Method and apparatus for thixotropic molding of semisolid alloys

Abstract

A method and apparatus for thixotropic molding of semisolid alloys are disclosed. The method comprises of feeding a dendritic-free feedstock bar into an extruder barrel, melting a terminal portion of the feedstock bar into a semisolid slurry by heating it to a temperature between its solidus and liquidus temperatures, and using the solid portion of the feedstock bar as an one-time “plunger” to inject the semisolid slurry into a mold cavity. The apparatus based on the present method, which is equipped without a regular extruder screw or plunger, uses a feedstock bar as a one-time plunger to inject semisolid slurries formed from a terminal portion of the same feedstock bar.

I claim:  
     
         1 . A method for thixotropic molding of semisolid alloys comprising of feeding a dendritic-free feedstock bar into an extruder barrel, melting a terminal portion of the said feedstock bar in a heating zone of the said barrel into a semisolid slurry by heating it to a temperature between its solidus and liquidus temperatures, and using the solid portion of the said feedstock bar as a one-time “plunger” to inject the said semisolid slurry into a mold cavity.  
     
     
         2 . The method of  claim 1  requires no regular extruder screw or plunger to perform thixotropic molding of semisolid alloys.  
     
     
         3 . The method of  claim 1  wherein the said extruder barrel contacts with the said semisolid slurry and is subjected to an elevated temperature and injection pressure only at its frontal portion.  
     
     
         4 . The method of  claim 1  wherein the said feedstock bar serves as both a feedstock of the said semisolid slurry and a one-time plunger simultaneously.  
     
     
         5 . The method of  claim 1  wherein the said extruder barrel has a cooling zone where leaked slurry is frozen into a sealant, which closes the clearance between the said feedstock bar and the inner surface of the said barrel.  
     
     
         6 . The method of  claim 1  wherein the said heating zone is periodically sealed by means of the formation of the said sealant at one end and a solid plug formed from the residual slurry in the discharge nozzle at the other end upon the completion of each shot, and, therefore, oxidation is prevented without using a protective gas within the said barrel.  
     
     
         7 . An apparatus for thixotropic molding of semisolid alloys according to the method of  claim 1 , which comprises: 
 a) an extruder barrel having a discharge nozzle at one end;    b) a feeding means to drive a dendritic-free feedstock bar into the said extruder barrel and toward the said discharge nozzle;    c) a heating means to generate a heating zone in the said extruder barrel to heat a terminal portion of the said feedstock bar to a temperature between its solidus and liquidus temperatures;    d) a cooling means to generate a cooling zone adjacent the said heating zone in the said extruder barrel to freeze slurry leaked from the said heating zone into a solid sealant;    e) a supporting means to withhold the shot pressure subjected to the heating portion of the said barrel.    
     
     
         8 . The apparatus of  claim 7  wherein the said extruder barrel is a tri-metallic cylinder constituting a bimetallic and a monometallic portion joined by welding.  
     
     
         9 . The apparatus of  claim 8  wherein the said bimetallic portion comprises of an outer shell made of one material with high strength at operating temperatures and a liner made of another material with high corrosive resistance to the semisolid slurry and shrunk-fit onto the said outer shell.  
     
     
         10 . The apparatus of  claim 7  wherein the said extruder barrel equipped with an O-ring at its rear to prevent air from entering the said barrel via the clearance between the said feedstock bar and the inner surface of the said barrel.  
     
     
         11 . The apparatus of  claim 7  wherein the said extruder barrel has relatively thin walls giving advantages in economy and precise temperature control.  
     
     
         12 . The apparatus of  claim 7  wherein the said feeding means comprises of a pair of unassisted wedges, which drives the said feedstock bar into the said barrel during forward motion and then slides freely back.  
     
     
         13 . The apparatus of  claim 7  wherein the said heating means comprises of a series of band resistance heaters attaching on the outer surface of the said extruder barrel.  
     
     
         14 . The apparatus of  claim 7  wherein the length of the said heating zone is adjustable by the number of band heaters used.  
     
     
         15 . The apparatus of  claim 7  wherein the said cooling means is a two-part cooling ring with internal circulating coolant.  
     
     
         16 . The apparatus of  claim 7  wherein the said supporting means comprises of a series of supporting hoops, each of which has a groove for housing a band heater.  
     
     
         17 . The apparatus of  claim 7  wherein the said extruder barrel, said feeding means, said heating means, said cooling means and said supporting means are within a barrel housing.  
     
     
         18 . The apparatus of  claim 7  wherein the said extruder barrel can be easily detached from the said barrel housing.  
     
     
         19 . An alloy switching method, without need of barrel opening and purging, via replacement of the current barrel with another barrel preloaded with a feedstock bar of another alloy.