DIE CASTING MACHINE WITH REDUCED STATIC INJECTION PRESSURE

Abstract

A die casting method initially injects an alloy with minimal volumetric contraction during solidification into a die cavity, monitors at least one of the pressure of the shot cylinder and the position of the plunger, and reduces the injection pressure during the final stage of filling of the die mold. A die casting machine includes a shot cylinder having one of a pressure detector located for detecting the hydraulic pressure applied to the cylinder or a position sensor for a plunger rod. The plunger rod includes a tip extending into a cold chamber, which receives a metal alloy having a minimal shrinkage characteristic. A source of hydraulic pressure is coupled to the shot cylinder by a control valve and a control circuit is coupled to said valve and to one of said detector or sensor for reducing the injection pressure near the end of an injection cycle.

Description

BACKGROUND OF THE INVENTION

The present invention relates to die casting machines and methods for controlling the injection pressure, particularly during the final stages of the injection process. Die casting machines inject metals, polymers, or other material in a controlled fashion into a mold (a.k.a. tool or die) that is clamped in a closed position by the machine. The metal is typically injected into a die using a hydraulic cylinder. For most metals, the metal is injected into the die with a controlled or predetermined velocity and/or pressure. Back-pressure from pushing the metal through a thin die entrance (i.e., gate) requires significant hydraulic force to overcome such resistance to flow. At the end of die filling, the hydraulic force of the injection cylinder applies hydrostatic pressure to the metal in the die. During solidification of the metal in the die, the metal undergoes a volumetric change that typically contracts the metal, causing porosity in the part known as shrinkage. Shrinkage is minimized through the injection of more molten metal via the high pressure applied to the injection cylinder. Frequently, especially in the casting of aluminum alloys, a higher pressure source is actuated on the head side of the injection cylinder, to further increase the force of the cylinder by a factor of up to five times the injection force used for the initial die filling.

When the cavity is completely filled during injection, and the cylinder applies force on the solidifying metal, the transferred pressure on the metal counteracts the clamped die pre-load force. When the hydrostatic transmitted force of the metal exceeds the clamping force, the die opens and the molten metal will be ejected under high pressure from the die resulting in flash. Flashing causes major process problems including: 1) variation in part size and dimensions, 2) damage to the die, and 3) frequent process stops to remove the sticking flash from the die.

The projected area of the casting, that is the surface area of the casting that is perpendicular to the closing axis of the die casting machine, is limited by the hydrostatic metal pressure of the solidifying metal. The product of the projected area and hydrostatic metal pressure cannot exceed the clamping force of the die casting machine. For example, a part with 100 sq. in. of projected area and 10,000 psi of applied metal pressure from the injection cylinder would have 1,000,000 pounds of separating force, or 500 tons. The 500 tons of force requires a die casting machine with 500 tons of clamping force to maintain the die closed during the casting process. This product of hydrostatic metal pressure and casting projected area constrains the size of parts that can be produced for a given size of die casting machine.

FIG. 1 shows a typical molten metal injection system including a hydraulic shot cylinder 10 having a piston 12 coupled to a plunger 14. The diagram represents part of an overall die cast machine which can be of a conventional commercially available design. Shot cylinder 10 is activated by a pressurized source of hydraulic fluid applied at an inlet 16. An outlet 18 releases hydraulic fluid from the shot cylinder to a reservoir 19 through valve 17. Plunger 14 extends into a cylindrical cold chamber 20, which has a molten metal inlet 22. Plunger 14 has a plunger tip 24, which typically has a smaller diameter than the diameter of the shot cylinder piston 12. Plunger tip 24 forces molten metal out of an exit end 26 of cold chamber 20. The exit end 26 of cold chamber 20 communicates with one or more runners 32 formed in die halves 30, 31. The width of runner(s) 32 is typically less than 1 inch. The runner(s) 32, in turn, each communicate with a gate 34 leading to the mold cavity 36 in die halves 30, 31. The mold cavity 36 typically will also communicate with one or more overflow cavities 38.

When the die cavity 36 is completely filled during injection, and the shot cylinder 10, applies force on the solidifying metal, the transferred pressure on the metal counteracts the clamped die pre-load force. If the hydrostatic transmitted force of the metal exceeds the clamping force, the die opens and the metal will flash. Thus, flashing occurs when molten metal is ejected under high pressure between the die halves and can cause major process problems including a variation in part size and dimensions, damage to the die, and frequent process stops to remove the sticking flash from the die halves before the next injection cycle.

