US 3516480 A
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Description (OCR text may contain errors)
June 23, 1970 J woLTERlNG 3,516,480
SHOT TUBE FOR A DIE CASTING TYPE MACHINE Filed June 17, 1968 I NVENTOR.
United States Patent US. Cl. 164-312 Claims ABSTRACT OF THE DISCLOSURE An improved shot tube structure for a die casting type machine of horizontal cold chamber design. The preferred embodiment of the shot tube has an outer sleeve fitted to an inner sleeve, the outer sleeve having a heat exchange insert fitted into a longitudinal slot along the bottom of the outer sleeve. The insert acts as a heat transmitter to receive and to dissipate heat from the bottom of the shot tube to the atmosphere as successive molten metal charges are ladled into and ejected from the shot tube. Such structure prevents warping or bowing of the shot tube during extended die casting runs of high melting point alloys.
This invention relates to die casting type machines and, more particularly, relates to die casting type machines of cold chamber design.
The two principal types of die casting machines are: (l) the gooseneck or submerged hot chamber machine, and (2) the cold chamber machine. The hot chamber die casting machine includes a furnace for holding molten metal and a goosenecked chamber submerged in the molten metal, a port being provided beneath the surface of the metal in the furance for charging the chamber. The goosenecked chamber communicates at one end with a die cavity and receives a piston at the other end. To die cast a product, the piston is driven into the gooseneck chamber to force metal around the gooseneck out into the die. This hot chamber machine is useful primarily with tin, lead and zinc based alloys. These alloys have relatively low melting points and, as such, the hot chamber process may be used because the continual contact of molten metal with the goosenecked chamber will not adversely affect the structure of that chamber over an extended period of time.
There are two main designs of cold chamber, die casting type machines, namely, the horizontal shot tube design and the vertical shot tube design. The shot tube chamber is not submerged in molten metal in either of these machines but receives successive molten metal charges as required.
In the vertical design cold chamber machine, the shot tube is vertically disposed and is open at its top end to provide entry for the molten metal charge. The top end also receives a primary plunger adapted to reciprocate into and out of the shot tube through the top end. An outlet port connected to the die cavity is provided toward the bottom of the shot tube. The bottom end of the shot tube may have a secondary plunger that prevents molten metal from running into the die cavity when the shot tube is being charged. The casting sequence includes the steps of ladling molten metal into the top of the shot tube (the metal forming a pool on top of the secondary plunger), and then advancing the primary plunger into the shot tube. As the primary plunger advances it pushes down the secondary plunger, thereby opening the outlet port to the die cavity and injecting the molten metal into the cavity. After injection has been completed the excess metal, which has formed a slug in the shot tube, is automatically sheared off by the rising secondary plunger and "ice is pushed up to the top of the shot tube where it is ejected. The secondary plunger then returns to its normal position ready for the next casting cycle.
In the horizontal shot tube design, the shot tube is arranged horizontally and communicates at one end with a die cavity, the other end being provided with a reciprocatable plunger. A port is provided in the shot tube intermediate the plunger and die cavity for receiving the molten metal. The casting sequence includes the steps of ladling the molten metal charge from a furnace into the shot tube (the metal forming a pool along the bottom of the shot tube), and then advancing the plunger to force the molten metal into the die cavity. As the plunger is advanced it seals the port and then forces the metal into the cavity.
The cold chamber process is often utilized for the high melting point metals and alloys such as aluminum, magnesium, copper and ferrous metal and their alloys. For example, the melting points of the aluminum and mag nesium alloys are in the vicinity of 1200 F., depending on composition, and the melting points of the brasses, that is, the copper-zinc alloys, is in the range of 1600 F. upward to 2300 F. Although a difference of 400 F. may not seem particularly large, at these high temperatures the difliculties encountered pyramid rapidly as the melting point increases, and the casting of metal under pressure at these high temperatures presents many problems not encountered at casting temperatures a few hundred degrees lower.
