Patent Application
20200180018
Amorphous alloys may have unique combinations of properties—among them, high strength, corrosion resistance, low friction, and magnetic properties—due to their atomic structure. But, amorphous alloys must be quenched rapidly to achieve the amorphous structure, and there are few forming/manufacturing processes that have been capable of producing amorphous alloy products that are technically and economically viable.
The die casting process has been used to cast amorphous alloys with mixed results. One of the challenges has been protecting the molten alloy from the melt process all the way from the crucible to the mold cavity. Many amorphous alloys are highly reactive with oxygen and/or nitrogen while in the molten state. The reaction forms oxides which become inclusions in the castings, resulting in reduced mechanical properties and surface imperfections.
To deal with the reactive nature of amorphous alloys, one approach has been to use vertical die cast machines, in which the entire melt system, pouring system, dies and molds, and casting ejection/removal/handling system are enclosed in a large vacuum chamber. Material feedstock is generally shuttled into the chamber through vacuum lock chambers, and the cast parts are likewise shuttled out of the chamber through similar vacuum lock chambers. Generally an individual ingot is melted directly in the ladle crucible for each shot. A limitation of vertical die cast machines is that cycle times tend to be long.
Horizontal die cast machines, which typically seal the die faces to each other and evacuate only the mold cavity with vacuum once the dies are closed, have also been used. One problem with such systems, however, is that of isolating the melt system while the dies are open. Typically, a “cold shot chamber”, often just called a “shot sleeve”, connects with the stationary die. A shot sleeve typically has an open port on the top through which the molten alloy is poured. The shot sleeve houses a plunger that retracts to open the port to receive the alloy, then pushes the alloy toward the dies and into the mold cavity to form the casting. For reactive alloys, the cold chamber may be enclosed by a vacuum cavity, which also houses a crucible into which an ingot is fed for each shot. However, while the dies are open (as is necessary to eject each casting), the plunger tip OD and shot sleeve ID are exposed to air (in particular, oxygen and nitrogen) and atmospheric pressure. The only seal between atmosphere and the shot sleeve/crucible vacuum chamber is the plunger OD itself, yet the necessary small gap between the plunger OD and the shot sleeve ID allows atmosphere to leak past and into the vacuum chamber. Once the dies are again closed, the atmosphere in both the mold cavity and the chamber must be drawn down to an acceptable vacuum level before melting of the next ingot can be initiated. Vacuum drawdown time can contribute to excessively long cycle time when using horizontal die casting machines with reactive amorphous alloys.
The reactive nature of amorphous alloys, plus their relative high melting temperatures, causes them to wet the iron-base alloys typically used in die casting, and in fact even to wet some ceramic materials. This can lead to the problem of the amorphous alloy “brazing” components together, particularly if those components are below the solidus temperature of the alloy, and if the melt stays in contact with said components for any significant time duration. This makes it difficult to use components such as valves to isolate certain regions (in particular, those containing melt) of the die casting system from contact with air.
Another problem with either vertical or horizontal systems as described is that an individual ingot is melted for each casting cycle. As such, the crucible must withstand repeated thermal cycles, which can cause crucibles breaks down and contribute contamination to the melt. Further, crucibles will typically eventually crack from thermal cycling. “Hot chambers”, which are crucibles or holding tanks that hold a large quantity of molten alloy and are maintained at a fairly constant temperature, would seem to be a better solution, but with amorphous alloys hot chambers have other limitations.
Many amorphous alloys have a higher melting point, or liquidus temperature, than other alloys, such as those based on aluminum or magnesium, that are commonly used in die casting. The liquidus temperatures of many are above the typical tempering temperature of the high-strength steels normally used in die casting machinery. At these elevated temperatures, steels soften and lose much of their strength. For this reason, steels often cannot be used in continuous, prolonged contact with molten amorphous alloys. For example, the iron-based hot chamber traditionally used with magnesium alloys is not used with amorphous alloys. The alloys, at or above their liquidus temperatures, react with iron-based materials. Elements from the iron-based materials contaminate the melt and reduce the properties of the final cast product. Further, degradation of the iron-based materials makes them wear rapidly and reduces their strength, preventing them from achieving their normal performance characteristics; for example, at the temperature required to melt many amorphous alloys, a pump in an iron-based hot chamber cannot be expected to survive long, or to generate as high a pressure as it would in its usual molten magnesium environment.
Some of the issues given are particularly problematic when it comes to die casting components that require thin cross-sections and/or “cosmetic” (highly-polished) as-cast surface finishes. Oxidation, porosity, inclusions, flow-related defects such as flow lines, laps, and cold shuts are unacceptable defects in such products.
Traditional die casting uses very high injection rates, with molten fluid flow velocities of 30 m/sec to 50 m/sec to prevent the molten alloy from cooling due to contact with the various passages that connect the crucible to the mold cavity. However, the turbulence induced by such velocities can cause void pockets and particulates from localized solidification of bubbles and spray droplets. If these defects “freeze”, or solidify, upon contact with the mold cavity, unacceptable defects are likely to result.
Further, solidified particulates are difficult to force through small cross-sections of molds, limiting designers' abilities to make lightweight components.