An example of a typical injection profile for the operation of the machine of FIG. 1 is shown in FIG. 2. The available force of the shot cylinder 10 is plotted versus the position of the plunger rod 14. In regime 1, the hydraulic cylinder pushes the molten metal in the cold chamber 20 towards the cavity 36. Typically, the cold chamber is not completely filled after pouring, so in this flow regime, an injection cylinder advances the metal to completely fill the cold chamber 20. There is little resistance to this fluid flow because the cold chamber is typically a cylinder with large diameter (greater than 1″). At the end of regime 1, when the cold chamber is filled, the separating force is now equal to the metal pressure in the cold chamber multiplied by the cross-sectional area of the cold chamber.

In regime 2, the runner 32 begins to fill with metal. The runner is substantially smaller in cross section than the cold chamber, typically less than 1″ in diameter. The smaller cross section begins to create back pressure in the metal within the runner and cold chamber, and thus the hydraulic fluid in shot cylinder 10. At the end of regime 2, the separating force increases by an amount equal to the metal pressure multiplied by the projected area of the runners plus the increased pressure applied to the cross-sectional area of the cold chamber.

At the end of regime 2 and the beginning of regime 3, the metal begins to flow into the part through the gate 34. The gate is relatively thin, with a thickness which can vary from 0.020″ to 0.500″, but is typically less than 0.100″ for most die castings.

During regime 3, metal pressure in the cavity rises from resistance to flow through thin sections of the part in mold cavity 36. The metal pressure in the cavity now begins to transmit onto the closed die halves (30, 31), which are held closed by the clamping force of the die cast machine. The hydraulic cylinder pressure in the head side of the shot cylinder also rises. Resistance to flow is not yet maximized because the metal can still flow within the cavity 36 and also into the overflows 38. If intensification (an increase in the force of the shot cylinder) is utilized, it is typically triggered during the final stages of regime 3. Intensification is essential for alloys such as aluminum that undergo volumetric shrinkage. By the end of regime 3, the die separating force has risen substantially.

In regime 4, the final sections of the mold cavity 36 and overflows 38 are filled.

Overflows are designed to create back pressure within the casting, and also capture metal ridden with gas, lubricants, defects, etc. The pressure rises yet again within the metal and the hydraulic cylinder, and the die separating force correspondingly increases because of the increased back pressure on the metal in the casting portion of the cavity.

In regime 5, the casting and overflows are completely filled, and the hydrostatic force in the metal rises to its maximum value. For a solidifying metal, the force peaks in the initial stages when the metal remains almost completely a fluid. When the metal begins solidifying, the pressure is only hydrostatically transferred to the projected area on the die in contact with molten metal. The metal pressure during this stage of the casting cycle, combined with the molten projected area of the casting, typically dictates the necessary clamping force and therefore size of the die casting machine. For metals that undergo volumetric shrinkage, the hydraulic cylinder may advance some distance because additional molten metal within the runner will enter the part cavity into the void created by the shrinking, solidifying metal. If the separating force in regime 5 exceeds the die clamping force, the die will flash, creating significant process related problems noted above.

Recent advances in materials technology have led to the development of alloys that experience minimal or no volumetric contraction during solidification or cooling. An example of this is a metallic glass alloy, such as described in U.S. Pat. Nos. 5,618,359 and 7,017,645, the disclosures of which are incorporated herein by reference. These alloys are viscous and require a relatively high injection force to push the metal through the gate(s) and fill the cavity. However, because there is little or no volumetric contraction of the solidifying material (shrinkage), the applied static force of the hydraulic cylinder when the cavity is full only serves to place limits on the size of part that can be made for a given die casting machine. Therefore, while a high dynamic force is required to fill the die, a high static force only serves to limit the projected area of parts that can be made on a given die casting machine.

SUMMARY OF THE INVENTION

A die casting machine injection system of the present invention decreases the hydraulic force during the final stages of injection of a molten alloy into a die, culminating with a final hydrostatic pressure on the alloy in the filled cavity that is less than the dynamic injection pressure. This is in contrast to current state of the art die casting machines which use either the same hydraulic force for injection and intensification, or a higher intensification force and do not decrease force during the final stages of injection and intensification.

A method of die casting parts according to the present invention includes initially injecting a molten alloy having the characteristics of minimal volumetric contraction during solidification into a die cavity during a filling stage, monitoring at least one of: 1) the pressure of the shot cylinder for injecting a molten alloy into the die cavity or 2) the position of the plunger rod for injecting the molten alloy into a die cavity, and reducing the injection pressure during the final stage of filling of the die mold for reducing the die separating force at the end of a molding cycle.

