Image forming apparatus employing LED head

Abstract: An image forming apparatus comprises: a photoreceptor drum (8); an LED head (10) for exposure provided along the width of the photoreceptor drum (8); a pin (19,20) provided on the LED head (10); a supporting base substance to which the photoreceptor drum (8) is attached; and a pin admitting groove (37,38) provided on the supporting base substance for admitting the pin. The LED head (10) is half-fixed on the supporting base substance.

Description: The invention relates to an image forming apparatus according to the preamble of claim 1. Such an apparatus is known from US 4 875 057. The image forming apparatus employs an LED (light emitting diode). More particularly, the present invention relates to an image forming apparatus such as a digital electrophotographic copying machine, a printer etc. For digital image forming apparatuses, an LED head is frequently used as exposure means for writing an image onto the photoreceptor surface which has already been charged. The LED head has a structure where pairs consisting of an LED device and SELFOC lens are linearly arranged in a direction parallel to the axis of a photoreceptor drum (that is, in a direction along the width of the photoreceptor drum). The number of pairs corresponds to the number of picture elements in the direction. Although the LED head is arranged in a direction along the axis of the photoreceptor drum as described above, the arrangement position of the LED head sometimes shifts with respect to the direction along the width of the photoreceptor because of a thermal expansion of the whole LED head caused by a light emission of the LED device. This means that a formed image is shifted from an intended position corresponding to an original, which shift is undesirable. However, in a conventional image forming apparatus for paper of smaller size, since the number of LED devices and the thermal expansion are small so that hardly any problems are caused, both ends of the LED head are fixed along the width of the photoreceptor on a supporting base substance to which the photoreceptor is attached. In an image forming apparatus for paper of larger size, since a greater number of LED devices are required because of a large number of the picture elements, an expansion and contraction, in actual size, of the LED heads and the shift of a formed image from an intended position corresponding to an original due to the thermal expansion cannot be ignored, so that, for example, there are occasions when the center of the original and that of the formed image do not correspond. US 4 875 057 discloses a modular optical printhead for hard copy printers. A supporting and registration plate contains a rectangular opening in which a lens is located. The plate is secured to the overall LED supporting structure of the printhead by end members which together with side members completely seal the LED's and connections from contaminants. The registration plate has at least two surfaces thereon which mate with surfaces in the associated apparatus to accurately position the plate. During the alignment process of the printhead, the lens and plate are adjusted to provide a focus at a fixed distance from one of the registration surfaces of the plate. Alignment of the lens across the axis of the LED array is provided by recessed set screws and a shim. In this prior art, effects due to thermal expansion are considered only within the optical printhead, but not with respect to the fixation of the printhead in relation to the photoconductor. It is the object of the invention to provide an image forming apparatus capable of forming a stable image. This object is accomplished in accordance with the features of claim 1, dependent claims are directed on preferred embodiments of the invention. An image forming apparatus is provided with a photoreceptor, an LED head for exposure provided along the width of said photoreceptor, first fixing means provided on said LED head, a supporting base to which said photoreceptor is attached, second fixing means provided on said supporting base and on which said first fixing means is fixed, and supporting means which support said LED head on said supporting base substance. When a fixing position of the first and second fixing means is the center of the LED head, although the both ends of the LED heads freely expand and contract in a direction of the thermal expansion caused by a light emission of the LED device, the center never shifts with respect to the photoreceptor. Thus, the center of the shift is fixed even when a formed image is shifted from an original due to the thermal expansion of the LED heads, so that a stable image is formed. This and other objects and features of this invention will become clear from the following description taken in conjunction with the preferred embodiments with reference to the accompanied drawings in which: Fig. 1 is a schematic view of an electrophotographic copying machine in which the present invention is incorporated; Fig. 2 is an enlarged view of a photoreceptor drum and an LED head; Fig. 3 is a view showing a condition of an engagement of the LED head and stays; Fig. 4 is a cross-sectional view thereof; and Fig. 5 is a perspective view of the LED head and the stays. An embodiment of the present invention will hereinafter be described with reference to the drawings. Fig. 1 shows a main portion of a digital copying machine in which the present invention is incorporated. In the figure, an original 3 is placed on a contact glass 2 provided on the upper surface of a copying machine 1. A scanning optical system 4 is provided below the contact glass 2. The scanning optical system 4 moves from a home position shown in the figure toward the right while scanning the original, and turning around at a predetermined position, it returns to the home position. Light L from the optical system 4 is directed to a CCD (charge coupled device) 6 through an image forming lens 5. After converted into an electric signal at the CCD 6, the light information is processed as image data at an image processing circuit 7. An output of the image processing circuit 7 is directed to an LED head 10 for exposure to expose the surface of a photoreceptor drum 8. A charging portion 9, the LED head 10 for exposure, a developing roller 11, a transferring portion 12, a separating portion 13, a cleaning portion 14 and a charge removing portion 15 are provided on the periphery of the photoreceptor drum 8 in the order named along a rotation direction thereof. The charging portion 9 charges the surface of the photoreceptor drum 8 so that its potential is approximately 700V. The LED head 10 forms a latent image on the surface of the photoreceptor drum 8 by exposing the charged surface of the photoreceptor drum 8 based on the above-described image information. The developing roller 11, which constitutes a part of the developing apparatus, provides toner onto the surface of the photoreceptor drum 8 and develops the latent image formed on the surface of the photoreceptor drum 8. The transferring portion 12 transfers the image on the surface of the photoreceptor drum 8 onto a sheet of paper provided by a paper feeder (not shown) so as to contact the surface of the photoreceptor 8. The separating portion 13 separates the sheet of paper on which an image is transferred from the surface of the photoreceptor drum 8 and directs it to a conveying path leading to a fixing portion (not shown). Fig. 2 is an enlarged view of the photoreceptor drum 8 and the LED head 10. In the figure, the numeral 16 represents an optical system where zooming is not performed such as a SELFOC lens, etc., the numeral 17 represents a substrate where the LED device is attached, and the numeral 18 represents a heat radiating plate serving as a heat sink and made of aluminum. The optical system 16 and the LED device make a pair. Many such pairs are arranged in an array in a direction perpendicular to the surface of Fig. 2 (that is, a direction along the axis of the photoreceptor drum 8). The numerals 19 and 20 represent pins attached to the LED head 10, and they are engaged with pin admitting grooves 37 and 38 of stays 21 and 22 to be subsequently described (see Fig. 5). Fig. 3 is a plan view showing a condition of an engagement of the LED head and the stays 21 and 22. Fig. 4 is a a cross-sectional view taken on a part thereof. In this embodiment, an engagement position of the LED head 10 and the stays 21 and 22 is the center of the LED head 10, that is, the center of the photoreceptor drum 8 along the axis thereof. The stays 21 and 22 are fixed with screws 29, 30, 31 and 32 on side plates 27 and 28 on which the photoreceptor drum 8 is fixed. As shown in Fig. 5, the pin admitting grooves 37 and 38 for admitting the pins 19 and 20 provided on the LED head are formed on the stays 21 and 22. Both ends of the LED head are fixed with pins 23 and 24 through elliptic holes 25 and 26 on L-shaped fixing plates 33 and 34 fixed on the side plates 27 and 28 with screws 35 and 36. In the above structure, the LED head 10 is heated by the turning on of the LED device thereof and is expanded when the heat is high. Relating an expansion in a lateral direction in Figs. 3 and 4, since a correspondence of the center of the LED head and the centers of the stays 21 and 22 is maintained by the engagement of the pins 19 and 20 of the LED head 10 and the pin admitting grooves 37 and 38 of the stays 21 and 22, the center of the LED head 10 never shifts with respect to the center of the photoreceptor drum 8. Thus, an image formed on transferring paper (not shown) through the photoreceptor drum 8 is relatively clear, since the image, although shifted on both sides, is not shifted in the center (that is, the center of the shift is fixed). In this embodiment, the positions of the center of the LED head 10 and the centers of the stays 21 and 22 correspond, and the positions of both sides of the LED head and both sides of the stays 21 and 22 shift. The shift becomes permittable by moving the pins 23 and 24 in the elliptic holes 25 and 26. Although the pins 19 and 20 are provided on the LED head 10, and the pin admitting grooves 37 and 38 are formed on the stays 21 and 22 in this embodiment, it may be possible to provide the pins on the stays 21 and 22 and to form the grooves on the LED head. Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced other than as specifically described.

