US 3344848 A
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Description (OCR text may contain errors)
Oct. 3, 1967 H. D.
DIE CASTING APPARAT ETAL 3,344,848
NON--TURBULENT4 FILL AND DUAL SHOT PLUNGER ARRANGEMENT Original Filed June 24, 1963 2 Sheets-Sheet 1 Lamm lll?
MEA/70H5 A NUR/v5 Octf 3, 1967 H D. HALL ETAL 3,344,848
DIE CASTING APP'ARATUS WITH NON-TURBULENT FILL AND DUAL SHOT PLUNGER ARRANGEMENT Original Filed June 24, 1963 2 Sheets-Sheet 2 INVENTORS United States Patent C) DTE CASTING APPARATUS WITH NON-TURBU- LENT FILL AND DUAL SHOT PLUNGER AR- RANGER/,ENT
Harry D. Hall, Birmingham, Mich., David C. Salatin, An-
derson, Ind., and Don R. Sutherland, Mount Clemens, and Clayton J. Trible, Berkley, Mich., assignors to General Motors Corporation, Detroit, Mich., a corporation of Delaware Griginal application June 24, 1963, Ser. No. 289,903, now Patent No. 3,279,383, dated Sept. 6, 1966. Divided and this application Apr. 25, 1966, Ser. No. 545,112
4 Claims. (Cl. 164-312) ABSTRACT OF THE DISCLGSURE Die casting apparatus including a nonrestricted gate which leads from the shot sleeve to the die cavity, a rst shot plunger for non-turbulently moving the molten metal charge from the shot sleeve to the die cavity and subjecting the charge within the cavity to pressure, and a second smaller plunger for breaking through the initially solidified shell which prevents the rst plunger from exerting continued pressure and subjecting the interior molten portion of the Solidifying charge to additional pressure.
This is a division of the patent application Serial No. 289,903, tiled June 24, 1963 and now Patent No. 3,270,- 383.
This invention relates to die casting and more particularly to a process for making improved die castings and to apparatus for carrying out the process.
Conventional die casting procedures involve injecting molten metal into a die cavity at a relatively high pressure in order of 10,00() pounds per square inch by means of a piston-like ram. Typically, the molten metal is forced through a restrictive sprue passage to cause the metal to enter the die cavity in particulate form and at a high velocity to facilitate filling of the die cavity. The molten metal therefore enters the die cavity in a highly turbulent state and the high pressure of the injection is relied on to compress the molten metal in the die cavity to obtainv reasonably dense castings. Die casting procedures of this type may be of the hot-chamber type, as is typically used in zinc die casting, in which the injection mechanism or gooseneck is submerged in a mass of molten metal or of the cold-chamber type, typically used in aluminum or magnesium casting, in which the injection mechanism is maintained in an atmospheric environment. The principal advantage in the above high pressure type of die casting is in the speed with which the castings are made since only a few seconds are involved in the entire procedure of injecting the molten metal into the die cavity, solidification of the casting, and its removal from the die. The principal disadvantage of this high pressure type die casting is that since as above indicated the metal is injected into the dies under high pressures and generally through a restricted sprue opening the metal enters the die cavity in a highly turbulent condition with the result that the castings tend to be porous and unsuitable for many purposes. Thus, for example, castings made by the abovementioned high pressure die casting are suitable for automotive body hardware, ornamental parts, and the like. However, due to the inherent porosity of such castings, they are generally unsuitable for making cylinder blocks, cylinder heads, pump housings, and the like.
Another well-known type of die casting generally referred to as low pressure casting involves mounting the dies over an enclosed pot of molten metal, providing a tube or stalk which extends from a point within the 3,344,848 Patented Oct. 3, 1967 ICC molten metal to the sprue of the die, and subjecting the molten metal to a superimposed pressure differential in the order of a few pounds per square inch whereby the molten metal is forced slowly up into the stalk and thence into the die cavity under essentially non-turbulent conditions. The principal advantage in this type of die casting is that a more dense casting is formed. The principal disadvantage resides in the fact that a typical casting cycle requires a considerable amount of time since the molten metal is injected into the die cavity slowly and slow solidication is provided for, to prevent premature solidication of the casting in the cavity.
It is the principal object of this invention to provide an improved die casting process whereby highly improved, dense castings are made at a relatively high speed and whereby the advantages of prior art procedures are realized and the disadvantages are eliminated. Another object of this invention is to provide apparatus for carrying out this process.