Turbulence can be eliminated by reducing the flow rate of the molten alloy, but then premature solidification is more likely to occur due to extended contact duration with the internal passages.
So, what is needed for effectively die casting reactive amorphous alloys is a system that:
All of the concepts in this application offer the following advantages:
The term “hot chamber” is used throughout. This is because in all concepts the supply of molten metal can come directly from either:
The concepts disclosed herein related to high-fidelity amorphous metal casting as well as a method of using a hot thank, hot chamber, etc. to inject or to supply a machine that injects amorphous alloys.
Such concepts may, however, also be used in other methods like investment casting, though die casting is generally referenced herethroughout.
The term “melt” is generally used as a noun, referring to the molten alloy of which the casting itself will be made.
In the molten state, amorphous alloys are generally reactive with air. The reaction products prevent the end casting products from achieving cosmetic finishes, and may degrade their mechanical properties as well. Thus, the systems embodied by these concepts ensure that from the time that the feedstock is melted, to the time that it has been injected and solidified in the mold cavity, it is never exposed to air, but rather exposed only to an inert environment. Exemplary inert environments include, but are not limited to, vacuum and argon gas.
Amorphous alloys in the molten state are also generally reactive with many other metals, including iron. The duration of exposure is a factor in the extent of reaction. Thus, in any area of the system in which the melt is in contact with an element of the system for more than a few seconds, that element should be made of a material that does not react with the melt. In general, certain ceramics are the best material choice for this purpose.
Ceramic components such as the feed tube(s) going from the crucible/hot tank to the shot chamber or valve, and the valve and valve bodies themselves, need to be heated as a minimum, above the solidus temperature of the alloy being cast. Induction heating will not work with ceramics, so the best method is believed to be resistive heating. Resistive heating may be used in conjunction with thermocouples in a feedback loop to achieve precise temperature control.
In the molten state, amorphous alloys exhibit fluid rheological properties that vary as a function of temperature. It is thus important to control the temperature of the melt being injected, at various locations throughout the system, by controlling the temperature of the surfaces with which the melt comes into contact. Controlling melt temperature thus is a method to prevent defects in the final casting product by preventing premature solidification, as well as ensuring that the mold cavity is able to completely fill before the melt solidifies. These concepts mention specific heating requirements that are unique to each concept. However, die heating and cooling are barely, if at all, mentioned in these concepts because, to some extent, they considered to be part of each concept. This statement applies to all concepts: The extent of die heating and cooling may vary depending on the efficiency of the various systems in delivering melt at the proper temperature and speed to the mold cavity. Dies, the mold cavity, and various components such as sprue bushings may be heated with fluids such as oil or heat transfer fluids, or with inductive or resistive electrical heating elements. Cooling may be accomplished with oils, water, or water-based heat transfer fluids. Depending on the needs at specific locations within the system, components may be continuously heated, continuously cooled, neither actively heated or cooled, or alternately heated and cooled with each cycle.
Each system is presumed to be capable of using recycled cast material with minimal reprocessing by feeding it into the crucible.
In some of these concepts, where a concept requires a pump in the hot chamber, EM pumps are cited as the preferred embodiment. However, in each case, the function of the pump is only to transfer the molten alloy to the PMV, shot sleeve, or mold cavity; the pump is not required to generate high pressure. (The fact that the final high pressure squeeze comes from another source is one reason that we think we can get away with old school, pump-in-hot-chamber technology with these high-melting temperature alloys.) We believe that EM pumps will work, or can be made to work with our alloys because they work with aluminum, and our liquidus temperatures aren't too higher than that of aluminum. However, if EM pumps won't work, ceramic centrifugal pumps or piston/sleeve submerged pumps, either made of ceramic materials, should work.
“Biscuit”—the portion of a casting that is where the melt first entered the mold cavity. The biscuit is waste material that is trimmed off the casting after its ejection from the mold cavity. The function of the biscuit is to serve as a sink for shrinkage in the critical areas of the casting, and to serve as a collector for gas bubbles and oxidized particulates that tend to be entrained in the last bit of melt to be injected into the die.
Feed tube—a tube connecting, and feeding melt between, a hot chamber an another element (e.g., a cold shot chamber).
Dies—two large plates that clamp together and provide the force required to constrain the pressurized melt during injection. Dies generally contain mold cavity inserts. Die casting machines generally have a moving, or ejector, die, and a stationary, or cover, die. The melt is generally first injected through the cover die. Dies must come together (close) to allow the melt to be injected into the mold cavity, and separate (open) to eject the solidified casting.
Mold cavity—the internal, formed surfaces within the dies that create the exterior surface of the finished casting itself. The mold cavity is generally constructed of mold inserts that are affixed to the dies, as well as various components such as cores and sliders that are used to create certain features.
Inert gas—a gas, or mixture of gases, that has little or no tendency to react chemically with the melt.
Cold chamber, or cold shot chamber—a piston-and-cylinder arrangement that injects melt into the mold cavity at high pressure. The cold chamber is generally maintained at a nominal temperature well below that of the melt itself.
Shot sleeve—the cylinder that houses the plunger. A shot sleeve generally has a fill port into which melt is poured. As the melt is rammed into the mold cavity by the plunger, the shot sleeve must withstand significant pressure.
Plunger—the piston in the shot sleeve.
