Method and Apparatus for Manufacturing Light Metal Alloy

1. Field of the Invention

The invention relates to a method and apparatus for manufacturing metal alloys, more particularly to a method and apparatus for manufacturing a light metal alloy by the process of injection molding the metal alloy when it is in a thixotropic (semi-solid) state.

2. Description of the Related Art

One conventional method used to produce molded metal alloys is the die cast method. The die cast method is disclosed in U.S. Pat. Nos. 3,902,544 and 3,936,298, both of which are incorporated by reference herein. The die cast method uses liquid metal alloys during casting and as a consequence, metal alloys produced from this method have low densities. Metal alloys having low densities are not desirable because of their lower mechanical strength, higher porosity, and larger micro shrinkage. It is thus difficult to accurately dimension molded metal alloys, and once dimensioned, to maintain their shapes. Moreover, metal alloys produced from die casting have difficulty in reducing the resilient stresses developed therein.

The thixotropic method improves upon the die casting method by injection molding a metal alloy from its thixotropic (semi-solid) state rather than die casting it from its liquid state. The result is a metal alloy which has a higher density than one produced from the die casting method.

A method and apparatus for manufacturing a metal alloy from its thixotropic state is disclosed in U.S. Pat. No. 5,040,589, which is incorporated by reference herein. A method of converting a metal alloy into a thixotropic state by controlled heating is disclosed in U.S. Pat. Nos. 4,694,881 and 4,694,882, both of which are incorporated by reference herein.

The system disclosed in U.S. Pat. No. 5,040,589 is an in-line system, in which the conversion of the metal alloy into a thixotropic state and the pressurizing of the same for the purposes of injection molding is carried out within a single cylindrical housing. With such a system, it is difficult to control the molding conditions, i.e., temperature, pressure, time, etc., and as a result, metal alloys of inconsistent characteristics are produced.

Moreover, the system of U.S. Pat. No. 5,040,589 requires that the metal alloy supplied to the feeder be in pellet form. As a consequence, if a molded metal alloy of undesired characteristics are produced by its system, recycling of the defective molded metal alloys is not possible unless the defective molds are recast in pellet form.

An improved system for manufacturing light alloy metals, which is capable of accurately producing molded metal alloys of specified dimensions within a narrow density tolerance, is desired. Further, a production process for light alloy metals which can consistently produce molded metal alloys of desired characteristics, and which can easily accommodate recycling of defective molded metal alloys would represent a substantial advance in this art.

An object of the invention is to provide a method and apparatus for producing metal alloys through injection molding.

Another object of the invention is to provide an improved injection molding system for metal alloys which is capable of producing molded metal alloys of accurate dimensions within a narrow density tolerance.

Still another object of the invention is to provide an injection molding system for light alloy metals which is capable of producing light alloy metals of desired characteristics in a consistent manner.

Still another object of the invention is to provide an injection molding system for light alloy metals which accommodates recycling of defective molded metal alloys easily.

These and other objects are accomplished by an improved injection molding system for metal alloys in which the steps of melting the metal alloy, converting the metal alloy into a thixotropic state, and injecting the metal alloy in the thixotropic state into a mold are carried out at physically separate locations.

The improved system comprises a feeder in which the metal alloy is melted and a barrel in which the liquid metal alloy is converted into a thixotropic state. An accumulation chamber draws in the metal alloy in the thixotropic state through a valve disposed in an opening between the barrel and the accumulation chamber. The valve selectively opens and closes the opening in response to a pressure differential between the accumulation chamber and the barrel.

After the metal alloy in the thixotropic state is drawn in, it is injected through an exit port provided on the accumulation chamber. The exit port has a variable heating device disposed around it. This heating device cycles the temperature near the exit port between an upper limit and a lower limit. The temperature is cycled to an upper limit when the metal alloy in the thixotropic state is injected and to a lower limit when the metal alloy in the thixotropic state is drawn into the accumulation chamber from the barrel.

A piston-cylinder assembly supplies the accumulation chamber with the pressure necessary to inject the metal alloy in the thixotropic state and with the suction necessary to draw in the metal alloy in the thixotropic state from the barrel.

Additional objects and advantages of the invention will be set forth in the description which follows. The objects and advantages of the invention may be realized and obtained by means of instrumentalities and combinations particularly pointed out in the appended claims.

The invention is described in detail herein with reference to the drawings in which:

FIG. 1 is a schematic illustration of a side view of the injection molding system according to a first embodiment of the invention;

FIGS. 2A and 2B illustrates the two positions of a ball valve used in the injection molding system of the invention;

FIG. 3 is a schematic illustration of a top view of the injection molding system according to a second embodiment of the invention;

FIG. 4 is a block diagram of an exemplary control circuit for the heating elements of the injection molding system according to the invention; and

FIG. 5 shows characteristic curves, corresponding to three solid/liquid ratios, achievable by the control circuit of FIG. 4.