These and other objects are carried out by apparatus generally of the cold-chamber type having a novel cornbination of machine elements including an enlarged gate of sucient volume to permit non-turbulent passage of the molten metal therethrough into the die cavity, a coldchamber type shot sleeve, a duplex shot plunger operable within the shot sleeve consisting of a relatively small diameter plunger reciprocally operable within a larger diameter plunger of substantially the same diameter as the shot sleeve, wherein the tip of the smaller plunger is adapted to be positioned substantially at the tip of the larger plunger in a first position thereof and to extend to a longitudinal position substantially beyond the tip of the larger plunger in a second position thereof, and hydraulic and control means for operating the two plungers as a unit when the plungers are in said first position and for operating the smaller plunger independently of the larger one to obtain said second position.
The process includes the steps of admitting a measured charge of molten metal into the shot sleeve, operating the duplex plunger as a unit to move the molten metal non-turbulently into the die cavity in the form of a solid front, maintaining a relatively high compacting pressure on the molten metal by means of the duplex unit until a relatively thin shell of molten metal has solidified adjacent the walls of the die cavity, the gating, and the duplex plunger tip to prevent further application of compacting pressure and thereafter actuating the smaller plunger independently of the larger one whereby its tip breaks through the solidified metal shell adjacent the tip of the smaller plunger to thereby subject the molten core portion of the metal in the die cavity to continued high pressure and continued filling until the metal has solidified.
Preferably, means are provided whereby the actuation of the smaller plunger is in response to the pressure built up against the larger plunger due to the aforementioned solidied metal shell having been formed adjacent the plunger and shot sleeve walls thereby preventing the larger plunger from subjecting the molten metal in the die cavity to further compression and whereby the solidifcation of the casting is not permitted to progress to a point which would involve solidiication of the core portion of the casting or to a point which would make it dificult for the smaller plunger to break through the solidied shell.
Other objects and advantages will be apparent from the following detailed description of the invention, reference being had to the accompanying drawings of which:
FIGURE 1 is a fragmentary cross-sectional view of die casting apparatus embodying this invention showing theV FIGURE 2 is a jragmentary cross-sectional view of the die cavity and plunger tip portion of the apparatus at an intermediate stage of the process of this invention just prior to the actuation of the smaller plunger.
FIGURE 3 is a fragmentary cross-sectional view similar to "thatof FIGURE 1 but after the actuation of the smaller plunger.
FIGURE 4 is a view of the apparatus shown in FIG- URE l together with hydraulic and electrical control apparatus shown in schematic form.
Referring now to the drawings and particularly to FIGURE 1, it will be observed that there is included in the fragmentary illustration of the die casting machine a cover die and an injector die 12 defining a die cavity 14 therebetween. The die member 10 is supported on the face of a stationary platen 16 by suitable bolts (not shown) which in turn is mounted on a base plate (not shown).
The stationary platen 16 also supports one end of each of four longitudinally extending tie bars 18, the opposite end of each of the tie bars being supported by a suitable upright stationary plate (not s'hown).
The movable ejector die y12 is supported on the movable platen 29 mounted on the tie rods 18 for longitudinal movement thereon. The movable platen 20 is movable reciprocally along the tie rods 1'8 between die open and closed positions by means of -a toggle device 21 as is well known in the art. Y
In the space within the movable die 1-2, there is provided an ejecting mechanism comprising a head plate 22 and a plurality of ejecting pins 24. The ejecting pins 24 are each secured at one end of the head plate 22 and lare slidably received in suitable bores formed in the ejector die 12. A plunger 26 has its forward end secured to the head plate 22 of the ejecting mechanism and extends rearwardly through an axially aligned bore in the die-12 and the movable platen 20. A bushing 28 having sealing rings 30 is provided to prevent leakage through the movable platen about the plunger 26. When the movable platen 20 is retracted and closely approaches the limits of its rearward movement, the plunger 26 is adapted to engage a stop (not shown) whereby its rearward movement is terminated. The continued movement of the platen to its rearward limit thereafter results in relative movement between the platen .20 -and the plunger 26 and between the ejector pins 24 and the movable die 12, the front ends of the pins being urged outwardly from the face of the die to dislodge the casting formed in the cavity.
Extending snugly through the aligned bores in the platen 16 and the cover die 18 is a shot sleeve 32 having a vertical extending opening 34 therein for admitting a charge of the molten metal into the shot sleeve in -any well-known manner. An important aspect of the invention resides in the provision of a wide gating 36 extending from the end of the shot sleeve 32 into the die cavity proper 14. As will hereinafter more fully be described, the gating 36 is of suflcient volume so that the molten metal may be moved into the die cavity 14 without signicant turbulence. As may be seen from the drawing, the gating 36 may be initially of the same dimensions as the shot sleeve to avoid turbulence as the molten metal changes from horizontal to vertical flow into the die cavity.
