This invention relates to a method and apparatus for single point peripheral low pressure direct melt injection casting of metal products and especially aluminum alloy of structural quality.
Heretofor there have been three main ways of casting metal products in metal dies or permanent moulds. In so-called die casting techniques, metal from a melting furnace is transferred to a holding furnace and is poured from the holding furnace into a ram chamber from which it is expressed into relatively thin walled dies having substantial water cooling associated therewith for controlling differential cooling conditions. Such dies in die casting techniques are normally of multiple form. Injection takes place under very high pressures at a central region and spreads through feed channels to edge filling of small component cavities. In bronze, zinc and aluminum die casting metals the maximum thickness which can be produced is of the order of about one-eighth of an inch, otherwise the heat dissipation from the relatively thin walled dies of the order of about half of an inch to 2 inches wall thickness about the die cavities requires massive cooling in order to maintain satisfactory production rates.
Ordinarily die castings weigh less than 10 pounds each. Single die castings weighing as much as 20 pounds have been made in die casting machines but in such event the throughput of metal is of such volume that the transfer of metal from the melting furnace to the holding furnace or ladle becomes a major handling operation exposed to oxide forming atmosphere interfering with production and metal quality. In particular this has been overcome by utilising combined melting and holding furnaces of the induction melting type especially for die casting metal and aluminum die castings, but the very large die casting machines required, while delivering a relatively high throughput per hour, are limited by the nature of the dies in the production rate and thus the thickness of the castings to be produced is severely limited in spite of high injection pressures. Injection occurs at the centre of the mould cavities so that in a large single casting this area becomes the last to cool and if such area in the product is the thickest part of the casting the cooling problem is so severe as to render die casting by central injection impractical. Multiple point injection in die casting for a large single casting has been attempted with some success but at extremely high die cost. Conventional die casting technique is not regarded by skilled persons as a practical process for the production of heavy castings of structural quality.
The casting of aluminum and other non-ferrous as well as ferrous metals has been accomplished in metal dies usually referred to as permanent moulds to provide product or casting wall thicknesses of heavier section than is attempted by die casting technique. Such moulds are usually organised on the casting floor with lever mountings and the like so that they may be the more easily manhandled in production. Ordinarily a permanent mould floor operation is fed metal by hand ladling or by a ladle operated by an associated crane or equivalent device working between a holding furnace and the pouring location. Substantial attention must be paid to the cooling of such dies. Generally large permanent moulds are made with a wall thickness about the cavity no greater than about 2 inches in order to facilitate a sufficient rapid reaction to a cooling medium.
An automatic permanent mould system of casting has been used in recent years, especially in England, for casting products of the order of 10 to 20 pounds weight in aluminum. The moulds may be of the single casting or single product type in which the moulds are supported over a holding furnace and the furnace is adapted by air pressure vertically to inject metal at pouring temperature upwardly into the centre of the product to be formed in the moulds. On cooling the vertical pouring spout is slightly mechanically retracted and the mould or the holding furnace is moved out of close relationship therewith whereupon the mould is separated by pneumatic or hydraulic devices to deliver the finished casting. It is always necessary in such casting arrangement to support the mould above the holding furnace. Where the weight of the product being produced is of the order of more than 10 pounds, the system suffers the disadvantage that operations must be interrupted substantially at the end of each hour to enable the holding furnace to be refilled and thereafter the holding furnace must drive the temperature of the metal to a satisfactory pouring temperature. It is this requirement of operations in a central vertical low pressure injection technique which cuts the rate of production in half and causes severe problems by loss of temperature in the dies or moulds. In addition, the moving of the holding furnace relative to the moulds for each casting gives rise to substantial mechanical and operating difficulties requiring a high degree of training by operators. Furthermore the dies for such casting technique, while of the permanent mould type slightly more massive than die casting dies, are limited in their weight in order to facilitate handling and the machanics of the apparatus to a total weight of the order of less than 10 times the weight of the casting produced and usually of the order of about six times such weight. Accordingly while it is not difficult to raise the temperature of the dies to the operating temperature the inordinate amount of cooling required, such as by water cooling, renders the accurate control of temperature uniformity in the dies extremely difficult at all times and impossible in a practical sense in many applications. While such central vertical injection technique minimises the temperature drop from the holding furnace to the dies to as low as five degrees the metal in the holding furnace will always have been transferred from a melting furnace and thus will have been exposed to the atmosphere giving rise to inclusion of oxides and impurities in the melt. The casting quality is highly variable leading to a higher percentage of rejects than gravity poured floor handled permanent mould technique.
