Method of welding laminates each having the structure of metal layer/thermally softenable insulating layer/metal layer
This application is a continuation of application Ser. No. 654,214, filed Sept. 25, 1984, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of resistance-welding laminates each having a basic three-layered structure consisting of metal layer/thermally softenable insulating layer/metal layer 
     2. Description of the Prior Art 
     Laminated materials obtained by combining a metallic material with various nonmetallic materials to improve various properties of the metallic material have been proposed. Examples of such laminated materials include a laminated material in which a synthetic resin film is formed, or is coated, on the surface of a metallic material so as to prevent corrosion of the metal; a laminated material for vibration prevention in which a thin layer of a vibration absorbing material such as rubber is formed inside a metallic material so as to provide good vibration prevention; and a light-weight laminated material consisting of an inner layer of a synthetic resin and a thin outer layer of a metal so as to provide a light-weight material with flexural stiffness equivalent to a metallic material. U.S. Pat. No. 4,313,996 (Feb. 2, 1982) discloses an example of a light-weight laminated material having a structure of metal/synthetic resin/metal. A structure having the following dimensions is disclosed: each metal skin layer is from about 2 to 20 mils (0.05-0.5 mm) thick, the ratio of the core thickness to skin thickness is less than 9:1, and the total laminate thickness is from about 5 to about 65 mils (0.13-1.65 mm). However, this reference does not describe the welding method. 
     The laminated metallic materials described above are all combined with electrically insulating materials such as synthetic resins. Therefore, two such laminated metallic materials cannot be welded by a conventional welding method since the overall structure is not conductive and so cannot be welded together. 
     In view of this, various improved welding methods have been proposed and actually practiced. FIG. 1 shows a welding method for projection welding laminates each having a metal/insulating material/metal structure. In this method, one laminate 1 consists of an internal insulating layer 4 sandwiched between metal layers 3a and 3b. Another laminate 2 consists of an internal insulating layer 6 sandwiched between metal layers 5a and 5b. These laminates 1 and 2 are to be welded together. Projection welding is performed such that electrodes 7 and 8 are brought into contact with the opposing metal layers 3b and 5a, and holding jigs 20a and 20b hold the upper and lower metal layers 3a and 5b. However, in a laminated metallic material obtained in this manner, only the metal layers 3b and 5a are welded. Metal layers 3a and 5b are not welded. Therefore, the welding strength is very weak. If the metal layer is a thin layer, the welded portion cannot serve a practical purpose. 
     FIG. 2 shows a known welding method utilizing a bypass circuit. According to this method, each of two laminates 9 and 10 to be welded has the following structure. The laminate 9 has an internal insulating layer 12 sandwiched between metal layers 11a and 11b. The laminate 10 consists of an internal insulating layer 14 sandwiched between metal layers 13a and 13b. A bypass circuit 15 is formed between the metal layers 11a and 13b. In the welding start period, a current flows through an electrode 16, the metal layer 11a, the bypass circuit 15, the metal layer 13b, and an electrode 17. Then, Joule heat is generated in the metal layers 11a and 13b and melts the internal insulating layers 12 and 14. According to this method, the melted portion of the insulating layers 12 and 14 near the electrode is made to flow transversely by the urging force of the electrodes 16 and 17. At the same time, the metal layers 11a and 13b are deformed and urged against the metal layers 11 b and 13a. As a result, a current flows directly between the electrodes 16 and 17 and the metal layers 11a, 11b, 13a and 13b to allow welding of the laminates. This method achieves excellent welding when applied to light-weight laminated materials in which the outer metal layers are thick and the internal insulating layers are thin. However, when this method is applied to a light-weight laminated material in which the outer metal layers are thin and the inner insulating layers are thick, problems are encountered. A light-weight laminated material of this type which can be welded by the method of the present invention is rendered light-weight by substituting the inner layer with a light material (normally insulating material) without decreasing the flexural stiffness. Such a light-weight laminated material is intended for use in a casing or the like. Therefore, deformation of the laminate at points other than the welding spot cannot be neglected. 
     Furthermore, in a light-weight laminate of this type, since the metal layer is thin, the outer metal layer is significantly damaged due to recessed deformation upon welding. Since the internal insulating layer generally consists of an organic material, it is decomposed and generates a gas at a melting point of the metal upon welding. This gas cannot diffuse out of the insulating layer due to the metal layers covering the insulating layer. The trapped gas then causes the problem of &#34;doming&#34;. This tendency becomes particularly notable when the insulating layer is thick. However, when the method shown in FIG. 2 is adopted, the area in which the internal resin layer melts is not confined to the vicinity of the electrodes but also extends to the area connecting the electrodes and the bypass circuit. Therefore, the amount of resin which is squeezed by the urging forces of the electrodes is undesirably increased. In addition, a large amount of decomposition gas is produced. The laminate is distorted or waved within a wide area having the welded portion as its center. A good weld cannot, therefore, be obtained. In the method shown in FIG. 2, every time the welding spot is changed, the position of the bypass circuit must be changed and adjusted. This involves complex procedures. Furthermore, the shape of laminated materials which may be welded is limited due to the apparatus. 