A die casting machine embodying the present invention includes a shot cylinder having one of a pressure detector located for detecting the hydraulic pressure applied to the cylinder or a position sensor for a plunger rod. The plunger rod includes a tip extending into a cold chamber, which receives a molten alloy having a minimal shrinkage characteristic. The machine also includes a source of hydraulic pressure, a control valve coupling said hydraulic pressure source to said shot cylinder, and a control circuit coupled to said valve and to one of said detector or sensor for reducing the injection pressure near the end of an injection cycle.

The resultant machine and operation greatly reduces the die separating forces at the end of a casting cycle where low shrinkage alloys are employed and allows the casting of larger parts with lower tonnage die casting machines. Larger projected areas of parts can be made on the same size of die casting machine that was previously limited by the higher final force. This increases the size and/or number of castings that may be made on a given die casting machine. The cost of the die casting machine can also decrease, because less clamping force is necessary to perform the process for a given sized part.

These and other features, objects and advantages of the present invention will become apparent upon reading the following description thereof together with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the injection system of a typical molten metal die casting machine;

FIG. 2 is a diagram showing an injection profile (force verses stroke) of such a typical die casting machine;

FIG. 3 is a schematic diagram of the injection system of a die casting machine incorporating the present invention;

FIG. 4 is a diagram showing an injection profile (force verses stroke) of the method of operation of the machine shown in FIG. 3; and

FIG. 5 is an electrical diagram in schematic and block form of the control system for the machine shown in FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In order to control the final pressure when molding an alloy which does not exhibit the characteristics of shrinking during solidification, i.e., an alloy such as a metallic glass alloy, the injection system shown in FIG. 3 and its method of control can be employed. It should be understood that the injection system of FIG. 3 is part of an overall die casting machine, which can be of the type disclosed in U.S. Published Patent Application 2003/0217829, the disclosure of which is incorporated herein by reference. The injection method and equipment of FIG. 3, however, can be incorporated in any conventional die casting machine to achieve the desired result of reducing the final injection pressure during the casting process.

In FIG. 3, an injection system 60 is disclosed which includes a shot cylinder 70 supplied on its intake side through an inlet 72 coupled to a control servo valve 74 and, in turn, coupled to a source of hydraulic fluid pressure 76. The net forward force provided by shot cylinder 70 can be reduced by decreasing the head-side pressure in the shot cylinder. A servo valve 74 (FIGS. 3 and 5), under the control of circuit 50, can be used to reduce the pressure during the final stages of filling from a high pressure source of hydraulic fluid 76, reducing the head pressure and, thus, the net forward force.

Shot cylinder 70 includes a piston 80 and plunger rod 82 extending therefrom having a plunger tip 84 which extends into a cold chamber 86 coupled to die halves 90 and 91. Cold chamber 86 has an inlet 88 through which molten alloy, such as a glass metal alloy is poured for filling the cold chamber 86 prior to the injection molding of the alloy into a die cavity 96 through gate 94 and outlet 89 of cold chamber 86. Die cavity 96 also communicates with an overflow 98. Cavity 96 forms, with die halves 90 and 91, the shape of a part to be molded. Restricting the hydraulic flow out of the rod-side outlet 71 of the shot cylinder 70 is a technique for controlling velocity of the shot cylinder plunger rod 82. A servo-hydraulic, flow-control valve 73 (FIGS. 3 and 5), such as the Parker TDL valve, controls the amount of hydraulic fluid exiting the rod-side of the shot cylinder into a reservoir 75. Restricting this exiting flow raises the pressure in the rod-side hydraulic fluid, also decreasing the net forward force of the shot cylinder. The servo-hydraulic valve 73 can completely restrict the flow from the rod-side of the shot cylinder, thus stopping the shot cylinder and decreasing its net forward force to zero. The servo-hydraulic valve can be controlled by different techniques; two examples include shifting the valve at a predetermined position of the shot cylinder based upon an input signal from a position sensor 54 (FIG. 5) associated with rod 82 or shifting the valve when the pressure (detected by pressure detector 52 shown in FIG. 5) rises above a selected level that is associated with the end of the part-cavity filling, regime 3 of FIG. 4. The FIG. 4 example of an injection profile using the invention shows the large injection forces in regime 3 necessary to inject the molten alloy through the thin orifice (gate) into the casting, which itself is quite thin. The available force is shown by the dotted line. However, when the casting is nearly filled, the force is reduced (dash-dot line). The force cannot be decreased too early in the process, otherwise the casting will not completely fill with molten alloy. One major benefit of the process is the reduced die separating force in regime 5, especially when compared with that of FIG. 2, regime 5.