It is the main object of the invention to provide a method and apparatus for the melting and direct injection of metal from a melting furnace into massive dies within a controlled atmosphere continuously at a melting rate matched to a casting rate in turn conforming to the cyclic heat loss rate of the dies.
It is another object of the invention to inject molten metal at a pouring temperature into a massive die mould at a single point of injection substantially at a periphery of the casting being formed in castings of greater than 15 pounds weight and especially in aluminum to provide structural aluminum products.
It is a further object of the invention to provide a direct melting injection casting centre embodying a melting furnace and casting machine integrated to enable operation on a continuous repetitive cycle basis and of a capital cost of the order of less than half the capital cost of a melting and die casting facility of equivalent throughput per hour.
It is a still further object of the invention to provide a continuous cyclic method, apparatus and system organised in a casting centre for the production of castings of non-ferrous metals and especially of aluminum alloys in which differences in section thickness may be as great as of the order of four to one or more and in which the maximum thickness of section to be produced may be greater than 1 inch.
It is a still further object of the invention to provide a method and apparatus in a direct metal injection casting system adapted for the casting of aluminum alloy products of greater than 174 inch section and of greater than 15 pounds weight on a continuous cyclic production basis to enable structural brackets, sections and components for buildings, bridges and the like and automobile engine components such as, for example, engine heads and machine parts of substantial weight and section thickness to be produced at costs and of strengths competitive with iron and steel castings and structural steel components.
It is a still further object of the invention to release molten metal from the injection point of the casting formed by using a bubble of gas to avoid the necessity of mechanical separation of pouring spout or injection nozzle and die.
Other objects of the invention will be appreciated by a study of the following drawings taken in conjunction with the accompanying specification.
In the drawings:
FIG. 1 is a side elevation of a typical arrangement of a direct melt injection casting centre according to the invention.
FIG. 2 is an end elevation of a typical casting machine according to the invention and as shown in FIG. 1.
FIG. 3 is a section on the line 3--3 of FIG. 2.
FIG. 4 is a plan view of the integrated melting holding and injection furnace and casting machine centre of FIG. 1.
FIG. 5 is a sectional view of a conventional induction melting and holding furnace modified to provide gas bubble injection release according to the invention.
FIG. 6 is a sectional view of the single injection point area of the casting machine dies and injection spout of the apparatus of FIGS. 1 to 4.
FIG. 7 is a perspective view partly broken away of one arrangement of permanent moulds or dies in the casting machine of FIGS. 1 to 4 partly cut away to reveal features of construction according to the invention.
FIG. 8 is a half section view of typical permanent mould dies of massive heat sink properties according to the invention and of the peripheral single point injection type as set forth herein.
Referring to FIGS. 1 to 4 the direct melt injection casting 10 of the invention has a casting floor 11 preferably formed of a rigid metal mesh 12 above pit floor 13. A continuous foundation frame 14 formed by beams 15, 16 and cross frames 17 rigidly connects to and supports the casting machine 18 and the induction melting and holding injection furnace 19 connecting by injection spout 20 to a rigid platen 21 associated with massive permanent mould die assembly 22.
The casting machine 18 is shown in more detail in FIGS. 2, 3 and 4 and comprises a heavy frame plate 23 having corresponding inner plates 24 and connected thereto by inner and outer flange plates 25, 26 to describe the triple box frame structure 27 comprising the central box frame cavity 28 having on each side thereof the box frame cavities 29 and 30. These cavities extend continuously about the operating opening 31 of the machine and are interrupted only by the massive platen 21. Laterally slidable and separable on the platen 21 are the side die or mould parts 32, 33 actuated by pneumatic ram cylinders 34, 35. An upper mould die part 36 is actuated by vertical pneumatic ram 37 and a lower or bottom die or mould part 38 is moveable vertically through a bottom die opening 39 in plate 31 by pneumatic ram cylinder 40. The dies 22 are shown in the closed position in FIGS. 2 and 3 and define by chain lines 41 a cavity representing one typical product such as a wheel having a circular rim part 42 and having an intermediate web and hub 43.
