Abstract:
An exposure device includes: a substrate having a first and second surface opposed to each other, wherein a light-emitting element array is mounted on the first surface; a protective sheet covering the second surface while exposing a part of the second surface in the vicinity of an edge of the substrate to define an exposed part of the second surface; a holder that supports a condenser lens array and that supports the substrate, with a gap formed between the holder and the exposed part of the substrate, to position the substrate such that light emitted from the light-emitting element array is condensed on a predetermined irradiation location through the condenser lens array; and a thermally-conductive sealant that covers the exposed part of the second surface of the substrate and seals the gap.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority based on 35 USC 119 from prior Japanese Patent Application No. 2014-112869 filed on May 30, 2014, entitled “EXPOSURE DEVICE AND IMAGE FORMATION APPARATUS”, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This disclosure relates to an exposure device and an image formation apparatus, and is suitable for application to electrophotographic printers (hereinafter also referred to as printers), for example. 
     2. Description of Related Art 
     In widespread printers as conventional printers, an electrostatic latent image is formed on the surface of a photoconductive drum by irradiating the drum&#39;s surface with light from an exposure device such as an LED head, which includes light-emitting elements such as light emitting diodes (LEDs) and a lens array, and then is developed to a toner image with application of toner to the electrostatic latent image. 
     Some of such exposure devices include, for example, a substrate with one of its surfaces equipped with an LED array chip having LEDs arranged linearly, a rod lens array configured to condense light emitted from an LED array chip, and a holder holding the substrate and the rod lens array such that the LED array chip on the substrate and the rod lens array face each other. In addition, in some exposure devices, an attachment is attached to the substrate so as to cover the other surface opposed to the one surface, and the substrate is fixed to the holder by bonding the holder and the attachment (see Patent Literature 1, for example). 
     In such an exposure device, the LED array chip mounted on the substrate emits light in an irradiation pattern based on an image to be printed. The emitted light is converged through the rod lens array, and the surface of a photoconductive drum disposed at a focus position of the rod lens array is exposed to the light, so that an electrostatic latent image is formed on the surface.
     [Patent Literature 1] Japanese Patent Application Publication No. 2012-66499 (FIG. 5)   

     SUMMARY OF THE INVENTION 
     In the manufacturing of such an exposure device, the substrate is fixed to the holder with an adhesive after the holder is positioned with the optical axis of the LED array chip aligned with the center of the rod lens array. For this reason, in the exposure device, the substrate needs to be formed slightly smaller than the holder in order to form a gap between them, and this gap remains after the bonding with the adhesive. In the exposure device, however, problems such as a reduction in the amount of light may arise if foreign matter such as toner enters a space at the one surface side of the substrate, namely, a space around the LED array chip and the rod lens array. To prevent this, the gap between the substrate and the holder is sealed with a sealant such as silicone. 
     Meanwhile, in some exposure devices, a protective sheet covering the entire other surface of the substrate is placed between the other surface of the substrate and the attachment. The protective sheet can prevent a circuit on the substrate from being physically damaged by the attachment and, particularly when the attachment is conductive like metal, can prevent a short circuiting of the circuit on the substrate if the sheet is insulative. In addition, by covering the entire other surface of the substrate with the protective sheet, the sheet can prevent foreign matter such as toner from entering the one surface side of the substrate via through-holes formed in the substrate. 
     On the other hand, due to its nature, the LED array chip of such an exposure device generates heat at the time of emitting light and transmits the heat to the substrate. However, since the substrate is not in contact with a thermally conductive component such as the holder and the other surface of the substrate is wholly covered with the protective sheet, the exposure device cannot release the generated heat to any other part efficiently. For this reason, the exposure device has a problem in that the LED array chip is heated to such an extremely high temperature as to degrade the performance of the chip, or even break the chip in some cases. 
     An embodiment of the invention aims to propose an exposure device and an image formation apparatus capable of securing a substrate position adjustment function and improving heat dissipation performance at the same time. 
     An aspect of the invention is an exposure device that includes: a substrate having a first and second surface opposed to each other, wherein a light-emitting element array is mounted on the first surface; a protective sheet covering the second surface while exposing a part of the second surface in the vicinity of an edge of the substrate to define an exposed part of the second surface; a holder that supports a condenser lens array and that supports the substrate, with a gap formed between the holder and the exposed part of the substrate, to position the substrate such that light emitted from the light-emitting element array is condensed on a predetermined irradiation location through the condenser lens array; and a thermally-conductive sealant that covers the exposed part of the second surface of the substrate and seals the gap. 
     According to the above aspect of the invention, the substrate can be positioned with respect to the holder so that the light emitted from the light-emitting element array attached to the substrate may be condensed appropriately through the condenser lens array. Further, thanks to the sealant that seals the gap between the substrate and the holder, it is possible to improve thermal conductivity between the holder and the exposed part defined in the second surface of the substrate. 
     Thus, the substrate can be positioned with respect to the holder while the position of the light-emitting element array is adjusted with respect to the condenser lens array. Further, thanks to the sealant that seals the gap between the substrate and the holder, it is possible to improve thermal conductivity between the holder and the exposed part defined in the second surface of the substrate. Thereby, this aspect of the invention can secure a substrate position adjustment function and improve heat dissipation performance at the same time. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view illustrating the configuration of a color printer. 
