Electrophotographic imaging apparatus with reduced packaging size

An electrophotographic imaging apparatus includes a plurality of photoconductive drums and a plurality of printhead assemblies. Each drum has an axis of rotation and a photoconductive peripheral surface. A plurality of the axes of rotation of the drums lie in a generally common plane. The peripheral surface of each drum rotates in a common direction defining an advance direction of the print medium. Adjacent drums are disposed at a first distance away from each other between the axes of rotation. Each printhead assembly is associated with a respective drum and is configured for generating a laser beam which impinges upon the respective drum along a line of incidence. The printhead assemblies are disposed at a second distance from one to another which is greater than the first distance.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to an electrophotographic imaging apparatus
 such as a laser printer, and, more particularly, to such an imaging
 apparatus including multiple printhead assemblies and photoconductive
 drums.
 2. Description of the Related Art
 An electrophotographic imaging apparatus, such as a laser printer, may be
 configured as a multi-color imaging apparatus for producing color images
 on a print medium. In a tri-color imaging apparatus, it is known to
 provide four separate toner reservoirs which contain different color
 toners. For example, a first toner reservoir may contain yellow toner; a
 second toner reservoir may contain magenta toner; a third toner reservoir
 may contain cyan toner; and a fourth toner reservoir may contain black
 toner. Each toner reservoir is positioned in association with a
 photoconductive (PC) drum on which a latent image is formed using a laser
 beam which is projected from a printhead assembly. The laser beam projects
 against the peripheral surface of the PC drum at an angle of incidence
 relative to a line extending through an axis of rotation of the PC drum
 and the point of incidence of the laser beam. The laser beam is scanned
 across the peripheral surface of the PC drum along a line extending
 parallel to the axis of rotation of the drum. In an image forming
 apparatus as described above, the printhead assemblies may be configured
 substantially identical to each other. Moreover, the printhead assemblies
 are configured to project the scanned laser beam at identical angles to
 the PC drum along lines of incidence which are positioned at the same
 relative position from one drum to another. Although such a configuration
 provides high quality printing, geometric constraints associated with such
 an arrangement tend to increase the packaging size of the imaging
 apparatus.
 What is needed in the art is an EP imaging apparatus which provides high
 quality printing with a reduced overall packaging size.
 SUMMARY OF THE INVENTION
 The present invention provides an electrophotographic imaging apparatus,
 such as a laser printer, with a reduced packaging size by increasing the
 distance between adjacent printhead assemblies and decreasing the distance
 between adjacent PC drums.
 The invention comprises, in one form thereof, an electrophotographic
 imaging apparatus for printing on a print medium, including a plurality of
 photoconductive drums and a plurality of printhead assemblies. Each drum
 has an axis of rotation and a photoconductive peripheral surface. A
 plurality of the axes of rotation of the drums lie in a generally common
 plane. Each peripheral surface rotates in a common direction defining an
 advance direction of the print medium. Each printhead assembly is
 associated with a respective drum and is configured for generating a laser
 beam which impinges upon the respective drum along a line of incidence.
 The laser beam is disposed at an angle of incidence relative to a
 perpendicular extending through the line of incidence and the axis of
 rotation. The angle of incidence sequentially increases from one printhead
 assembly to another along the advance direction.
 The invention comprises, in another form thereof, an electrophotographic
 imaging apparatus including a plurality of photoconductive drums and a
 plurality of printhead assemblies. Each drum has an axis of rotation and a
 photoconductive peripheral surface. A plurality of the axes of rotation of
 the drums lie in a generally common plane. The peripheral surface of each
 drum rotates in a common direction defining an advance direction of the
 print medium. Adjacent drums are disposed at a first distance away from
 each other between the axes of rotation. Each printhead assembly is
 associated with a respective drum and is configured for generating a laser
 beam which impinges upon the respective drum along a line of incidence.
 The printhead assemblies are disposed at a second distance from one to
 another which is greater than the first distance.
 An advantage of the present invention is that the packaging size of the
 electrophotographic imaging apparatus is reduced.