Claims: An image forming apparatus comprising a photoreceptor (8), an LED-head (10) for exposure provided along the width of the photoreceptor (8), first fixing means (19, 20) provided on said LED-head (10), second fixing means (21, 22, 37, 38) provided on the supporting base, to which the first fixing means (19, 20) is fixed, and a supporting base to which the photoreceptor (8) is attached, characterized in that the first fixing means (19, 20) are provided on the LED-head (10) at a fixing position corresponding to the center of the photoreceptor (8), and supporting means are provided for supporting the LED-head on the supporting base and allowing a shift of both ends of the LED-head along the width of the photoreceptor. An apparatus according to claim 1, characterized in that the second fixing means comprises a stay (21, 22) attached to the supporting base. An apparatus according to claim 1, characterized in that the image forming apparatus is an electrophotographic copying machine. An apparatus according to claim 1, characterized in that the first fixing means is a pin and the second fixing means is a pin admitting groove.

Inventors: KONDOU AKIHIRO, TAKADA AKIHIRO, KONDOU, AKIHIRO, TAKADA, AKIHIRO
Applicants: MITA INDUSTRIAL CO LTD, MITA INDUSTRIAL CO. LTD.
Application year: 1991
Main classification: G03G15
IPC codes: B41J2, H04N1, G03G15