Patent Publication Number: US-2023138809-A1

Title: Expandable display device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. Application No. 17/329,364, filed on May 25, 2021 in the U.S. Patent and Trademark Office, which is a continuation of U.S. Application No. 16/726,463, filed on Dec. 24, 2019 in the U.S. Patent and Trademark Office, which is a divisional of U.S. Application No. 15/724,395, filed on Oct. 4, 2017 in the U.S. Patent and Trademark Office, which claims priority under 35 U.S.C. § 119 from, and the benefit of, Korean Patent Application No. 10-2016-0129096, filed in the Korean Intellectual Property Office on Oct. 06, 2016, the contents of which are herein incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     1. Technical Field 
     Embodiments of the present disclosure are directed to an expandable display device. More particularly, embodiments of the present disclosure are directed to a sliding display device that does not use a roller. 
     2. Discussion of the Related Art 
     Interest in larger and clearer display devices and display devices that can be easily moved and stored has increased. A flexible display panel, which includes a flexible substrate such as a plastic film and a plurality of pixels disposed thereon, can be easily bent. 
     Recently, a rollable display device, a foldable display device, an expandable display device, etc., that use a flexible display panel have been under development. Among those stated above, an expandable display device includes a sliding mechanism combined with a flexible display panel, and the sliding mechanism controls a visible display unit of the expandable display device. 
     SUMMARY 
     Embodiments of the present disclosure can provide an expandable display device that limits damage to a flexible display panel by reducing stress applied to the flexible display panel when the flexible display panel slides. 
     An exemplary embodiment provides an expandable display device that includes a flexible display panel and a bending guide and slider. The flexible display panel includes a first side on which a display unit is provided, and a second side opposite to the first side. A bending guide includes an external guide that faces part of the first side and has a first curved side, and an internal guide that faces part of the second side and has a second curved side. A slider is attached to one end portion of the flexible display panel and that slides on the bending guide. A radius of the first curved side is greater than a radius of the second curved side. When the slider moves toward the bending guide, the first side contacts the first curved side and is bent by the first curved side, and when the slider moves away from the bending guide, the second side contacts the second curved side and is bent by the second curved side. 
     A curvature center of the first curved side may correspond to a curvature center of the second curved side, and a distance between the first curved side and the second curved side may be greater than a thickness of the flexible display panel. The flexible display panel may include a portion that is separated from the first curved side the external guide and the second curved side of the internal guide when the slider is stationary. 
     The flexible display panel may include a first end portion attached to the slider, a second end portion opposite to the first end portion, and a bent area bent by the bending guide. In a reduction mode, the flexible display panel may include a first part between the first end portion and the bending area, and a second part between the bent area and the second end portion and that overlaps the first part. 
     The flexible display panel may be supported by a housing. The housing may include the bending guide, a first support plate that supports the first part in reduction mode, and a receiver that receives the second part in reduction mode. 
     The slider may include a fixed portion that covers the first end portion, and a linear guide connected to the fixed portion and that overlaps the receiver. The linear guide may include a second support plate that overlaps the first support plate in reduction mode and supports the first part in an expansion mode. The slider may be attached to the housing by an elastic member. The elastic member may be deformed under tension in expansion mode or may be compressively deformed in reduction mode. 
     The external guide may include a first friction reducing layer disposed on the first curved side, and the internal guide may include a second friction reducing layer disposed on the second curved side. Alternatively, the external guide may include a first bearing portion disposed on the first curved side, and the internal guide may include a second bearing portion disposed on the second curved side. 
     Another embodiment provides an expandable display device that includes a flexible display panel, a first slider, and a second slider. A flexible display panel includes a first side on which a display unit is provided and a second side opposite to the first side, and is supported by a housing. A first slider includes a first external guide that faces part of the first side and has a third curved side and a first internal guide that faces part of the second side and has a fourth curved side. A second slider includes a second external guide that faces another part of the first side and has a fifth curved side and a second internal guide that faces another part of the second side and has a sixth curved side. A radius of the third curved side is greater than a radius of the fourth curved side. When the first slider moves toward the housing, the first side contacts the third curved side and is bent by the third curved side, and when the first slider moves away from the housing, the second side contacts the fourth curved side and is bent by the fourth curved side. 
     A curvature center of the third curved side may correspond to a curvature center of the fourth curved side, and a distance between the third curved side and the fourth curved side may be greater than a thickness of the flexible display panel. When the first slider is stationary, the flexible display panel may include a portion that is separated from the third curved side of the first external guide and the fourth curved side of the first internal guide. 
     The first slider may include a first receiver connected to the first external guide and the first internal guide. The first receiver may overlap the housing and may receive part of the flexible display panel in a reduction mode. 