 Another advantage is that the distance between adjacent printhead
 assemblies may be increased by either sequentially increasing the angle of
 incidence or sequentially moving the line of incidence from one
 photoconductive drum to another along an advance direction.
 Yet another advantage is that the printhead assemblies may be precisely and
 easily located relative to one another in the image forming apparatus
 using precisely located support channels attached to a frame.

DETAILED DESCRIPTION OF THE INVENTION
 Referring now to the drawings, and more particularly to FIG. 1, there is
 shown a simplified schematic illustration of a printhead assembly 10 and
 photoconductive drum 12 which are incorporated into one embodiment of an
 electrophotographic (EP) imaging apparatus 14 of the present invention. In
 the embodiment shown, EP imaging apparatus 14 is in the form of a laser
 printer, but may be configured as a different imaging apparatus such as an
 EP photocopier, etc.
 PC drum 12 rotates about an axis of rotation 16, and defines a nip 18 with
 a back-up roller 20. Drum 12 includes a photoconductive peripheral surface
 22 on which a latent image is formed, in known manner. The latent image is
 transferred to either an intermediate transfer member such as a
 photoconductive belt (not shown), or directly to a print medium such as
 paper (not shown) which travels through nip 18 in advance direction 24.
 Advance direction 24 is generally tangent to direction of rotation 26 of
 drum 12 at nip 18, and thus is generally defined by direction of rotation
 26.
 Printhead assembly 10 includes a housing 28 carrying a laser source 30,
 polygon mirror 32, turn-around mirror 34, first lens 36, fold-down mirror
 38 and second lens 40. Laser source 30 generates a laser beam in a known
 manner which impinges upon polygon mirror 32. Polygon mirror 32 rotates
 about an axis of rotation 42 at a predetermined rotational speed. As
 polygon mirror 32 rotates, the laser beam generated by laser source 30
 scans across turn-around mirror 34 (i.e., in a direction perpendicular to
 the drawing of FIG. 1), thereby ultimately causing the laser beam to scan
 across peripheral surface 22 of drum 12, as will be described hereinafter.
 The laser beam then passes through first lens 36 and is reflected at a
 generally right angle using fold-down mirror 38. The laser beam then
 passes through second lens 40 and exits through aperture 44. The travel
 path of the laser beam is thus represented by dashed line 46.
 Laser beam 46 is projected against peripheral surface 22 of PC drum 12 at a
 point of incidence 48, and is scanned across peripheral surface 22 along a
 line of incidence 50 as polygon mirror 32 rotates. A parallel 52 is
 disposed generally parallel to advance direction 24; and a normal 54
 extends generally perpendicular to advance direction 24. Point of
 incidence 48 and line of incidence 50 are each disposed at an angle
 .alpha. relative to parallel 52 and advance direction 24. Angle .alpha.
 defines a perpendicular 56 which extends generally perpendicular to line
 of incidence 50 (i.e., extending through line of incidence 50 and axis of
 rotation 16 and thus perpendicular to peripheral surface 22). Laser beam
 46 is disposed at an angle of incidence .beta. relative to perpendicular
 56 as laser beam 46 is scanned across line of incidence 50. The specular
 component of light reflected from peripheral surface 22 of drum 12 is
 disposed at a complementary angle .beta. relative to perpendicular 56.
 Referring now to FIG. 2, there is shown another embodiment of an EP imaging
 apparatus 60 of the present invention. EP imaging apparatus 60 corresponds
 to a tricolor printer with four separate toner reservoirs 62A, 62B, 62C
 and 62D which respectively contain different color toners. In the
 embodiment shown, toner reservoir 62A contains yellow toner; toner
 reservoir 62B contains magenta toner; toner reservoir 62C contains cyan
 toner; and toner reservoir 62D contains black toner. Each toner reservoir
 62A-62D is associated with a respective PC drum 12 which is configured the
 same as PC drum 12 shown in FIG. 1. Each drum 12 includes an axis of
 rotation 16 which lies in a common plane 64. Drums 12 thus define a
 substantially linear advance direction 24 of ITM 66 in the form of a belt.
