Abstract:
Disclosed is a key sheet having a base sheet equipped with a hard resin plate and a key sheet manufacturing method, wherein a reduction is achieved in the thickness of the base sheet and in the effort and cost for the molding. Further, accurate depressing operation is realized without impairing the outward appearance of the key sheet. The key sheet has a hard base plate having a communicating groove connecting through-holes with each other. The communicating groove has a connecting portion formed so as to be continuous and integral with operating portions formed respectively at the through-holes. Thus, there is no need to form inlets for a rubber-like elastic material respectively corresponding to the through-holes in the mold. As a result, the mold structure can be simplified to thereby achieve a reduction in cost. Further, since the entire front or back surface of the hard base plate is not covered with the rubber-like elastic material, it is possible to achieve an overall reduction in the thickness of the base sheet.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a pushbutton switch key sheet for use in an operating portion of various apparatuses, such as a mobile phone, a personal digital assistant (PDA), a car navigation apparatus, or a car audio apparatus, and more particularly to a key sheet having keytops on a rigid base sheet. 
   2. Description of the Related Art 
   As a pushbutton switch key sheet for use in an operating portion of various apparatuses, there is widely used one in which keytops are attached by adhesive to a base sheet formed of a rubber-like elastic material (See JP 11-144549 A). Recently, however, from a viewpoint of outward appearance and diversity in design, there has been increasing a demand for a key sheet using a base sheet using as the base a hard resin plate instead of a base sheet formed of a rubber-like elastic material (See JP 2003-178639 A). 
   In such a key sheet, a base sheet is formed such that a plurality of through-holes are formed in a thin hard resin plate, and each of the through-holes is covered with a thin-walled operating portion consisting of a rubber-like elastic material. A keytop is fixed to each operating portion by an adhesive or the like, thus a key sheet is formed (See JP 2003-178639 A, the first embodiment (paragraph 0014, FIG. 1)). 
   This conventional key sheet, however, has a problem in that its molding takes effort, resulting in a rather high cost. That is, to manufacture the base sheet, it is necessary to form in the mold an inlet for the rubber-like elastic material corresponding to each through-hole of the hard resin plate. After molding, it is also necessary to perform gate cutting for each through-hole. This leads to a rather complicated mold structure, resulting in high cost and an increase in operational burden after molding. 
   Further, when the operating portions are formed by injection molding using a rubber-like elastic material such as a thermoplastic elastomer, there are formed relatively large protrusion-like molding marks on the front or the back surfaces of the operating portions. That is, when the molding marks protrude on the front surfaces of the operating portions, the keytops, which are attached by adhesive or the like, are attached obliquely, thereby impairing the outward appearance of the key sheet. When the molding marks protrude on the back surfaces of the operating portions, the contact switch of the printed circuit board is turned on/off by the molding marks at the time of depression of the keytops, so there is a fear of accurate depressing operation being hindered. Further, in the case of an illumination type key sheet, the existence of the molding marks in the operating portions leads to unevenness in illumination of the keytops, so there is also a fear of the outward appearance of the key sheet when the illumination as been impaired. 
   As means for solving the above-mentioned problems, there has been proposed a key sheet in which a rubber-like elastic material film is formed so as to cover the entire front or back surface of a hard resin plate while filling each through-hole (See JP 2003-178639 A, second embodiment (paragraphs 0016, 0017, FIG. 2), and third embodiment (paragraphs 0018, 0019, FIG. 3)). 
   In this conventional key sheet, however, the rubber-like elastic material film covers one surface of the hard resin plate, which means that the thickness of the rubber-like elastic material film is added to the thickness of the hard resin plate, with the result that the thickness of the base sheet as a whole increases. In particular, this conventional key sheet is not applicable to an apparatus of which a reduction in thickness on the order of millimeters or less is required, as in the case of an electronic device. 