Shot—a specific volume of melt that is injected into the mold cavity to form the casting.
Waterfalling—a condition in which melt flows down a surface, often leaving artifacts such as gas bubbles and solidification-induced particulates and layers in the casting.
“Advance”—movement of a plunger, pump, or melt itself that causes melt to progress toward the mold cavity
“Retract”—movement of a plunger, pump, or melt itself that causes melt to progress away from the mold cavity
“Metering”—pumping, or allow the transfer, of a predetermined volume of melt (a “shot”) that will completely fill the mold cavity with a calculated, small amount of excess
Define and give values for solidus and liquidus temperature
The concepts disclosed herein relate to atmospheric control (including sealing) and mechanical devices necessary to make the systems and methods work. Further aspects include maintaining a melt temperature throughout the cycle, injecting the melt at a (slower) rate to reduce turbulence and flow-related spraying, reducing cycle time (e.g., below 30 seconds), e.g., reducing the time to at or around 15 seconds, thereby increasing efficiency in terms of costs and the die casting process, and isolating melt from the atmosphere during the processes.
Where the atmosphere in the mold cavity is inert gas, that gas may be used at roughly atmospheric pressure, or its pressure may be increased to provide “counter pressure”—that is, pressure greater than atmospheric pressure. One benefit of counter pressure is that it may be used to control the properties of the advancing front of melt. The tendency of the melt to “wet” the mold cavity surface is affected by counter pressure. Another benefit of counter pressure is that to the extent that there are any gas bubbles in the melt, counter pressure will compress those bubbles to a smaller size. Further, flow-induces effect such as cavitation, which can cause damage to the mold cavity surfaces and leave defects in the casting, are suppressed by counter pressure. Each concept disclosure has a table that identifies whether use of counter pressure is a viable option with the given configurations.
The concepts disclosed herewith include different ways of metering each shot for each casting that is made. In some of them it has a metering pump. In others the plunger tip itself does the metering. In others there's a couple of valves that do the metering. Each describes a way of controlling how much volume is taken out of the hot chamber and put into the shot sleeve for each casting.
Generally, in each concept the melt never sees, never is in contact with any metal that's at a lower temperature than its solidest temperature and never in contact with any atmosphere that it would react to from the time that it's molten until the time that it solidifies in the die, in the mold cavity. The material is never being transferred to a cold chamber per se—or a cold shot sleeve. It gets all the way to the cavity in an environment that is heated. Further, it may be transferred in protected environment (e.g., vacuum) to the die cavity.
In some cases parts are lined with ceramic material.
Concepts A through C all involve a valve means adjacent to the cover die to seal the melt supply/hot chamber from exposure to air while the dies are open. The valving mechanisms require mating contact between two components, and when they come into contact, there is initially molten metal between them. There is a danger that the molten metal may solidify and “braze” the two together; thus, it is necessary to ensure that any molten metal between the two never drops below its solidus temperature. In these concepts, then, the conduits between the hot chamber and the cover die are all heated above the solidus temperature; inner surfaces, at least, are ceramic to prevent the melt from reacting with those surfaces. The plunger and shot chamber (where used) are also heated, and made of, or coated with, ceramic.
In a conventional “cold” shot chamber system, the plunger and shot sleeve are generally steel, and thus their exposure to melt must be of a very short duration. This limits the orientations that can be used, and generally dictates that the melt be poured into an opening on the top side of the shot sleeve. In Concepts A-C, since the shot sleeve and plunger are by necessity heated, and constructed of ceramic, these requirements do not exist. So, a variety of plunger and feed orientations are possible. For example, a plunger may be vertical, pointed up, and the feed port in the shot sleeve may be in constant contact with the melt. This may lead to advantages such as quicker cycle times and less likelihood of turbulent flow of melt into the shot chamber.
Concept A:
Melt is supplied by a hot chamber, protected by an inert atmosphere from contact with air, to a plunger that is housed in a shot chamber. In addition to driving the melt into the mold cavity, the plunger has two novel functions, 1.) to meter the volume of melt being injected and 2.) to seal the hot chamber/gooseneck from intrusion by air while the dies are open. The melt may be driven from the hot chamber to the plunger by a pump, by gravity in versions in which the pressure differential between the hot chamber and the mold cavity is either positive or zero, or by gas pressure in versions in which a positive pressure differential exists between the hot chamber and the mold cavity. Squeeze pin(s) are necessary to provide the desired high pressure at the end of the injection cycle. There are two possible combinations of atmospheric protection for the melt:
The design of the plunger/metering valve requires gravity to transfer the melt from the metering chamber to the shot chamber. Thus, the plunger axis must be in orientations 1 or 2 (see definitions at the end of this document) as defined herein. This requirement also limits the inert atmosphere in the mold cavity to vacuum only.
Concept B:
Melt is supplied by a hot chamber, protected by an inert atmosphere. There is no plunger; melt is driven into the mold cavity by either a pump, or (as in Concept A, depending on whether pressure differential conditions allow these methods) by gravity or gas pressure. A valve allows the melt to enter the mold cavity once the dies are closed and the proper inert atmosphere has been established in the mold cavity; the valve closes prior to the dies opening to protect the melt in the hot chamber/gooseneck from reaction with air. In one version, the pump may meter the melt volume injected into the mold cavity; in other versions, vacuum/gas shutoff valves in the die(s) are relied upon to control the melt volume. Squeeze pin(s) are necessary to provide the desired high pressure at the end of the injection cycle; the valve may be used to withstand the pressure generated by the squeeze pins. The inert atmosphere combinations are similar to those of Concept A, but use of inert gas in the mold cavity is also possible.
