Patent Publication Number: US-2023162636-A1

Title: Display device

Description:
TECHNICAL FIELD 
     The present invention relates to display devices, and more specifically, to a display device having a foldable display surface. 
     BACKGROUND ART 
     Previously known folding portable terminals incorporate a display device having a body that is to be developed so as to increase its display region. However, two displays are incorporated in these conventional examples. In contrast to this, a recently developed type is foldable displays, which are incorporated in smartphones. The characteristic of organic EL displays, i.e., being foldable, is applied to smartphones, which are known as foldable smartphones or other names. 
     For instance, Patent Literature discloses a foldable display device having a stack of, in sequence, a display panel, a first adhesive, a polarizer, a second adhesive and a window, and being bendable along a bending axis. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2017-126061 
     SUMMARY OF INVENTION 
     Technical Problem 
     In the technique in Patent Literature 1, folding the device causes positional shifts between the stacked layers, thus possibly rendering the display surface flexural. 
     One aspect of the present invention aims to provide a display device that can avoid flection in its display surface even when the display surface undergoes folding. 
     Solution to Problem 
     To solve the above problem, a display device according to one aspect of the present invention includes the following: a display module incorporating at least a display panel, including a stack of a plurality of function layers structured mutually slidably, and structured in a foldable manner; and a case having a first bezel and a second bezel respectively attached to one end and another end of the lowermost layer of the display module with a slide mechanism interposed, the case having a hinge provided between the first bezel and the second bezel, the case housing the display module in a foldable manner, wherein the case houses the display module in a foldable manner in response to selection between a closed state where the first bezel and the second bezel are adjacent to each other with the hinge interposed, and an open state where the first bezel and the second bezel are spaced from each other with the hinge interposed, the case is provided with a coupling portion that couples the hinge and the first bezel together in such a manner that the hinge is located above the uppermost layer of the display module when the case is brought into the open state, and the case is provided with a coupling portion that couples the hinge and the second bezel together in such a manner that the hinge is located above the uppermost layer of the display module when the case is brought into the open state, the case has a cover portion provided in each of the first bezel and the second bezel, and covering one end and another end of the uppermost layer of the display module, the cover portion is provided with an engaging portion that engages with an engaged portion provided at the one end and the another end of the uppermost layer of the display module, and in conjunction with an operation from the closed state of the case into the open state of the case, the engaging portion engages with the engaged portion and pulls the uppermost layer of the display module in such a manner that the uppermost layer is in parallel. 
     Advantageous Effect of invention 
     The aspect of the present invention can provide a display device that can avoid flection in its display surface even when the display surface undergoes folding. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a front view of a display device according to one embodiment of the present invention viewed from its display surface. 
         FIG.  2    illustrates the display device in  FIG.  1    viewed from its non-display surface. 
         FIG.  3    is a side view of the display device in  FIG.  1   . 
         FIG.  4    is a side view of the display device in  FIG.  1    and is a side view of the display device viewed from a direction orthogonal to the direction in  FIG.  3   . 
         FIG.  5    is a schematic view a specific configuration of a display module included in the display device in  FIG.  1   . 
         FIG.  6    is a side view of the same display device as that in  FIG.  3   . 
       The upper part of  FIG.  7    is a side view of a display device  1  with its case  10  closed, that is, with its display surface folded, and the lower part of the drawing is a side view of the display device  1  with the case  10  being opened from the closed state. 
         FIG.  8    illustrates a display device according to a comparative configuration, which is not included in one aspect of the present invention. 
         FIG.  9    is a front view of a display device according to another embodiment of the present invention viewed from its display surface. 
         FIG.  10    illustrates the display device in  FIG.  9    viewed from its non-display surface. 
         FIG.  11    is a side view of the display device in  FIG.  9   . 
         FIG.  12    is a side view of the display device in  FIG.  1    and is a side view of the display device viewed from a direction orthogonal to the direction in  FIG.  11   . 
         FIG.  13    illustrates the display device in  FIG.  11    viewed laterally with only a region provided with an engaging portion being enlarged, and the display device in  FIG.  3    viewed laterally with only a region provided with an engaging portion being enlarged. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
     A display device according to one aspect of the present invention will be described with reference to  FIG.  1    through  FIG.  8   .  FIG.  1    is a front view of a display device  1  according to this embodiment viewed from its display surface.  FIG.  2    illustrates the backside of the display device  1  viewed from its non-display surface, which corresponds to the backside of the display surface in  FIG.  1   .  FIG.  3    a side view of the display device  1 .  FIG.  4    is a side view of the display device viewed from a direction orthogonal to the direction in  FIG.  3   . 
     The display device  1  according to this embodiment is a so-called foldable display device having a foldable display module. The display device  1  can be used as, but not limited to, a portable terminal, including a smartphone and a tablet terminal. 
     The display device  1  includes a case  10  and a display module  20 , as illustrated in  FIG.  1    and  FIG.  2   . 