The casting machine may be built up of 1 inch or preferably 2 inch plate. The rigid platen 21 ought to be of the order of greater than 6 inches in thickness having regard to concepts of operation according to the invention in which the control of the heat cycle of the process and thus the control of the process itself derives from relating the proportions of the equipment to the weight of the casting. This approach to the casting of metal has delivered a reliability of performance in practice according to the invention which has achieved a simplicity of operation heretofor not known.
The invention contemplates that the weight of the massive dies or moulds should not be less than 50 times the weight of the casting to be produced and should preferably be of the order of 100 times such weight. The weight of the casting machine itself should always be not less than 10 times the weight of the dies or moulds and for the sake of rigidity and desirable inertia properties ought to be of the order of 20 to 30 or more times the weight of the dies or moulds. The furnace 19 of FIG. 1 through a foundation connection 44 to the casting machine 18 must be rigidly anchored relative to the casting machine and any vibration absorbed in the removable injection spout structure 20. Preferably the cavities 28, 29 and 30 of the casting machine are subjected to air circulation entering the bottom openings 45 and escaping from the upper vents 46, which latter preferably embody dampers or valve means and fans (not shown). Said cavities should contain thermometers at least during initial trials of an installation so that at operating temperatures at the casting centre the hot side or the furnace side of the casting machine may be free of distortion relative to the cold or off side by control of air passage through the cavities.
By having a substantial amount of metal available in the casting machine itself and in the dies relative to the heat input to the dies and the machine effected by the continuous production of castings, the entire casting machine achieves a substantially uniform distribution of heat throughout the metal thereof about the opening 31. The dies to by virtue of their large mass develop a greater uniformity of temperature in their casting defining surfaces than could otherwise be accomplished with thin wall dies and greater attention to heating and cooling correction according to the prior art. The use of a substantial body of metal in the dies and in the casting machine itself enables cooling or heating correction to be applied substantially only for the purpose of achieving differences in thickness in the walls of the casting produced and in maintaining and or controlling overall die temperature to a satisfactory operating temperature having regard to pouring temperature.
It is intended that the platen 21 should be of a weight greater than that of the die or mould assembly and that the unsupported length in any direction be less than four times the thickness thereof. The massive character of the platen and the mould parts is not to be underestimated in its influence on the consistency of producing uniform quality castings in a manner free of any sudden changes in conditions at any point in the casting cycle. The invention recommends that the dies and the casting machine be as large as is practical to increase uniformity of casting produced, increase control and reduce the length of the casting cycle.
The exterior surface area of the dies in order to provide a heat sink characteristic must be greater than the surface area of the part being cast. Depending upon the surface intricacies of the part being cast it will not necessarily be obvious that dimensionally larger dies will have a larger surface area. In more particular the die material will have an ability to radiate heat according to the color and texture of the surface and the temperature of the surface. The heat conductivity of the metal of the die will usually be less than the heat conductivity of the metal in the mould cavity. Generally the exterior surface area of the dies should be many times greater than the surface area of the casting multiplied by the ratio of the die conductivity and casting conductivity. When the die shape is such that this cannot be done then coolig is used to remove heat from the die and not from a particular casting surface. Cooling design for permanent moulds and die casting dies is approached differently for, because of thin die walls, cooling design must be related to die cavity surface temperature at each point.
The next consideration is to take into account the heat lost to the dies by the metal forming the casting while the casting is in the dies. For example, assuming a 25 pound aluminum alloy casting poured at a pouring temperature 1,250°F and a temperature drop through solidification while in the dies prior to opening of the dies of the order of 400° F., or say 500°F. The amount of heat energy lost to the dies can be computed by considering the amount of heat energy necessary to raise such weight of aluminum from the lower temperature to the higher temperature. It is of interest that the usual estimates of solidification times in the casting of aluminum in metal dies are somewhat midleading when compared with actual experience in the present process. Especially in the case of aluminum there is a tendency to assume that a substantial thickness, say of the order of 1 inch or more, will take much longer to set than actually will be the case in practice. In pilot operations, according to the invention operating with massive dies of the heat sink type as described herein, the time from pouring to solidification for satisfactory strength for removal of the part from the die has been found to be of the order of between 60 and 180 seconds per inch of casting thickness depending upon pouring temperature and die temperature. From this derives a basis of a cyclic rate of heat input into the dies which the dies by their outer surfaces must have the ability to dissipate by radiation.