     A radius of the fifth curved side may be greater than a radius of the sixth curved side. When the second slider moves toward the housing, the first side may contact the fifth curved side and may be bent by the fifth curved side, and when the second slider moves away from the housing, the second side may contact the sixth curved side and may be bent by the sixth curved side. 
     A curvature center of the fifth curved side may correspond to a curvature center of the sixth curved side, and a distance between the fifth curved side and the sixth curved side may be greater than a thickness of the flexible display panel. When the second slider is stationary, the flexible display panel may include a portion that is separated from the fifth curved side of the second external guide and the sixth curved side of the second internal guide. 
     The second slider may include a second receiver connected to the second external guide and the second internal guide. The second receiver may overlap the housing, and may receive another part of the flexible display panel in a reduction mode. 
     A curvature of the third curved side may correspond to a curvature of the fifth curved side, and a curvature of the fourth curved side may correspond to a curvature of the sixth curved side. On the other hand, the curvature of the third curved side may be different from the curvature of the fifth curved side, and the curvature of the fourth curved side may be different from the curvature of the sixth curved side. 
     Another embodiment provides an expandable display device that includes a flexible display panel and a first slider. The flexible display panel includes a first side on which a display unit is provided and a second side opposite to the first side, and supported by a housing. The first slider includes a first external guide, a first internal guide that faces the first external guide, and a first curved space between the first external guide and the first internal guide. A width of the first curved space is greater than a thickness of the flexible display panel. When the first slider moves toward the housing, the first side contacts a first curved side of the first external guide and is bent by the first curved side, and when the first slider moves away from the housing, the second side contacts a second curved side of the first internal guide and is bent by the second curved side. 
     A radius of the first curved side may be greater than a radius of the second curved side, and a curvature center of the first curved side may correspond to a curvature center of the second curved side. The flexible display panel may include a portion that is separated from the first curved side of the first external guide and the second curved side of the first internal guide when the first slider is stationary. The first slider may include a first receiver connected to the first external guide and the first internal guide, and the first receiver may overlap the housing and receives part of the flexible display panel in a reduction mode. 
     The expandable display device may further include a second slider that includes a second external guide, a second internal guide that faces the second external guide, and a second curved space between the a second external guide and the second internal guide. A width of the second curved space may be greater than a thickness of the flexible display panel. When the second slider moves toward the housing, the first side may contact a third curved side of the second external guide and is bent by the third curved side, and when the second slider moves away from the housing, the second side may contact a fourth curved side of the second internal guide and is bent by the fourth curved side. 
     A radius of the third curved side may greater than a radius of the fourth curved side, and a curvature center of the third curved side may correspond to a curvature center of the fourth curved side. The flexible display panel may include a portion that is separated from the third curved side of the second external guide and the fourth curved side of the second internal guide when the second slider is stationary. The second slider may include a second receiver connected to the second external guide and the second internal guide, and the second receiver may overlap the housing, and may receive another part of the flexible display panel in a reduction mode. 
     According to the exemplary embodiments, the stress applied to the flexible display panel may be reduced. Therefore, the damage to the flexible display panel may be reduced, and the durability and lifespan characteristics of the flexible display panel may be improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a perspective view of a reduction mode of an expandable display device according to an exemplary embodiment. 
         FIG.  2    is a partially enlarged cross-sectional view of a flexible display panel in an expandable display device shown in  FIG.  1   . 
         FIG.  3    is a cross-sectional view of an expandable display device along line III-III of  FIG.  1   . 
         FIG.  4    is a cross-sectional view of an expandable display device along line IV-IV of  FIG.  1 .   
         FIG.  5    is a perspective view of an expansion mode of an expandable display device according to an exemplary embodiment. 
         FIG.  6    is a cross-sectional view of an expandable display device along line VI-VI of  FIG.  5 .   
         FIG.  7    is a partially enlarged view of an expandable display device according to a comparative example. 
         FIG.  8   ,  FIG.  9   , and  FIG.  10    are partially enlarged views of an expandable display device shown in  FIG.  2   . 
         FIG.  11    is a partially enlarged view of an expandable display device according to another exemplary embodiment. 
         FIG.  12    is a partially enlarged view of an expandable display device according to another exemplary embodiment. 
         FIG.  13    is a cross-sectional view of a reduction mode of an expandable display device according to another exemplary embodiment. 
         FIG.  14    is a cross-sectional view of an expandable display device switching to an expansion mode from a reduction mode according to an exemplary embodiment of  FIG.  13   . 
         FIG.  15   ,  FIG.  16   , and  FIG.  17    are partially enlarged views of an expandable display device shown in  FIG.  13   . 