     The case  10  has the following: a first bezel  11  and a second bezel  12  respectively attached to one end and the other end of the lowermost layer of the display module  20  with a slide mechanism  113  (see  FIG.  3   ) interposed therebetween; and a hinge  13  provided between the first bezel  11  and the second bezel  12 , and the case  10  houses the display module  20  in a foldable manner. 
     The first bezel  11  has a bottom  110 , which is fiat, a cover portion H 1 , and coupling portions  112 . The slide mechanism  113  (see  FIG.  3   ) is disposed on the bottom  110 . The cover portion  111  is provided on a side of the bottom  110  opposite to a side adjacent to the second bezel  12 . The cover portion  111  is provided so as to extend from the bottom  110  and to cover one of the ends of the uppermost layer of the display module  20 . Part of the cover portion  111  faces a window film  21 , which constitutes the uppermost layer of the display module  20  outside the display area of the display module  20 . The coupling portions  112  are provided the respective fight and left ends, which are outside a region where the display module  20  is placed, of a side of the bottom  110  adjacent to the second bezel  12 . The coupling portions  112  are provided at a predetermined height from a surface of the bottom  110  where the display module  20  is placed and are coupled to the hinge  13  at distal ends  112   a,  the details of which will be described later on. 
     The second bezel  12  has a bottom  120 , which is flat, a cover portion  121 , and coupling portions  122 . A slide mechanism  123  (see  FIG.  3   ) is disposed on the bottom  120 . The cover portion  121  is provided on a side of the bottom  120  opposite to a side adjacent to the first bezel  11 . The cover portion  121  is provided so as to extend from the bottom  120  and to cover the other end of the uppermost layer of the display module  20 . Part of the cover portion  121  faces the window film  21 , which constitutes the uppermost layer of the display module  20  outside the display area of the display module  20 . The coupling portions  122  are provided at the respective right and left ends, which are outside the region where the display module  20  is placed, of a side of the bottom  120  adjacent to the first bezel  11 . The coupling portions  122  are provided at a predetermined height from a surface of the bottom  120  where the display module  20  is placed and are coupled to the hinge  13  at distal ends  122   a,  the details of which will be described later on. 
     Here, the coupling portions  112  and  122  of the case  10  are provided in such a manner that the hinge  13  is located above the window film  21 , constituting the uppermost layer of the display module  20 , when the case  10  is brought into an open state, where, as illustrated in  FIG.  1    and  FIG.  2   , the first bezel  11  and the second bezel  12  are spaced with the hinge  13  interposed therebetween. It is noted that the coupling portions  112  and  122 , although respectively integrated with the first bezel  11  and the second bezel  12  in this embodiment, may be separated from the respective bezels. Such a separate coupling portion also needs to couple the hinge  13  and the first bezel  11  together in such a manner that the hinge  13  is located above the window film  21 , constituting the uppermost layer of the display module  20 . 
     It is also noted that the cover portions  111  and  121 , although respectively integrated with the first bezel  11  and the second bezel  12  in this embodiment, may be separated from the respective bezels. 
     The hinge  13  couples the first bezel  11  and the second bezel  12  together. To be specific, the hinge  13  includes the following: a pair of hinge bodies  13   a  provided so as to sandwich the display module  20 ; and a rotation shaft  13   b  provided at one end and the other end of each of the pair of hinge bodies  13   a,  and coupled to be able to rotate the distal ends of the coupling portions  112  and  122 . The pair hinge bodies  13   a  couples the first bezel  11  and the second bezel  12  together and is disposed on both of the right and left sides the first bezel  11  and second bezel  12 . The left hinge body  13   a  is coupled to the left coupling portion  112  and coupling portion  122  with the rotation shafts  13   b  interposed therebetween, and this hinge body  13   a  has a predetermined length, as described later on. The right hinge body  13   a  is likewise coupled to the right coupling portion  112  and coupling portion  122  with the rotation shafts  13   b  interposed therebetween, and this hinge body  13   a  has a predetermined length, as described later on. The hinge  13  is disposed laterally out of the first bezel  11  (and the second bezel  12 ), as illustrated in  FIG.  4   . 
     The first bezel  11  and the second bezel  12  can rotate (pivot) about the rotation shafts  13   b  of the hinge  13 . This enables the display module  20 , disposed across the bottom  110  of the first bezel  11  and the bottom  120  of the second bezel  12 , to undergo selection between a closed state, where the display module  20  is folded with the display surface protruding so that the first bezel  11  and the second bezel  12  become close to each other with the hinge  13  interposed therebetween, and an open state, where the folded display module  20  is developed to be flat so that the first bezel  11  and the second bezel  12  are spaced from each other with the hinge  13  interposed therebetween. The closed or folded state is illustrated in the upper part of  FIG.  7   . 
     The display module  20  incorporates at least a display panel and includes a stack of a plurality of function layers structured mutually slidably. 