     U.S. patent application Ser. No. 559,239 (June 21, 1966) discloses a method of welding conductors which are not coated with an insulator to a lead wire which is coated with an insulator. External heaters are connected to the electrodes to soften the insulators on the lead wire. Thus, the conductors and the lead wire are forcibly brought into contact with each other and are welded together. In this method, one part to be welded is the insulated wire, and the others are conductors which are not coated with an insulator. In such a welded article, the decomposition gas produced by heating the insulating material is freely discharged outside the article, so that a problem of &#34;doming&#34; is avoided. 
     U.S. Pat. No. 3,155,809 (Nov. 3, 1964) discloses a spot-welding or resistance-welding method. According to this method, when conductors coated with an insulator, particularly flexible ribbon-type cables, are electrically coupled, a pair of heated electrodes is used to clamp the conductors so as to soften the insulators. Then, the metal portions are physically brought into contact with each other. A welding current is made to flow between the electrodes to allow spot welding or resistance-welding. According to this method, as in the method disclosed in U.S. patent application Ser. No. 559,239, members to be welded are insulator coated conductors. With such members, the decomposition gas produced upon heating the insulators is freely discharged outside the structure, so that there is no doming problem. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a resistance-welding method, in which laminates each having a structure of metal/thermally softenable insulating material/metal are resistance-welded in such a manner that doming due to gas formation in the insulating material is prevented such that the reproducibility of the process and welding strength are improved and deformation of laminates at points other than the welded spots is prevented, thereby providing a welded structure having an excellent outer appearance. 
     In order to achieve the above object of the present invention, there is provided a resistance-welding method comprising the steps of: 
     preparing two laminates in which each metal layer has a thickness of 0.02 to 0.5 mm, and the total thickness of the metal layers is 1/3 to 2/3 of the total thickness of the laminate; 
     holding a pair of electrodes for resistance-welding an overlapped portion of the laminates at a temperature higher than a softening temperature of insulating layers, and urging said electrodes against surfaces at a welding spot of the laminates to soften the insulating layers, thereby contacting every adjacent metal layer; and 
     supplying power to the pair of electrodes while all the adjacent metal layers are in contact with each other, thereby resitance-welding the laminates. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view showing a conventional method of resistance-welding laminates employing projection welding; 
     FIG. 2 is a schematic view showing a conventional method of resistance-welding laminates utilizing a bypass circuit; 
     FIG. 3 is a schematic view showing a method of resistance-welding laminates according to an embodiment of the present invention; 
     FIG. 4 is a sectional view of laminates welded by the method shown in FIG. 3; 
     FIG. 5 is a schematic view showing a method of resistance-welding laminates according to another embodiment of the present invention; and 
     FIG. 6 is a sectional view showing laminates welded by the method shown in FIG. 5. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The resistance-welding method of the present invention is adapted to weld two laminates each having a basic structue of metal layer/thermally softenable insulating layer/metal layer. A laminate to be welded by the method of the present invention is intended for use in a casing or the like and is a light-weight substitute for a metal plate. In this laminate, the inner layer of a metal plate is replaced with a light-weight insulating layer, so that the overall laminate can be rendered light without degrading its flexural stiffness. 
     Each metal layer of such a laminate may consist of a metal such as iron, lead, zinc, or aluminum; an alloy thereof with other elements such as carbon, chromium, or titanium; or such metals or alloys with various conductive surface-treated layers formed by metal plating or chemical treatment. Each metal layer is selected to have a thickness of 0.02 to 0.5 mm. The ratio of the total thickness of the two metal layers of each laminate is 1/3 to 2/3 of the total thickness of the laminate. In order not to degrade the flexural stiffness, each metal layer must have a certain thickness. However, to provide a light laminate, each metal layer should be as thin as possible. If each metal layer is thin, the insulating layer must be thick to keep thickness of the laminate uncharged. The thicker the insulating layer, the more gas it generates upon welding. In this case, the thin metal layer may not be able to withstand the pressure of the gas and deforms. Therefore, the welded spot may have an unsatisfactory strength and a poor outer appearance and be easily deformed. In view of this, each metal layer must have a thickness above a predetermined thickness. However, if a metal layer is too thick, when the insulating layer is heated through the metal layers, the heat is dissipated by the metal layers to a great extent, resulting in a long welding time. Therefore, each metal layer must have a thickness less than a predetermined value. 
