Patent Grant
7828042
The present invention relates to a method and apparatus for die-casting magnesium components from molten magnesium using a hot runner system in which both temperature and flow are controlled.
Magnesium is an attractive material for application in motor vehicles because it is both a strong and lightweight material. The use of magnesium in motor vehicles is not new. Race driver Tommy Milton won the Indianapolis 500 in 1921 driving a car with magnesium pistons. A few years after that magnesium pistons entered mainstream automotive production. By the late 1930's over 4 million magnesium pistons had been produced. Even in the early days of car production, the weight-to-strength ratio of magnesium, compared with other commonly-used materials, was well-known.
Considering the recent increase in fuel prices driven largely by increased global demand, more attention is being given to any practical and economically viable step that can be taken to reduce vehicle weight without compromising strength and safety. Accordingly, magnesium is increasingly becoming an attractive alternative to steel, aluminum and polymers, given its ability to simultaneously meet crash-energy absorbing requirements while reducing the weight of vehicle components. Having a density of 1.8 kg/L, magnesium is 36% lighter per unit volume than aluminum (density=2.70 kg/L) and is 78% lighter per unit volume than steel (density=7.70 kg/L). Magnesium alloys also hold a competitive weight advantage over polymerized materials, being 20% lighter than most conventional glass reinforced polymer composites.
Beyond pistons, numerous other vehicle components are good candidates for being formed from magnesium, such as inner door panels, dashboard supports and instrument panel support beams. In the near-term it is anticipated that components made from magnesium for high volume use in the motor vehicle might also include powertrain, suspension and chassis components.
The fact that the surface “skin” of die-cast magnesium has better mechanical properties over the bulk than more commonly used materials, thinner (ribbed) and lighter die-castings of magnesium can be produced to meet their functional requirements. Such components can have sufficiently high strength per unit area to compete with more common and heavier aluminum and plastic components. Furthermore, magnesium has considerable manufacturing advantages over other die-cast metals, such as aluminum, being able to be cast closer to near net-shape thereby reducing the amount of material and associated costs. Particularly, components can be routinely cast at 1.0 mm to 1.5 mm wall thickness and 1 to 2 degree draft angles, which are typically ½ that of aluminum. The extensive fluid flow characteristics of magnesium offers a single, large casting to replace a plurality of steel fabrications. Magnesium also has a lower latent heat and reduced tendency for die pick-up and erosion. This allows a reduced die-casting machine cycle time (˜25% higher productivity) and 2 to 4 times longer die life (from 150-200,000 to 300-700,000 shots) compared with that of aluminum casting.
However, the use of magnesium in automotive components is burdened with certain drawbacks. While magnesium is abundant as a natural element, it is not available at a level to support automotive volumes. This situation causes hesitation among engineers to design and incorporate magnesium components. On the occasion when the magnesium is selected as the material of choice, designers fail to integrate die-casting design with manufacturing feasibility in which the mechanical properties, filling parameters, and solidification profiles are integrated to predict casting porosity and property distribution.
The raw material cost of magnesium relative to other commonly used materials is also an impediment to mass implementation in the automotive industry. Current techniques for casting parts from magnesium make expanding the use of magnesium into a broader array of products less attractive. Presently, all large die-castings are produced in high pressure, cold-chamber machines where the metal is injected from one central location. This approach results in inferior material properties and waste material.
Accordingly, in order to make the use of magnesium in the production of vehicle components more attractive to manufacturers, a new approach to product casting is needed. This new approach is the focus of the present invention.
The present invention represents an advancement in the technique of casting components from magnesium and other metals. The primary objective of the present invention is to provide a multi-point injection, hot runner system for introducing molten magnesium into production die cavities at a controlled temperature and flow rate. The method and apparatus of the present invention provides an approach that minimizes waste while maximizing manufacturing repeatability. This provides a cost-effective and practical solution to the problems ordinarily associated with current approaches to die-casting magnesium.
This invention accomplishes these and other objectives by utilizing a self-contained and enclosed system that maximizes control over heat and molten metal flow while minimizing contamination. The system utilizes a gooseneck in which a plunger draws molten metal from a molten metal crucible and directs the molten metal to a hot runner assembly via a machine nozzle. The molten metal exits the hot runner assembly into a mold cavity through the tip of the hot runner which is provided to gate directly on or very near the part surface.
Each of the machine nozzle, the hot runner assembly, and the hot runner tip is heated by adjacent heating elements which may be coil heaters, tubular heaters or band heaters. By providing such an array of heaters the temperature of the molten metal can be readily and accurately maintained.