       FIG.  5    is a schematic view of a specific configuration of the display module  20 . As illustrated in  FIG.  5   , the display module  20  includes the following for instance, sequentially disposed: a display panel  24  (function layer) constituting the lowermost layer; a touch panel  23  (function layer); a polarizing plate  22  (function layer); and the window film  21  (function layer) constituting the uppermost layer. These individual layers are joined by adhesive layers  25   a,    25   b  and  25   c  (hereinafter, generically referred to as an adhesive layer  25 ). That is, the adhesive layer  25   a  is provide between the window film  21  and the polarizing plate  22 , the adhesive layer  25   b  is provided between the polarizing plate  22  and the touch panel  23 , and the adhesive layer  25   c  is provided between the touch panel  23  and the display panel  24 . These adhesive layers  25  each can be composed of a well-known transparent adhesive (OCA). Further, the display panel  24  includes a light-emitting layer, an anode, a cathode, and other things, all of which are not shown. The light-emitting layer can be composed of, but not limited to, OLEDs; the light-emitting layer may be composed of QLEDs. It is noted that the uppermost layer of the display module  20  is not limited to a window film; the uppermost layer may be a protective film stacked further on a side of the window film adjacent to the display surface with an adhesive layer interposed therebetween. 
     In the display module  20  with a stack having such a structure as described above, the window film  21 , that is, the uppermost layer, is provided with engaged portions  30  (see  FIG.  3   ), which is one characteristic element in this embodiment, that can engage with engaging portions  111   a  and  121   a  of the cover portions  111  and  121 , provided respectively in the first bezel  11  and the second bezel  12 . 
     Each engaged portion  30  is composed of a protrusion provided in a region on the outside of the display area on a surface of the window film  21  adjacent to the display surface. To be specific, the engaged portion  30  is provided at one end and the other end of the window film  21 . The engaged portion  30  can protrude, but not limited to, 0.5 mm long and 1 mm wide. The window film  21  is about 100 μm thick for instance. 
     The engaged portions  30  may be integrated with the window film  21  or may be fastened to the window film  21  as separate components. Such a separate component may be made of the same material as the window film  21  or may be made of a material different from that of the window film  21 . The window film  21  is desirably made of PI or PET, both of which are transparent and flexible. It is noted that although the engaged portions  30  can be made of a material different from that of the window film  21 , the engaged portions  30  are desirably made of PI or PET, both of which are transparent and flexible. 
     In contrast, the cover portions  111  and  121  are provided with engaging portions  111   a  and  121   a  that engage with the engaged portions  30 . The engaging portion  111   a  of the cover portion  111  is composed of a recess provided in a region facing the engaged portion  30  extending along the end side of one of the ends of the display module  20 . Further, the engaging portion  121   a  of the cover portion  121  is composed of a recess provided in a region facing the engaged portion  30  extending along the end side of the other end of the display module  20 . 
     Here,  FIG.  6    is a side view of the same display device as that in  FIG.  3   .  FIG.  6    illustrates the length of each portion of the case  10  necessary for description in addition to the display area of the display module  20 . 
     As illustrated in  FIG.  6   , the first bezel  11  and the second bezel  12  are coupled together in a higher position than the bottoms  110  and  120  by the hinge  13 . To be specific, L h2  is a length from the surface of the bottom  110 , where the display module  20  is set, to the center of the rotation shaft  13   b,  by which the coupling portion  112  or  122  provided on the bottom  110  or  120  is coupled. Here, L P  is a length from the center of the rotation shaft  13   b  by which the coupling portion  112  is coupled to one of the ends of the display module  20  fastened to the bottom  110  of the first bezel  11 . Likewise, L P  is a length from the center of the rotation shaft  13   b  by which the coupling portion  122  is coupled to the other end of the display module  20  fastened to the bottom  120  of the second bezel  12 . Further, L h1  is a length between the rotation shafts  13   b  provided at one end and the other end of the each hinge body  13   a.    
     The display module  20  is fastened to the slide mechanisms  113  and  123  in such a manner that the length from one end to the other end of the display module  20  placed in the first bezel  11  and second bezel  12  is equal to a length 2L P +L h1 , as shown in  FIG.  6   , when the case  10  is open. 
     The upper part of  FIG.  7    is a side view of the display device  1  with the case  10  closed, that is, a side view of the display device  1  folded. The lower part of  FIG.  7    is a side view of the display device  1  developed from its folded state. When the display device  1  having the foregoing lengths shown in  FIG.  6    at the respective locations is folded, the display module  20  has a slip (positional shift) occurring with a length denoted by S in the upper part of  FIG.  7   . The slip occurs because the adhesive layer  25 , joining the layers together, deforms. Further, the degrees of deformation in the adhesive layers  25   a  to  25   c  are different from other when the case  10  remains folded, as clearly seen from  FIG.  7   . Here, the difference in length between the lowermost layer and uppermost layer in a portion where the display module  20  bends is determined by (2×π×r 1 −2×π×r 2 )/2=π×(r 1 −r 2 ) when the shape of the bent portion is formed approximately into a semicircle, where r 1  denotes the curvature radius of the uppermost layer, where r 2  denotes the curvature radius of the lowermost layer. The difference in curvature radius between the lowermost layer and the uppermost layer, which is denoted as r 1 −r 2 , is equal to the thickness, D, of the display module  20  (see  FIG.  5   ), and thus, the slip length S between both ends of the display module  20  can be expressed as π×D/2. 