Concept C:
As in Concept A, melt is supplied by a hot chamber, protected by an inert atmosphere, to a plunger that is housed in a shot chamber. In this case, the plunger does not meter the melt, but does have a tip that seals and functions as a valve to protect the melt from exposure to air while the dies are open. Again, squeeze pins are needed to provide high pressure. The feed options and inert atmosphere combinations are the same as in Concept B.
The concepts above, and various feed and orientation options are outlined in more detail in the table below:
An example injection cycle for a non-metering pump is as follows:
In the example above, since the pump is non-metering, it may be left on continuously so that melt is always in contact with the shot sleeve feed port. The same is true of gravity or gas pressure feed.
The advantages of this system are:
A potential drawback of this system is that the requirements on the PMV are more demanding than they are on most other systems/elements; the PMV must hold vacuum, yet also be exposed to molten alloy flowing past it, and must be maintained at above the alloy solidus temperature.
Because of the need for a valve that is exposed to melt near the supply and also must hold vacuum, this disclosed approach has not been attempted or known. (Most vacuum valves in vacuum die cast systems are at the top of the die, at the last point reached by the inflowing melt, and as such are exposed to much lower temperature.
The overall system concept is shown below in Figures A1 and A2 (sectional views):
In accordance with another embodiment, inert gas is used in the crucible along with vacuum in the die (see A2 below). A2 is mechanically similar to Concept A1; the only differences pertain to atmosphere control. For example, the crucible/hot chamber is under constant pressure from an inert gas, such as argon. In addition to the previously noted advantages, due to the positive pressure in the crucible/hot tank, A2 does not rely heavily on the extent to which the PMV can hold vacuum.
The PMV must hold vacuum in the die, yet also be exposed to molten alloy flowing past it, and must be maintained at slightly above the alloy liquidus temperature.
In another embodiment, an inert gas is in the crucible, still vacuum in the die, but the melt is driven by gas pressure from the crucible and a pump is not used (see Figure below). Here, this concept is using the pressure of the inert gas to drive melt into that shot sleeve metering chamber or valve. So, while the above two embodiments have a non-metering pump in the crucible, e.g., this embodiment—with no pump in the crucible—uses the inert gas pressure in the crucible (which would be slightly higher than the atmosphere—e.g., 15 PSI absolute) to drive the melt into the metering chamber in the plunger. The pressure difference in the crucible (just over atmospheric, at about 15 psia) and that in the vacuum-evacuated mold cavity (essentially zero psia) pushes the melt into the PMV metering chamber.
In yet another embodiment, an inert gas is in the hot chamber but now instead of having vacuum in the die, an inert gas is introduced into the die as a different means of having atmospheric control. This gas may be used for what we call counter pressure. That is, there is a positive pressure in the die and the gas is pushing against that; this positive pressure has some beneficial effects as far as the front of the melt is concerned.
The front of the melt is the first part of the melt that is advancing into the mold cavity. So counter pressure has some effects on the surface tension of the melt, and affects the way that melt front behaves. It makes it behave better with respect to not breaking up and not spraying as it comes out of the gates, for example, and not getting turbulent.
This inert gas introduced into the mold cavity may or may not be controlled as counter pressure, however. But it's possible it may be quicker to get rid of oxygen and nitrogen in the mold cavity by first applying vacuum, then applying an inert gas and then possibly vacuum again, then inert gas again. For example, the first time vacuum is applied to the die cavity, say, 99% of the oxygen and nitrogen are removed, but then to get the rest of it out, one may either keep on pulling vacuum to get that last 1% out, or quickly fill that vacuum volume with inert gas, and it still has 1% air in it and 99% inert gas, then suck out that inert gas and oxygen combination again. This may reduce it down to 0.1% oxygen and nitrogen, if you apply the same level of vacuum to it.
In Summary, in addition to the previously noted mechanical features referenced above (e.g., see A2), this embodiment describes the use of inert gas and positive pressure in the mold cavity)
The die is purged by vacuum first, then filled with inert gas (e.g., argon) to provide counter pressure during injection to help prevent turbulence and breakdown of the melt front.
The die fill process may involve a single step each of vacuum and inert gas fill, or multiple steps such as vacuum/inert gas fill/re-vacuum/re-inert gas fill to further reduce the oxygen level in the mold cavity.
As with Concepts A1 and A2, but unlike the third, a pump pushes fluid upon command to the metering chamber; a low pressure pump such as an EM pump is an acceptable method. In this embodiment it is not necessary for the pump to meter the volume of melt, because the PMV performs that function.
It can introduce a positive pressure to prevent those bubbles from ever forming to begin with which is also an advantage.
This system may require additional overflow areas for the counter pressure gas, and/or pressure relief valves that limit the maximum pressure and vent the inert gas back to the inert gas source to recycle it.
