Compound and process for an injection molding
This is a continuation of copending application Ser. No. 07/731,000 filed on Jul. 16, 1991 now abandoned which is a division of application Ser. No. 555,089, filed Jul. 18, 1990 pending. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention is related to a compound which is used in an injection molding process for producing precision machine components of metal or an alloy that are small in size and intricate in form. 
     According to a conventional injection molding process, sintered products manufactured by a powder metallurgy are produced by first pressing a metal or alloy powder and then sintering the part. However, it is difficult to manufacture such products which are three-dimensional, intricate in form, and have thin walls or knife-edge sections. 
     In order to overcome the aforementioned drawbacks associated with the conventional processes, a sintering process has been proposed in Japan Patent Laid-Open application Nos. 57-16,103, and 57-26,105, in which an injection-molding compound comprised of a metal or alloy powder and a binder are injection-molded in a metal die. The injection-molded material is heated to remove the binder component, and then is sintered to produce the final product. 
     Although the above improved process provides products having a higher sintering density because it utilizes metal or alloy powders having less than ten μm average particle diameter, there are still some problems associated therewith. Sufficiently high injection speed needed for high productivity cannot be achieved. Furthermore, the binder materials typically cannot be efficiently removed from the mold. When injection speeds are increased, the products become porous thereby adversely affecting the mechanical properties of the sintered product. Moreover, the binder-removal requires a relatively long time to complete and the removal temperatures are relatively high. This, in turn, causes the parts to crack, swell and/or deform during the sintering operation. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a compound for an injection-molding process in which metal or a metal alloy along with a binder material are used to mold precise parts, said process having improved binder removal properties and being capable of higher production rates. 
     In order to achieve these and other objects of this invention, studies of binder compositions and the weight ratio of their constituents and mixing ratio (in terms of volume ratio) of metal or metal alloy powders were carried out. It was found that the following composition and mixing ratios achieve the aforementioned objects of this invention. The green component used in the process contains one or more metals or metal alloy powders for sintering and a binder component that includes 10-80% by weight of a low density polyethylene; 10-80% by weight of a paraffin group wax; 5-35% by weight of an ester of boric acid; and 0.1-5% by weight of polyoxyethylenealkylester group or polyoxyethylenealkylether group. The volume ratio of the sintering-powder consisted of 30-70% of one or more than one type of metal or metal allow powder and the 70-30% of a binder. 
     The sintering powder which can be used in this invention can be one or more types of powder consisting of pure iron, stainless steel, carbonyl iron, or pure cobalt. The low density polyethylene can be any type of commercially available material. Paraffin-group wax can be simply pure paraffin wax. The ester of boric acid can be selected from one or more of the triglycoldiborate-group; trialkylborate-group; glycerolborate-group; or alkinediborate with the trialkylborate group being preferred. More specifically, the triglycoldiborate-group can be 1,6-bis (5-ethyl-4-propyl-1,3,2-diboxabora-2-cyclohexyloxy) hexane, or 1,4-bis (5-ethyl-4-propyl-1,3,2-dioxabora-2-cyclohexyloxy) butane. As for the trialkylborate-group, any one from trimethylborate, triethylborate, tributhylborate, or triamyborate can be chosen. Any one from glycerolboratestearate, or polyoxyethyleneglycerolboratepalmitate can be selected as the glycerolborate-group. As for the alkinediborate-group, a methyldiborate or ethyldiborate can be chosen. 
     The borate-ester can be used as one or a mixture of these materials. When it is mixed with other components, it will be preferable to dissolve it with solvents such as benzene, toluene, or xylene to prepare a solution of 60-80% by weight. This will enhance the mixing efficiency of metal powder with organic binders. Hence, the binder component contains a certain amount of solvent as a constituent of said binder. For both polyoxyethylenealkylester-group and polyoxyethylenealkylether-group, commercial available type of surface activators having a formula such as RCOO(C 2  H 4  O) n  H and RO(C 2  H 4 ) n  H, respectively can be utilized. 
     The equipment and facility which are usually employed for the molding of plastics can also be used to injection mold powders prepared according to the present invention, under the following operation conditions; the molding temperature is 80°-200° C., the injection speed is 150-250 mm/second, and the injection pressure is 500-2,000kg/cm 2 . Although the above injection speed is more than two times faster than the conventional injection speed, it was found that the mechanical properties of final products were not adversely affected. 
