Patent Publication Number: US-6906274-B2

Title: Movable contact unit having press-down projections

Description:
TECHNICAL FIELD 
   The present invention relates to a movable contact unit provided with press-down projections used in a control panel and the like of a variety of electronic apparatuses. 
   BACKGROUND OF THE INVENTION 
   With the increase in variety of electronic apparatuses for portable use such as cellular phones, portable audio-video equipment, and the like in recent years, there is continued advancement in multi-functional capability as well as reduction in size, thickness and weight. Movable contact units of a type having a plurality of movable contacts retained on an insulating plastic film are used in large number as switches for control panels of the electronic apparatuses, as they can be made thin, and provide good tactile response as well as stable electrical contact. The movable contacts are made of electrically conductive metal sheets formed into a dome-like shape. 
   Since these apparatuses are being carried and used anywhere, the switches are operated so frequently that they require movable contact units of high durability and stable tactile response in the switching operation. 
   Referring now to  FIG. 7 , a description is provided hereinafter of a conventional movable contact unit of such kind used for the switch of a control panel. 
   In the conventional movable contact unit, as shown in  FIG. 7 , a plurality of dome-like movable contacts  3  are arranged independently with respect to one another, with their upper surfaces retained by adhesion of adhesive layer  2  formed on an underside surface of flexible insulating base film  1  having an external shape formed into a predetermined shape. 
   Movable contacts  3  are sandwiched between separator  4  made of an insulating film having a surface treated with release agent and base film  1 . Separator  4  is held adhered to adhesive layer  2  on base film  1  in a manner that it covers the entire underside surface of base film  1 . 
   As separator  4  is placed in close adhesion to base film  1  in a manner to completely cover the outer peripheries of movable contacts  3 , it prevents corrosion of movable contacts  3 . In addition, separator  4  also prevents adhesive layer  2  on base film  1  from sticking to other surfaces and gathering foreign objects unintendedly during transportation and in the storage. 
   The conventional movable contact unit further has small cylindrical parts  5 A, which are fabricated by die-cutting a plastic film into a cylindrical shape, and bonded to base film  1  by adhesive resin  5 B composed of epoxy acrylate, i.e. a kind of resin curable by ultraviolet rays. Each of cylindrical parts  5 A and adhesive resin  5 B compose press-down projection  5 . 
     FIG. 8  shows a structure of the conventional movable contact unit in the actual application. After separator  4  is peeled off, base film  1  bearing movable contacts  3  is attached to wiring board  7  using adhesive layer  2  so that movable contacts  3  are aligned in a manner to face with their respective sets of stationary contacts  6  ( 6 A and  6 B). 
     FIG. 8  depicts wiring board  7  with the movable contact unit attached to it. The center portion of movable contact  3  confronts the respective one of central stationary contacts  6 A, and the peripheral edge of movable contact  3  lies on the corresponding outer stationary contact  6 B, so that each movable contact  3  and the corresponding stationary contacts  6  compose a single switch. In addition, actuator button  8  of the apparatus is arranged above press-down projection  5 . 
   A switch for control panel using the conventional movable contact unit constructed as above operates in a manner which will be described now with reference to FIG.  9 . 
   When actuator button  8  is depressed in a direction shown by an arrow in  FIG. 9 , the depressing force is given on the upper surface of cylindrical press-down projection  5 , which is in contact to underside surface  8 A of actuator button  8 . The depressing force is thus applied to the center portion of movable contact  3  through press-down projection  5  and base film  1 . 