     The engaging portions  111   a  and  121   a  of the cover portions  111  and  121  engage with the engaged portions  30 , provided in the window film  21 , thus eliminating a slip in the window film  21  resulting from such folding. To be specific, in conjunction with an operation from the folded state of the case  10  to the developed state of the case  10 , the engaging portions  111   a  and  121   a  engage with the engaged portions  30  and pull the window film  21  in such a manner that the window film  21  is in parallel. In detail, as illustrated in the lower part of  FIG.  7   , the cover portions  111  and  121  individually pull the window film  21  in directions denoted by the open arrows by π×D/2 from region C, where the two hinges  13  are linked, to thus undo the slip. 
     Here, the hinges  13  are located in a higher position than the bottoms  110  and  120  of the first bezel  11  and second bezel  12  by L h2 , as earlier described. Hence, the display module  20  with the case  10  open is 2L P +L h1  long, but the display module  20  with the case  10  folded is apparently 2L P +L h1 −2L h2  long, which is shorter than it is in the open state. Accordingly, gaps are generated between the window film  21 , the engaging portions  111   a  and  121   a  of the cover portions  111  and  121 , and the engaged portions  30 , thereby being able to avoid the window film  21  from a break resulting from excessive pull. 
     Further, the slide mechanisms  113  and  123 , fastening the display panel  24  on the bottom surface of the display module  20 , at this time slides the display panel  24  with respect to the window film  21  in accordance with the open/closed state of the case  10 , thus being able to correct the difference in length between the top surface and bottom surface of the display module  20  resulting from a slip. 
     The slide mechanisms  113  and  123  include the following: a corresponding one of lowermost-layer support plates  113   a  and  123   a  composed of a flat plate; and a corresponding one of springs  113   b  and  123   b  having their respective ends connected to the lowermost-layer support plates  113   a  and  123   a,  and their respective other ends connected to the bezels  11  and  12 . The lowermost-layer support plates  113   a  and  123   a  are fastened to the display panel  24 , constituting the lowermost layer of the display module  20 , with an adhesive material or other things. The springs  113   b  and  123   b  are compressed when the case  10  is folded, and the spring  113   b  and  123   b  are stretched by elasticity when the case  10  is developed. This enables the lowermost-layer support plates  113   a  and  123   a  to slide along the bottoms  110  and  120  of the respective bezels. The lowermost-layer support plates  113   a  and  123   a  are desirably composed of a flat plate of metal that is easy to process into a thin plate, such as SUS or aluminum. It is noted that guide mechanisms may be provide that guide the sliding of the lowermost-layer support plates  113   a  and  123   a.  It is noted that plate springs may be provided as the slide mechanisms  113  and  123  instead of the lowermost-layer support plates  113   a  and  123   a  and of the springs  113   b  and  123   b.    
     It is noted that the engaged portions  30  of the window film  21  do not engage with the engaging portions  111   a  and  121   a  of the cover portions  111  and  121  at the time of the folding of the case  10 . However, such contact as not to produce a force from the first bezel  11  and second bezel  12  toward the display module  20  is allowable. 
       FIG.  8    illustrates a display device  1000  according to a comparative configuration, which is not included in one aspect of the present invention.  FIG.  8    illustrates a hinge  1300  that is not located in a higher position, but is flush with the bottom of a first bezel  1100  and the bottom of a second bezel  1200 , unlike the hinge  13  according to this embodiment (see  FIG.  6   ). Further, no cover portions  111  and  121  as well as no engaging portions  111   a  and  121   a  according to this embodiment are provided, and unlike this embodiment, no engaged portions  30  are provided in the uppermost layer of the display module  20  (see  FIG.  6    and other things). 
     In the display device  1000  according to the comparative configuration in  FIG.  8   , where the configuration of this embodiment is not applied, closing a case  1010 , followed by folding the display surface causes each layer to slip to mitigate a stress occurring between the frontside and backside of a display module  1020 . The case  1010 , if opened as is, opens with each layer of the display module  1020  remaining slipped. Hence, if opening and closing are repeated, interlayer expansion called buckling in the direction of display occurs in a folded portion of an uppermost layer  1021  of the display module  1020  and of a group of staked layers thereunder in  FIG.  8   , thus causing flection in the folded portion of the display surface. Such bending that the display surface protrudes particularly causes the highest degree of slippage in the uppermost layer of the display module and causes a high degree of buckling as well. If buckling occurs, a bend in the form of a protrusion is produced on the display surface of the display module  1020 , thus causing delamination, thereby degrading display quality. It is noted that the amount of slippage depends not on a curvature radius in the folded state, but on the thickness D (see  FIG.  5   ) and is expressed as π×D/2 as earlier described. 