     When the precise products having the above composition is heated and degassed in the equipment, an inert or reduction gas for powders which is easily oxidized can be used; while air or inert gas can be used for sintering the mixed powders which cannot be as easily oxidized during the sintering process. In any case, the binder-removal can be achieved at temperatures of between 250°-300° C. using a heating rate of 12°-30° C./hour. When using the conventional composition of mixed compound, it is required to treat the compound at relatively higher temperatures such as 400°-550° C. with a slower heating rate of 1°-10° C./hour. Therefore, in the practice of the present invention, high temperature heat treatment and slow heating rates are unnecessary, resulting in a great improvement in the efficiency of the binder-removal process. 
     Reasons for limiting the aforementioned mixing composition, in terms of the noted volume and weight ratios, are based on several factors of concern. The volume ratio of metal or alloy powder as a sintering-powder is, as mentioned before, 30-70%. This is due to the fact that (1) if the volume ratio is less than 30%, the fluidability of the compound will deteriorate during the injection process to a point where the injection-molding operation cannot be successfully completed, and (2) the compacting density of the sintering powder into the injection-molded products must be low, resulting in a final sintered product having a relatively low density. On the other hand, if the volume ratio of the sintering powder exceeds 70%, the strength of the injection-molded products is lower, cracks will be formed in the part due to surface-shrinkage. 
     If the low density polyethylene is less than 10% by weight of the binder, both the strength and shape-stability of the injection-molded products are reduced and cracks will form on the surface of the molded products. If the low density polyethylene is more than 80% by weight of the binder, the time needed for the complete removal of the binder is unreasonably long. 
     Moreover, one reason for using 10-80% by weight of the paraffin-group wax stems from the fact that if the paraffin-group wax is contained with less than 10% by weight, the injection-moldability of the material becomes poor, and both the temperature and the time for complete removal of the binder will be extended. If it exceeds more than 80% by weight, the molded products will exhibit reduced mechanical strength and poor shape-stability, and the molded part becomes difficult to handle. 
     The reason for defining the amount of borate-ester as being 5-35% by weight is to 1) improve the mixing efficiency of the sintering powder, 2) stabilize the binder-removal process, and 3) enhance the density and the shape-stability of the final sintered products. Therefore, if the borate-ester is less than 5% by weight, the final product will possess porosity defects due to poor mixing. At the same time, if it exceeds 35% by weight, the strength of the products is weakened. 
     Finally, as also mentioned above, the polyoxyethylenealkylester-group or polyoxyethylenealkylether-group is defined as being 0.1-5% by weight. This is based on the fact that if it is less than 0.1% by weight, the injection speed must be relatively high, thus producing porosity defects in the product. If it exceeds 5% by weight, the strength of the sintered products is weakened. 
     It is also demonstrated that adding stearic acid with less than 20% by weight to the present compound, the removability of the molded products from the metal die is improved without effecting the final product. 
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
     Sintered parts having sharp edges and thin wall configurations were injection-molded using various binders with atomized stainless steel powder (17Cr-4Ni-Fe, SUS 630), having a 15 μm diameter average particle size. Details of these compositions are set forth as Examples 1-6 in Table 1 below. The atomized stainless steel powder was added to variously prepared binders, mixed and injection-molded into a gear-like shape. Characteristics of the injection-molded part are listed in Table 2 below. 
     The weight changes of the product before and after the heating operation were measured in a nitrogen gas atmosphere. The surfaces of the products were examined and tested to determine when the binder residue was less than 1% by weight of the final product&#39;s total weight after binder-removal. The results of the test are also presented in Table 2 as a function of both heating temperature and time. 
     The molded products, whose surface appearances were evaluated as &#34;good&#34; in Table 2, were then sintered at 1,250° C. for 1 (one) hour. It was observed that this procedure provided excellent sintered parts. Plate-shape test samples were prepared according to standards issued by &#34;Japan Society of Powder and Powder Metallurgy&#34; in order to test the strength of the sintered products. The strength tests were repeated five times. The average tensile strength and elongation values for the parts are listed in Table 3 below. 
     From the noted examples 1 through 6, it can be seen that all of the compounds are suitable for use in a high speed injection-molding, and that binder-removal can be completed at a relatively low temperature of about 250° C. over shortened periods of time lasting about 16-26 hours. The appearance of the molded parts were also evaluated as being excellent. 
     In another embodiment of the invention, powders listed in Table 4 below were added to a binder in 60:40 volume ratio. The binder consisted of a low density polyethylene (fluidability 200) 20% by weight, paraffin-group wax 60% by weight,. borate-ester 18% by weight and polyoxyethylenealkylester (molecular weight is about 900) 2% by weight. The injection-moldability of this example showed similarly good results as the previously noted examples. 
     After the binder-removal process was completed at 250° C. for 16 hours, the appearances of the products were evaluated excellent as in the previous examples. Furthermore, the molded components were sintered under the conditions listed in Table 4, followed by testing of the mechanical strength. The results are listed in Table 4. 