   DISCLOSURE OF THE INVENTION 
   The present invention has been made in view of the above-mentioned problems in the prior art. It is an object of the present invention to provide a key sheet of the type whose base plate is equipped with a hard base plate, such as a hard resin plate, in which it is possible to achieve a reduction in effort and cost for molding while achieving a reduction in thickness, and a method of manufacturing such a key sheet. Another object of the present invention is to provide a key sheet which helps to realize accurate depressing operation without impairing the outward appearance thereof, and a method of manufacturing such a key sheet. 
   In order to attain the above-mentioned objects, the present invention provides a key sheet including: a base sheet having a hard base plate with a plurality of through-holes; operating portions formed of a rubber-like elastic material and elastically supporting key tops at the through-holes so as to allow displacement under pressurization, characterized in that the hard base plate has a communicating groove leading to a plurality of through-holes, and in that a connecting portion connecting operating portions is integrally formed in the communicating groove. 
   In this key sheet, there is formed in the hard base plate a communicating groove leading to the plurality of through-holes to connect the plurality of through-holes as a series of spaces, and there is integrally formed in the communicating groove a connecting portion connecting the operating portions with each other so as to be continuous with the operating portions. As a result, there is no need to form in the mold inlets for the rubber-like elastic material corresponding to the individual through-holes, so the mold structure is simplified, thereby achieving a reduction in cost. Further, since a plurality of operating portions are integrally formed through the connecting portion of the communicating groove, so there is no need to cover the front or back surface of the hard base plate with a rubber-like elastic material, making it possible to achieve a general reduction in the thickness of the base plate. 
   The key sheet is formed such that the upper surfaces of the operating portions elastically supporting the keytops are formed at in-hole positions that are lower than the surface of the hard base plate. In this construction, even if the keytops are fixed to the operating portions through the intermediation of adhesion layers, it is possible to reduce the height of the fixed keytops since the upper surfaces of the operating portions are at in-hole positions that are one step lower than the surface of the hard base plate, thus making it possible to achieve a reduction in the thickness of the key sheet. 
   In the above-described key sheet, the connecting portion may be formed so as to be within the thickness of the hard base plate. In this structure, the connecting portion does not protrude from the surface of the hard base plate, so it is possible to achieve a reduction in the thickness of the key sheet. Further, it is also possible to form on the connecting portion a buffer portion protruding from the surface of the hard base plate. In this structure, when the key sheet is pressurized by an external force, the buffer portion comes into elastic contact with the printed circuit board, so it is possible to avoid large-pressure contact of the hard base plate; thereby preventing damage or deformation of the printed circuit board. 
   The above-described key sheet has a molding mark on the connecting portion. In this construction, even in the case in which the operating portions are formed through injection molding of a thermoplastic elastomer, which leaves a relatively large molding mark, the molding mark is on the connecting portion and on none of the operating portions. As a result, there is no fear of the keytops being obliquely fixed due to protrusion of molding marks on the upper surfaces of the operating portions to thereby impair the outward appearance of the key sheet, or of the contact switch being erroneously turned on due to protrusion of molding marks on the lower surfaces of the operating portions. Further, when the key sheet of the present invention is formed as an illumination type key sheet, it is also possible to prevent unevenness in the illumination of the keytops due to molding marks on the operating portions. 
   In the above-described key sheet, a recess is formed in the connecting portion, and a molding mark exists in the recess. In this construction, even if the connecting portion and the operating portions are formed by injection molding of a thermoplastic elastomer, which leaves a relatively large molding mark, the molding mark remains within the depth of the recess, and does not protrude beyond the surface of the base sheet. As a result, the key sheet can be installed on the printed circuit board of an apparatus in a stable attitude without involving looseness with respect to the printed circuit board. 