     To address such a problem, the display device  1  according to this embodiment is configured such that the engaging portions  111   a  and  121   a,  provided in the cover portion  111  and  121 , engage with the engaged portions  30  of the display module  20  and pull the display module  20  to a normal length, i.e., 2L P +L h1  every time the case  10  opens, and hence, a slip resulting from folding is corrected every time. 
     Here, the stacked layers under the window film  21  of the display module  20  are pulled by the adhesive layers (not shown) joining the respective layers in synchronization with the window film  21 , and hence, their individual slips are corrected. 
     The display device  1  with the foregoing configuration can avoid flection in its display surface even when the display surface undergoes folding. 
     First Modification 
     Although this embodiment has described a configuration where the engaged portion  30  of the display module  20  extends along the end side of one end of the display module  20  and the end side of the other end of the same, the present invention is not limited to this configuration. In some aspects for instance, a plurality of engaged portions may be arranged at intervals along the end side of one end of the display module  20  and the end side of the other end of the same. 
     Second Modification 
     The display device  1  according to this embodiment is configured such that as illustrated in  FIG.  3   , the display module  20  is exposed on a side surface of the case  10  where the hinge  13  is provided (the same holds true for the left side surface). However, this aspect is non-limiting; the side surface where the hinge  13  is provided may be provided with a side wall, like the side surfaces of one end and the other end of the case  10 . 
     Third Modification 
     Although the window film  21  is used in this embodiment, a foldable glass window may be used instead. 
     Second Embodiment 
     Another aspect of the present invention will be described with reference to  FIG.  9    through  FIG.  13   . It is noted that for convenience in description, components having the same functions as those described in the first embodiment will be denoted by the same signs, and their description will not be repeated. 
       FIG.  9    is a front view of a display device  1 ′ according to the second embodiment viewed from its display surface.  FIG.  10    illustrates the backside of the display device  1 ′ viewed from its non-display surface, which corresponds to the backside of the display surface in  FIG.  9   .  FIG.  11    a side view of the display device  1 ′.  FIG.  12    is a side view of the display device  1 ′ viewed from a direction orthogonal to the direction in  FIG.  11   . 
     The display device  1 ′ according to this embodiment includes a case  10 ′ provided with, instead of the recesses used as the engaging portions  111   a  and  121   a  according to the first embodiment, engaging portions  111   a ′ and  121   a ′ composed of protrusions protruding toward the side surface of the uppermost layer of the display module. Further, instead of the protrusions used as the engaged portions  30  according to the first embodiment, engaged portions  30 ′ are included that are composed of recesses that engage with the protrusions of the engaging portions  111   a ′ and  121   a′.    
     To be specific, the engaging portions  111   a ′ and  121   a ′ are provided in cover portions  111 ′ and  121 ′ of respective bezels, i.e., a first bezel  11 ′ and a second bezel  12 ′, of the case  10 ′ where a pair of coupling portions  112  and a pair of coupling portions  122 , both provided so as to sandwich a display module  20 ′, are provided. The engaging portions  111   a ′ and  121   a ′ are composed of protrusions protruding toward the side surface of a window film  21 ′ of the display module  20 ′.  FIG.  9    and  FIG.  10    illustrate an example where protrusions constituting the engaging portions  111   a ′ and  121   a ′ are provided at four sites around the window film  21 ′ of the display module  20 ′. 
     In contrast, the window film  21 ′ of the display module  20 ′ is provided with the engaged portions  30 ′ composed of recesses formed by cutting the outer edges of the side surface. The engaged portions  30 ′ are provided in respective regions facing the engaging portions  111   a ′ and  121   a ′ at the forgoing four sites in total. 
     Here,  FIG.  11    illustrates the length of each portion of the case  10 ′ necessary for description in addition to the display area of the display module  20 ′. It is noted that L h1 , L h2 , and L P  shown in  FIG.  11    are the same as those described in the first embodiment, and that their description will be thus omitted. 
     Further, like that according to the first embodiment, the display module  20 ′ housed in the case  10 ′ is fastened to the slide mechanisms  113  and  123 . The display module  20 ′ is 2L P +L h1  long when the case  10 ′ is open, as illustrated in  FIG.  11   . 
     The display device  1 ′ is configured such that the display surface of the display module  20 ′ can be folded, like the display device  1  according to the first embodiment. Further, the window film  21 &#39;s of the display module  20 ′ at this time has a slip (positional shift) occurring with a length denoted by S in the display device  1  in the upper part of  FIG.  7   , described in the first embodiment. Accordingly, in this embodiment, the engaging portions  111   a ′ and  121   a ′ of the first bezel  11 ′ and second bezel  12 ′ engage with the engaged portions  30 ′ of the window film  21 ′, thus eliminating the slip. To be specific, in conjunction with an operation from the folded state of the case  10 ′ to the developed state of the case  10 ′, the engaging portions  111   a ′ and  121   a ′ engage with the engaged portions  30 ′ and pull the window film  21 ′ in such a manner that the window film  21 ′ is in parallel. In detail, the cover portions  111 ′ at two sites of the first bezel  11 ′ and the cover portion  121 ′ at two sites of the second bezel  12 ′ individually pull, by π×D/2, the window film  21 ′ in such directions (directions denoted by open arrows in  FIG.  11   ) as to move the window film  21 ′ away from a location (region where the two hinges  13  are linked) constituting the protrusion of the fold. Accordingly, the slip is eliminated, and the window film  21 ′ gets back to a parallel state. 