         FIG.  18    is a cross-sectional view for a reduction mode of an expandable display device according to another exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure will be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the disclosure are shown. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present disclosure. 
     The drawings and description are to be regarded as illustrative in nature and not restrictive, and like reference numerals may designate like elements throughout the specification. 
     In the drawings, the thickness of layers, films, panels, regions, etc., may be exaggerated for clarity. 
     It will be understood that when an element such as a layer, film, region, or substrate is referred to as being “on” another element, it can be directly on the other element or intervening elements may also be present. 
     Herein, when two or more quantities are described as being substantially the same as each other, it is to be understood that the quantities are identical to each other, indistinguishable from each other, or distinguishable from each other but functionally the same as each other as would be understood by a person having ordinary skill in the art. 
       FIG.  1    is a perspective view of a reduction mode of an expandable display device according to an exemplary embodiment, and  FIG.  2    is a partially enlarged cross-sectional view of a flexible display panel in an expandable display device shown in  FIG.  1   . Referring to  FIG.  1    and  FIG.  2   , an expandable display device  100  according to an exemplary embodiment includes a flexible display panel  10 , a housing  20 , and a slider  30 . 
     The flexible display panel  10  may be one of an organic light emitting display panel, a liquid crystal display panel, an electrophoretic display panel, or an electronic ink display panel. According to an embodiment, the flexible display panel  10  includes a flexible substrate  11 , a display unit  12  provided on the flexible substrate  11 , and an encapsulator  13  that encapsulates the display unit  12 . 
     According to an embodiment, the display unit  12  includes a plurality of driving circuits and a plurality of pixels, and displays an image by a combination of light output by the plurality of pixels. The flexible display panel  10  includes a polarization film as an antireflection member, and includes a touch sensor for sensing user touches. 
     According to an embodiment, the flexible display panel  10  includes a first side S 10  on which the display unit  12  is provided, and a second side S 20  opposite the first side S 10 . The first side S 10  is referred to as a display side, and the second side S 20  is referred to as a non-display side. 
       FIG.  3    is a cross-sectional view of an expandable display device along line III-III of  FIG.  1   , and  FIG.  4    is a cross-sectional view of an expandable display device along line IV-IV of  FIG.  1   . 
     Referring to  FIG.  1    to  FIG.  4   , according to an embodiment, the flexible display panel  10  includes a first end portion EG 1  and a second end portion EG 2  provided on the respective opposite sides while unfurled, and is attached to the housing  20  and the slider  30 . The first end portion EG 1  is attached to the slider  30 , and the second end portion EG 2  is attached to the housing  20 . 
     According to an embodiment, the housing  20  includes a first support plate  21 , a receiver  22  separated from and provided below the first support plate  21  by a predetermined distance , and a bending guide  23  provided on one side of the first support plate  21  and the receiver  22 . The one side is shown as a left side in  FIG.  3   , however, embodiments are not limited thereto. For example, the one side can be a right side. 
     According to an embodiment, the bending guide  23  includes an external guide  24  that faces part of the first side S 10  and that includes a first curved side CS 1 , and an internal guide  25  that faces the second side S 20  and that includes a second curved side CS 2 . The first curved side CS 1  and the second side CS 2  face each other with a curved space CP therebetween. Note that the curved space CP is only shown in those drawings where the space is visible ( FIGS.  8 - 12   ). In a cross-sectional view, the first curved side CS 1  and the second curved side CS 2  are semicircular, and a curvature center of the first curved side CS 1  corresponds to a curvature center of the second curved side CS 2 . A radius of the first curved side CS 1  is greater than a radius of the second curved side CS 2 . 
     According to an embodiment, the flexible display panel  10  is disposed between the first curved side CS 1  and the second curved side CS 2  and is attached to the housing  20 , and includes a bent area (BA) bent by the bending guide  23 . The bent area (BA) of the flexible display panel  10  can be defined as a portion disposed between the first curved side CS 1  and the second curved side CS 2 . 
     According to an embodiment, the bent area (BA) has a semicircular shape in the cross-sectional view. A distance between the first curved side CS 1  and the second curved side CS 2 , i.e. the width of the curved space CP, is greater than a thickness of the flexible display panel  10 , and is constant along a circumferential direction of the curved sides CS 1  and CS 2 . 
     According to an embodiment, in reduction mode, the flexible display panel  10  includes a first part PA 1  between the first end portion EG 1  and the bent area (BA), and a second part PA 2  between the bent area (BA) and the second end portion EG 2 . The second part PA 2  is opposite from first part PA 1 , and the first part PA 1  and the second part PA 2  are planar. The first part PA 1  is visible to a user, and a size of the display unit  12  visible to the user in reduction mode corresponds to a size of the first part PA 1 . 