     The absolute and relative thickness of each metal layer must thus be defined to fall within predetermined ranges in order to maintain the properties of the laminates, to perform satisfactory welding, and to improve the strength and outer appearance of the welded spot. The specific ranges were determined by the present inventors in experiments under different conditions and will be described with reference to Example 4. Although U.S. Pat. No. 4,313,996 specifies the dimensions of the metal layers and insulating (synthetic resin) layers having the same structure as that of the present invention, they were not determined for the purpose of performing satisfactory resistance-welding and providing satisfactory strength and outer appearance at the welded spot. 
     The material of the thermally softenable insulating layer contained in each laminate can be a thermoplastic resin such as a polyolefin, polyvinyl chloride, polyvinylidene chloride, polyester, polyamide, polycarbonate, polyvinyl acetate, polyacetal, polystyrene, ABS resin, methyl polymethacrylate, or a fluorine-containing resin. Of these materials, preferred examples in view of formability and weldability may include polyolefins such as homopolymers or copolymers of an α-olefin, e.g., ethylene, propylene, 1-butene, 1-pentene, 3-methyl-1-butene, 1-hexene, 4-methyl-1-pentene, 1-heptene, or 1-octene; copolymers of these α-olefins with small amounts of monomers, e.g., vinyl acetate, acrylic acid, methacrylic acid, methyl acrylate, or methyl methacrylate; or graft-modified polyolefins obtained by grafting the above polyolefins with monomers such as vinyl acetate, acrylic acid, methacrylic acid, maleic acid, fumaric acid, methyl acrylate, ethyl acrylate, ethyl maleate, or maleic anhydride. The material of the thermally softenable insulating layer may be a rubbery material such as natural rubber, a butadiene-styrene copolymer, nitrile rubber, chloroprene rubber, polyisoprene, butyl rubber, or polyisobutylene. Instead of these materials, the thermally softenable insulating layer may consist of other insulators which can be softened and made to flow upon being heated. 
     The insulating layer and the metal layer can be laminated by conventional methods. For example, the insulating layer can be melted and adhered to the metal layer. Alternatively, the insulating layer and the metal layer can be adhered together with an adhesive. 
     The method of resistance-welding two laminates each having the above-mentioned structure will be described with reference to the embodiment shown in FIG. 3. 
     One laminate 21 has an internal thermally softenable insulating layer 22 and metal layers 23a and 23b sandwiching it. Another laminate 24 has an internal thermally softenable insulating layer 25 and metal layers 26a and 26b sandwiching it. The overlapping portion of these laminates 21 and 24 forms a welding spot. Electrodes 27 and 28 are arranged at the two surfaces of the welding spot. Heaters 29 and 30 are arranged around the electrodes 27 and 28, respectively. The heaters 29 and 30 heat the electrodes 27 and 28 and keep them at a temperature higher than the softening temperature of the insulating layers 22 and 25. When the insulating layers consist of polypropylene, the electrodes 27 and 28 are kept at a temperature higher than the softening temperature of polypropylene, i.e., 165° C. The heated electrodes 27 and 28 are brought into contact with the laminates 21 and 24. Then, inside the respective laminates, heat from the electrodes 27 and 28 is conducted from the metal layers 23a and 26b to the insulating layers 22 and 25, and the insulating layers 22 and 25 are sequentially fluidized. The fluidized portions of the insulating layers mainly overlap the portions immediately below the metal layers since the electrodes are in point contact or in surface contact close to point contact with the metal layers. As a result, the electrodes urged against the laminates squeeze the fluidized portions of the insulating layers and come closer to each other. The insulating layers are further squeezed out at the fluidized portions, and the metal layers 23a and 25b are deformed, so that the metal layers 23a, 23b, 26a and 26b are brought into contact with each other. The squeezed portions of the insulating layers bulge around the welding spot and are cooled and solidified. 
     Another method for softening and fluidizing the insulating layers may be adopted wherein heated punches or the like are pressed toward a prospective welding spot of laminates to fluidize the insulating layers, the fluidized insulating layers are squeezed by the pressure of the punches, and electrodes are then brought into contact with the welding spot. However, this method is not suitable for welding laminates according to the present invention for the following reason. When the heated punches are removed and the electrodes are brought into contact with the metal layers, the deformed metal layers spring back and are separated from each other so the insulator flows into the space formed between the separated metal layers. Then, power cannot be supplied to the metal layers. Furthermore, due to the two-step stress exerted by the heated punches and electrodes, a thin matal layer may be torn away. 