     Further, like that according to the first embodiment, the display module  20 ′ with the case  10 ′ open is 2L P +L h1 −2L h2  long because the hinge  13  is in a higher position than the bottoms  110  and  120  of the first bezel  11 ′ and second bezel  12 ′ by L h2 , but the display module  20 ′ with the case  10 ′ folded is apparently 2L P +L h1 −2L h2  long, which is shorter than it is in the open state. Accordingly, gaps are generated between the window film  21 ′, the engaging portions  111   a ′ and  121   a ′ of the cover portions  111 ′ and  121 ′, and the engaged portions  30 ′, thereby being able to avoid the window film  21 ′ from a break resulting from excessive pull. 
     As described above, the display device  1 ′ according to this embodiment is configured such that the engaging portions  111   a ′ and  121   a ′ of the first bezel  11 ′ and second bezel  12 ′ are hooked onto the engaged portions  30 ′ of the display module  20 ′ to thus pull the display module  20 ′ to a normal length, i.e., 2L P +L h1 , every time the case  10 ′ opens, and hence, a slip is corrected every time. 
     Here, the stacked layers under the window film  21 ′ of the display module  20 ′ are pulled by the adhesive layer  25 , joining the individual layers, in synchronization with the window film  21 ′, and hence, their individual slips are corrected. 
     The display device  1 ′ with the foregoing configuration can avoid flection in its display surface even when the display surface undergoes folding. 
     Further, the engaging portion  111   a ′ or  121   a ′ is provided at two sites, i.e., one end and the other end of the cover portion  111  or  121  in an orthogonal direction (the direction of an arrow illustrated in  FIG.  12   ) orthogonal to the direction (the open arrow in  FIG.  11   ) where the engaging portion  111   a ′ or  121   a ′ pulls the window film  21 , constituting the uppermost layer of the display module. It is noted that although  FIG.  12    illustrates the engaging portion  111   a ′ at two sites in the first bezel  11 ′, the engaging portion  121   a ′ is likewise provided at two sites, i.e., one end and the other end of the second bezel  12 ′ in the orthogonal direction. This enables the window film  21  to be pulled more suitably. 
     Further, the display device  1 ′ according to this embodiment can be further thinned than the display device  1  according to the first embodiment. The details will be described with reference to  FIG.  13   . It is noted that for simplification, the illustration of the adhesive layer  25  is omitted in  FIG.  13   . 
       FIG.  13    illustrates the display device  1 ′ according to this embodiment viewed laterally with only a region provided with an engaging portion being enlarged and illustrates the display device  1  according to the first embodiment viewed laterally with only a region provided with an engaging portion being enlarged.  FIG.  13    illustrates the display device  1  according to the first embodiment on the left side of the drawing sheet, and the display device  1  on the right side of the drawing sheet. The display device  1  according to the first embodiment needs cover portions structured so as to cover a surface of the window film  21  adjacent to the display surface. The display device  1 ′ in contrast needs no such structure, thus enabling the thickness TH′, of the first bezel  11 ′ and second bezel  12 ′ to be smaller than the thickness, TH, of the first bezel  11  and second bezel  12 . 
     Modification 
     Although the engaged portions  30 ′ composed of recesses are provided in only the uppermost layer (window film  21 ) in the second embodiment, recesses formed by cutting the outer edges of the side surface of the polarizing plate  22  may be provided not only in the uppermost layer, but also in the polarizing plate  22  thereunder as engaged portions  30 ′. Further, in this case, the adhesive layer  25   a,  interposed between the window film  21  and the polarizing plate  22 , can be also provided with recesses formed by cutting the outer edges of the side surface of the adhesive layer  25   a,  as engaged portions  30 ′. 
     Further, when the polarizing plate  22  as well is provided with engaged portions  30 ′, the size of the recesses constituting the engaged portions  30 ′ may be different between the window film  21  and the polarizing plate  22 . Folding the display module  20 ′ in the aforementioned manner causes the window film  21  to have the greatest buckling. In other words, there is a difference in buckling degree between the window film  21  and the polarizing plate  22 . Accordingly, in view of this difference, different recesses constituting the engaged portions  30 ′ may be formed to have different sizes. 