The high melting point alloys of copper, aluminum, magnesium and iron generally cannot be cast in a submerged plunger or goosenecked machine because the steel or cast iron surface of the plunger and cylinder is submerged in the furnace at all times, thereby contaminating the alloy and causing the production of inferior castings. Thus, the hot or submerged chamber machine is most often used for the casting of the low melting point alloys such as alloys of zinc, tin, and lead; and the cold chamber machines are used for the casting of higher melting point alloys such as alloys of aluminum, magnesium, copperzinc and iron. In both of these basic types of machines the back or die ends are essentially identical. The primary difference is in the injection system. The furnace that holds the molten metal at casting temperature is not an integral part of the cold chamber type as it is of the hot chamber type. In the cold chamber machine the metal is ladled, or charged, either manually or automatically, into the shot tube or injection cylinder prior to each shot and is then forced by the plunger into the die. On the submerged plunger machine, the furnace is actually attached to the machine and the plunger and cylinder are submerged into molten metal. At the end of each shot, metal flows into the cylinder; and at the start of the next cycle, it is forced by the plunger into the die.
The shot tubes for die casting type machines of the cold chamber design are commonly made of heat-treated tool steel to Withstand the accelerated Wear and thermal shock encountered when used with the high melting point metals. When high temperature melting point metals are poured or ladled into a horizontal design cold chamber machine, that is, a die casting type machine that employs a horizontally positioned shot tube, the metal does not entirely fill the shot tube but lies or pools along the bottom of the tube prior to commencement of the forward plunger stroke. Heat from the molten metal is rapidly conducted into that portion of the horizontal shot tube where the metal is pooled, thereby creating an uneven or nonsymmertical temperature gradient in the chamber so that the molten 'metal contacted portion of the tube is many degrees hotter than the portion not contacted by the molten metal (the portion above the liquid level). Due to this temperature gradient, which becomes more and more pronounced over extended production runs of high melting point metals, it has been experienced that in cold chamber die casting shot tubes of the horizontal type the bottom portion of the shot tube distorts, warps, or bananas along its axis. Thus bowed, passage of the plunger through the horizontal shot tube from one end to the other will, at the very least, accelerate wear of both the tube and plunger head. Depending upon the degree of warp or how, the plunger may become bound in the horizontal shot tube sufficiently to cause substantial machine downtime. Under extreme circumstances the plunger may actually freeze in the horizontal shot tube so that the plunger head actually has to be bored out, thereby ruining the plunger. Such occurrences, of course, may affect quality of the castings themselves by reason of charge loss until discovered and will certainly reduce efiiciency of the casting operation. This problem does not obtain in hot chamber die casting type machines nor in cold chamber die casting type machines of vertical shot tube design because in the hot chamber machine the goosenecked shot tube is at substantially the same temperature as the molten metal since it is submerged in the molten metal. In the vertical cold chamber machine the molten metal charge does not pool on the bottom or inner surface of the shot tube but. pools on top of the secondary plunger before being forced into the die and thus is in peripheral contact with the vertical shot tube so that no temperaure gradient across a cross section of the shot tube is established.
Accordingly, it has been an objective of this invention to provide an improved shot tube or injection cylinder for a cold chamber die casting type machine of horizontal shot tube design that will substantially obviate the problem of shot tube bow or 'warpage during extended runs of that type machine.
This objective has been achieved by providing, in preferred embodiment form, a shot tube having an outer sleeve fitted to an inner sleeve. The outer sleeve carries a heat exchange insert fitted into a longitudinal slot along the bottom of the outer sleeve. The insert acts as a heat transmitter to receive, to transmit, and to dissipate heat from the bottom of the shot tube to the atmosphere as successive metal charges are ladled into the tube. This structure prevents warping or bowing of the hot tube during extended usage when utilized in a die casting type machine of horizontal cold chamber design.
Other objectives and advantages of this invention will be more apparent from the following detailed description taken in conjunction with the drawings in which:
FIG. 1 is an axial cross-sectional view, somewhat diagrammatic, of a die casting type machine of horizontal cold chamber design having a shot tube formed in accordance with this invention;
FIG. 2 is a partially broken way perspective view of that shot tube illustrated in FIG. 1;
FIG. 3 is a cross-sectional view taken along lines 33 of FIG. 1; and
FIG. 4 is a bottom view of the shot tube illustrated in FIG. 1.