In the discussion of the preferred embodiment which follows, a metal alloy is produced by injection molding from a magnesium (Mg) alloy ingot. The invention is not limited to a Mg alloy and is equally applicable to other types of metal alloys. Further, specific temperature and temperature ranges cited in the description of the preferred embodiment are applicable only to a system producing a Mg alloy, but could readily be modified in accordance with the principles of the invention by those skilled in the art in order to accommodate other alloys. For example, a Zinc alloy becomes thixotropic at about 380.degree. C.-420.degree. C.

FIG. 1 illustrates an injection molding system 10 according to a first embodiment of the invention. The system 10 has four substantially cylindrical sections--a feeder 20, a barrel 30, a cylinder 40, and an accumulation chamber 50. A metal alloy, e.g., Mg alloy, is supplied to the feeder 20. The feeder 20 is provided with a mixer 22 and a heating element 25 disposed around its outer periphery. The heating element 25 may be of any conventional type and operates to maintain the feeder 20 at a temperature high enough to keep the metal alloy supplied through the feeder 20 in a liquid state. For a Mg ingot, this temperature would be about 600.degree. C. or greater. The mixer 22 is driven by a stirrer motor 23 for the purposes of evenly distributing the heat from the heating element 25 to the metal alloy supplied to the feeder 20.

The liquid metal alloy is subsequently supplied to the barrel 30 by way of gravity through an opening 27 which may optionally be supplied with a valve serving as a stopper (not shown). The barrel 30 has a plurality of heating elements 70a-e disposed along the length of the barrel 30. The heating elements 70a-e maintain the barrel at temperatures at and slightly below the melting point of the liquid metal alloy supplied from the feeder 20. For an injection molding system 10 designed for a Mg ingot, heating pairs 70a and 70 b would be maintained at a temperature of about 600.degree. C.; a heating pair 70c would be maintained at a temperature of about 580.degree. C.; and heating pairs 70d and 70e would be maintained at a temperature of about 550.degree. C. Heating pairs 70a-70e induce, a thermal slope to the metal alloy flowing through the barrel 30. The purpose of the thermal slope is to convert liquid metal alloy entering the barrel 30 into a metal alloy in the thixotropic state at the exit of the barrel 30.

The barrel 30 also has a physical slope or an inclination. The inclination, preferably between 30.degree. and 90.degree., is necessary to supply the metal alloy in the thixotropic state to the accumulation chamber 50 by the force of gravity. The barrel 30 is also provided with a mixer 32 which is driven by a stirrer motor 33. The mixer 32 is provided to assure that the ratio of solid and liquid is consistent throughout the metal alloy in the thixotropic state. Plural mixing blades attached to the rotating shaft may of course be used.

The metal alloy in the thixotropic state exits the barrel 30 into an accumulation chamber 50 through a ball valve 60. The ball valve 60 operates in response to a pressure differential between the accumulation chamber 50 and the barrel 30. The pressure within the barrel 30 remains somewhat constant, but the pressure within the accumulation chamber 50 is determined by the position of a piston 45 disposed in the cylinder 40. When the piston 45 is displaced inwardly, the pressure in the accumulation chamber 50 increases (and becomes higher than that of the barrel 30) and the ball valve 60 closes off an opening 37 between the barrel 30 and the accumulation chamber 50. When the piston 45 is displaced outwardly, the pressure in the accumulation chamber 50 decreases and is lower than that of the barrel 30, and the ball valve 60 opens. A seal 41, e.g., an O-ring, is provided at the outer periphery of the piston 45 to maintain the pressure within the accumulation chamber 50 and to prevent leakage of metal alloy in the thixotropic state drawn into the accumulation chamber 50.

The operation of the ball valve 60 is shown in greater detail in FIGS. 2A and 2B. FIG. 2A shows the position of the ball valve 60 when the piston 45 is displaced outwardly. In this case, the opening 37 between the barrel 30 and the accumulation chamber 50 is opened as the ball element 65 of the ball valve 60 moves away from the opening 37. A ball valve stop 62 is provided to confine the ball valve movement away from the opening 37. On the other hand, when the piston 45 is displaced inwardly, as shown in FIG. 2B, the pressure inside the accumulation chamber 50 increases and the ball element 65 of the ball valve 60 is forced to lodge up against the opening 37 and thereby close off fluid communication between the barrel 30 and the accumulation chamber 50.

