Abstract:
An improved method and apparatus for use in semi-solid molding. A hydraulic clamping mechanism producing substantially equal clamping force on the four comers of a moveable platen is utilized to clamp a pair of die halves together to form a die cavity for use in semi-solid molding. In one exemplary embodiment, clamping force is created when a plurality of retractable tie bars affixed to a moving platen are locked to a stationary platen and hydraulic pressure is utilized to tension the tie bars.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application is related to and claims the benefit under 35 U.S.C. § 119 (e) of U.S. Provisional Application Ser. No. 60/178,204 filed Jan. 26, 2000. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to forming products from semi-solid metals, and more particularly to a method and apparatus for effectively and efficiently forming semi-solid castings or slurried material castings.  
           [0004]    2. Description of the Related Art  
           [0005]    Semi-solid forming of light metals for net-shape or near-net-shape manufacturing produces high strength, low porosity components with the economic cost advantages of die casting.  
           [0006]    Semi-solid forming, or semi-solid molding (SSM) offers the process economies of die casting and mechanical properties that approach those of forgings. Additionally, SSM capitalizes on the non-dendritic morphology of the semi-solid stock metal to produce parts of high quality and strength. SSM can be utilized to cast parts with thinner walls than those produced by squeeze casting, due to the globular alpha grain structure and consequently improved flow properties of the semi-solid metal.  
           [0007]    The SSM process has been shown to hold tighter dimensional capabilities than any other aluminum molding process. The dimensional capabilities of this process have intensified demand for SSM components due to the potential for significant cost savings, reduction of machining and quicker cycle times for higher production rates. In addition to high strength and minimal porosity, SSM parts exhibit less part-to-die shrinkage than die cast parts and very little warpage. SSM produces castings extremely close to the desired net-shape which reduces and can even eliminate secondary machining operations.  
           [0008]    To exhibit optimal thixotropic properties, the stock material utilized in SSM must be converted from a dendritic morphology to a globular morphology. One technique to obtain a globular morphology utilizes stirring during solidification of a molten metal alloy. Stirring during solidification can be performed utilizing a mechanical stirring process, or electromagnetic stirring. As is known in the art, electromagnetic stirring utilizes an applied rotating electromagnetic field to induce fluid flow in liquid metal without any physical contact by a mechanical stirring device.  
           [0009]    Stirring the molten metal alloy during solidification causes a shearing of the stock material microstructure and causes the morphology of the alloy to change from dendritic to globular. Commonly, after stirring and the consequent change in morphology has occurred, the alloy is cooled and completely solidified into a billet. The raw billet is then cut into a plurality of slugs to obtain the desired volume of raw material for a particular casting application.  
           [0010]    If solidified billets and slugs are utilized in the SSM process, they are reheated, e.g., by an induction heater, to a semi-solid form before being placed in a die casting machine. The semi-solid alloy exhibits higher viscosity relative to molten metal and flows more like plastic in injection molding than molten metal in die casting. Generally, semi-solid metal exhibits laminar flow characteristics as it enters the die casting machine.  
           [0011]    The rounded or globular shaped particles of semi-solid metal easily slide over each other making it possible to form complex shapes with low applied injection force. Semi-solid molding can produce complex shaped components without the excessive scrap produced when such components are formed by machining. Semi-solid molding produces net-shaped or near-net-shaped components and thus greatly reduces required machining. Parts produced using semi-solid molding have higher tensile strength and ductility relative to die casting and can include thin walled portions due to the flow properties of the semi-solid stock material.  
           [0012]    Semi-solid metal forming has many advantages over die casting. The reduced temperature of the semi-solid metal charge relative to the molten metal utilized in die casting places less heat extraction requirements on the die or mold and thus decreases mold and die wear and lessens solidification shrinkage. Additionally, the lower temperature of semi-solid metal relative to molten metal in die casting leads to shorter solidification time in semi-solid molding. Furthermore, the higher viscosity of semisolid metal relative to the molten metal used in die casting leads to less turbulent mold and die filling, less gas entrainment, less porosity, and lower occurrence of other solidification defects. The less turbulent flow of semi-solid metal allows for improved material utilization in forming small components due to the accurate introduction of metal into the forming die.  