The concept of operating with massive dies can best be understood by imagining a solid sphere of metal having an electrical heater in the middle. While the rate of heat input is greater than the rate of heat dissipation from the outer surfaces the temperature at the outer surfaces will rise. Since the rate of heat dissipation by radiation from a surface increases with temperature we may regard the outer surfaces as increasing in their cooling ability or efficiency as the temperature thereof increases. The sphere acts as a heat storage device especially at higher temperatures. Thus assuming that the heater is turned on for 1 minute then turned off for 1 minute on a cyclic basis, the outer surfaces will achieve a stable outer surface temperature gradient and the inner core surface next the heater will drop to a temperature always above the outer surface temperature and during heating will rise to a temperature at the heating surfaces less than the temperature of the heater.
The casting should be delivered from the dies at the highest average temperature at which the casting has structural integrity. This can be accomplished with thick section castings of the invention at a casting release temperature of about 80 per cent of melting temperature in less than 2 minutes from start of pouring. The mould cavity surface temperature of the dies just prior to pouring should be less than casting release temperature but otherwise as high as possible. The higher the average temperature of operation limited by the structural integrity casting release temperature, the higher will be the minimum temperature or relaxation temperature of the die cavity surfaces just prior to pouring. At lower thermal levels of cyclic operation the relaxation die surface temperature will be much less than the casting release temperature. It is for this reason that the flow of metal in the dies and casting uniformity are assisted by operating with hot dies even in conventional die casting and permanent mould techniques.
The pouring time must be sufficiently rapid to avoid undue temperature drop as the metal is distributed. In thicker section castings of the kind considered herein this is more a consideration of the heat conductivity of the aluminum itself rather than heat loss of the aluminum derived from a chilling of the aluminum by contact with the walls of the cavity. For this reason peripheral single point injection casting techniques of the invention require pouring rates filling the mould from the lower regions as by bottom pouring or injection to fill the mould in less than 20 seconds and preferably in less than 10 seconds. After the pouring time for filling the cavity a curing interval of time is permitted during which solidification occurs through heat loss to the moulds to a casting release temperature representing a heat loss of the aluminum to the moulds of a predictable quantity within a predictable period of time. After casting release the dies are held in open position and lose heat from their exposed cavity surfaces as well as from the remaining radiating surfaces thereof so that when in the open position the rate of heat loss from the dies is greater than the rate of heat input to the dies in the closed position. Thus, for example, a 2 minute casting time including pouring and curing may accomplish the same heat input as the dies may lose in heat energy if held in the open position for say 1 minute for which the total cyclic period may then be 3 minutes adapted for repetitive operation on this cycle continuously. A loss of one cycle or even a number of cycles in succession will not materially effect operating conditions with massive dies of the invention because of the storage of heat energy in the dies. Thus assuming a cavity surface die temperature prior to pouring of 650° rising to 750° maximum during curing and falling again to 650° just prior to next pouring, an omission of three or four pouring cycles may result in a loss in die cavity surface temperature to the next casting to be poured of less than 50°. Experience has shown that the loss of a cycle with massive dies operated at a desirably high casting temperature may be as small as 5° per cycle but will ordinarily be of the order of 10° to 20°. Recovery to the desired operating level of casting die temperature will be made by a number of cycles less than the number of cycles omitted.