     In order to compare the results listed in Table 4, a metal powder without any binding composition was press-sintered using conventional procedures known in the prior art and similar strength tests were performed. The results are listed in Table 5 below. From a comparison of the results shown in Tables 4 and 5, it can be clearly seen that the presently invented compound exhibits far superior characteristics when compared to similar products made by the more conventional prior art process. 
     Another example using polyoxyethylenealkylether (molecular weight is approximately 900) instead of polyoxyethylenealkylester showed similarly excellent results as exhibited in Tables 1 through 4. 
     In accordance with the teachings of this invention, excellent injection-molded parts can be produced using a metal or alloy sintering powder in a high speed injection process. The binder-removal procedure is substantially shortened without adversely effecting any of the final sintered product properties. Accordingly, manufacturing powder-sintered products through this injection-molding process can be achieved economically to produce very intricately shaped parts having thin walls and sharp edges. 
     
                                           TABLE 1                                 
__________________________________________________________________________
        binder composition (by weight %)                                  
        low density                                                       
        polyethylene             polyoxyethylene-                         
        fluidability             alkylester      low molecule             
        250                                                               
           200                                                            
              50 paraffin-group wax                                       
                           borate-ester                                   
                                 (molecule: ca. 900)                      
                                           stearic acid                   
                                                 polypropylene            
__________________________________________________________________________
example 1                                                                 
        20 -- -- 60        18    2         --    --                       
example 2                                                                 
        40 -- -- 50         8    2         --    --                       
example 3                                                                 
        20 -- -- 60        13    2          5    --                       
example 4                                                                 
        -- 75 -- 15         8    2         --    --                       
example 5                                                                 
        -- 50 -- 16        30    4         --    --                       
example 6                                                                 
        40    -- 50        9.8   0.2       --    --                       
comparison 1                                                              
        -- 20 -- 60        19    1         --    --                       
comparison 2                                                              
        -- 20 -- 60        19    1         --    --                       
comparison 3                                                              
        -- 11 -- 82         6    1         --    --                       
comparison 4                                                              
        -- 75 --  9        15    1         --    --                       
comparison 5                                                              
        -- 82 -- 11         6    1         --    --                       
comparison 6                                                              
        --  9 -- 75        15    1         --    --                       
comparison 7                                                              
        -- 30 -- 30        39    1         --    --                       
comparison 8                                                              
        -- 27 -- 70         2    1         --    --                       
comparison 9                                                              
        -- 20 -- 55        16    9         --    --                       
comparison 10                                                             
        40 -- -- 40        20    --        --    --                       
conventional 1                                                            
        -- -- -- --        --    --        --    60                       
conventional 2                                                            
        -- -- 5  90        --    --        --    --                       
conventional 3                                                            
        -- 20 -- --        --    --        10    --                       
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         binder composition (by weight %)                                 
         partially                   mineral-system                       
                                              binder mixing ratio         
         saponified wax                                                   
                  dibutylphthalate                                        
                           ethyleneacrylate                               
                                     oil solvent                          
                                              (volume %)                  
__________________________________________________________________________
example 1                                                                 
         --       --       --        --       34                          
example 2                                                                 
         --       --       --        --       39                          
example 3                                                                 
         --       --       --        --       32                          
example 4                                                                 
         --       --       --        --       50                          
example 5                                                                 
         --       --       --        --       32                          
example 6                                                                 
         --       --       --        --       39                          
comparison 1                                                              
         --       --       --        --       75                          
comparison 2                                                              
         --       --       --        --       27                          
comparison 3                                                              
         --       --       --        --       34                          
comparison 4                                                              
         --       --       --        --       40                          
comparison 5                                                              
         --       --       --        --       34                          
comparison 6                                                              
         --       --       --        --       44                          
comparison 7                                                              
         --       --       --        --       40                          
comparison 8                                                              
         --       --       --        --       36                          
comparison 9                                                              
         --       --       --        --       34                          
comparison 10                                                             
         --       --       --        --       40                          
conventional 1                                                            
         15       25       --        --       44                          
conventional 2                                                            
         --       --       3         2        38                          
conventional 3                                                            
         70       --       --        --       34                          
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                       TABLE 2                                                     
______________________________________                                    
                binder-removalability                                     
                  heating heating                                         
        injection-                                                        
                  temp.   time                                            
        moldability                                                       
                  (°C.)                                            
                          (hours) appearance                              
______________________________________                                    
example 1 good        250     26    good                                  
example 2 good        250     16    good                                  
example 3 good        250     16    good                                  
example 4 good        250     18    good                                  
example 5 good        250     16    good                                  
example 6 good        250     16    good                                  
comparison 1                                                              
          good        250     18    partially                             
                                    deformed                              
comparison 2                                                              
          no molding  --      --     --                                   
comparison 3                                                              
          low strength of                                                 
                      250     16    deformation                           
          molded product                                                  