           [0013]    The molten metal utilized in die casting often must be injected into the bottom of the die. If the molten metal in die casting were injected into a non-bottom portion of the die, the extremely low viscosity of the molten metal would allow portions of the molten metal to leak into the die prior to the injection process taking place. This does not present a problem in SSM because of the higher relative viscosity of semi-solid metal.  
           [0014]    Semi-solid molding is currently performed utilizing a die casting machine with a toggle clamping structure as is known in the art. Toggle clamps are utilized to join mating dies and to resist the forces created by the high pressure injection of the semisolid or molten metal as required by SSM or die casting respectively. The molten metal in die casting is typically shot into the die at between 100 and 250 inches/second, while semi-solid metal is typically introduced into the die at a speed of 5-10 inches/second. Toggle mechanisms typically comprise four over center knuckles and apply clamping force to the four comers of the movable platen of the die casting machine. The mechanical nature of this clamping mechanism and the structure of an individual die can lead to unequal force application at the four comers of the movable platen. Die designers must balance part placement within the die to avoid exacerbating the problem of unequal force application. Furthermore, the toggle clamping structure utilized in die casting machines substantially increases the footprint of the casting machine.  
           [0015]    Die casting machines utilized in SSM processes include two die carrying platens. One of the die carrying platens is stationary and is connected to the injection unit which injects the semi-solid metal into the die cavity. The other of the die carrying platens is a movable platen which is moved into engagement with and clamped to the stationary platen to engage the two die halves and receive and form the metal during the forming process. The movable platen is moved out of engagement with the stationary platen so that the completed part may be removed from the dies.  
           [0016]    What is needed in the art is a semi-solid metal forming method and apparatus which allows semi-solid metal forming to be accomplished utilizing machines of smaller size than die casting machines and which produces uniform clamping force on the die carrying platens.  
         SUMMARY OF THE INVENTION  
         [0017]    The present invention provides an improved method and apparatus for use in semi-solid molding, wherein it is desired to perform semi-solid molding utilizing a machine smaller in size than the previously utilized die casting machines and which produces equal clamping force on all four comers of the movable platen. The current invention replaces the toggle clamping mechanism of a die casting machine with a hydraulic clamping mechanism of the type used in plastic injection molding, requiring less space than the toggle clamping mechanism. The hydraulic clamping mechanism of the current invention not only decreases the size of the semi-solid molding machine, but also produces uniform clamping force. The hydraulic clamp can be, e.g., a hydro-mechanical clamp.  
           [0018]    The invention, in one form thereof, comprises a semi-solid molding machine having a hydro-mechanical clamping apparatus. In an exemplary embodiment of the current invention, the hydro-mechanical clamping apparatus comprises four retractable tie bars affixed to a movable platen. The retractable tie bars include distal tie bar lugs which are engaged by a rotary locking mechanism when the movable platen is moved into contact with the stationary platen. The rotary locking mechanism locks each tie bar to a hydraulically actuated piston. Hydraulic pressure applied to the piston tensions the retractable tie bars and clamps the movable platen to the stationary platen.  
           [0019]    An advantage of the present invention is the uniform clamping pressures supplied by the hydro-mechanical clamping mechanism.  
           [0020]    Another advantage of the present invention is the ability to provide increased tool and die life due to uniform clamping pressure which eliminates die twisting and other problems which decrease die longevity.  
           [0021]    A further advantage of the present invention is the ease of part removal due to the increased die access area produced by the retractable tie bar clamping mechanism of the present invention.  
           [0022]    Yet another advantage of the present invention is the ability to easily increase the stroke of the machine. With the retractable tie bar design of the present invention, the only modification required to extend the machine&#39;s stroke is to extend the base and the shuttle cylinders. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention itself will be better understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:  
         [0024]    [0024]FIG. 1 is a side elevational view of a semi-solid molding machine according to one embodiment of the current invention;  
         [0025]    [0025]FIG. 2 is a top plan view thereof;  
         [0026]    [0026]FIG. 3 is a partial sectional view of the semi-solid metal injection unit;  
         [0027]    [0027]FIG. 4 is a partial sectional view of an alternative embodiment of a semi-solid molding injection unit;  
         [0028]    [0028]FIG. 5 is a side elevational view partially in section of a semi-solid molding machine according to one embodiment of the current invention;  
         [0029]    [0029]FIG. 6 is an end elevational view of the movable platen; and  
         [0030]    [0030]FIG. 7 is a sectional view of the tie bar locking mechanism. 