         FIG. 2  is a schematic view illustrating the configuration of an image drum unit. 
         FIG. 3  is a schematic view illustrating the configuration of an LED head. 
         FIG. 4  is a schematic perspective view illustrating the configuration of the LED head. 
         FIG. 5  is another schematic perspective view illustrating the configuration of the LED head. 
         FIGS. 6A and 6B  are schematic sectional views illustrating the configuration of the LED head. 
         FIGS. 7A and 7B  are other schematic sectional views illustrating the configuration of the LED head. 
         FIG. 8  is a schematic view illustrating the configuration of a manufacturing jig. 
         FIGS. 9A and 9B  are schematic views illustrating the configuration of a protection sheet according to other embodiments. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Descriptions are provided hereinbelow for embodiments based on the drawings. In the respective drawings referenced herein, the same constituents are designated by the same reference numerals and duplicate explanation concerning the same constituents is omitted. All of the drawings are provided to illustrate the respective examples only. 
     [1. Configuration of Color Printer] 
     As illustrated in a left side view of  FIG. 1 , color printer  1  is a color electrophotographic printer, and is configured to print a desired color image on paper sheet P of A3 size or A4 size, for example. 
     Color printer  1  which is an image formation apparatus has various components arranged inside a substantially box-shaped printer chassis  2 . Incidentally, in the following description, a right-edge part in  FIG. 1  is the front face of color printer  1 , and an upward/downward direction, a leftward/rightward direction, and a forward/rearward direction are used and are defined as color printer  1  is viewed while facing the front face of the printer. 
     Controller  3  has overall control of color printer  1 . Controller  3  is connected to a host apparatus (not illustrated) such as a personal computer by wire or wirelessly via a communication processor (not illustrated). Upon receiving a command to print a color image to be printed together with an image data of the color image from the host apparatus, controller  3  executes printing processing to form a print image on a surface of paper sheet P. 
     Sheet housing cassette  4 , sheet feeder  5 , and sheet-color measurement unit  6  are arranged inside printer chassis  2  in its lowermost part. Sheet housing cassette  4  is configured to house paper sheets P. Sheet feeder  5  is configured to pick up and feed one at a time paper sheets P stacked in sheet housing cassette  4 . Sheet-color measurement unit  6  is configured to measure the color of paper sheet P thus fed. Sheet feeder  5  is located at a front upper side of sheet housing cassette  4 , and includes rollers such as hopping roller  7  and paired register rollers  8 , a guide for guiding paper sheet P, and the like. Hopping roller  7  is disposed at the front upper side of sheet housing cassette  4  and has a central axis extending in the leftward/rightward direction, and paired register rollers  8  are disposed above hopping roller  7 . 
     Under the control of controller  3 , sheet feeder  5  rotates hopping roller  7 , paired register rollers  8 , and the like to pick up and take in one at a time paper sheets P housed in sheet housing cassette  4 , advance the sheet to a front upper part of the printer, and then flip it over toward the rear at a position substantially in the center in the upward/downward direction of printer chassis  2  and near the front edge thereof. Sheet-color measurement unit  6  measures the color of paper sheet P and sends the measurement result to controller  3 . 
     Transfer belt unit  10  is disposed inside printer chassis  2  above sheet housing cassette  4  in such a way as to traverse the inside of the entire printer chassis  2  in the forward/rearward direction. Transfer belt unit  10  includes: front and rear rollers  11  each having the shape of a slender cylinder and having a central axis extending in the leftward/rightward direction; and transfer belt  12  stretched around front and rear rollers  11 . Transfer belt  12  is in the form of an endless belt which is wide in the leftward/rightward direction, and travels along with the rotation of rollers  11 . Under control of controller  3 , transfer belt unit  10  rotates rollers  11  and thereby drives transfer belt  12  to transport paper sheet P transferred from sheet feeder  5  rearward on the upper surface of transfer belt  12 . 
     Meanwhile, above transfer belt unit  10 , i.e., in a part of printer chassis  2  higher than the center thereof, four image drum units  15 C,  15 M,  15 Y, and  15 K (hereinafter, these are referred to as image drum units  15  collectively) are arranged in that order from the rear side toward the front side. In other words, image drum units  15  of the respective colors are disposed in a so-called tandem arrangement. 
     Incidentally, image drum units  15 C,  15 M,  15 Y, and  15 K respectively correspond to the colors of cyan (C), magenta (M), yellow (Y), and black (K). In addition, image drum units  15 C,  15 M,  15 Y, and  15 K have the same configuration except for the colors of toner corresponding thereto. Image drum units  15  which are image formation units each have substantially the shape of a box relatively long in the leftward/rightward direction so as to suit to the horizontal width of paper sheet P. 
     Further, in printer chassis  2 , Light Emitting Diode (LED) heads  16 C,  16 M,  16 Y, and  16 K (hereinafter, these are referred to as LED heads  16  collectively) which are exposure devices are arranged to correspond to image drum units  15 C,  15 M,  15 Y, and  15 K respectively. LED heads  16  each have the shape of a rectangular solid long in the leftward/rightward direction. Each LED head  16  has LED elements inside which are arranged in the leftward/rightward direction, and is configured to cause the LED elements to emit light according to a light emission pattern based on image data fed by controller  3 . Image drum units  15  are located very close to LED heads  16  when mounted in printer chassis  2 , and are subjected to exposure processing by the light from LED heads  16 . 
