Patent Publication Number: US-2020277222-A1

Title: Method for manufacturing cover glass, cover glass, and display device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from Japanese Patent Application No. 2019-035699 filed on Feb. 28, 2019, the entire subject matter of which is incorporated herein by reference. 
     BACKGROUND OF INVENTION 
     Technical Field 
     The present invention relates to a method for manufacturing a cover glass, a cover glass, and a display device. 
     Background Art 
     Patent Literature 1 discloses “a cover glass that covers a display panel of a display device, the cover glass including a front surface that does not face the display panel, a back surface that faces the display panel, a front chamfered portion that is a chamfered portion on the front surface side, and a back chamfered portion that is a chamfered portion on the back surface side, wherein surface roughness Ra of the front chamfered portion exceeds 100 nm, and surface roughness Ra of the back chamfered portion is 100 nm or less” ([Claim  1 ]). 
     In the cover glass according to Patent Literature 1, Patent Literature 1 describes that “the surface roughness Ra of the back chamfered portion  13   b  is 100 nm or less, so that occurrence of cracking in the cover glass  12  can be prevented, and the cover glass  12  can have excellent impact resistance in an edge portion thereof” (paragraph [0024]). 
     In addition, in the cover glass according to Patent Literature 1, Patent Literature 1 describes that “the surface roughness Ra of the front chamfered portion  13   a  exceeds 100 nm, so that occurrence of gradation can be prevented, and defective display in the edge portion can be prevented” (paragraph [0021]).
     Patent Literature 1: WO 2017/208995 A1   