     According to the method of the present invention, before or when the metal layers 23a, 23b, 26a and 26b are brought into direct contact with each other, a current is supplied to the electrodes 27 and 28 to resistance-weld the laminates 21 and 24. As a result, as shown in FIG. 4, a uniformly bulged insulating layer is formed to surround a directly welded spot 31. The welded spot has a high welding strength and a good outer appearance. The inventors think that this is attributed to the following reason. According to the present invention, since both the electrodes 27 and 28 are heated, the insulating layers are softened within a short time to prevent the generation of a decomposition gas from the insulating layers. Generation of the decomposition gas is further prevented by using laminates 21 and 24 having insulating layers of a small thickness. Therefore, since welding can be performed without adverse influence from a decomposition gas, reproducibility of the welding process and the welding strength are improved, doming is prevented, and a good outer appearance is obtained. 
     Electrodes 27 and 28 may be made of a material ordinarily used in resistance welding and, preferably, one of the materials described hereinafter. 
     The material is generally one having an electric resistance lower than that of the material to be welded. When the material to be welded is, for example, iron, an alloy such as a copper-based chromium alloy is used so as to suppress heat generation due to contact resistance between the electrodes and the material to be welded and to prevent fusing. However, since laminates of the present invention have insulating layers between metal layers, the contact resistance between the metal layers upon welding is unstable. Even if the tip shape of the electrode is slightly changed, the contact resistance is significantly influenced and the welded area becomes unstable. For this reason, with conventional copper-based chromium alloy electrodes, dressing of electrode tips is performed after welding 30 spots to assure satisfactory welding performance. According to the present invention, although electrodes of this alloy can be used, if metal layers consist of iron or an iron alloy, at least the tips of the electrodes can be made of molybdenum having a resistance equivalent to that of iron. When electrodes of such materials are used, the electrodes generate heat upon being powered so as to facilitate melting of insulators utilizing the contact resistance at the interface between the metal layers. For this reason, the above described problem can be eliminated, and the dressing interval of electrodes can be extended to welding 300 or more spots. 
     When molybdenum electrodes are used to resistance-weld iron members, the electrodes and iron fuse and the iron members cannot be welded. However, with laminates having a structure of iron layer/insulating layer/iron layer, the contact resistance at the interface is high. Thus, the contact resistance between iron and molybdenum is considered to be suppressed, and consequently, fusing of the electrodes is prevented. 
     When molybdenum electrodes are used, series-welding can be performed. When series-welding of laminates according to the present invention is performed, using electrodes made of copper-based chromium alloy, the contact resistance inside the laminates is inevitably unstable. Consequently, the welding strength is too great at one spot while it is too small at another spot, thus preventing welding. However, with molybdenum electrodes heating by the contact resistance can be negated by the heat generated by the electrodes, and uneven welding strength can be avoided. Series-welding is thus facilitated. 
     FIG. 5 shows another embodiment of a welding method according to the present invention. An object of this emodiment is to eliminate deformation of one surface of the welded spot so as to provide a smooth welded spot when viewed from one side. In order to achieve this object, a holding jig 40 is provided around an electrode 28 on the same side of a welding spot on which a smooth surface is to be formed. The holding jig 40 must be arranged parallel to the tip of the electrode 28, have a smooth surface in contact with a laminate 24 and have an area wider than the thermally deformed area of the welded portion. The material of the electrode 28 at the side of the holding jig 40 preferably has a greater hardness than that of the electrode 27 at the other side in order to prolong the electrode life. As in the case of the electrode 28, the holding jig 40 is heated to and kept at a temperature higher than the softening temperature of insulating layers so as to improve the welding speed and the appearance of the welded spots. When welding is performed using the holding jig 40, even if the insulating layers are heated and softened, the metal layer 26b in contact with the holding jig 40 is not deformed and is kept smooth, as shown in FIG. 6. Therefore, the welded spot of the finished, welded laminates have one smooth side and a very good outer appearance. In order to provide the same effect, a back bar electrode can be used in place of the holding jig. 
     The embodiments shown in FIGS. 3 to 6 are described with reference to spot-welding. However, the present invention can also be applied to seam-welding or projection-welding. 
     The present invention will now be described by way of its Examples. 
     EXAMPLE 1 
     Resistance-welding of laminates was performed by the method shown in FIGS. 3 and 5. The welding results together with the materials, dimensions and welding conditions are shown in Table 1. Two metal plates and insulating layers of laminates were adhered with a modified polyolefin adhesive layer having an adhesive functional group. 