Allows a high quality vacuum and positively-pressured inert gas atmosphere to be built up in the mold cavity before introducing the melt.
In one embodiment, the overall concept is to have a plunger that meters the shots, so the plunger itself has two functions. It pushes the melt into the mold cavity but it also meters the shot itself so it allows use of any kind of pump in the hot chamber. This disclosure uses amorphous metals with the hot chamber concept, which has never been done before, and it does solve some problems. For example, it solves that problem of repeated thermal cycling and individual one-shot melts.
In the figure shown in Concept A1, we're pumping some melt to the shot chamber, but because we're no longer melting an individual ingot which is perfectly sized to the size of shot that we need, we now use a method of metering the amount of melt that is given to the shot chamber of the shot sleeve. In this case, the plunger itself is that metering method.
As shown in the figures above and below, the angle of the plunger is shown at about 45 degrees. The reason for that is with this particular concept, it requires gravity so that once the metering chamber is full, then the plunger is put in a different position that requires gravity to allow the melt to progress down to the next session of the chamber (gravity feed). So that's the reason for the inclined plunger. The pump in this case it can be something like an EM (electromagnetic) pump, which in this case would not need to have its own metering. Pumps can be used in conjunction with a sensor such as an EM sensor where you control the current that's sent to the pump based on what the sensor says the volume is or has been. So those two things in combination can be a metering pump. But a metering pump can be just used in an on/off mode to supply a chamber as long as that chamber has a fixed volume.
The biscuit would be part of the casting that's injected. The configuration as shown is a draft angle that's easily ejectable. So instead of having a conventional sort of round biscuit with a little bit of a draft angle on the sides of it, it would be shaped a little less like that so that it's ejectable.
In Concept A1, it's all vacuum system and so there wouldn't be air, and that melt would find its own level and the plunger would approach it until just hits melt and it would start pushing it in.
In an embodiment, that there can be a plurality of rotating circular dies such as that pumping and flowing it simply keeps going on constantly.
Squeeze pins are also shown in the figures. Squeeze pins in die casting are used to increase the pressure generally at the end of the cycle—e.g., at the end of the injection cycle. Basically they are piston-like devices that either extend into the mold cavity a little bit or can be forced by the melt to retract into their bores a little bit. The same pins can be used as ejector pins, so once the casting has solidified and once the die is open and the casting is ready to be ejected, the squeeze pins are used to push the casting back out. Squeeze pins can perform that dual role.
In one embodiment, in the bore that squeeze pin resides in, a spring in that bore pushes on the squeeze pin to form a spring-loaded pin. In some embodiments, by putting a preload on it, the plunger is made to retract with a predetermined pressure.
Position 1 shows an angled shot chamber and the plunger. The tube that branches off down to the lower right is the feed tube that connects with the hot chamber itself (the source of molten alloy). In position 1, the plunger tip seals off the hot chamber. All these components are maintained above the liquidus temperature of the alloy because when this plunger tip is pressed against that ceramic seat, we don't want the alloy to serve as a brazing material and braze the plunger tip to the seat. So it is maintained above the solidus temperature. Also in the position 1 shown, the dies are open and the plunger tip itself is acting as a valve to seal everything else—everything upstream of that if you will—from the atmosphere. Then once the dies are closed and we have pulled a vacuum on the dies, the plunger is pulled back to position 2, which is the fill position.
In position 2 there's annular sealing around the OD of the plunger—that is, there is a very small gap between the plunger OD and the shot sleeve ID that functions as a seal—and so that volume shown there is going to define the metering volume. So in position 2 the pump in the hot chamber is actuated to fill that volume with melt. Once it fills, which will just take a fraction of a second, we move on to position 3.
In position 3, the large OD of the plunger has closed off the feed tube that goes down to the hot chamber. Now the volume that was in that annular area around that neckdown area of the plunger, it's now able to gravity feed past the plunger tip and into the mold area—that is, into that biscuit area.
Then finally in position 4, once we know that all the melt has gravity-fed past that plunger tip empirically; (e.g., based on a predetermined amount of time, e.g., 0.05 sec)—then the plunger is advanced. Once the plunger enters that final diameter within the shot sleeve, then it becomes a piston instead of a valve and it is driven further forward. Now this is where the squeeze pins may come into play. If you just push that plunger tip all the way until it bottomed out on the mating valve seat, if then the fluid volume that was in that shot was a little bit low, the pressure in the die cavity won't build up. On the other hand, if it was a little bit high, the plunger tip wouldn't be able to stroke fully and seal on the valve seat. Thus, the squeeze pins may be employed to compensate for any differences in volume. As such, they could be simply preloaded by a spring, or they could be preloaded hydraulically or pneumatically to a certain pressure. The key is that they are able to be pushed in by the pressure that the plunger generates when it bottoms out on the seat, and compensate for any variation in volume.
In yet another embodiment, no plunger and no pump are provided. Instead, it has just a valve that is right next to the biscuit—that is, right next to the casting. This valve is simply open and shut, so there's a source of pressure. That source of pressure is the pressure differential. There's a higher pressure in the hot chamber than there is in the die cavity because one has inert gas, the other has vacuum in it.