   When the depressing force exceeds an inflectional strength of movable contact  3  of the dome-like shape, it bends into an inverted shape with a click-feeling, and an underside surface in the center of movable contact  3  comes in contact with central stationary contact  6 A. This makes an electrical connection between central stationary contact  6 A and outer stationary contact  6 B on wiring board  7  via movable contact  3 . 
   When the depressing force is removed, movable contact  3  regains its original shape by an elastic restoring force of its own. This separates the underside surface in the center of movable contact  3  from central stationary contact  6 A, to restore the state of electrical isolation between the central stationary contact  6 A and the outer stationary contact  6 B of wiring board  7 . By allowing the optimum click-feeling in the depressing operation, the above structure provides the control panel switch with good tactile response. 
   Due to the rapid-paced advancement in the multi-functional capability in addition to downsizing of portable apparatuses in the recent years, a number of functions assigned to a single switch increases, which consequently increases frequency of depressing operation of the switch. There is thus growing demand for improvement of durability to the switching operation. 
   In other words, the desire continues to increase for a movable contact unit designed to increase strength of adhesion of press-down projection  5  to base film  1  with steadiness of the tactile response even after repeated operations. 
   SUMMARY OF THE INVENTION 
   The present invention addresses the above problems, and it is intended to provide a movable contact unit having press-down projections which can maintain the press-down projections in a securely bonded condition to a base film without increasing a number of components used and necessitating a complicated manufacturing process. 
   To achieve the above object, the movable contact unit of this invention comprises a plurality of dome-like movable contacts made of electrically conductive metal sheets capable of making an inflective action when depressed, a base film having an adhesive layer for retaining upper surfaces of the plurality of movable contacts, and a plurality of press-down projections bonded by adhesive to an upper surface of the base film in positions corresponding to center portions of the respective movable contacts, wherein the upper surface of the base film and underside surfaces of the press-down projections are each provided with a pretreated layer. The above structure has an advantage of strengthening the bonding between the adhesive and each of the pretreated layers formed on the adhering surfaces of the base film and the press-down projections, so as to provide the movable contact unit with outstanding reliability in maintaining the bonded condition for a long duration. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a sectioned front view of a movable contact unit with press-down projections according to an exemplary embodiment of the present invention. 
       FIG. 2  is a sectioned view of a portion of a control panel switch provided with the movable contact unit according to the exemplary embodiment of this invention. 
       FIG. 3  is a drawing illustrating operation of the control panel switch shown in FIG.  2 . 
       FIG. 4  is a drawing illustrating a method of measuring bonding strength of the press-down projections of the movable contact unit according to the exemplary embodiment of this invention. 
       FIG. 5  is a graphical representation showing a test result of the bonding strength corresponding to number of depressing operations. 
       FIG. 6  is a graphical representation showing another test result of the bonding strength corresponding to number of depressing operations under high temperature and high humidity environment. 
       FIG. 7  is a sectioned front view of a movable contact unit of the related art. 
       FIG. 8  is a sectioned view of a portion of a control panel switch provided with the movable contact unit of the related art. 
       FIG. 9  is a drawing illustrating operation of the control panel switch of the related art. 
   

