in Figs. 28 and 29 in which the transfer roller 60 is reindexed for each image, this variation can be compensated for by making drum 1 slightly larger in diameter than the combined diameter of roller 60 at its operating temperature and receiving sheet 75. This refinement is shown in Figs. 30 and 31. According to Figs. 30 and 31, the circumference of drum 1 is made larger by an amount that will compensate for the width of the nip, i.e., the distance between the points of engagement and disengagement. In the Figs. 30-31 example, the circumference of drum 1 should equal the circumferential distance between the points of discontinuity (the distance between the point A in Fig. 30 at engagement and point D in Fig. 31 at disengagement) plus 4/3rds of the length (in the in-track direction) of the receiving sheet 75. Since the roller and drum are reindexed for each image the circumference of the roller 60 is immaterial, needing to be only big enough to accept sheet 75. With this arrangement, exposure station 3 is 180° from the center of the nip, point F, and the discontinuities felt by drum f 1 occur when point C (Fig. 30) and point B (Fig. 31) are being written. Since they occur on the boundary between images (Fig. 2) they do not affect image quality. The overall arrangement. Figs. 28-31, provides the high quality registration of Figs. 6-10 but with a simpler mechanism. Figs 32-34 show an alternative embodiment of the invention in which the disengagement of the transfer and imaging members is obtained by a discontinuity in the imaging member. According to Figs. 32 and 33, a photoconductive drum 201 includes a drum support, for example, a metallic cylinder 212 having an axis of revolution 213 and a gap or trough 214 in the periphery of the cylinder, which gap or trough runs generally parallel to the axis 213. A photoconductive sheet 217 is secured around the periphery of the cylinder 212. The photoconductive sheet 217 is clamped in the gap or trough 214 by suitable clamps 222 and 223. This is generally a known construction for a photoconductive drum. The sheet 217 itself is a multilayer structure well known in the art having at least one photoconductive layer and a conductive layer coated on a suitable support. Electrical contact to the conductive layer can be made at clamp 222 or 223. The use of a flexible photoconductive sheet has many known advantages compared to coating a photoconductive material directly on the drum. For example, the photoconductive sheet can be manufactured by ordinary coating technologies for coating webs which are considerably less expensive and very highly developed compared to drum coating approaches. Further, the photoconductive sheet can be easily replaced when its useful life is over, with or without removing the drum from the apparatus. As illustrated in Fig. 33, the trough or gap 214 does not extend to the full ends of the drum 201 End portions 231 and 232 of drum 201 are complete cylinders. End portions 231 and 232 can be formed as part of the same cylinder 212 with the gap 214 cut or cast in the manufacturing operation. Alternatively, end portions 231 and 232 can be separate disks mounted about axis 213. Drum 201 is driven by a motor 235 connected by a shaft 236 to cylinder 212. Shaft 236 also includes an encoder 237 which monitors the angular position of drum 201. In operation, the photoconductive surface of drum 201 (the photoconductive surface of sheet 217) is rotated by motor 235 through a series of stations to form a series of different color toner images according to a process well known in the art. Drum 201 is first charged at charging station 202, then imagewise exposed at an exposure station, for example laser exposure station 203, to create a series of electrostatic images. Each of the electrostatic images is toned by a toner of a different color by toning stations 204, 205, and 206 to create a series of different color toner images which, when superimposed, will give the desired multicolor toner image. To assure that the images are formed on the periphery of drum 201 and not attempted to be formed in the gap 214, the angular position of the drum is monitored by a sensing device 283 which senses a mark or other indicia 282 once each revolution (Fig. 33) . Indicia 282 and sensing device 283 can also be used to assure accurate registration. An encoder 237 produces pulses responsive to drum rotation which are superposed on a clock in a logic and control 270 to control each start of scan and other actions, all as is well known in the art. A transfer roller or drum 211 is positioned to engage drum 201 forming a nip 230 and to be driven thereby. A receiving sheet 210 is fed from a receiving sheet supply 216 to transfer drum 211 where it is secured to its periphery. The series of different color toner images are transferred in registration to the surface of receiving sheet 210 in the nip 230. The transfer drum 211 makes one revolution for each toner image transferred to receiving sheet 210. After the series of toner images has been transferred to receiving sheet 210 to create the multicolor toner image, the receiving sheet is separated from transfer drum 211 by a separating claw 218 and conveyed by a sheet transport device 219 to a fixing device 220 and then to an output tray 221, as is well known in the art. The photoconductive surface of drum 201 is cleaned at cleaning station 213 for reuse. As mentioned with respect to the other embodiments, conventional electrostatic transfer of ordinary toner images is commonly carried out with light or no pressure between the receiving sheet 210 and the photoconductive surface of drum 201. However, such transfer has shown to be inferior for transferring extremely small toner particles, for example particles as small as 3.5 microns and smaller. An effective approach to transferring such fine toner particles involves a combination of heat and pressure in the transfer nip 230. In this