Flow of the molten metal is regulated by use of the gooseneck plunger which incorporates an internal reciprocating piston to selectively draw molten metal from a crucible into which the plunger is at least partially submerged. Once the gooseneck plunger channel is filled with molten metal the direction of the piston is changed and the molten metal is forced under pressure out of the gooseneck and into the machine nozzle. A preferred and accurate pressure is maintained by the amount of force applied by the piston upon the molten metal. This pressure is maintained evenly throughout the system such that the molten metal moves at a constant, regulated flow out of the gooseneck and through the machine nozzle, the hot runner assembly, the hot runner tip, and into the cavity.
To maintain this constant pressure or zero pressure difference by avoiding the return of molten metal back into the gooseneck when the piston extracts or moves to apply pressure to the molten metal, a one-way (or non-return) check valve is also incorporated into the plunger to prevent such an outflow. During the extraction step a thermal valve (“TV”) is formed at the tip of the hot runner assembly, thus preventing flow of molten metal from the mold cavity and back into the hot runner tip. With this arrangement the molten metal is retained in and completely fills entire feeding system. This is necessary because magnesium molten metal needs to be present in the machine nozzle at all times, before and after each shot.
Attached to the check valve is a pipe which is fitted so as to allow the inflow of molten metal only from the upper portion of the molten metal-holding crucible. This assures that only that metal most free of impurities will be used in the casting process.
The formed thermal valve blocks the flow of molten metal back into the hot runner tip during ejection and application of the die lubricant. The thermal valve is formed at the tip of the hot runner. The tip of the hot runner is fitted with a heating element so that its temperature may be heated to a regulated temperature. The operation of the hot runner tip is closely controlled as it also determines the cycle time of the process. The temperature of the hot runner tip may be computer controlled by a pre-programmed feedback controller.
Temperature regulation of the hot runner tip is necessary to form the thermal valve which is a blockage of solid metal between the in-flowing molten metal and the mold cavity. The blockage is formed to prevent the back-flow of molten metal out of the cavity and back into the machine nozzle or gooseneck at the end of the shot. In addition, no pressure difference arises in the feeding system after the shot. The formation of the thermal valve is accomplished by a balance of both temperature regulation and tip opening geometry.
With respect to temperature regulation, if the temperature at the hot runner tip is too high for the given metal, then the thermal valve will not form at the end of the shot and the molten metal will back-flow into the gooseneck, allowing air to enter the system and compromising the quality of the component made from the next shot since exposure to air results in a component having relatively high porosity while exposure to oxygen leads to oxidation of the resulting component.
On the other hand, if the temperature at the tip is too low for the given metal, then the thermal valve will be oversized and will be formed too deep into the hot runner assembly. This condition will make it very difficult to re-melt the blockage for the next casting shot since an excessive amount of heat will be needed to re-melt the thermal valve. The needed excess amount of heat will make it difficult for the formation of a thermal valve in the subsequent shot.
Between these two extremes is the ideal temperature condition under which a blockage of solid metal is formed in the immediate area of the opening at the tip. The ideal amount of blockage is that which will prohibit the back-flow of molten metal into the machine nozzle or gooseneck after the die opens and will not excessively block the molten metal passageway during the next shot after the die is re-closed.
With respect to tip opening geometry, if the opening is too small, then the amount of molten metal allowed to pass will be too small to fill the cavity during the shot and the heat necessary to assure the flow of metal will be too great to permit the formation of a blockage. If the opening is too large, the flow of molten metal through the apparatus will be too fast to allow time for the heating element of the thermal valve to provide proper temperature regulation of the passing metal. Accordingly, the temperature of the thermal valve and the size of the tip opening are related and must be balanced to form the appropriate blockage at the tip opening.
By providing a method and apparatus according to the present invention, several advantages are achieved. First, the quality and consistency of die castings is improved. This is because less air is entrained, avoiding the chemical reaction of air with the molten metal. Second, reductions in cycle time and machine clamping force are achieved. Cycle time is reduced because molten metal is already present at a desired pressure and temperature within the entire course of the feeding system. A reduction in clamping force is achieved because the opening of the hot runner tip is small compared with the traditional sprue which is ordinarily much larger. Third, less waste and less recycling of material is achieved.
Other advantages and features of the invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and the appended claims.
For a more complete understanding of this invention, reference should now be made to the embodiment illustrated in greater detail in the accompanying drawings and described below by way of examples of the invention wherein:
In the following figures, the same reference numerals will be used to refer to the same components. In the following description, various operating parameters and components are described for one constructed embodiment. These specific parameters and components are included as examples and are not meant to be limiting.