   DESCRIPTION OF THE INVENTION 
   Referring to FIG.  1  through  FIG. 6 , description is provided hereinafter of an exemplary embodiment of the present invention. 
   (Exemplary Embodiment) 
   Like reference numerals are used to denote like structural components as those described in the background techniques section, and their details will be skipped. 
     FIG. 1  shows movable contact unit  100  provided with press-down projections according to the present invention. As shown in  FIG. 1 , base film  11  composed of a flexible resin film. (made of PET) having an external shape formed into a predetermined shape has an underside surface (hereinafter referred to as first film surface)  112  and an upper surface (hereinafter referred to as second film surface)  111 . Second film surface  111  is covered with a resin layer serving as a pretreated layer (hereinafter designated first pretreated layer)  13  formed by coating Ester resin. 
   Base film  11  has adhesive layer  12  covering an entire surface of first film surface  112 , on which movable contacts  14  made of electrically conductive metal sheets formed into a dome-like shape are bonded securely. The plurality of movable contacts  14  are positioned independently with respect to one another into a predetermined layout, with their upper surfaces retained by adhesion. Movable contacts  14  positioned independently here means that they are not in contact with each other. 
   Base film  11  may be so prefabricated that portions corresponding to respective movable contacts  14  are convexed to fit the dome-like shape of movable contacts  14 . 
   Separator  15  made of an insulating plastic film having a surface treated with agent is adhered to adhesive layer  12  to cover the entire area of first film surface  112  of base film  11  in the same manner as the related example, in order to prevent corrosion of movable contacts  14 , gathering of foreign objects and the like. 
   In addition, small cylindrical parts (i.e. press-down members)  16  are bonded by adhesive resin  18  on their underside surfaces  162  to second film surface  111  of base film  11  in respective positions corresponding to the center portions of dome-like movable contacts  14 , to thus compose press-down projections  19  of this exemplary embodiment of the invention. Each of small cylindrical parts  16  made of a PET film has resin layer (hereinafter designated second pretreated layer)  17  composed of Ester resin coated on its underside surface  162 . Second pretreated layer  17  is bonded to base film  11  with adhesive resin  18 . Accordingly, small cylindrical part  16 , adhesive resin  18  and second pretreated layer  17  compose each of press-down projections  19 . On the other hand, adhesive resin  18  bonds between second pretreated layer  17  on small cylindrical part  16  and first pretreated layer  13  on base film  11 , since base film  11  has first pretreated layer  13  formed over second film surface  111 . A main ingredient of adhesive resin  18  used in this exemplary embodiment is urethane acrylate, which is a kind of UV-curable resin. 
   What has been described above is the structure of movable contact unit  100  provided with a plurality of movable contacts  14 , base film  11 , and press-down projections  19  in order of the manufacturing process. 
   Next,  FIG. 2  shows a control panel switch assembled with the movable contact unit having press-down projections according to this exemplary embodiment. Separator  15  is peeled off first from the movable contact unit having press-down projections, which bears separator  15  adhered to it as shown in FIG.  1 . The movable contact unit is attached to wiring board  7  using adhesive layer  12  on the underside of base film  11 , while movable contacts  14  are aligned over wiring board  7  having an array of stationary contacts  6  ( 6 A and  6 B), in a manner that movable contacts  14  face the corresponding sets of stationary contacts  6 . 
   Here, each of movable contacts  14  is positioned with its peripheral edge lying on the outer stationary contact  6 B so that the bottom center portion confronts the central stationary contacts  6 A. Each of movable contacts  14  thus composes individual switch in combination with the corresponding stationary contacts  6 . 
   Actuator buttons  8  of the apparatus are then arranged above press-down projections  19  in a corresponding manner. Accordingly, small cylindrical parts  16  come in contact with actuator buttons  8  at their upper surfaces  161 . 
   In the control panel switch of the above structure, depression of actuator button  8  in the direction of an arrow shown in  FIG. 3  inflects movable contact  14  into an inverted shape, and causes the center portion of movable contact  14  lying on outer stationary contact  6 B to come into contact with central stationary contact  6 A, so as to make switching operation. 
   Evaluation 
   Description is given next of test methods and results of various comparison evaluations conducted on the movable contact unit having press-down projections according to this exemplary embodiment. 
   Describing first pertains to details of measurements made on a plurality of samples produced for bonding strength between press-down projections  19  and base film  11 . 
   1. Embodied Samples  1  Through  4   
   Adhesive resin  18  used in the samples for the evaluation of bonding strength is a kind of resin having a main ingredient of urethane acrylate with four levels of hardness ranging between HDD80 and HDD65 at intervals of HDD5 as measured by a type D durometer specified in JIS Standard K-7215 (hereinafter referred to as durometer hardness), after the resin has been cured. 
   For this evaluation, tests were conducted on specially made samples provided with round dome-like movable contacts  14  having 4 mm in outer diameter, and press-down projections  19  having five different outer diameters ranging from 0.5 mm to 2.5 mm at intervals of 0.5 mm. 
   2. Related Art Samples 
   Equivalent tests were carried out on separately prepared samples of the related art, which employ epoxy acrylate resin with durometer hardness of HDD80 as adhesive resin  5 B, and provided with same movable contacts  14  and press-down projections  19  as those of the embodied samples. 
   Test 1. Measurement of Bonding Strength 
   The samples prepared for measurement of bonding strength are as shown in FIG.  4 . Movable contacts  14  and separator  15  placed on base film  11  were removed from each sample of movable contact units  100  described in the above exemplary embodiment. In addition, the convexed portions in the areas where movable contacts  14  were located on base film  11  were flattened, and made the base film into generally a flat plate-like shape. After the above preparation, a shearing load was applied in a direction indicated by an arrow in  FIG. 4  on the side of small cylindrical part  16  bonded to first film surface  111  of each sample, and a strength of the bonding was measured when press-down projection  19  came off. 
   The results are shown in Table 1. 
   