     
                                           TABLE 1                                 
__________________________________________________________________________
                               Welding condition                          
                                               Ther-                      
                                               mally                      
       Laminate                Upper   Lower   de- Con-*                  
       First metal                                                        
               Second metal                                               
                       Insulating                                         
                               electrode                                  
                                       electrode                          
                                               formed                     
                                                   tact                   
       layer   layer   layer   Tem-    Tem-    area                       
                                                   area                   
                                                       Weld-              
           Thick-  Thick-  Thick-                                         
                               pera-   pera-   (diam-                     
                                                   (diam-                 
                                                       ing Eval-          
   Meth-                                                                  
       Mate-                                                              
           ness                                                           
               Mate-                                                      
                   ness                                                   
                       Mate-                                              
                           ness                                           
                               ture                                       
                                  Mate-                                   
                                       ture                               
                                          Mate-                           
                                               eter                       
                                                   eter                   
                                                       re- ua-            
No.                                                                       
   od  rial                                                               
           (mm)                                                           
               rial                                                       
                   (mm)                                                   
                       rial                                               
                           (mm)                                           
                               (°C.)                               
                                  rial (°C.)                       
                                          rial in mm)                     
                                                   in mm)                 
                                                       sult               
                                                           tion           
__________________________________________________________________________
1  FIG. 3                                                                 
       iron                                                               
           0.2 iron                                                       
                   0.2 poly-                                              
                           0.42                                           
                               280                                        
                                  Cu--Cr                                  
                                       280                                
                                          Cu--Cr                          
                                               10   4  uni-               
                                                           o              
       foil    foil    pro-                            form               
                       pyl-                            pro-               
                       ene                             jec-               
                                                       tion               
2  FIG. 5                                                                 
       iron                                                               
           0.2 iron                                                       
                   0.2 poly-                                              
                           0.42                                           
                               300                                        
                                  Cu--Cr                                  
                                       240                                
                                          Cu--Cr                          
                                               12  35  one o              
       foil    foil    pro-                            smooth             
                       pyl-                            side               
                       ene                                                
__________________________________________________________________________
 o . . . Very good outer appearance (Primary welding voltage: 160 V; power
 cycle number: 5 c/s; electrode pressure: 400 kg)                         
 *The contact area between the holding jig and the laminates              
 
    
     EXAMPLE 2 
     Various welding conditions were set wherein the heating temperature of the electrodes and the softening temperature of the insulating layers were varied, and laminates were welded by the method shown in FIG. 3. 
     The obtained results are shown in Table 2 together with the structure of the laminates and the welding conditions. A welder used in Example 2 was an AC spot welder 3111-3, available from Durex Co., Ltd., West Germany, connected to a power source control apparatus A-11, available from the same company. In order to allow a variable voltage to be applied to the primary side of the welding transformer, variable-voltage transformers of 200 V and 20 A were added. Upper and lower welding electrodes (of 12 mm diameter) of the welder had truncated conical shapes having a top diameter of 4 mm and a cone angle of 60°. Ring heaters of 600 W capacity having temperature controllers were mounted on these electrodes. 
     
                                           TABLE 2                                 
__________________________________________________________________________
        Laminate                            Welding condition             
        First metal layer                                                 
                   Second metal layer                                     
                              Insulating layer                            
                                            Upper electrode               
             Thickness  Thickness     Thickness                           
                                            Temperature                   
No.                                                                       
   Method                                                                 
        Material                                                          
             (mm)  Material                                               
                        (mm)  Material                                    
                                      (mm)  (°C.)                  