     Further, image drum units  15 C,  15 M,  15 Y, and  15 K are respectively connected to toner cartridges  18 C,  18 M,  18 Y, and  18 K (hereinafter, these are referred to as toner cartridges  18  collectively) located above them. Toner cartridges  18  are each a hollow container long in the leftward/rightward direction which houses a powdery toner of the corresponding color and is incorporated with a predetermined stirring mechanism. 
     Incidentally, transfer rollers  13 C,  13 M,  13 Y, and  13 K (hereinafter, these are referred to as transfer rollers  13  collectively) are arranged in transfer belt unit  10  at four locations between front and rear rollers  11  and immediately below respective image drum units  15 . To put it another way, an upper part of transfer belt  12  is nipped between image drum units  15  and the corresponding transfer rollers  13 . Incidentally, each transfer roller  13  is designed to be capable of being charged. 
     Controller  3  causes toner to be fed from toner cartridges  18  to image drum units  15 . Controller  3  also causes each LED head  16  to emit light such that the light forms a light emission pattern according to image data fed by a host apparatus (not illustrated). In response to this, image drum units  15  form toner images according to the light emission patterns of their corresponding LED heads by using the toner fed by toner cartridges  18 , respectively, and transfer the toner images onto paper sheet P one by one (to be described in detail later). Thereby, the toner images of four colors according to the image data are sequentially transferred onto paper sheet P being transported by transfer belt unit  10 . 
     Fixation unit  20  is disposed behind transfer belt unit  10 , i.e., near the center in the upward/downward direction of printer chassis  2  and near the rear edge thereof. Fixation unit  20  includes heating roller  21  and pressure roller  22 . Heating roller  21  has the shape of a cylinder having a central axis extending in the leftward/rightward direction, and has a heater inside. Pressure roller  22  has the shape of a cylinder similarly to heating roller  21 , and presses its upper surface against the lower surface of heating roller  21  with a predetermined pressing force. 
     Fixation unit  20  is configured to heat heating roller  21  and rotate heating roller  21  and pressure roller  22  in their predetermined directions under the control of controller  3 . Thereby, fixation unit  20  applies heat and pressure onto paper sheet P, which is transferred from transfer belt unit  10 , i.e., on which the toner images of four colors are laid, so as to fix the toner to the sheet, and then transfers the sheet to a rear upper part of the printer. 
     Delivery unit  24  is disposed behind and above fixation unit  20 . Delivery unit  24  is formed by a combination of rollers (not illustrated) each having a central axis extending in the leftward/rightward direction, a guide for guiding paper sheet P, and the like. Under the control of controller  3 , delivery unit  24  rotates its rollers as needed to transport paper sheet P transferred from fixation unit  20  to the rear upper part of the printer, and then flip it over toward the front and discharge it to discharge tray  2 T formed on the upper surface of printer chassis  2 . 
     In this way, when executing a printing process, color printer  1  causes image drum units  15  of the respective colors to form toner images with light emitted by LED heads  16 , respectively, and sequentially transfers the toner images onto paper sheet P. 
     [2. Configuration of the Image Drum Unit] 
     Next, the configuration of each image drum unit  15  is described. As illustrated in the schematic sectional view of  FIG. 2 , most of the outer periphery of image drum unit  15  is covered with frame  31 , and image drum unit  15  has a relatively large space inside. 
     Photoconductive drum  35  which is an electrostatic latent image carrier is disposed inside image drum unit  15  at a lower center thereof. Photoconductive drum  35  has the shape of a cylinder with a central axis extending in the leftward/rightward direction, and is supported by frame  31  to be rotatable about its central axis. Incidentally, photoconductive drum  35  is rotated in a direction denoted by arrow R 1  (R 1  direction) by a driving force transmitted from a motor not illustrated. 
     A relatively large region of a part, corresponding to the lower surface of photoconductive drum  35 , of frame  31  is left open. Thus, image drum unit  15  when mounted in printer chassis  2  ( FIG. 1 ) can bring the lower surface of photoconductive drum  35  into contact with transfer belt  12  or paper sheet P placed on transfer belt  12 . In addition, an exposure hole long in the leftward/rightward direction is bored in a part of frame  31  immediately above photoconductive drum  35 . 
     Charging roller  36  is disposed behind and above photoconductive drum  35 . Charging roller  36  has the shape of a cylinder having a diameter smaller than photoconductive drum  35 . Charging roller  36  is made of a semiconductive elastic material, for example, and has a circumferential side surface in contact with circumferential side surface  35 S of photoconductive drum  35 . Charging roller  36  is thereby configured to evenly charge a location of contact of circumferential side surface  35 S with this roller. 
     Development roller  38  is disposed ahead of and above photoconductive drum  35 . Development roller  38  has the shape of a cylinder having a diameter smaller than photoconductive drum  35 . Development roller  38  is made of semiconductive urethane rubber made by controlling, as needed, the electrical resistance of a urethane rubber material by adding thereto a conductive material such as carbon, for example, and is designed to be capable of being charged. The circumferential side surface of development roller  38  is in contact with circumferential side surface  35 S of photoconductive drum  35  on the rear side of the development roller, and is in contact with feed roller  39  on the front side thereof. Feed roller  39  has the shape of a cylinder having a diameter slightly smaller than development roller  38 , and is made of a semiconductive silicone foam sponge, for example. 