     
       
         
             
             
           
             
                 
               TABLE 1 
             
           
          
             
                 
                 
             
             
                 
               Bonding Strength 
             
          
         
         
             
             
             
             
             
             
          
             
                 
               Related art 
               Embodied 
               Embodied 
               Embodied 
               Embodied 
             
             
                 
               Samples 
               Samples 1 
               Samples 2 
               Samples 3 
               Samples 4 
             
             
               Dia. of 
               Hardness: 
               Hardness: 
               Hardness: 
               Hardness: 
               Hardness: 
             
             
               Press- 
               HDD80 
               HDD80 
               HDD75 
               HDD70 
               HDD65 
             
          
         
         
             
             
             
             
             
             
             
             
             
             
             
          
             
               down 
               Bonding 
               Judge- 
               Bonding 
               Judge- 
               Bonding 
               Judge- 
               Bonding 
               Judge- 
               Bonding 
               Judje- 
             
             
               Projection 
               strength 
               ment 
               strength 
               ment 
               strength 
               ment 
               strength 
               ment 
               strength 
               ment 
             
             
                 
             
          
         
         
             
             
             
             
             
             
             
             
             
             
             
          
             
               0.5 mm 
               9 
               X 
               9 
               X 
               9 
               X 
               7 
               X 
               5 
               X 
             
             
               1.0 mm 
               13 
               O 
               19 
               O 
               16 
               O 
               14 
               O 
               10 
               O 
             
             
               1.5 mm 
               18 
               O 
               26 
               O 
               23 
               O 
               21 
               O 
               20 
               O 
             
             
               2.0 mm 
               23 
               O 
               34 
               O 
               31 
               O 
               28 
               O 
               25 
               O 
             
             
               2.5 mm 
               28 
               O 
               39 
               O 
               37 
               O 
               35 
               O 
               31 
               O 
             
             
                 
             
             
               (Unit of bonding strength: N)  
             
          
         
       
     
   
   Since the desired bonding strength is 10 N (Newtons) or greater as required for the practical application, a mark “O” is used in the above table when the bonding strength is 10 N or greater, and another mark “X” when the strength is less than 10 N. 
   Upon examination of the individual tested samples, it was confirmed initially that press-down projections  5  had been torn off between the surfaces of base films  1  and adhesive resins  5 B shown in  FIG. 7  in the related art samples. It was thus considered that the bonding strength depends on the adhesion between those components. 
   On the other hand, the embodied samples  1  through  4  showed that press-down projections  19  have been torn off together with base films  11  at the areas where the projections  19  are bonded. 
   This result indicates strong adhesion between base films  11  made of PET and first pretreated layers  13  formed of coated Ester resin, as well as cylindrical parts  16  and second pretreated layers  17 , and also strong adhesion between first pretreated layers  13  and UV-cured adhesive resin  18  composed of urethane acrylate or between second pretreated layers  17  and the UV-cured adhesive resin  18 . 
   First pretreated layers  13  and second pretreated layers  17  formed of Ester resin, when provided in the above manner, improve wettability of base films  11  made of PET and cylindrical parts (i.e. press-down members)  16  to the adhesive resin, thereby providing the strong bonding therebetween. 
   As shown in Table 1, the embodied samples  1  through  4  have comparatively higher bonding strength than the related art samples, except for the one having press-down projections  19  of 0.5 mm in the outer diameter, and the higher the durometer hardness of adhesive resin  18 , the greater the bonding strength obtained by it. It was determined here that adhesive resin  18  gets better adhesion to base film  11  the greater the hardness of it becomes after cured by the ultraviolet rays. 
   However, the embodied samples  3  and  4  were not superior in strength than the related art samples when outer diameters of their press-down projections  19  were 0.5 mm or less and 1.0 mm or less respectively. In consideration of the torn-off conditions, it was determined that press-down projections  19  in the diameter of 1.0 mm can satisfy the standard value (i.e. 10 N or greater in the bonding strength), and have durability equal to or greater than the related art devices even when unevenness of the adhesive and the like are taken into account. 
   Furthermore, the results showed lack of the bonding strength even on the embodied samples  1  through  3  having adhesive resin  18  of durometer hardness HDD80 to 70 when press-down projections  19  were 0.5 mm in diameter, although they had sufficient strength when the diameter was 1.0 mm to 2.5 mm. 
   The related art samples also showed similar results. In consideration of the fact that movable contacts  14  are 4 mm in outer diameter, and according to the above test results, it was confirmed desirable that press-down projections  19  have a diameter equal to or larger than ¼ of the diameter of movable contacts  14 . Likewise, press-down projections  19  may have a surface area equal to or larger than {fraction (1/16)} of the movable contacts  14  because they are ¼ in the outer diameter. 
   Test 2. Evaluation of Tactile Response 
   Next, same samples as the embodied samples  3  were used for depressing operation, and their tactile responses were evaluated by comparison. 
   Table 2 shows the results. A mark “0” is used for a sample exhibiting good tactile response and another mark “X” for any sample exhibiting poor tactile response in Table 2. 
   