                                                   Material               
__________________________________________________________________________
3  FIG. 3                                                                 
        steel*                                                            
             0.2   steel*                                                 
                        0.2   polypropylene                               
                                      0.42   25    Cu--Cr                 
        plate      plate                                                  
4  FIG. 3                                                                 
        steel*                                                            
             0.2   steel*                                                 
                        0.2   polypropylene                               
                                      0.42  220    Cu--Cr                 
        plate      plate                                                  
5  FIG. 3                                                                 
        steel*                                                            
             0.2   steel*                                                 
                        0.2   polypropylene                               
                                      0.42  240    Cu--Cr                 
        plate      plate                                                  
6  FIG. 3                                                                 
        steel*                                                            
             0.2   steel*                                                 
                        0.2   polypropylene                               
                                      0.42  260    Cu--Cr                 
        plate      plate                                                  
7  FIG. 3                                                                 
        steel*                                                            
             0.2   steel*                                                 
                        0.2   polypropylene                               
                                      0.42  280    Cu--Cr                 
        plate      plate                                                  
8  FIG. 3                                                                 
        steel*                                                            
             0.2   steel*                                                 
                        0.2   polypropylene                               
                                      0.42  300    Cu--Cr                 
        plate      plate                                                  
9  FIG. 3                                                                 
        steel*                                                            
             0.2   steel*                                                 
                        0.2   polypropylene                               
                                      0.42  300    Cu--Cr                 
        plate      plate                                                  
10 FIG. 3                                                                 
        steel*                                                            
             0.2   steel*                                                 
                        0.2   polypropylene                               
                                      0.42  300    Cu--Cr                 
        plate      plate                                                  
11 FIG. 3                                                                 
        steel*                                                            
             0.2   steel*                                                 
                        0.2   polypropylene                               
                                      0.42  300    Cu--Cr                 
        plate      plate                                                  
12 FIG. 3                                                                 
        steel*                                                            
             0.2   steel*                                                 
                        0.2   polypropylene                               
                                      0.42  300    Cu--Cr                 
        plate      plate                                                  
13 FIG. 3                                                                 
        steel*                                                            
             0.2   steel*                                                 
                        0.2   polypropylene                               
                                      0.42  300    Cu--Cr                 
        plate      plate                                                  
__________________________________________________________________________
         Welding condition      Welding result                            
                     Thermally  Time                                      
                     deformed                                             
                           Contact                                        
                                before                                    
         Lower electrode                                                  
                     area  area current                                   
                                     Welding                              
         Temperature (diameter                                            
                           (diameter                                      
                                flow pressure                             
No. Method                                                                
         (°C.)                                                     
                Material                                                  
                     in mm)                                               
                           in mm)                                         
                                (second)                                  
                                     (kg/spot)                            
                                          Evaluation                      
                                                Remarks                   
__________________________________________________________________________
3   FIG. 3                                                                
          25    Cu--Cr                                                    
                     --    4    60 or                                     
                                     --   x     No current                
                                more            flow                      
4   FIG. 3                                                                
         220    Cu--Cr                                                    
                     --    4    60 or                                     
                                     --   x     1 minute or               
                                more            more before               
                                                current flow              
5   FIG. 3                                                                
         240    Cu--Cr                                                    
                     12    4    20    80  Δ                         
                                                Doming around             
                                                welded spot               
6   FIG. 3                                                                
         260    Cu--Cr                                                    
                     10    4    6    112  o     Excellent                 
7   FIG. 3                                                                
         280    Cu--Cr                                                    
                     10    4     1&gt;  122  o     Excellent                 
8   FIG. 3                                                                
         300    Cu--Cr                                                    
                     10    4     1&gt;  140  o     Excellent                 
9   FIG. 3                                                                
         240    Cu--Cr                                                    
                     10    4    1    --   Δ                         
                                                Doming around lower       
                                                welded spot side          
10  FIG. 3                                                                
         200    Cu--Cr                                                    
                     10    4    3    --   Δ                         
                                                Doming around lower       
                                                welded spot side          
11  FIG. 3                                                                
         150    Cu--Cr                                                    
                     10    4    3    --   Δ                         
                                                Doning around lower       
                                                welded spot side          
12  FIG. 3                                                                
         100    Cu--Cr                                                    
                     --    4    60 or                                     
                                     --   x     1 minute or more          
                                                before current flow       
13  FIG. 3                                                                
          25    Cu--Cr                                                    
                     --    4    60 or                                     
                                     --   x     1 minute or more          
                                more            before current            
__________________________________________________________________________
                                                flow                      
 *Electrolytically chromiumtreated plate of soft cold rolled steel (Tin   
 free steel)                                                              
 o . . . Very good outer appearance                                       
 Δ . . . Doming caused by gasification of the contained resin upon  
 welding                                                                  
 x . . . Not practically usable (Primary welding voltage: 160 V; power    
 cycle number: 5 c/s; electrode pressure: 400 kg)                         
 
    
     In Example 2, when the upper and lower electrodes were not heated (sample No. 3), a welding current did not flow. For this reason, a short-circuit, as shown in FIG. 2, was added. However, thermal deformation at the welded point was significant and thermal deformation also occurred in an area between the welded point and the short-circuit of the current. Thus, the only welded laminates obtained were those not suitable for practical use. For sample Nos. 4, 12 and 13 (temperature: 220° C. or 300° C.), the time before current flow after application of pressure on the electrodes was 1 minute or more. This is not practical from the viewpoint of work speed. For sample Nos. 5 and 9 to 11 (temperature: 240° C.), heating of the inner resin layer at the welding point and squeezing of the melted resin were insufficient. Due to the high temperature applied during welding, the insulating layer at the welding point is gasified to urge upward the metal layer around the welded point, thereby causing doming, which is not desirable from the viewpoints of outer appearance and welding strength. For sample Nos. 6 to 8 (temperature of 260° C. or higher), an excellent welding operation could be performed. However, if the electrode temperature is too high, wear of the electrode tips is significant. Therefore, the electrode temperature preferably falls within the range of 260° to 300° C. 