     Sheet-shaped development blade  40  is disposed ahead of and above development roller  38 . Development blade  40  is made of metal such as stainless steel or phosphor bronze, or a rubber material such as silicone rubber. The rear upper edge of development blade  40  is fixed to the inside of frame  31 , and a slight clearance is defined between the front lower edge of the blade and the circumferential side surface of development roller  38 . 
     Further, spacers  45  are arranged above and at both left and right sides of photoconductive drum  35 . The sizes of spacers  45 , their attachment positions with respect to frame  31 , and the like are so optimized that the distance between the circumferential side surface of photoconductive drum  35  and LED head  16  is set at a desired length by bringing the lower surface of LED head  16  into contact with the upper surfaces of the spacers (to be described in detail later). 
     For printing of an image on paper sheet P under the control of controller  3 , image drum unit  15  having the above configuration rotates photoconductive drum  35  in the R 1  direction, rotates charging roller  36 , development roller  38 , and feed roller  39  in a direction denoted by arrow R 2  (R 2  direction), and charges charging roller  36  and development roller  38 . Incidentally, image drum unit  15  causes photoconductive drum  35 , charging roller  36 , and development roller  38  to rotate without sliding over each other. 
     First, a rear upper part of circumferential side surface  35 S of photoconductive drum  35  is evenly charged by charging roller  36 , and then with the rotation of the drum in the R 1  direction, this charged part reaches the vicinity of the upper edge of the photoconductive drum and faces LED head  16 . With this event, circumferential side surface  35 S of photoconductive drum  35  is exposed to light emitted by LED head  16  in the light emission pattern based on image data, and thereby an electrostatic latent image according to the image data is formed on the drum. 
     On the other hand, by way of feed roller  39 , toner fed from toner cartridge  18  is attached to the circumferential side surface of development roller  38  which is rotated in the R 2  direction. Next, excess toner is shaved off the development roller by development blade  40 , so that toner is attached to the circumferential surface of the roller uniformly in the form of a thin film. 
     Photoconductive drum  35  is further rotated in the R 1  direction and, near the front edge of the drum where it comes into contact with development roller  38 , the toner formed in the form of a thin film on the circumferential side surface of development roller  38  is attached only to the part of circumferential side surface  35 S corresponding to the electrostatic latent image formed on the drum. In this manner, a toner image according to the image data is formed on circumferential side surface  35 S of photoconductive drum  35 . Incidentally, the toner image formed on circumferential side surface  35 S at this time is an image representing only one color component (that is, any of cyan, magenta, yellow, and black) that this image drum unit  15  handles. 
     Then, photoconductive drum  35  is further rotated in the R 1  direction, and thus the toner image reaches the vicinity of the lower edge of the drum. In this event, controller  3  causes transfer belt unit  10  ( FIG. 1 ) to transport paper sheet P to under image drum unit  15 , and charges transfer roller  13  with the polarity opposite to that of the toner. Accordingly, image drum unit  15  nips paper sheet P between charged transfer roller  13  and a part of photoconductive drum  35  where the toner image is formed, and transfers this toner image onto paper sheet P. Incidentally, if toner is left on circumferential side surface  35 S of photoconductive drum  35  after the toner image is transferred on paper sheet P, such toner is removed by a cleaning device not illustrated. 
     In this way, image drum unit  15  can form a toner image on circumferential side surface  35 S of photoconductive drum.  35  using the exposure function of LED head  16  while causing LED head  16  to face the vicinity of the drum. 
     3. Configuration of the LED Head 
     Next, the configuration of LED head  16  is described. As illustrated in  FIGS. 3, 4, 5, 6, and 7 , LED head  16 , which is an exposure device, as a whole has the shape of a rectangular solid long in the leftward/rightward direction, and has a configuration where various components are attached to holder  51  which is a holder or a support member. Incidentally, FIG.  3  is a sectional view illustrating LED head  16  as well as photoconductive drum  35  as viewed from the front side.  FIG. 4  is a perspective view illustrating LED head  16  as viewed from the obliquely-left front upper side.  FIG. 5  is an enlarged perspective view illustrating a part of  FIG. 4  on an enlarged scale.  FIG. 6A  is a sectional view taken along line A 1 -A 2  of  FIG. 3  and  FIG. 6B  is a partial enlarged view of  FIG. 6A .  FIG. 7A  is a sectional view taken along line B 1 -B 2  of  FIG. 3  and  FIG. 7B  is a partial enlarged view of  FIG. 7A . 
     Holder  51  has, as its central part, basal part  51 A in the form of a plate long in the leftward/rightward direction and thin in the upward/downward direction. Side parts  51 B in the form of a plate long in the leftward/rightward direction and thin in the forward/rearward direction extend upward from both front and rear sides of basal part  51 A. Hole part  51 AH in the form of a slit long in the leftward/rightward direction is bored at substantially the center of basal part  51 A in the forward/rearward direction such that it penetrates basal part  51 A in the upward/downward direction. Side parts  51 B each have the shape of a substantially flat plate without any steps or the like. In addition, light shield plates  52  stand upward on the upper surface of basal part  51 A and at both left and right sides of hole part  51 AH Incidentally, holder  51  is manufactured by subjecting a steel sheet in the form of a flat plate to a cutting process, bending process, and the like, and its manufacturing cost is relatively low. 