     
       
         
             
             
             
           
             
                 
               TABLE 2 
             
           
          
             
                 
                 
             
             
                 
               Tactile Response to Operation 
                 
             
          
         
         
             
             
             
          
             
               Diameter of 
               Related art Samples 
               Embodied Samples 3 
             
             
               Press-down 
               Hardness: HDD80 
               Hardness: HDD70 
             
             
               Projection 
               Judgment 
               Judgment 
             
             
                 
             
             
               0.5 mm 
               O 
               O 
             
             
               1.0 mm 
               O 
               O 
             
             
               1.5 mm 
               O 
               O 
             
             
               2.0 mm 
               O 
               O 
             
             
               2.5 mm 
               X 
               X 
             
             
                 
             
             
               Marks for judgment: “O” denotes good tactile response, and “X” denotes poor tactile response.  
             
          
         
       
     
   
   As shown in Table 2, both the related art samples and the embodied samples  3  exhibited the same result of tactile response to the depressing operation. In other words, the samples having press-down projections  19  ranging from 0.5 mm to 2.0 mm in outer diameter gave excellent click feelings, and the samples having 2.5 mm diameter gave poor click feelings. 
   These poor tactile responses are considered attributable to the outer diameter of press-down projection  19  bonded to round dome-like movable contact  14 , in that the diameter of press-down projection  19 , if made closer to the outer diameter of movable contact  14 , overlies near the rounded surface of movable contact  14 , which impedes the springy click motion of the dome-like movable contact  14  although it is still inflective. 
   The above results also showed that the tendency of changes in the tactile response is not dependent upon presence or absence of the coating of Ester resin serving the pretreated layer, or properties of the UV-curable resin material, but it depends only on size of press-down projection  19 . In other words, the same results were obtained on the samples irrespective of kind of the UV-curable resin, between Urethane material and epoxy-base material. 
   According to the above result of examination on the tactile response, it was found that press-down projections  19  of ½ or less in diameter of the movable contacts  14  is desirable. It is therefore desirable that press-down projections  19  have a relative surface area of ¼ or less to movable contacts  14 . 
   Test 3. Durability Test 
   As an evaluation of durability, test was carried out next on the bonding strength with respect to number of depressing operations. 
   Test Method 
   First, separator  15  was peeled off, and the movable contact unit having the press-down projections was placed on wiring board  7 , to compose a panel switch as shown in FIG.  2 . Next, samples of the switch were operated by depressing and releasing press-down projections  19  from the direction shown by an arrow in  FIG. 3  with a thrusting force of 3 N. The operation was repeated by counting each inflection and restoration of movable contact  14  as one cycle. On the other hand, bonding strengths were measured by the method described previously with reference to  FIG. 4 , at the start of testing, after completion of operations for 100,000 cycles, 500,000 cycles, 1,000,000 cycles, and 2,000,000 cycles. 
   Test samples 
   The tests were performed on samples prepared using press-down projections  19  having an outer diameter of 1.5 mm, and four different types of adhesive resin  18  in the hardness of HDD 80, 75, 70 and 65 (hereinafter designated as embodied samples  5  to  8 ). 
   In addition, the same tests were also conducted for the comparison purpose on the related art samples prepared by using press-down projections  19  of 1.5 mm diameter among those samples described above. 
   Test Result 1 
   The results are shown in Table 3. 
   