     According to an embodiment, the first support plate  21  is provided below the first part PA 1  of the flexible display panel  10 , and supports the first part PA 1 . A size of the first support plate  21  is equal or similar to the size of the first part PA 1  in reduction mode. The first support plate  21  is manufactured from a thin metal plate. 
     For convenience of illustration, according to an embodiment, the bent area (BA) of the flexible display panel  10  is shown in contact with the first curved side CS 1  and the second curved side CS 2  in  FIG.  3   , although the distance between the curved sides CS 1  and CS 2  is greater than the thickness of the flexible display panel  10  so that the flexible display panel  10  can move freely between the curved sides CS 1  and CS 2 . 
     According to an embodiment, the receiver  22  includes an internal space that is connected with an internal space of the bending guide  23 , i.e., the curved space CP between the first curved side CS 1  and the second curved side CS 2 . In reduction mode, the receiver  22  receives the second part PA 2  of the flexible display panel  10 . 
     For convenience of illustration, according to an embodiment, the second part PA 2  of the flexible display panel  10  is shown in contact with the receiver  22  in  FIG.  3    and  FIG.  4   , although a height of the internal space of the receiver  22  is greater than the thickness of the flexible display panel  10  in the cross-sectional view. For example, in the cross-sectional view, the height of the internal space of the receiver  22  is equal to the distance between the first curved side CS 1  and the second curved side CS 2 . Therefore, the flexible display panel  10  can move freely in the internal space of the receiver  22 . 
     According to an embodiment, the slider  30  supports the first end portion EG 1  of the flexible display panel  10 , and slides in a first direction DR1 with respect to the housing  20 . The slider  30  includes a fixed portion  31  that is fixed to the first end portion EG 1  of the flexible display panel  10 , and a linear guide  32  that is connected to one side, i.e., the left side with respect to  FIG.  3   , of the fixed portion  31  and that overlaps the receiver  22 . 
     According to an embodiment, the fixed portion  31  is extends parallel to a second direction DR2, and is opposite from the bending guide  23  with respect to the first part PA 1  of the flexible display panel  10 . A plurality of pad electrodes are provided on the first end portion EG 1  of the flexible display panel  10 . In this case, an output terminal portion of a flexible circuit board  14  includes a pad portion, and a printed circuit board (PCB)  15  connects to an input terminal portion of the flexible circuit board  14 . 
     According to an embodiment, the first end portion EG 1  of the flexible display panel  10  and the flexible circuit board  14  is bent or curved inside the fixed portion  31 , and the printed circuit board (PCB)  15  is disposed on the linear guide  32 . In  FIG.  3   , reference numeral  33  refers to a buffer member that protects the first end portion EG 1  of the flexible display panel  10  and the flexible circuit board  14 . 
     According to an embodiment, the linear guide  32  includes a bottom portion  321  that has a substantially plate-like shape in contact with an upper side of the receiver  22 , a plurality of side walls  322 ,  323 , and  324  connected to an edge of the bottom portion  321 , and a second support plate  325  supported by the side walls  322 ,  323 , and  324 . 
     According to an embodiment, the side walls  322 ,  323 , and  324  include a first side wall  322  that faces the fixed portion  31  in the first direction DR1, and a second side wall  323  and a third side wall  324  that face each other in the second direction DR2. The second support plate  325  is disposed below the first support plate  21  in reduction mode, and is manufactured from a thin metal plate. 
     According to an embodiment, the linear guide  32  is combined with the housing  20  by a rail portion  34  in a protrusion and groove combining structure. For example, protrusions can be formed on a lower side of the bottom portion  321  of the linear guide  32  and outer sides of the second side wall  323  and the third side wall  324 , and grooves to be combined with the protrusions may be formed in part of the housing  20  that contacts the linear guide  32 . The rail portion  34  is parallel with the first direction DR1. 
     According to an embodiment, the linear guide  32  can be combined with the housing  20  by an elastic member  40 .  FIG.  3    shows a case in which an elastic member  40  is disposed between the internal guide  25  of the housing  20  and the first side wall  322  of the slider  30 , however, positions of the elastic member  40  are not limited thereto. 
       FIG.  5    is a perspective view of an expansion mode of an expandable display device according to an exemplary embodiment, and  FIG.  6    is a cross-sectional view of an expandable display device along line VI-VI of  FIG.  5   . 
     According to an embodiment, referring to  FIG.  5    and  FIG.  6   , the slider  30  slides in a positive (+) first direction DR1 from the housing  20 , pulls the first end portion EG 1  of the flexible display panel  10  in the positive (+) first direction DR1 along the slider  30 , and releases the flexible display panel  10  from the housing  20 . 