     Rod lens array  53  which is a condenser lens array is mounted in hole part  51 AH. Rod lens array  53  as a whole has them shape of a rectangular solid long in the leftward/rightward direction, and has multiple minute lenses arrayed in the leftward/rightward direction. These lenses have such an optical characteristic as to converge light emitted from LED array chip  56  to be described later. 
     In addition, LED substrate unit  54  is mounted in holder  51  above rod lens array  53  such that it is sandwiched between front and rear side parts  51 B. LED substrate unit  54  includes substrate  55 , LED array chip  56 , protective sheet  57 , and attachments  58 . 
     Substrate  55  is made of a so-called glass epoxy substrate and has the shape of a plate long in the leftward/rightward direction and thin in the upward/downward direction. Substrate  55  has such a configuration that wiring layers each having a predetermined wiring pattern formed thereon are stacked in the upward/downward direction. The length in the forward/rearward direction of substrate  55  is shorter than the distance between side parts  51 B of holder  51 , and the length in the leftward/rightward direction thereof is shorter than the distance between light shield plates  52  of holder  51 . 
     Incidentally, a part of the wiring pattern is exposed on each of the upper and lower surfaces of substrate  55 , and through holes for connecting the wiring patterns of the layers to each other in a vertical direction are provided at appropriate locations in substrate  55 . Moreover, connector  55 C for connecting a wiring member is attached to a predetermined position on the upper surface of substrate  55 . 
     LED array chip  56  which is a light-emitting element array is attached to the lower surface of substrate  55 . In LED array chip  56 , light-emitting points at which the elements emit light downward are arrayed in the leftward/rightward direction at predetermined minute intervals. LED array chip  56  is physically fixed, through die bonding for example, and electrically connected to the wiring pattern formed on the lower surface of substrate  55 . 
     Protective sheet  57  is made of an insulating material. Protective sheet  57  has the shape of a film which is thin in the upward/downward direction and long in the leftward/rightward direction, and which has a length in the forward/rearward direction (i.e., width) narrower than substrate  55 . In addition, holes, notches, and the like are formed in protective sheet  57  as needed so that connector  55 C and the like may be exposed on the sheet without being covered therewith. Protective sheet  57  is laid over the upper surface of substrate  55 , namely, over the surface opposite to the lower surface on which LED array chip  56  is mounted, in such a way that both front and rear edges of protective sheet  57  are respectively spaced from both front and rear edges of substrate  55  in the forward/rearward direction. 
     Attachments  58  which are attachment members are each formed by subjecting a thin steel sheet as thin as about 0.1 to 0.3 [mm] to a cutting process and a bending process, for example. Attachment  58  is conductive due to the nature of a steel sheet, and can exert elastic force upon the application of an external force such as a bending force. With such an elastic force, attachment  58  can attach protective sheet  57  to substrate  55  while pinching the sheet between the attachment and the substrate, as illustrated in  FIGS. 6 and 7 . Incidentally, because attachment  58  is made of a steel sheet, it can stably keep its shape and elastic force even under a high temperature environment, unlike a resin material or the like. 
     Further, coil springs  59  are attached to the upper surface of basal part  51 A to stand upward near both left and right edges of holder  51 . When compressed in the upward/downward direction, coil springs  59  exert an elastic force to try to extend in the upward/downward direction. When coil springs  59  at their upper ends are fixed to the printer chassis ( FIG. 1 ) side while being compressed in the upward/downward direction, they press the lower surface of basal part  51 A of holder  51  against spacers  45  ( FIG. 3 ) on the image drum unit  15  side. Thereby, coil springs  59  can adjust the distance between the circumferential side surface of photoconductive drum  35  and the lower edge of rod lens array  53  to a desired distance and keep this desired distance. 
     In this way, LED substrate unit  54  of LED head  16  is fixed to holder  51  in the state where protective sheet  57  is laid over the upper surface of substrate  55  at an appropriate position and attachments  58  are attached thereto. 
     [4. Process of Manufacturing the LED Head] 
     Next, a process of manufacturing LED head  16  is described mainly in terms of a process of manufacturing LED substrate unit  54  and a process of mounting LED substrate unit  54  in holder  51 . 
     In a process of assembling LED substrate unit  54 , firstly, LED array chip  56  is attached to the lower surface of substrate  55 , and then protective sheet  57  is laid over the upper surface of substrate  55 . In this event, the position of protective sheet  57  with respect to substrate  55  is appropriately set by using a jig (not illustrated). Thereby, as illustrated in  FIGS. 4, 5, 6 , and the like, both front and rear edges of protective sheet  57  are positioned inward of both front and rear edges of substrate  55  so that exposed parts  55 E, being front and rear circumferential portions of substrate  55  on its upper surface, may be exposed. 
     Subsequently, in LED substrate unit  54  in a part above substrate  55  where protective sheet  57  is laid over the substrate, attachment  58  is elastically deformed by application of external force to widen the distance between the two edges of attachment  58  in the forward/rearward direction. While its deformed shape is kept, attachment  58  is moved downward and then released from the external force. Thereby, while securing the insulation properties of the upper surface of substrate  55  and protecting the upper surface thereof against physical damage by use of protective sheet  57 , LED substrate unit  54  can fix the position of protective sheet  57  by use of attachment  58 . 