comparison 4                                                              
          high injection                                                  
                      250     80    good                                  
          pressure                                                        
comparison 5                                                              
          no molding  --      --     --                                   
comparison 6                                                              
          good        500     80    deformation                           
comparison 7                                                              
          low strength                                                    
                      --      --     --                                   
          and fracture of                                                 
          molded product                                                  
comparison 8                                                              
          binder      250     18    porous                                
          separation                                                      
comparison 9                                                              
          low strength                                                    
                      --      --     --                                   
          and fracture of                                                 
          molded product                                                  
comparison 10                                                             
          binder      280     24    porous                                
          separation                                                      
conventional 1                                                            
          high injection                                                  
                      500     70    good                                  
          pressure                                                        
conventional 2                                                            
          low strength of                                                 
                      300     24    partially                             
          molded product            deformed                              
conventional 3                                                            
          large shrinkage                                                 
                      500     70    crack                                 
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                       TABLE 3                                                     
______________________________________                                    
             mechanical properties                                        
             after sintering                                              
               tensile strength                                           
                           elongation                                     
test sample    (kg/mm.sup.2)                                              
                           (%)                                            
______________________________________                                    
example 1      121         13.6                                           
example 2      122         13.0                                           
example 3      123         12.7                                           
comparison 4   113         13.7                                           
conventional 1 115         13.2                                           
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                                           TABLE 4                                 
__________________________________________________________________________
                                         strength of sintered product     
                                         tensile strength                 
                                                 elongation               
sintering powder                                                          
         average particle size                                            
                   process      sintering condition                       
                                         (kg/mm.sup.2)                    
                                                 (%)                      
__________________________________________________________________________
stainless steel                                                           
         15    μm                                                      
                   gas-atomized powder                                    
                                1350° C. × 2                 
                                         96.3    68.7                     
(SUS 316L)                                                                
pure iron                                                                 
         5     μm                                                      
                   carbonyl powder                                        
                                1350° C. × 2                 
                                         24.8    15.9                     
Fe--0.5P 5     μm                                                      
                   carbonyl iron powder                                   
                                1300° C. × 1                 
                                         36.7    34.9                     
         -300  mesh                                                       
                   27P--Fe crushed powder                                 
50Co--Fe 5     μm                                                      
                   carbonyl iron powder                                   
                                1350° C. × 2                 
                                         136.1    3.1                     
         4.5   μm                                                      
                   reduced Co powder                                      
Fe--2Ni  5     μm                                                      
                   carbonyl iron powder                                   
                                1300° C. × 1                 
                                         43.5    33.5                     
         5     μm                                                      
                   carbonyl nickel powder                                 
pure Co  4.5   μm                                                      
                   reduced Co powder                                      
                                1350° C. × 2                 
                                         46.2    28.1                     
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                                           TABLE 5                                 
__________________________________________________________________________
                                         strength of sintered product     
                                         tensile strength                 
                                                 elongation               
sintering powder                                                          
         average particle size                                            
                   process      sintering condition                       
                                         (kg/mm.sup.2)                    
                                                 (%)                      
__________________________________________________________________________
stainless steel                                                           
         15    μm                                                      
                   gas-atomized powder                                    
                                1350° C. × 2                 
                                         63.1    58.2                     
(SUS 316L)                                                                
pure iron                                                                 
         5     μm                                                      
                   carbonyl powder                                        
                                1350° C. × 2                 
                                         16.2    14.0                     
Fe--0.5P 5     μm                                                      
                   carbonyl iron powder                                   
                                1300° C. × 1                 
                                         35.0    30.1                     
         -300  mesh                                                       
                   27P--Fe crushed powder                                 
50Co--Fe 5     μm                                                      
                   carbonyl iron powder                                   
                                1350° C. × 2                 
                                         124.5    2.0                     
         4.5   μm                                                      
                   reduced Co powder                                      
Fe--2Ni  5     μm                                                      
                   carbonyl iron powder                                   
                                1300° C. × 1                 
                                         41.2    29.4                     
         5     μm                                                      
                   carbonyl nickel powder                                 
pure Co  4.5   μm                                                      
                   reduced Co powder                                      
                                1350° C. × 2                 
                                         42.1    19.0                     
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     While this invention has been explained with reference to the structure disclosed herein, it is not confined to the details as set forth and this application is intended to cover any modifications and changes as may come within the scope of the following claims.