For example, melt is drawn to the mold cavity by the pressure difference in the crucible (just over atmospheric, at about 15 psia) and that in the vacuum-evacuated mold cavity (essentially zero psia).
To isolate the crucible/hot tank from contamination by the atmosphere when the dies are open, there is a valve adjacent to the biscuit.
Unlike the previous concepts, there is no ability to meter the shot. The valve is simply left open until the mold cavity is full.
After the mold cavity is full, the valve is shut, and shortly thereafter hydraulically-driven squeeze pins are to be activated to increase the final mold pressure. The purpose of increasing pressure is to minimize porosity in the casting, and in doing so, increase mechanical properties and improve the surface finish of the casting. The valve, since it is not a plunger per se, cannot generate this pressure, but nevertheless must withstand it.
The injection cycle is as follows:
The advantages of this system are:
As with previous concepts, a potential drawback of this system is that the requirements on the valve are more demanding than they are on most other systems/elements; the valve must hold vacuum in the die, yet also be exposed to molten alloy flowing past it, and must be maintained at slightly above the alloy liquidus temperature.
There are other potential drawbacks are unique to this embodiment (illustrated as Concept 5 below). One is that since the melt volume is simply drawn in by vacuum, it is not positively controlled/metered. Also, the fill rate is not controlled as it is with a PMV or plunger type system, and may be too slow or too fast.
Such an approach is not known and has not been done before, probably because of the need for a valve that is exposed to melt near the supply and also must hold vacuum. (Most vacuum valves in vacuum die cast systems are at the top of the die, at the last point reached by the inflowing melt, and as such are exposed to much lower temperature.)
The valve protects the melt while the dies are open, and also allows a high quality vacuum to be built up in the dies in the interval while they are closed but before the valve is opened. The best way to achieve a vacuum seal in this situation is with a ceramic-to-ceramic face seal (in this case on conical faces) as opposed to a small-gap (leaky) seal as would be typical when sealing between the OD of the plunger and the ID of the shot sleeve (as has been tried in the past).
It is critical that the melt does not solidify in the valve area. This is the reason that the valve is designed and oriented so that no internal surfaces are horizontal (so that the melt cannot pool), and that the valve must be maintained close to, or above, liquidus temperature. As such, all valve body internal surfaces and the valve itself must be a ceramic material which the melt will not wet.
The best ceramic material is considered to be zirconia. Other options may include alumina, magnesia, and silica; specific examples are Al2O3+MgO and Al2O3+SiO2 ceramics.
The overall system concept is shown below:
An injection cycle for a non-metering pump is as follows:
The table above gives the cycle for a pump; however, it is the same for gravity- or pressure-feed systems.
In accordance with embodiments, the pump may be an EM pump, centrifugal pump, piston pump, or any pump that can survive long term exposure in the melt. High pressure is not a requirement.
It is useful to control the pump flow rate, but not necessary to meter the shot. The valve may be simply left open until the mold cavity is full.
The advantages of this system are:
As with previous concepts, a potential drawback of this system is that the requirements on the valve are more demanding than they are on most other systems/elements; the valve must hold vacuum in the die, yet also be exposed to molten alloy flowing past it, and must be maintained at slightly above the alloy liquidus temperature.
Generally, this disclosed approach is not known, most likely because of the need for a valve that is exposed to melt near the supply and also must hold vacuum. (Most vacuum valves in vacuum die cast systems are at the top of the die, at the last point reached by the inflowing melt, and as such are exposed to much lower temperature. Further, the melt does not go through the valve itself; rather, the valve control mechanism is designed to shut the valve just before the melt actually passes through it.)
The valve protects the melt while the dies are open, and also allows a high quality vacuum to be built up in the dies in the interval while they are closed but before the valve is opened. The best way to achieve a vacuum seal in this situation is with a ceramic-to-ceramic face seal (in this case on conical faces) as opposed to a small-gap (leaky) seal as would be typical when sealing between the OD of the plunger and the ID of the shot sleeve (as has been tried in the past).
It is critical that the melt does not solidify in the valve area. This is the reason that the valve must be maintained close to, or above, liquidus temperature.
The best ceramic valve material is considered to be fused silica. Other options may include aluminum oxide, and aluminum titanate. The feed tube and hot chamber linings, and hot chamber pump materials may be made of various ceramic materials including fused silica, aluminum oxide, aluminum titanate, zirconium oxide, and magnesium oxide; specific examples are Al2O3+MgO and Al2O3+SiO2 ceramics.
The overall system concept is shown below:
The above “Concept B2” shows an inert gas in crucible/hot tank, with melt driven into mold cavity by a pump, and with a valve to isolate crucible/hot tank from atmosphere while dies are open.
In the above illustrated Concept B2 there is no shot chamber or plunger but instead the system has a valve in place of a plunger. This concept provides the melt from the hot chamber purely by pressure and the hot chamber is positively pressured. As described above, the mold cavity has a vacuum so that once the valve is opened, the mold cavity fills based on that pressure differential. Once the mold is full, the valve is simply shut and then the squeeze pins are actuated.