     According to an embodiment, in expansion mode, the slider  30  has a predetermined maximum sliding distance, and the flexible display panel  10  maximizes the size of the first part PA 1  that is visible to the user. For example, in expansion mode, the second end portion EG 2  of the flexible display panel  10  is an edge of the bent area (BA), and most of the flexible display panel  10  except for the bent area (BA) is the first part PA 1 . In expansion mode, the size of the display unit corresponds to the size of the expanded first part PA 1 . 
     In this instance, the slider  30  slides by a force applied by a user’s hand or an external power source such as a motor. The second support plate  325  of the slider  30  is pulled out in the positive (+) first direction DR1 from the first support plate  21 , and supports the expanded first part PA 1  together with the first support plate  21 . A mechanism such as a stopper that fixes the position of the slider  30  in expansion mode is provided to the housing  20  and the slider  30 . 
     According to an embodiment, the elastic member  40  has an initial state in reduction mode, and is deformed under tension in expansion mode. In this case, the slider  30  can quickly return to reduction mode from expansion mode by a restoring force of the elastic member  40 . Alternatively, the elastic member  40  can have an initial state in expansion mode, and can be compressed and deformed in reduction mode. In this case, the slider  30  can quickly return to expansion mode from reduction mode by the restoring force of the elastic member  40 . 
     According to an embodiment, the expandable display device  100  according to an exemplary embodiment bends the flexible display panel  10  using a bending guide  23  that is a non-rotating member instead of a rotating roller. The expandable display device  100  according to an exemplary embodiment reduces the stress applied to the flexible display panel  10  compared to an expandable display device that uses a roller according to a comparative example. 
       FIG.  7    is a partially enlarged view of an expandable display device according to a comparative example. 
     Referring to  FIG.  7   , according to an embodiment, an expandable display device according to a comparative example includes a roller (RO) with a predetermined curvature radius, and the flexible display panel  10  includes a bent area (BA) in contact with the roller (RO). In a comparative example, the flexible display panel  10  maintains close attachment to the roller (RO), so a constant tension is continuously applied to the flexible display panel  10 . 
     That is, a constant force, i.e., stress, is applied to the flexible display panel  10  during the sliding operation and in both reduction mode and expansion mode. Therefore, in a comparative example, the flexible display panel  10  can be easily damaged by the continuously applied stress. 
       FIG.  8   ,  FIG.  9   , and  FIG.  10    are partially enlarged views of an expandable display device shown in  FIG.  3   . 
     Referring to  FIG.  8   ,  FIG.  9   , and  FIG.  10   , in the expandable display device  100  according to an exemplary embodiment, the bending guide  23  includes a first curved side CS 1  and a second curved side CS 2  with different curvature radii, and the flexible display panel  10  includes a bent area (BA) bent by the first curved side CS 1  and the second curved side CS 2 . 
     Referring to  FIG.  8   , according to an embodiment, when the slider  30  slides in the positive (+) first direction DR1 and the expandable display device  100  switches to expansion mode from reduction mode, the second side S 20  of the flexible display panel  10  moves with respect to the second curved side CS 2 . 
     That is, according to an embodiment, the second side S 20  of the flexible display panel  10  is in contact the second curved side CS 2  and is bent according to the curvature of the second curved side CS 2 , and the first side S 10  is separated from the first curved side CS 1  of the external guide  24 . When the slider  30  pulls away from the bending guide  23 , an internal curvature of the bent area (BA) corresponds to the curvature of the second curved side CS 2 . 
     Referring to  FIG.  9   , according to an embodiment, when the slider  30  slides in the negative (-) first direction DR1 and the expandable display device  100  switches to reduction mode from expansion mode, the first side S 10  of the flexible display panel  10  moves in contact with the first curved side CS 1 . 
     That is, according to an embodiment, the first side S 10  of the flexible display panel  10  is in contact with the first curved side CS 1  and is bent according to the curvature of the first curved side CS 1 , and the second side S 20  is separated from the second curved side CS 2  of the internal guide  25 . When the slider  30  pushes toward the bending guide  23 , an external curvature of the bent area (BA) corresponds to the curvature of the first curved side CS 1 . 
     According to an embodiment, a separation distance between the bent area (BA) and the first curved side CS 1  in  FIG.  8    and a separation distance between the bent area (BA) and the second curved side CS 2  in  FIG.  9    is about 0.1 times the thickness of the flexible display panel  10 . The flexible display panel  10  of  FIG.  8    and the flexible display panel  10  of  FIG.  9    are shown to have a constant height difference, and in an actual expandable display device  100 , height variations of the flexible display panels  10  are negligible. 