     
    
       [0031]    Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates one preferred embodiment of the invention, in one form, and such exemplification is not to be construed as limiting the scope of the invention in any manner.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0032]    Referring now to the drawings and particularly to FIG. 1, there is shown semi-solid molding machine  30  including base  22 , semi-solid stock material injection unit  26 , stationary platen  14  and movable platen  12 . Machine  30  is generally formed of clamp end  100  and shot end  102 . Shot end  102  is of the type generally utilized in die casting machines, while clamp end  100  is of a type commonly utilized in injection molding machines. Machine  30  is formed as a hybrid of, e.g., HPM die casting machine model M-900-A, and HPM NEXT WAVE 2200 ton injection molding machine. Stationary platen  14  and movable platen  12  carry die halves  32 . Machine  30  includes accumulators  18  and intensifier  16  as is known in the art. Retractable tie bars  28  are affixed to movable platen  12  and are engageable in locking units  20 . Movable door  24  selectively limits access to the die area of machine  30 .  
         [0033]    [0033]FIG. 2 illustrates machine  30  including retractable tie bars  28  and movable platen shuttle cylinders  34 . Movable platen shuttle cylinders  34  are diagonally opposed and thus provide increased access to dies  32 . As illustrated in FIG. 6, stationary platen  14  includes four rotary locking bushings  38  while movable platen  12  includes four retractable tie bars  28  affixed at one end of movable platen  12 . Retractable tie bars  28  are each received into one of rotary locking bushings  38  when movable platen  12  is moved into position to engage stationary platen  14  and operatively join dies  32 . Rotary locking bushing  38  is rotated and locks the respective retractable tie bar  28  in engagement with a matching lug  40  on a secondary tie bar positioned between lug  40  and hydraulic clamp piston  44 .  
         [0034]    Tie bar lugs  78  each include four lobes which pass through channels  90  of rotary locking bushings  38 . After entry of the four lobes on each tie bar lug  78  through channels  90 , rotary locking bushing  38  is rotated 45° so that the partial flanges located between channels  90  engage the lobes of tie bar lugs  78  and thereby retain retractable tie bars  28 .  
         [0035]    Levers  86  are operably affixed to rotary locking bushings  38  as illustrated in FIG. 6. Levers  86  are hingedly connected to respective linkages  84  which are operably connected to locking cylinders  82  via pistons  88 . Locking cylinders  82  are operable to actuate pistons  88  which results in substantially vertical movement of linkages  84  and rotation of levers  86  and rotary locking bushings  38 . In this way, locking cylinders  82  may be utilized to rotate locking bushings  38  and thereby lock and unlock retractable tie bars  28 .  
         [0036]    As illustrated in FIG. 7, matching lug  40  is connected to hydraulic piston  44  and, when retractable tie bars  28  are locked, pressurized hydraulic fluid is supplied to face  92  of hydraulic clamp piston  44  from a hydraulic fluid source (not shown). The hydraulic fluid places matching lug  40 , rotary locking bushing  38  and retractable tie bar  28  in tension and clamps movable platen  12  to stationary platen  14 . Since the hydraulic clamping cylinders are located on stationery platen  14  they can be rigidity piped, avoiding the need for moving hydraulic hoses. In one exemplary embodiment, each clamping cylinder has a transducer that monitors pressure. If a pressure drop is detected, the system tries to compensate to prevent uneven tie bar loading. If the system can not compensate, the machine will shut down and diagnose the problem. While the clamping mechanism of the current invention has been described as a hydro-mechanical clamping mechanism utilizing retractable tie bars, other hydraulic clamping mechanisms, such as those used in straight hydraulic injection molding machines, may be utilized in accordance with the teaching of the present invention.  