     Summary 
     A display device  1 ,  1 ′ according to a first aspect of the present invention includes the following: a display module  20 ,  20 ′ incorporating at least a display panel, including a stack of a plurality of function layers structured mutually slidably, and structured in a foldable manner; and a case  10 ,  10 ′ having a first bezel  11 ,  11 ′ and a second bezel  12 ,  12 ′ respectively attached to one end and another end of the lowermost layer (display panel  24 ) of the display module  20 ,  20 ′ with a slide mechanism  113 ,  123  interposed, the case  10 ,  10 ′ having a hinge  13  provided between the first bezel  11 ,  11 ′ and the second bezel  12 ,  12 ′, the case  10 ,  10 ′ housing the display module in a foldable manner, wherein the case  10 ,  10 ′ houses the display module  20 ,  20 ′ in a foldable manner in response to selection between a closed state where the first bezel  11 ,  11 ′ and the second bezel  12 ,  12 ′ are adjacent to each other with the hinge  13  interposed, and an open state where the first bezel  11 ,  11 ′ and the second bezel  12 ,  12 ′ are spaced from each other with the hinge  13  interposed, the case  10 ,  10 ′ is provided with a coupling portion  112  that couples the hinge  13  and the first bezel  11 ,  11 ′ together in such a manner that the hinge  13  is located above the uppermost layer (window film  21 ,  21 ′) of the display module  20 ,  20 ′ when the case  10 ,  10 ′ is brought into the open state, and the case  10 ,  10 ′ is provided with a coupling portion  122  that couples the hinge  13  and the second bezel  12 ,  12 ′ together in such a manner that the hinge  13  is located above the uppermost layer (window film  21 ,  21 ′) of the display module  20 ,  20 ′ when the case  10 ,  10 ′ is brought into the open state, the case  10 ,  10 ′ has a cover portion  111 ,  121  provided in each of the first bezel  11 ,  11 ′ and the second bezel  12 ,  12 ′, and covering one end and another end of the uppermost layer (window film  21 ,  21 ′) of the display module  20 ,  20 ′, the cover portion  111 ,  121  is provided with an engaging portion  111   a,    121   a,    111   a ′,  121   a ′ that engages with an engaged portion  30 ,  30 ′ provided at the one end and the another end of the uppermost layer (window film  21 ,  21 ′) of the display module  20 ,  20 ′, and in conjunction with an operation from the closed state of the case  10 ,  10 ′ into the open state of the case  10 ,  10 ′, the engaging portion  111   a,    121   a,    121   a ′ engages with the engaged portion  30 ,  30 ′ and pulls the uppermost layer (window film  21 ,  21 ′) of the display module  20 ,  20 ′ in such a manner that the uppermost layer (window film  21 ,  21   a ′) is in parallel. 
     The foregoing configuration can provide a display device that can avoid flection in its display surface even when its display module undergoes folding. To be specific, even when a slip (positional shift for stress mitigation) occurs between stacked layers of the display module due to folding, engaged portions provided in the uppermost layer of the display module engage with engaging portions provided in the respective bezels in conjunction with case opening with respect the hinge, that is, in conjunction with folding development, thus pulling the uppermost layer. This can avoid the slip, caused at the time of the folding, from turning into interlayer expansion and remaining after the development. The foregoing configuration can thus provide a display device that can avoid flection in its display surface. 
     The display device  1 ,  1 ′ according to a second aspect of the present invention is preferably configured, in the first aspect, such that the display module  20 ,  20 ′ further includes an adhesive layer  25  provided between two adjacent function layers, and the two adjacent function layers are structured mutually slidably by a deformation of the adhesive layer  25 . 
     The foregoing configuration enables the display module  20 ,  20 ′ to be folded and can eliminate not only a slip in the uppermost layer, but also slips in the plurality of stacked layers under the uppermost layer as a result of the engagement made between the engaged portion and engaging portion at the time of the development, thus pulling the uppermost layer of the display module  20 ,  20 ′. 
     The display device  1  according to a third aspect of the present invention is preferably configured, in the first or second aspect, such that the hinge  13  includes a pair of hinge bodies  13   a  provided to sandwich the display module, and a rotation shaft  13   b  provided at one end and another end of each of the pair of hinge bodies  13   a,  and coupled to be able to rotate a distal end  112   a,    122   a  of the coupling portion  112 ,  122 . 
     The foregoing configuration enables opening and closing firmly with the case  10 ,  10 ′ stable. 
     The display device  1 ,  1 ′ according to a fourth aspect of the present invention is configured, in the first to third aspects, such that the slide mechanism  113 ,  123  slides the lowermost layer (display panel  24 ) of the display module  20 ,  20 ′ with respect to the uppermost layer (window film  21 ,  21 ′) of the display module  20 ,  20 ′ in accordance with an open/closed state of the case  10 ,  10 ′. 
     The forgoing configuration can eliminate flection in the display module  20 ,  20 ′ also from the lowermost layer (display panel  24 ) of the display module  20 ,  20 ′ in conjunction with the opening of the case  10 ,  10 ′. 