     Incidentally, as illustrated in  FIGS. 3 and 4 , multiple attachments  58  are mounted in LED substrate unit  54  such that they are arranged at almost predetermined intervals in the leftward/rightward direction, i.e., in the direction in which the light-emitting points of LED array chip  56  are arrayed. 
     Next, in LED head  16 , LED substrate unit  54  is mounted in holder  51  by using manufacturing jig  70  illustrated in  FIG. 8 . Manufacturing jig  70  includes: spacers  71  arranged one by one at the lower side of the head and near both left and right edges thereof; chucks  72  arranged at the upper side of the head; photosensors  73  arranged at the lower side of the head; controller  74 ; display  75 ; and jig connector  76 . 
     Spacers  71  correspond to spacers  45  ( FIG. 3 ) of image drum unit  15 . The position of LED head  16  in manufacturing jig  70  is set by bringing the lower surface of basal part  51 A of holder  51  into contact with the upper surfaces of spacers  71 . Chucks  72  have a suction function, and are capable of sucking up the upper surface of LED substrate unit  54  and holding the substrate part. In addition, chucks  72  are configured to be capable of moving in the upward/downward direction or in the leftward/rightward direction, adjusting their angles, and the like while holding LED substrate unit  54 . 
     Photosensors  73  are equivalent to photosensitive drum  35 . Photosensors  73  each have photosensing elements for receiving light on its upper surface, and the distance between the upper surfaces of spacers  71  and the upper surface of photosensor  73  is adjusted appropriately. Photosensor  73  is configured to receive light incident thereon from the above, generate a light reception signal according to the amount of received light, and send it to controller  74 . Controller  74  is configured to form a predetermined display screen by using figures, graphs, and the like based on the received light reception signal, and display it on display  75  to notify an operator and the like of the amount of receiving light. 
     Jig connector  76  is attached to the leading end of a wiring member electrically connected to controller  74 , and includes terminals (not illustrated), and is formed in such a shape as to be able to be engaged with connector  55 C of substrate  55 . Jig connector  76  can supply LED array chip  56  of LED substrate unit  54  with electrical signals for causing the LED elements to emit light by connecting jig connector  76  to connector  55 C of substrate  55  and bringing their terminals into continuity with each other. 
     For manufacturing LED head  16 , firstly, rod lens array  53  and two coil springs  59  are attached to holder  51 . Incidentally, a sufficient amount of adhesive  60  is used to bond rod lens array  53  to holder  51  so as to close the gap between holder  51  and rod lens array  53 , or predetermined filler is filled between the gap therebetween. 
     Next, holder  51  is mounted to manufacturing jig  70  in the state where coil springs  59  are compressed in the upward/downward direction. Here, coil springs  59  exert an elastic force while their upper ends are in contact with predetermined positions of manufacturing jig  70 . Coil springs  59  thereby apply a downward force on holder  51  to press the lower surface of basal part  51 A against the upper surfaces of spacers  71  and bring them into contact with each other without any gap between them. Since the distance between the upper surfaces of spacers  71  and the upper surfaces of photosensors  73  is set appropriately, manufacturing jig  70  can fix holder  51  at an appropriate position temporarily. 
     Next, manufacturing jig  70  adsorbs LED substrate unit  54  in the state where chucks  72  are moved above holder  51 . In addition, jig connector  76  of manufacturing jig  70  is connected to connector  55 C of substrate  55  in LED substrate unit  54 . 
     In the above state, by manipulation by an operator, manufacturing jig  70  moves chucks  72  downward to roughly align the position of LED substrate unit  54  with an ideal mounting position in holder  51  first. Then, manufacturing jig  70  supplies a predetermined signal to LED array chip  56  of LED substrate unit  54  through jig connector  76  so as to cause the elements of LED array chip  56  to emit light. The light outputted from LED array chip  56  as diverging rays at this time is converged by rod lens array  53  and irradiated on the upper surfaces of photosensors  73 . 
     Manufacturing jig  70  lets an operator and the like visually check through display  75  the amount of light received by photosensors  73 . At this time, with reference to the amount of receiving light, the operator finely adjusts the positions and angles of chucks  72  holding LED substrate unit  54  so that the position of LED substrate unit  54  may be adjusted to an optimal position, that is, a position where the sensors can receive a desired amount of light. Thereby, LED substrate unit  54  can cause the optical axis of LED array chip  56  to be directed in parallel with the central axis of rod lens array  53 , and can set the distance between the lower edge of LED array chip  56  and the upper edge of rod lens array  53  at an ideal distance throughout a main-scanning direction (i.e., the leftward/rightward direction). 
     In the above state, LED substrate unit  54  is fixed to the ideal position with respect to holder  51  by filling adhesive  60  ( FIG. 6 ) in front and rear side portions of each attachment  58  and between front and rear side parts  51 B of holder  51 . Adhesive  60  is, for example, an ultraviolet cure adhesive designed to be cured in a relatively short period of time when applied to a location to be bonded in the form of liquid or gel and then irradiated with ultraviolet light. 
     Next, in LED head  16 , sealant  61  made of silicone is filled in gaps between LED substrate unit  54  and front and rear side parts  51 B of holder  51  so as to fill in these gaps ( FIGS. 7A and 7B ). Thereby, LED head  16  can substantially seal space  51 S which is surrounded by basal part  51 A and front rear side parts  51 B of holder  51  and the lower surface of substrate  55 . 