     SUMMARY OF INVENTION 
     As described above, Patent Literature 1 discloses a cover glass in which there is a difference between surface roughness Ra of a chamfered portion (front chamfered portion) on one main surface side and surface roughness Ra of a chamfered portion (back chamfered portion) on the other main surface side. 
     Specifically the cover glass according to Patent Literature 1 is, for example, manufactured as follows. 
     “A glass sheet is ground and chamfered by use of a chamfering wheel with coarse grain size (for example, grit number #600), so as to form a front chamfered portion  13   a  and a back chamfered portion  13   b . After that, only the back chamfered portion  13   b  is ground by use of the chamfering wheel with fine grain size (for example, grit number #6000). Thus, a cover glass  12  in which the surface roughness Ra of the front chamfered portion  13   a  exceeds 100 nm and the surface roughness Ra of the back chamfered portion  13   b  is 100 nm or less can be obtained” (paragraph [0042]). 
     However, when the grain size of the chamfering wheel (grindstone) is changed from #600 to #6000 without any other grit number, the chamfering wheel (grindstone) is damaged. In fact, it is however necessary to increase the grit number from #600 gradually in multistage steps until finally reaching the grit number #6000. Such a manufacturing process is very complicated. 
     Therefore, an object of the present invention is to simply and easily obtain a cover glass in which there is a difference between surface roughness Ra of a chamfered portion on one main surface side and surface roughness Ra of a chamfered portion on the other main surface side. 
     The present inventors performed extensive studies, and found that the object could be achieved by using the following configuration. 
     That is, the present invention provides the following [1] to [11]. 
     [1] A method for manufacturing a cover glass, comprising: 
     covering partial regions of both main surfaces of a glass sheet with a mask material, the partial regions being opposed to each other; 
     etching the glass sheet having the partial regions covered with the mask material by use of an etchant, thereby obtaining a small-piece glass sheet having chamfered portions in the both main surfaces; and 
     further chamfering at least a part of one main surface of the small-piece glass sheet, thereby providing a difference between surface roughness Ra of the chamfered portion on the one main surface side and surface roughness Ra of the chamfered portion on the other main surface side. 
     [2] The method for manufacturing a cover glass according to [1], comprising further chamfering a side face portion of the small-piece glass sheet. 
     [3] The method for manufacturing a cover glass according to [1] or [2], wherein a plurality of the mask materials are disposed in a main surface direction of the glass sheet. 
     [4] The method for manufacturing a cover glass according to [3], wherein an interval between the mask materials adjacent to each other in the main surface direction of the glass sheet is equal to or less than a thickness of the glass sheet. 
     [5] The method for manufacturing a cover glass according to any one of [1] to [4], wherein the glass sheet has a thickness of 0.5 mm to 2.5 mm. 
     [6] The method for manufacturing a cover glass according to any one of [1] to [5], wherein: 
     the etchant is an aqueous solution containing hydrogen fluoride; and 
     a content of the hydrogen fluoride in the etchant is 2 mass % to 10 mass %. 
     [7] The method for manufacturing a cover glass according to any one of [1] to [6], wherein the etchant has a temperature of 10° C. to 40° C. 
     [8] The method for manufacturing a cover glass according to any one of [1] to [7], wherein the glass sheet having the partial regions covered with the mask material is a glass sheet that has been subjected to an antiglare treatment. 
     [9] The method for manufacturing a cover glass according to any one of [1] to [8], comprising subjecting the small-piece glass sheet to a chemical strengthening treatment after the chamfering. 
     [10] A cover glass obtained by the method according to any one of [1] to [9]. 
     [11] A display device, comprising the cover glass according to [10]. 
     According to the present invention, it is possible to simply and easily obtain a cover glass in which there is a difference between surface roughness Ra of a chamfered portion on one main surface side and surface roughness Ra of a chamfered portion on the other main surface side. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a sectional view of a glass sheet covered with mask materials. 
         FIG. 2  is a sectional view of a small-piece glass sheet obtained by etching. 
         FIG. 3  is a sectional view of the small-piece glass sheet subjected to chamfering. 
         FIG. 4  is a sectional view of an on-vehicle display device. 
         FIG. 5  is a perspective view of a test body. 
         FIG. 6  is a sectional view taken on line A-A in  FIG. 5 . 
         FIG. 7  is a plan view of the test body. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiment of the present invention are described below with reference to the drawings. However, the present invention is not limited to the following embodiments. Various modifications and replacements may be made on the following embodiments without departing from the scope of the present invention. 
     Surface roughness Ra (arithmetic average roughness) is a value measured in accordance with JIS B 0601:2001. 
     [Method for Manufacturing Cover Glass] 
     A method for manufacturing a cover glass in an embodiment of the present invention (hereinafter also simply referred to as “manufacturing method of the present invention”) is a method for manufacturing a cover glass as follows. That is, the method for manufacturing a cover glass includes: covering partial regions of both main surfaces of a glass sheet with a mask material, the partial regions being opposed to each other; etching the glass sheet having the partial regions covered with the mask material by use of an etchant, thereby obtaining a small-piece glass sheet having chamfered portions in the both main surfaces; and further chamfering at least a part of one main surface of the small-piece glass sheet, thereby providing a difference between surface roughness Ra of the chamfered portion on the one main surface side and surface roughness Ra of the chamfered portion on the other main surface side. 
     The manufacturing method of the present invention is a method for obtaining a cover glass in which there is a difference between surface roughness Ra of a chamfered portion on one main surface side and surface roughness Ra of a chamfered portion on the other main surface side. 
     In the background art, such a cover glass can be obtained in a complicated manner in which chamfering is performed with a grindstone whose grain size is increased gradually in multistage steps (paragraph [0042] in Patent Literature 1). 
     On the other hand, chamfering is performed only once in the manufacturing method of the present invention. Thus, such a cover glass can be obtained simply and easily. 
     In addition, in the background art, work for cutting a large-size glass sheet to obtain a plurality of small-size glass sheets is required as a stage prior to chamfering. 