   In the above-described key sheet, a plurality of operating portions and a connecting portion are formed of a translucent rubber-like elastic material, and recesses for accommodating inner light sources to be provided on the apparatus are formed in the connecting portion. In this construction, the light emitted from the inner light sources is guided from the translucent connecting portion to the operating portions, making it possible to brightly illuminate the keytops. Further, in this key sheet, the accommodating recesses have molding marks. In this construction, even if the connecting portion and the operating portions are formed through injection molding of a thermoplastic elastomer, which leaves a relatively large molding mark, no molding mark protrudes beyond the surface of the base sheet due to the depth of the accommodating recesses and the inner light sources. As a result, the key sheet can be installed on the printed circuit board of an apparatus without involving looseness. 
   Further, in order to achieve the above-mentioned objects, the present invention provides a key sheet manufacturing method in which a hard base plate having a plurality of through-holes is placed in a mold, operating portions blocking the through-holes is formed through molding of a rubber-like elastic material, key tops are then fixed to the operating portions. This method is characterized by including the steps of: forming the hard base plate having a plurality of through-holes and a communicating groove leading to a plurality of through-holes; forming, integrally on the hard base plate, the plurality of operating portions and a connecting portion connecting the operating portions with each other by pouring the rubber-like elastic material from an inlet of the mold, with the inlet being open at a position corresponding to one of the through-holes or the communicating groove; and fixing the key tops respectively to the operating portions. 
   This key sheet manufacturing method includes the steps of: forming a hard base plate having a plurality of through-holes and a communicating groove leading to the plurality of through-holes; and integrally forming on a hard base plate a plurality of operating portions and a connecting portion connecting the operating portions with each other by pouring a rubber-like elastic material into an inlet of a mold having the inlet at a position corresponding to one of the through-holes or a position corresponding to the communicating groove. Thus, there is no need to form in the mold inlets for the rubber-like elastic material corresponding to the individual through-holes. As a result, the structure of the mold can be simplified, thereby achieving a reduction in cost. Further, a plurality of operating portions are integrally formed through the connecting portion of the communicating groove, so that there is no need to cover the front surface or back surface of the hard base plate with a rubber-like elastic material, making it possible to manufacture a thin key sheet. Further, when using a mold whose inlet is open at a position corresponding to the communicating groove, even if the operating portions are formed through injection molding of a thermoplastic elastomer, which leaves a relatively large molding mark, the molding mark exists on the connecting portion and on none of the operating portions, so there is no disadvantage of the keytops being obliquely fixed to thereby impair the outward appearance of the key sheet due to protrusion of molding marks on the upper surfaces of the operating portions, or of the contact switch being erroneously turned on due to protrusion of molding marks on the lower surfaces of the operating portions. Further, when the key sheet obtained by this manufacturing method is formed as an illumination type key sheet, the key sheet can be one that is free from unevenness in illumination of the keytops due to molding marks on the operating portions. 
   In the step of forming the operating portions and the connecting portion in the key sheet manufacturing method described above, various types of molding methods, such as injection molding, compression molding, and transfer molding, are applicable according to the type of selected rubber-like elastic material forming the operating portions, etc. Further, the two-color molding method is also applicable to the step of molding the hard base plate and the step of molding the operating portions, etc. of the key sheet manufacturing method described above. 
   The communicating groove of the hard base plate in each aspect of the present invention described above is formed as a bottomed groove by partially removing the thickness of the hard base plate, or as a hole-shaped groove extending through the thickness of the hard base plate. 
   Further, the key sheet in each aspect of the present invention described above may be formed as a small-pitch key sheet in which a plurality of closely arranged keytops are collectively exposed through a single opening with no partition frame provided in the casing of an apparatus. In this construction, since the casing has no partition frame, it is possible to make the keytop operating surfaces so much the larger, thereby achieving an improvement in terms of operability. 
   In the key sheet and the key sheet manufacturing method of the present invention, it is possible to achieve a reduction in production cost through simplification of the mold structure although the base sheet is thin, so it is possible to realize a key sheet which is superior in thinness at low cost. Further, it is possible to realize a key sheet superior not only in thinness but also in outward appearance and operability at low cost. 