As illustrated in FIG. 1, a typical die casting type machine of horizontal cold chamber design includes a horizontally positioned shot tube 11 with a plunger head 12 disposed in one end thereof. At the other end the shot tube 11 communicates with a die cavity 13 defined by a stationary die half 14- and a movable die half 15. The leading end of the shot tube 11 extends into the stationary die half 14 and is positioned therein by flange 19 integral with the shot tube. The movable half 15 is reciprocable by apparatus, not shown, into and out of engagement with the fixed half 14 to permit removal of cast parts from the die cavity 13. Each half 14, 15 of the die may be comprised of a shoe 16 and an impression block 17, the shoe and impression block defining the die cavity 13, to more readily effect changing of the die cavity configuration. The die assembly 14, 15 may also be pro vided with ejector pins 18 for ejecting the cast part from the movable die halfs cavity when the halves are separated.
The horizontally positioned shot tube 11 is provided with an orifice or port 21 intermediate the retracted or charging position of the plunger 12, as illustrated in FIG. 1, and the die 14, 15. It is by means of the orifice 21 that molten metal 22 is charged by ladle 23 into the horizontal shot tube chamber and, thereafter, forced by the plunger head 12 into the die cavity 13.
In the machine illustrated, when a charge of molten metal 22 is ladled into the horizontally disposed shot tube 11, the metal momentarily runs out, or pools, on the bottom of the shot tube, as at 24, before the plunger 12 is actuated forward to force the molten metal into the die cavity 13. During this period of time substantail heat is transferred by direct conduction from the molten metal pool 24 to that bottom portion 25 of the shot tube 11 on which the pool is supported because of the intimate contact between the molten metal and the bottom. Thus, the bottom 25 of the shot tube 11 becomes substantially hotter than other portions of the tube because those other portions are not in intimate contact with the molten metal, and are, therefore, only heated by convection and indirect conduction. That is, portions of the shot tube 11 other than the bottom 25 are insulated from the molten metal by the atmosphere in the shot tube because the amount of heat transferred is substantially greater from the molten metal to the shot tube in those areas where the molten metal is in intimate contact with the shot tube than in those areas where it is not. On extended production runs of die cast parts the shot tube 11 grows hotter and hotter as successive molten metal charges are ladled into the shot tube chamber 26 and forced therefrom into the die cavity 13. Without the advantageous structure of this invention, as the bottom 25 of shot tube 11 increases in temperature, it would tend to bow or warp in an upward or concave manner so that the inner periphery of the bottom of the shot tube is no longer parallel with the axis of the shot tube nor the axis of the plungers movement. Pronounced warpage would cause the plunger 12 to stick intermediate its cycle within the shot tube 11 or may even keep it from moving at all, as discussed above.
A solution to this bowing or warping problem in the shot tube is provided by the novel shot tube 11 structure of this invention. The preferred embodiment of the shot tube 11 is formed of an inner cylinder 28 and an outer cylinder 29 that are concentric. The concentric cylinders 28, 29 are sized to fit together in intimate contact throughout their axial length, see FIG. 2. The inner cylinder 28 preferably is formed of a high grade tool steel to withstand thermal shock. The outer cylinder 29 is preferably formed of cold rolled steel and is present to lend structural rigidity to the inner cylinder 28. Such a dual cylinder shot tube structure permits the shot tube 11 to be more economically produced because high grade tool steel is substantially more expensive than cold rolled steel. The bottom portion 25 of the outer cylinder 29 is provided with aslot or groove cut through the cylinder 29 wall such as the dovetailed slot 30. Thus, the slot 30 is cut into the outer cylinder 29 on the underside of that cylinder 'm the area where the molten metal 22 normally pools at 24 in the inner cylinder 28 after it has been ladled into the shot tube 11, that is, on that side of the outer cylinder 29 opposite the port 21. The slot 25 preferably extends from immediately under the pouring port 21 of the shot tube 11 forward to the die halves 14, 15 and advantageously extends between the retracted position of the plunger 12 (see FIG. 1) and the die.
While the depth of slot 30 has been illustrated as being cut through the thickness of the outer cylinder 29, it will be understood that the slot may be cut into the outer cylinder without completely cutting through the wall of the cylinder 29, that is, to a depth less than the thickness of the cylinder wall. Alternatively, the slot 30 may be cut to a depth greater than the thickness of the outer cylinder 29 wall so that it extends into the wall of the inner cylinder 28, too. However, it is not desirable to provide a slot of a depth equal to the wall thickness of the combined inner (28)-outer (29) cylinder shot tube structure for the reason that joints are not desirable on the inner surface of the inner cylinder.