     At this time, sealant  61  is filled to come into contact with both side parts  51 B of holder  51  and parts of the upper surface of substrate  55  that are exposed without being covered with protective sheet  57 , namely, parts around the front and rear edges of substrate  55  (these are referred to as exposed parts  55 E in this disclosure). 
     Due to the nature of silicone, sealant  61  is made relatively soft and has a thermal conductivity higher than the material for protective sheet  57 . Thus, once filled, sealant  61  is deformed flexibly to substantially seal the gaps between substrate  55  and holder  51  and come into contact with a relatively large area of each of exposed parts  55 E and side parts  51 B. 
     Thereby, as illustrated by arrow Q 1  in  FIG. 7B , sealant  61  can effectively transmit the heat of substrate  55  to holder  51 . Moreover, because sealant  61  is flexible but as rigid and strong as gel, it can keep well its shape at the time of being filled without flowing off like a liquid. 
     Incidentally, LED head  16  is designed to prevent an incident of external light coming in from the leftward/rightward direction, the entrance of foreign matter, and the like by using left and right light shield plates  52  of holder  51  ( FIG. 3 ). 
     In this way, in LED head  16 , exposed parts  55 E not covered with protective sheet  57  are defined in the upper surface of substrate  55 . Then, in the process of mounting LED substrate unit  54 , sealant  61  seals the gaps between substrate  55  and side parts  51 B of holder  51 , and is widely brought into contact with exposed parts  55 E and side parts  51 B. 
     Incidentally, in LED head  16 , if LED substrate unit  54  should be displaced from its ideal position with respect to holder  51  after manufacturing, sealant  61  is removed and attachments  58  of a relatively low cost are cut off and broken with a nipper or the like. Thereby, according to LED head  16 , it is possible to take out substrate  55  and LED array chip  56  of relatively high cost from holder  51  without damaging these, and reuse and mount these to new holder  51  or reused holder  51 . 
     [5. Operation and Effect] 
     In the manufacturing of LED head  16  of color printer  1  according to this embodiment with the above configuration, LED substrate unit  54  is formed by laying protective sheet  57  over the upper surface of substrate  55 , on which LED array chip  56  is mounted, so that exposed parts  55 E ( FIGS. 5, 6, and 7 ) may be defined therein, and attaching attachments  58 . 
     Next, in LED head  16 , the position of LED substrate unit  54  is adjusted with respect to holder  51 , in which rod lens array  53  is mounted, by use of manufacturing jig  70 . In this adjusted state, attachments  58  are fixedly attached with adhesive  60 , and the gaps between substrate  55  and holder  51  are sealed with sealant  61 . 
     Thus, according to LED head  16 , it is possible to position LED substrate unit  54  so that photosensors  73  can receive an optimal amount of light emitted from LED array chip  56  and converged by rod lens array  53 . Accordingly, when installed in printer chassis  2 , LED head  16  can irradiate circumferential side surface  35 S of photoconductive drum  35  ( FIG. 2 ) with an optimal amount of light outputted from LED array chip  56  and appropriately converged through rod lens array  53 . 
     In addition, at this time, LED substrate unit  54  can prevent electrical contact (i.e., electrical continuity) between attachments  58  and the upper surface of substrate  55  with protective sheet  57 , and prevent minute foreign matter such as toner from entering space  51 S through the through-holes formed in substrate  55 . 
     Further, because sealant  61  to seal the gaps between substrate  55  and holder  51  is in contact with a large area of each of exposed parts  55 E on the upper surface of substrate  55  and side parts  51 B of holder  51 , LED head  16  can effectively transmit heat between substrate  55  and holder  51 . 
     Thereby, LED head  16  can transmit heat generated by the emission of light by LED array chip  56  to substrate  55 , then smoothly transmit the heat from exposed parts  55 E of substrate  55  to side parts  51 B of holder  51  via sealant  61 , and then diffuse the heat from the outer circumferential surface of holder  51  toward a surrounding space. In other words, LED head  16  can transmit heat generated by the emission of light by LED array chip  56  to holder  51  effectively without confining the heat inside substrate  55  or space  51 S ( FIG. 7 ), and thus can prevent deterioration in the performance (such as light-emission efficiency) of LED array chip  56 , reduction in the life of the chip, damage on the chip, and the like due to the heat. 
     Further, LED head  16  can increase transmission efficiency of heat from substrate  55  to holder  51  while using sealant  61  made of silicone as in the conventional case, only by putting a twist on the shape of protective sheet  57  so that exposed parts  55 E may be defined in the upper surface of substrate  55 . In other words, it is possible to manufacture LED head  16  at the same cost as a conventional LED head because there is no need to add a member for transmitting heat from substrate  55  to holder  51  to an LED head with a conventional configuration, nor use a sealant and the like of a nature different from that of a conventional LED head. 
     To put it another way, sealant  61  made of silicone having moderate flexibility and insulation properties is conventionally used for the purpose of filling the gaps between substrate  55  and holder  51  originally, that is, while no attention is paid to its thermal conductivity. Against such a background, while paying attention to the fact that silicone has good thermal conductivity, the embodiment forms exposed parts  55 E in substrate  55  and uses sealant  61  also as a thermal conductor in LED head  16 . The embodiment can thereby improve the heat dissipation property drastically while keeping the cost the same as before. 