   The above description of the present invention should not be construed restrictively; the objects, advantages, features, and uses of the present invention will become still more apparent from the following description given with reference to the accompanying drawings. Further, it should be understood that all appropriate modifications made without departing from the gist of the present invention are covered by the scope of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
       FIG. 1  is an outward perspective view of an electronic device equipped with a key sheet according to one of first through third embodiments of the present invention; 
       FIG. 2  is a plan view of the back surface of a key sheet according to the first embodiment of the present invention; 
       FIG. 3  is a sectional view taken along the line III-III of  FIG. 2  for showing the key sheet in use; 
       FIG. 4  is a sectional view taken along the line IV-IV of  FIG. 2  for showing the key sheet in use; 
       FIG. 5  is a sectional view of a mold for illustrating a key sheet manufacturing method of the present invention; 
       FIG. 6  is a plan view of a back surface of a key sheet according to the second embodiment of the present invention; 
       FIGS. 7A and 7B  are sectional views of the key sheet of  FIG. 6 , in which FIG.  7 A is a sectional view of the key sheet taken along the line VII-VII, and  FIG. 7B  is an enlarged main portion sectional view; 
       FIG. 8  is a plan view of the back surface of a key sheet according to the third embodiment of the present invention; 
       FIG. 9  is a sectional view of the key sheet taken along the line IX-IX of  FIG. 8 ; 
       FIG. 10  is a plan view of a back surface of a key sheet according to a fourth embodiment of the present invention; 
       FIG. 11  is an outward perspective view of an electronic device equipped with a key sheet according to the fourth embodiment of the present invention; 
       FIGS. 12A and 12B  are sectional views of the key sheet of  FIG. 10 , in which  FIG. 12A  is a sectional view of the key sheet taken along the line XIIA-XIIA, and  FIG. 12B  is an enlarged main portion sectional view; 
       FIG. 13  is an enlarged main portion sectional view of a modification of the embodiments of the present invention; and 
       FIG. 14  is a sectional view (corresponding to  FIG. 3 ) of a modification of the embodiments of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In the following, embodiments of the key sheet and the key sheet manufacturing method of the present invention will be described with reference to the drawings. In the following description, the present invention will be described, by way of example, as applied to a pushbutton key sheet applicable to an electronic device. 
   First Embodiment 
   FIGS.  1  Through  5   
   As shown in  FIGS. 1 and 2 , a key sheet  11  of the first embodiment is of a type in which keytops  12  are exposed through individual operation openings  3  formed in a casing  2  of an electronic device  1 . This electronic device  1  being a mobile phone, a PDA, a car navigation apparatus, or a car audio apparatus, or any other electronic device having a pushbutton switch key sheet is within the scope of the present invention. 
   As shown in  FIGS. 2 through 4 , the key sheet  11  is composed of a plurality of keytops  12  and a base sheet  13 . Of these, the keytops  12  are fixed to the base sheet  13  through the intermediation of adhesion layers  14  ( FIG. 3 ). When the key sheet  11  is formed as an illumination type key sheet, the material of the keytops  12  is one which is entirely or partially translucent. In this case, the material of the adhesion layer  14  is also translucent. In particular, the material of the adhesion layer  14  is colorless and transparent. 
   The base sheet  13  is equipped with a hard base plate  15  formed of a translucent hard resin as a base. The hard base plate  15  is composed of an annular outer frame portion  16  and a lattice-like inner frame portion  17  extending on the inner side of the annular outer frame portion  16 . 
   On the outer side of the outer frame portion  16 , there is formed a holding/receiving portion  18  formed of a rubber-like elastic material. The back surface of the casing  2  of the electronic device  1  and a printed circuit board  4  are contained in the casing  2  pressurize and hold the holding/receiving portion  18  therebetween, whereby the key sheet  11  is attached to the electronic device  1 . 