     Referring to  FIG.  10   , according to an embodiment, when a position of the slider  30  is fixed such that the expandable display device  100  is maintained in either reduction mode or expansion mode, substantially no force is applied to the flexible display panel 10as compared to the mode changing operations shown in  FIG.  8    and  FIG.  9   . Therefore, the bent area (BA) of the flexible display panel  10  can be freely unfurled between the first curved side CS 1  and the second curved side CS 2 . 
     According to an embodiment, as described above, the expandable display device  100  according to an exemplary embodiment uses the above-noted bending guide  23  instead of a roller to reduce the stress applied to the flexible display panel  10 . Therefore, damage to the flexible display panel  10  may be reduced, and durability and lifespan of the flexible display panel  10  may be improved. 
       FIG.  11    is a partially enlarged view of an expandable display device according to another exemplary embodiment. 
     Referring to  FIG.  11   , in the expandable display device  200  according to another exemplary embodiment, the external guide  24  includes a first friction reducing layer  26  disposed on the first curved side CS 1 , and the internal guide  25  includes a second friction reducing layer  27  disposed on the second curved side CS 2 . 
     According to an embodiment, the first friction reducing layer  26  and the second friction reducing layer  27  may be metal layers with smooth surfaces, or may be a region of the bending guide  23  to which a surface treatment has been applied to reduce the friction. The first friction reducing layer  26  and the second friction reducing layer  27  can reduce friction with the flexible display panel to improve the sliding characteristics of the flexible display panel. 
     The expandable display device  200  according to another exemplary embodiment is formed with substantially a same configuration as an exemplary embodiment described above with respect to  FIGS.  1 - 10    except for the first friction reducing layer  26  and the second friction reducing layer  27 . 
       FIG.  12    is a partially enlarged view of an expandable display device according to another exemplary embodiment. 
     Referring to  FIG.  12   , according to an embodiment, in the expandable display device  300  according to another exemplary embodiment, the external guide  24  includes a first bearing portion  28  disposed on the first curved side CS 1 , and the internal guide  25  includes a second bearing portion  29  disposed on the second curved side CS 2 . 
     According to an embodiment, the first bearing portion  28  is configured with a plurality of ball bearings arranged in a circumferential direction of the first curved side CS 1 , and the second bearing portion  29  are configured with a plurality of ball bearings arranged in a circumferential direction of the second curved side CS 2 . The first bearing portion  28  and the second bearing portion  29  can reduce friction with the flexible display panel by rotations of the ball members to improve the sliding characteristics of the flexible display panel. 
     The expandable display device  300  according to another exemplary embodiment has substantially a same configuration as an exemplary embodiment described above with respect to  FIGS.  1 - 10    except for the first bearing portion  28  and the second bearing portion  29 . 
       FIG.  13    is a cross-sectional view of a reduction mode of an expandable display device according to another exemplary embodiment, and  FIG.  14    is a cross-sectional view of an expandable display device according to an embodiment of  FIG.  13    switching to expansion mode from reduction mode. 
     Referring to  FIG.  13    and  FIG.  14   , the expandable display device  400  according to another exemplary embodiment includes a flexible display panel  10 , a housing  50 , a first slider  60 , and a second slider  70 . The expandable display device  400  according to another exemplary embodiment is a dual-expandable display device that can expand on both sides. 
     According to an embodiment, the housing  50  includes a third support plate  51  that supports the flexible display panel  10 , and a fourth support plate  52  disposed below the third support plate  51  by a predetermined distance. The first slider  60  and the second slider  70  are disposed opposite each other in the first direction DR1 with the housing  50  therebetween. The first slider  60  includes a first bending guide  61  and a first receiver  65 , and the second slider  70  includes a second bending guide  71  and a second receiver  75 . 
     According to an embodiment, the first bending guide  61  includes a first external guide  62  that faces part of the first side S 10  and has a third curved side CS 3 , and a first internal guide  63  faces part of the second side S 20  and has a fourth curved side CS 4 . The third curved side CS 3  and the fourth curved side CS 4  face each other with a first curved space CP 1  therebetween. In the cross-sectional view, the curvature center of the third curved side CS 3  corresponds to the curvature center of the fourth curved side CS 4 , and the radius of the third curved side CS 3  is greater than the radius of the fourth curved side CS 4 . 