Traditionally hot chamber die casting is a relatively low pressure process because the pump is submerged in the hot chamber, and because at such a high temperature, the components of the pump can't take a whole lot of stress. It is typically a piston or plunger type pump. So the hot camber process is typically relatively low pressure, say 500 to 1500 PSI or thereabout. But in the last 10-15 years, industry has realized, especially in aluminum products, that they need a high pressure squeeze to get high quality castings. So some known processes involve injecting with the low pressure pump in the hot chamber, then freezing in this sprue area to provide essentially a valve (a stopper) there. As soon as that sprue area has cooled, or actively cooled, as soon as that melt freezes there but before the rest of the melt in the casting solidifies, they'll use squeeze pins to jack the pressure up (to maybe 10,000 PSI). It has been found that high pressurization makes a difference between getting good mechanical properties, and especially low porosity properties, in castings, and getting bad properties.
In this disclosure, the melt is introduced in the die cavity with a low pressure gas differential but then once the cavity fills, we close that valve—that same valve that is used to isolate the hot chamber from the atmosphere with the die opening. So once the cavity is full, we close that valve and activate the squeeze pins to increase the pressure in the melt in the mold cavity, before it solidifies. That's crucial to this process.
In one embodiment, a metering pump is used instead of gas to drive the melt from the hot chamber into the die, into the mold cavity.
In another embodiment, a non-metering pump is used to drive the melt in.
In either case, vacuum valves in the mold cavity may apply vacuum to the cavity when fluid melt is not being pushed in, and then when the fluid hits those valves, the molten fluid, it freezes up quickly and basically seals off the cavity. At that point you can further apply pressure.
In one embodiment, the vacuum is in both the hot chamber and in the die, and inert gas is used in the hot chamber and vacuum in the die.
In another embodiment, the crucible/hot chamber is under a vacuum environment, but does not require an inert gas system.
The pump may be a metering pump, for example (i.e., a pump that delivers the melt from the hot chamber to the plunger to the shot chamber, and is capable of delivering a specific volume of melt).
An example injection cycle for a metering pump is as follows:
In the example above, since the pump is metering (i.e., positive displacement), it may be held in position while the plunger shot chamber is not filling, so that melt is always in contact with the shot sleeve feed port.
The various combinations of atmosphere and feed methods are shown in the table below:
The advantages of this system are:
A potential drawback of this system is that the requirements on the plunger are more demanding than they are on most other systems/elements; the plunger tip must hold vacuum, yet also be exposed to the molten alloy flowing past it, and must be maintained at above the alloy solidus temperature.
Because of the need for a valve that is exposed to melt near the supply and also must hold vacuum this disclosed approach has not been attempted or known. (Most vacuum valves in vacuum die cast systems are at the top of the die, at the last point reached by the inflowing melt, and as such are exposed to much lower temperature.
The plunger tip, serving as a valve, protects the melt while the dies are open, and also allows a high quality vacuum to be built up in the dies in the interval while they are closed but before the valve is opened. The best way to achieve a vacuum seal in this situation is with a ceramic-to-ceramic face seal (in this case on conical faces) as opposed to a small-gap (leaky) seal as would be typical when sealing between the OD of the plunger and the ID of the shot sleeve (as has been tried in the past).
It is critical that the melt does not solidify in the plunger area. This is the reason that the plunger must be maintained close to, or above, liquidus temperature.
In the particular configuration shown, the plunger tip seals against a separate valve seat. This seat is made of ceramic. A separate valve seat is considered to be the ideal configuration, as it may exhibit a different wear rate, or necessitate different material properties, than that of the shot sleeve. However, as the shot sleeve in this concept must also be either made of ceramic, or lined with ceramic, the valve seat alternatively could be formed integrally into the shot sleeve.
The best ceramic valve material is considered to be fused silica. Other options may include aluminum oxide, and aluminum titanate. The feed tube and hot chamber linings, and hot chamber pump materials may be made of various ceramic materials including fused silica, aluminum oxide, aluminum titanate, zirconium oxide, and magnesium oxide; specific examples are Al2O3+MgO and Al2O3+SiO2 ceramics.
The overall system concept is shown below:
For illustrative purposes, the shot chamber is shown as being oriented at a 45 degree angle so as to reduce negative effects such as waterfalling or bubbles.
System Description: A Low Pressure Pump in the Hot Chamber Feeds a Metered Shot to a “Cold” Shot Chamber (or Cold Shot Sleeve), which Forces the Melt into the Mold at Hither Pressure. Inert Gas Pressure on Back Side of Plunger Prevents Air Intrusion while Dies are Open.
The injection cycle is as follows:
The above described embodiment provides a way of keeping atmosphere from getting in and contaminating the melt in cases where we don't want a chamber that has to be heated above the solidest temperature.
The melt is going to be about a thousand degrees C./about 1800 degrees F. and about 1500 where iron-based materials get red hot. At such temperatures the materials lose almost if not all of their strength properties. (At about 1200 degrees F. is where most materials start to degrade in strength.) Thus, when trying to use a ferrous alloy at those temperatures, it would have no strength whatsoever and wouldn't be able to obtain high pressure out of it. Further, it would scour and scratch easily and there would be braising of the alloy to the steel.
Thus, this embodiment aims to keep the shot sleeve below that temperature range, dump the molten alloy into it very quickly, and inject it very quickly.