A bypass conduit 100 couples outlet 71 to inlet 72 by means of a control valve 102 for further controlling the force applied by the plunger rod 82 to plunger tip 84. As shown in FIG. 5, the control circuit 50 has outputs which provide signals to control servo valve 74, bypass valve 102, and discharge valve 73. Circuit 50 receives pressure information from detector 52 and rod 82 position information from sensor 54. Circuit 50 is programmed to control the pressure on piston 80 of shot cylinder 70 for controlling the movement of plunger rod 82 and, therefore, the pressure applied to the metallic alloy in cold chamber 86 by plunger tip 84 according to the desired characteristics as shown in the pressure profile of FIG. 4. Each specific part defined by mold cavity 96 may require a specific pressure profile also the use of different glass metal alloys may result in different operating pressure profiles. One typical profile is illustrated in FIG. 4. Because the flow is restricted out of the rod-side outlet 71 of the shot cylinder 70, the net force of the cylinder is near zero, as shown at regime 5 in FIG. 4.

The ratio of the pressure on the head and rod side of the shot cylinder is inversely proportional to the area of the head and the annular area on the rod-side of the cylinder. For example, a shot cylinder with a 4″ diameter head and a 2″ diameter rod would have a head area of 12.6 sq. in. and an annular area of 9.4 sq. in. Therefore, if the head pressure was 3000 psi, the rod-side pressure would be 4000 psi. To limit the increase in pressure on the rod-side of the cylinder, a 1:1 head to annular area shot cylinder can be utilized. Furthermore, the 1:1 ratio cylinder could be utilized to decrease net forward force to zero by not only completely restricting the flow-out of the rod-side, but also by allowing hydraulic fluid to flow between the head and rod side of the cylinder by opening bypass valve 102 in the coupling (bypass) circuit 100. The pressure on the head and rod side are, therefore, equal, known as a regenerative mode. In this regenerative mode, if the head-side of the cylinder has a larger area than the rod annular area, there is a net forward force (a reduced force compared to the capability of the cylinder), and with a 1:1 head to annular area cylinder, the net forward force is zero.

A shot cylinder will also have a net-forward force of zero if the cylinder has fully extended. The stroke of the shot cylinder piston 80 of the die casting machine can be controlled to reach its limit (detected by a limit sensor) during the overflow filling regime of cavity fill thus stopping the shot cylinder.

Each of these techniques can decrease the force of the cylinder, anywhere from 0-100% of the dynamic force. The important criteria is to decrease the force within a short period time, on the order of 10 ms or less, or to stop the injection after the part is filled, but the overflows and therefore the complete cavity have not yet filled. In order to increase the time window of decreasing force and/or decreasing velocity, the overflows can be designed to be filled over a longer time frame.

It will become apparent to those skilled in the art that various modifications to the preferred embodiment of the invention as described herein can be made without departing from the spirit or scope of the invention as defined by the appended claims.

Claims

1. An injection system for a die casting machine comprising: a shot cylinder having a plunger rod and a hydraulic fluid inlet for receiving pressurized hydraulic fluid for moving said plunger rod;one of a pressure detector located for detecting the hydraulic pressure applied to said shot cylinder and a plunger rod position sensor for determining the position of said plunger rod;a cold chamber with an inlet for receiving a molten alloy having a low shrinkage characteristic, said plunger rod extending into said cold chamber for injecting a molten alloy into a die cavity;a source of hydraulic pressure for coupling to said shot cylinder;a control valve coupling said hydraulic pressure source to said inlet of said shot cylinder; anda control circuit having an output coupled to said valve and an input coupled to at least one of said detector and sensor and responsive to input signals for providing a control signal to said valve for reducing the injection pressure applied by said plunger rod near the end of an injection cycle.

2. The injection system as defined in claim 1 wherein said shot cylinder includes a hydraulic fluid outlet and a second valve coupled to said outlet, wherein said second valve is coupled to said control circuit and responsive to signals therefrom for reducing the injection pressure applied by said plunger rod near the end of an injection cycle.