     On the other hand, in the manufacturing method of the present invention, a plurality of small-size glass sheets can be obtained at one time by etching without cutting. Also in this respect, the manufacturing method of the present invention is simple and easy. 
     The manufacturing method of the present invention is described below more in detail with reference to  FIG. 1  to  FIG. 3 . 
     (Masking) 
       FIG. 1  is a sectional view of a glass sheet  1  having partial regions covered with a mask material  5 . 
     The glass sheet  1  has one main surface  1   a  and the other main surface  1   b . First, partial regions of the both main surfaces of the glass sheet  1  are covered with the mask material  5 , partial regions being opposed to each other. That is, a partial region of the main surface  1   a  of the glass sheet  1  and a partial region of the main surface  1   b  opposed thereto are covered with the mask material  5 . The partial regions opposed to each other preferably have the same shape (the same size) as each other. 
     Examples of glass of the glass sheet  1  include soda lime glass, aluminosilicate glass (SiO 2 —Al 2 O 3 —Na 2 O based glass), and the like. In the case where a chemical strengthening treatment is performed as described below, a glass for chemical strengthening which is based on aluminosilicate glass (for example, “Dragontrail (registered trademark)” may be also used suitably. 
     The thickness (represented by the reference sign tin  FIG. 1 ) of the glass sheet  1  is preferably 0.5 mm to 2.5 mm and more preferably 0.7 mm to 2.0 mm for the reasons described below. 
     The dimensions of the main surfaces (the main surface  1   a  and the main surface  1   b ) of the glass sheet  1  are set appropriately. 
     An antiglare (AG) treatment may be performed on the glass sheet  1  to be covered with the mask material  5 . A method for the AG treatment is not limited particularly. Examples of the method include a method of etching a surface layer of the glass sheet  1 ; a method of applying coating liquid including fine particles and a matrix to a surface of the glass sheet  1  and hardening the matrix; and the like. 
     The material of the mask material  5  is not limited particularly as long as it is a material resistant to an etchant described below. A material common in the background art may be selected and used suitably. 
     A film-like mask material is, for example, used as the mask material  5 . As a specific example, an acid-resistant PET (polyethylene terephthalate) material coated with an acrylic pressure-sensitive adhesive may be used suitably. 
     A curable resin may be applied to the glass sheet  1  by use of a bar coater or the like, and hardened to form the mask material  5 . Examples of the curable resin include a UV-curing resin and a thermosetting resin. Examples of the UV curable resin include an acrylate based radical polymerizable resin and an epoxy based cation polymerizable resin. Examples of the thermosetting resin include an epoxy resin, a phenolic resin, a urea resin, a melamine resin, an unsaturated polyester resin, a polyurethane resin, a diallyl phthalate resin, a silicone resin, and an alkyd resin. The UV curable resin is preferred because its curing rate is so high that the tact time can be shortened. 
     As shown in  FIG. 1 , a plurality of mask materials  5  may be disposed in the main surface direction (the left/right direction in  FIG. 1 ) of the glass sheet  1 . In this manner, a plurality of small-piece glass sheets  12  (see  FIG. 2  and  FIG. 3  described below) can be obtained from the glass sheet  1 . 
     On this occasion, each interval (represented by the reference sign G in  FIG. 1 ) between the mask materials  5  adjacent to each other in the main surface direction of the glass sheet  1  is preferably equal to or less than the thickness t (equal to or less than one times as large as the thickness t), preferably equal to or less than ½ times as large as the thickness t, and more preferably equal to or less than ⅓ times as large as the thickness t. On the other hand, the interval G is preferably equal to more than 1/10 times as large as the thickness t, and more preferably equal to more than ⅛ times as large as the thickness t. 
     In the case where the interval G between the mask materials  5  adjacent to each other is within the aforementioned range, a chamfered portion  12   c , a side face portion  12   e  and a chamfered portion  12   d  of each obtained small-piece glass sheet  12  can form a convex curve (curved surface) easily. 
     (Etching) 
     Next, the glass sheet  1  having the partial regions covered with the mask material  5  is etched with an etchant. Thus, a portion of the part of the glass sheet  1 , which is not covered with the mask material  5 , is dissolved by the etchant so that a small-piece glass sheet  12  smaller than the glass sheet  1  can be obtained. 
       FIG. 2  is a sectional view of the small-piece glass sheet  12  obtained by etching. The small-piece glass sheet  12  has chamfered portions in the both main surfaces. 
     That is, the small-piece glass sheet  12  has a chamfered portion  12   c  on one main surface  12   a  side and a chamfered portion  12   d  on the other main surface  12   b  side. 
     The small-piece glass sheet  12  further has a side face portion  12   e  which is connected to the chamfered portion  12   c  and the chamfered portion  12   d.    
     As shown in  FIG. 2 , the chamfered portion  12   c , the side face portion  12   e  and the chamfered portion  12   d  are connected to one another to thereby form a convex curve (curved surface). 
     Here, “convex” means that any straight line parallel to the thickness direction (the up/down direction in  FIG. 2 ) of the small-piece glass sheet  12  and the outline of the small-piece glass sheet  12  intersect each other at two or less points. In the case of not the convex curve but a concave curve where they intersect each other at three or more points, it is extremely difficult to process not only the chamfered portion  12   c  during the chamfering process described below. 
     In a region of the glass sheet  1  (see  FIG. 1 ), which is not covered with the mask material  5 , dissolving starts on the both main surface sides and advances gradually toward the central portion. The central portion tends to remain without being dissolved, as compared with the both main surface sides. Thus, the chamfered portion  12   c , the side face portion  12   e  and the chamfered portion  12   d  can form a convex curve. 
     In addition, the chamfered portion  12   c , the side face portion  12   e  and the chamfered portion  12   d  are formed not by grinding with a grindstone but by etching with an etchant. Therefore, they form a smooth curve (curved surface). As a result, the chamfered portion  12   c , the side face portion  12   e  and the chamfered portion  12   d  have very small surface roughness Ra. 
     The etchant is not limited particularly. The etchant is, for example, an aqueous solution containing hydrogen fluoride. 
     In this case, the content of the hydrogen fluoride in the etchant is preferably 2 mass % to 10 mass %. 
     In the case where the content of the hydrogen fluoride in the etchant is 2 mass % or higher, the processing time by etching can be shortened comparatively so that processing can be performed with good productivity. 
     On the other hand, in the case where the content is 10 mass % or lower, a variation in etching rate for each of the obtained small-piece glass sheets  12  can be reduced so that processing can be performed uniformly. 