     
       
         
             
             
             
             
             
             
           
             
               TABLE 3 
             
             
                 
             
             
                 
               Related 
                 
                 
                 
                 
             
             
                 
               art 
               Embodied 
               Embodied 
               Embodied 
               Embodied 
             
             
               Test Samples 
               samples 
               sample 5 
               sample 6 
               sample 7 
               sample 8 
             
             
                 
             
           
          
             
               Coating resin 
               None 
               Ester resin 
               Ester resin 
               Ester resin 
               Ester resin 
             
             
               Adhesive 
             
             
               resin 
             
             
               (UV-curable 
             
             
               resin) 
             
             
               Basic 
               Epoxy 
               Urethane 
               Urethane 
               Urethane 
               Urethane 
             
             
               composition 
               Acrylate 
               Acrylate 
               Acrylate 
               Acrylate 
               Acrylate 
             
             
               Hardness 
               HDD80 
               HDD80 
               HDD75 
               HDD70 
               HDD65 
             
             
               Number of 
             
             
               Operations 
             
             
               Start 
               18 
               26 
               23 
               21 
               20 
             
             
               100,000 
               16 
               24 
               21 
               20 
               19 
             
             
               cycles 
             
             
               500,000 
               14 
               17 
               20 
               18 
               18 
             
             
               cycles 
             
             
               1,000,000 
               12 
               13 
               19 
               18 
               17 
             
             
               cycles 
             
             
               2,000,000 
               5 
               10 
               17 
               15 
               15 
             
             
               cycles 
             
             
                 
             
             
               Numerical values represent the bonding strengths (N).  
             
          
         
       
     
   
     FIG. 5  is a graphical representation of the results shown in Table 3 for the purpose of easy reference. 
   As is obvious from Table 3 and  FIG. 5 , the embodied sample  5  having durometer hardness of HDD80 had the largest initial bonding strength, and they exhibited a tendency of decreasing the bonding strength as the number of operating cycles increases. However, these samples showed better performance on all of the tests up to 2,000,000 cycles as compared to the results of the related art sample. 
   This tendency of the embodied sample  5  is considered attributable to the excessive stiffness of adhesive resin  18 , which makes it difficult to absorb the repeated pressure of inflection and restoration of movable contact  14  caused by the depressing operations, thereby resulting in the decrease of bonding strength. 
   However, the bonding strength of the embodied sample  5  after the completion of 2,000,000 cycles was 10 N, which remains still within a range of the desirable bonding strength for the practical use. 
   On the other hand, it was found that the embodied samples  6  through  8  prepared with the adhesive material of HDD75 to 65 in the durometer hardness have high durability, as they maintain bonding strengths higher than 80% of the initial bonding strengths after 1,000,000 cycles of operation, and higher than 70% even after the completion of 2,000,000 cycles of operation. 
   These results of the embodied samples  6  through  8  seem to be the effects of elasticity provided by the low hardness of adhesive resin  18  composed of urethane acrylate, in that adhesive resin  18  functions as a buffer to absorb the repeated pressure of the depressing operation mentioned above. 
   Test Result 2 
   Further tests were performed to evaluate the bonding strength under the high temperature and high humidity environment. Those tests were carried out since the elasticity of adhesive resin  18  might be obtained owing to the porous structure of the material or might show high moisture permeability. 
   Similar tests were conducted and measurement results of bonding strength against the number of operations were recorded on samples prepared in the same manner as the embodied samples  5  through  8  as well as the related art samples corresponding to them. 
   Bonding strengths were measured after the test samples were subjected to the 100,000 cycles of depressing operation under an environment of 60° C. in temperature and 90 to 95%-RH in humidity as the test condition. 
   The results are shown in Table 4 and a graph in FIG.  6 . 
   
     
       
         
             
             
             
             
             
             
           
             
               TABLE 4 
             
             
                 
             
             
                 
               Related 
                 
                 
                 
                 
             
             
                 
               art 
               Embodied 
               Embodied 
               Embodied 
               Embodied 
             
             
               Sample 
               sample 
               sample 5 
               sample 6 
               sample 7 
               sample 8 
             
             
                 
             
           
          
             
               Coating resin 
               None 
               Ester resin 
               Ester resin 
               Ester resin 
               Ester resin 
             
             
               Adhesive 
             
             
               resin 
             
             
               (UV-curable 
             
             
               resin) 
             
             
               Basic 
               Epoxy 
               Urethane 
               Urethane 
               Urethane 
               Urethane 
             
             
               composition 
               Acrylate 
               Acrylate 
               Acrylate 
               Acrylate 
               Acrylate 
             
             
               Hardness 
               HDD80 
               HDD80 
               HDD75 
               HDD70 
               HDD65 
             
             
               Number of 
             
             
               Operations 
             
             
               Start 
               18 
               26 
               23 
               21 
               20 
             
             
               100,000 
               6 
               11 
               18 
               16 
               10 
             
             
               cycles 
             
             
                 
             
             
               Numerical values represent the bonding strengths (N).  
             