     EXAMPLE 3 
     Laminates and welder as used in Example 2 were used for welding by the method shown in FIG. 5. Welding was performed by restricting the contact area between the holding jig and the laminates to portions directly above and below the thermally deformed area. The obtained results are shown in Table 3 below, together with the structure of the laminates and the welding conditions. 
     
                                           TABLE 3                                 
__________________________________________________________________________
        Laminate                            Welding condition             
        First metal layer                                                 
                   Second metal layer                                     
                              Insulating layer                            
                                            Upper electrode               
             Thickness  Thickness     Thickness                           
                                            Temperature                   
No.                                                                       
   Method                                                                 
        Material                                                          
             (mm)  Material                                               
                        (mm)  Material                                    
                                      (mm)  (°C.)                  
                                                   Material               
__________________________________________________________________________
14 FIG. 5                                                                 
        steel*                                                            
             0.2   steel*                                                 
                        0.2   polypropylene                               
                                      0.42  300    Cu--Cr                 
        plate      plate                                                  
15 FIG. 5                                                                 
        steel*                                                            
             0.2   steel*                                                 
                        0.2   polypropylene                               
                                      0.42  300    Cu--Cr                 
        plate      plate                                                  
16 FIG. 5                                                                 
        steel*                                                            
             0.2   steel*                                                 
                        0.2   polypropylene                               
                                      0.42  300    Cu--Cr                 
        plate      plate                                                  
17 FIG. 5                                                                 
        steel*                                                            
             0.2   steel*                                                 
                        0.2   polypropylene                               
                                      0.42  300    Cu--Cr                 
        plate      plate                                                  
18 FIG. 5                                                                 
        steel*                                                            
             0.2   steel*                                                 
                        0.2   polypropylene                               
                                      0.42  300    Cu--Cr                 
        plate      plate                                                  
19 FIG. 5                                                                 
        steel*                                                            
             0.2   steel*                                                 
                        0.2   polypropylene                               
                                      0.42  300    Cu--Cr                 
        plate      plate                                                  
20 FIG. 5                                                                 
        steel*                                                            
             0.2   steel*                                                 
                        0.2   polypropylene                               
                                      0.42  300    Cu--Cr                 
        plate      plate                                                  
21 FIG. 5                                                                 
        steel*                                                            
             0.2   steel*                                                 
                        0.2   polypropylene                               
                                      0.42  300    Cu--Cr                 
        plate      plate                                                  
22 FIG. 5                                                                 
        steel*                                                            
             0.2   steel*                                                 
                        0.2   polypropylene                               
                                      0.42  300    Cu--Cr                 
        plate      plate                                                  
23 FIG. 5                                                                 
        steel*                                                            
             0.2   steel*                                                 
                        0.2   polypropylene                               
                                      0.42  300    Cu--Cr                 
        plate      plate                                                  
24 FIG. 5                                                                 
        steel*                                                            
             0.2   steel*                                                 
                        0.2   polypropylene                               
                                      0.42  300    Cu--Cr                 
        plate      plate                                                  
__________________________________________________________________________
                                Welding result                            
         Welding condition      Time                                      
                     Thermally                                            
                           Contact                                        
                                before                                    
         Lower electrode                                                  
                     deformed                                             
                           area current                                   
                                     Welding                              
         Temperature area  (diameter                                      
                                flow pressure                             
No. Method                                                                
         (°C.)                                                     
                Material                                                  
                     (diameter                                            
                           in mm)                                         
                                (second)                                  
                                     (kg/spot)                            
                                          Evaluation                      
                                                Remarks                   
__________________________________________________________________________
14  FIG. 5                                                                
         300    Cu--Cr                                                    
                     14    12   1&gt;   140  o     Rough lower               
                                                surface                   
15  FIG. 5                                                                
         240    Cu--Cr                                                    
                     14    12   1&gt;   138  o     Rough lower               
                                                surface                   
16  FIG. 5                                                                
         200    Cu--Cr                                                    
                     14    12   3    --   Δ                         
                                                Rough lower               
                                                surface                   
17  FIG. 5                                                                
         150    Cu--Cr                                                    
                     14    12   6    --   Δ                         
                                                Rough lower               
                                                surface                   
18  FIG. 5                                                                
         100    Cu--Cr                                                    
                     --    12   60 or                                     
                                     --   x                               