Another important aspect of the invention resides in a duplex plunger indicated generally as 38 which consists of a relatively larger outer plunger 40 snugly and reciprocally received in the shot sleeve 32 and a relatively small plunger 42 reciprocally received within a longitudinal bore of the larger plunger 48 centrally thereof. The larger plunger 40 is slidably supported on a plurality of tie rods 44, each bolted at one end thereof to the stationary platen d6, by means of the flange 45 integrally attached to the plunger 40. To the opposite ends of the tie rods 44 there is bolted a hydraulic cylinder 46 including a piston stop 47 attached to the end thereof and having a piston 48 reciprocally positioned therein. The piston 48 is attached to a connecting rod 50 which is slidably mounted on the' tie rods 44 by means of the integral ange 52. The flange 52 is connected to the flange 45 of the plunger 40 by means of the cylinder 54 welded thereto whereby the piston 48 is connected to the plunger 40 by means of the connecting rod 50, the anges 45 and 52, and the cylinder 54. The flanges 45 and 52 have aligned bores therein which receive the bushings 56 and are welded thereto. These in turn slidably receive the tie rods 44. The cylinder 54 also forms a hydraulic cylinder including the piston stop 55 attached to the rod 50, which contains a piston 58 attached to the smaller plunger piston 42.
It will thus be observed that reciprocal motion of the piston 48 is operable to move the smaller or inner plunger 42 and the larger or outer plunger 40 as a unit. On the other hand, the reciprocal motion of the piston 58 is operative to reciprocate the inner plunger 42 independently of the motion of the outer plunger 40. As will be hereinafter described, hydraulic Iand control means are provided for moving the outer plunger 40 and the inner plunger 42 in a predetermined sequence in .accordance to the requirements of the process of this invention.
Referring to FIGURE 4, an example of suitable hydraulic mechanism for practicing the process of this invention includes a gear pump 62 having an input line 64 connected to an oil sump 66 and lan output line 68 connected to a relief valve 69 discharging to the sump; to an accumulator 70; to a rst line 72 connected to the portion of the cylinder 46 outwardly of the piston 48 through a ow control valve 73, a two-way solenoid operated valve 76, the hydraulic line 79` and the hydraulic line 8-4; and to a second line 78 connected to the portion of the cylinder 46 outwardly of the piston 48 through the flow control valve 580, the solenoid operated four-way valve 812 and the lines 77 and 84. The line 718 is also connected to that portion of the cylinder 46 inwardly of the piston 48 through the flow control valve 80', the four-way solenoid operated valve 82 and the line 83. 'Ihe outlet 68 of the gear pump 62 is also connected to a third line 86 which connects with the portion of the cylinder 54 outwardly of the piston 58 through the four-way solenoid operated valve 188 and the reducing valve 90. The line 86 is also connected to the portion of the cylinder 54 inwardly of the piston 58 through the four-way valve 8-8 and the line 89. The portion of the cylinder 46 inward of the piston 48 is also connected to the oil sump 66 through the line 83, the four-way valve 82, and the line 81. The portion of the cylinder 46 outward of the piston 4'8 is also connected to the oil sump through line 84, the four-way valve 82 and the line 81. Similarly, the portion of the cylinder 54 inward of the piston 5-8 is connected to the oil sump 66 by means of the line 89, the four-way valve 1818, and the line v87; and the portion of the cylinder 54 outwardly of the piston 58 is connected to the sump 66 through the line 92, the four-way valve 88 and the line y87. The character and operation of the electrical components for operating the hydraulic apparatus will be apparent from FIGURE 4 and the detailed description of the operation of the mechanism to be made hereinafter.