   Formed in the inner frame portion  17  are a plurality of accommodation recesses  17   a , where there are accommodated inner light sources  5 , such as chip light-emitting diodes (LEDs), protruding from the printed circuit board  4  of the electronic device  1 . 
   Formed in the hard base plate  15  are a plurality of through-holes  19  defined by the outer frame portion  16  and the inner frame portion  17  in correspondence with the fixing positions of the keytops  12 . In the portions of the inner frame portion  17  between the adjacent through-holes  19 , there are formed bottomed communicating grooves  17   b , which are formed by partially reducing the plate thickness. 
   The through-holes  19  are blocked by operating portions  20  formed of a rubber-like elastic material and adapted to floatingly support the keytops  12  so as to allow displacement under pressurization. The adjacent operating portion  20  is formed serially and integrally through connecting portions  21  filling the communicating grooves  17   b . The upper surfaces of the operating portions  20  are situated within the through-holes  19 , and are one step lower than the surface of the hard base plate  15 . The keytops  12  are fixed to these upper surfaces, which are one step lower, through the intermediation of adhesion layers  14  formed within these upper surfaces. On the lower surfaces of the operating portions  20 , there are formed pushers  20   a , the forward end portions of which are opposed to contact disc springs  6  constituting the contact switches of the printed circuit board  4 . 
   Next, the materials of the components of the key sheet  11  will be described. 
   The material of the hard base plate  15  of the base sheet  13  is rigid enough not to cause the key sheet  11  to be bent by the weight of the keytops  12  even when the electronic device  1  is set upright, or it is placed obliquely or with the keytop  12  side thereof facing downwards. More specifically, examples of the materials for the hard base plate  15  include a polycarbonate resin, a polymethyl methacrylate resin, a polyproylene resin, polystyrene-based resins, polyacrylic-based copolymer resins, polyolefin-based resins, an acrylonitrile butadiene styrene resin, polyester-based resins, epoxy-based resins, polyurethane-based resins, a polyamide resin, and silicone-based resins. The key sheet  11  of this embodiment is an illumination type key sheet having accommodation recesses  17   a  for the inner light sources  5  in the hard base plate  15 . Since light is guided through the plate thickness of the hard base plate  15 , there is used, in particular, a colorless transparent material for the hard base plate  15 . More specifically, of those, a transparent polycarbonate resin or a transparent polypropyrene resin is particularly used as a preferable material. 
   As the rubber-like elastic material forming the holding/receiving portion  18 , the operating portion  20 , and the connecting portions  21 , it is possible to use a thermosetting elastomer with satisfactory resiliency, such as silicone rubber, isoprene rubber, ethylene propylene rubber, butadiene rubber, chloroprene rubber, or natural rubber. Further, it is also possible to use as this rubber-like elastic material a thermoplastic elastomer such as styrene-based, ester-based, urethane-based, olefin-based, amide-based, butadiene-based, ethylene-vinyl-acetate-based, fluoro-rubber-based, isoprene-based, or chlorinated polyethylene based thermoplastic elastomer. Of these, silicone rubber, styrene-based thermoplastic elastomers, and ester-based thermoplastic elastomers make it possible to obtain a key sheet  11  with particularly satisfactory resiliency for the operating portions  20  and high durability. 
   The keytops  12  may be formed of the same material as the hard base plate  15 , the operating portions  20 , and the connecting portions  21 . Further, due to the rigidity of the hard base plate  15 , it is also possible to use as the hard base plate  15  a heavy material, such as metal or wood. Further, as the adhesion layers  14  for the adhesion of the keytops  12 , it is possible to use an ultraviolet setting type adhesive that cures in seconds. 