As indicated in FIGS. 1 and 4, the invention contemplates the direct connection of an induction melting and holding furnace 19 through injection spout 20 rigidly connected to platen 21 in such manner that melting injection furnace 19 forms a part of the apparatus and process entering into the cyclic conditions described. Thus furnace 19 loaded with ingot through loading door 47 must have a melting rate at least matching the cyclic throughput rate of the casting machine component 18 of the casting centre apparatus 10. As seen in FIG. 5 the melting section 48 connects by submerged passage 49 with the holding section 50, the latter having induction heating electrode assemblies 51 adapted to control the temperature of the molten metal to a value of plus or minus 5°F. The loading of ingot in conjunction with the delivery of melt from the holding section 50 must operate between selected levels such as the upper level 52 and the lower level 53 which will exist in both the melting and holding chambers 48, 50. A cycle amount of melt, hereinafter referred to as a shot, for example of 25 pounds is delivered up the injection spout structure 20 to the casting machine 18 by the simultaneous increase of gaseous pressure such as by the gas pressure tubes 54 into chambers 48, 50 to a pressure of the order of between 10 and 30 pounds per square inch and for a period of time determining the quantity of melt forced up the spout 20. Preferably nitrogen is kept in chambers 48, 50 and the pressure thereof controlled by nitrogen pressure control lines to tubes 54 in a conventional manner. The furnace is of a known design which is totally enclosed. The power rating of the electrical inductors of the furnace must not only be capable of melting aluminum at the desired rate but also of raising the temperature thereof once melted at the desired rate to a pouring temperature. The holding section melt should be between 50 and 100 times the weight of the casting and the melting section melt should be between 100 and 200 times the weight of the casting. Furthermore the ingot weight should be approximately equal to the weight of the casting though slightly faster melting rates can be achieved by using smaller sizes.
The present invention provides an improvement in a conventional combined melt and holding induction type furnace of the enclosed injection type by enabling an automatic release of unsolidified metal from the casting without mechanical aid and by providing a gaseous valve in the manner indicated in FIG. 6. The spout base 55 of furnace 19 embodies heaters indicated by hatching 56 about a tubular liner 57. Removeable spout 20 embodies end flanges 58, 59 converging downwardly in their surfaces to enable the spout to be lifted and removed. The flange 59 is bolted to the heavy flange 60 of spout base structure 55 with a thick asbestos washer 61 of the order of about one inch in thickness but compressed by tightening flange bolts (not shown), and likewise flange 58 connects by bolts 62 rigidly to platen 21 through a thick asbestos washer 63. Preferably such asbestos washers embody glass fibre such as glass cloth and are sealed in place by the liberal application of thick die coat mixture at the time of assembly.
Some degree of mild shock and vibration is dampened in the connection between the furnace and the casting machine. Because the dies move slowly and their weight is small relative to the weight of the machine, shock vibration or movement is a lesser problem than necessary inspection, cleaning or removal of the spout for other reasons. The removeable connecting spout may be replaced easily with the loss of a minimum number of cycles, such as three or four cycles, should this become necessary. It is important in the present process that at a point where a difficulty may occur the structure must provide for ready replacement. No attempt should be made to make such components so reliable that they will not be replaced for a period of time. The success of the process relies upon the continual servicing of two critical areas and thus it is intended that the pouring spout be replaceable as a usual practice in the operation of the process substantially on a daily basis.
The spout embodies a gas injection tube 64 adapted for the injection of a high pressure bubble of gas into the injection passage 65 of tubular liner 66 surrounded by electrical heaters shown by hatch lines 67 within the rigid outer metal casing 68. Injection throat structure 69 seats in angular bore 70 of platen 21 and comprises an outer shell 71 containing a ceramic lining 72 having a converging throat passage 73 terminating in the upwardly turned throat 74 surrounded by a replaceable sealing washer 75 having an enlarged opening 76. The replaceable washer 75 is seated within a washer recess 77 formed in platen 21. The laterally moving dies 32, 33 embody the mouth die parts 78 bolted thereto by bolts 79 and formed preferably of cast iron being adapted to be replaced as a result of any scouring that may occur and defining a mouth passage 79 having a restriction 80 defined by the rising tongue portion 81. The mouth is defined by an opening 82 registering with the opening 76 of washer 75 and throat 74.