     In order to further enhance those effects, the content of the hydrogen fluoride in the etchant is more preferably 4 mass % to 8 mass %. 
     In order to reduce the variation in etching rate and perform uniform processing on the obtained small-piece glass sheets  12 , the temperature of the etchant is preferably 10° C. to 40° C., and more preferably 20° C. to 30° C. 
     The etching method is not limited particularly, but the glass sheet  1  having the partial regions covered with the mask material  5  is preferably immersed in the etchant. 
     The immersing time (etching time) in the etchant may be changed suitably depending on the sheet thickness of the glass sheet  1 . The immersing time becomes longer with increase in thickness of the glass sheet  1 . 
     For example, when the thickness of the glass sheet  1  is 0.5 mm to 2.5 mm, the etching time is preferably 20 minutes or longer, and more preferably 30 minutes or longer, and the etching time is preferably 600 minutes or shorter, and more preferably 300 minutes or shorter. 
     After the etching, the mask material  5  is removed suitably. 
     (Chamfering) 
     Next, one main surface of the small-piece glass sheet  12  obtained by etching is further chamfered. Specifically, the chamfered portion  12   c  on the one main surface  12   a  side of the small-piece glass sheet  12  is, for example, chamfered by grinding with a chamfering wheel (grindstone) or the like. 
       FIG. 3  is a sectional view of the small-piece glass sheet  12  which has been subjected to chamfering. By the chamfering, another chamfered portion  13   c  different from the curved chamfered portion  12   c  is formed in the small-piece glass sheet  12  as shown in  FIG. 3 . 
     The chamfered portion  13   c  formed by grinding with the chamfering wheel (grindstone) or the like has a rougher face than the original chamfered portion  12   c  formed by etching. 
     On the other hand, chamfering is not performed on the chamfered portion  12   d  on the other main surface  12   b  side. Therefore, the curved chamfered portion  12   d  formed by etching is kept as it is. 
     In this manner, in the small-piece glass sheet  12  subjected to the chamfering, there is a difference between surface roughness Ra of the chamfered portion  13   c  on the one main surface  12   a  side and surface roughness Ra of the chamfered portion  12   d  on the other main surface  12   b.    
     Chamfering may be performed on the side face portion  12   e  of the small-piece glass sheet  12  in the same manner. Thus, another side face portion  13   e  different from the curved side face portion  12   e  is formed in the small-piece glass sheet  12 . 
     In the case where chamfering is performed on the side face portion  12   e , the side face portion  13   e  formed newly is shaped into a straight line substantially parallel to the thickness direction of the small-piece glass sheet  12 . Thus, the small-piece glass sheet  12  can be attached easily when it is attached as a cover glass to a display device. 
     The chamfering conditions (grit number of the grindstone, processing rate, grinding amount, and the like) may be selected suitably depending on desired surface roughness Ra of the chamfered portion  13   c  (desired surface roughness Ra of the chamfered portion  13   c  and desired surface roughness Ra of the side face portion  13   e  when the side face portion  13   e  is formed). 
     For the chamfering, as long as the number of steps does not increase excessively to complicate the work, rough processing may be first performed with a grindstone with rough grain size, and then, finishing processing may be performed with a grindstone with fine grain size. 
     (Chemical Strengthening Treatment) 
     In the manufacturing method of the present invention, a chemical strengthening treatment may be performed on the small-piece glass sheet  12  after the aforementioned chamfering. Even when the chemical strengthening treatment is performed, the value of the surface roughness Ra is normally unchanged. 
     When the chemical strengthening treatment is performed, a glass for chemical strengthening is used as a glass. 
     According to a typical method for the chemical strengthening treatment, a glass is immersed in molten salt of KNO 3  for an ion exchange treatment, and then cooled down to the vicinity of room temperature. The treatment conditions such as the temperature of the molten salt of KNO 3 , the immersing time, and the like may be set so that a desired surface compressive stress (CS) of a compressive stress layer and a desired thickness (DOL) of the compressive stress layer can be obtained. 
     The surface compressive stress (CS) of the compressive stress layer is preferably 500 MPa or more, more preferably 650 MPa or more, and even more preferably 750 MPa or more, and the surface compressive stress (CS) of the compressive stress layer is preferably 1,200 MPa or less. 
     The thickness (DOL) of the compressive stress layer is preferably 10 μm or more, more preferably 15 μm or more, even more preferably 25 μm or more, and particularly preferably 30 μm or more. The thickness (DOL) of the compressive stress layer is preferably 50 μm or less. 
     When the chemical strengthening treatment is performed, the small-piece glass sheet  12  which has been subjected to the chemical strengthening treatment serves as a cover glass  12  (described below). 
     On the other hand, when the chemical strengthening treatment is not performed, the small-piece glass sheet  12  which has been chamfered but has not been subjected to the chemical strengthening treatment serves as a cover glass  12  (described below). 
     [Cover Glass] 
     Hereinafter, the cover glass  12  is described below with reference to  FIG. 3 . In the following description, parts the same as (or corresponding to) those in the small-piece glass sheet  12  which has been chamfered are referenced correspondingly. 
     The cover glass  12  has a front surface  12   a  which does not face a display panel  104  (see  FIG. 4 ) described below, and a back surface  12   b  which faces the display panel  104  on the opposite side to the front surface  12   a.    
     Further, the cover glass  12  has a front chamfered portion  13   c  which is a chamfered portion on the front surface  12   a  side, a back chamfered portion  12   d  which is a chamfered portion on the back surface  12   b  side, and a side face portion  13   e  which is connected to the front chamfered portion  13   c  and the back chamfered portion  12   d.    
     The thickness of the cover glass  12  is preferably 0.5 mm to 2.5 mm, and more preferably 0.7 mm to 2.0 mm. In the case where the thickness of the cover glass  12  is within the aforementioned range, durability against bending fracture in the back surface  12   b  can be enhanced in a head impact test described below. 
     The outer shape and dimensions of the cover glass  12  may be determined suitably depending on its use. For example, the outer shape is rectangular. 
     The dimensions of the cover glass  12  are, for example, 100 mm to 900 mm in its longitudinal direction and 40 mm to 500 mm in its lateral direction when the outer shape is rectangular. The dimensions are preferably 100 mm to 800 mm in the longitudinal direction and 40 mm to 300 mm in the lateral direction. 
     The size of the front surface  12   a  or back surface  12   b  of the cover glass  12  is, for example, preferably 5 inches to 20 inches. 