3. The injection system as defined in claim 1 wherein said shot cylinder includes a hydraulic fluid outlet and a bypass conduit coupled between said inlet of said shot cylinder and said outlet and a bypass valve coupled in the bypass conduit, said bypass valve coupled to said control circuit and responsive to signals therefrom for reducing the injection pressure applied by said plunger rod near the end of an injection cycle.

4. The injection system as defined in claim 3 and including a second valve coupled to said outlet, wherein said second valve is coupled to said control circuit and responsive to signals therefrom for reducing the injection pressure applied by said plunger rod near the end of an injection cycle.

5. The injection system as defined in claim 1 wherein said molten alloy is a metal glass alloy.

6. A method of die casting parts comprising the steps of: injecting a molten alloy having the characteristics of minimal volumetric contraction during solidification into a die cavity during a filling stage;monitoring at least one of the pressure of a shot cylinder for injecting the molten alloy into the die cavity or the position of a plunger rod for injecting the molten metal into the die cavity; andreducing the injection pressure applied by the shot cylinder and plunger rod during a final stage of filling of the die mold for reducing the die separating force near the end of a molding cycle.

7. The method as defined in claim 6 wherein said reducing step comprises limiting the injection pressure at the head end of the shot cylinder near the end of said molding cycle.

8. The method as defined in claim 7 wherein said limiting step includes providing a valve between a high pressure source of hydraulic fluid and the head end of the shot cylinder and controlling the valve to limit the pressure of fluid applied to the shot cylinder and the resulting injection pressure of molten alloy.

9. The method as defined in claim 7 wherein the shot cylinder has an outlet at a side of a piston opposite the head end and said limiting step further comprises providing a valve coupled to the outlet and controlling the valve to limit the pressure applied by the shot cylinder to the die cavity.

10. The method as defined in claim 7 wherein the shot cylinder has an outlet at a side of a piston opposite the head end and said limiting step further comprises providing a bypass conduit between the head end of the shot cylinder and the outlet and a valve coupled in the bypass conduit and controlling the valve to control the pressure applied by the shot cylinder in injecting molten metal into the die cavity during a molding cycle.

11. The method as defined in claim 6 wherein said molten alloy is a metal glass alloy.

12. A die casting machine comprising: a pair of dies defining at least one die cavity for a part to be molded;a clamping system for clamping said dies together during the injection of a molten alloy into said die cavity;a shot cylinder having a plunger rod and a hydraulic fluid inlet for receiving pressurized hydraulic fluid for moving said plunger rod;a cold chamber with an inlet for receiving a molten alloy having a low shrinkage characteristic and an outlet coupled to said die cavity, said plunger rod extending into said cold chamber for injecting a molten alloy into said die cavity;a source of hydraulic pressure for coupling to said inlet of said shot cylinder; anda control system in communication with said source of hydraulic pressure for reducing the injection pressure applied by said plunger rod near the end of an injection cycle.

13. The die casting machine as defined in claim 12 and further including one of a pressure detector located for detecting the hydraulic pressure applied to said shot cylinder and a plunger rod position sensor for determining the position of said plunger rod.

14. The die casting machine as defined in claim 13 and further including a control valve coupling said hydraulic pressure source to said inlet of said shot cylinder.

15. The die casting machine as defined in claim 14 wherein said control system includes a control circuit having an output coupled to said valve and an input coupled to at least one of said detector and sensor and responsive to input signals for providing a control signal to said valve for reducing the injection pressure applied by said plunger rod near the end of an injection cycle.

16. The die casting machine as defined in claim 15 wherein said shot cylinder includes a hydraulic fluid outlet and a second valve coupled to said outlet, wherein said second valve is coupled to said control circuit and responsive to signals therefrom for reducing the injection pressure applied by said plunger rod near the end of an injection cycle.

17. The die casting machine as defined in claim 15 wherein said shot cylinder includes a hydraulic fluid outlet and a bypass conduit coupled between said inlet of said shot cylinder and said outlet and a bypass valve coupled in the bypass conduit, said bypass valve coupled to said control circuit and responsive to signals therefrom for reducing the injection pressure applied by said plunger rod near the end of an injection cycle.

18. The die casting machine as defined in claim 17 and including a second valve coupled to said outlet, wherein said second valve is coupled to said control circuit and responsive to signals therefrom for reducing the injection pressure applied by said plunger rod near the end of an injection cycle.

19. The die casting machine as defined in claim 18 wherein said molten alloy is a metal glass alloy.