     Further, even if through-holes should be formed in exposed parts  55 E of substrate  55 , LED head  16  can prevent foreign matter from dropping in space  51 S through the through-holes thanks to sealant  61  covering the upper surfaces of these parts. Furthermore, owing to its moderate rigidity, sealant  61  can keep covering the gaps between substrate  55  and holder  51  and exposed parts  55 E without flowing off to space  51 S through the through-holes. 
     According to the configuration described so far, in LED head  16  of color printer  1 , LED substrate unit  54  is formed by laying protective sheet  57  over the upper surface of substrate  55 , on which LED array chip  56  is mounted, so that exposed parts  55 E may be defined therein, and attaching attachments  58 . Then, in the state where the position of LED array chip  56  is adjusted with respect to rod lens array  53  of holder  51 , LED substrate unit  54  is bonded to side parts  51 B of holder  51  with adhesive  60 . Then, the gaps between substrate  55  and holder  51  are sealed with sealant  61 . Thereby, LED head  16  can improve thermal conductivity between exposed parts  55 E and side parts  51 B, and transmit and dissipate heat, generated in LED array chip  56  and transmitted to substrate  55 , via exposed parts  55 E and sealant  61 . 
     [6. Other Embodiments] 
     Note that what is described in the above embodiment is a case where protective sheet  57  is rectangular in shape. However, the invention is not limited thereto. For example, the width in the forward/rearward direction of protective sheet  57  may be enlarged only at locations where attachments  58  are mounted and reduced at parts other than these locations, as illustrated in  FIG. 9A . Thereby, it is possible to prevent unnecessary continuity or short circuiting of substrate  55  due to attachments  58 , and increase the area of exposed parts  55 E to further improve thermal conductivity between substrate  55  and sealant  61 . 
     Alternatively, for example, protective sheet  57  may have such a shape as to match locations where through-holes  55 T are formed on the upper surface of substrate  55 , that is, such a shape as to positively cover through-holes  55 T and positively expose other parts, as illustrated in  FIG. 9B . In this case, even if there should be an error in the filling of sealant  61 , it is possible to avoid such a risk that foreign matter drops in space  51 S through exposed through-holes  55 T. 
     Further, what is described in the above embodiment is a case where attachments  58  are made of a thin steel sheet which is conductive. However, the invention is not limited thereto. For example, attachments  58  may be made of a non-conductive material such as a resin material. In this case, protective sheet  57  can prevent substrate  55  from being damaged by the contact of attachments  58 . 
     Further, what is described in the above embodiment is a case where sealant  61  is conductive. However, the invention is not limited thereto. For example, if an insulating material is applied on the upper surface of substrate  55 , sealant  61  may be made of a non-insulating material. In this case, protective sheet  57  may also be made of a non-insulating material as long as it is capable of physically protecting the upper surface of substrate  55 . In sum, any sealant  61  can do as long as it never interferes with the electrical operation of substrate  55  and it can conduct heat from exposure parts  55 E of substrate  55  to side parts  51 B of holder  51  highly efficiently. 
     Further, what is described in the above embodiment is a case where attachments  58  are bonded to side parts  51 B of holder  51  when LED substrate unit  54  is mounted in holder  51 . However, the invention is not limited thereto. For example, substrate  55  may be directly bonded to side parts  51 B of holder  51 . Alternatively, LED substrate unit  54  may be mounted in holder  51  by engaging side parts  51 B with an engaging body of a predetermined shape formed in each attachment  58 . 
     Further, what is described in the above embodiment is a case where the invention is applied to LED heads  16  of the respective color for image drum units  15  of the respective color which are arranged in series in the forward/rearward direction in tandem type color printer  1 . However, the invention is not limited thereto. For example, the invention may be applied to LED heads mounted in other type color printers such as a four-cycle type color printer. 
     Further, what is described in the above embodiment is a case where four LED heads  16  corresponding to the colors of yellow, magenta, cyan, and black are mounted in printer chassis  2  of color printer  1  for color printing. However, the invention is not limited thereto. For example, three or less LED heads  16  or five or more LED heads  16  may be mounted in printer chassis  2  according to the number of colors of toner to be used in a color printer. Alternatively, one LED head  16  may be mounted in a monochrome printer for monochrome printing. 
     Further, in the above embodiment, the invention is applied to color printer  1  as an image formation apparatus. However, while not limited thereto, the invention is applicable to apparatuses such as a facsimile machine, a multi function product (MFP), and a copier as long as they have LED head  16  as an exposure device as in the case of color printer  1 . 
     Further, the invention is not limited to the embodiment and other embodiments described above. In other words, the invention is applicable to an embodiment in which a part of, or all of, the embodiment and other embodiments described above are used in combination, or an embodiment in which a part of these embodiments is extracted. 
     Further, what is described in the above embodiment is a case where color printer  1  as an image formation apparatus includes LED head  16  as an exposure device including substrate  55  as a substrate, protective sheet  57  as a protective sheet, attachment  58  as an attachment member, holder  51  as a holder, and sealant  61  as a sealant. However, the invention is not limited thereto. An image formation apparatus may include an exposure device including a substrate, a protective sheet, an attachment member, a holder, and a sealant of other various configurations. 
     The invention is usable in a wide range of apparatuses including color printers and copiers which use light from one or more LEDs for exposure.