 Each PC drum 12 is respectively associated with a printhead assembly 10,
 which are substantially identically configured relative to each other in
 terms of structural components.
 According to an aspect of the present invention, printhead assemblies 10
 and PC drums 12 are arranged relative to each other to reduce the
 packaging size of each EP imaging apparatus 60. More particularly, each
 printhead assembly 10 is configured to project a laser beam against a
 corresponding drum 12 at a different angle of incidence .beta.1, .beta.2,
 .beta.3 or .beta.4. Angles of incidence .beta.1-.beta.4 sequentially
 increase from one printhead assembly 10 to another along advance direction
 24 of EP imaging apparatus 60. That is, from an upstream end of ITM 66
 associated with toner reservoir 62A to a downstream end of ITM 66
 associated with toner reservoir 62D, the angle of incidence
 .beta.1-.beta.4 sequentially increases from one printhead assembly 10 to
 another along advance direction 24.
 It has been found that the angle of incidence .beta. may maximally vary
 between approximately 2 and 10.degree. relative to perpendicular 56 and
 still provide high quality printing. In the embodiment shown in FIG. 2,
 the angle of incidence .beta. generally varies between 5 and 8.degree.,
 and increases by a value of 0.75.degree. from one printhead assembly 10 to
 another in advance direction 24. In particular, .beta.1 equals
 approximately 5.degree.; .beta.2 equals approximately 5.75.degree.;
 .beta.3 equals approximately 6.5.degree.; and .beta.4 equals approximately
 7.25.degree.. By increasing the angle between adjacent printhead
 assemblies 10, the distance between adjacent printhead assemblies (using
 any arbitrary reference location) is increased from one printhead assembly
 10 to another in advance direction 24. This in turn allows PC drums 12 to
 be placed closer together relative to each other (using, e.g., a center to
 center distance from one axis of rotation to another). The overall
 packaging size of EP imaging apparatus 60 is thus reduced since the
 distance between PC drums 12 is reduced.
 In the embodiment of FIG. 2, each printhead assembly 10 has a second lens
 40 which is placed at a distance of approximately 120 mm from peripheral
 surface 22 of a corresponding PC drum 12. Laser beam 46 projects against
 each PC drum 12 at an angle .alpha. of approximately 85.degree.. Since the
 angle .alpha. remains constant, while the angle .beta. changes from one
 printhead assembly 10 to another, it is apparent that the relative
 mounting location of each printhead assembly 10, as well as the angular
 orientation of each printhead assembly 10, varies a corresponding amount
 within EP imaging apparatus 60. To precisely locate each printhead
 assembly 10, EP imaging apparatus 60 includes a plurality of support
 channels 68 (FIG. 3) which are precisely located on and attached to frame
 70 of EP imaging apparatus 60. Each support channel 68 extends generally
 perpendicular from frame 70, and includes one or more mounting flanges 72
 which extend parallel to and are disposed against frame 70. Each mounting
 flange 72 is suitably attached with frame 70, such as by a toxing process,
 riveting, welding, or the like. Each support channel 68 is suitably
 configured, such as with precisely located attachment holes, keying, etc.
 (not shown) to precisely locate and orient a corresponding printhead
 assembly 10.
 In the embodiment shown in FIG. 2, each support channel 68 is mounted at a
 distance of approximately 147 mm from peripheral surface 22 of the
 corresponding PC drum 12. The 0.75 increase in the angle of incidence
 between each adjacent printhead assembly 10 along advance direction 24
 provides an additional 1.9 mm spacing between adjacent printhead
 assemblies 10. More particularly, in the embodiment shown in FIG. 2, the
 spacing between axis of rotation 16 of adjacent PC drums 12 is 101 mm and
 the spacing between adjacent printhead assemblies 10 is 102.9 mm.