The slot 30 in the outer cylinder 29 of the shot tube 11 is provided with a mating dovetailed, bar-like, heat exchange insert 33 for acting essentially as a heat transmitter and promoting the dissipation of excess heat to the atmosphere from the bottom 25 of the shot tube 11 when successive molten metal charges are ladled into the shot tube.
Insert 33 is preferably provided with a U-shaped coolant passage 34 having an inlet 35 and an outlet 36, the inlet and outlet being positioned adjacent that end of the insert furthest removed from the die halves 14, 15, so that the insert can more efficiently transmit and dissipate heat away from the bottom of the shot tube 11. The inlet 35 and outlet 36 are threaded to receive suitable couplings, not shown, for relating the passage 34 to suitable coolant lines, not shown. The bar-like insert 33 is preferably closely fitted into the slot 30 so as to establish a tight and full fit for its entire length to ensure good heat transfer from the inner cylinder 28 to the insert 33. That is, the insert 33 is preferably configured in cross section to completely fill the cross-sectional area of the slot 30 cut in the outer cylinder 29, see FIG. 3.
Alternatively, or in addition to the coolant passage 34, the insert 33 may be provided with fins that extend or hang down below the periphery of the outer cylinder 29. Such fins may be positioned either parallel to the inserts length or transverse to it, and may be air cooled by forced air if desired. Any other type of heat transmitting structure may be substituted for the coolant passage 34 or the fins, the primary objective being to transmit and to dissipate the excess heat from the bottom 25 of the shot tube 11 more rapidly than heat is dissipated from the top of the shot tube to balance out the uneven rates of heat transfer from the metal charge 24 to the tube.
One of the main reasons it is preferred to use a dual shot tube 11 structure, that is, a structure utilizing an inner cylinder 28 with an outer cylinder 29, is that this combined structure permits the heat exchange insert 33 to be easily mated with the shot tube by a practical method of fabrication. In making the shot tube 11 of this invention it is preferred that the outer cylinder 29 be completely separated from the inner cylinder 28. The dovetail slot 30 is then cut through the outer cylinder 29 wall, the narrow end 41 of the slot being on the outer surface of the cylinder 29. An insert 33 of the desired material is then cut from rectangular bar stock with the width of the insert and, hence, sides 42, dimensioned and configured to fit flush with the sides of the dovetail slot 30. Holes 43 are then tapped at the interface 44 of the dovetail slot 30 and insert 33, and set screws 45 inserted in those holes to maintain the insert in position with the slot. At this point, the outer cylinder 29 is still separate from the inner cylinder 28 and the insert sides 42 are mated and closely fitted with the slot sides in the outer cylinder wall. However, the top 46 of the insert 33 is still flat, and the bottom 41 of the insert is still flat.
The next step in forming the shot tube structure involves boring the inside of the outer cylinder 29 so as to make the radius of the inserts top surface 46 conform to the inner radius of the outer cylinder 29. After the boring step is completed the inner cylinder 28 can be fitted within the outer cylinder 29 so that the top surface 46 of the insert 33 is closely fitted to and mated with the outer surface of the inner cylinder 28, thereby maintaining intimate contact of the inner cylinder with the heat exchange insert so as to insure eificient transmission of heat out of the bottom 25 of the shot tube 11.
With this method of fabrication, even if the coefficient of expansion for the heat exchange insert material is substantially different from the coeflicient of expansion for the inner (28)-outer (29) cylinder material, thereby tending to create gaps between the insert 33 and cylinders 28, 29 at elevated temperatures, the insert will always remain fixed in place within the dovetail slot 30 and optimum heat transfer will be maintained out of the bottom of the shot tube 11.
The material from which the heat exchange insert 33 is made is primarily dependent upon the melting point of that metal being cast. The higher the melting point of the casting metal the higher must be the melting point of the insert 33 material. The primary criteria on which the insert 33 material is selected are that it must have a higher degree of thermal conductivity than the material from which the cylinders 28, 29 are fabricated, and it must be able to retain sufficient structural strength under operating conditions (for example, temperature) so it does not substantially deform under prolonged use at those operating conditions.