In cyclic operation, with the dies at satisfactory moulding temperature and the metal at satisfactory pouring temperature, low pressure nitrogen or argon is injected into the furnace by tubes 54 to develop a pressure in the melting and holding chambers 48, 50 of between 10 pounds per square inch and 30 or more pounds per square inch for a period of time delivering the desired amount of metal having regard to the resistance to metal flow to the moulds. This can be achieved not only by a timer being placed on the valve for nitrogen or argon under pressure but also by controlling the injection of a bubble amount of higher pressure nitrogen to injection tube 64 responsive to a sudden rise in pressure in holding chamber 50. The bubble of gas rises quickly in passage 65 whle expanding due to heat to the restriction 80 thus forcing a runback of excess aluminum to the holding furnace. pressure in chambers 48, 50 is reduced to atmospheric by release through tubes 54 before casting release.
The restriction 80 and throat passages should be of constant cross sectional area. The restriction 80 should be above the level of the injection outlet into the mould and should be thinner than the thinnest section of the casting.
The gas bubble injection tube 64 may as an alternative be located in the holding section 50 of the furnace so long as the outlet of the injection tube is close to the injection passage 65, such as under the sloped surface 84 of a ceramic furnace insert 85. The latter traps a gas bubble injected by a tube inserted through the holding section cover 86 as indicated by chain lines 87. Thus the gas injection bubble tube may be located in the furnace or in the removable spout structure 20. In this way the molten metal can be released from the passages 65, 73, 74 and the throat of passage 79 at the moment the mould is filled.
As indicated in FIG. 7 the mould parts may comprise the upper and lower die parts 36, 39 and the side die parts 32, 33 in which a flange 88 of the upper die overlies upper surfaces 89 of the side dies as indicated in FIG. 8 but in which said overlapped region Is characterised by very small air passages 90 extending radially preferably in the upper surfaces of the side dies thus to enable escape of air from the cavity 91 as the cavity is filled. Accordingly, subject only to resistance to exit of air from the upper regions of the closed dies, nitrogen of predetermined pressure allowed into the furnace by the pressure tubes 54 will cause the metal to flow upwardly into the die cavity as described. The leading surfaces of the metal during flow will be under a pressure determined by the leakage resistance in the leak passages 90 so that the air pressure within the die cavity can be adjusted always to be less than the pressure in the furnace, in which event the metal upon hitting the passages 90 and quickly freezing because of their small size, blocks such leakage passages. Thus the pressure in the melt and in the die cavity filled with metal as well as in the bubble injection tube 64 will rise or peak suddenly, metal will freeze in restriction 80 and the molten metal between the restriction 80 and the furnace is immediately returned to the furnace by a higher pressure nitrogen bubble.
In the specific form of dies indicated in FIGS. 7 and 8 the die cavity is adapted to form a cast aluminum automobile wheel being typical of an application of a casting centre of the invention. The side die blocks 32, 33 are massive and are driven by hydraulic rams 91, 92 threaded into mounting blocks 93 adapted to be bolted, such as by bolts 94, to die blocks 32, 33. The latter slide over the upper surface 95 of the platen 21 between the guide runners 95 adjustably bolted thereto for the purpose of die alignment. The simple massive structure illustrated works particularly well with graphite lubrication alone even though the side die parts may each weigh of the order of 250 pounds, 500 pounds, half of a ton or more. The larger surfaces reduce the unit surface pressure between the die parts and the platen thus reducing wear problems.
It is intended that the side dies be heated by metal encased electric heaters inserted into the die blocks and connected by electrical bus bar ducting 96 to flexible electrical cables 97. The heaters in the side dies should be of a total kilowatt rating of the order of about one kilowatt per pound of casting to be cast in one cycle. Thus dies for a 20 pound wheel casting should have electric heaters in the side dies of the order of 20 kilowatts though half this amount of power may be adequate to hold die temperature once achieved. The first criterion is that the electric heater power input should be equal to the radiation heat loss of the side dies. The second and more important criterion is that the heat input should be sufficient to permit electrical heating of the dies from the cold condition within a sufficiently short period of time, say within one hour, as to enable casting to proceed at a temperature close to operating temperature.
For most casting shapes suitable for the invention an outer ring-like form will be desirable and should be defined by the side dies for in such event the rim or peripheral structure of the casting serves as a feeder for the casting itself by single point injection from the peripheral mouth 83. The tendency in many castings of large configuration is to have some sort of hub structure which generally will define a region of thicker material as, for example, in the region 98 of the cavity 91 of FIG. 8.