With reference to
The hot chamber 10 includes a casting die 12. The casting die 12 includes a cover half 14 and an ejector half 16, a hot runner assembly 18 partially recessed within the cover half 14 of the casting die 12, a gooseneck 20, a shot plunger 21 operatively associated with the gooseneck 20, and a machine nozzle 22 fitted between the hot runner assembly 18 and the gooseneck 20. A substantial portion of the gooseneck 20 is submerged within a crucible 24 of molten metal.
Referring now to
With reference still to
The hot runner tip 38 is provided to establish thermal valving in the apparatus 10 whereby a thermal plug (shown in
The hot runner body 26 is positioned in a hot runner body cavity 40 which is recessed within the cover half 14 of the casting die 12. The hot runner body 26 is held in place by a support ring 42 which may be fastened to the cover half 14 of the casting die 12 by conventional means such as by mechanical fasteners 44, 44′.
It is important in the operation of the apparatus 10 that the molten metal be maintained at high temperatures at all stages between the crucible 24 and the die 12. Accordingly, a series of insulators and heaters are provided to maintain the needed temperatures. To this end the hot runner assembly 18 includes both insulators and heaters. A hot runner body insulator ring 46 is fitted between the hot runner body 26 and the support ring 42. A thermal valve insulator ring 49 is fitted between the hot runner tip 38 and the cover half 14 of the casting die 12. The hot runner body insulator ring 46 and the thermal valve insulator ring 49 are formed from a known insulating material.
To keep the hot runner assembly 19 as uniform a temperature as possible external heaters are applied. As illustrated in
In addition or as an alternative to the use of band heaters as illustrated in
Referring now to the hot runner tip 38, this component is illustrated in sectional view in
The hot runner tip heater 54 is provided to keep the hot runner tip 38 at a pre-selected temperature such that the metal at the end 41 may flow freely into the mold cavity during the plunger shot but will form a solid blockage once the shot is completed. Accordingly, there is a temperature differential between the end 41 and the hot runner tip 38. This temperature differential means that the area of the opening of the hot runner tip 38 into the mold cavity will be cooler than the rest of the hot runner tip 38, thus allowing the molten metal in the immediate area of the tip to cool and become solidified locally in the area of the tip. This arrangement prevents molten metal from leaking from the cavity and back into the hot runner tip 38 at the end of the shot.
The temperature differential is dependent upon the metal being used to make the cast component. By way of example, magnesium alloy (for example, AZ91) becomes solid at 470° C. and is fully molten at temperatures over 595° C. Accordingly, the temperature of the hot runner tip 38 must be such that the metal therein is molten to allow it to flow. Conversely, the temperature at the end 41 of the hot runner tip 38 that is open to the mold cavity must be cooler than that of the rest of the hot runner tip 38 and specifically must approach, but not necessarily meet, the temperature of 470° C. at which magnesium alloy is solid. Of course, the temperature of the thermal valve 38 may be adjusted up or down depending on the metal alloy being used.
As illustrated in
The machine nozzle 22 is illustrated in
As noted above, it is important to establish and maintain desired temperatures at all points between the crucible 24 and the die 12. Accordingly, the machine nozzle 22 is also provided with a heating element. Two forms of heating elements are illustrated in
Delivery of the molten metal from the crucible 24 to the machine nozzle 22 is accomplished by the gooseneck 20. The gooseneck 20 is detailed in sectional view in
The molten metal passageway 78 includes an inlet end 80 and an outlet end 82. The inlet end 80 is in fluid communication with the plunger cylinder 76 by way of a molten metal channel 84. The outlet end 82 terminates at a plunger molten metal outlet port 86. The plunger molten metal outlet port 86 is preferably of a conical configuration so as to mate snugly with the outer cone 68 of the machine nozzle molten metal input end 64.
The shot plunger 21 includes a piston head 88 and a plunger drive shaft 90 which selectively drives the piston head 88. The plunger drive shaft 90 reciprocates within the plunger cylinder 76. A pair of sacrificial metal rings 89 and 89′ is fitted to the piston head 88. The rings 89 and 89′ are sacrificial and are intended to be worn instead of the piston head 88 during operation. Accordingly, the need to replace the piston head 88 at regular intervals is avoided. The plunger drive shaft 90 is attached to a plunger drive mechanism 91 (shown in
The plunger cylinder 76 includes a molten metal inlet end 92. A check valve assembly 94 is fitted to the molten metal inlet end 92 at the base of the gooseneck 20 for controlling entry of the molten metal into the plunger cylinder 76 from the crucible 24. The check valve assembly 94 is needed to make repeatable castings per casting shot by assuring that the hot runner assembly 18 and the gooseneck 20 are always filled with molten metal.