approach, the receiving sheet 210 is heated, for example, by external heating device 225 and internal heating device 226, to a temperature which causes the toner to soften and transfer under pressure to the surface of receiving sheet 210. For especially high quality work, receiving sheet 210 can have a thermoplastic overcoating which is heated to its softening point by heating devices 225 and 226. The heat of the thermoplastic coating heats the toner in the nip 230 causing it to soften or sinter. Some of the toner embeds in the softened thermoplastic layer and the rest of the toner adheres to the toner so embedded to fully transfer the toner image. To produce high quality multicolor images, it is necessary that the toner images be transferred in extremely accurate registration. To make full use of the fine toner particles available and the accuracy of the laser scanning image forming process, superposition of the images should be accurate to one one-thousandth of an inch. We have found such accuracy can be maintained with the structure shown in Fig. 32 with transfer drum 211 being driven by engagement between receiving sheet 210 and the photoconductive surface of drum 201. However, when multiple copies are made, the registration between drum 211 and drum 201 drifts over usage. This is due to slight changes in the circumference of drum 211 as it is heated, variations in the thickness of the receiving sheet 210, and tolerances in general. With the gap or trough 214 determining the location of the images on drum 201, such drift must be eliminated. The drum 211 also includes a trough 245 in which the ends of the transfer sheet 210 are secured. These securing means cannot be allowed to drift into the image areas of photoconductive drum 201. To solve this problem a separate motor 251 is fixed to a shaft 252 of transfer drum 211. Motor 251 rotates transfer drum 211 when the trough 214 is in the nip 230. To accomplish this, transfer drum 211 is of multipart construction as seen best in Fig. 34. More specifically, transfer drum 211 includes a central portion 240 which has a trough 245. A pair of vacuum drop-off bars 248 and 249 are mounted in trough 245 for securing the ends of receiving sheet 210. On each side of central portion 240 are outer portions 241 and 242 which are complete cylinders, not having the trough 245. Outer portions 241 and 242 are fixed to central portion 240 and rotate with it. Outer portions 241 and 242 have a diameter slightly more than central portion 240, but slightly less than the diameter of central portion 240 and receiving sheet 210 in combination. Outside of outer portions 241 and 242 are a pair of disks or rim-riders 256 and 257 which are also complete cylinders, are slightly less in diameter than are outer portions 241 and 242 and are free to rotate on shaft 252. Shaft 252 is spring-loaded toward drum 201 (as illustrated in Fig. 33) to provide the desired amount of pressure between the image bearing surface of drum 201 and receiving sheet 210. In operation, the periphery of drum 201 is large enough to hold a series of, for example, 3 or 4 color images. The color images are positioned on the periphery of drum 201 by laser exposure device 203 to exactly register on receiving sheet 210. That is, the starts of scan of each image are separated by a distance equal to the circumference of roller 203, counting the compacted receiving sheet 210. Enough space is inserted between images to account for trough 245. In general, such timing is well-known in the color image forming art and utilizes encoder 237, mark 282 and sensor 283 as described above. The electrostatic images are toned by toner stations 204, 205 and 206 to create, for example, cyan, magenta and yellow toner images. While the second and third electrostatic images are being formed by exposure station 203, the first toner image is reaching transfer nip 230 where it is transferred to receiving sheet 210. Transfer drum 211 is driven by the periphery of drum 201 which contacts receiving sheet 210 and, when facing trough 245, outer portions 241 and 242. The second and third toner image (and a fourth toner image in a four color system) are transferred on the second and third revolutions of transfer drum 211 to form a multicolor image on receiving sheet 210. As the leading edge of the receiving sheet 210 leaves the nip 230, with the last toner image being transferred to receiving sheet 210, claw 218 is moved to engage the periphery of drum 211 and vacuum drop-off bar surface 251 to strip the receiving sheet 210 therefrom. The vacuum applied to vacuum drop-off bar 249 is relieved just before claw 218 engages the leading edge of sheet 210 to permit such separation. After the third toner image has been transferred to receiving sheet 210, the trough 214 enters the nip 230 as shown in Fig. 32. At this point, neither central portion 240 nor outer portions 241 or 242 are in engagement with the periphery of drum 201 and therefore are no longer rotated by it. The axial separation between drum 201 and drum 211 is maintained by rim riders 256 and 257 which engage end portions 231 and 232 of drum 201. Since rim riders 256 and 257 are free to rotate on shaft 252, their engagement with drum 201 does not affect the rotation of roller 211 (central portion 240 and outer portions 241 and 242). Drum 201 and drum 211 can be castered (and gimbaled) together to provide a constant nip width across the nip. Maintaining the axial separation with rim riders 256 and 257 obviates the need to reestablish the caster axis. As the trough 214 enters the nip 230, motor 251 is engaged, to drive drum 211 to continue its rotation in a counterclockwise direction as seen in Fig. 32. The first part of this rotation completes the separation of receiving sheet 210 from the periphery of central portion 240, driving it onto sheet transport 219. The motor 251 continues to rotate transfer drum 211 to bring trough 245 around to a