     An antireflection film may be provided on the front surface  12   a  of the cover glass  12 . The thickness of the antireflection film is, for example, about 100 nm to 300 nm. As the material of the antireflection film and the method for forming the same, for example, a material and a deposition method described in paragraphs [0029] to [0030] of Patent Literature 1 may be used. 
     However, in the case where the antireflection film is formed in the front chamfered portion  13   c , gradation may occur in the front chamfered portion  13   c . The occurrence of the gradation may be recognized as poor appearance (poor appearance in an edge portion). 
     In order to prevent the occurrence of gradation and prevent poor appearance in an edge portion, surface roughness Ra of the front chamfered portion  13   c  is preferably more than 100 nm, more preferably 140 nm or more, even more preferably 170 nm or more, and particularly preferably 210 nm or more. The surface roughness Ra is preferably 500 nm or less, and more preferably 400 nm or less. 
     A cover glass for an on-vehicle display device is required to have impact resistance high enough not to be cracked by a head portion of a passenger colliding therewith when a vehicle crash occurs. 
     In the case where a head portion of a passenger collides with an edge portion of the cover glass  12 , a large stress is generated in the back chamfered portion  12   d . Due to the generated stress, the cover glass  12  may be cracked from a flaw (made during processing) in the back chamfered portion  12   d  as a start point. 
     In order to prevent cracking of the cover glass  12  and provide excellent impact resistance in an edge portion, surface roughness Ra of the back chamfered portion  12   d  is preferably 100 nm or less, more preferably 70 nm or less, even more preferably 30 nm or less, and particularly preferably 10 nm or less. The surface roughness Ra is preferably 0.1 nm or more. 
     The cover glass  12  can be obtained through the aforementioned masking, etching and chamfering (further optionally the chemical strengthening treatment). The back chamfered portion  12   d  of the cover glass  12  obtained thus has not undergone grinding with a grindstone or the like. Therefore, the state after the etching is kept as it is, and thus, the number of fine cracks (also referred to as “micro-cracks”) is very small. Thus, cracking is further prevented, and the impact resistance in the edge portion is more excellent. 
     However, the micro-cracks are very fine cracks. There is no suitable means for grasping the present state of the micro-cracks in the back chamfered portion  12   d . Therefore, the features of the back chamfered portion  12   d  of the cover glass  12  cannot be specified directly from its physical structure or properties. 
     In addition, an extremely large number of trials and errors are required to find out some index other than the micro-cracks based on a large number of measurements repeated using various devices in order to grasp the features of the back chamfered portion  12   d . It is therefore not practical to find out such an index. 
     Next, an on-vehicle display device which is mounted on a vehicle is described as a display device using the cover glass  12  with reference to  FIG. 4 . 
     The on-vehicle display device is, for example, a car navigation device, or a rear seat entertainment (RSE) device on which passengers at rear seats can watch video and so on. 
     The car navigation device is often used in a state where it is provided to stand on an exterior portion of a dash board or it is embedded in the dash board. 
     The RSE device is often used in a state where it is attached to the back side of a front seat. 
     However, the display device is not limited to such an on-vehicle display device. 
     [On-Vehicle Device] 
       FIG. 4  is a sectional view of an on-vehicle display device  100 . 
     The on-vehicle display device  100  has a housing  106  for receiving members thereof. A backlight unit  102  is mounted on a housing bottom sheet  107  which is a bottom sheet of the housing  106 . A display panel  104  is mounted on the backlight unit  102 . The display panel  104  is, for example, a liquid crystal panel. An opening portion is formed in the housing  106 . 
     The configurations of the backlight unit  102  and the display panel  104  are not limited particularly. Common configurations may be used. The material and so on of the housing  106  (including the housing bottom sheet  107 ) are also not limited particularly. 
     The on-vehicle display device  100  may have, for example, an organic EL panel, a PDP, an electronic ink type panel, or the like. The on-vehicle display  100  may have a touch panel or the like. 
     The cover glass  12  is pasted on the display panel  104  through a pressure-sensitive adhesive layer  14 . The cover glass  12  functions as a protective member for the display panel  104 . 
     The pressure-sensitive adhesive layer  14  is preferably transparent like the cover glass  12 , and there is preferably a small difference in refractive index between the cover glass  12  and the pressure-sensitive adhesive layer  14 . Examples of the pressure-sensitive adhesive layer  14  include a layer made of a transparent resin obtained by curing a liquid curable resin composition, and an OCA (Optical Clear Adhesive) film or tape. The thickness of the pressure-sensitive adhesive layer  14  is, for example, 5 μm to 400 μm, and preferably 50 μm to 200 μm. 
     Examples 
     The present invention is described specifically below with reference to examples thereof. However, the present invention is not limited to the following examples. 
     &lt;Manufacturing Cover Glass&gt; 
     Cover glasses  12  in Case 1 to Case 4 were manufactured according to the manufacturing method of the present invention described with reference to  FIG. 1  to  FIG. 3 . Case 1 to Case 4 are examples of the present invention. 
     &lt;&lt;Masking&gt;&gt; 
     First, a glass for chemical strengthening (“Dragontrail” made by AGC Inc.) subjected to an AG treatment was prepared as the glass sheet  1 . The thickness t of the glass sheet  1  was set to vary among Case 1 to Case 4 as shown in the following Table 1. 
     Next, the mask materials  5  were disposed on the main surface  1   a  and main surface  1   b  of the glass sheet  1  as shown in  FIG. 1 . A film (resistant to acid) of a PET material coated with an acrylic pressure-sensitive adhesive was used as the mask materials  5 . The interval G between the mask materials  5  adjacent to each other was set at ½ of the thickness t of the glass sheet  1 . 
     &lt;&lt;Etching&gt;&gt; 
     The glass sheet  1  coated with the mask materials  5  was immersed in an etchant, thereby performing etching. Thus, small-piece glass sheets  12  which were small in size were obtained. 
     Each obtained small-piece glass sheet  12  had a chamfered portion  12   c , a side face portion  12   e  and a chamfered portion  12   d , these forming a convex curve (curved surface) as shown in  FIG. 2 . 
     An aqueous solution containing 6 mass % of hydrogen fluoride was used as the etchant. The temperature of the etchant was set at 25° C. The etching time varied depending on the thickness t of the glass sheet  1  as shown in the following Table 1. 
     After the etching, the mask materials  5  were removed. 
     &lt;&lt;Chamfering&gt;&gt; 