     The display device  1 ,  1 ′ according to a fifth aspect of the present invention is preferably configured, in the fourth aspect, such that the slide mechanism  113 ,  123  includes the following: a lowermost-layer support plate  113   a,    123   a  fastened to the lowermost layer (display panel  24 ) of the display module  20 ,  20 ′, and provided in a corresponding one of the first bezel  11 ,  11 ′ and the second bezel  12 ,  12 ′ so as to be able to slide; and a spring  113   b,    123   b  having one end connected to the lowermost-layer support plate, and another end connected to a corresponding one of the first bezel  11 ,  11 ′ and the second bezel  12 ,  12 ′. 
     In the foregoing configuration, the compression and stretch of the spring  113   b,    123   b  causes the lowermost-layer support plate  113   a,    123   a  to slide with respect to the first bezel  11 ,  11  or the second bezel  12 ,  12 ′. Further, a slip of the lowermost layer (display panel  24 ) of the display module  20 ,  20 ′ can be eliminated as a result of this slide. 
     The display device  1  according to a sixth aspect of the present invention is preferably configured, in the first to fifth aspects, such that the engaging portion  111   a,    121   a  is composed of a recess provided in the cover portion  111 ,  121 , and the engaged portion  30  is composed of a protrusion that engages with the recess. 
     In the foregoing configuration, the engaged portion  30 , which is a protrusion provided in the uppermost layer (window film  21 ) of the display module  20 , engage with the engaging portion  111   a,    121   a,  which is a recess provided in the cover portion  111 ,  121 , and in conjunction with the opening of the case  10  with respect to the hinge  13 , the cover portion  111 ,  121  pulls the uppermost layer (window film  21 ) of the display module  20  in such manner that the uppermost layer (window film  21 ) is in parallel. This achieves a display device without flection in its display surface. 
     The display device  1 ′ according to a seventh aspect of the present invention is preferably configured, in the first to fifth aspects, such that the engaging portion  111   a ′,  121   a ′ is composed of a protrusion protruding toward a side surface of the uppermost layer (window film  21 ′) of the display module  20 ′, and the engaged portion  30 ′ is composed of a recess that engages with the protrusion. 
     In the foregoing configuration, the engaging portion  111   a ′,  121   a ′, which is a protrusion provided in the cover portion  111 ′  121 ′, engages with the engaging portion  111   a ′,  121   a ′, which is a recess provided in the uppermost layer (window film  21 ′) of the display module  20 ′, and in conjunction with the opening of the case  10 ′ with respect to the hinge  13 , the cover portion  111 ,  121  pulls the uppermost layer (window film  21 ′) of the display module  20 ′ in such a manner that the uppermost layer (window film  21 ′) is in parallel. This achieves a display device without flection in its display surface. 
     The display device  1 ′ according to an eighth aspect of the present invention is preferably configured, in the first to seventh aspects, such that the engaging portion is provided at two sites in the cover portion  111 ′,  121 ′, the two sites being one end and another end in an orthogonal direction ( FIG.  12   ) orthogonal to a direction where the engaging portion pulls the uppermost layer (window film  21 ′) of the display module  20 ′. 
     In the foregoing configuration, the engaging portion engages with the engaged portion at the two sites in each of the bezels  11 ′ and  12 ′, thereby enabling the display module to be pulled more suitably. 
     The display device  1 ,  1 ′ according to a ninth aspect of the present invention is preferably configured, in the first to eighth aspects, such that the coupling portion  112 ,  122  and the cover portion  111 ,  121 ,  111 ′,  121 ′ are integrated with each of the first bezel  11 ,  11 ′ and the second bezel  12 ,  12 ′. 
     In the foregoing configuration, such integration can reduce the number of components. 
     The display device  1 ,  1 ′ according to a tenth aspect of the present invention is configured, in the first to ninth aspects, such that the display module can incorporate a touch panel  23 , a polarizing plate  22 , and a window film  21 ,  21 ′ provided sequentially on the display panel  24 . 
     The present invention is not limited to the foregoing embodiments. Various modifications can be devised within the scope of the Claims. An embodiment that is obtained in combination, as necessary, with the technical means disclosed in the respective embodiments is also included in the technical scope of the present invention. Furthermore, combining the technical means disclosed in the respective embodiments can form a new technical feature. 
     REFERENCE SIGNS LIST 
     
         
           1 ′ display device 
           10 ,  10 ′ case 
           11 ,  11 ′ first bezel 
           12 ,  12 ′ second bezel 
           13  hinge 
           13   a  hinge body 
           13   b  rotation shaft 
           20 ,  20 ′ display module 
           21 ,  21 ′ window film 
           22  polarizing plate 
           23  touch panel 
           24  display panel 
           25  adhesive layer 
           30 ,  30 ′ engaged portion 
           111   a,    121   a,    111   a ′,  121   a  engaging portion 
           110 ,  120  bottom 
           111 ,  121 ,  111 ′,  121 ′ cover portion 
           112 ,  122  coupling portion 
           113 ,  123  slide mechanism 
           113   a,    123   a  lowermost-layer support plate 
           113   b,    123   b  spring 
         C folding position