   Next, a method of manufacturing the key sheet  11  will be described. The keytops  12  and the base sheet  13  are produced separately. In producing the base sheet  13 , the hard base plate  15  is obtained by molding, such as injection molding. When a thermosetting elastomer is selected as the rubber-like elastic material for the holding/receiving portion  18 , the operating portions  20 , and the connecting portions  21 , the hard base plate  15  is placed in a cavity  24   a  of a mold  24  composed of an upper mold  22  and a lower mold  23 . When a thermoplastic elastomer is selected, the hard base plate  15  is placed in the cavity  24   a  of the mold  24  for injection molding. Then, liquid elastomer is poured in from inlets  24   b  opening above the communicating grooves  17   b  to effect molding. At this time, the communicating grooves  17   b  communicate with the adjacent through-holes  19 , so the liquid elastomer is poured into both through-holes  19 . Thereafter, a post processing according to the material, whereby it is possible to obtain the base sheet  13 , in which the operating portions  20 , and the connecting portions  21  are integrally formed on the hard base plate  15 . Instead of this manufacturing method, there is also a possible method in which the hard base plate  15  and the operating portions  20 , and the connecting portions  21  are formed by two-color molding. Finally, predetermined keytops  12  are fixed to the respective operating portions  20  through the intermediation of the adhesive layers  14 , whereby the key sheet  11  of this embodiment is obtained. 
   Next, the operation and effect of the key sheet  11  of this embodiment and the manufacturing method thereof will be described. 
   The communicating grooves  17   b  leading to the plurality of through-holes  19  are formed in the hard base plate  15  to connect the plurality of through-holes  19  as a series of spaces, and the connecting portions  21  connecting the operating portions  20  to the communicating grooves  17   b  are formed integrally as moldings connected to the operating portions  20 , so there is no need to form in the mold  24  inlets  24   b  corresponding to the individual through-holes  19 . As a result, the mold structure can be simplified, thereby achieving a reduction in cost. Further, since the plurality of operating portions  20  are integrally formed through the connecting portions  21  in the communicating grooves  17   b , there is no need to cover the front surface or the back surface of the hard base plate  15  with a rubber-like elastic material, thereby achieving a general reduction in the thickness of the base sheet  13 . 
   The upper surfaces of the operating portions  20  elastically supporting the keytops  12  are formed at the in-hole positions of the through-holes  19  lower than the surface of the hard base plate  15 . As a result, it is possible to reduce the height by which the fixed keytops  12  protrude from the hard base plate  15 , thereby achieving a reduction in the thickness of the key sheet  11 . 
   Since the connecting portions  21  are formed within the plate thickness of the hard base plate  15 , the connecting portions  21  do not protrude beyond the surface of the hard base plate  15 , thereby achieving a reduction in the thickness of the key sheet  11 . 
   Even in the case in which the connecting portions  21  have inlets  24   b  for the rubber-like elastic material and in which the operating portions  20  are formed by injection molding of thermoplastic elastomer, which leaves relatively large molding marks, the molding marks exist on none of the operating portions  20  but on the connecting portions  21 . Thus, there is no disadvantage of the keytops  12  being obliquely fixed due to the protrusion of molding marks on the upper surfaces of the operating portions  20  to thereby impair the outward appearance of the key sheet  11 . Further, nor is there fear of the contact disc springs  6  being erroneously turned on due to the protrusion of the molding marks on the lower surfaces of the operating portions  20 . Further, even when the key sheet  11  is formed as an illumination type key sheet, it is also possible to avoid unevenness in illumination of the keytops  12  since there are no molding marks on the operating portions  20 . 
   Since the hard base plate  15  is formed of a translucent resin, the light emitted from the inner light sources  5  is guided from the accommodation recesses  17   a  throughout the plate thickness of the translucent hard base plate  15 , so that it is possible to brightly illuminate the entire surface of the key sheet  11 . Thus, even when the use of a separate light guide plate is abolished, it is possible to brightly illuminate the keytops  12 . 