     The small-piece glass sheet  12  obtained by the etching was chamfered. 
     More specifically, the chamfered portion  12   c  on the one main surface  12   a  side and the side face portion  12   e  in the small-piece glass sheet  12  were ground by use of a chamfering wheel (grindstone). Thus, another chamfered portion  13   c  and another side face portion  13   e  which were rougher than the curved chamfered portion  12   c  and the curved side face portion  12   e  were formed in the small-piece glass sheet  12  as shown in  FIG. 3 . 
     Specifically in the chamfering process, rough processing (grindstone grit number: #325, processing rate: 1,200 mm/min, grinding amount: 0.4 mm) was performed by use of a chamfering wheel (grindstone) with rough grain size. After that, finishing processing (grindstone grit number: #600, processing rate: 800 mm/min, grinding amount: 0.1 mm) was performed by use of a chamfering wheel (grindstone) with fine grain size. 
     &lt;&lt;Chemical Strengthening Treatment&gt;&gt; 
     A chemical strengthening treatment was applied to the small-piece glass sheet  12  which had been chamfered. The chemical strengthening treatment was performed by immersing the whole of the glass sheet into molten salt of KNO 3  so as to form a compressive stress layer with a thickness (DOL) of 35 μm and a surface compressive stress (CS) of 750 MPa. 
     In the aforementioned manner, the cover glasses  12  in Case 1 to Case 4 were obtained. 
     The value of the surface roughness Ra of the front chamfered portion  13   c  and the value of the surface roughness Ra of the back chamfered portion  12   d  in each of the cover glasses  12  in Case 1 to Case 4 are shown in the following Table 1. 
     The surface roughness Ra was measured by a laser microscope “VK-9500” made by Keyence Corporation in accordance with JIS B 0601:2001. A cutoff value λ c  was set at 0.25 mm. 
     In this manner, in each of Case 1 to Case 4, the cover glass  12  in which there was a difference between the surface roughness Ra of the front chamfered portion  13   c  and the surface roughness Ra of the back chamfered portion  12   d  could be obtained simply and easily without cutting a large-size glass sheet or increasing the grain size of a grindstone in multistage steps (equal to or more than three steps). 
     &lt;Manufacturing Test Body&gt; 
     A test body  200  of an on-vehicle display device was manufactured using each of the cover glasses  12  in Case 1 to Case 4 in order to perform a test for making a rigid body model collide therewith (also referred to as “head impact test”). 
     The test body  200  is described with reference to  FIG. 5  to  FIG. 7 . In  FIG. 5  to  FIG. 7 , parts the same as (or corresponding to) those of the on-vehicle display device  100  in  FIG. 4  are referenced correspondingly, and description thereof may be omitted. 
       FIG. 5  is a perspective view of the test body  200 .  FIG. 6  is a sectional view taken on line A-A in  FIG. 5 .  FIG. 7  is a plan view of the test body  200 . 
     As shown in  FIG. 5  and  FIG. 6 , the test body  200  has a housing bottom sheet  107 . Four housing frames  109  having ribs attached thereto internally are disposed on a periphery of the housing bottom sheet  107 . A housing  106  having a rectangular recess portion in its central region is formed by the housing bottom sheet  107  and the four housing frames  109 . A backlight unit  102  and a display panel  104  are disposed inside the housing  106 . 
     As shown in  FIG. 6 , a top-side edge portion of the backlight unit  102  is covered with an L-shaped member  208  having an L-shape in section. The top surface of the L-shaped member  208  and a bottom-side edge portion of the display panel  104  are bonded to each other through a double-sided tape  207 . Therefore, between the display panel  104  and the backlight unit  102 , there is an air gap (1.5 mm) corresponding to the total thickness of the L-shaped member  208  and the double-sided tape  207 . A pressure-sensitive adhesive layer  14  is pasted on the top surface of the display panel  104 . The bottom surface of the cover glass  12  and the top surface of the housing frame  109  are pasted to each other through a double-sided tape  115 . A housing edge frame  110  is disposed outside the edge face of the cover glass  12  and on the top surfaces of the housing frames  109 . The housing edge frame  110  is also pasted to the housing frames  109  through the double-sided tape  115 . 
     As shown in  FIG. 5  and  FIG. 6 , plate-like housing protrusion portions  111  are provided in the four sides of the housing bottom sheet  107  so as to be continuously connected to the housing bottom sheet  107 . A recess portion is formed on the back side (on the opposite side to the backlight unit  102 ) of the housing bottom sheet  107  by the housing bottom sheet  107  and the four housing protrusion portions  111 . A part of a cushion material  321  enters into the recess portion. The cushion material  321  is disposed on a support plate  215  which is a flat plate. The housing  106  is supported by the cushion material  321 . Two pieces of “CF45” (thickness: 25.4 mm) made by K. C. C. Shokai Co., Ltd. put on top of each other are used as the cushion material  321 . In a state where the housing  106  is supported by the cushion material  321 , one ends of fixation portions  301  are bonded to a pair of housing protrusion portions  111  opposed to each other by bolts  311 . The other ends of the fixation portions  301  are bonded to the support plate  215  by bolts  311 . Thus, the housing  106  including the housing protrusion portions  111  is fixedly positioned by the fixation portions  301 . 
     As for each fixation portion  301  which is a plate-like member having an L-shape in section, the dimensions represented by L 1  to L 4  in  FIG. 5  were set as L 1 : 20 mm, L 2 : 50 mm, L 3 : 100 mm, and L 4 : 20 mm. 
     The dimensions represented by H 1  to H 3  and W 1  to W 3  in  FIG. 7  were set as H 1 : 120 mm, H 2 : 150 mm, H 3 : 250 mm, W 1 : 173 mm, W 2 : 250 mm, and W 3 : 350 mm. 
     The other portions were set as follows.
         Pressure-sensitive adhesive layer  14  . . . OCA (“MHM-FWD” made by Nichiei Kakoh Co., Ltd., thickness: 150 μm)   Display panel  104  . . . alternative in which polarizing plates (material: TAC) were pasted on the both sides of a soda lime glass (having a thickness of 1.1 mm and a dimension of 173 mm×120 mm) was used.   Backlight unit  102  . . . alternative in which a bottom surface and four side faces of a plate-like body  102   a  (material: PC, thickness: 4 mm, dimensions: 117 mm×170 mm) were covered with a concave body  102   b  (material: aluminum, thickness: 1 mm) was used.   Double-sided tape  207  . . . material: PET, tape width: 5 mm, thickness: 0.5 mm   L-shaped member  208  . . . material: PVC, thickness: 1 mm, one side length of L-shape: 5 mm   Housing frame  109  . . . material: ABS, thickness: 2 mm   Housing edge frame  110  . . . material: ABS, thickness: 2.5 mm, sheet width: 5 mm   Double-sided tape  115  . . . material: PET, thickness: 0.5 mm   Fixation portion  301  . . . material: iron (SS400), thickness: 1.0 mm   Bolt  311  . . . material: iron   Cushion material  321  . . . two pieces of “CF45” made by K. C. C. Shokai Co., Ltd. (thickness: 25.4 mm) put on top of each other   Support plate  215  . . . material: iron, thickness: 9 mm   Housing bottom sheet  107  and housing protrusion portion  111  . . . material: iron, thickness: 1.15 mm       