     According to an embodiment, the second bending guide  71  includes a second external guide  72  that faces another part of the first side S 10  and has a fifth curved side CS 5 , and a second internal guide  73  faces another part of the second side S 20  and has a sixth curved side CS 6 . The fifth curved side CS 5  and the sixth curved side CS 6  face each other with a second curved space CP 2  therebetween. Note that the first and second curved spaces CP 1 , CP 2  are only shown in those drawings where the spaces are visible ( FIGS.  15 - 16   ). In the cross-sectional view, the curvature center of the fifth curved side CS 5  corresponds to the curvature center of the sixth curved side CS 6 , and the radius of the fifth curved side CS 5  is greater than the radius of the sixth curved side CS 6 . 
     According to an embodiment, the first receiver  65  is connected to the first bending guide  61 , and includes an internal space connected with an internal space of the first bending guide  61 , i.e., the first curved space CP 1  between the third curved side CS 3  and the fourth curved side CS 4 . The second receiver  75  is connected to the second bending guide  71 , and includes an internal space connected with the internal space of the second bending guide  71 , i.e., the second curved space CP 2  between the fifth curved side CS 5  and the sixth curved side CS 6 . 
     According to an embodiment, the third curved side CS 3  and the fifth curved side CS 5  have the same radius, and the fourth curved side CS 4  and the sixth curved side CS 6  may have the same radius. The first receiver  65  overlaps the second receiver  75 , and may be provided between the third support plate  51  and the fourth support plate  52  of the housing  50 . The housing  50  further includes a guide plate  53  between the first receiver  65  and the second receiver  75 . 
     According to an embodiment, the flexible display panel  10  includes a first end portion EG 1  and a second end portion EG 2  at opposite ends thereof. The first end portion EG 1  of the flexible display panel  10  slides between the third curved side CS 3  and the fourth curved side CS 4  of the first bending guide  61 , and is in contact with the first slider  60 . The second end portion EG 2  of the flexible display panel  10  slides between the fifth curved side CS 5  and the sixth curved side CS 6  of the second bending guide  71 , and is in contact with to the second slider  70 . 
     According to an embodiment, the first bent area BA 1  is a portion disposed between the third curved side CS 3  and the fourth curved side CS 4 , and is bent by the first bending guide  61 . The second bent area BA 2  is a portion disposed between the fifth curved side CS 5  and the sixth curved side CS 6 , and is bent by the second bending guide  71 . 
     According to an embodiment, a distance between the third curved side CS 3  and the fourth curved side CS 4  is greater than the thickness of the flexible display panel  10 , and is constant along the circumferential direction of the curved sides CS 3  and CS 4 . A distance between the fifth curved side CS 5  and the sixth curved side CS 6  is greater than the thickness of the flexible display panel  10 , and is constant along the circumferential direction of the curved sides CS 5  and CS 6 . 
     According to an embodiment, in reduction mode, the flexible display panel  10  includes a third part PA 3  between the first end portion EG 1  and the first bent area BA 1 , a fourth part PA 4  between the first bending area BA 1  and the second bending area BA 2 , and a fifth part PA 5  provided between the second bending area BA 2  and the second end portion EG 2 . 
     According to an embodiment, the third part PA 3  and the fifth part PA 5  overlap the fourth part PA 4 , and the third part PA 3  and the fifth part PA 5  are received in the first receiver  65  and the second receiver  75 , respectively. The fourth part PA 4  is visible to a user, and in reduction mode, the size of the display unit visible to a user corresponds to the size of the fourth part PA 4 . 
     According to an embodiment, the first slider  60  slides in the negative (-) first direction DR1 with respect to the housing  50 , and the second slider  70  slides in the positive (+) first direction DR1 with respect to the housing  50 . 
     According to an embodiment, the first end portion EG 1  of the flexible display panel  10  moves in the negative (-) first direction DR1 along the first slider  60 , and part of the flexible display panel  10  is unfurled from the first slider  60 . Further, the second end portion EG 2  of the flexible display panel  10  moves in the positive (+) first direction DR1 along the second slider  70 , and part of the flexible display panel  10  is unfurled from the second slider  70 . 
     According to an embodiment, the first slider  60  and the second slider  70  have a predetermined maximum sliding distance, and in expansion mode, the flexible display panel  10  maximizes the size of the fourth part PA 4  visible to the user. 
     For example, according to an embodiment, in the expansion mode, the first end portion EG 1  of the flexible display panel  10  corresponds to an edge of the first bent area BA 1 , and the second end portion EG 2  corresponds to an edge of the second bent area BA 2 . Most of the flexible display panel  10 , except the first bending area BA 1  and the second bending area BA 2 , is part of the fourth part PA 4 . The size of the display unit visible to the user in expansion mode corresponds to the expanded size of the fourth part PA 4 . 