 FIG. 4 illustrates another embodiment of an EP imaging apparatus 80 of the
 present invention. As indicated above, it has been found that an angle of
 incidence between 2 and 10.degree. provides high quality printing. EP
 imaging apparatus 80 is configured as a tri-color laser printer with four
 printhead assemblies 10 as shown. Printhead assemblies 10 are arranged to
 maximize the distance between adjacent printhead assemblies 10 while still
 staying within the 2 to 10.degree. angle of incidence limit providing good
 performance. By maximizing the distance between adjacent printhead
 assemblies 10, the corresponding distance between adjacent PC drums 12 is
 also fully minimized. The angle of incidence .beta. from one printhead
 assembly 10 to another in advance direction 24 increases by a value of
 2.66.degree. from one printhead assembly 10 to another in advance
 direction 24. Utilizing a spacing of about 147 mm from each support
 channel 68 to an associated PC drum 12, this results in an additional 6.8
 mm between adjacent printhead assemblies 10 as compared with a
 conventional arrangement wherein laser beams 46 are arranged parallel to
 each other within EP imaging apparatus 80.
 FIG. 5 illustrates yet another embodiment of an EP imaging apparatus 90 of
 the present invention which provides a reduced package size. In the
 embodiment shown in FIG. 5, the angle of incidence .beta.1-.beta.4
 respectively associated with PC drums 12 remains constant. Nonetheless,
 printhead assemblies 10 are arranged to provide a greater distance
 therebetween, thus reducing the overall packaging size of EP imaging
 apparatus 90. This is accomplished by increasing the angle .alpha. of each
 perpendicular 56A, 56B, 56C and 56D from one PC drum 12 to another in
 advance direction 24. As the angle .alpha. increases from one printhead
 assembly 10 to another in advance direction 24, the associated printhead
 assembly 10 must of course be repositioned within EP imaging apparatus 90
 to maintain a constant distance relative to PC drum 12 as well as a
 constant angle of incidence .beta.1-.beta.4. In the embodiment shown, each
 PC drum 12 has a diameter of approximately 30 mm, and the angular change
 of each laser beam 46 results in the line of incidence 50 of each PC drum
 12 being sequentially offset from one drum 12 to another along advance
 direction 24 between 0.100 and 0.800 mm, preferably between 0.150 and
 0.300 mm, and more preferably about 0.262 mm. That is, the point of
 incidence 48 and line of incidence 50 shifts in a counter-clockwise
 direction about 0.262 mm along peripheral surface 22 from one drum 12 to
 another in advance direction 24. This corresponds to a 1.degree. change in
 angle on a 30 mm diameter PC drum. This results in the spacing between
 adjacent printhead assemblies 10 being increased compared to the spacing
 between adjacent PC drums 12 with an overall reduced packaging size.
 EP imaging apparatus 14, 60, 80 and 90 have been detailed for the case in
 which the axes of rotation of the drums lie in a common plane. However,
 the present invention is equally applicable to the case in which the drums
 are located in a shallow, convex arc; or to the case in which a single
 drum (black, for example) is moved out of the plane of the remaining
 drums, such as may be done when printing in a black only mode. For
 example, in another embodiment (not shown because of minuscule tolerance
 differences between previous figures), the black PC drum associated with
 developer 62D in FIG. 1 is dropped by 1 mm with respect to the plane of
 the axes of the remaining color PC drums. This results in a 1 mm
 translation of the entire black station in a direction downward and
 perpendicular to the plane 64 (FIG. 2) established by the remaining PC
 drums. Here the black station consists of print head 10, PC drum 12,
 developer 62D, and transfer roll 20. The angle of incidence of the laser
 beam on the black drum is no different than in the case where all drums
 are located in a common plane.
 While this invention has been described as having a preferred design, the
 present invention can be further modified within the spirit and scope of
 this disclosure. This application is therefore intended to cover any
 variations, uses, or adaptations of the invention using its general
 principles. Further, this application is intended to cover such departures
 from the present disclosure as come within known or customary practice in
 the art to which this invention pertains and which fall within the limits
 of the appended claims.