     &lt;Evaluation of Impact Resistance in Edge Portion (Head Impact Test)&gt; 
     Using the test body  200  manufactured thus, a head impact test was preformed and the impact resistance in an edge portion of the cover glass  12  was evaluated. 
     The support plate  215  of the test body  200  was placed on a horizontal plane. A not-shown spherical rigid body model (material: iron, diameter: 165 mm, mass: 19.6 kg) was made to fall from a height of 793 mm and collide at a collision position P (see  FIG. 7 ) in the front surface  12   a  of the cover glass  12  at a collision speed of 3.944 m/s so that energy at the collision reached 152.4 J. 
     As for the testing method, “Attachment  28 : Technical Standard of Impact Absorption of Instrument Panel” of “Article  20 : Riding Device” in “Maintenance Standard of Road Transportation Vehicles” (hereinafter simply referred to as “Standard”) represented by the Ministry of Land, Infrastructure and Transport was referred to. In this “Standard”, a spherical rigid body model (material: iron, diameter: 165 mm, mass: 6.8 kg) is shot to collide at a collision speed of 6.7 m/s so that energy at the collision reaches 152.4 J. 
     That is, in the head impact test using the test body  200 , the energy at the collision was made equivalent to that in “Standard”. 
     Deceleration of the rigid body model is stipulated not to exceed 784 m/s 2  (80 G) continuously for 3 ms (milliseconds) or more. It was confirmed that any test performed this time satisfied this stipulation. 
     In view from top of the test body  200 , the collision position P (see  FIG. 7 ) on the cover glass  12  that the rigid body model was made to collide with was closer to one of the fixation portions  301  than the central position and 1 mm inside from the endmost portion of the cover glass  12 . 
     Test bodies  200  using the cover glasses  12  in Case 1 to Case 4 were manufactured, and the head impact test was performed on each of the test bodies  200 . 
     As a result of the test, a cover glass  12  which was not cracked was evaluated as “A”, and a cover glass  12  which was cracked was evaluated as “B”. The evaluations were described in the following Table 1. If a cover glass  12  is evaluated as “A”, the cover glass  12  can be evaluated as excellent in impact resistance in an edge portion thereof 
     &lt;Evaluation of Poor Appearance in Edge Portion&gt; 
     An antireflection film having a thickness of 243 nm was formed on the front surface  12   a  of the cover glass  12  in each of Case 1 to Case 4 by sputtering. On this occasion, it was confirmed that an antireflection film was also formed on the front chamfered portion  13   c.    
     The antireflection film was specifically an antireflection film in which a total of four layers of niobium oxide and silicon oxide were deposited sequentially from the cover glass  12  side. The antireflection film was formed by the manner described in paragraphs [0105] to [0106] in JP 2016-029474 A. 
     Next, a cover glass was removed from a commercially available on-vehicle display device for a rear seat, and the cover glass  12  on which the antireflection film was formed was attached in place to the on-vehicle display device. The used on-vehicle display device for a rear seat was a display device which was of a type in which an edge portion of the cover glass was not received in a housing but was exposed therefrom (see  FIG. 4 ). Therefore, an edge portion of the attached cover glass  12  was not received in the housing but was exposed therefrom. Next, on the following conditions 1 to 3, it was checked whether the edge portion of the cover glass  12  developed a color in gradation to sparkle or not. 
     Condition 1: The cover glass standing perpendicularly to the ground was observed from a place at a distance of 80 cm. 
     Condition 2: The cover glass was observed within a range of 45° at most in a vertical direction from a perpendicular plane to the cover glass. 
     Condition 3: The cover glass was observed on the condition that indoor illuminance was set at 1500 lx (lux). 
     As a result, a cover glass where no gradation was observed was evaluated as “A”, and a cover glass where gradation was observed was evaluated as “B”. The evaluations are described in the following Table 1. If a cover glass is evaluated as “A”, the cover glass can be evaluated as capable of preventing poor appearance in an edge portion. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Case 1 
                 Case 2 
                 Case 3 
                 Case 4 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Thickness t [mm] of glass sheet 
                 0.7 
                 1.1 
                 1.3 
                 2.0 
               