   The adhesion layers  14  are applied not to the entire upper surfaces of the operating portions  20  but partially to portions within the upper surfaces thereof, so that when the keytops  12  are depressed, the operating portions  20  undergo elastic deformation on the outer periphery side of the adhesion layers  14 , making it possible for the keytops  12  to be displaced. Thus, there is no need to form thick-walled protrusions on the upper surfaces of the operating portions  20  in order to raise the back surfaces of the keytops  12  from the above-mentioned upper surface, whereby the base sheet  13  can be so much the thinner and lighter. 
   The holding/receiving portion  18  is formed over the entire periphery of the outer frame portion  16  of the hard base plate  15 , and is held between the back surface of the casing  2  and the printed circuit board  4 . As a result, watertightness of the electronic device  1  may be secured. 
   Second Embodiment 
   FIGS.  6 ,  7 A, and  7 B 
   As shown in  FIGS. 6 ,  7 A, and  7 B, a key sheet  25  according to the second embodiment differs from the key sheet  11  of the first embodiment in that recesses  21   a  are formed in the connecting portions  21 , and molding marks  21   b  exist in the recesses. Otherwise, the operation and effects of this embodiment are the same as those of the first embodiment. 
   That is, in this embodiment, the holding/receiving portion  18 , the operating portions  20 , and the connecting portions  21  are formed by injection molding of thermoplastic elastomer, which leaves relatively large molding marks  21   b . However, even if injection molding of a thermoplastic elastomer is effected, the molding marks  21   b  remain within the depth of the recesses  21   a , and do not protrude beyond the surface of the base sheet  13 . Thus, it is advantageously possible to install the key sheet  25  in a stable attitude without involving any looseness with respect to the printed circuit board  4 . 
   Third Embodiment 
   FIGS.  8  and  9   
     FIGS. 8 and 9  show a key sheet  26  according to the third embodiment. The key sheet  26  of the third embodiment differs from the key sheet  11  of the first embodiment in the construction of communicating grooves  27   a  of an inner frame portion  27  and connecting portions  28 . That is, each bottomed communicating groove  27   a  formed in the hard base plate  15  leads to three through-holes  19 , with the connecting portion  28  being integrally formed so as to connect them together. 
   In the connecting portions  28 , there are formed accommodation recesses  28   a  for the inner light sources  5 , which means that the key sheet  26  is an illumination type key sheet, in which light is guided to the operating portions  20  through the wall thickness of the connecting portions  28 . Thus, of the materials mentioned with reference to the first embodiment, a colorless and transparent material is used as the material as the rubber elastic material forming the connecting portions  28 . More specifically, a colorless and transparent silicone rubber is used in this embodiment. Further, for light guidance through the connecting portions  28 , of the materials as mentioned with reference to the first embodiment, one differing from the connecting portions  28  in refractive index is used as the material of the hard base plate  15  of this embodiment. More specifically, a white polycarbonate resin is used in this embodiment. By thus forming the connecting portions  28  of a colorless and transparent material and the hard base plate  15  of a white material, the light from the inner light sources  5  is guided from the connecting portions  28  to the operating portions  20  while being reflected at the interface between the connecting portions  28  and the hard base plate  15 , making it possible to brightly illuminate the keytops  12 . Further, since the distance from the accommodation recesses  28   a  to the operating portions  20  is the same all over, it is also possible to attain uniform illumination for the keytops  12 . The light entering the hard base plate  15  is diffused by the white pigment, and is not absorbed. 
   Further, the connecting portions  28  are formed of a colorless and transparent silicone rubber, which is a thermosetting elastomer. Thus, unlike a thermoplastic elastomer, which leaves relatively large molding marks, no large molding marks are left. Thus, it is possible to use the mold  24  of the first embodiment as shown in  FIG. 5 , in which the inlets  24   b  are open above the communicating grooves  17   b . In this embodiment, however, it is also possible to use a mold in which inlets are open above the operating portions  20 . 