position at which vacuum drop-off bar 249 can receive a new receiving sheet from receiving sheet supply 216 and also to position transfer drum 211 at a home position to receive the next set of images as the trailing portion of trough 214 reaches the nip and the periphery of transfer drum 201 again engages transfer drum 211 to drive it for three more revolutions to receive three new images on the new receiving sheet. Depending on the size of trough 214, it is desirable to drive transfer drum 211 by motor 251 at a substantially increased rotational speed compared to its speed when it is driven by the periphery of drum 201. This allows a relatively narrow trough 214, but still manages to completely rotate transfer drum 211 to its home position for the next set of images. According to a preferred embodiment, transfer drum 211 can be rotated more than one time while the trough 214 is in nip 230 to completely secure the next receiving sheet to the periphery of transfer drum 211. This has the advantage of fully securing the sheet to the periphery so that it is securely held by both vacuum drop-off bars 248 and 249 for the entire transfer of the first toner image. This operation is controlled by logic and control 270 (Fig. 33) which receives pulses from encoder 237 on shaft 236 of drum 201 and from an encoder 238 on shaft 252 of transfer drum 211 and controls the engagement and speed of motor 251. Encoder 238 can be used to position drum 211 by counting pulses from the actuation of motor 251. However, for the most precise location of transfer drum 211 at its home position to receive each set of toner images, a mark or other indicia 272 is located on transfer drum 211 and is sensed by a sensor 273, for example an optical sensor, which sends a signal to logic and control 270 that transfer roller 211 has reached its home position. The home position for transfer drum 211 can be a position in which the drum 211 is stopped with the trough 245 in the nip 230. As the trailing portion of trough 214 reaches the nip 230, the periphery of drum 201 will engage outer portions 241 and 242 of transfer drum 211 and begin its rotation with the leading edge of the transfer sheet engaging a portion of the periphery following the trailing edge of the trough 214 into nip 230. With this approach, the rotational position of drum 201 does not have to be monitored to correctly reengage the drums (although it is monitored for image formation) . However, engagement can be made smoother if motor 251 is allowed to drive drum 211 after drum 211 reaches its home position at a speed dependent upon the rotational position of drum 201 (as determined by logic and control 270 from sensor 283 and encoder 237) . With such a control, the parameters of the system can be chosen so that drum 211 is moving when it engages with drum 201 at substantially the speed of drum 201. A clutch can be used in the drive train of motor 251 which disengages motor 251 at the instant of engagement of drum 201 and drum 211. However, a one-way clutch on motor 251 which permits drum 201 to over drive it also will be effective and requires less exact timing. Fig. 34 illustrates an alternative embodiment to motor 251 driving drum 211 while mounted on shaft 252. A recessed gear 290 is driven through a timing belt and motor (not shown) to drive roller 211. With this approach, the gear 290 can be fixed to shaft 252 or to roller 211. If it is fixed to roller 211, drum 211 can be allowed to rotate with respect to shaft 252. This would eliminate encoder 238 using only mark 272 and sensor 273 for control of drum 211. With all of the Fig. 32-34 embodiments, the transfer drum 211 is positioned at its home position before receiving each series of toner images. The transfer drum thus is reindexed for each set of images thereby eliminating any problem of gradual drift of the trough 245 into image areas of drum 201. According to preferred embodiments, a rapid rotation of transfer drum 211 can be utilized to firmly secure the next receiving sheet to the transfer drum 211 before the first image of the following series is transferred. According to an alternative embodiment, image drum 201 can be smaller in size and carry only a single image on its periphery at a time. In this approach, transfer drum 211 would be reindexed before transfer of each image. This approach is comparable with the approach illustrated, for example, in Figs. 28 and 29, and has the aspect that color registration is now dependent upon motor 251 , s accuracy in reindexing drum 211. Thus, for this embodiment, motor 251 should be a high quality stepper motor and sensor 273 must be precise. In the original approach described with respect to Figs. 32-34, where all three images are transferred before reindexing, the reindexing prevents drift as described and determines the location of the image on the page, but does not control color registration and therefore need not as precise a motor and sensing means. Athough this invention has its greatest benefit when the transfer member is driven by the image member during transfer, the concept of reindexing between images or groups of images can be used even though the transfer drum is otherwise driven during transfer. Although this invention can be utilized with any method of transfer, it is particularly useable with transfers involving substantial amounts of pressure in which rotation of the transfer roller by the transfer drum during transfer is desirable. When the transfer also includes the utilization of heat, changes in the parameters of the system due to expansion of both the image member and transfer member from the heat associated with the transfer member make particularly advantageous use of the invention. The invention has been described in detail with particular reference to a preferred embodiment thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention as described hereinabove and as defined in the appended claims.