The process of this invention stated generally in terms of FIGURES l, 2, and 3 involvesrst returning the duplex plunger 38 to a cycle commencement position in which the plunger 40 is withdrawn sufficiently from the shot sleeve to expose the inlet 34. When operated as ,a unit the tips of each of the plungers are substantially aligned to form a composite tip of more or less conventional shape. A measure charge of the molten metal is poured into the shot sleeve which is suflicient to lill the die cavity 14 as well as the enlarged gating portion 36 when the plunger 38 reaches the end of its stroke. Hydraulic fluid is then caused to enter the hydraulic cylinder 46 on the side thereof outward of the piston 4S whereby the duplex plunger 38 including the outer plunger 4()l and the inner plunger 42 moves as a unit forward toward the die cavity to non-turbulently move the molten metal into the die cavity 14. The volume of the gating 36 and the speed of the duplex plunger is such as to provide a lineal velocity through the gate not in excess of about 50 feet per second, i.e., a non-turbulent ow, whereby a solid front of the metal moves into and fills the cavity. Just before the die cavity 14 is lled with molten metal, additional hydraulic pressure is applied outwardly of the piston 48 whereby a squeeze in the neighborhood of 5,000-10,000 pounds per square inch is applied to the metal in the die cavity 14. The molten metal begins immediately to solidify in the die cavity and the gating adjacent the metal Walls of the die. After the lapse of a short period of time in the vicinity of a fraction of a second, a relatively thin layer of the molten metal 124 is formed adjacent the metal walls of the die within the die cavity 14 as well as the gating 36 and over the tip of the duplex plunger as shown in FIGURE 2. The formation of this metal shell prevents the further application of pressure by the duplex plunger tip on the molten metal since this metal shell creates an annular barrier 126 about the periphery of the duplex plunger tip. This condition is evidenced by a marked increase in the hydraulic pressure developed in the cylinder 46 outwardly of the piston 48. Electrical and hydraulic control mechanism is provided to cause this increase in pressure to then admit hydraulic pressure into the cylinder 54 outwardly of the smaller piston 58 whereby the smaller inner plunger 42 is caused to move forward and break through the shell 124 as shown in FIGURE 3 and whereby the core or internal molten portions of the casting is subjected to continued pressure on the order of 10,000 pounds per square inch to further feed molten metal into the die cavity to compensate for shrinkage until the molten metal has solidied, and thereby produce a high quality dense casting. After the metal has solidiiied, the hydraulic pressure outwardly of the smaller piston 58 is relieved and hydraulic pressure is applied to the cylinder 54 inwardly of the piston 58 whereby the smaller plunger 42 is withdrawn from the casting. The dies are then opened and the continued pressure of the duplex plunger upon the solidified gating biscuit forces the latter from the cover die 10. Thereafter the pressure in the cylinder 46 outwardly of the piston 48 is relieved and hydraulic pressure is applied inwardly of the piston 48 whereby the entire duplex plunger is withdrawn from the shot sleeve so as to expose the inlet 34r preparatory to another casting cycle. Withdrawal of the smaller plunger 42 prior to the withdrawal of the larger plunger 40 is essential to efficiently eject the casting.
It will be apparent that the process described has a number of important advantages. As -described above, the molten metal is moved into the die cavity slowly and nonturbulently in the form of a substantially solid front so Athat atmospheric air in the die cavity is expelled through the die vents ahead of the rising front of molten metal thus minimizing casting porosity due to air entrapment. The slow progressive illing of the die cavity operates to seal the die vents and the die parting line crevices at low pressure and thus hashing (liquid metal escaping from the die cavity through the parting line and vents) is minimized. Thus, the process is particularly advantageous in producing castings with dies which are somewhat deteriorated at the parting lines. Further, the solid shell 124 is formed substantially at lower pressures and the shell functions to contain the molten metal within the shell when the high pressure is applied by the inal squeeze of the larger plunger and the smaller plunger thus reducing the necessary die clamping force by as much as 50%. Moreover, the vhigh pressures `applied by the smaller plunger causes the shell 124 to lie up snugly against the casting cavity to obtain improved casting di- ,mensions A very important aspect of the process is that the inner portions of the casting are subject to continued pressure until complete solidication has been substantial- CII ly completed whereby shrinkage and porosity of the casting is minimized.
The process described may be advantageously practiced using the hydraulic and electrical apparatus shown in FIGURE 4. At the commencement of 'a casting cycle, the dies 1G and 12 have been closed by actuation of the toggle mechanism 21 operated by the usual hydraulic means and the duplex plunger 38 has been moved outwardly so that the plunger exposes the shot sleeve opening 34. The shot sleeve is then charged with molten metal. At this time, the operator moves the switch 96 to the plunger forward position shown in FGURE 4. In this position of the switch the contact 118 is open whereby the large plunger return solenoid 119 of the four-way double solenoid valve 82 is de-energized whereby the portion of the cylinder 46 inwardly of the piston 48 is open to the oil sump 6-5 through the hydraulic lines S3 and 81. The contact 102 thereof is closed to prepare the circuit of the solenoid 106 of the two-way valve 76. The contact 94 thereof is closed and is operative to energize the forward motion solenoid 98 of the four-way valve 82 to open the portion of the cylinder 46 outwardly of the piston 48 to hydraulic pressure from the gear pump 62 through the hydraulic line 78, the flow control valve S0, and the lines 77 and 84. The closure of the contact 94 also energizes the timer 99 to close the normally open contacts 101 and 103 by means of the coil 109 and as will be seen is necessary at a subsequent stage of the cycle to return the smaller plunger 42 from its forward position.