In a slightly different embodiment, the ball valve 60 may be provided with a biasing element, e.g., a spring. In such a case, the ball element 65 may be biased towards either the open or the closed position. It is preferable to provide such a biasing element in larger injection molding systems for producing metal alloys.

In still another slightly different embodiment, the ball valve 60 may be electronically controlled, in which the opening and closing of the ball valve would be synchronized with the displacement motion of the piston 45.

As shown in FIG. 1, heating elements 70f-70i and heating element 80 are also provided along the lengths of the cylinder 40 and the accumulation chamber 50. Heating elements referenced and prefixed by the numeral 70 are resistance heating elements. In the preferred embodiment of the injection molding system for producing a Mg alloy, heating pairs 70f-70i are preferably maintained at temperatures of 550.degree.-570.degree. C. in order to maintain the metal alloy in a semi-solid state.

The heating element 80 is an induction coil heater and is used to cycle the temperature at an exit port 57 of the accumulation chamber 50 between temperatures 550.degree. C. and 580.degree. C. One cycle is approximately 30 seconds to one minute. As the temperature at the exit port 57 is cycled, the characteristic of the metal alloy in the thixotropic state near the exit port 57 is varied. For example, the exit port 57 at a temperature of 550.degree. C. would cause the metal alloy in the thixotropic state to have a higher solid to liquid ratio compared with the situation in which the exit port 57 is at a temperature of 580.degree. C.

The purpose of raising the solid to liquid ratio of the metal alloy in the thixotropic state at the exit port 57 during the outward stroke of the piston 45 is to solidify the metal alloy in the thixotropic state near the exit port 57 sufficiently to function as a plug for the accumulation chamber 50. During the inward stroke of piston 45, the temperature at the exit port 57 cycled to a higher temperature (e.g., 580.degree. C.) so that the metal alloy in the thixotropic state at the exit port 57 will take on a characteristic with a lower solid/liquid ratio and thereby allow the metal alloy in the thixotropic state to be easily injected through the exit port 57.

The injection of the metal alloy in the thixotropic state is made through the exit port 57 into a mold (not shown). Molded metal alloys of desired characteristics are retained and molded metal alloys of undesired characteristics are recycled to the feeder 20. The defective molded metal alloys (e.g., density of molded metal alloy outside a predetermined range, surface blemish, etc.) are recycled "as is" and need not be reformed into any particular shape, since the system according to the invention melts the metal alloy supplied thereto before further processing.

The control of the heating elements 70, the cycling of the induction coil heating element 80, and the timing of the piston stroke are implemented electronically based on the following. The heating elements 70 are resistance heating elements. Electric current is supplied through the heating elements 70 sufficiently to maintain the heating elements 70 at their desired temperatures. The cycling of the induction coil heating element 80 is synchronized with the piston stroke. An outward piston stroke should be synchronized with the lower temperature and an inward piston stroke should be synchronized with the upper temperature. The control of the piston stroke is accomplished in a conventional manner.

The following table gives representative dimensions for a large, medium and small injection molding systems for metal alloys.

The dimensions given in the above table are exemplary and are provided to give guidance on how scaling for large, medium and small systems should be carried out. In the table, d indicates the inside diameter and l indicates the length. All dimensions are in millimeters (mm).

FIG. 3 is a top view illustration of a second embodiment of the injection molding system of the present invention. This embodiment is identical to the first embodiment except for the barrel 30. The barrel 30 in FIG. 3 is positioned horizontally with respect to the cylinder 40 and the accumulations chamber 50. Since gravity no longer supplies the force necessary to advance the metal alloy in the thixotropic state flowing in the barrel 30, a plurality of screw elements 34 driven by the motor 33 is provided. The screw elements 34 advance the metal alloy in the thixotropic state to accumulate near the opening 37 adjacent to the ball valve 60. The mixer 32 is provided on the same shaft 35 which rotates the screw elements 34. (In FIG. 3, the shaft 35 is shown to be separated by the feeder 20, because the shaft 35 runs underneath the feeder 20.) Therefore, the motor 33 operates to power both the screw elements 34 and the mixer 32. Other features of this embodiment are identical to the first embodiment.

Both the first and second embodiments may also have a pressure device attached to the barrel 30 to slightly pressurize the barrel. Such pressure is much less than the pressure used in the cylinder 40 and the accumulation chamber 50.

In all of the embodiments of the invention it is desired to have a temperature gradient between the portion of the barrel 30 in which the metal alloy enters the barrel 30 and the portion of the opening 37 where the metal alloy in the thixotropic state exits the barrel 30. The temperature gradient is necessary in order to produce the metal alloy in the thixotropic state. An exemplary manner of producing the temperature gradient is shown in FIGS. 4 and 5. As seen in FIG. 4, the control apparatus includes a control device 100 and a power supply circuit 102. The power supply circuit is connected to each of the heating element pairs 70a-70iand supplies different currents for the resistive heaters. Thus, a larger current (or a current supplied for a longer time, or a combination of current value and time) supplied from the power supply to a particular heating element or pair, say pair 70a, results in a larger heating effect in the resistive heater pair.