                                more                                      
19  FIG. 5                                                                
          25    Cu--Cr                                                    
                     --    12   60 or                                     
                                     --   x                               
                                more                                      
20  FIG. 5                                                                
         300    Cu--Cr                                                    
                     14    35   1&gt;   140  o     Smooth lower              
                                                surface                   
21  FIG. 5                                                                
         260    Cu--Cr                                                    
                     14    35   1&gt;   140  o     Smooth lower              
                                                surface                   
22  FIG. 5                                                                
         240    Cu--Cr                                                    
                     14    35   1&gt;   132  o     Smooth lower              
                                                surface                   
23  FIG. 5                                                                
         220    Cu--Cr                                                    
                     14    35   1&gt;   130  Δ                         
                                                Smooth lower              
                                                surface                   
24  FIG. 5                                                                
         150    Cu--Cr                                                    
                     14    35   3    --   Δ                         
                                                Smooth lower              
                                                surface                   
__________________________________________________________________________
 *Electrolytically chromiumtreated plate of soft cold rolled steel (Tin   
 free steel)                                                              
 o . . . Very good outer appearance                                       
 Δ . . . Doming caused by gasification of the contained resin upon  
 welding                                                                  
 x . . . Not practically usable (Primary welding voltage: 160 V; power    
 cycle number: 5 c/s; elctrode pressure: 400 kg)                          
 
    
     In Example 3, the welding performance was influenced by the temperature of the upper and lower electrodes, as in the case of Example 2. Among laminates in which the contact area with the holding jig is smaller than the thermally deformed area, those which resulted in satisfactory welding (sample Nos. 14 to 17) caused ring-like thermal deformation in the lower surface of the welded point and could not provide one smooth side. Those which had a contact area with the holding jig larger than the thermally deformed area had a wider suitable electrode temperature range (for the lower electrode) of 240° to 300° C., shifted toward lower temperatures than those in Example 2. Substantially no thermal deformation was observed around the lower surface of the welded spot welded within this temperature range, and one smooth side was provided. 
     EXAMPLE 4 
     Welding was performed following the same procedures as those in Example 1 for laminates in which the thickness of each metal layer of each laminate and the ratio of the total thickness of the two metal layers of each laminate to the thickness of the laminate fell inside and outside the ranges defined according to the present invention. The obtained results are shown in Table 4 together with the structure of the laminates and the welding conditions. 
     
                                           TABLE 4                                 
__________________________________________________________________________
        Laminate                            Welding condition             
        First metal layer                                                 
                   Second metal layer                                     
                              Insulating layer                            
                                            Upper electrode               
             Thickness  Thickness     Thickness                           
                                            Temperature                   
No.                                                                       
   Method                                                                 
        Material                                                          
             (mm)  Material                                               
                        (mm)  Material                                    
                                      (mm)  (°C.)                  
                                                   Material               
__________________________________________________________________________
25 FIG. 3                                                                 
        iron 0.2   iron 0.2   polypropylene                               
                                       0.42 280    Cu--Cr                 
        foil       foil                                                   
26 FIG. 3                                                                 
        iron 0.2   iron 0.2   polypropylene                               
                                      1.0   280    Cu--Cr                 
        foil       foil                                                   
27 FIG. 3                                                                 
        iron 0.8   iron 0.8   polypropylene                               
                                      1.6   280    Cu--Cr                 
        foil       foil                                                   
__________________________________________________________________________
                                Welding results                           
         Welding condition      Time                                      
                     Thermally                                            
                           Contact                                        
                                before                                    
         Lower electrode                                                  
                     deformed                                             
                           area current                                   
                                     Welding                              
         Temperature area  (diameter                                      
                                flow pressure                             
No. Method                                                                
         (°C.)                                                     
                Material                                                  
                     (diameter)                                           
                           in mm)                                         
                                (second)                                  
                                     (kg/spot)                            
                                          Evaluation                      
                                                Remarks                   
__________________________________________________________________________
25  FIG. 3                                                                
         280    Cu--Cr                                                    
                     10    4    1&gt;   122  o                               
26  FIG. 3                                                                
         280    Cu--Cr                                                    
                     10    4    6    --   x     Surface                   
                                                metal layer               
                                                broke down                
27  FIG. 3                                                                
         280    Cu--Cr                                                    
                     10    4    60 or                                     
                                     --   x     1 minute or               
                                more            more before               
                                                current                   
__________________________________________________________________________
                                                flow                      
 o . . . Very good outer appearance                                       
 Δ . . . Doming caused by gasification of the contained resin upon  
 welding                                                                  
 x . . . Not practically usable                                           
 
    
     In Example 4, with a sample (No. 25) in which the thickness of the metal layer and the ratio of the total thickness of the metal layers to the thickness of the laminate fell within the ranges defined by the present invention, an excellent welding performance was obtained. However, with a sample (No. 26) having thicker insulating layers because of a smaller ratio of total thickness of the metal layers compared to the thickness of the laminate, and with a sample (No. 27) having thicker metal layers, no satisfactory results were obtained.