     In this instance, according to an embodiment, the first slider  60  and the second slider  70  can slide by the power of a user’s hand, or by an external power source such as a motor. A mechanism, such as a stopper that fixes the positions of the first slider  60  and the second slider  70  in expansion mode, is provided between the housing  50  and the first slider  60  and between the housing  50  and the second slider  70 . 
       FIG.  15   ,  FIG.  16   , and  FIG.  17    are partially enlarged views of an expandable display device shown in  FIG.  13   . 
     Referring to  FIG.  15   , according to an embodiment, to switch to expansion mode from reduction mode, the first slider  60  slides in the negative (-) first direction DR1 and the second slider  70  slides in the positive (+) first direction DR1. In this process, the second side S 20  of the flexible display panel  10  slides in contact with the fourth curved side CS 4  and the sixth curved side CS 6 . 
     That is, according to an embodiment, regarding the first bending guide  61 , the second side S 20  is in contact with the fourth curved side CS 4  and is bent according to the curvature of the fourth curved side CS 4 , and regarding the second bending guide  71 , the second side S 20  is in contact with the sixth curved side CS 6  and is bent according to the curvature of the sixth curved side CS 6 . Regarding the first bending guide  61 , the first side S 10  is separated from the third curved side CS 3 , and regarding the second bending guide  71 , the first side S 10  is separated from the fifth curved side CS 5 . 
     Referring to  FIG.  16   , according to an embodiment, to switch to reduction mode from expansion mode, the first slider  60  slides in the positive (+) first direction DR1 and the second slider  70  slides in the negative (-) first direction DR1. In this process, the first side S 10  of the flexible display panel  10  slides in contact with the third curved side CS 3  and the fifth curved side CS 5 . 
     That is, according to an embodiment, regarding the first bending guide  61 , the first side S 10  is in contact with the third curved side CS 3  and is bent according to the curvature of the third curved side CS 3 , and regarding the second bending guide  71 , the first side S 10  is in contact with the fifth curved side CS 5  and is bent according to the curvature of the fifth curved side CS 5 . Regarding the first bending guide  61 , the second side S 20  is separated from the fourth curved side CS 4 , and regarding the second bending guide  71 , the second side S 20  is separated from the sixth curved side CS 6 . 
     Referring to  FIG.  17   , according to an embodiment, when the positions of the first slider  60  and the second slider  70  are fixed and the expandable display device is maintained in either reduction mode or expansion mode, substantially no force is applied to the flexible display panel  10  as compared to the mode switching operation. Therefore, the first bent area BA 1  can be freely unfurled between the third curved side CS 3  and the fourth curved side CS 4 , and the second bent area BA 2  can be freely unfurled between the fifth curved side CS 5  and the sixth curved side CS 6 . 
     Referring to  FIG.  13    and  FIG.  14   , according to an embodiment, the expandable display device  400  according to another exemplary embodiment has a substantially same configuration as an exemplary embodiment described with reference to  FIGS.  1 - 10    except for having two sliders  60  and  70  that are combined with the housing  50  and can be expanded on both sides. 
       FIG.  18    is a cross-sectional view for reduction mode of an expandable display device according to another exemplary embodiment. 
     Referring to  FIG.  18   , according to an embodiment, in the expandable display device  500  according to another exemplary embodiment, the first bending guide  61  and the second bending guide  71  have different radii. That is, the third curved side CS 3  can have a radius that can be greater than or less than that of the fifth curved side CS 5 , and the fourth curved side CS 4  can have a radius that is greater than or less than that of the sixth curved side CS 6 .  FIG.  18    shows an example in which the third curved side CS 3  and the fourth curved side CS 4  respectively have radii that are greater than those of the fifth curved side CS 5  and the sixth curved side CS 6 . 
     The expandable display device  500  according to another exemplary embodiment has a substantially same configuration as an exemplary embodiment described with reference to  FIGS.  13 - 17    except for the curvature differences between the first bending guide  61  and the second bending guide  71 . 
     Regarding the above-described embodiment of  FIGS.  13 - 17    and exemplary embodiment of  FIG.  18   , the first slider  60  may include a friction reducing layer or a bearing portion provided on the third curved side CS 3  and the fourth curved side CS 4 , and the second slider  70  may include a friction reducing layer or a bearing portion provided on the fifth curved side CS 5  and the sixth curved side CS 6 . The friction reducing layer is the same as the first friction reducing layer or second friction reducing layer according to an exemplary embodiment of  FIGS.  10 - 11   , and the bearing portion is the same as the first bearing portion or the second bearing portion according to an exemplary embodiment of  FIG.  12   . 
     While this disclosure has been described with reference to what are presently considered to be practical exemplary embodiments, it is to be understood that the disclosure is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.