Metals such as, for example, copper, the refractory metals, and their alloys are useful because of their ability to accept, to transmit, and to dissipate heat, that is, their relatively high thermal conductivity, at high working temperatures and their ability not to deform at those temperatures. The refractory metals and refractory metal alloys are particularly useful when die casting very high melting point metals because of, in addition to their good thermal conductivity, their very high melting points and good ability to retain structural strength at high temperature conditions. For example, in die casting metals like copper, copper-zinc alloys and similar melting point metals the insert 33 may be fabricated of copper but in die casting of metals like gray iron, stainless steel and titanium the insert 33 preferably is fabricated of a refractory metal such as molybdenum, tungsten or the like. Refractory metals found particularly useful in this invention include molybdenum, tungsten, columbium, and tantalum although other conductive refractory metals or refractory metal alloys may also be used.
Only one heat exchange insert 33 positioned on the bottom 25 of the shot tube 11 has been illustrated and described in detail; this for the reason that the most pronounced problems giving rise to warping or bowing of the shot tube arise due to the higher temperature of the shot tube at the bottom than elsewhere in its configuration. The problem normally can be solved with the location of only one heat exchange insert 33 along the bottom of the shot tube 11 as illustrated. However, under certain limited operating conditions the heat exchange insert 33 located on the bottom of the shot tube 11 may, in elfect, remove too much heat from the bottom of the shot tube so as to cause a reverse bowing or wraping condition. That is, the shot tube would tend to bow or warp upwardly. This, of course, would be due to substantially cooler temperatures along the bottom of the shot tube 11 than are present along the top of the shot tube. Such a problem may be effectively overcome by providing a secondary heat exchange insert, not shown, along the top of the shot tube. By varying the amount of coolant passing through the botom insert 33 relative to the amount passing through the top insert, the temperature gradient can be elfectively maintained within that range whereby no bowing or warping of the shot tube occurs, that is, where an equilibrium condition is established. It is to be expected that substantially less coolant will be required for the top insert, not shown, than will be required for the bottom insert 33 to reach and to maintain equilibrium conditions under these circumstances.
Although the invention has been particularly described in its preferred embodiment, it will be obvious to those skilled in the art that other modifications and adaptions of the invention may be made without departing from the spirit and scope of the claims. Having completely described my invention, what I desire to claim and protect by Letters Patent is:
1. In a die casting type machine including a stationary die half and a movable die half cooperating to form a mold cavity, a horizontal chamber adjacent said stationary die half that defines a bore communicating with said cavity, a plunger reciprocable in said bore for introducing molten material from said bore into said cavity, and means in said chamber for charging said molten material into said bore, said bore including a substantial area adjacent said charging means which is adapted to normally support said molten material in a pool on the bottom of said bore prior to reciprocation of said plunger, the improvement comprising a slot along the bottom portion of said horizontal chamber of a depth less than the thickness of the wall of said chamber, said slot being positioned beneath said area and extending a length at least about equal to the length of said area, and a heat exchange insert formed of a material having a higher degree of thermal conductivity than the material of said chamber and closely fitted into and extending substantially the entire length of said slot, said insert adapted to permit excess heat from the bottom of said chamber to be dissipated more rapidly than from the top thereof to aid in balancing out the heat transfer in said chamber from a series of molten charges and, thereby, discourage warping or bowing of said chamber during a die casting run.
2. An improvement as set forth in claim 1 wherein said insert is cooled by coolant flow.
3. An improvement as set forth in claim 1 including heat transfer fins mounted to said insert.
4. An improvement as set forth in claim 3 including fan means associated with said fins for directing forced air onto said fins.
5. An improvement as set forth in claim 1 where the material of said insert is selected from the group consisting of refractory metals, copper, and their alloys.
6. An improvement as set forth in claim 5 wherein said material is a refractory metal selected from the group consisting of molybdenum, tungstun, columbium, tantalum, and their alloys.
7. An improvement as set forth in claim 1 wherein said chamber comprises an inner sleeve, and
an outer sleeve, said slot being in said outer sleeve.
8. An improvement as set forth in claim 1 wherein said slot and said insert are of mating dovetail configuration. I
9. An improvement as set forth in claim 1 wherein said slot and said insert extend at least from about the axial location of said means adapted for charging said molten material into said bore to about said stationary die half.
10. An improvement as set forth in claim 1 wherein said slot and said insert extend from about the retracted position of said plunger to about said stationary die half.
References Cited ROBERT D. BALDWIN, Primary Examiner US. Cl. X.R. J