          
         
       
     
   
   As is obvious from Table 4 and  FIG. 6 , it was confirmed that all of the embodied samples  5  through  8  maintain high levels of bonding strength in absolute value as compared with the related art sample, although the embodied samples  8  and  5  show a tendency of decrease in the relative bonding strength similar to that of the related art sample. 
   The reduction in strength of the embodied sample  8  is thought to be due to an increase in hygroscopic property of adhesive resin  19  as it might be formed into a porous structure or a dynamic structure with elasticity, as discussed above. The reduction in strength of the embodied sample  5  is thought to be due to the stiffness of cured adhesive resin  19 , as was seen in the previous test on the bonding strength to the number of operations. 
   The embodied samples  6  and  7  have very high durability, as they maintain bonding strengths higher than 75% of the initial bonding strengths even after the test. 
   According to the confirmation tests for the bonding strengths of press-down projections  19  to base film  11 , as discussed above, it was verified that the invention can strengthen the bonding strengths while maintaining the excellent durability by the above structure, in which base film  11  made of PET having round dome-like movable contacts  14  is provided with first pretreated layer  13  formed of coated Ester resin on it upper surface, small cylindrical parts  16  made of PET and constituting press-down projections  19  are each provided with second pretreated layer  17  also formed of coated Ester resin on the underside surface, and base film  11  and press-down projections  19  are bonded together via these pretreated layers. 
   In addition, the productivity can be improved to make the movable contact unit less expensive when UV-curable urethane acrylate resin is used as adhesive resin  18 , since it has fast curing reaction. 
   Furthermore, adhesive resin  18  can be formed with optimum elasticity when its hardness is maintained within the range of HDD80 and 65 in the durometer hardness, so as to make it function as a buffer to the force applied during repeated depressing operation, and thereby improving the durability in addition to the bonding strength. 
   Like advantages can also be expected even when other types of adhesive resin are used so long as the hardness is maintained to the same range of HDD80 and 65 in the durometer hardness, so as to provide the optimum elasticity as the adhesive resin with buffering function. 
   Press-down projections  19  can provide excellent and stable operational response when their outer diameter is designed to be ¼ to ½ the outer diameter of the round dome-like movable contacts  14 . This can provide a large area of adhesion for press-down projections  19  to ensure strong adhesion and to maintain the adhesion steady for a prolonged duration. A ratio of the surface area of adhesion between {fraction (1/16)} and ¼ is thus suitable. 
   However, outer diameter of press-down projections  19  between ⅜ and ½ of the outer diameter of dome-like movable contacts  14  is rather desirable, if adhesive resin  18  used is an UV-curable urethane acrylate resin having HDD65 in the durometer hardness. A desirable relative surface area in this case is between {fraction (9/64)} and ¼. 
   In this exemplary embodiment, although what has been described is an example in which pretreated layers are formed by coating Ester resin, this is not restrictive and that the pretreated layers can be formed by coating Urethane resin. The coating of Urethane resin can also improve wettability with the adjoining resin. In addition, use of the above-said urethane acrylate resin as adhesive to form adhesive resin  18  can further improve adhesion between the preprocessing layers and adhesive resin  18  since they are the same type of material. 
   According to this exemplary embodiment, what has been discussed is the structure in which pretreated layers formed of coated Ester resin or Urethane resin are provided on the surfaces of the press-down projections and the base film that come to contact with each other, and these pretreated layers are bonded with UV-curable urethane acrylate resin. This structure improves the wettability between the pretreated layers and the adhesive to strengthen their adhesion, and achieves the movable contact unit having press-down projections that can keep the adhesion for a long duration. 
   As discussed, the present invention has an outstanding advantage of providing the highly reliable movable contact unit having press-down projections which can maintain the press-down projections in a securely bonded condition to the base film without increasing a number of components used and necessitating a complicated manufacturing process.