The check valve assembly 94 includes an inlet end 96 and an outlet end 98. Between the inlet end 96 and the outlet end 98 of the check valve assembly 94 is a check valve ball 100. The check valve ball 100 is shown in its closed position on a check valve ball seat 102. A molten metal inlet tube 104 is optionally though preferably fitted to the inlet end 96 of the check valve assembly 94. This arrangement allows for purer molten metal to be drawn from the upper end of the crucible 24 than might be drawn from the lower end of the crucible 24.
The check valve ball 100 is movable between the illustrated closed position where the check valve ball 100 is positioned on the check valve ball seat 102 and an open position (not shown) where the check valve ball 100 is lifted off of the check valve ball seat 102. Particularly, molten metal is drawn from the crucible 24 into the plunger cylinder 76 when the piston head 88 is moved in a direction away from the molten metal inlet end 92 by suction. This action urges the check valve ball 100 to be moved from its closed position, resting upon the check valve ball seat 102, to its open, molten metal-passing position (not shown) whereupon molten metal may be allowed to pass through the check valve assembly 94 unrestricted by the check valve ball 100. Once the plunger cylinder is filled with molten metal, the piston head 88 is moved in an opposite direction, that is, it is moved toward the molten metal inlet end 92. This movement forces the molten metal against the check valve ball 100 such that it is moved against and seated upon the check valve ball seat 102. The molten metal is then forced through the molten metal channel 84, into the molten metal passageway 78, through the outlet end 82 and into the machine nozzle 22.
As noted above with reference to
The gooseneck body 74′ is configured so as to eliminate the need of having to change sacrificial rings. Accordingly, the piston head 88′ is provided without sacrificial rings. This is accomplished by use of a ceramic liner 105. The ceramic liner is a sleeve that is shrink-fitted within the gooseneck body 74′. The ceramic liner 105 may be composed of a variety of ceramic materials, but preferably is composed of a silicon nitride material such as SN-240 manufactured by Kyocera. Other ceramic materials may be used as an alternative to silicon nitride. By using a ceramic liner in the gooseneck 20′ the metal-to-metal wear of the arrangement of the gooseneck 20 is eliminated.
Regardless of whether the gooseneck 20 or the gooseneck 20′ is used, once the molten metal enters the machine nozzle 22 its movement is continued by the action of the piston head 88 through the machine nozzle 22 and into the hot runner body 26. Passing through the hot runner body 26, the molten metal next proceeds through the hot runner tip 38 and into a cavity in the die 12. This procedure represents the most fundamental aspect of the invention. The molten metal proceeds from the gooseneck 20 through to the casting die 12 with both the temperature and the rate of flow being fully controlled by external operations (not shown).
A flow-chart of the method of the present invention is set forth in
At Step B the piston head moves toward its injection position thereby closing the check valve and forcing the molten metal through the feeding system. The temperature of the hot runner tip is raised to a point above the melting point of the selected metal and this, in combination with the pressure of the molten metal now being forced through the system, causes the thermal valve to re-melt, permitting the flow of molten metal through the system and into the mold cavity.
Once the piston head has moved to its fully injected position at Step C whereby the mold cavity is filled, the temperature of the hot runner tip is allowed to drop to a point below the melting point of the selected metal. This allows the thermal valve to form within the hot runner tip. The formed part may be removed after cooling.
At Step D, the thermal valve remains in its blocking position, thus allowing the piston head to again move to its extracting position for the next shot.
The single port injector arrangement shown in
Referring first to
The position of the injectors 108 and 108′ relative to a cover die half 114 and an ejector die half 116 is illustrated in
The open ends of the thermal valves 112 and 112′ are positioned at a part line 126 between the cover die half 114 and the ejector die half 116. A component forming cavity 128 is formed between the cover die half 114 and the ejector die half 116. Manifold heaters 129, 129′, 129″ and 129′″ are embedded within the manifold body 120 to maintain the desired level of heat. It is to be understood that a greater or lesser number of heaters 129, 129′, 129″ and 129′″ may be used as desired and as necessary to achieve an adequate thermal profile. The illustrated heaters are of the tubular variety, but it should be understood that other configurations may be applicable.