The various combinations of atmosphere and feed methods are shown in the table below:
The unique advantages of this system are:
The shot chamber or shot sleeve doesn't necessarily have to be maintained above the solidest temperature of the alloy.
Typically when the dies are open, air can leak past the plunger into the melt chamber and it will contaminate the melt. Once the dies are closed it can also take a long time to draw a vacuum and suck the air back out of that chamber. This disclosure solves the challenge of making the process more efficient by simply pressurizing that chamber that houses the crucible/ladle with inert gas at slightly higher than atmospheric pressure (e.g., 15-16 psi atmospheric).
In accordance with an embodiment, the shot chamber is oriented somewhere between vertical and horizontal (e.g., pointing upwards toward the die, as opposed to being horizontal as a conventional shot chamber normally is).
The hot chamber pump with the shuttle piston is shown below in
The overall system concept in Configuration D1, with the plunger in orientation 4 (angled, pointed up) is shown below in
The overall system concept in Configuration D1, with the plunger in orientation 3 (horizontal) is shown below in
The terminology for Concept D is shown below (note that the shot sleeve, insulating spacer, and bushing are sectioned for clarity):
The images below are typical of both concept D1 and D2; the only difference in D2 is that the mold cavity is first evacuated with vacuum, then filled with inert gas.
In this position (position 1, above), the dies are open and basically we're just holding the plunger where it's stroked out, at its last stroke, or retracted a little bit, so it doesn't maintain contact with the biscuit as it's solidifying or as it's still real hot. In this position, this plunger backside area connects to the port that supplies it with inert gas and the inert gas also goes down and fills the feed tube. So everything is surrounded by inert gas that's at a slightly higher pressure than atmospheric pressure.
Position four shows where the piston first begins to open up the feed tube to that backside chamber and now inert gas can push the melt, or really just allow the melt to fall back down the feed tube and back into the hot chamber.
System Description: A Low Pressure Pump Feeds Melt from a Hot Chamber to a Metering Chamber, which Feeds a Cold Shot Chamber. The Metering Chamber Volume is Defined by the Space Between Two Valves. The Valves Also Isolate the Melt in the Feed Tube/Hot Chamber from Exposure to Air while the Dies are Open. Final High Pressure Injection is Provided by the Cold Shot Chamber.
The injection cycle is as follows:
Note that the pump may be “on” as long as the lower valve is closed (which allows the upper valve to be open). However, it may not be necessary to run the pump for such a large percentage of the overall cycle.
The various combinations of atmosphere and feed methods are shown in the table below:
The advantages of this system are:
The overall system concept (version with horizontal shot sleeve) is shown in
This embodiment provides the ability of getting a metered shot by putting two valves in the system prior to the shot chamber. The valves are between the hot chamber and the shot chamber. In this concept, the plunger doesn't have any means of the sealing against atmosphere. The lower most of those two valves will be closed when the dies are open. When the dies are open, atmosphere can enter into the plunger cavity. Once the dies are closed, a vacuum is pulled on the mold cavity that will also suck the air out of the plunger cavity (i.e. the shot chamber). In the meantime, the top valve is opened and the feed tube between the two valves fills up with a specific volume. That volume is defined by the length and the diameter of that feed tube. Once vacuum has been established, with the die closed, the bottom valve is opened and the melt is allowed through and then shot it into the dies.
In accordance with an embodiment, this disclosed concept uses gravity feed. In another embodiment, the same two valve configuration is used with a pump.
A nozzle may be used to prevent wetting of feed port in top-fill configuration:
An example valve is shown below:
As for the materials and type of valve, in one embodiment, both the body of the valve and the valve stopper itself are made from ceramic. In another embodiment, at least the valve stopper is made of ceramic. In yet another embodiment, the valve stopper and stem are made of ceramic. The valve needs to be heated above the solidest temperature continuously. In an embodiment, the valve may be manufactured such that the area of the seat of this valve and its angle is such that there isn't a surface that's horizontal, so that the melt never touches a surface that's horizontal. The valve has to be kept heated so that the material never braises, it never solidifies and braises the valve to the body.
To get rid of any air in or around the valves, when the dies are open, the plunger is left in its fully extended position so that there's only a small gap between the plunger and the inside diameter of the shot chamber. Any air that leaks in will be done slowly so it doesn't create thermal shock for that valve. Then there is drawing vacuum on the dies once the dies are closed. Once vacuum on the dies is being drawn, the plunger is pulled back so that it's open to the feed port and therefore open to this passage below the bottom of the valve and suck all the air out of there.
System Description: Combination hot chamber/cold shot chamber system. Final injection is provided by a cold shot chamber. A valve in the feed tube, proximal to the shot chamber, isolates the melt from atmosphere while the dies are open. The shot must be metered by the hot chamber pump and/or control system.
(Note: This system is similar in many respects to Concepts D and E. The use of a valve is similar to that of Concept E, but since there is only one valve, shot metering must be performed by a means other than a metering chamber. As with Concept D, the metering function is provided by either by a positive displacement hot chamber pump, or by a non-positive displacement pump combined with flow sensor(s) and a control system.
The injection cycle is as follows:
The various combinations of atmosphere and feed method are shown in the table below:
The unique advantages of this system are:
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2017/055616 | 10/6/2017 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62404974 | Oct 2016 | US |