Actuation of the solenoid 98 causes hydraulic pressure to be admitted into the cylinder 46 through the lines 77 and 84 whereby the duplex plunger moves relatively slowly into the shot sleeve as a unit moving the molten metal ahead of it non-turbulently and with a solid front. The flow of hydraulic fluid is controlled by the flow control valve to insure a solid fill ilow velocity of about 50 lineal feet per second or less. As the duplex plunger nears the end of its stroke, it closes the limit switch 104 by the action of the cam 10S attached to the flange 52 acting on the switch arm 107. The actuation of the limit switch 194 energizes the solenoid 106 of the two-way valve 76 whereby the valve opens and additional hydraulic iiuid with the aid of the accumulator 70 is moved into the cylinder 76 outwardly of the piston 43 through the lines 72, 79, and 84. This increases the speed of the duplex plunger at the very end of its stroke and subjects the metal in the cavity to an increased filling velocity. Since the gating is now submerged, the solid front till is not disturbed by this increased velocity and the resulting higher pressure.
When the pressure in the cylinder 46 outwardly of the piston -84 builds up to a pre-set relatively higher pressure due to the die cavity having been lled and the molten metal therein having been compressed, the pressure switch 108, which is exposed to the pressure in the outward portion of the cylinder 46 through the line SS, is closed whereby the timer 110, including the coil 112 and the contact 113, is energized to close the contact 113. When the timer times out, which involves a time delay in the neighborhood of a fraction of a second, suicient to allow the solidification of the layer 124 and 12-6, the timer energizes the solenoid 114 of the four-way valve 88 through the timer 99 which as described above had been closed when the timer 99 was energized. As a conse'- quence, the portion of the cylinder 54 outward of the piston S8 is subjected to hydraulic pressure through the pressure reducing valve 90, the line 92, the Valve 88 and the line 86 and the smaller plunger 42 is caused to move forward and break through the solidified shell 124 as shown in FIGURE 3 to subject the molten material to continued pressure and to cause the shell to lie up snugly against the cavity walls and to compensate for metal shrinkage.
7 A short time before the dies 10 and 12 are to be opened, the timer 99 times out thereby deenergizing the solenoid 114 and energizing the solenoid 116 of the four-way valve ,88. This causes the hydraulic pressure outwardly of the piston 58 to be relieved through the lines 92 and 87 and for hydraulic pressure to be applied inwardly of the piston 58 through the lines 86 and 89. As a result, the plunger 42 is caused to be withdrawn from the solidified casting biscuit and returned to its original position as shown in FIGURE 4. Thereafter, other mechanism is actuated (not shown) to open the dies and effect ejection of the casting by the action of the ejector pins 24 associated with the bar 22 and plunger 26 in a well-known manner. While the dies are being opened, however, continued pressure applied by the duplex plunger forces the casting biscuit 127 (FIGURE 3) from the cover die. After the dies are fully open, the operator moves the switch 96 to a return position. As a consequence, the contact 94 is opened deenergizing the solenoid 98 and the timer 99. As a result, the hydraulic pressure outwardly of the large hydraulic piston 48 is relieved. The contact 102 is opened deenergizing the solenoid 106 and the contact 118 is closed energizing the return solenoid 119 whereby hydraulic pressure is applied inwardiy of the piston 48 through the lines 78 and 83 to return the duplex piston to its original position. The return of the piston 48 to its original position causes the limit switch 104 to open whereby the cycle is completed.
It will be understood that the control mechanism described above to illustrate the invention is a simplified version of control apparatus which may be used. HOW- ever, it will be obvious to those skilled in the art that it is desirable to integrate the operation of the apparatus described with the control of the operation of the entire die casting apparatus and to install additional control and safety features.
Although the invention has been described in terms of a specific embodiment, it will be understood that various modifications may be made within the scope of the invention.