   Fourth Embodiment 
   FIGS.  10 ,  11 ,  12 A, and  12 B 
   A key sheet  29  according to the fourth embodiment is a modification of the key sheet  26  of the third embodiment. As shown in  FIG. 11 , the key sheet  29  is formed as a small-pitch key sheet, in which the plurality of keytops  30  are collectively exposed through the operation opening  7  with no partition frame formed in the casing  2  of the electronic device  1 . The interval between the keytops  30  is very small and ranges, for example, approximately from 0.15 mm to 0.2 mm; the distance between the keytops and the operation opening  7  is approximately as small as that. 
   Further, as shown in  FIGS. 10 ,  11 ,  12 A, and  12 B, molding marks  28   b  exist in the accommodation recesses  28   a  of the connecting portions  28 . That is, in this embodiment, the operating portions  20  and the connecting portions  28  are formed through injection molding of a thermoplastic elastomer. More specifically, a colorless and transparent styrene-based thermoplastic elastomer is used as the material. If the holding/receiving portion  18 , the operating portions  20 , and the connecting portions  28  are formed through injection molding of a thermoplastic elastomer, which leaves relatively large molding marks  28   b , the molding marks  28   b  remain within the accommodation recesses  28   a . Thus, it is advantageously possible to install the key sheet  29  in a stable attitude without involving any looseness with respect to the printed circuit board  4 . Further, specifically speaking, the hard base plate  15  is formed of a black and light-blocking polypropylene resin. Thus, while a part of it is absorbed by the hard base plate  15 , the light emitted from the inner light sources  5  enters the operating portions  20  through the connecting portions  28 , making it possible to illuminate the keytops  30  effectively only from the back side thereof. 
   Modifications of the Embodiments 
   Modifications of the above-described embodiments will be described below. 
   While in the above-described embodiments the communicating grooves  17   b ,  27   a  are formed without allowing them to extend through the wall thickness of the inner frame portion  17 ,  27 , it is also possible to form them as through-holes as shown in  FIG. 14 . Further, while in the above-described embodiments the communicating grooves  17   b ,  27   a  are formed in the back surface of the hard base plate  15 , it is also possible to form them in the front surface of the hard base plate  15 , with the connecting portions  21 ,  28  being formed therein. 
   While in the above-described embodiments the connecting portions  21 ,  28  do not protrude from the back surface of the hard base plate  15 , as shown, for example, in  FIG. 13 , in the case of the key sheet  11  of the first embodiment, it is also possible to form on the connecting portions  21  buffer portions  31  downwardly protruding from the back surface of the hard base plate  15 . In this construction, when the key sheet  11  is pressurized by an external force, the buffer portions  31  come into elastic contact with the printed circuit board  4 , making it possible to avoid large-pressure contact of the hard base plate  15  (inner frame portion  17 ) and to suppress damage or deformation of the printed circuit board  4 . 
   In the above-described embodiments, it is possible to form the keytops  12 ,  30  of a rubber-like elastic material, such as a thermoplastic resin, a thermosetting resin, silicone rubber, or a thermoplastic elastomer. Further, it is possible to form on the keytops  12 ,  30  display portions indicating characters, figures, symbols, etc. in ink or by plating or the like. Further, the keytops  12 ,  30  may also be formed as open-character-illumination type keytops or character-illumination type keytops. Further, the three-dimensional configuration of the keytops  12 ,  30  may be other than that described above. 
   While in the above-described embodiments the operating portions  20  are rectangular in plan view, they may also be of a circular, an elliptical, or some other polygonal configuration. Further, the configuration of the base sheet  13  and the configuration of the connecting portions  21 ,  28  are not restricted to those of the above-described embodiments; it is also possible to form them in some other configurations. 
   While the key sheet  11 ,  25 ,  26 ,  29  of the above-described embodiments is used in the electronic device  1 , it may also be formed as the key sheet of some other apparatuses, such as a PDA or a remote controller.