         [0037]    As illustrated in FIG. 3, semi-solid metal injection unit  26  is affixed to stationary platen  14  and provides semi-solid stock material to dies  32 . Shot sleeve  50  includes injection port  46  into which semi-solid metal is inserted. Semi-solid metal inserted into shot sleeve  50  can, for example, take the form of a slug  58  of thixotropic aluminum. Thixotropic stock material may also be inserted into injection port  46  from a nearby mixing station as described in  Semi-Solid Metal Process Eliminates Preformed Billets,  Die Casting Management, March 1998, page 31-33, the disclosure of which is herein explicitly incorporated by reference.  
         [0038]    Plunger  52  contacts the stock material inserted into shot sleeve  50  and is utilized to introduce the stock material into cavity  60  formed by dies  32 . In one exemplary embodiment, plunger  52  includes a beryllium tip. Plunger  52  is affixed to piston rod  54  while piston rod  54  is affixed to piston  56 . In operation, piston  56  is actuated by pressurized hydraulic fluid in chamber  62  thereby causing movement of plunger  52  to inject stock material  58  into die  32 . If desired, a vacuum can be utilized to remove air from die  32  and assist in filling the die with semi-solid metal  58 .  
         [0039]    [0039]FIG. 4 illustrates a further embodiment of semi-solid injection unit  26 . The semi-solid metal injection unit  26  illustrated in FIG. 4 includes plunger  52  affixed to end  53  of piston rod  54  which is affixed to piston  56 . Piston  56  is further connected to stroke adjustment screw  48 . Stroke adjustment screw  48  is contained within stroke adjustment housing  64  and may be advantageously utilized to adjust the stroke length of plunger  52  depending upon the current application. Generally, the shot cylinder stroke of a semi-solid machine is  20 - 30 % longer than that of a die casting machine. Stroke adjustment screw  48  may advantageously allow injection unit  26  to accommodate the shot cylinder stroke of either a die casting or a semi-solid molding machine. Check valve  72  includes spring  74  and spring retainer  76 . Guide plate  66  is affixed between packing shim  68  and packing retainer  70  and is useful for guiding piston rod  54 .  
         [0040]    After a forming operation has taken place, retractable tie bars  28  are unlocked and retract with movable platen  12 . As the mold opens, retractable tie bars  28  move back out of the way and allow for quick, easy and possibly automated mold changes as well as fast and easy removal of large parts. After the part is removed, movable platen  12  is returned to operable molding position, with each tie bar lug  78  engaged by rotary locking bushing  38 , and hydraulic piston  44  supplying tension thereto.  
       EXAMPLE  
       [0041]    In one application of the current invention, the parts to be formed comprise large shallow discs which will weigh between 3.9 and 10.3 pounds after casting. The semi-solid stock material will be injected into the center of the die, and the following machine configuration will be utilized to effectively form the part.  
         [0042]    Shot sleeve diameters of five inches and six inches are operative to receive slugs having diameters of four inches and five inches respectively. Recommended process pressures for this application are up to 30,000 psi. Prior to injection of semisolid metal, the dies are heated to delay premature solidification during filling. Additionally, to avoid premature solidification, filling speeds should be as fast as possible without producing turbulent flow. The air volume to be vented from the shot sleeve should be minimized by minimizing the shot sleeve diameter and positioning the ram within one-half inch of the slug.  
         [0043]    For this application, a two hundred fifty ton ram is utilized to inject the slug. However, providing tonnage 25-35% larger will prolong press life. A ram velocity of three to thirty inches per second is recommended. Transition from velocity profile to dwell pressure should be less than 0.1 seconds with an intensifier being employed to accommodate shrinkage during solidification. Ram hydraulics producing 1500-2000 psi with intensification at the cylinders provides adequate hydraulic pressure for this example application. The recommended clamp tonnage for this application is 2200 tons to resist the maximum forming pressure, although additional tonnage may be utilized to provide increased safety.  
         [0044]    A dwell time of five seconds should be sufficient, but dwell times up to ten seconds may be required to allow for sufficient solidification. The shot sleeve and injection ram tip should be preheated and the shot sleeve temperature can be controlled utilizing heat transfer oil.  
         [0045]    While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.