In operation, the molten metal is forced from the machine nozzle (not shown) and into and through the manifold inlet 122, into and through the manifold passageway 124, into and through the injectors 108 and 108′, and into the cavity 128. The resulting casting is a part, generally illustrated as part 130, shown in
In the event that a part is to be cast that has a more complex configuration than part 130, an alternative arrangement, shown in
The twin injector hot runner manifold 136 includes a pair of spaced apart injectors 138 and 140. The positioning of the injectors 138 and 140 is made to improve the filling pattern of the die. As illustrated, the injectors 138 and 140 are of different lengths as is required to form the desired complex part. The injector 138 comprises a hot runner tip 142 and a hot runner 144. The injector 140 comprises a hot runner tip 146 and a hot runner 148. As with the thermal valves 110 and 110′ and the hot runners 112 and 112′ discussed above with respect to
The position of the injectors 138 and 140 relative to a cover die half 150 and an ejector die half 152 are illustrated in
Positioned adjacent to the cover die half 150 is a manifold body 158 which includes a manifold inlet 160. The manifold inlet 160 is in fluid communication with a manifold passageway 162. The manifold passageway 162 is itself in fluid communication with each of the injectors 138 and 140. A machine nozzle (not shown) is fluidly connected with the manifold inlet 160 in the same manner as discussed above with respect to machine nozzle 22.
The open ends of the hot runner tips 142 and 146 are positioned at a part line 164 between the cover die half 150 and the ejector die half 152. A component forming cavity 168 is formed between the cover die half 150 and the ejector die half 152. A pair of manifold heaters 170 and 170′ is embedded within the manifold body 158 and function as the series of manifold heaters 129, 129′, 129″ and 129′″ embedded within the manifold body 120 shown in
In operation, the molten metal is forced from the machine nozzle (not shown) and into and through the manifold inlet 160, into and through the manifold passageway 162, into and through the injectors 138 and 140, and into the mold cavity 168. The resulting casting is a relatively complex, multi-dimensional part, generally illustrated as part 172, shown in
A further use of the invention disclosed herein is shown in
With reference to
A manifold inlet 184 is provided on the manifold 178. Molten material is injected into the manifold inlet 184 from the machine nozzle (not shown) in the same manner as discussed above with respect to the use and operation of the machine nozzle 22. The manifold inlet 184 is in fluid communication with a manifold passageway 186. The manifold passageway 186 is in fluid communication with each of the injectors 180, 180′, 182, 182′, 182″ and 182′″.
The multiple injector mold assembly 176 further includes an ejector die half 190 and a cover die half 188. The injectors 180, 180′, 182, 182′, 182″ and 182′″ are recessed substantially within the cover die half 188. Between the cover die half 188 and the ejector die half 190 is defined a die cavity 192.
In operation, the molten metal is forced from the machine nozzle (not shown) and into and through the manifold inlet 184, into and through the manifold passageway 186, into and through the injectors 180, 180′, 182, 182′, 182″ and 182′″, and into the die cavity 192. The resulting casting (not illustrated) is a highly complex, large, multi-dimensional part. According to the various configurations discussed above, a wide variety of castings having different configurations and sizes may be made according to the teachings of the present invention.
The foregoing discussion discloses and describes an exemplary embodiment of the present invention. One knowledgeable in this area will readily recognize from such discussion, and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the true spirit and fair scope of the invention as defined by the following claims.
This invention was made with United States Government support awarded by the following program, agency and contract: NIST Advanced Technology Program, the United States Department of Commerce, Contract No. 70NANBOH3053. The United States has certain rights in this invention.
| Number | Name | Date | Kind |
|---|---|---|---|
| 1964324 | Korsmo | Jun 1934 | A |
| 3270383 | Hall et al. | Sep 1966 | A |
| 3652073 | Lewis | Mar 1972 | A |
| 4408651 | Smedley et al. | Oct 1983 | A |
| 4595044 | Caugherty et al. | Jun 1986 | A |
| 4690198 | Hardey et al. | Sep 1987 | A |
| 5244033 | Ueno | Sep 1993 | A |
| 5299623 | Yaffe et al. | Apr 1994 | A |
| 6305921 | Grams et al. | Oct 2001 | B1 |
| 6405784 | Takizawa et al. | Jun 2002 | B2 |
| 6405785 | Gellert et al. | Jun 2002 | B1 |
| 6505674 | Sample et al. | Jan 2003 | B1 |
| 6634412 | Murray et al. | Oct 2003 | B1 |
| 6830094 | Fink | Dec 2004 | B2 |
| 6945308 | Jones | Sep 2005 | B1 |
| 6945310 | Hirai et al. | Sep 2005 | B2 |
| 20020121355 | Bigler et al. | Sep 2002 | A1 |
| Number | Date | Country | |
|---|---|---|---|
| 20080115907 A1 | May 2008 | US |