               
                 Etching time [min] 
                 58 
                 92 
                 108 
                 167 
               
            
           
           
               
               
               
               
               
               
            
               
                 Front chamfered portion 
                 Ra [nm] 
                 380 
                 261 
                 298 
                 335 
               
               
                 Back chamfered portion 
                 Ra [nm] 
                 85 
                 80 
                 76 
                 71 
               
            
           
           
               
               
               
               
               
            
               
                 Impact resistance in edge portion 
                 A 
                 A 
                 A 
                 A 
               
               
                 Poor appearance in edge portion 
                 A 
                 A 
                 A 
                 A 
               
               
                   
               
            
           
         
       
     
     &lt;Summary of Evaluation Results&gt; 
     As is apparent from the results shown in Table 1, impact resistance in an edge portion was excellent and poor appearance in the edge portion was prevented when the cover glasses  12  in Case 1 to Case 4 were used. 
     
       
         
           
               
               
             
               
                   
               
             
            
               
                  1 
                 Glass sheet 
               
               
                  1a 
                 One main surface of glass sheet 
               
               
                  1b 
                 The other main surface of glass sheet 
               
               
                  5 
                 Mask material 
               
               
                  12 
                 Small-piece glass sheet (cover glass) 
               
               
                  12a 
                 One main surface of small-piece glass sheet (front surface 
               
               
                   
                 of cover glass) 
               
               
                  12b 
                 The other main surface of small-piece glass sheet (back 
               
               
                   
                 surface of cover glass) 
               
               
                  12c 
                 Chamfered portion on one main surface side of small-piece 
               
               
                   
                 glass sheet 
               
               
                  12d 
                 Chamfered portion on the other main surface side of small- 
               
               
                   
                 piece glass sheet (back chamfered portion of cover glass) 
               
               
                  12e 
                 Side face portion of small-piece glass sheet 
               
               
                  13c 
                 Chamfered portion on one main surface side of small-piece 
               
               
                   
                 glass sheet (front chamfered portion of cover glass) 
               
               
                  13e 
                 Side face portion of small-piece glass sheet (side face 
               
               
                   
                 portion of cover glass) 
               
               
                  14 
                 Pressure-sensitive adhesive layer 
               
               
                 100 
                 On-vehicle display device 
               
               
                 102 
                 Backlight unit 
               
               
                 104 
                 Display panel 
               
               
                 106 
                 Housing 
               
               
                 107 
                 Housing bottom sheet 
               
               
                 109 
                 Housing frame 
               
               
                 110 
                 Housing edge frame 
               
               
                 111 
                 Housing protrusion portion 
               
               
                 115 
                 Double-sided tape 
               
               
                 200 
                 Test body 
               
               
                 207 
                 Double-sided tape 
               
               
                 208 
                 L-shaped member 
               
               
                 215 
                 Support plate 
               
               
                 301 
                 Fixation portion 
               
               
                 311 
                 Bolt 
               
               
                 321 
                 Cushion material 
               
               
                 G 
                 Interval between mask materials adjacent to each other 
               
               
                 P 
                 Collision position 
               
               
                 t 
                 Thickness of glass sheet