The invention contemplates the use of air cooling alone where required in the manner indicated in FIG. 8 in which the upper die 36 is connected by bolts 99 to mounting flange 100 threaded on to the upper ram 101 of upper cylinder 37 and the bottom or lower die 39 is likewise bolted as at 102 to the mounting flange 103 connected to lower ram 104 of the pressure cylinder 40 (FIG. 2). A centre hole may be provided in the casting form by upstanding die insert 105 embodying a downwardly extending tubular portion 106. The upper pilot portion 107 slides within a bore 108 of the upper die 36 to seat against the lower end 109 of the upper ram 101. Cooling is accomplished by the insertion of copper, aluminum or sodium rods 110 extending into the upper and lower dies to develop a surface contact of such rods with the metal of the dies at least equal to the upper and lower surface area 111, 112 of the upper and lower dies. A copper, aluminum or bronze ring 113 is then fastened to the outer surfaces 114, 115 of the upper and lower dies by bolts 116 and a plurality of circular thin fins 117 are cut into same with a lateral slot over said fins in the region of an inlet air fitting 118 and an outlet air fitting 119 located 180° from the inlet fitting as seen in FIG. 7. By this means high pressure air of the order of 100 to 300 pounds per square inch is injected into the rings 113 and flows between the fins to the outlet fittings 119 for cooling of the metal of the rings 113. Heat conducting rods 110 fit tightly through the rings 113 and into the dies so as not to obstruct flow in the air passages 120 between the fins 117. Copper sleeves or tubes 121, 122 serve to communicate heat from the central bores 123, 124 of the upper and lower dies to the rings 113. In this way the operating temperature in the middle regions of the die may be held to as low as 300° or more below the mould surface temperature of the side dies at moulding temperatures. The velocity of air flow in the rings is a function of air pressure which can be adjusted to provide a closer control of this difference in temperature to enable operation on a consistent cyclic basis. Generally during heating up of the dies, the dies are held in the closed position and during operation it will be unnecessary to use electric heating unless one is holding over a number of casting cycles before beginning again in which event it is best to hold the dies closed.
It has been found that when operating with full metal dies without the use of resin cured sand inserts or so-called biscuit mould inserts, high production rates can be achieved and casting quality can be controlled to the highest degree. The use of mould inserts of other materials becomes unnecessary in the present process except for the formation of tubular passages. This, however, requires the use of operators' hands within the jaws of hot dies which is to be avoided.
It is preferred, according to the present process, to form the die parts of nodular type cast iron and high alloy irons of toughness rather than brittleness and cast to define the final surfaces of the die cavity. It has been found that a mere graphite coating on such surfaces is superior in avoiding die scouring and attachment by hot aluminum. Where machining is done on the die surfaces the machining should be kept relatively rough. If such surfaces are to be highly polished it is preferred to cut into the cast surface a distance about one quarter of an inch to define such finished polished surface.
Generally every attempt should be made to use a graphite coating on a polished surface and while this can be done quite satisfactorily on the cooler die areas such as in the mid regions if defining thick product sections, where a high finish is required on the peripheral areas of the die cavity, it may be necessary to resort to conventional die coatings. It is of interest that the early pilot operations in the development of the present process were rendered almost unsatisfactory by too great an attention to die coatings as compared with necessary attention to the continuous delivery of sufficient heat to the dies by operating at a satisfactory production rate. Those die coatings which have a tendency to cake give rise to severe problems in high production rates. As soon as possible attempts should be made to use graphite coating for in this way the rate of heat transfer to the dies will be more uniform and will be less subject to a substantial variety of temperature conditions which can arise from the use of different die coatings and thickness thereof within any one die assembly. Thus the danger in the use of conventional die coatings is that they can mask and alter the correct cyclic operation of massive dies. While successful castings can be obtained a predictable controllable cyclic operation of the present process can be distorted by die coatings which interfere with heat transfer at the die surface. A sufficiently high die temperature combined with injection pressure and pouring temperature correctly provide for sufficient running ability of metal to fill the cavity. If consistent dureable die coatings are used then their use is permissable but such should be necessary only for extending die life.