1. In a die casting apparatus the combination comprising a stationary cover die and a movable ejector die movable between die open and die closed positions cooperatively dening a cavity therebetween, a shot sleeve having one end extending into the cover die, said dies including a substantially nonrestricted passage extending upwardly from said end of said shot sleeve to said cavity, said shot sleeve having a molten metal charging opening externally of said cover die, a first shot plunger having a Vbetween a retracted position in which the tip forms the central portion of the first plunger tip and an advanced position substantially beyond said first plunger tip, means for advancing said first plunger from its retracted position to its advanced position while the second plunger is maintained in its retracted position.
2. In a die casting apparatus the combination comprising a stationary cover die and a movable ejector die movable between die open and die closed positions cooperatively defining a cavity therebetween, a shot sleeve having one end extending into the cover die, said dies including a substantially nonrestricted passage extending upwardly from said end of said shot sleeve to said cavity, said shot sleeve having a molten metal charging opening externally of said cover die, a first shot plunger having a tip reciprocable within said sleeve between a retracted position in which said opening is exposed for receiving a charge of molten metal and an advanced position near the said one end of said sleeve, said first plunger including a second smaller plunger reciprocable within a longitudinal bore in said first plunger substantially centrally thereof between a retracted position in which the tip forms the central portion of the first plunger tip and an advanced position substantially beyond said first plunger tip, means for advancing said first plunger from its retracted position to its advanced position while the second plunger is maintained in its retracted position, means for increasing the velocity of said first plunger as it closely approaches its advanced position, means for advancing and retracting said second plunger while said first plunger is in said advanced position, and means for maintaining said forward pressure on said first plunger while said dies are being opened.
3. In a die casting apparatus the combination comprising a stationary cover die and a movable ejector die movable between die open and die closed positions cooperatively defining a cavity therebetween, a shot sleeve having one end extending into said cover die, said dies including a substantially nonrestricted passage extending from said end of the shot sleeve to said cavity, said shot sleeve having a molten metal charging opening externally of said cover die, a first shot plunger having a tip reciprocable within said sleeve movable between a retracted position in which said opening is open for receiving a charge of molten metal and an advanced position near said end of said sleeve, said first plunger including a second smaller plunger reciprocable within a longitudinal bore in said rst plunger substantially centrally thereof between a retracted position in which its tip forms the central portion of said first plunger tip and an advanced position substantially beyond said first plunger tip, a first hydraulic means for operating said first plunger and a second hydraulic means for operating said second plunger, control means for operating said first and second hydraulic means, for advancing said first plunger at a relatively slow velocity adapted to move a charge of molten metal in the form of substantially a solid front into said cavity while said second plunger is maintained in its retracted position, for increasing the velocity of said first plunger as it closely approaches its advanced position adapted to subject the molten metal in the cavity at a relatively high pressure, for advancing said second plunger to its advanced position and into the molten metal while said first plunger is in said advanced position in response to a predetermined pressure build-up in said first hydraulic means resulting from the solidification of a layer of molten metal at the periphery of said first plunger, for retracting said second plunger a predetermined time after the die cavity has been filled and after the metal has solidified and for maintaintaining pressure on said first plunger while said dies are being opened.
4. In a die casting apparatus the combination comprising a stationary cover die and a mov-able ejector die movable between die open and die closed positions cooperatively defining a cavity therebetween, a shot sleeve having one end thereof extending into said cover die, said die including a substantially nonrestricted passage extending from said one end of the shot sleeve to said cavity, said sleeve having a molten metal charging opening externally of said cover die, a first plunger having a tip reciprocable within said sleeve movable between a retracted position in which said opening is open and receiving a charge of molten metal and an advanced position near said end of the sleeve, hydraulic means .for operating said first plunger, said first plunger including a second smaller plunger reciprocable within a longitudinal bore in said first plunger substantially central thereof between a retracted position in which its tip forms the portion of said first plunger tip and an advanced position substantially beyond said first plunger tip, 'and control means for operating the said first and second plungers, for advancing said rst plunger at a relatively slow velocity adapted to move a charge of molten metal in the form of substantially solid front into References Cited said cavity While said second plunger is maintained in UNITED STATES PATENTS its retracted position and for applying pres-sure to the molten metal in said cavity, for advancing said second 2265333 12/1941 22h68 plunger to its advanced position and into the molten metal 5 65g while said first plunger is in said advanced position in respouse to a predetermined pressure developed in said hydraulc means resulting from the solidication of a layer J SPENCER OVERHOLSER Pr'mary Examiner of molten metal at the periphery of said plunger. R. S. ANNEAR, Assistant Examiner.