Claims: I Claim: 1. Multicolor image forming apparatus comprising: a movable image member(l;201), means for moving said image member along a path(118;235), means for forming a series of electrostatic images on said image member(2,3;202,203), means for applying different color toners to said electrostatic images to form a series of different color toner images(20,21,22;204,205,206), transfer means for transferring said toner images to a receiving surface in registration to form a multicolor image, characterized in that said transfer means includes: a transfer member(5;60;211) defining or supporting said receiving surface on its periphery, said transfer member being movable repeatedly through a path which path includes a portion in whic said transfer member is in driving engagement and driven by said image member during transfer of an image to said receiving surface and a portion in which said transfer member is out of driving engagement with said image member, and means for moving said transfer member at least a portion of the time it is out of such driven engagement and for timing said movement with the movement of said image member to register said toner images on the receiving surface(17,12,13,30;117; 251,270,273,283). 2. The multicolor image forming apparatus according to claim 1 wherein said transfer member is a transfer drum and said means for moving said transfer member includes means for rotating said transfer drum. 3. Apparatus according to claim 2 wherein said transfer drum includes timing indicia and said apparatus includes means for sensing the presence of said timing indicia at a predetermined drum location and said apparatus includes means for determining that an image to be transferred is at a predetermined image location and said means for rotating said drum includes means for rotating said drum until said timing indicia is located at said predetermined location and for rotating said drum from said predetermined drum location in timed relation to the determination that an image to be transferred is at the predetermined image location. 4. Apparatus according to claims 1 or 3 wherein said means for moving said transfer member is a stepper motor. 5. Apparatus according to claim 3 wherein said determining means also is used to control the formation of said electrostatic images. 6. Apparatus according to claim 2 wherein said receiving surface is a surface of a receiving sheet secured to the periphery of said drum. 7. Apparatus according to claim 2 wherein said receiving surface is the periphery of said drum. 8. Apparatus according to claim 2 wherein said means for rotating said drum includes means for rotating said drum to a predetermined position and for stopping at that position and for beginning rotation of said drum from said predetermined position in timed relation to the position of said imaging member in its endless path. 9. Apparatus according to claim 6 including means for preventing engagement of said image member and said transfer drum, said means separating said image member and said transfer drum by a distance less than the thickness of a receiving sheet secured to the periphery of said drum. 10. Apparatus according to claim 9 wherein said transfer drum includes timing indicia and said apparatus includes means for sensing the presence of said timing indicia at a predetermined drum location and said apparatus includes means for determining that an image to be transferred is at a predetermined image location and said means for rotating said drum includes means for rotating said drum until said timing indicia is located at said predetermined location and for rotating said drum from said predetermined drum location in timed relation to the determination that an image to be transferred is at the predetermined image location. 11. Apparatus according to claim 10 wherein said determining means also is used to control the formation of said electrostatic images. 12. Apparatus according to claim 1 wherein said transfer means includes means for separating said transfer member and said image member and for reengaging said transfer member and said image member. 13. Apparatus according to claim 12 wherein said separating and reengaging means is cam means which moves in timed relationship with the movement of said transfer member. 14. Apparatus according to claim 2 wherein said transfer means includes cam means fixed for rotation with said drum for separating said drum and said image member and for controlling reengagement of said drum and said image member. 15. Apparatus according to claim 6 wherein said transfer means includes means for heating a receiving sheet carried on the periphery of said drum. 16. Apparatus according to claim 15 wherein said transfer means includes means for heating a heat softenable outer layer of said receiving sheet. 17. Apparatus according to claim 2 including means for applying a pressure greater than 10 psi between the image member, receiving sheet and the drum. 18. Apparatus according to any preceding claim wherein said image member is a photoconductive drum. 19. Apparatus according to claim 2 wherein apparatus includes means for separating and for reengaging said transfer drum and said image member and said means for rotating includes means for rotating said transfer drum at a speed such that the outer surface of said receiving sheet and the surface of said image member are moving at the same speed when reengaged. 