Each of the heating pairs 70a-70e heats a respective localized zone in the barrel 30. By controlling the current (and/or time) supplied to the heating pairs 70a-70e, the amount of heat in each zone of the barrel 30 adjacent the respective heating pair may be controlled. While only five heating pairs 70a-70eare shown provided for the barrel 30, the barrel 30 is preferably equipped with between seven to ten separately controllable heating zones, each corresponding to a separately controllable heating pair.

Preferably, the control device is programmable so that the desired solid/liquid ratio characteristic R1, R2, R3 of the metal alloy in the thixotropic state may be achieved as seen in FIG. 5. Control device 100 may, for example, comprise a microprocessor (with an associated input device such as a keyboard, not shown) which may be easily and quickly reprogrammed to changed the resultant solid/liquid ratio depending on the type of finished mold product desired. FIG. 5 shows three characteristic curves for three different values, R1, R2, and R3 of the solid/liquid ratio. The abscissa of the graph in FIG. 5 is labeled "a, b, . . . e" corresponding to the position of the respective heating pairs 70a, 70b . . . 70e in FIGS. 1 and 3. The ordinate of FIG. 5 represents the varying temperature range which may be employed. It should be appreciated that all values of the temperature used for the heating pairs 70a, 70b . . . 70e are within the range of 550.degree. C. to 580.degree. C. necessary to maintain the metal alloy in its thixotropic state. Further, it will be noted that the values of the temperature associated with the position of heating pair 70a are approximately the same (580.degree. C.) for all the curves since these values are near the value of the metal alloy as it enter the barrel 30 from the feeder 20. By selecting a ratio R1, as contrasted with R3, one may achieve a larger solid/liquid ratio and thus achieve a more dense resultant metal alloy in the thixotropic state and a more dense molded product. The heating element pairs 70f-70i are all typically controlled to have a temperature equal to the temperature of the heating pair 70e, i.e., there is no temperature gradient between heating pairs 70f-70i.

FIG. 4 also shows an electrically actuated valve 104 which may be used instead of the ball valve 60. The electrically actuated valve 104 has two positions, one permitting communication between the barrel 30 and accumulation chamber 50 and the other blocking such communication. The valve is controlled by the power supply circuit as shown by the dotted line 106. Two limit switches S1 and S2 are used to open and close valve 104. These limit switches are shown implemented in the form of two photodetectors 108 and 110 and associated light sources 112 and 114 (i.e., photodiodes). Detector 108 provides an output signal along line 116 to the control device 100 whenever the light beam from the source 112 is interrupted by the piston 45 moving outwardly (to the right in FIGS. 1 and 3) and thus acts as a first switch S1. In response to this signal the control valve 104 is opened permitting the metal alloy in the thixotropic state to enter the accumulation chamber 50 from the barrel 30. Also, this same signal may be used to direct the power supply circuit to cool down the induction coil heating element 80 to a relatively low temperature (550.degree. C.) thus permitting the solid/liquid ratio of the metal alloy in the thixotropic state which is adjacent the exit port 57 to increase and thus form a plug.

When the piston 45 reaches its outermost position as shown by the dotted lines 45' in FIGS. 1 and 3, the second limit switch (light source 114 and photodetector 110) is actuated for delivering a signal along line 118 to the control device 100 thus acting as a second switch S2 (e.g., see FIG. 4). In response to this signal, the control device 100 directs the power supply circuit 102 to close valve 104 and to raise the temperature of the induction coil heating element 80 to thereby lower the solid/liquid ratio of the metal alloy in the thixotropic state in the region of the exit port 57 and unplug the exit port 57 to permit injection to take place upon the inward movement of the piston 45.

In the above described manner, the gradient temperature may be selectively controlled, and the induction coil heating element 80 may be controlled in synchronism with the movement of the piston 45. Moreover, in the case of an electronically actuated valve, the valve opening and closing may also be controlled in synchronism with the movement of the piston 45.

While particular embodiments according to the invention have been illustrated and described above, it will be clear that the invention can take a variety of forms and embodiments within the scope of the appended claims. For example, the photodetectors and light sources may be replaced by mechanical micro-switches, or the position of the piston 45 may be inferred by measuring pressure changes within the accumulation chamber 50. Alternatively, an encoder (e.g. photoencoder) may be used to detect the position of the shaft 45.