Removal of heat from the casting is a complex phenomena involving the interaction of many factors including the specific heats of the die and casting metals, conductivity of the die metal, shape and surface area of the die, ambient air temperature and the influence of air circuits over the outer surfaces of the die. In general, the average rise in temperature of the die will be directly proportional to the drop in temperature of the casting, the ratio of the mass of the casting to the mass of the die and the ratio of the specific heat of the casting to the specific heat of the die. If the mass of the die is 50 times that of the casting and the ratio of specific heats is of the order to two to one (as is the case with 380 Alloy and ductile iron), then a 400°F. temperature drop in the casting will result in an average temperature rise of 16°F. in the die.
The influence of the other factors mentioned above will cause the temperature rise at the hot face of the die to be greater than the average temperature rise. This is desirable provided the change in hot face temperature does not set up stresses in the casting. In practice it has been found that if the ratio of die mass to casting mass is greater than about 50 then the change in hot face temperature can be maintained within 200° and as low as 50° if die temperature at pouring is high enough.
A cyclic rate of operation is similar to the cyclic firing of an internal combustion engine and the dissipation of heat through the walls of the cylinder. The present heat source is casting metal which drops in temperature about 400°F. in the case of aluminum alloy. At one inch inside the die face tests showed that the maximum temperature achieved should be about 100° less than casting release temperature and that massive dies permit the pouring temperatures of the dies at the die face to be no lower than 200° below release temperature. Massive dies further permit a maximum temperature gradient to a remote point across the die opposite the point of injection to be held lower than 10° at operating temperatures and for metal runs of the order of four feet in length through the die to such remote point.
Both the volume and exposed surface area of the die serving each unit area of casting surface thereof per unit thickness of casting against said surface should be the same for all die surfaces. Where the geometry of a die part does not readily permit this, as with the upper and lower dies in the present drawings, then air cooling can substitute for die mass and cooling surface. Nevertheless air cooling can be eliminated in the present example and the present process has been so operated by making such upper and lower dies in the form of long cylinders. Thus, for example, assume the cavity surface area multiplied by the average casting thickness facing the top and bottom dies is four times the cavity area and casting thickness facing the side dies, the top and bottom dies should then be four times the mass of the side dies and as well the surface area thereof should be four times the surface area of the side dies. It is this aspect of the invention which permits the casting of different thicknesses and heavy sections without special provision for cooling or treating of particular surfaces. Thus the invention, when calling for all die parts being of the same effective mass and exposed surface area per unit of casting surface area and unit thickness of casting defined thereby, contemplates that mass and surface area can be reduced by air cooling to achieve the same effective value which latter enables cyclic operation while sufficient mass stores heat to maintain operating temperature and releases heat from exposed surface area at a rate generally corresponding to the rate of heat release from the casting to the dies.
Not all aluminum alloys are suitable for both induction melting and metal die casting. Generally an aluminum alloy of 7.5 to 9.5 per cent silicon and 3.0 to 4.0 per cent copper and impurities (max), 2.0 % Fe, 0.10 % Mg, 0.50 % Mn, 1.0 % Zn, 0.50 % Ni, others each 0.15 per cent and totalling 0.50 per cent and designated by the trade name 380 Alloy (USA) or ASTM number B45-46T Alloy SC7 is best adapted for the present process. This alloy has high strength and may be modified in silicon to lower values since runability as in conventional die casting is not a problem in the present process. This type of alloy does not require special heat treatment, develops tensile strength and yield properties comparable to structural steels, and is an ideal aluminum alloy composition for induction melting.
The casting machine 18 is only one form useable in a casting centre of the invention. In the casting of standard 10 or 12 foot aluminum structural sections of H or U type, injection is made uphill from one end. The casting machine design is extended outwardly from the off side opposite the furnace and inclines upwardly at about forth 5°. Structural brackets, bridge posts and highway lamp enclosures require mould die arrangements and actuation similar in principle but different in detail from the casting machine disclosed herein by way of example. The casting centre of the invention is adapted for the production of a variety of structural parts especially from aluminum and of a part weight exceeding 20 pounds permitting a greater potential use of this metal in machinery and engine parts such as in construction, farm and industrial equipment.