20. Apparatus according to claim 18 including means for heating said receiving sheet to facilitate transfer of said toner images. 21. Apparatus according to claim 20 wherein said heating means heats at least a heat softenable outer layer of said receiving sheet. 22. Multicolor image-forming apparatus comprising: a moveable image member, means for moving said image member along a path, means for forming a series of electrostatic images on said image member, means for applying different color toners to said electrostatic images to form a series of different color toner images. transfer means for transferring said toner images to a receiving sheet in registration to form a multicolor image on said sheet, characterized in that said transfer means includes a transfer drum having a periphery for supporting said receiving sheet around a portion of said periphery, leaving a portion of said periphery uncovered by said sheet, means for urging said drum toward said image member, means for preventing engagement of said drum and said image member, said preventing means including means for providing a spacing between said drum and said image member that is less than the thickness of said receiving sheet when compacted between said drum and said image member so that said drum is driven by said image member through said sheet, and means for rotating said drum independent of said engagement between the receiving sheet and the image member when said uncovered portion of the periphery of said drum is facing said image member to index said drum for receipt by said receiving sheet of each of said series of images. 23. Apparatus according to claim 22 wherein said means for preventing engagement of said drum and said image member includes a pair of disks coaxially mounted with said drum outside opposite ends of said drum, said disks being radially larger than said drum by an amount equal to the desired spacing between the drum and the image member. 24. Apparatus according to claim 23 wherein said disks and said transfer drum are mounted on a common shaft, said shaft being fixed to said transfer drum but rotatable relative to said disks and wherein said rotating means includes means for rotating said shaft to rotate said transfer drum. 25. Multicolor image forming apparatus comprising: a rotatable image drum, having an axis of rotation, a periphery and a gap in said periphery running generally parallel to said axis, means for creating electrostatic images on said periphery, means for applying different colored toners to said electrostatic images to create different color toner images, and transfer means for transferring said toner images in registration to a receiving surface to create a multicolor toner image, characterized in that said transfer means includes: a transfer drum positioned to engage, form a nip with, and be rotated by the periphery of said image drum, means for securing a receiving sheet to a portion of the periphery of said transfer drum for receiving said series of toner images in registration as said transfer drum is repeatedly rotated by said image drum, and means for reindexing said transfer drum when said gap in said drum passes through said nip. 26. Multicolor image forming apparatus according to claim 25 further including means for separating said receiving sheet from said transfer drum and means for securing a new receiving sheet to said transfer drum while said transfer drum is being reindexed. 27. Apparatus according to claim 25 wherein said image drum is large enough to hold an entire series of toner images, one after another around said periphery of said image drum. 28. Apparatus according to claim 25 wherein said image drum is designed to hold only a single image on its periphery at a time and said transfer drum is reindexed between toner images that are superposed on a single receiving sheet. 29. A method of transferring a series of toner images from a moving image member in registration to a heat softenable outer layer of a receiving sheet supported on the outer periphery of a transfer drum, said method including heating said heat softenable layer to at least its softening point and rotating said transfer drum to provide repeated pressure contact between said images and said softened layer to transfer said images thereto, said method being characterized in that the step of rotating said drum includes providing a separation between said drum and image member between transfers and reindexing said drum with respect to the location of the next image on said image member during such separation. 30. The method according to claim 29 wherein the toner images of the series are of different color and said reindexing step is carried out between transfers of each of said images in registration to the heat softenable layer to form a multicolor image. 31. The method according to claim 29 wherein said reindexing step is carried out only between after transfer of all images of a series.

Inventors: JAMZADEH FEREIDOON SHAHJAHAN, JOHNSON KEVIN MICHAEL, SHOUP CLAY EDWARD, YOUNG TIMOTHY JOHN, JAMZADEH, FEREIDOON, SHAHJAHAN, JOHNSON, KEVIN, MICHAEL, SHOUP, CLAY, EDWARD, YOUNG, TIMOTHY, JOHN
Applicants: EASTMAN KODAK CO, EASTMAN KODAK COMPANY
Application year: 1991
Main classification: G03G15
IPC codes: G03G15